ultrastructure of neuronal circuitry in sympathetic ganglia

MICROSCOPY RESEARCH AND TECHNIQUE 35146-156 (1996)

Ultrastructure of Neuronal Circuitry in Sympathetic Ganglia YOSHINORI KAWAI Department of Anutomy and Neurobwlogy, Wakayama Medical College, Wakayama 640, Japan

KEY WORDS Superior cervical ganglion, Dendrodendritic synapse, Dendritic glomerulus, Ad- renergic connection

ABSTRACT To elucidate the intraganglionic circuitry in sympathetic ganglia, attempts have been made to define the nature and source of those neuronal elements that establish synaptic connections there. Intracellular labeling of sympathetic cells is of particular value for this purpose. Dendrites of principal neurons in the rat superior cervical ganglion exhibit a varying complexity in their morphology and arborization. Some dendrites show specializations such as a glomerular plexus, where extensively-branched dendritic collaterals form synaptic connections comprising not only axodendritic synapses between preganglionic axons and principal cell dendrites, but also dendrodendritic synapses between principal cell dendrites. A few of these may represent reciprocal synapses. Most presynaptic elements of adrenergic synapses observed by conventional methods appear to represent these specialized dendritic collaterals of principal neurons. These presynaptic dendrites may be an important addition to the conventional scheme of intraganglionic synaptic organization. However, there seem to be extreme species and even strain differences in the number of these adrenergic synapses, and in the sophistication of the specialized local circuits within sympathetic ganglia. Sympathetic ganglia may thus function as more than a simple relay station, with specialized neuronal circuitry that may be involved in the modulation of cholinergic transmission. 0 1996 Wiley-Lisa, Inc.

INTRODUCTION Classically, the sympathetic ganglia have been re-

garded as relay stations, where preganglionic axons derived from the spinal cord establish synaptic contacts with the principal (postganglionic) neurons whose axons leave the ganglion and project to peripheral targets (Ramon y Cajal, 1911). The transmitter of preganglionic axons is acetylcholine, whch acts on nicotinic receptors on the membrane of the principal neurons to generate spikes, whereas the principal cells use noradrenaline as their transmitter, which acts on adreno- ceptors of, for example, the smooth muscles of peripheral target organs (Langley, 1893). The concept of a relay station remains largely valid for sympathetic ganglia of mammals and nonmammals. However, several more recent lines of evidence suggest that the sympathetic ganglion is more than a simple relay station in terms of physiology, pharmacology, and anatomy (Dun, 1980; Jacobowitz, 1970; Skok, 1973). This review attempts to redefine the identity of ganglionic neuronal elements and their connections in order to provide morphological evidence for a modulatory intraganglionic circuitry in the superior cervical ganglion (SCG) of adult rats.

The sympathetic ganglia had been thought to contain only two kinds of cells: principal neurons and Schwann cells (de Castro, 1932; Ramon y Cajal, 1911). Histofluorescence methods introduced by Falck and Hillarp have revealed the existence of a new group of cells: small intensely-fluorescent (SIF) cells, (Falck and Hillarp, 1962) also termed small granule-containing cells or chromaffin cells (Jacobowitz, 1970; Norberg et al., 1966). These SIF cells have cell bodies approxi-

mately 10-20 pm in diameter, and several processes. Electron microscopic studies have shown that SIF cells contain many large granular vesicles (200-300 nm in diameter) that resemble those seen in chromaffin cells in the adrenal medulla, and that they receive synaptic inputs from preganglionic axons (Norberg et al., 1966). There is also evidence that they make efferent contacts with the principal cells (Matthews and Raisman, 1969; Williams, 1967). Thus, SIF cells may play a role as interneurons in the sympathetic ganglia (Williams, 1967) and provide the morphological basis for an inhibitory postsynaptic potential observed in the SCG (Li- bet, 1976). However, SIF cells may not be the only candidates for generating inhibitory postsynaptic potentials (Dun, 1980). Quantitative estimates of the number of SIF cells and of their efferent connections suggest that their synaptic influence on principal neurons is limited to relatively small areas (Williams and Jew, 1983). A close relationship between SIF cell populations and fenestrated capillaries suggests the alter- native possibility that SIF cells may perform an endo- crine-like function. Thus, their definitive function remains to be elucidated (Williams and Jew, 1983).

