Fine structural aspects of the synaptic organization of the spinal trigeminal nucleus (pars interpolaris) of the cat

Free postsynaptic densities in the hippocampus of the female rat

The number of synapses in the adult, female hippocampal CA1 region fluctuates naturally across the estrous cycle in an ovarian steroid-dependent manner. This phasic variation in synapse number occurs without identifiable degenerating synapses. Ultrastructural correlates of the dynamic aspect of this synapse loss and synapse formation thus remain undescribed. During early development, one hallmark of synaptogenesis is the presence of free postsynaptic densities (PSDs). Here we report that the incidence of free PSDs in CA1 fluctuates across the rat estrous cycle. The number of free PSDs is greatest on the afternoon of proestrus and is significantly decreased on the afternoon of estrus, 24 h later. We hypothesize that these free PSDs reflect synapse turnover in the adult CA1 region.

Electron microscopy of immunoreactivity patterns for glutamate and gamma-aminobutyric acid in synaptic glomeruli of the feline spinal trigeminal nucleus (Subnucleus Caudalis)

We studied the ultrastructure of the synaptic organization in the feline spinal trigeminal nucleus, emphasizing specific neurotransmitter patterns within lamina II of the pars caudalis/medullary dorsal horn. Normal adults were perfused, and Vibratome sections from pars caudalis were processed for electron microscopy. Ultrathin sections were reacted with antibodies for the excitatory neurotransmitter glutamate (Glu) and for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) by using postembedding immunogold techniques. Both single- and double-labeled preparations were examined. Results with single labeling show that Glu-immunoreactive terminals have round synaptic vesicles and form asymmetric synaptic contacts onto dendrites. GABA-immunoreactive axon terminals and vesicle-containing dendrites have pleomorphic vesicles, and the axon terminals form symmetric contacts onto dendrites and other axons. Double labeling on a single section shows glomeruli with central Glu-immunoreactive terminals that are presynaptic to dendrites, including GABA+ vesicle-containing dendrites. These Glu+ terminals are also postsynaptic to GABA+ axon terminals, and these GABA-immunoreactive terminals may also be presynaptic to the GABA+ vesicle-containing dendrites. Quantitative analyses confirm the specificity of the Glu and GABA immunoreactivities seen in the various glomerular profiles. The results suggest that a subpopulation of Glu-immunoreactive primary afferents (excitatory) may be under the direct synaptic influence of a GABA-immunoreactive intrinsic pathway (inhibitory) by both presynaptic and postsynaptic mechanisms.

Ischemia-induced delayed-onset paraplegia is accompanied by an unusual form of synaptic degeneration in the lumbosacral segments: an experimental light and electron microscopic study in dogs

We studied the effect of high thoracic aorta cross-clamping, complete transverse section of the spinal cord at Th6 level, and combined hemisection at Th6 level followed later by high thoracic aorta cross-clamping upon the morphology and number of identified presynaptic knobs in lumbosacral segments in dogs. In animals surviving 48-72 hours after high thoracic aorta cross-clamping the occurrence of an unusual form of boutons accompanied by periboutonal halo in L3-S1 segments was found. According to the bouton size and light as well as electron microscopic appearance, four types, i.e., light giant (T1), dark enlarged (T2), light giant with periboutonal halo (T3), and giant disintegrating (T4) boutons were detected after 48 and 72 hour reperfusion. The appearance of four boutonal types in the lumbosacral segments is caused by spinal cord ischemia secondary to high thoracic aorta cross-clamping followed by 48 or 72 hour reperfusion. At the end of the sixth reperfusion day no signs of enlarged and giant boutons were detected in L3-S1 segments. A statistically significant increase of enlarged and giant boutons was noted at the end of the third reperfusion day in comparison with 48 hour survival. After spinal cord transection at midthoracic (Th6) level, followed by 72 hour survival, no such unusual synaptic knobs could be found in L3-S1 segments. The laminar distribution pattern of T1-T4 types based on light microscopic analysis and confirmed electron microscopically is characteristic and strictly bound to those spinal cord gray matter layers which serve as main termination sites of the descending cortical, brain stem, as well as long propriospinal projections in the lumbosacral segments (laminae V-VII). A statistically significant increase of enlarged and giant boutons was found in the intermediate zone (lamina VII). Hemisection at midthoracic level (Th6) followed later by 30 minute high thoracic aorta cross-clamping and 48 hour reperfusion caused a marked decrease of enlarged and giant boutons in L3-S1 segments on the hemisectioned side in comparison with the contralateral one. Large amounts of irregularly arranged round vesicles and tubular profiles were disclosed in the boutonal matrix of T1, T3, and T4 types in L3-S1 segments of animals subjected to 30 minute high thoracic aorta cross-clamping followed by 72 hour reperfusion. Accumulation of tubular and membranous materials was invariably seen in the bulbous enlargement of the terminal axonal branch.(ABSTRACT TRUNCATED AT 400 WORDS)

