c. j. wilson, h. t. chang and s. t. kitai- firing patterns and synaptic potentials of identified...

8/3/2019 C. J. Wilson, H. T. Chang and S. T. Kitai- Firing Patterns and Synaptic Potentials of Identified Giant Aspiny Interneuro

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The Journal of Neuroscience, February 1990, 70(2): 508-519

Firing Patterns and Synaptic Potentials of Identified Giant AspinyInterneurons in the Rat NeostriatumC. J. Wilson, H. T. Chang, and S. T. Kita iDepartment of Anatomy and Neurobiology, University of Tennessee, Memphis College of Medicine, Memphis, Tennessee38163

Intracellular recordings were made in viwo from 9 giant as-piny neurons in the neostriatum of urethane-anesthetizedrats. The cells were identified by intracellular staining withHRP or biocytin. The neurons exhibited morphological fea-tures typical of neostriatal cholinergic interneurons. Six ofthe cells were obtained from intact animals, while 3 wererecorded from rats with ipsilateral hemidecortications.

Giant aspiny neurons were characterized by their slowirregular but tonic (3-l O/set) spontaneous act ivi ty and long-duration action potentials. Examination of the underlyingmembrane potential trajectories during spontaneous firingrevealed that individual action potentials were triggered fromspontaneous small (l-5 mV) depolarizing potentials. Thesespontaneous potentials exhibited the voltage sens itivi ty ofordinary EPSPs. They were much less frequent during the80-200 msec pause in tonic afferent input that follows theexcitation evoked by cortical or thalamic stimulation, andwere decreased in frequency in decorticate animals. Theirrise times and half-widths matched those expected for uni-tary synapt ic potentials placed proximally on the surface ofthe neurons. Low-intensity stimulation of neostriatal affer-ents produced small short-latency EPSPs that appeared tobe composed of responses identical to the spontaneousdepolarizing potentials. The latencies of the EPSPs evokedfrom the cerebral cortex and thalamus were consistent witha monosynaptic input from both structures, but the maximalsize of the EPSPs was much smaller than that evoked inspiny neurons, suggesting that a smaller number o f afferentinputs make synapses with each of the aspiny cells.

Giant aspiny neurons exhibited much larger input resis-tances and longer time constants than spiny neostriatal neu-rons. They also exhibited relatively linear steady-state cur-rent-voltage relationship compared to spiny projection cells.Input resistances ranged from 71-105 MS, and time con-stants ranged from 17.8-28.5 msec. Analysis of the chargingtransients in response to current pulses yielded estimatesof dendritic length of approximately 1 length constant. Re-petitive firing of the neurons was limited by a powerful spikeafterhyperpolarization and by a strong spike frequency ad-aptation.The sensitiv ity of the giant aspiny interneuron to a rela-

Received June 7, 1989; revised July 27, 1989; accepted July 28, 1989.Th is work wa s supported by NIH grant NS20473 and RCDANS01078 to C.J.W.,AGO5944 to H.T.C., and NS20702 to S.T.K.Correspondence should be addressed to Charles J. Wilson, Department ofAnat-omy and Neurobiology , University of Tenn essee , Memphis, 875 Monroe Avenue,Memphis, TN 38 163.

Copyright 0 1990 Socie ty for Neuro scienc e 0270-6474/90/100508 -12$02.00/O

tivel y small number of proximal afferent synaptic contacts,its tonic firing, and its widespread dendritic and axonal f ieldsplace it in an excellent position to act as a modulator of theexcitabili ty of neostriatal projection neurons in advance ofthe onset of movement-related neostriatal act ivi ty.

Between 6 and 9 cell categorieshave been constructed for theneuronsof the mammalian neostriatum basedon their somato-dendritic morphology in sectionsstainedwith the Golgi method(e.g., Kemp and Powell, 1971; DiFiglia et al., 1976; Chang etal., 1982). n thesestudies, he main group ofprojection neurons,the spiny cells, are viewed asa singlecategory of cells, and theremaining cells are all aspiny neurons. Most of the aspiny cellsare believed to be interneurons, and the differences n the num-bersof cell classes escribedby the various authors areprimarilyin their schemesor dividing up the interneurons. This hasbeena difficult task because,despite their morphological and cyto-chemical diversity, the aspiny neurons are relatively rare. Theyaccount for as little as 5% of the total cell population in ratsand cats e.g., Kemp and Powell, 1971; Graveland and DiFiglia,1985).An enormoussamplingbiasagainst he projection cellswouldbe required to obtain a reasonablesample of aspiny cells byrandom sampling.Despite heir smallsize, the neostriatal spinyneurons are relatively resistant to damageby microelectrodes,so there is no samplingbias against hem that would facilitaterecording from aspiny neurons. In addition, there have beennoneurophysiologicalcriteria to allow identification of aspiny neu-rons during intracellular recordings experiments. Eliminationof spiny projection neuronsafter identifying them by antidromicactivation has not been reliable. For reasons hat are not yetclear, many spiny projection neurons dentified by intracellularinjection of HRP, and even by following their axons from theneostriatum, do not respond antidromically to stimulation ofsubstantia nigra or globus pallidus (e.g., Liles, 1974; Fuller etal., 1975; Prestonet al., 1980; Changet al., 1981). It is thereforenot surprising that there have been no intracellular recordingstudiesof the properties of identified neostriatal interneurons.Nevertheless, he physiological properties of aspiny neuronsare of some functional importance. For example, among thevarious types of aspiny neurons s the cholinergic neuron, whoseactivity has ong been postulated as the focus of synaptic inter-actions responsible or the therapeutic effect of a variety oftreatments n Parkinsonsdisease,Huntingtons disease,ardivedyskinesia,and other disorders e.g.,see eview by Groves, 1983).Recognition of the importance of aspiny cellsand of their varietyin the neostriatum is rapidly increasing as new cytochemicalmarkers reveal categoriesof cells that stain specifically for var-


