Fine structural survey of the rat's brainstem sensory trigeminal complex

Parallel Inhibitory and Excitatory Trigemino-Facial Feedback Circuitry for Reflexive Vibrissa Movement

Animals employ active touch to optimize the acuity of their tactile sensors. Prior experimental results and models lead to the hypothesis that sensory inputs are used in a recurrent manner to tune the position of the sensors. A combination of electrophysiology, intersectional genetic viral labeling and manipulation, and classical tracing allowed us to identify second-order sensorimotor loops that control vibrissa movements by rodents. Facial motoneurons that drive intrinsic muscles to protract the vibrissae receive a short latency inhibitory input, followed by synaptic excitation, from neurons located in the oralis division of the trigeminal sensory complex. In contrast, motoneurons that retract the mystacial pad and indirectly retract the vibrissae receive only excitatory input from interpolaris cells that further project to the thalamus. Silencing this feedback alters retraction. The observed pull-push circuit at the lowest-level sensorimotor loop provides a mechanism for the rapid modulation of vibrissa touch during exploration of peri-personal space.

Inhibitory gating of vibrissal inputs in the brainstem

Trigeminal sensory nuclei are the first processing stage in the vibrissal system of rodents. They feature separate populations of thalamic projecting cells and a rich network of intersubnuclear connections, so that what is conveyed to the cortex by each of the ascending pathways of vibrissal information depends on local transactions that occur in the brainstem. In the present study, we examined the nature of these intersubnuclear connections by combining electrolytic lesions with electrophysiological recordings, retrograde labeling with in situ hybridization, and anterograde labeling with immunoelectron microscopy. Together, these different approaches provide conclusive evidence that the principal trigeminal nucleus receives inhibitory GABAergic projections from the caudal sector of the interpolaris subnucleus, and excitatory glutamatergic projections from the caudalis subnucleus. These results raise the possibility that, by controlling the activity of intersubnuclear projecting cells, brain regions that project to the spinal trigeminal nuclei may take an active part in selecting the type of vibrissal information that is conveyed through the lemniscal pathway.

Plasticity of cerebral metabolic whisker maps in adult mice after whisker follicle removal--II. Modifications in the subcortical somatosensory system

The follicles of whiskers C1-3 were removed from the left side of the snout of adult mice. Adjacent whiskers B1-3 and D1-3 were stimulated while local rates of glucose utilization were measured with the [14C]2-deoxyglucose method two, four, eight, 64, 160 and approximately 250 days after follicle removal. Local metabolic activity in the trigeminal sensory brainstem and somatosensory thalamus was compared with that of unoperated mice with the same stimulation and of mice with the same lesion that had all whiskers clipped. Actual rates of glucose utilization were measured in brainstem subnuclei caudalis and interpolaris whereas metabolic activation was only assessable by colour-coded imaging in brainstem nucleus principalis and in the thalamic ventrobasal complex. Whisker stimulation activated the somatotopically appropriate loci in brainstem and thalamus. In addition, the territory deprived by follicle removal was metabolically activated in subnuclei caudalis and interpolaris at all time intervals examined. The activation was statistically significant in subnucleus interpolaris at two days, indicating that the metabolic representations of whiskers neighbouring the lesion rapidly expanded into the deprived territory. Nucleus principalis showed a broad metabolic activation at two and four days that was absent at the longer time intervals examined. Instead, at approximately 250 days the metabolic representations of the whiskers adjacent to the lesion were enlarged into the deprived territory as in the subnuclei. Since metabolic whisker representation in the ventrobasal complex appeared to have changed in the same fashion, follicle removal apparently resulted in congruent modifications of the whisker map in the three nuclei of termination as well as in the thalamic relay at the longest time interval examined. Since metabolic responsiveness of the deprived barrels in barrel cortex of the same animals increased statistically significantly only several months after follicle removal, the novel neural responses in the brainstem were not effectively transmitted to barrel cortex immediately and the slowly evolving cortical modifications are more likely to be associated with regrowth of the connectivity of primary neurons. By contrast, unmasking of hitherto suppressed inputs may underlie the early expansion of metabolic whisker representation in the brainstem.

A comparative and ultrastructural study of synaptic contacts established by primary sensory fibers in the spinal trigeminal nucleus of the rat

A quantitative evaluation of the types of afferent synaptic contacts in the pars oralis, using transganglionic degeneration and a comparison of previous data obtained from the pars interpolaris (Lapa & Bauer, 1992), of the rat was performed. Following left inferior alveolar nerve transection or partial pulpectomy of the first and second left lower molar teeth well-defined degenerating terminals appeared bilaterally. In both experiments, the majority of these afferent synapses formed single asymmetric contacts with intermediate and distal dendritic segments in the pars oralis. Fewer contacts were observed with dendritic spines, proximal dendritic segments, perikarya, and other terminals. Double and multiple synaptic contacts, preferentially with small dendritic profiles, were also found. Pars oralis showed higher density of degenerating terminals and higher proportion of the contralateral contacts than pars interpolaris suggesting that it is a prime input area and that it may play a role in the bilateral management of sensory information. Pars oralis showed a higher density of contacts with intermediate and distal dendritic segment and a lower density of double contacts in comparison to the pars interpolaris. Partial pulpectomy revealed a distribution in synaptic types similar to that following IAN transection suggesting that sensory fibers conveying pain-related stimuli are not distinguished from fibers of other sensory modalities as to preference of synaptic contacts. The overall pattern demonstrates a structural organization of the sensory inputs to the spinal trigeminal nucleus regarding the bilateral handling of sensory information.