Axon hillocks and initial segments in spinal trigeminal nucleus with emphasis on synapses including axo-axo-axonic contacts

As a part of a continuing study of the feline spinal trigeminal nucleus, the fine structure and synaptic arrangements on the axon hillock and axon initial segment of neurons in this region are described here. Transmission electron microscopy has been used to characterize qualitatively the axon hillock and initial segment and associated synapses in pars interpolaris. Axon hillocks and initial segments are easily identified in continuity with somata or as isolated profiles in the neuropil, and they receive synaptic contacts: these we regard as axo-axonic. The presynaptic terminals contain either mainly round or mainly flattened synaptic vesicles and have Type I (asymmetric) or Type II (symmetric) thickenings respectively at their contacts with the axon hillock or initial segment. I report here also the unusual arrangement of three separate axons in a serial synaptic complex. Some of the round vesicle Type I contacts onto the axon hillock-initial segment region also receive Type II contacts from one or more flattened vesicle terminals, thus forming an axo-axo-axonic complex. These flattened vesicle terminals lack the usual features of a presynaptic dendrite. It has been shown that in this nucleus some round vesicle terminals, especially those postsynaptic to flattened vesicle terminals, are primary afferents from the periphery. Therefore the round vesicle terminal presynaptic to the axon hillock-initial segment region, some of which are included in the axo-axo-axonic complex may also be a primary afferent directly contacting the spike generator area of the relay neuron and under presynaptic control of a flattened vesicle synapse. The latter may possibly be an intrinsic contact. This strategic situation of round vesicle terminals and the axo-axo-axonic complex at the axon hillock or initial segment has major implications relevant to the overall output of these neurons.

Light and electron microscopic localization of calcitonin gene-related peptide immunoreactivity in lamina II of the feline trigeminal pars caudalis/medullary dorsal horn: a qualitative study

Calcitonin gene-related peptide (CGRP) is a neuropeptide that is associated with a subset of primary afferent fibers and appears to have a role in nociception. The purpose of the present study was to perform a qualitative light, and especially electron microscopic (LM and EM), examination of CGRP-immunoreactivity (IR) within lamina II (substantia gelatinosa) of the feline pars caudalis/medullary dorsal horn (PC/MDH) of the spinal trigeminal nucleus. The LM investigation revealed massive CGRP-IR within lamina II outer, with fewer fibers that traversed lamina II inner. The EM preparations showed CGRP-IR in small, thinly myelinated and unmyelinated axons, preterminal axons, and in axon terminals that formed asymmetric synaptic contacts onto small dendritic profiles. The terminals with CGRP-IR were often the central element within glomeruli, where the terminal usually formed 2 or more asymmetric synaptic specializations onto 1 or more dendrites. Many of these postsynaptic dendrites contained synaptic vesicles. Other profiles were seen forming presynaptic contacts onto the terminal with CGRP-IR, and these profiles most likely represent presynaptic dendrites and/or other axon terminals of intrinsic origin. The synaptic association of terminals showing CGRP-IR with vesicle-containing dendrites, presynaptic dendrites, and/or other axon terminals suggests that terminals with CGRP-IR are especially susceptible to modulation.