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The Journal of Neuroscience, February 1990, IO(2 ) 509

ious other neuroactive substances known to be important inbrain function.Extracellular recordings have the advantage of allowing thecollection of very large samples, in which even rare cell typesmay be detected. Most neurons in the neostriatum have verylow firing rates and may be silent for long periods of time. Theirsilence is interrupted by episodal firing that may be associatedwith performance of a learned task (e.g., DeLong, 1973). Thisfiring pattern has previously been shown to be typical of thespiny projection neurons (Wilson and Groves, 1981). A mi-nority of neurons show a tonic firing pattern, characterized byaverage fir ing rates as high as 10 Hz, with increases and decreasesin rate that may be temporally related to the presentation of asensory cue used as a signal for the animals to perform a learnedtask (Kimura et al., 1984). The identity of the neurons f iring inthis pattern and their position within the circuitry of the neostri-atum are unknown, but they have been presumed to be aspinyneurons.Over the past 8 years of intracellular recording and stainingof neostriatal neurons, we have collected a sample oflarge aspinyneurons similar enough in their morphological and physiologicalproperties to be considered representative of a single cell type.These cells appear to correspond to the tonically active neuronsdescribed by Kimura et al. (1984). These labeled neurons alsoshow morphological features identical to those of the large cho-line acetyltransferase-containing neurons described in immu-nocytochemical studies (Bolam et al., 1984; Phelps et al., 1985;DiFig lia, 1987). In this report we describe the spontaneous fir-ing, synaptic responses to afferent stimulation, and some of themembrane properties of these cells. A deta iled description ofthe morphological features of the cells will be reported else-where.Materials and MethodsThe cells were impaled in the course of intracellular recording experi-ments designed for other purposes. These cells were found only veryoccasionally (1 per year on average), and so no experiment was specif-ically designed for their analysis. Consequently, they were obtainedunder a variety of conditions. In all cases, the cells were recorded inmale rats, weighing between 250 and 550 gm and anesthetized withUrethane (1.3 gm/kg) supplemented with ketamine (35 mg/kg, i.m.,every 1 hr) or a combination of ketamine and xylazine (35 mg/kg ke-tamine, 6 mg/kg xylazine, i.m., every 1 hr). The animals were fixed ina stereotaxic device, and stimulating electrodes were implanted andattached to the skull with dental cement. The cerebrospinal fluid wasdrained, and the animal was suspended by a clamp placed on the baseof the tail. Body temperature was maintained at 36.5 t 1C. In 2 of the9 animals, the ipsilateral cerebral cortex had been removed acutely(between 2 and 6 hr earlier) by aspiration. A similar cortical lesion hasbeen made 3 d preceding the experiment in another animal. The corticallesions were very large, including the entire rostra1 pole of the cortex,and the frontal and parietal cortices lateral to within 1 mm o f the rhinalsulcus and medial to within 0.5 mm of the midline. The corpus callosumwas anatomically intact in all 3 cases, and the presence of synapticresponses evoked by stimulation of contralateral cerebral cortex stim-ulation was verified in 2 of these.Stimulating electrodes were placed in the cerebral peduncle at thelevel of the substantia nigra or in the substantia nigra itself (6 rats), theipsilateral cerebral cortex (4 rats), the ipsilateral parafascicular thalamicnucleus (6 rats), or the contralateral cerebral cortex (2 rats). Stimulatingelectrodes were pairs of insect pins (00 gauge) insulated except for 0.25mm at the tips with epoxylite. Stimuli were single bipolar 0.1 msecduration current pulses, 0.0 l-l .O mA in amplitude.Intracellular recordings were obtained using glass micropipettes bro-ken under microscopic control to have tip diameters less than 0.5 pm.They were filled with 4% HRP (Sigma Type VI) in 0.5 M potassiummethylsulfate and 0.05 M Tris buffe r (pH 7.6) or with 2% biocytin in 1M potassium acetate (Horikawa and Armstrong, 1988). They had re-

sistances between 30 and 80 MQ. Cells were labeled by passage ofpositive current pulses, 1 O-3.0 nA in amplitude and 150 msec in du-ration, at a rate of 3/set fo r 2-20 min.After completion of the recording experiment, the brains were fixedby intracardial perfusion of an aldehyde fixative and stored in fixa tiveovernight. Sections through the region of the injected neurons were cutat a thickness of 50 pm on a Vibratome and collected into phosphatebuffer or buffered saline. Sections containing cells injected with b iocytinwere treated in a solution of avidin-biotin-HRP comnlex (ABC. 1: 100dilution, Vector) and 0.25% T&on-X100 in PBS for4 hr.All sectionswere reacted with DAB (3-diaminobenzidine, 0.05%, and hydrogenperoxide, 0.03%) for 5-25 min. Sections containing injected neuronswere identified by microscopic observation in buffer and were treatedwith osmium tetroxide (0.1-l% depending upon whether they were tobe prepared for electron microscopy), dehydrated, and embedded inplastic between glass microscope slides and coverslips. Neurons weredrawn at 600 x using a 60 x oil-immersion objective (n.a. 1.4) and adrawing tube.ResultsIdentification of neuronsNine cells were identified as giant aspiny neurons on the basisof their morphological featuresafter intracellular staining withHRP (7 cells) or biocytin (2 cells). These representabout 2.6%of the sample of approximately 350 neostriatal neurons westained ntracellularly over the same ime period. On the basisof observationson Nissl-stainedsections rom the same nimalsand from the previous work of others (e.g., Kemp and Powell,197 ), cells arge enough o belong o the samecell class s heseneuronsaccount for only l-2% of the total neuron populationin the rostra1 head of the rat caudate putamen, where the re-cordings were made. Thus, there is no sampling bias againstthese cells, and in fact, cells of this type are probably slightlyoverrepresented n intracellular recording experiments,perhapsdue to their largesize. The possibility of a samplingbias n theirfavor is also suggested rom the quality of the recordings. Ingeneral, ntracellular recordings rom giant aspiny neuronswereof higher quality and were maintained for longer periods thanrecordings from medium spiny neurons, which made up theoverwhelming majority of intracellularly stained neostriatalneurons.All of the neurons whose esponses re reported herehad membrane potentials in excessof 50 mV and action po-tentials in excessof 55 mV when firing spontaneously.All ofthe neurons had action potential amplitudes n excessof theirmembranepotentials (spikeovershoot). Four of them were heldfor over 2 hr with no sign of deterioration.The morphological features of a representative giant aspinyneuron as seenafter intracellular staining are shown n Figure1. The somatawere elongatedand generally aligned n the ros-trocaudal direction. Measurementsof somatic size are difficultbecause he cell bodiescannot easily be differentiated from the2-4 large primary dendrites, but the cells ranged in cross-sec-tional area from 445 to 586 pm2 (mean, 506 pm2). For com-parison, a sampleof 6 1 neurons wascollected from sectionsofrat neostriatum stainedusing mmunocytochemistry for cholineacetyl transferase ChAT), cut in the sameplane, treated simi-larly as he intracellularly stainedneurons,and measured n thesameway. The cells n this sample anged rom 308 to 700 pm2in crosssection, with a mean of 446 pm2and a standard devia-tion of 86.2. Thus, the injected neurons n this sampleare ap-proximately the samesize as the cholinergic intemeurons andare thus among the largestneurons n the rat neostriatum. Theintrasomatic placementof the recording electrode n the neuronshown n Figure 1 s ndicated by the peroxidase-positive eryth-rocytes that mark the track of the electrode.