The peripheral and central projections of the Edinger-Westphal nucleus in the rat. A light and electron microscopic tracing study

The peripheral and central efferent projections of the rostral part of the Edinger-Westphal nucleus in the rat were investigated at the light and electron microscopic level by means of iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin and retrograde tracer injections of Fast blue and Nuclear yellow into the facial nucleus and into the principal olive. Two pathways leaving the rostral part of the Edinger-Westphal nucleus were studied, a peripheral and a central descending pathway. Fluorescent experiments demonstrated that the central pathway fibers originated from distinct individual Edinger-Westphal neurons. These neurons were mainly distributed throughout the rostral part of the Edinger-Westphal nucleus and had fusiform cell bodies. The neurons rarely form collateral projections. The central descending pathway left the Edinger-Westphal nucleus medially and terminated bilaterally in the principal olive, in the subnuclei A, B and C of the inferior olive and ipsilaterally in the medial accessory olive. The central pathway also terminated contralaterally in the lateral parabrachial nucleus, the facial nucleus, the trigeminal brainstem nuclear complex, the lateral reticular nucleus and the rostroventral reticular nucleus. The projection to the facial nucleus provides evidence for the existence of a polysynaptic loop forming the central part of the corneal blink reflex. Projections from the Edinger-Westphal nucleus to the cerebellar cortex or the deep nuclei, as described in cat and primate, could not be confirmed. The peripheral pathway left the Edinger-Westphal nucleus ventrally and terminated on dendrites of ciliary ganglion cells, along smooth muscle cells of ciliary ganglion associated arterioles and in the proximity of ciliary ganglion associated venules. The central and peripheral terminals that originate in the Edinger-Westphal nucleus all had similar ultrastructural features: clear, round vesicles and electron dense mitochondria. The terminals originating from the central descending pathway were often found to be arranged in glomerular-like structures. The central and peripheral terminals made asymmetric synaptic membrane specializations (Gray type one), except terminals innervating the ciliary ganglion associated vessels, which showed no synaptic contacts.

The barrelettes--architectonic vibrissal representations in the brainstem trigeminal complex of the mouse. I. Normal structural organization

The organization of the brainstem trigeminal complex (BTC) of the mouse is described, with emphasis on the normal organization of the vibrissal representations. Thionin staining for Nissal substance was employed to reveal the cytoarchitecture. Cytochrome oxidase histochemistry was used to reveal the chemoarchitecture. Golgi impregnation methods, in combination with thionin staining, were used to examine the neuronal dendritic morphology within a defined cytoarchitectonic context. An in vitro horseradish peroxidase labelling method was used to study the distribution and morphology of primary trigeminal afferent terminals within the BTC. The BTC consists of four distinct subnuclei: principalis (nVp), oralis (nVo), interpolaris (nVi), and caudalis (nVc). The present study shows that these sub-nuclei can be distinguished from each other on the basis of several anatomical criteria, including the distribution and density of neuronal size classes, histochemical staining intensity, morphology and orientation of neuronal dendrites, and size and texture of primary afferent terminal arbors. Anatomical manifestation of vibrissal representations within the BTC can be described in nVp, nVi, and nVc, but not in nVo. Within the three subnuclei where they are found, anatomical vibrissal representations are composed to architectural subunits that form an overall pattern homeomorphic to the pattern of vibrissae on the face of the animal. Each sub-unit forms a cylindrical tube running in a rostrocaudal orientation within the BTC. These sub-units will be called barrelettes. Cytologically, each barrelette consists of cell-dense "sides," surrounding a practically cell-free "hollow." Individual sub-units are separated by narrow, cell-free "septa." Histochemically, each subunit is manifested as a discrete patch of positive-staining reaction products. Differential interference contrast optics shows that these patches correspond precisely to the barrelette hollows. Evidence is presented to show that the barrelettes are the functional units for the processing of vibrissal sensory information. Terminal arborizations of individual primary afferents seem to be confined to the hollow of single barrelettes. The majority of neurons that form the sides of a barrelette have bitufted dendritic arbors, which project predominantly into the barrelette hollow, although a minority of neurons, particularly in nVi and nVc, also extend part of their dendritic arbors into adjacent barrelette hollows. The barrelette hollows are thus the principal neuropil region in which primary afferents and their target neurons interact. Contacts are made mainly between en passant varicosities and terminal boutons on primary afferent collaterals and dendritic spines and shafts of second order neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Fine structure of the lateral mammillary projection to the dorsal tegmental nucleus of Gudden in the rat