Ultrastructural characterization of substance-P-immunoreactive synaptic terminals in the cat's normal and rhizotomized trigeminal subnucleus caudalis

Deafferenting injuries often cause transient or permanent physiological alterations within the central projection field of affected primary afferent fibers. Aberrant sensory perceptions, dysesthesias, and hyperalgesias represent the clinical sequelae of such injuries; however, the results of experimental deafferentations have been subject to a variety of interpretations (Rodin and Kruger, 1984b). Neurochemical studies show an increased sensitivity of partially deafferented neurons to substance P (SP). Our previous studies (Hoffmann et al., 1991) documented, primarily at the light-microscopic level, a moderate transient loss of SP-immunoreactive (SPIR) boutons in the trigeminal subnucleus caudalis (Vc)--a loss that seemed to preferentially affect the slightly larger, possibly complex boutons with multiple contacts. However, despite the elimination of the trigeminal input, the larger boutons reappeared. In the present study, therefore, we examined Vc using electron-microscopic immunocytochemistry, in order to document these changes over time and to clarify the structure and relationships of this population of boutons. SPIR boutons occurred in lamina I and II degrees of the substantia gelatinosa of Vc, ranged in size from 1 to 5 microns in diameter, and displayed mixed populations of clear and dense-core vesicles. Most formed single or multiple axodendritic junctions, but a significant number engaged in axoaxonic contacts with both SPIR-labeled and unlabeled terminals. A small number appeared to be the central element of a typical glomerulus, particularly in lamina II degrees. Three to seven days following an ipsilateral retrogasserian rhizotomy, synaptic degeneration was evident in the substantia gelatinosa and often involved glomerular terminals. However, most of these were SPIR-negative and occurred primarily in lamina II degrees. Those SPIR boutons that displayed degenerative features often made single or multiple axodendritic contacts, and in some instances were scalloped. By 30 days, most remaining SPIR boutons were small, with a lower incidence of contacts; however, some of these were axoaxonic. In addition, many SPIR terminals were only very lightly stained--a feature not encountered to such an extent in the contralateral Vc. At 45 days, complex SPIR boutons were again evident in the field, and some showed densely packed vesicles. An increased incidence of clusters of two to four SPIR axoaxonic contacts was also observed. Finally, almost all SPIR boutons encountered at this stage were intensely stained. It is suggested that these alterations represent a compensatory neuroplastic response on the part of overlapping cervical and cranial primary afferents to the partial deafferentation resulting from the interruption of the trigeminal root.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural organization of the interstitial subnucleus of the nucleus of the tractus solitarius in the cat: identification of vagal afferents

This electron microscopic study, based on serial section analysis, describes the synaptic organization of the interstitial subnucleus of the nucleus of the solitary tract and identifies the terminals of the vagal primary afferents utilizing degeneration and HRP transport. The interstitial subnucleus contains sparsely scattered cell bodies, numerous dendrites and axon terminals, and bundles of unmyelinated and myelinated axons. The cell bodies which are small in diameter have an organelle poor cytoplasm and a large invaginated nucleus. Axon terminals can be classified into two main types according to their vesicular shape. The first type contains clear, round vesicles and can be further subdivided into two subgroups on the basis of their morphology and the size of their vesicles. In the first subgroup the terminals are small, contain a few mitochondria and their vesicles are densely packed with an homogeneous size. In the second subgroup the terminals which vary from small to large, contain many mitochondria and contain round vesicles which are heterogeneous in size. The second main terminal type consists of axon terminals containing pleomorphic vesicles which are associated with asymmetrical or symmetrical synaptic contacts on dendrites. Axo-axonic contacts are present in the interstitial subnucleus. In general, the presynaptic axon terminals contain pleomorphic vesicles and the postsynaptic elements contain round vesicles of varying size. In some dendrites, identified by the presence of ribosomes, groups of round and/or pleomorphic vesicles are found associated with synaptic contacts. These dendrites are presynaptic to conventional dendrites and postsynaptic to axon terminals. After removal of the nodose ganglion, degenerative alterations are seen only at the caudal and middle levels of the interstitial subnucleus. Degeneration occurs in a few myelinated axons and in axon terminals which usually contain a mixture of small and larger round, clear vesicles. After HRP injection into the vagus nerve, the HRP reaction product is visible in axon terminals filled with clear, round vesicles which are heterogeneous in size. The labelled axon terminals establish single or multiple synaptic contacts. This study demonstrates that terminals of vagal primary afferents consist principally of terminals of the second subgroup. The morphology of these terminals are compared to primary afferents in the brainstem and spinal cord.

