ACTA NEUROBIOL. EXP. 1982. 42: 195-201

Short communication

RESPONSES OF RED NUCLEUS NEURONS TO PERIPHERAL STIMULATION IN CHLORALOSE ANESTHETIZED CATS

Janusz RAJKOWSKI

Department of Neurophysiology, Nencki Institute of Experimental Biology, Pasteura 3, 02-093 Warsaw, Poland

Key words: red nucleus, unit activity, chloralose

Abstract. Spontaneous and evoked activity of red nucleus neurons is described in chloralose anesthetized cats. I t is suggested that some effects, e.g. burst discharges and prolonged inhibitory-excitatory res- ponses, were induced by the use of chloralose.

In chloralose anesthetized animals a prolonged modulation of neuronal activity through peripheral stimulation was recorded in various struc- tures, i.e. red nucleus (RN) (8, l l), interpositus nucleus (IN) (2) and others (1, 4, 9). The aim of the present study was to investigate in more detail the influence of chloralose anesthesia upon cell activity in the somatosensory system.

Experiments were performed on 25 cats anesthetized with alpha-chlo- ralose (80 mglkg). The animals were paralysed with flaxedil and artifi- cially ventilated. In three animals no relaxant was used to enable assessment of anesthetic depth (through observing paw withdrawal reflex, corneal reflexes, respiratory rate etc.). End-tidal C02 and body tempera- ture were continuously monitored throughout the experiment. Three insulated steel needles exposed at the tip were inserted into the right (i.e. contralateral to the recording side) IN to test orthodromic responses of the RN neurons. In some cats the right rubrospinal tract was stimulat- ed with bipolar electrodes at the C2 level to permit antidromic identifi- cation of RN cells. Nucleus interpositus and spinal cord stimulations were

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carried out with single square pulses of 0.1 ms duration and current intensity of between 40-150 PA. Both forelimbs and the right hindlimb were stimulated with single square pulses (0.5 ms; 0.4-1.2 mA) delivered to the central footpad by a pair of subcutaneous needle electrodes. Ex- tracellular single unit recordings from the left RN were performed with tungsten microelectrodes, inserted vertically. With exception of a few cases the recording began 4 h after surgery had been completed. An ANOPS laboratory computer was used to obtain interval histograms (IHs) for spontaneous activity, or poststimulus-time histograms (PSTHs) (32 stimulus presentations, 0.5, 1, or 2s scan length). At the end of the experiment, the position of the microelectrode was marked by electso- coagulation for later histological analysis.

A sample of 285 units was analysed during these experiments. They represented a wide spectrum of resting discharge frequencies, ranging

a

d l

e int.

Fig. 1. Sample of red nucleus neuron activity: a, interval hi.stograms of eight spon- taneously active neurons recorded in a single experiment; each histogram is ex- pressed as a rough envelope of the original ANOPS recording (see inset); b and c, olscilloscopic recordings of a neuron with rhytmic background discharges (b) and burst activity (c); d-k, complete survey of PSTH patterns following footpad sti- mulation record,ed from different units in a single experiment (bin width 2 ms, stimulus onset at zero time). Time from chloralose administration (in minutes): d,

180; e, 190; f, 320; g, 330; h, 500; i, 580; j, 620; k, 790.

from 0 to 1101s. In Fig. l a sample IHs, recorded from 8 cells in a single experiment, are shown superimposed. About two-third of all the record- ed neurons were spontaneously active, with an average rate of spikes 431s; the remaining neurons did not show any spontaneous activity.

Typical responses of RN neurons to contralateral forelimb stimulation, expressed as facilitation or depression of the neuronal firing, were re- corded in form of PSTHs (Fig. Id-k). Three most prominent features were distinguished in the responses: (i) a short-latency excitation, (ii) an inhibition and (iii) a long-latency, late excitation. The most typical patt- erns of responses are specified below; they represent different units and all of them could occur in any single experiment. The response pattern consisting of an early excitation alone occurred in 27010 of neurons (Fig. If), they were mostly the units that had no spontaneous activity. The two component (excitatory-inhibitory) response occurred in 19010 of

Short-latency excitat ion

Inhibition

Long-latency excitat ion 10 e I units I

ms

Latency Duration

Fig. 2. Response parameters of right red nucleus neurons to pad stimulation: a-e, latency and f-j, duration of each of three components (early excitation, inhibition, late excitation). All histograms are related to the left forelimb stimulation, except for b and g, right forelimb, and c and h, right hindlimb. Because of stimulus site

unspecificity the late components evoked from only one leg are presented.

neurons (Fig. l e and g). The majority of recorded units (44"/o) responded with a late burst of activity (Fig. l d and h-k). In most of belonging to that group spontaneously active units, the response pattern consisted of a short-latency burst, an inhibition and a long-latency burst (Fig. l i) , often followed by later, less pronounced, modulation in discharge fre- quency. On the other hand, in neurons lacking background activity the response appeared in form of an early and late excitation (Fig. l j ) , or sometimes a late excitation alone. Additionally. in 4OIo of all the recorded neurons inhibition was found as the only response. Also other, less pro- nounced forms of responses could be distinguished.