The histofluorescence method has also demonstrated that there are abundant catecholaminergic nerve terminals or varicosities throughout the sympathetic ganglion, whose origin and possible significance have been the subjects of a vigorous controversy (Jacobowitz,

Received September 12, 1994; accepted in revised form November 14, 1994. Address reprint requests to Yoshinori Kawai, Department of Anatomy and

Neurobiology, Wakayama Medical College, 27 Kyu-bancho, Wakayama 640, Ja- Pa.

0 1996 WILEY-LISS, INC.

NEURONAL CIRCUITRY IN SYMPATHETIC GANGLIA 147

1970; Norberg and Sjoqvist, 1966). Electron microscopic studies have also shown adrenergic nerve terminals and their synapses in sympathetic ganglia (Mat- thews, 1983). These adrenergic fibers may represent processes of the principal neurons, since their terminals contain small vesicles, some of which are dense- cored, resembling those seen in adrenergic terminals in peripheral tissues (Gordon-Weeks, 1988). The number of these nerve terminals does not appear to decrease after preganglionic denervation (Quilliam and Tama- rind, 1972). However, it is not clear whether these fibers are derived from axonic or dendritic collaterals of the principal neurons. To elucidate the neuronal circuitry involving the processes of principal neurons and of preganglionic axons, it was necessary to define the source of the adrenergic nerve fibers in the sympathetic ganglion. Intracellular recording and labeling with appropriate tracers followed by electron microscopy (Kawai et al., 1993) made it possible to address this problem directly, and to compare the morphology of sympathetic neurons thus revealed with that described by classical Golgi studies (de Castro, 1932; Ra- mon y Cajal, 1911). MORPHOLOGY OF SYMPATHETIC NEURONS

The morphology of sympathetic neurons at the light microscopic level was described in detail by early his- tologists using silver impregnation methods (de Castro, 1932; Ramon y Cajal, 1911). More recently, studies using intracellular labeling methods in smaller mammals (mouse, rat, guina pig, rabbit, and hamster) (McLachlan, 1974; Purves and Lichtman, 1985) confirmed the overall morphology of sympathetic neurons, but failed to confirm or overlooked the specific dendritic morphology or other detailed features described by earlier authors. Since the materials of the classical workers were prepared mostly from larger mammals including man, it was possible that this discrepancy reflected species differences in the morphology of sympathetic neurons. However, our recent study using intracellular labeling with biocytin showed that the characteristics of rat SCG neurons surprisingly resem- bled those of larger mammals (Kawai et al., 1993). Al- though considerable species differences in, for example, the incidence of certain types of cells have been observed, the detailed morphology and dendritic configu- rations of sympathetic neurons are confirmed, as described below.

General: Cell Bodies and Processes The cell body of the sympathetic neuron is generally

ovoid to polygonal in shape, with a longer diameter of 20-60 pm. Cell body contours are sometimes smooth but often irregular, and their shapes depend upon the nature of the emerging processes. Two types of processes (axons and dendrites) are clearly distinguish- able when intracellular labeling is complete. Almost all cells have single axons that leave the ganglion after traveling long distances without branching. Rarely ( ~ 1 % of rat SCG neurons), axons of large neurons give off collaterals a t their origins or at some distance from their origins. The remaining processes (dendrites) are distributed within the ganglion. Sympathetic neurons

have a variable number of dendrites (2-15). Most dendrites are thick and long, with moderate branching (Fig. 1D). Some dendrites show little or no branching over a rather long distance, thus resembling the axon (Fig. 1 0 . Thinner finger-like processes often emerge from cell bodies (Fig. 1A). Thus, dendrites of sympathetic neurons exhibit extreme variations of complexity in arborization and morphology, as shown in Figure 1. In this respect, we completely agree with a notion of de Castro (1932): I ‘ . . . it follows that the most essential points in the morphology of the sympathetic neurones are the aspect of their dendrites, their behaviour, dis- tribution and relations to the corresponding prolonga- tions of the other intrinsic cells of the ganglia.”