The synaptic organization of projections from the lateral mammillary neurons within the dorsal tegmental nucleus of Gudden is studied in the rat with the aid of anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) and visualized with tetramethylbenzidine. The dorsal tegmental nucleus consists of the pars ventralis (TDV) and the pars dorsalis (TDD). The normal neuropil of the dorsal tegmental nucleus contains three classes of axodendritic terminals, that is, terminals containing round, flat, and pleomorphic vesicles. They make up 44%, 5%, and 51%, respectively, of all axodendritic terminals in the TDV, and 62%, 1%, and 37% in the TDD. Injection of WGA-HRP into the lateral mammillary nucleus permits ultrastructural recognition of many anterograde labeled terminals within both the TDV and TDD. In the TDV, 81% of the labeled terminals contain round synaptic vesicles and make asymmetric synaptic contacts. A few of the labeled terminals contain pleomorphic vesicles and make symmetric synaptic contacts. More than 50% of the labeled terminals contact intermediate dendrites (1-2 microns diameter). In the TDD, almost all labeled terminals are small, contain round vesicles, and make asymmetric synaptic contacts. These terminals mainly contact intermediate as well as distal (less than 1 micron diameter) dendrites. There are only a few labeled terminals with pleomorphic vesicles and no terminals with flat vesicles. The termination pattern of the lateral mammillary neurons in the TDV is similar to that in the TDD. Anterograde labeled axon terminals often contact retrograde labeled dendrites in the TDV. No reciprocal connections are present in the TDD. These results suggest that the TDV and the TDD receive mainly excitatory and a few inhibitory inputs from the lateral mammillary nucleus. The TDV neurons also have direct reciprocal connections with the lateral mammillary neurons.

Ultrastructure of the mammillotegmental projections to the ventral tegmental nucleus of Gudden in the rat

This study examines the termination pattern of axons from the medial mammillary nucleus within the ventral tegmental nucleus of Gudden (TV) in rats by using anterograde transport of horseradish peroxidase conjugated with wheat germ agglutinin (WGA-HRP) and visualized with tetramethylbenzidine. The neuropil of TV contains three classes of axodendritic terminals, that is, terminals containing round, flat, and pleomorphic synaptic vesicles. These types make up 55.6%, 26.1%, and 18.3%, respectively, of all normal axodendritic terminals. Injection of WGA-HRP into the medial mammillary nucleus permits ultrastructural recognition of anterogradely labeled terminals within the TV. More than 80% of the labeled terminals contain round synaptic vesicles and form asymmetric synaptic contacts, whereas about 16% contain flat synaptic vesicles with symmetric synaptic contacts. There are a few labeled terminals with pleomorphic vesicles and only a few axosomatic terminals. Almost all labeled terminals are small, having diameters of less than 1.5 microns. Compared with the distributions of normal and labeled terminals with round vesicles, there is an increase of the percentage of labeled terminals with round vesicles on the intermediate dendrites (1-2 microns diameter) and a decrease on the distal dendrites (less than 1 micron diameter). Anterogradely labeled axon terminals often contact retrogradely labeled dendrites. These results suggest that the medial mammillary neurons send mainly excitatory as well as a few inhibitory inputs to the dendrites of TV and have direct reciprocal contacts with the TV neurons.

Structure-function relationships in rat brainstem subnucleus interpolaris: IV. Projection neurons

In a companion paper (Jacquin et al., '89), the structure and function of local circuit (LC) neurons in spinal trigeminal (V) subnucleus interpolaris (Sp Vi) were described. The present report provides similar data for 44 projection neurons in Sp Vi. Of these, 25 thalamic, 16 cerebellar, 2 superior collicular, and 1 inferior olivary projecting neurons were studied. The majority responded to vibrissa(e) deflection, and all except 4 of these had multivibrissae receptive fields. The remainder were responsive to either guard hair deflection or indentation of glabrous skin. Latencies to V ganglion shocks were suggestive of monosynaptic activation from the periphery. Sp Vi projection neurons were topographically organized in a manner consistent with that of their primary afferent inputs. Nonvibrissa sensitive cells had diverse morphologies. Morphometric analyses of the more heavily sampled thalamic and cerebellar projecting, vibrissa(e)-sensitive cells indicated the following. (1) As compared to LC neurons, projection neurons had bigger receptive fields, cell bodies, dendritic trees, and axons; less circular dendritic trees; a greater preponderance of spiny dendrites and fewer axon collaterals in Sp Vi. (2) Dendritic tree extent correlated significantly with receptive field size, thus suggesting that dendritic tree size is one mechanism contributing to receptive field size in vibrissae-sensitive projection neurons. (3) V thalamic cells had significantly bigger receptive fields and dendritic trees, and also give off more local axon collaterals, than V cerebellar neurons. Collicular and inferior olivary projecting neurons shared structural and functional attributes with other Sp Vi long-range projecting cells. Structure-function relationships exist for vibrissa-sensitive projection neurons in Sp Vi. The relevant parameters correlating with projection neuron morphology are receptive field size and projection status, whereas for Sp Vi LC neurons the relevant correlative parameter is peripheral receptor association.