Plasticity in innervation of the rat superior colliculus by transplanted retinae as a result of eye removal at maturity

When the superior colliculus of a rat is innervated by inputs from both eyes as well as a retina transplanted intracranially over one tectum at birth, the tectal projection from the transplant is confined mainly to the superficial surface of the superior colliculus. The transplant-derived fibers possess a simple morphology, lacking terminal arborizations. If the contralateral eye is removed one month after transplantation, these fibers can be induced to arborize into the denervated portion of the superior colliculus over the next month. This demonstration of sprouting in a mature sensory relay system raises the possibility that an enhancement of behavioral responses mediated by transplanted retina might also occur. In turn, this may provide an ideal system to study the correlation between anatomical changes in transplant axons and changes in behaviors mediated by transplant activity.

Synaptic connections of a low-threshold mechanoreceptive primary neuron within the trigeminal subnucleus oralis

The central axon of a primary afferent neuron that responded to indentation of the glabrous skin of the lower lip in a slowly adapting fashion was intra-axonally injected with horseradish peroxidase. The labeled terminal within the subnucleus oralis was examined electron microscopically. The labeled ending had a pale axoplasm and contained clear spherical synaptic vesicles. The labeled ending formed a synaptic triad with a dendrite and an unlabeled axonal ending with pleomorphic vesicles (a mixture of oval, flattened and dense core vesicles). The labeled primary ending was presynaptic only to the dendrite, while the unlabeled ending was presynaptic to both the dendrite and the labeled primary ending.

Age-related remodeling of glutamic-acid decarboxylase-labeled elements in deafferented piriform cortex of rats

Olfactory bulb (OB) removal has been shown to result in plasticity in the piriform cortex (PC) that is age dependent. We are studying this phenomenon using immunoelectron microscopy of glutamic acid decarboxylase immunoreactivity (GAD, the enzymatic precursor for GABA) at selected postnatal ages and in adults with emphasis on short survival times of 4-7 days after OB ablation. Normally GAD-labeled synaptic terminals form type II symmetric contacts onto unlabeled dendrites and GAD-labeled dendrites receive type I, asymmetric contacts from unlabeled terminals (Westenbroek, et al., 1988a). The OB lesion results in degenerating terminals with type I contacts onto unlabeled and onto GAD-labeled dendrites. Type I postsynaptic sites may be seen partially contacted by or entirely devoid of degenerating terminals and occasionally may be apposed to variable degrees by normal unlabeled or by GAD-positive terminals. Subsequently, some GAD-labeled terminals may form asymmetric type I contacts usually with unlabeled dendrites and rarely with GAD-labeled dendrites. The findings are most common in the youngest subjects and essentially absent in the adult subjects. A sequence of reinnervation of deafferented type I sites by GAD-labeled terminals is suggested for the formation of this "atypical" synapse and the sequelae of this reorganization are discussed.

Contralateral degeneration in the cat spinal trigeminal nucleus following unilateral retrogasserian trigeminal rhizotomy

Retrogasserian trigeminal rhizotomy was used to study the central projections and patterns of degeneration in the spinal trigeminal nucleus (STN). At survival times of 3-20 days, reduced silver stains show extensive degeneration throughout the ipsilateral STN and in addition, well delineated degeneration was identified in the periobex region of the contralateral STN that varied with survival time. The results suggest that primary afferents may contribute to this contralateral projection.