Dendritic Arborization Dendrites of sympathetic neurons show various de-

grees of complexity in their morphology. Some dendrites give rise to distinctive collaterals or spine-like processes. These collaterals establish either a simple or a complicated dendritic apparatus, referred to as “receptor plates” or “dendritic glomeruli,” respectively, in the earlier literature (de Castro, 1932). Dendrites with their collaterals usually travel along intercellular nerve trunks (fasciculi). Some of these appear to en- close single cell bodies of principal neurons (Kawai et al., 19931, forming “a pericellular dendritic nest” (de Castro, 1932). Dendritic glomeruli are complicated in- terminglings of collaterals encapsulated by Schwann cell processes. They are usually situated near the cell bodies of the principal neurons and are 20-50 pm in diameter (Fig. 1A). It seems that dendritic branches of several ganglion cells are associated in each glomerulus (Fig. 2A); thus, such glomeruli are termed “pluri- cellular dendritic glomeruli” (de Castro, 1932). Larger glomeruli contain part of the cell bodies of nearby neurons, but in most cases they constitute a neuropil in which principal cell dendrites, preganglionic axons, and processes of Schwann cells are densely intermingled with the nuclei of Schwann cells (Fig. 3). Some glomeruli are situated close to cell bodies of principal neurons, and appear to be within a dilatation of the pericellular Schwann cell capsules (Fig. 1B); these cells with this type of glomeruli are called cells with “mono- cellular glomeruli,” or “knapsack cells” (de Castro, 1932; Ramon y Cajal, 1911). The incidence of cells associated with mature dendritic glomeruli is, however, very small (<lo%) in rat SCG under normal conditions.

Close Apposition Between Dendrites Observations of two neighboring principal neurons

labeled with tracer made it possible to demonstrate the close apposition of dendritic collaterals derived from different cells (Fig. 2). Two types of apposition were noted: one occurred along intercellular nerve trunks (fasciculi), and the other in a larger intercellular neuropil encapsuled by Schwann cells. The former seemed to correspond to the “receptor plates” (Fig. 2B; de Cas- tro, 19321, and the latter to “dendritic glomeruli” (Fig. 2A; de Castro, 1932). De Castro (1932) mentioned that “from the functional standpoint the glomeruli can be regarded as small intraganglionic nuclei formed by

Fig. 1. Camera lucida drawings of rat superior cervical ganglion (SCG) neurons. Biocytin is intracellularly introduced into the cells and visualized with diaminobenzidine reaction following avidin-bi- otin horseradish peroxidase complex incubation (Horikawa and Arm- strong, 1988; Kawai et al., 1993). A: Cell with a dendritic glomerulus (g). Some dendrites give off numerous collaterals whose terminals and varicosities establish a bushy glomerulus. Several fine, finger-like processes emerge from the cell body. B Cell with dendritic glomeru-

lus (9). The glomerulus is situated very close to the cell body. C: Cell with simple dendritic arborization. Dendrites give rise to a few branchings. D Typical SCG cell. Dendrites give rise to moderate branchings. Spine-like processes are noted on some dendrites. These short collaterals correspond to "receptor plates." All cells have single axons (asterisks) that leave SCGs via one of the postganglionic nerves without branching. Bar, 50 pm, A-D.


Fig. 2. Two examples showing interrelations between dendrites derived from two neighboring SCG neurons (gray and black). A In- volvement of dendrites from both cells in a glomerulus (arrow). Den- dritic collaterals derived from a primary dendrite of one cell (in black)

are in close apposition (arrow) with glomerular dendrites of another cell (in gray). B Dendrites derived from two cells form close appositions (arrows) in intercellular fascicular tracts. Axons are indicated by asterisks. Bar, 50 km, A and B.

neurones in isodynamic association since they all receive the same nervous impulse through preganglionic fibers ending in the glomeruli.”

and thus have left several issues unresolved (Mat- thews, 1983). For example, is there a definitive crite- rion to distinguish cholinergic SYnaDses from intrinsic

NEURONAL CIRCUITRY IN THE SYMPATHETIC GANGLION

(adrenergic) synapses? What is the exact source of the presynaptic elements of intrinsic synapses?