From each histogram recorded in the experiments, the latency and duration of each response component were measured. The distributions of these response parameters, comprising all the units recorded, are presented in Fig. 2. The receptive field of most units was wide, involving the whole body, and only in 18O/o of cells was the receptive field restrict- ed to one leg. Stimulation of different legs resulted in significant diffe- rences only between t'le latencies of the early excitation, and the late components were essentially unchanged when altering the stimulation site. Moreover, the response patterns of different RN units, even spatially distant ones, were almost identical, when recorded close in time (see 11). Although somatosensory stimulation was most frequently employed alone, other stimulus modalities (acoustic or visual) were also checked, and those evoked essentially similar response patterns. Moreover, stimulation of the contralateral IN which typically elicited a monosynaptic excitation, was also followed in 46O/o of neurons by a late response, composed of inhibition and subsequent excitation (see also 10). I t seems that differen- ces between the characteristics of these responses reflect only different stimulus effectiveness.

Each experiment could be characterized according to the predominant pattern of spontaneous activity and the predominant response pattern. These two characteristics were found to be correlated with each other which is expressed in the following classification: in 4 experiments no activity was recorded except for injury potentials; in 5 preparations with low background activity, only short-latency excitations could be evoked; in 6 preparations showing regular spontaneous discharges, the cells responded to stimulation with an excitatory-inhibitory pattern; in 10 cats the neurons displayed usually bursts in their spontaneous activity, with irregular and long-lasting interburst intervals, and in these experi- ments the long-latency excitatory response prevailed over the other patterns of responses.

The background activity was changing throughout the experimental session. Unfortunately, that aspect was investigated casually and docu-

mented in form of oscilloscope recording - allowing only for some ge- neral remarks to be made here. The changes could be traced only in those recording sessions which lasted at least 8h. At the begining of the experiment, a few hours after chloralose administration most cells were silent and they responded only to peripheral stimulation or dischar- ged in consequence of injury of the neuron. These neurons which were spontaneously active, had a tendency to fire with long and rather regular intervals (Fig. lb). However, several hours after injection of the anes- thetic the neuronal activity had changed: the number of spontaneously active neurons increased and they began to discharge with high fre- quency bursts (Fig. lc).

The changes in spontaneous activity throughout the experiment were parallelled by changes in pattern of response. Figure Id-k presents a complete survey of PSTH patterns recorded from different neurons in a single experiment. Typically, 4 to 8 h after the injection of the anesthe- tic the responses consisted of a simple early excitation (Fig. If) , or an early excitation followed by a long-lasting inhibition (Fig. l e and g). Later, 6 to 12 h after administration of chloralose, the long-lasting, long- latency secondary excitation appeared (Fig. 1 i-k). Still later, 10-15 11

from the begining of the experiment, both inhibition and late excitation shortened and subsequent bursts occured (Fig. lk).

The described typical patterns of responses fit well with the similar results of other authors, concerning RN (8). This pattern of RN activity is a precise reproduction of that existing in IN (3, and our own, un- published observations), which is expected in light of the known rela- tionships that exist between the two structures (12). An activity describ- ed in this and in the other cited papers is characterized by some very specific features: (i) spontaneous activity occurs in bursts, (ii) response forms characteristic response pattern with prolonged inhibitory-excita- tory sequence a rd (iii) the reaction involves numerous brain stem neurons in a synchronized manner (11). All these features seem typical for chlora- lose preparation, as opposed to some other experimental conditions (e.g. under barbiturates, our own observation). However it should be noted here, that spontaneous burst activity or late burst responses were also observed in other preparations (5, 7).

Additionally, it was observed that both the spontaneous activity pattern and the response pettern changed with time after induction of chloralose, thus these patterns can be related to the assessed depth of anesthesia. "Profound" anesthesia resulted in suppression of both the spontaneous activity and the response to stimulation. "Deep" anesthesia was characterized by regular background activity and short-latency excitatory or excitatory-inhibitory response. Spontaneous burst activity

and all the response patterns with late burst can be considered as typical for "moderate" anesthesia. "Light" anesthesia was marked by the shor- tening of spontaneous bursts and shortening of excitatory-inhibitory-ex- c~tatory complex. It is our suggestion that different patterns characte- rize different anesthesia levels, rather than other factors which may influence the response. This assumption is based on results of our pre- vious experiments (ll), when the reverse process was observed, i.e. the typical chloralose neuronal activity, characterized by spontaneous burst activity and excitatory-inhibitory-excitatory response, developed in con- sequence of chloralose injection.

Although it has been suggested that similar compound responses occur in unanesthetized animals (8), our further observations (in preparation), and also experiments of Armstrong concerning IN (2, 3) show that there are significant differences in realctivity between normal and chloralose preparation. Response to somatcrsensory stimuli in unanesthetized cats is characterized by short and labile activity-inactivity sequence and small receptive field, whereas in the cat under chloralose it appears in charac- teristically generalized form.

In conclusion, the observed characteristic compound and long-lasting reactions of RN neurons could be interpreted as a consequence of chlo- ralose anesthesia.

This investigation was supported by Project 10.4.1.01 of the Polish Academy of Sciences.

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Accepted 10 October 1981