Reorganization of GABAergic synapses in the viper optic tectum following retinal deafferentation

The immunocytochemical analysis of the viper optic tectum was carried out with an anti-gamma-aminobutyric acid (anti-GABA) antiserum following retinal deafferentation for survival times ranging from 10 to 90 days. The ultrastructure of the SGFS neuropil revealed that among the two types of axon terminals, namely pleiomorphic (P) and spherical (S) boutons, a subtype of the latter (S2), corresponding to the retinal terminals, was found to degenerate at varying rates. In most cases, this resulted in the glial engulfment of the presynaptic partner, leaving the postsynaptic differentiation free (FPSD). Beyond the two first months, the asymmetrical freed postsynaptic differentiations (FPSDs) were progressively and partly reafferented by GABA-positive P axon terminals through a sliding process. Three months postoperative, the number of GABA-positive P axon terminals which, in normal animals, establish asymmetrical contacts (1-2%), was found to increase to approximately 10%. The postsynaptic differentiation may represent a mismatched receptor area for the new competing presynaptic partners. The functional implications of such 'axonal terminal sprouting' are discussed.

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.

Intersubnuclear connections within the rat trigeminal brainstem complex

Prior intracellular recording and labeling experiments have documented local-circuit and projection neurons in the spinal trigeminal (V) nucleus with axons that arborize in more rostral and caudal spinal trigeminal subnuclei and nucleus principalis. Anterograde tracing studies were therefore carried out to assess the origin, extent, distribution, and morphology of such intersubnuclear axons in the rat trigeminal brainstem nuclear complex (TBNC). Phaseolus vulgaris leucoagglutinin (PHA-L) was used as the anterograde marker because of its high sensitivity and the morphological detail provided. Injections restricted to TBNC subnucleus caudalis resulted in dense terminal labeling in each of the more rostral ipsilateral subnuclei. Subnucleus interpolaris projected ipsilaterally and heavily to magnocellular portions of subnucleus caudalis, as well as subnucleus oralis and nucleus principalis. Nucleus principalis, on the other hand, had only a sparse projection to each of the caudal ipsilateral subnuclei. Intersubnuclear axons most frequently traveled in the deep bundles within the TBNC, the V spinal tract, and the reticular formation. They gave rise to a number of circumscribed, highly branched arbors with many boutons of the terminal and en passant types. Retrograde single- or multiple-labeling experiments assessed the cells giving rise to TBNC intersubnuclear collaterals. Horseradish peroxidase (HRP) and/or fluorescent tracer injections into the thalamus, colliculus, cerebellum, nucleus principalis, and/or subnucleus caudalis revealed large numbers of neurons in subnuclei caudalis, interpolaris, and oralis projecting to the region of nucleus principalis. Cells projecting to more caudal spinal trigeminal regions were most numerous in subnuclei interpolaris and oralis. Some cells in lamina V of subnucleus caudalis and in subnuclei interpolaris and oralis projected to thalamus and/or colliculus, as well as other TBNC subnuclei. Such collateral projections were rare in nucleus principalis and more superficial laminae of subnucleus caudalis. TBNC cells labeled by cerebellar injections were not double-labeled by tracer injections into the thalamus, colliculus, or TBNC. These findings lend generality to currently available data obtained with intracellular recording and HRP labeling methods, and suggest that most intersubnuclear axons originate in TBNC local-circuit neurons, though some originate in cells that project to midbrain and/or diencephalon.

Structure-function relationships in rat brainstem subnucleus interpolaris: III. Local circuit neurons