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Figure 1. A photomicrograph of anintracellularly stained giant aspiny neu-ron in a coronal section from the ratneostriatum. The section is stained withosmium tetroxide, which accounts forthe staining of the capsular fiber bun-dles. The site of recording in this neu-ron is apparent due to the numerousred blood cells, stained darkly becauseof their peroxidase act ivi ty, along theelectrode t rack. Only a small part of thedendritic field is visible in this micro-graph.

The large dendritic trunks branched repeatedly to fill a spaceextending approximately 500 pm in the dorsoventral and me-diolateral directions, and 750-1000 pm in the rostrocaudal di-rections. This arrangement of dendrites is shown in coronalsection in the drawing in Figure 2. This dendritic field orien-tation in the rostrocaudal direction is probably partly the resultof the locations of our neurons. All the recordings were madein the rostra1head of the caudate-putamen,where afferent andefferent fibers are all aligned ostrocaudally. Thus, the dendritictrees of the aspiny neurons may conform to the generalorien-tation of the tissue n this region of the neostriatum. The distaldendrites of the aspiny neuronsexhibited numerous fine, shortsidebranches,varicosities,and spine-likeappendages. ear theirterminals, many dendritesbranched repeatedly to form a com-plex terminal thicket, similar to those reported for neurons nthe substantia nigra (Grofova et al., 1982). The axons of theaspiny cells n this sampleall arose rom a primary dendrite atleast20 pm from the soma. n the early part of their trajectories,the axons appeared dentical to the dendrites from which they

arose. t wasonly from observation of the more distal branchingpattern that their axonal identity could be ascertained.Theycoursed farther between branch points than did the dendritesand gave rise to equal-sizeddaughter branches.The axonal ar-borizations of the neurons were very denseand extended overa large region of the neostriatum, usually even larger than thevolume occupied by the dendritic field.In intracellular recordings,giant aspiny neuronscould be dis-tinguished from the more commonly encountered spiny pro-jection neurons in 2 ways. One was their tonic spontaneousfiring, which wasespeciallyevident in intact animals.The otherwas the long duration and characteristic shapeof their actionpotentials.Spontaneousjring patternIn contrast to the more common spiny neurons, which weregenerallysilent or exhibited phasicactivity, all of the giant aspinyneurons ecorded n intact animals ired tonically in the absenceof stimulation. The firing of these cells was not periodic or


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The Journal of Neuroscience, February 1990, IO(2) 511

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bursty, but was a maintained irregular firing at rates of 3-lo/sec.This firing, asseen n intracellular recordings, s illustratedin Figure 3A. Spontaneousaction potentials were seen o arisefrom small spontaneousdepolarizing potentials, with ampli-tudes of 0.5-5.0 mV. These depolarizing potentials likewisewere irregular in occurrence, and most were subthreshold forgeneration of action potentials. Larger spontaneouspotentialsoften aroseby summation of the smallerones, as ndicated byan inflection in the rising phase of the depolarization, and aprolongation of its time course. As the membranepotential ofthe giant cells was never more than 5 mV from the actionpotential threshold, and was more commonly only 1 or 2 mV

Figure 2. Reconstruction of the entiredendritic tree of the cell shown in Fig-ure 1. The extent of the dendritic fieldin the mediolateral and dorsoventraldirections is apparent. Nineteen sec-tions, each 50 lrn thick, were requiredfor reconstruction of the dendritic fieldin the rostrocaudal direction. The axonof this cell had a series of successivebifurcations, with resulting branches ofapproximately equal size. All branchesof the axon became extremely fine afteronly a few such bifurcations. Only thefirs t few of the largest axonal branchesare shown.

below the value required to trigger a spike, the spontaneousdepolarizingpotentials often could trigger action potentials.Thistoo is in contrast with the more common spiny projection neu-rons, whosemembranesare commonly more than 10 mV fromthreshold in the same preparation. Hyperpolarization of themembraneof the giant aspiny neuronsby only a few millivoltscompletely suppressedhe spontaneous iring, as shown n Fig-ure 3B. However, even very large hyperpolarizations (20-30mV below the baseline)could not alter the rate of occurrenceof spontaneousdepolarizing potentials. These potentials didsometimesbecome slightly larger during the passage f hyper-polarizing currents, however (Fig. 3B). Passage f large depo-


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512 Wison et al. - Neostriatal Aspiny Neurons

I0 ml20 ms

Figure 3. Evidence that the spontaneous depolarizing potentials areEPSPs. A, Spontaneous depolarizing potentials and action potentials inthe absence of stimulat ion in a giant aspiny neuron. B, The same neuronduring application of 0.5 nA hyperpolarizing current. Action potentialsare completely suppressed. The incidence of depolarizing potentials isunchanged. No amount of hyperpolarization could alter the occurrenceof spontaneous depolarizing potentials in aspiny cells. Often, the am-plitudes o f the potentials were increased by hyperpolarization of theneuron, as they appear to be in B. C, Dependence of the depolarizingpotentials on extrinsic afferent act ivi ty. Stimulation of the cerebral pe-duncle (at arrow) evokes an EPSP in the same neuron shown in A andB, followed by a 50-l 50 msec period of reduced incidence of sponta-neous depolarizations and a corresponding reduced incidence of spon-taneous action potentials. This period corresponds to the inhibi toryperiod known to occur in corticostriatal and thalamostriatal neuronsafter stimulation of this kind.larizing currents, which led to rhythmic firing in aspiny neurons(see below) did not reverse the depolarizing potentials observedin intact animals.

Measurement of the rise times and half-widths of the spon-taneous depolarizing potentials was performed for 3 neuronsfrom in tact animals, in which a large number of these wererecorded. Rise times and half-widths measured for depolarizingpotentials that appeared to be simple and isolated did not dif ferfor the 3 cells. Rise times varied from 1.7 to 4.7 msec (mean =2.7), and half-widths from 4.4 to 17.0 msec (mean = 9.3 msec).Normalization for the average time constant for these 3 neurons

I0mV40 msFigure 4. Spike waveform in the giant aspiny interneuron. Five spon-taneously occurring action potentials are superimposed. This cell wasrecorded from an animal with acute decortication, accounting for therelatively low incidence of spontaneous depolarizing potentials. Theneuron was firing irregularly at a mean rate of about 2/set, with actionpotentials arising from the few depolarizing potentials that remained.A membrane hyperpolarization lasting SO-120 msec is observed tofollow every action potential. Although only about 2 mV in amplitude,this hyperpolarization was suff icien t to prevent spontaneous action po-tentials at intervals less than 50 msec. The baseline membrane potential(-63 mV) is indicated by the dashed line. The threshold for actionpotentials was 1.8 mV above the baseline.