Synaptic substrates for enkephalinergic and serotoninergic interactions with dental primary afferent terminals in trigeminal subnucleus interpolaris: an immunocytochemical study using peroxidase and colloidal gold

Pain processing in the trigeminal complex has been thought to reside primarily in the spinal subnucleus caudalis (Vc). However, trigeminal tractotomies eliminating primary afferent input to Vc and severance of secondary trigemino-thalamic fibers from Vc do not disturb pain perception from the central face and oral cavity. Furthermore, large numbers of neurons that are highly responsive to noxious stimuli and suppressed by inputs from the periaqueductal gray and raphe complex have been identified in subnuclei interpolaris (Vi) and oralis (Vo). Therefore, the purpose of this study was to assess the distribution and spatial arrangements of nociceptive modulatory transmitters with nociceptive afferents and trigemino-thalamic relay cells in the rostral portion of the spinal trigeminal nuclear complex. The dental pulp contains predominantly nociceptors that project to all three subdivisions of the trigeminal spinal complex. These projections were visualized by anterograde transganglionic transport of horseradish peroxidase or by degeneration following administration of toxic ricin to the pulp chambers. The spatial arrangements of dental primary afferents with enkephalinergic (ENK) and serotoninergic (5HT) inputs was then assessed by employing avidin-biotin peroxidase and protein-A colloidal gold double-labeling immunocytochemistry. Trigemino-thalamic relay cells were also labeled by retrograde transport of HRP after stereotaxic injections into the ventrobasal thalamus. ENK and 5HT immunoreactivity was found in the ventrolateral quadrant and lateral margin of Vi, together with the adjacent interstitial nucleus (IN). This activity extended from the caudal pole of Vi and the periobex region, where it was most dense, rostrally to a position approximately 2.9 mm from the Obex. Neither ENK nor 5HT immunoreactivity was observed in Vo. Primary dental afferents projected into the ventromedial quadrant of rostral Vi and were found in the ventrolateral quadrant and dorsal aspect of the subnucleus farther caudally. They appeared as simple boutons with single contacts or as larger, sometimes scalloped terminals that formed multiple contacts. Postsynaptic elements were usually small dendritic profiles, although relay cell somata rarely received primary afferent inputs. Many primary afferents entered areas of synaptic clustering and contacted enkephalinergic dendrites, some of which were also postsynaptic to serotoninergic synapses. Alternatively, primary afferents contacted unlabeled processes that were also postsynaptic to the enkephalinergic element to form a triad arrangement. The least common occurrence was axo-axonic contacts in which enkephalinergic synapses were presynaptic to primary afferents. Both enkephalinergic and serotoninergic synaptic categories displayed round vesicles and generally formed asymmetric junctions.(ABSTRACT TRUNCATED AT 400 WORDS)

A survey of the cytology and synaptic organization of the insular trigeminal-cuneatus lateralis nucleus in the rat, including an identification of spinal afferent inputs

The cytology and synaptic organization of the insular trigeminal-cuneatus lateralis (iV-Cul) nucleus was examined in the rat. In addition, the ultrastructural morphology and synaptic connectivity of anterogradely labeled spinal afferent axons terminating in iV-Cul were examined following injection of horseradish peroxidase (HRP) into the cervical spinal cord. The uniformity of the ultrastructural features of iV-Cul neurons supports the presence of a homogeneous neuronal population. The most prominent feature of the iV-Cul neuropil is the presence of numerous interdigitating astrocytic processes, which extensively isolate neuronal somata and processes. iV-Cul contains a heterogeneous population of axonal endings that can be separated into three categories, depending upon whether they contain predominantly spherical-shaped agranular synaptic vesicles (R endings), predominantly pleomorphic-shaped agranular synaptic vesicles (P endings), or a heterogeneous population of dense-core vesicles (DC endings). The R endings represent the majority of axonal endings in iV-Cul and establish asymmetrical axodendritic and axospinous synaptic contacts, primarily along the distal portions of the dendritic tree. P endings establish symmetrical axosomatic, axodendritic, and axospinous synaptic contacts and exhibit a more generalized distribution along the somadendritic tree. DC terminals establish asymmetrical axodendritic synaptic contacts with distal dendritic processes and are the least frequently observed endings in the iV-Cul neuropil. Numerous synaptic glomeruli, exhibiting a single large central R bouton that establishes multiple axodendritic or axospinous synapses, characterize the iV-Cul neuropil. Axoaxonic synapses are conspicuously absent from the iV-Cul neuropil and glomeruli. The anterograde HRP labeling of spinal afferent axons that terminate in iV-Cul indicates that the terminals along these fibers are of the R type and that they are engaged predominantly in synaptic glomeruli. The results of this study indicate that the synaptic organization of iV-Cul is distinctly different from that of neighboring somatosensory nuclei, and supports the recent suggestion that this nucleus should be considered a separate precerebellar spinal relay nucleus in the lateral medulla.