Intracellular recording, electrical stimulation, receptive field mapping, and intracellular injection of horseradish peroxidase were used to assess the response properties, collateral projections, and morphology of 44 local circuit (LC) neurons in the subnucleus interpolaris (Sp Vi) of the trigeminal brainstem complex of the rat. LC neurons were defined as those with axons restricted to brainstem areas receiving trigeminal primary afferent fibers. Thus, none were antidromically activated from the thalamus, tectum, or cerebellum, and their axons could be seen terminating exclusively within the trigeminal brainstem complex or reticular formation. All neurons sampled were discharged by innocuous or noxious mechanical stimulation of a restricted portion of the face or mouth. They were classified functionally as sensitive to vibrissae (N = 22), nociceptors (N = 9), guard hairs (N = 7), hairy skin (N = 3), or periodontia (N = 3). Fifty percent of the stained neurons were vibrissa sensitive. Twenty-one of these 22 responded to deflection of only one vibrissa. The remaining functional groups also had small receptive fields. Intracellular staining revealed a consistency in vibrissa-sensitive LC morphology. Somata were small to medium in size and multipolar. Their axons had an initial transverse trajectory and gave off recurrent collaterals which arborized extensively in the region of the soma. The parent axon then bifurcated. One branch traveled rostrally to subnucleus principalis while the other branch traveled caudally to subnucleus caudalis. The branches periodically sent collaterals into regions of the trigeminal complex corresponding to the transverse position of the soma. Dendrites extended 440 +/- 140 microns rostrocaudally, forming a tree with a transverse perimeter of 459 +/- 226 microns. Distal dendrites were thin and sinuous, had few spines, and extensively arborized adjacent to the soma. They ended in multiple swellings connected by slender processes. The stereotyped morphology of vibrissa-sensitive LC neurons differed from the variable morphologies of LC neurons activated by nociceptors, guard hairs, hairy skin, or periodontia. Although no group of neurons in one of these categories displayed a distinguishing morphological characteristic, they collectively had features which distinguished them from the vibrissa-sensitive neurons. Non-vibrissa-responsive neurons generally had more expansive, but less circular, dendritic and recurrent axonal arbors; dendrites had more spines, and axons often sent endings into the reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

Reorganization of neuronal connections following CNS trauma: principles and experimental paradigms

The present review summarizes how the nervous system responds to trauma. The goal is to provide an introduction to the problems, techniques, experimental paradigms, current issues, and future promise. The review is especially designed for basic scientists and clinicians who are not currently involved in research on CNS reorganization, and for students just entering the field. The review characterizes the secondary degenerative events that occur after trauma, and the types of growth that commonly occur. A standard terminology is set forth with criteria for differentiating between related phenomena. Experimental methods are described that can be used documenting reorganization of circuitry. The principles that determine whether a given process will or will not occur are summarized, and some of the factors that may regulate the nature and extent of growth are considered. Research strategies are outlined that have been used to evaluate whether reorganization of circuitry is functionally significant. Finally, future directions in research and clinical application are discussed, focusing especially on the efforts to facilitate regeneration, and the work on transplants of CNS tissue to facilitate growth of surviving connections, and to replace tissue destroyed by trauma.

Ulex europaeus agglutinin-I binding to dental primary afferent projections in the spinal trigeminal complex combined with double immunolabeling of substance P and GABA elements using peroxidase and colloidal gold

Ulex europaeus agglutinin I (UEA-I) is a plant lectin with an affinity for L-fucosyl residues in the chains of lactoseries oligosaccharides associated with medium- and smaller-diameter dorsal root ganglion neurons and their axonal processes. These enter Lissauer's tract and terminate within the superficial laminae of the spinal cord overlapping projections known to have a nociceptive function. This implies that the surface coatings of neuronal membranes may have a relationship with functional modalities. The present investigation further examined this concept by studying a neuronal projection with a nociceptive function to determine whether fucosyl-lactoseries residues were incorporated in its primary afferent terminals. Transganglionic transport of horseradish peroxidase (HRP) following injection into tooth pulp chambers was employed to demonstrate dental pulp terminals in the trigeminal spinal complex, while peroxidase and fluorescent tags were used concomitantly to stain for UEA-I. Double immunolabeling for substance P (SP) and gamma-aminobutyric acid (GABA) using peroxidase and colloidal gold allowed a comparison of the distribution of a known excitatory nociceptive transmitter with that of UEA-I binding in specific subnuclei. Synaptic interrelationships between UEA-I positive dental pulp primary afferent inputs and specific inhibitory terminals were also examined. SP immunoreactivity occurred in laminae I and outer lamina II (IIo) of subnucleus caudalis (Vc) and in the ventrolateral and lateral marginal region of the caudal half of subnucleus interpolaris (Vi), including the periobex area in which Vi is slightly overlapped on its lateral aspect by cellular elements of Vc. The adjacent interstitial nucleus (IN) also showed an intense immunoreactivity for this peptide antibody. UEA-I binding displayed a similar distribution pattern in both Vc and Vi, but extended into lamina IIi and the superficial part of Lamina III in Vc. Dental pulp terminals were found to have a comparable distribution; however, many extended into the dorsal portion of the caudal half of Vi and the ventromedial quadrant of rostral Vi. Electron-microscopic analysis showed that transganglionically labeled dental pulp terminals contained ovoid, complex membrane-bound vacuoles laden with transported HRP. The preterminal axon and synaptic membranes of those dental pulp terminals located in zones of Vc and Vi displaying an affinity for UEA-I were usually characterized by a patchy, electron-dense coating of the peroxidase tag. SP was demonstrated ultrastructurally with Protein-A colloidal gold (3-nm particles), whereas GABA immunoreactivity was revealed by the avidin-biotin-peroxidase method.(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.