Ultrastructural analyses have elucidated the in- Cholinergic Synapses traganglionic connections of sympathetic ganglia Random electron microscopic observation consis- (Elfvin, 1963a,b, 1971; Kondo et al., 1980). These stud- tently detects a certain type of synaptic structure in ies have proven that the ultrastructural organizations normal SCG, defined by ultrastructural criteria (Peters of synaptic and other connections are very complicated, et al., 1991). Presynaptic elements contain many small,

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Fig. 3. Correlated light and electron micrographs showing dendritic glomeruli. Electron micrographs of dendritic glomeruli in a and b correspond to enclosed regions (dotted rectangles) in A and B, respectively. Note that dendritic glomeruli consist of neuropil in which

labeled dendrites and other neuronal processes are mingled with Schwann cells, demarcated by connective tissue septa. Bar, 20 km, A and B. Bar, 10 km, a and b.

round, clear vesicles about 50 nm in diameter and a smaller number of larger dense-core vesicles about 100 nm in diameter (Fig. 4A,B). The cores of the large vesicles are round and situated centrally. Since this type of nerve ending degenerates rapidly and disappears after preganglionic denervation, these presynaptic ele-

ments are considered to be preganglionic axons of cholinergic type (Grillo, 1966; Quilliam and Tamarind, 1972). Postsynaptic elements usually appear to be dendritic profiles of principal neurons (Fig. 4A). In rare cases, they are the soma of principal neurons or SIF cells. In the mouse, preganglionic axons often establish


synaptic contacts with cell bodies (Yokota and Yamau- chi, 1974). In some cases, postsynaptic profiles contain vesicle populations (Fig. 4B,D,E). Some vesicles seem to be clustered toward synaptic membrane specializations (Fig. 4E). This arrangement may represent a reciprocal synapse.

Adrenergic Synapses A number of authors have described adrenergic syn-

apses in SCG and other sympathetic ganglia, using false transmitter labeling or conventional fixation procedures (Dail and Evan, 1978; Grillo, 1966; Raisman et al., 1974; Taxi et al., 1969; Watanabe, 1967; Yokota and Burnstock, 1983a). Adrenergic nerve terminals can be easily identified by preloading the ganglia with 5-hydroxydopamine, because vesicles in presynaptic elements are densely labeled with the marker (Dail and Evan, 1978; Watanabe, 1967; Yokota and Burnstock, 1983a). Also, in nonlabeled materials, by using conventional fixation procedures (buffered glutaraldehyde- paraformaldehyde-osmium fixative), one can distinguish adrenergic synapses from cholinergic synapses by the presence of small dense-core vesicles in presynaptic elements, as well as by the persistence of adrenergic nerve terminals after preganglionic denervation (Grillo, 1966; Raisman et al., 1974; Tamarind and Quilliam, 1971; Taxi et al., 1969). Presynaptic elements of adrenergic synapses contain varying num- bers of small vesicles about 50 nm in diameter, but none of the large, dense-core vesicles consistently found in preganglionic (cholinergic) nerve endings and in SIF cells (Fig. 4C,D). A few small vesicles in the presynaptic elements seem to have a central core or granule. The proportion of clear-to-dense-core vesicles, however, varies greatly with conditions of fixation. Adrenergic synapses in rat SCG (Sprague-Dawley (SD) strain) constitute about 15% of the total synapses by our quantitative estimation (unpublished observations). However, the incidence of adrenergic synapses has been variously reported in the literature. Several authors (Ostberg et al., 1976; Raisman et al., 1974; Tamarind and Quilliam, 1971; Voyvodic, 1987) have estimated that adrenergic synapses represent 5-10% of the total synapses in rat SCG (Wistar and SD strains). Ramsay and Matthews (1985) reported virtually no adrenergic synapses in normal rat SCG (Wistar strain). Grillo (1966) presented a value of 18% for rat SCG. Rabbit SCG has been reported to contain more adrenergic synapses (about 25%) (Dail and Evan, 1978; Tamarind and Quilliam, 19711, whereas feline and guinea pig SCGs contain few adrenergic synapses (2-4%) (Purves, 1976; Tamarind and Quilliam, 1971). It is very likely that these discrepancies reflect species and strain differences. Several authors have reported that even in the same strain of animals, experimental manipulations (denervation and/or axotomy) may cause changes in the number of adrenergic synapses (Quilliam and Tama- rind, 1972; Ramsay and Matthews, 1985; Yokota and Burnstock, 1983b). Adrenergic nerve terminals thus seem to exhibit considerable plasticity in intragagli- onic synapse formation.