(19 msec) yielded a mean rise time of 0.14 and a mean half-width of 0.49. Comparison with the published theoretical find-ings on pass ive neuron models and with EPSPs of known lo-cation on motor neurons and rubrospinal neurons (e.g. , Rall etal., 1967; Murakami et al., 1977) suggests that ifthe spontaneousdepolarizations were synaptic in origin, they must be locatedvery proximally.

Several observations suggested that the spontaneous depo-larizing potentials were synaptic in origin. First, the frequencyof the spontaneous potentials and the rate of spontaneous firingof the neurons were much lower in the cells recorded in animalswith acute cortical lesions. Such lesions are known to removea tonic exci tatory influence on the neostriatal spiny neurons(Wilson et al., 1983b). Second, a decrease in the occurrence ofspontaneous depolarizing potentials was seen to coincide withthe phasic pause in neostriatal afferent synaptic act ivi ty that isknown to follow stimulation of cerebral cortex, thalamus, orcerebral peduncle. This is illustrated in Figure 3C. The initialef fec t of stimulation of the cerebral peduncle is seen in thatfigure to be a short-lasting EPSP which triggers a single actionpotential. For a period of SO-100 msec thereafter, the rate ofoccurrence of spontaneous depolarizing potentials is decreasednearly to zero. Subsequently, these potentials, and action po-tentials, occur at an accelerated rate for a period of 20-50 msec.This sequence, observed in all of the cells recorded in intactrats, corresponds in time to the sequence of long-lasting hyper-polarization and rebound depolarization that occur in spinyneostriatal neurons following this same stimulation. Those re-sponses of the spiny neuron have been shown to be due to aperiod of decreased a fferent synaptic activity followed by anincreased synaptic bombardment from afferen t fibers in the ce-rebral cortex and thalamus (Wilson et al., 1983b). In spiny neu-rons, as well as in the giant aspiny cells, these late componentsof the response to synchronous cortical or thalamic afferentact ivi ty are absent af ter acute decortication.


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The Journal of Neuroscience, February 1990, 70(2) 513

A

Action potential shapeThe duration of action potentials of giant aspiny neurons rangedfrom 1.7 to 4.0 msec (measured at the base) and were usuallylonger than 2.0 msec in duration. This is substantially longerthan the 1.0-2.0 msec duration of action potentials of spinyneurons recorded in the same animals. The broader action po-tentials were not an artifact of depolarization due to damagedone by the microelectrode, because they were not substantiallyaltered when the cells were arti ficia lly hyperpolarized and actionpotentials were elicited using brief depolar izing pulses. In ad-dition to their long durations, the action potentials of the giantcells had a characteristic waveform that distinguished them fromthe spiny cells. The shape difference was so pronounced that itwas used to correctly predict the identity of al l but the firstneuron in the sample. The action potential had a very fast risingphase, and a gradual decay but without any marked shoulderor inflection. These action potential features are most clearlyseen in the example in Figure 4, which shows 5 superimposedspontaneous action potentials. The data shown in Figure 4 weretaken from an animal with a unilateral acute decortication, sothe rate of occurrence of spontaneous depolariz ing potentialswas very low, and the neuron was firing irregularly at a low rate(


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A ILL0 mV40 msC

Figure 6. Spontaneous depolarizations and minimal evoked responses in a giant aspiny interneuron. A, Firing in the absence of a stimulus.Numerous small depolarizing potentials are seen to occur in an irregular pattern. Most o f them are subthreshold for action potentials, but sometrigger spikes. B, The same neuron, but responding to an electrical stimulus applied to the thalamic stimulating site. A single action potential arisesfrom an EPSP. The EPSP amplitude (approximately 2 mV) is sliahtlv lamer than that of the suontaneous depolarizing ootentials. Uuver truce is_DC-coupled, at the same gain as A; lower trace s AC-coupled,-and m&h higher gain to show the shape-of the EPSP and the spontaneousdepolarizations. C-E, The EPSP evoked by stimulation near threshold for the response appears to be made up of small , all-or-none depolarizingpotentials similar to those observed spontaneously. Six successive sweeps, arranged in groups of 2, are shown at high gain to illustrate the responseto a stimulus applied to the cerebral peduncle. On one trial (in D), there is no response to the stimulus at all. In C and D, the response consists ofdepolarizing potentials similar to the ones occurring spontaneously. They differ from each other in amplitude and latency but have similar timecourses. In the last 2 traces, shown in E, the responses are nearly identical.

followed by the 2 shown in D, and finally the 2 trials shown inE. On one stimulus presentation shown in D, there was noresponse t all, while EPSPsevoked on the other 5 trials variedin amplitude and latency but had similar time courses.Theyalso resembled the spontaneousdepolarizing events, and ananalysis of their rise times and half-widths yielded identicalvalues to thoseobtained for spontaneous epolarizing potentialsrecorded in the sameneurons (seeabove). It might be arguedthat the stimulus had no effect and that the responses hown nFigure 6, C-E, were simply the chanceoccurrenceof the spon-taneous depolarizing responses.However, responses o lowerstimulus ntensities n the sameneuronsshow much lower fre-quency of occurrence of depolarizing potentials. Furthermore,increasing he stimulus ntensity beyond that used n these rialsproduced even more reliable responses, nd finally larger andlonger-lasting responses uch as the one shown in Figure 6B.Thus, the effect of increasedstimulus ntensity near thresholdfor the EPSP is best describedas ncreasing he probability ofoccurrence of depolarizing potentials indistinguishable fromthose that occur spontaneously. The larger EPSPs that occurwith larger stimuli may result from the summation of severalof the smallerdepolarizing potentials occurring simultaneously.

Responseso intracellularly applied current pulsesInput resistances ere determined using hyperpolarizing pulsesbecause f the low firing threshold of the aspiny neurons,whichcaused the cells to fire action potentials in response o evensmall depolarizations. Input resistance measurementswereavailable in 6 cellsand ranged rom 60 to 105MO. This is muchgreater than observed in spiny neurons in the sameanimals,which ranged from approximately 20 to 45 MO. In addition,the aspiny cellsshowedmuch more linear steady-state esponsesto hyperpolarizing current than is typical for neostriatal spinyneurons.Analysis of the repetitive firing of giant aspiny neurons inresponse o current pulsesand of their transient responsesointracellular current pulseswas completed n 4 cells,2 of whichwere recorded n animalswith acute decortication. The analysiswas much easier n the cells recorded in decorticate animalsbecause esponses ere not confounded by a high rate of spon-taneous iring, but the resultswere essentially dentical in the 4cells. An example showing the repetitive firing of one of thecells s shown in Figure 7. In Figure 7, A-C, firing in responseto stimulus pulses anging from 0.1 to 1 O nA are shown. Spike