Synaptic organization of primary axons in trigeminal nucleus oralis

This report examines the morphology and synaptic connections of small-diameter primary trigeminal axons that terminate in the border zone (BZ) and ventrolateral (VL) subdivisions of rat trigeminal nucleus oralis (Vo). Primary axons were made visible for light and electron microscopic analysis by utilizing the method of anterograde transport of horseradish peroxidase. BZ receives the terminal arborizations of two different populations of small-diameter primary axons. One of these arises from unmyelinated parent fibers and terminates in the dorsal one-half of BZ, while the other has small myelinated parent branches that arborize throughout the subdivision. Terminating within VL are the arbors of a second population of small myelinated primary axons. The endings of all three populations of primary axons lie in synaptic glomeruli. Endings in both subdivisions derived from small myelinated parent fibers lie centrally in glomeruli. Those in VL form axodendritic synapses on numerous dendritic shafts and spines, while endings in BZ glomeruli make at least one axodendritic synapse on one or two dendritic shafts. Endings of unmyelinated primary axons in BZ lie at the periphery of glomeruli where each forms a single axodendritic synapse on a central dendrite. It is at these asymmetrical axodendritic synapses that these three populations of primary axons are thought to transfer their inputs directly to the dendritic arbors of second-order BZ and VL neurons. Common to all three glomeruli is one or more small axonal endings filled with flattened synaptic vesicles that establish axoaxonic synapses on the primary ending as well as axodendritic synapses on the dendritic element(s) receiving primary input. In view of their symmetrical to intermediate synaptic contacts, these endings are thought to belong to axons derived from at least one source that can inhibit or diminish the firing rate of second-order BZ and VL neurons in response to primary input.

An electron microscopic analysis of the morphology and connectivity of individual HRP-labeled slowly adapting vibrissa primary afferents in the adult rat

The ultrastructure of 4 slowly adapting vibrissa primary afferent central terminal arbors was examined following intracellular injection of horseradish peroxidase (HRP). The terminals were found to contain clear round vesicles and formed primarily asymmetric synapses on dendritic shafts and spines. Few examples of synaptic glomeruli, with the labeled axon as the central element, were identified.

Uptake and transneuronal transport of horseradish peroxidase-wheat germ agglutinin by tooth pulp primary afferent neurons

Horseradish peroxidase-wheat germ agglutinin (HRP-WGA) applied to proximal stumps of tooth pulp primary afferent neurons in rats was taken up and transported transneuronally to neurons in the ipsilateral trigeminal brainstem nuclear complex. The results of this study suggest that HRP-WGA transport may be a novel means of labeling both primary and higher order neurons that transmit tooth pulp sensory information in the rat and may be used to investigate the fine structure and synaptic contacts of central nervous system neurons that receive tooth pulp afferent input.

Fine structural survey of Gudden's tegmental nuclei in the rat: cytology and axosomatic synapses

The fine structure of neuronal somata and axosomatic synapses in each subnucleus of Gudden's tegmental nuclei was studied by use of electron microscopy. The pars principalis of the ventral tegmental nucleus of Gudden (TVP) is composed of oblong or triangular, medium sized neurons (11.8 x 22.6 microns, 211.4 microns 2) containing many mitochondria, lysosomes, Golgi apparatus, and rough endoplasmic reticulum composing Nissl bodies. The light oval nucleus with a prominent nucleolus is centrally situated, and indentations of its nuclear envelope are recognized in all neurons. The neuron in the pars ventralis of the dorsal tegmental nucleus of Gudden (TDV) is similar to that in the TVP, but its average size is significantly smaller (10.0 x 18.8 microns, 151.4 microns 2), and its organelles are also less well developed. The pars dorsalis of the dorsal tegmental nucleus of Gudden (TDD) is composed of spindle shaped, small neurons (6.9 x 16.2 microns, 85.1 microns 2) characterized by their irregular shaped nucleus with its invaginated envelope. These neurons have a thin rim of cytoplasm, poorly developed organelles and no Nissl bodies. The average number of axosomatic terminals in a sectional plane is 9.9 in the TVP, 9.6 in the TDV and 2.6 in the TDD, and the bouton covering ratio is 24.3% in the TVP, 26.5% in the TDV and 7.4% in the TDD. The respective percentages of round, flat and pleomorphic type axosomatic terminals were estimated, and the flat type terminals were found to be dominant in the TVP, the pleomorphic type terminals in the TDV, and the round type terminals in the TDD.

GABA distribution in a pain-modulating zone of trigeminal subnucleus interpolaris

A recent model for control of spinal and medullary nociceptive neurons (Basbaum and Fields, 1984) incorporates a gamma-aminobutyric acid-ergic (GABA-ergic) cell into this circuitry and indicates that such elements could act as one substrate for presynaptic inhibition of primary afferents. This concept is supported by a variety of pharmacological and electrophysiological studies. We therefore examined the distribution of GABA-ergic activity in trigeminal subnucleus interpolaris (Vi) by focusing on the types of cells, together with dendritic and synaptic profiles, that are immunocytochemically labeled with an antiserum against glutamic acid decarboxylase (GAD). GAD occurred throughout Vi but was most concentrated in the ventrolateral quadrant and interstitial nucleus. It was localized to groups of small neurons with two to three primary dendrites, and within numerous punctate profiles suggestive of synaptic elements. Electron microscopy revealed labeled dendrites, some of which were postsynaptic to scalloped terminals of presumptive primary afferents. Other labeled dendritic elements, which were quite variable in size, engaged both GAD-labeled and unlabeled synapses. Most GAD synapses displayed clear round vesicles and formed contacts with unlabeled perikarya and a variety of dendritic processes. Numerous GAD-positive synapses were also incorporated into axoaxonic clusters, in which the GAD element was presynaptic to scalloped terminals. Others engaged in serial arrays with other unlabeled terminals, which, in turn, were presynaptic to dendrites. Occasionally, GAD synapses formed contacts with GAD-positive dendrites. These data show that GABA is localized to a variety of neuronal elements in ventrolateral Vi and the interstitial nucleus. These occur in spatial arrangements providing an anatomical substrate for postsynaptic modulation of activity in this area. GABA terminals also appear to be involved in a presynaptic inhibitory mechanism, which may, in some instances, affect transmission in primary afferents.