Source of Adrenergic Synapses Evidence that the main source of the adrenergic ter-

minals is dendritic branches of the principal neurons has recently been presented by using intracellular labeling in rat SCG (Kawai et al., 1993). Many dendritic collaterals in the glomerulus establish synaptic connection not only with preganglionic axons but also with dendritic profiles of other principal cells (Fig. 5). As expected, many dendritic terminals are postsynaptic to what are apparently preganglionic nerve endings (Fig. 5A; cf. Fig. 4A). A few of them are observed to contain vesicles clustered toward the synaptic membrane specialization (Fig. 5B; cf. Fig. 4E). Some dendritic terminals exhibit presynaptic elements on what are apparently dendritic profiles (Fig. 5C; cf. Fig. 4C). Postsynaptic dendrites, in some cases, contain vesicle populations (Fig. 5C). Some vesicles seem to be clustered toward the synaptic membrane specialization, thus appearing to establish reciprocal synapses (Fig. 5D). In dendritic glomeruli, dendrodendritic synaptic contacts or other nonsynaptic membrane contacts (zonula adherens, see below) underlie the close appositions observed between dendrites a t the light microscopic level (Figs. 2, 3, and 5). Dendrites in other regions (intercellular fascicular regions) are found to be mostly postsynaptic to preganglionic nerve endings, and presynaptic dendrites are rare in comparison to glomerular regions.

Synaptic vesicles and vesicles not associated with membrane contacts found in these labeled dendrites and in nonlabeled dendritic profiles have very similar features (Figs. 4B-D, 5C,D). They are mostly agranu- lar small, round vesicles with diameters of 30-70 nm. One or two vesicles per profile seem to have a granule, and appear to be small, dense-core vesicles. Often no granular vesicles are seen. We noted that neither the large, dense-core vesicles seen in cholinergic nerve endings nor the larger granular vesicles seen in SIF cells were associated or intermingled with these small- vesicle populations. Multivesicular bodies (200-300 nm in diameter) or vacuoles (100-200 nm in diameter) are often present near the clusters of small vesicles in the principal cells. It would be difficult to distinguish dendritic collaterals of principal cells from SIF cell processes if a monoamine marker such as 5-hydroxydopamine were to label these vacuoles.

Vesicle Population and Membrane Contact Vesicle clusters are frequently observed in the den-

drites and cell somata of sympathetic ganglion neurons (Grillo, 1966; Matthews, 1983). The characteristics of these vesicles resemble those seen in presynaptic elements of adrenergic synapses (Grillo, 1966). Several studies have demonstrated some vesicles that seem to be associated with dendritic membranes devoid of Schwann cell coverings (Dail and Evan, 1978; Mat- thews, 1983; Taxi et al., 1969). Therefore, it has been hypothesized that noradrenaline may be released from these nonsynaptic regions into the extracellular space (Elfvin, 1971). However, it seems unlikely that many other vesicle clusters in the dendrites and cell somata are readily releasable in response to membrane excita-

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Fig. 4. Electron micrographs showing synapses (indicated by open arrows) in rat SCGs fixed by conventional furation procedures. A hesynaptic element contains numerous small vesicles and several large, dense-core vesicles (arrowheads). Since this type of nerve ending rapidly disappears following preganglionic denervation, the synapse is thought to be a cholinergic type of preganglionic origin. Postsynaptic element is a dendritic profile (normal SCG). B Features of presynaptic element resemble those in A. Apparent postsynaptic dendrite contains a vesicle population (asterisk, normal SCG). C: Pre- synaptic element, resisting degeneration 6 days after preganglionic denervation, contains small vesicles, but no large, dense-eore vesicles.