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The Journal of Neuroscience, February 1990, 70(2) 515

20 Ins\

Figure 7. Repetitive iring character-isticsof a giantaspiny nterneuron. -C, Responsesf a neuron o constant-currentpulses f 3differentamplitudes.D, Frequency-intensityelation or thefull series f nulses. ome f whichare(/r: shownn A-? Frequenciesalculatedfrom he irst (openquares) andsecond*0.2 0.4 0.6 0.8 1.0 1s 1.4 (filled squares) intervalsare plotted oI(pA) show he spike requencyadaptation,which s present t all firing rates.E,Responsef the same euron o a 0.3nA depolarizing urrentpulse fter thecellwas njectedwith HRP by passage

afterhyperpolarization was much more pronounced when su-perimposedupon the depolarizing current pulses ut had a du-ration similar to that seenwith spontaneousaction potentials(cf Fig. 4) and rhythmic firing beganat interspike intervals of80-100 msec,corresponding o the duration of the spike after-hyperpolarization (Fig. 7A). Spike frequency, measuredas theinverse of the first interspike interval, increased inearly overthe range hat could be examined, asshown n Figure 70. Unlikeneostriatal spiny neurons,which showvery little spike requencyadaptation under thesecircumstances, iant aspiny neuronsex-hibited a pronounced spike frequency adaptation, which can beseen n the comparison of frequenciescalculated from the firstand second nterspike intervals in Figure 70. Thesepropertieswere sharedby all 4 neurons.In 2 of the cells examined in detail, and in several others inwhich repetitive firing was not analyzed, injection of HRP wasfollowed by a change n the firing of the cells.After injection ofHRP, thesecells exhibited a broadening of action potentials, areduction in spikeafterhyperpolarization, an increasen the rateof spontaneousactivity, and the occurrence of partial actionpotentials. An example of this is shown in Figure 7E for thesameneuron whose responses re shown n the rest of Figure7. Partial action potentials, but still distinctly all-or-none, oc-curred at very short intervals, and sometimeswere visible asinitial components of the full action potentials (Fig. 7&J. Theamplitude of the full action potentials wasnot affected, but theirduration was greatly increased,and the firing rate, as countedby the occurrenceof the full spikes,was also greatly increased.

of positivecurrent pulsesor 10 min.The spike afterhyperpolarization isgone,and smallaction potentials reintercalated etweenhe full spikesthelatterof whichcontinue o fireat aboutthenormal ate). n thiscondition, pikerepolarization as lsoslowed, nd hemembrane otential responseo theonset f the pulsewasmore apid.Theamplitudeof the action potentialandthe nput resistancef the cellwere otvery different from the responseb-servedbefore ntracellularnjection.

The input resistanceof the neuronswas not greatly altered (thecurrent pulse in Fig. 7E was the sameas that in Fig. 7C), butthe membranepotential transient response ad a much steeperonset (compare Fig. 7, C and E). These effects of intracellularinjection of HRP (and presumably Tris and potassium ons aswell) lasted l-10 mitt, after which the firing of the neuronsreturned to their preinjection state.Responseso step currents were analyzed in 4 neuronsusingthe method described by Rall (1969). As before, 2 of theseneuronswere from animalssubjected o acute decortication. Anexample showing a typical result is shown in Figure 8. Thetransient used or the analysisshown s in the inset at the upperright. In the lower graph, the transient is shown after normal-ization for size and resting membranepotential. The log of thenormalized transient is plotted as a function of time. This isequivalent to plotting the log of the derivative of the transientif the measurementof the steady-statemembrane potential isaccurate Rall, 1969) but it is esssensitive o noise.This meth-od hasbeen usedpreviously with striatal neuronsby Kita et al.(1984). The membrane ime constant, estimated rom the slopeof the late linear portion of the transient in Figure 8, was 18.6msec. A secondexponential process,with a time constant of2.6 mseccould alsobe extracted from this transient. Both offsetand onset ransientswere analyzed for eachneuron, and at least4 transients from each neuron were analyzed. Onset and offsettransients for hyperpolarizing current pulses ess han 0.5 nAsuperimposedexactly for all cells. With current pulses n thisrange, the input resistances nd time constantsof the neurons


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516 Wison et al. * Neostriatal Aspiny Neurons

Figure 8. Analysis o f time constantsof a giant aspiny neuron from the tran-sient response to a hyperpolarizing cur-rent step. The transient is shown in theinset at the upper right. The upper traceis the membrane potential transient, andthe lower trace shows the current step.The amplitude of the current was 0.2nA. The input resistance of this neuronwas 97 MCI as measured from this pulse(at 100 msec after onset). The sametransient is plotted below as the log ofthe proportional deviation of the mem-brane potential from its final value(again measured 100 msec after onset).The membrane time constant was de-termined as the slope o f the regressionline through the late linear componentof the transient (line labeled 70 = 18.6msec). Deviations from this line wereagain plotted on the same coordinates,and a regression line calculated for thelater linear portion of this data set (linelabeled 71 = 2.6 msec). This was takento be the first equalizing time constant.Higher-order exponential componentsof the response were not extracted.

10.0 mV

Time (ms)

did not changesystematically with the size of the current pulseused to measure hem. For example, in the neuron shown inFigure 8, the input resistance obtained with the transient shownwas 97 MO, and the time constant 18.6 msec.Analysis of theresponse o a 0.4 nA pulse yielded an input resistanceof 100Ma and a time constant of 21.5, while the responseo a 0.3 nApulsegave an input resistanceof 9 1 MO and a membrane imeconstant of 17.8 msec.The 2 exponential componentsextractedfrom the transient shown in Figure 8, and others from thisneuron, satisfied the constraint described by Kawato (1984)that E,IE, < 2*(7,/r,), whereE, andE, are the coefficients,and7, and 70are the time constantsof the first 2 exponential pro-cesses.Transients satisfying this constraint may be analyzedusing he methods developed for equivalent cylinder modelsofneurons o yield estimatesof the dendritic length and dendriticdominance ratio. That analysiswas performed as describedbyKawato (1984) for the neuron in Figure 8, and it yielded anestimate of 1.2 length constants or the dendritic length and adendritic dominance of 11 O.Time constants for the other neurons analyzed in this waywere 25.2, 22.8, and 11.0. None of these cells met Kawatoscriterion for analysisusingan equivalent cylinder model. Theyyielded electrotonic lengthsof 1.5, 1.4, and 1.1 ength constantswhen calculated using the simpler procedure of Rall (1969),which is valid when the dendritic dominance s large. The tran-sients could also be fi t using a model with nonuniform mem-brane resistance.When analyzed using Kawatos somatic shuntmodel, the results from these cells again indicated dendriticlengthsbetween 1 Oand 1.5, but the estimate or dendritic dom-inance was ower, being less han 2 for one cell and between 2and 4 for the others, with somatic resistivity between 3 and 4times less han that of the dendrites (the wide tolerances aredue to the sensitivity of the measuredcoefficients o noiseand