Direct connections of primary trigeminal afferent axons with trigeminocerebellar projection neurons in the border zone of rat trigeminal nucleus oralis

This study presents electron microscopical evidence for direct synaptic contact between primary trigeminal axonal (PR) endings and trigeminocerebellar projection neurons (TCPNs), in the border zone (BZ) of rat trigeminal nucleus oralis. The combined techniques of anterograde degeneration following trigeminal sensory root rhizotomy and retrograde transport of horseradish peroxidase subsequent to injections into the orofacial tactile portions of crura I and II of the cerebellar hemispheres were utilized. Degenerating PR endings lie centrally in glomeruli where they form axodendritic synapses on higher order dendrites of identified BZ TCPNs. It is at these synapses that the PR endings are probably transferring tactile input to BZ TCPNs for direct relay to the cerebellar cortex. Transmitter release at the axodendritic synapses may be modified within the glomeruli by axoaxonic synapses between terminals containing flattened synaptic vesicles and the PR ending.

The organization of trigeminotectal and trigeminothalamic neurons in rodents: a double-labeling study with fluorescent dyes

Retrogradely transported fluorescent dyes (fast blue and diamidino-dihydrochloride yellow) were used to compare the distributions of trigeminofugal neurons that project to the superior colliculus and/or the thalamus in three rodent species. The objective was to determine what the projection and collateralization patterns of these trigeminofugal pathways are and whether they are similar among different species. In each anesthetized animal, one dye was injected into the superior colliculus and the other into the topographically congruent area of the thalamus. Counts of the numbers of yellow, blue, and double-labeled neurons were made throughout the trigeminal complex: principalis, pars oralis, pars interpolaris, and pars caudalis. Trigeminothalamic projections were similar in each of the rodent species studied. The densest concentration of retrogradely labeled neurons was in principalis, with substantially fewer neurons in pars interpolaris, and fewer still in pars oralis and pars caudalis. These neurons were generally small and tended to have round or fusiform somata. A common pattern was also noted among the three species for trigeminotectal neurons. Most trigeminotectal projections originated from neurons in pars interpolaris, somewhat fewer from pars oralis, and the fewest from principalis and pars caudalis. These neurons tended to be the largest in each subdivision and were often multipolar. Following paired injections of the tracers, double-labeled neurons were scattered throughout the sensory trigeminal complex and had morphologies characteristic of single-labeled trigeminotectal neurons. Although comparatively few double-labeled neurons were observed in any species, most of those seen were restricted to the ventrolateral portion of pars interpolaris, a position that corresponds to the representation of the vibrissae. These data indicate that, regardless of the rodent species, the vast majority of labeled trigeminal neurons project either to the superior colliculus or the thalamus, but not to both targets. This might be expected on the basis of the very different behavioral roles these structures play. On the other hand, a subpopulation of trigeminal neurons exists (mainly in pars interpolaris) that does project to both the superior colliculus and the thalamus, perhaps because both structures require some of the same somatosensory information to perform their behavioral functions.

Morphological alterations in subcortical vibrissal relays following vibrissal follicle destruction at birth in the mouse

Morphological modifications of two subcortical vibrissal relays were analyzed, following destruction of vibrissal follicles in newborn mice. The volume of the nucleus interpolaris (NI) of the trigeminal nuclear complex in the brainstem decreased by 33%, while the number of its neuronal perikarya decreased only moderately. Vibrissal deafferentation caused no shrinkage of the ventrobasal complex (VB). In the damaged medial vibrissal part of VB (VBm), however, neuronal density was higher than normal, indicating the prevention or retardation of physiologically programmed cell death in the afferentation deprived thalamic somatosensory relay station. It is suggested that the difference in neuron density produced by deafferentation is related to the states of maturation at birth of the two subcortical vibrissal relays. Following vibrissal deafferentation the basic organization of the synaptic neuropil appeared to be similar to the control. Quantitative electron microscopic (EM) analysis revealed, however, an increased number of axon terminals with ovoid synaptic vesicles in both deafferented relay stations. The increased density of gamma-aminobutyric acid (GABA)-immunostained boutons observed in the VBm following vibrissal deprivation suggested a compensatory increase most probably of the inhibitory axon endings. Quantitative EM analysis also provided evidence that many or most of the specific afferent terminals in the damaged VBm were not identical with but were substitutes for the original "vibrissal" specific afferents. Forty percent of all "specific" afferents were shown to be modified corticothalamic terminals. The modification and the resemblence of some cortical endings to specific afferents demonstrated the morphogenetic plasticity of synaptogenesis in these terminals during development as well as the importance and inductive potential of the postsynaptic target in the differentiation of presynaptic axon terminals.