A few small vesicles seem to contain granules (arrow). Postsynaptic element is a dendritic profile. The synapse is thus considered to be adrenergic and of intrinsic origin (denervated SCG). D Presynaptic element contains only small vesicles, a few of which appear to be dense-core (arrows). Postsynaptic element also contains small vesicles of similar features (asterisk) (SCG treated with noradrenaline in vitro). E: Synapse in which both sides of elements contain vesicles clustered at membrane specializations (reciprocal synapse). Right element contains small vesicles and large, dense-core vesicles (arrowheads), and thus appears cholinergic. Left element contains only small vesicles (asterisk). Bar, 0.5 pm, applicable to A-E.

Fig. 5. Electron micrographs showing several types of synapses (open arrowheads) involving biocytin-labeled dendritic collaterals sampled from dendritic glomeruli. A Axodendritic synapse. Labeled dendrite is postsynaptic to an element that contains small vesicles and several large, dense-core vesicles (arrowheads). Characteristics are very similar to those shown in Figure 4A,B. This type of synapse predominates in rat SCG. B Possible reciprocal synapse between a preganglionic nerve ending and a principal cell dendrite. Unlabeled element contains large, dense-core vesicles (arrowheads), and thus

appears to be a preganglionic nerve terminal. Labeled dendrite contains small vesicles (asterisk) clustered to the membrane specialization. C: Dendrodendritic synapse. Labeled dendritic collateral contains small vesicles, a few of which are dense-core (arrow), and establishes a synaptic contact (open arrow) with a larger dendritic profile containing vesicle populations (asterisks). D Possible reciprocal dendrodendritic synapse. Both sides of synaptic elements contain only small vesicles. A few seem to be dense-core (arrows). Bar, 0.5 pm, applicable to A-D.

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Fig. 6. Electron micrographs showing membrane attachment structures (arrows) between principal cell dendrites. A Zonula adherens between dendritic profiles (normal SCG). Conventional fixation

tion, because these clusters are situated at some distance from the membranes, and the dendrites are almost completely ensheathed by Schwann cell processes. It is indeed conceivable that these vesicles represent a storage site for noradrenaline, a main transmitter of principal neurons. Also, it is very likely that localized collections of these vesicles in dendrites of sympathetic ganglion cells give rise to the beaded fluorescent appearance of abundant adrenergic nerve terminals or varicosities detected by histof luorescence methods. However, the location of these vesicles at ultrastructural levels raises several questions. What is their destination? Why do sympathetic cells possess such abundant vesicles in the dendrites? Are the vesicles related to synapse formation?

Specialized attachment sites, zonulae adherentes, between dendritic profiles (dendrodendritic contact) are also of frequent occurrence in the sympathetic ganglion (Elfvin, 1971; Grillo, 1966; Matthews, 1983). These contacts are also found between dendrites and cell soma (dendrosmatic contact). The membrane specialization resembles that of synaptic connections (Fig. 6). However, at these contacts no vesicle clusters are specifically associated with the membrane of either side. Were vesicle clusters associated with a zonula adherens, the structural complex might be interpreted as synaptic, at least morphologically. Kiraly et al. (1989) speculated that some functional implications underlie these membrane contacts. They observed that such dendrodendritic contacts often involve preganglionic

procedure. B Membrane attachment structure between a labeled dendrite and a nonlabeled dendritic profile. Bar, 0.5 p,m, applicable to A and B.

nerve endings, thus contributing a more complex “triad” structure. This is consistent with our hypothe- sis that the close appositions between dendrites seen in “receptor plates’’ represent zonulae adherens rather than dendrodendritic synapses. The exact functional importance of this nonsynaptic contact between nervous elements remains to be elucidated.