of the resulting variation among traces taken from the samecell). The somatic shunt conductancemeasuredn this way wasgreater for the cells ecorded n the intact animal. The cell fittingthe uniform resistivity model and shown in Figure 8 was re-corded in an acutely decorticate rat.DiscussionIdentity of the cellsIn someespeciallysimple views of neostriatal architecture, thecells are divided into only 2 categories, arge cells and smallcells.The variety of morphological eaturesseenn Golgi-stainedneostriatal cells has always argued against his simple scheme,but recent advances n the identification of cell classesn theneostriatum using mmunocytochemical techniqueshave madeit untenable. Although most medium-sized neurons are spinyprojection neurons, medium-sized ntemeurons of a variety oftypes can be demonstrated by staining designed o localize so-matostatin, GABA, neuropeptide Y, and a variety of othermarkers (e.g., see eview by Gerfen, 1988). Likewise, there areseveral different kinds of large neurons. Probably the largestcellsare those hat contain choline acetyltransferase e.g.,Bolamet al., 1984; Phelpset al., 1985). However, there are other cellswith large somata which are not positive for ChAT. One classof thesecells s a projection neuron with morphological featuressimilar to globus pallidus cells (Bolam et al., 1981). Anotherconsistsof the larger instancesof the classof GABAergic inter-neuron that contains he calcium-buffering protein parvalbumin(Cowan et al., 1987).The cellsdescribed n this study had somata hat placed hemat the extreme upper end of the somatic size spectrum in therat neostriatum. Their somatic sizesand somatodendritic mor-phology matched that of neostriatal cholinergic neurons. In ad-


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The Journal of Neuroscience, February 1990, IO(Z) 517

dition, we have examined the cytological features, distributionof synapses on the cel l surfaces, and the morphology and syn-aptic targets of these cells, and observed that they show exactlythe features reported previously for ChAT-containing neuronsin the neostriatum (these results will be published in detail else-where). While these observations do not rule out the possibilityof an unknown interneuron type with morpholog ical featuresidentica l to the cholinergic interneuron, it is most likely thatthe cells reported here are neostriatal cholinergic neurons.The spontaneous firing patterns of these neurons are almostidentica l to those of the tonically firing neurons in the neostri-atum of awake monkeys as described by Kimura et al. (1984).Since tonically firing neurons represent a small proportion ofall cells recorded in behaving animals, it has been speculatedthat these cells correspond to cholinergic interneurons (Kimuraet al., 1984). While the diversity of neostriatal aspiny neuronsmay cast suspicion on such a speculation, the results presentedhere support the view that large, almost certainly cholinergic,interneurons do fire tonically in the fashion described in extra-cellu lar recording studies. On the other hand, the proportion oftonically active neurons recorded in behaving animals is muchlarger than the proportion of cholinergic interneurons in thesame animal (4 1% of the sample reported by Kimura, 1986). Itis therefore likely that there are also noncholinergic tonicallyfiring neurons yet to be demonstrated.Aferent inputIn behaving monkeys, tonically firing neostriatal neurons re-spond differently from the majori ty of neurons during perfor-mance of a learned motor task. Whereas most neurons fire in aburst of action potentials time-locked to the onset of the move-ment, the tonically firing neurons are activated (or inhibited)by presentation of the sensory stimulus that acts as a signal tothe animal that he should begin the movement (Kimura, 1986).It seems likely, therefore, that these neurons receive differentafferent information than the spiny cells, as well as being dif-ferent in the way that they process the information they doreceive. The intracellular recording experiments here do notprovide a direct demonstration of any such difference. The la-tency of EPSPs evoked in giant interneurons by stimulation ofcerebral cortex, cerebral peduncle, or thalamus were well withinthe range observed in recordings of spiny neurons made in thesame animals (Wilson et al., 1982, 1983a, b; Wilson, 1986).There are reasons to suspect some differences, however. Thelarge size of the dendritic field of the giant aspiny interneuronbrings the cel l into potential contact with many more corticalaxons than possible for spiny neurons. Despite this, the giantaspiny cells appear to receive input from fewer cortical axonsthan the spiny cells. There are several observations that pointto this conclusion. Firstly, responses to cort ical inputs wereneither as large nor as reliably observed in giant interneurons.In two of the neurons reported here, there was no response tothe cortical stimulation sites availab le in the experiment. Inthese same animals, a ll spiny neurons, including some recordedvery near the giant cells, showed large and reliable responses tothe same cortical stimul i. In both of these aspiny cells, an EPSPwas observed in response to stimulation of the cerebral pedun-cle, suggesting that they received cortical inputs from some re-gion of the cortex other than those areas stimulated. None ofthe EPSPs to afferent stimulation were as large as those routinelyobserved in spiny neuron. Bishop et al. (1982), reporting on onelarge interneuron, interpreted the lack of a response in that cel l