Morphology and synaptic connections of unmyelinated primary axons in the border zone of rat trigeminal nucleus oralis

This anterograde horseradish peroxidase study examines the morphology and synaptic connections of a population of primary axons which terminate in the dorsal one-half of the border zone (BZ) of rat trigeminal nucleus oralis. Unmyelinated parent fibers in the spinal V tract enter BZ directly and each terminate by continuing as a sparsely branched, long caudally directed strand containing several axonal endings. Primary endings lie in glomeruli where each forms an asymmetrical synapse on a central dendrite. Other glomerular components include two types of non-primary endings. One contains flattened synaptic vesicles, and forms a symmetrical synapse on either the primary ending or the central dendrite, while the other contains pleomorphic synaptic vesicles and establishes a symmetrical synapse on the central dendrite.

Exocytosis from large dense cored vesicles outside the active synaptic zones of terminals within the trigeminal subnucleus caudalis: a possible mechanism for neuropeptide release

It has been hypothesized that chemical interactions between neurons in the central nervous system can occur in the absence of well defined synaptic complexes, but morphological correlates have been difficult to find. The present study demonstrates exocytotic release from large (70-130 nm) dense cored vesicles at structurally nonspecialized areas along the plasmalemma of structurally different categories of terminals and occasionally from dendrites and axons within the neuropil of the trigeminal subnucleus caudalis. In rats, the marginal (lamina I) and substantia gelatinosa (lamina II) layers contain the central terminals of primary afferent fibers from the infraorbital nerve that supply the skin and whiskers (vibrissae). Different types of interneurons are also present and may modify the input being relayed to higher centers. While exocytotic profiles were present in control animals, they increased significantly (P less than 0.01) on the ipsilateral side 1-24 h after a unilateral skin lesion in the vibrissae area. A second increase (P less than 0.001) occurred 14-15 days after the lesion. Virtually all examples of large vesicle exocytosis were observed at structurally nonspecialized sites while those at the active synaptic zones involved small clear vesicles. Substance P-like immunofluorescence, present in controls and on the ipsilateral side during the first 6 days, subsequently declined until 4 weeks after surgery when some recovery was noted. The increase in large vesicle exocytosis and the decrease in substance P are interpreted to reflect functional adjustments of different neurons in response to the lesion. The exocytosis involving large dense cored vesicles may serve to deliver transmitters and/or neuropeptide modulators to appropriate receptors in a wider area than release into a specialized synaptic cleft would allow.

Trigeminotectal and other trigeminofugal projections in neonatal kittens: an anatomical demonstration with horseradish peroxidase and tritiated leucine

The trigeminal projection to the superior colliculus in neonatal kittens was studied by using both anterograde and retrograde neuroanatomical tracing techniques. Trigeminothalamic observations also were made. In the first series of experiments, horseradish peroxidase was injected into the superior colliculus in kittens on the day of parturition and in adult cats. Retrogradely labeled cells were found throughout the contralateral sensory trigeminal complex: the greatest numbers of cells were concentrated in pars oralis, with fewer in the principal nucleus, and fewer still in pars interpolaris and pars caudalis. Thus, the distribution pattern of trigeminotectal cells in neonates is similar to that in adult animals. In the second series of experiments, we injected tritiated leucine into the rostral portion of the spinal trigeminal nucleus in neonatal kittens and adult cats and compared the laminar and spatial distribution of anterogradely transported label in the superior colliculus and thalamus. Terminal label was observed in both structures in animals as young as 1-2 days postpartum. The label in the superior colliculus was overwhelmingly contralateral and formed a tier of discontinuous patches in the stratum griseum intermediale and, in a more diffuse manner, in the stratum griseum profundum. Most of the patches were located in the rostral 80% of the superior colliculus and were 60-280 micron in width. Although the size of the patches was smaller in the neonates, their distribution was similar to that in adult cats. Thus, with the exception of the difference in patch size, the terminal pattern of trigeminotectal projections is essentially adultlike at birth. The dense pattern of contralateral terminal label in the arcuate division of the ventrobasal complex also was similar to that of the adult cat, as was the trigeminal projection to the supraoculomotor gray. These data indicate that the development of the spatial organization of a major ascending somatosensory pathway to the superior colliculus (and to the thalamus) is largely a prenatal event. It is likely that the further maturation of these systems during postnatal life is limited to fine changes in axonal terminals and synaptic formation within prenatally determined terminal territories. The in utero maturation of these trigeminofugal projections is necessary to enable the newborn kitten to utilize the perioral tactile cues necessary for early orientation and suckling behaviors.