Intraganglionic Neuronal Circuitry It is confirmed that synaptic connections in the sym-

pathetic ganglion are more complex than previously believed. Axodendritic (cholinergic) synapses between preganglionic axons and principal cell dendrites are considered to be responsible for fast excitatory postsynaptic potentials, generating spikes for relay transmission (Skok, 1973). This type of synapse represents about 85% of all synapses in normal rat SCG (SD strain; our unpublished observations). This cholinergic relay transmission may be modulated to various de- grees by the specialized dendritic apparatus. Dendritic collaterals in glomeruli form synaptic contacts with dendrites of nearby principal neurons. In a few cases, the connections are interpretable as reciprocal. It is probable that these connections modulate fast excitatory signals propagating along dendritic membranes. More direct modulation may take place at reciprocal synapses between preganglionic nerve endings and dendritic collaterals, since membranes of preganglionic nerve endings have been reported to have a-adrenocep- tors (Dun and Karczmar, 1977; Kafka and Thoa, 1979).


This dendritic modulation via synaptic connections may be inhibitory (Brown and Caulfield, 1979; our unpublished observations), thus possibly underlying the slow inhibitory postsynaptic potentials observed in the sympathetic ganglion (Libet and Kobayashi, 1974). The modulatory influence may be much more complicated, because the principal cells contain various neu- ropeptides in addition to noradrenaline (Lundberg et al., 1982).

Presynaptic dendrites have been reported in various regions of the central nervous system, such as the spinal dorsal horn (Gobel et al., 19801, the substantia nigra (Wassef et al., 19811, the lateral geniculate nucleus (Hamos et al., 1985), thalamic nuclei (Deschenes et al., 1985; Ralston and Herman, 19691, and the olfactory bulb (Pinching and Powell, 1971b). In the substantia nigra, dendritic release of dopamine has been demonstrated (Geffen et al., 1976), and there is evidence that the released dopamine has inhibitory effects (Cuello and Iversen, 1978). Dendrodendritic synapses in the lateral geniculate nucleus (Hamos et al., 1985), the thalamus (Deschenes et al., 1985), and the olfactory bulb (Mori et al., 1981) have been thought to be involved in the local GABAergic inhibitory circuit. It is also interesting that glomerular dendritic branches of SCG cells resemble those of the mitral cells or tuRed cells in the olfactory bulb in terms of morphology at both light and electron microscopic levels (Pinching and Powell, 1971a,b). There is likely to be similar functional importance with the presynaptic dendrites of the peripheral ganglion cells.

Synapses Involving SIF Cell Processes The possibility that SIF cells contribute presynaptic

elements of adrenergic synapses has been frequently suggested (Dail and Evan, 1978; Williams and Jew, 1983). Since the number of SIF cells in rat SCG is very small and their processes never exceed 150 pm in length in intracellularly-labeled preparations (our unpublished observations), the occurrence of synapses in which SIF-cell processes were the presynaptic elements would be restricted to the immediate neighborhood of SIF-cell groups, and the overall incidence would be very small (Williams and Jew, 1983). Indeed, synapses involving apparent SIF-cell processes are seldom en- countered in random observation. The incidence of synapses involving presumed SIF-cell processes was < 1% in rat SCG, and labeled dendrites were never observed to make synaptic or nonsynaptic contacts with apparent SIF-cell processes in our studies. Thus, presynaptic elements of adrenergic synapses in the sympathetic ganglion are hypothesized to be derived mainly from specialized dendrites of principal neurons.

CONCLUSIONS One of the most notable features underlying modu-

latory intraganglionic transmission is the specialized dendritic apparatus of principal neurons. The specialized dendrites serve not only as receptors for incoming signals from the central nervous system (relay transmission), but also as an intraganglionic efferent system via synaptic connections (modulatory transmission). However, it appears likely that the sophistication of

this specialized neuronal circuitry may vary not only between ganglia in different regions of the sympathetic system, but also between homologous ganglia in different species or strains. Even between ganglia of the same strain of a species under different conditions, the incidence of adrenergic connections appears to vary. This apparent plasticity of adrenergic synapses may pose significant problems in further understanding transmission in sympathetic ganglia.

ACKNOWLEDGMENTS The author is particularly grateful to Professor

Emiko Senba for her continuing support of the present research. This research was supported, in part, by Grants-in-Aid for Scientific Research from the Minis- try of Education, Science, and Culture of Japan.

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