as suggesting that cortical inputs did not impinge on giant cellsat all.A second reason for suspecting that cortical inputs to giantcells involve fewer axons than those going to projection neuronscomes from the appearance ofthe EPSPs themselves. Even EPSPsnear maximal for these responses appear to be made up ofdiscrete elemental responses that are identical in shape and sizeto spontaneously occurring synaptic potentials. These elementa lunits have the appearance of unitary synaptic potentials. Themaximal evoked response is made up of only a few (l-l 0) ofthese smaller EPSPs. The large size and rapid time course ofthese responses suggest that they may arise from synapses lo-cated on the soma or the most proximal portions of the den-drites. If they are unitary synaptic potentia ls from proximalsynapses, then the maximal cortical input in the giant intemeu-rons arises from activation of only a few cortical axons, manyfewer than could be accounted for by a possible difference inthe receptive surface areas of spiny and aspiny neurons. If, asreasoned above, the cortical input to giant aspiny neurons arisesfrom only a very small subset of corticostriatal axons, it is verylikely that these axons represent a special group or cortical ef-ferents with special properties, and which could account for theunique responses of the giant interneurons to sensory cues dur-ing behavioral experiments.Integrative propertiesThe differences in evoked afferent EPSPs in the spiny neuronsand giant aspiny interneurons also suggest essential differencesin the way these neurons process synaptic input. Spiny neuronsreceive many convergent excitatory synaptic connections fromwidespread regions of the cerebral cortex, thalamus and otherareas of the brain. The effects of ind ividual synaptic events areusually not strong enough to allow their identification in ordi-nary intracellular recording experiments. Correlated activity inmany inputs is very important in determining the firing of spinyneurons, as indicated by their late responses to afferent stimu-lation (Wilson et al., 1983b). These later response componentsare much less visible in the responses of giant aspiny cells.Spiny neurons have resting potentials far from the actionpotential threshold, at least in immobile animals, and actionpotentials in these cells arise from large synaptic potentials thatare due to combined action of many synaptic potentials (Wilsonand Groves, 198 1). In contrast, some individual excitatory syn-aptic events are much more powerful in giant aspiny neurons,and appear as spontaneous, easily recorded depolarizations. Theaspiny neurons membrane potentials are normally within a fewmillivolts of the spike threshold, so that these unitary synapticpotentials may individually trigger action potentials. Thus thegiant aspiny neuron is very sensitive to a few cortical and tha-lamic axons, while the spiny neurons around it are samplingfrom a much larger population of axons and are relatively in-sensitive to activity in each one.The sensitivity of the aspiny neurons to individual synapticinputs suggests that these inputs are placed in an electrotonica llyproximal region of the dendritic tree. Measurement of the elec-trotonic length of the dendrites using current pulses indicatedthat these cells do not have an especially compact dendr iticfield. This measurement is made using the somatic to dendr itictransfer of charge, and so is not very re liab le for the predictionof the effective electrotonic distance for synaptic potentialstransferring charge in the opposite direction (e.g., Wilson, 1988).However, asymmetry of electrotonic distance for signals prop-


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agating in opposite directions in the dendritic field is usuallyassociated with voltage sensitive conductances in the dendrites.There is no indication that such conductances are contributinggreatly to the propagation of synaptic potentials in the giantaspiny neurons. Large depolarizations or hyperpolarizations ofthe somatic membrane by applied current affected the sizes ofthe spontaneous and evoked depolariz ing potentials in the wayexpected for simple synaptic potentials. The somata of the giantaspiny neuron exhibited little nonlinear behavior, at least incomparison to the spiny neuron. This interpretation is also sup-ported by our analysis of the shapes of the spontaneous depo-larizations, which suggest that they have a very proximal, per-haps somatic, placement on the neurons. Thus it is most likelythat the spontaneous depolarizations, and the excitatory syn-aptic potentials observed in these neurons, are produced bysynaptic contacts on the somata or proximal parts of the den-dritic trees of the cells. These findings are consistent with theobservation that asymmetrical afferent-type synaptic contactsare present on proximal parts of cholinergic interneurons (e.g.,Phelps et al., 1985).The long time constant, high input resistance, and the place-ment o f the action potential threshold so close to the baselinemembrane potential of the giant aspiny neurons also make themsensitive to excitatory synaptic inputs with proximal locations.

RepetitivejringAlthough tonically active, giant aspiny neurons in the neostria-turn have not been observed to fire rhythmically under ordinaryconditions. The reason for this is apparent upon examinationof the membrane potential trajectory of the neurons during theirspontaneous activity. The firing of the neurons is patterned bythe occurrence of spontaneous depolarizing potentials, ratherthan by pacemaker potentials whose cyclic generation is resetby the action potential generation. For the cells to fire rhythmi-cally, either the spontaneous potentials would have to becomerhythmic, or the cel l would have to be depolarized so that itsfiring was determined by spike afterhyperpolarization (as hap-pens during rhythmic firing in response to long artificially ap-plied current pulses). Sti ll, spike afterhyperpolarization appar-ently acts to lim it the firing rate that can be maintained by thecells in response to their normal inputs. In the cells studied here,the majori ty of spontaneous depolarizing potentials weresubthreshold for spike generation, with only a small proportion,usually a combination of more than one smaller depolarizationattaining threshold for action potential generation. During the80-100 msec period of afterhyperpolarization that follows aspike, spontaneous depolarizations are even less likely to fireaction potentials. Thus under most conditions, the spike after-hyperpolarization is sufficient to lim it firing of the aspiny neu-rons to rates of 16/set or less. Maintained firing at these rateswill engage another lim iting factor, the spike frequency adap-tation that was consistently seen in these cells during repetitivefiring.The mechanisms of these two fi ring rate limit ing factors maybe related, since they were both reversibly deactivated by theprolonged depolarizing current pulses used for injection of HRP.Two additional effects of these currents were (1) an increase inthe duration of action potentials, and (2) an increase in linearityof the transient response to depolarizing current pulses resultingin an increase in the apparent time constant of the initia l partof the charging curve. All of these effects disappeared a fewminutes after injection of HRP was discontinued. Because of

their short duration the effects of intracellular injection are morelikely to be due to potassium load ing rather than loading withHRP. It is possible that the potassium concentration was re-stored in a few minutes, but the HRP remained in the cytoplasmof the cells for several hours, as indicated by intracellular stain-ing.Efects on neostriatal outputThe effect exerted by the giant aspiny intemeurons on the spinyprojection cells of the neostriatum is unknown. Studies of theeffects of acetylcholine on spiny neurons have suggested a pos-sible fast nicotinic excitation, and also a muscarinic responsethat results in a decreased sensitivity to excitatory synaptic ac-tivat ion (e.g., Dodt and Misgeld, 1986; Akaike et al., 1988). Inaddition, however, there are reports of modulatory effects ofmuscarinic agonists, both on the duration of calcium-dependentplateau potentials and, at higher doses, on input resistance (e.g.,Misgeld et al., 1986). Because the giant cells are tonically active,they are in a position to maintain a relatively constant level ofcholinergic input to the spiny cells, and so are in a position toact as continuous modulators of the excitabili ty of the outputcells. Their greater sensitivity to the few corticostriatal inputsthey receive may underly their responses to sensory stimul iassociated with the timing of voluntary movements (Kimura,1986) and results in changes in their f iring prior to those of thespiny neurons during the initia tion of movement. This mayserve to prepare the projection neurons for the afferent inputbarrage that is more directly related to the movement itself, andso create a sensory-motor preparatory set within the circuitryof the neostriatum.