Morphology and synaptic connections of myelinated primary axons in the ventrolateral region of rat trigeminal nucleus oralis

Neurons in the ventrolateral (VL) subdivision of rat trigeminal nucleus oralis (Vo) have most of their dendritic arbors confined within this region. This study examines the morphology and synaptic connections of a population of myelinated primary trigeminal axons that arborize within VL and are in a position to provide input directly to VL neurons. Primary axons were visualized for light and electron microscopic analysis by injecting 30% horseradish peroxidase (HRP) in 2% dimethylsulfoxide (DMSO) into the sensory root of the trigeminal nerve and allowing 24-36 hours for the anterograde transport of HRP into the terminal axonal arbors. This population is characterized by its cone-shaped terminal arbors, which generate many axonal endings (2-8 micron in diameter) along unmyelinated terminal strands. These arbors arise from collaterals emanating from thinly myelinated (2-5 micron in diameter) parent branches descending in the spinal V tract, which, on the basis of their size, are considered to be small myelinated (A sigma) primary trigeminal axons. HRP-labeled P endings belonging to this population of primary axons are scalloped, filled with spherical to ovoid (40-70 nm in diameter) synaptic vesicles, and lie centrally in glomeruli where they make asymmetrical axodendritic synapses on dendritic shafts and spine heads. It is at these synapses that this population of primary trigeminal axons is probably transferring its input directly to the dendritic arbors of VL neurons. The dendritic shafts and spine heads also receive symmetrical to intermediate axodendritic synapses from endings containing flattened (70 X 29 nm) synaptic vesicles. These terminals also establish axo-axonic synapses on the P ending. Other synaptic components found less often in the glomeruli include small terminals containing oval (14-23 nm) synaptic vesicles that establish symmetrical to intermediate synapses on the P ending, boutons containing pleomorphic (35-80 nm) synaptic vesicles that form symmetrical to intermediate synapses on the P ending as well as on dendritic shafts, and small peripheral endings containing round (20-40 nm) synaptic vesicles that establish asymmetrical synapses on dendritic shafts.

Morphology and synaptic connections of small myelinated primary trigeminal axons arborizing among neurons in the border zone of rat trigeminal nucleus oralis

The anterograde transport of horseradish peroxidase (HRP) was used to examine the morphology and synaptic connections of a morphologically distinct group of small-diameter primary trigeminal axons that arborize throughout the border zone (BZ) of rat trigeminal nucleus oralis. Thinly myelinated parent branches (0.75-1.5 micron in diameter) descending in the spinal V tract (SVT) were seen to issue medially directed collaterals that entered BZ, where they branched and eventually terminated by giving rise to thin terminal strands characterized by several relatively widely spaced axonal endings. Based on the size and morphology of the parent branches in SVT, in the root entry zone, and in the sensory root of the trigeminal nerve, these primary axonal (P) endings are considered to be derived from small-diameter myelinated primary trigeminal axons (SDMA). The P endings measured 1-2 micron in diameter and contained numerous agranular spherical (40-60 nm) synaptic vesicles. In the BZ neuropil, most P endings lay in glomeruli, where each formed at least one asymmetrical axodendritic synapse on a dendritic shaft. It is at these synapses that this group of primary axons is thought to transfer its input directly to the dendritic arbors of BZ neurons. A small (0.5-1.5 micron) axonal (F) ending filled with flattened synaptic vesicles (29 X 60 nm) was observed to form at least one symmetrical to intermediate axoaxonic synapse on the P ending, as well as at least one axodendritic synapse on the same dendritic shaft receiving the primary input. Some F endings only contacted dendritic shafts. In view of their symmetrical to intermediate synaptic contacts, F endings are thought to belong to axons derived from at least one source that can inhibit or diminish the firing rate of BZ neurons in response to SDMA input. This would be accomplished either postsynaptically through the axodendritic synapses on the dendritic shafts, and/or presynaptically through the axoaxonic synapses on the P endings.

The organization of the neonatal rat's brainstem trigeminal complex and its role in the formation of central trigeminal patterns

The present study delimits the relationship of primary trigeminal afferents to their targets, the brainstem trigeminal nuclei of the neonatal rat. Previously, the brainstem trigeminal complex of the rat has been subdivided on the basis of either cytoarchitectonics or patterns of succinic dehydrogenase activity into the principal sensory nucleus and the three subnuclei of the spinal trigeminal nucleus, oralis, interpolaris, and caudalis. In this paper, we demonstrate that each of these subdivisions can also be identified by its pattern of primary trigeminal afferents. In addition, we demonstrate that the terminations of these afferents are distributed in a punctate fashion which correlates with vibrissae-related patterns of histochemical staining. Further, vibrissae removal in the neonatal rat at any age studied results in a corresponding deafferentation of both the principal sensory nucleus and all subnuclei of the spinal trigeminal nucleus. This same procedure has a graded, age-dependent effect on the vibrissae-related pattern of cytochrome oxidase staining in somatosensory cortex. On this basis, we conclude that vibrissae-related pattern formation in the central trigeminal system can be best understood in terms of a single "sensitive" period for the entire system. We hypothesize that this is the period during which an interaction normally occurs between primary trigeminal afferents and target neurons of the principal sensory nucleus.