ReferencesAkaike, A., M. Sasa, and S. Takaori (1988) Muscarinic inhibition asa dominant role in choline& regulation of transmission in the cau-date nucleus. J. Pharm. Exp. 246: 1129-l 136.Bishop, G. A., H. T. Chang, and S. T. Kitai (1982) Morphologicaland physiological properties of neostriatal neurons: An intracellularhorseradish peroxidase study in the rat. Neuroscience 7: 179-l 9 1.Bolam, J. P., J. F. Powell, S. Totterdell, and A. D. Smith (1981) Asecond type of striatonigral neuron: A comparison between retro-gradely labelled and Golgi-stained neurons a t the light and electronmicroscopic levels. Neuroscience 6: 2 14 l-2 157.Bolam, J. P., B. H. Wainer, and A. D. Smith (1984) Characterizationof cholinergic neurons in the rat neostriatum. A combination of cho-line acetyltransferase immunocvtochemist ry, Golgi-impregnation andelectron-microscopy. Neuroscience 12: 7 1 -7 18: -Ghana. H. T.. C. J. Wilson. and S. T. Kitai (1982) A Golai studv ofrat Geostriatal neurons: Light microscopic analysis. J. Camp. Neural.208: 107-126.Cowan, R. L., C. J. Wilson, and P. C. Emson (1987) Parvalbumin ispresent in GABA containing interneurons of the rat neostriatum. Sot.Neurosci. Abstr. 13: 1573.DeLong, M. R. (1973) Putamen: Ac tiv ity of single units during slowand ranid arm movements. Science 179: 1240-l 242.DiFiglia,- M. (1987) Synaptic organization of cholinergic neurons inthe monkey neostriatum. J. Comp. Neurol. 225: 245-258.DiFiglia, M., P. Pasik, and T. Pasik (1976) A Golgi study of neuronaltypes in the neostriatum of monkeys. Brain Res. 114: 245-256.Dodt, H. U., and U. Misgeld (1986) Muscarinic slow excitat ion andmuscarinic inhibition of synaptic transmission in the rat neostriatum.J. Physiol. (Lond.) 380: 593-608.Fuller, D. R. G., C. D. Hull, and N. A. Buchwald (1975) Intracellularresponses of caudate output neurons to orthodromic stimulation. BrainRes. 96: 337-34 1.Galarraga, E., J. Bargas, A. Sierra, and J. Aceves (1989) The role ofcalcium in the repetitive firing of neostriatal neurons. Exp. Brain Res.(in press).


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Gerfen, C. R. (1988) Synaptic organization of the striatum. J. Elect.Microsc. Tech. 10: 265-28 1.Graveland, G. A., and M. DiFiglia (1985) The frequency and distri-bution of medium-sized neurons with indented nuclei in the primateand rodent neostriatum. Brain Res. 327: 308-3 11.Grofova, I., J. M. Deniau, and S. T. Kitai (1982) Morphology of thesubstantia nigra pars reticulata projectionneurons intracell;iarly la-beled with HRP. J. Comn. Neurol. 208: 352-368.Groves, P. M. (1983) A theory o f the functional organization of theneostriatum and the neostriatal control o f voluntary movement. BrainRes. Rev. 5: 109-132.Horikawa, H., and W. E. Armstrong (1988) A versatile means ofintracellular labeling: Injection of biocytin and its detection with av-idin conjugates. J. Neurosci. Methods 25: l-l 1.Kawato, M. (1984) Cable properties of a neuron model with non-uniform membrane resistiv ity: J. Theor. Biol. 1 I I: 149-169.Kemo. J. M.. and T. P. S. Powell (1971) The structure of the caudatenucleus of the cat: Light and electron microscopy. Phil. Trans. R.Sot. London [Biol .] 262: 383-40 1.Kimura, M. (1986) The role of primate putamen neurons in the as-sociation of sensory stimuli with movement. Neurosci. Res. 3: 436-443.Kimura, M., J. Raijkowski, and E. Evarts (1984) Tonically dischargingputamen neurons exhibit set-dependent responses. Proc. Natl. Acad.Sci. USA 81: 4998-5001.Kita, T., H. Kita, and S. T. Kitai (1984) Passive electrical membraneproperties of rat neostriatal neurons in in vitro slice preparation. BrainRes. 300: 129-139.Liles, S. L. (1974) Single-unit responses of caudate neurons to stim-ulation of frontal cortex, substantia nigra and entopeduncular nucleusin cats. J. Neurophysiol. 37: 254-265.Misgeld, U., P. Calabresi, and H. U. Dodt (1986) Muscarinic mod-ulation of calcium-dependent plateau potentials in rat neostriatal neu-rons. Pfliigers Arch. 407: 482-487.Murakami, F., N. Tsukahara, and Y. Fujito (1977) Analysis of unitaryEPSP mediated by the newly formed cortico-rubral synapses after

lesion of the nucleus interpositus of the cerebellum. Exp. Brain Res.30:233-243.Phelps, P. E., C. R. Houser, and J. E. Vaughn (1985) Immunocyto-chemical localization of choline acetyltransferase within the rat neo-striatum: A correlated light and electron microscopic study of cholin-ergic neurons and synapses. J. Comp. Neurol. 238: 286-307.Pickel, V. M., K. K. Sumal, S. C. Beckley, R. J. Miller, and D. J. Reis(1980) Immunocytochemical localization of enkephalin in the neo-striatum of rat brain: A light and electron microscopic study. J. Comp.Neurol. 189: 721-740.Preston, R. J., G. A. Bishop, and S. T. Kitai (1980) Medium spinyneurons projection from the rat striatum: An intracellular horseradishperoxidase study. Brain Res. 185: 253-263.Rall, W. (1969) Time constants and electrotonic length of membranecylinders and neurons. Biophys. J . 9: 1483-l 508.Rall, W., R. E. Burke, T. G. Smith, P. G. Nelson, and K. Frank (1967)Dendritic location of synapses and possible mechanisms for the mono-synaptic EPSP in motoneurons. JI Neurophysiol. 30: 1169-l 193.Wilson, C. J. (1986) Postsvnaotic ootentials evoked in sninv neostri-atal projection neurons by stimula%on of ipsilateral and contralateralcortex. Brain Res. 367: 20 l-2 13.Wilson, C. J. (1988) Cellular mechanisms controlling the strength ofsynapses. J. Electr. Microsc. Tech. 10: 293-313.Wilson, C. J., and P. M. Groves (198 1) Spontaneous firing patternsof identified spiny neurons in the rat neostriatum. Brain Res. 220:67-80.Wilson, C. J., H. T. Chang, and S. T. Kitai (1982) Origins of post-synaptic potentials evoked in identified rat neostriatal neurons bystimulation in substantia nigra. Exp. Brain Res. 45: 157-l 67.Wilson, C. J., H. T. Chang, and S. T. Kitai (1983a) Origins of post-synaptic potentials evoked in spiny neostriatal projection neurons bythalamic stimulation in the rat. Exp. Brain Res. 51: 2 17-226.Wilson, C. J., H. T. Chang, and S. T. Kitai (1983b) Disfacilitationand long-lasting inhibition of neostriatal neurons in the rat. Exp. BrainRes. 51:217-226.

c. j. wilson, h. t. chang and s. t. kitai- firing patterns and synaptic potentials of identified...

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