A longitudinal study of the effects of retinal ablation on the organization of the retinal target lamina of the optic tectum in the teleost, Rutilus rutilus

The optic tecta of 55 Rutilus rutilus were examined at intervals varying from 2 days to 4 years after unilateral retinal ablation. Qualitative ultrastructural examination of the retinal target lamina of the optic tectum (stratum fibrosum et griseum superficiale, SFGS) revealed that an initial period of degeneration and glial reaction, each of which could take one of a variety of forms and which lasted for 1-3 months after ablation, was followed by the temporary formation of heterologous synapses which persisted for a further 1-12 months. This in turn was followed by the degeneration of these synapses during the second year after ablation. Quantitative analysis at the level of the light microscope revealed a shrinkage of the SFGS throughout the level of the light microscope revealed a shrinkage of the SFGS throughout the first 14 postoperative months with no further reduction taking place thereafter. Analysis at the ultrastructural level revealed that this shrinkage was due to the disappearance, and not to the reduction in size, of pre- and postsynaptic profiles accompanied by glial reaction. No signs of collateral sprouting were seen throughout the survival period. Thus, partial deafferentation of the SFGS leads in the long run to a marked impoverishment of its neuronal network, without any apparent compensation.

Electrophysiological responses of trigeminal root ganglion neurons in vitro

The membrane electrical properties of neurons and their responses to endogenous compounds or other neuroactive substances were investigated in vitro with intracellular recording techniques in slices of trigeminal root ganglia of guinea-pigs. The mean resting membrane potential of these neurons was -60 mV. Intracellular injections of hyperpolarizing current pulses evoked time-dependent rectification with varying degrees of dependence on membrane voltage in 107 of 110 neurons. Membrane potential oscillations were observed following the termination of the hyperpolarizing pulses and after similar injections of depolarizing current. This phenomenon appeared to be voltage-dependent at levels that were subthreshold for spike genesis; the more pronounced oscillations were evident at the more depolarized levels and were insensitive to tetrodotoxin applications. Two groups of neurons could be distinguished on the basis of certain characteristics in their action potentials. The majority exhibited short duration (0.6 ms) spikes with mean amplitude of 72 mV in response to intracellular depolarizing current. The brief (3 ms) afterhyperpolarizations that followed such spikes were blocked by intracellular injections of Cs+ or by bath applications of tetraethylammonium. Action potentials in the minority group exhibited a hump in their repolarization phase. The humped spikes had a mean peak amplitude of 78 mV and a longer duration (2 ms). Both the duration (6 ms) and the amplitude (16 mV) of the afterhyperpolarization were significantly greater in this latter group of neurons. Some fast spikes were easily blocked whereas others, including humped spikes, were resistant to tetrodotoxin (10(-6) M). Spikes which were resistant, were also not affected by perfusion with Co2+ (10(-3) M) and were reduced in amplitude during perfusion with Na+-deficient solution. Bath applications of S-glutamate (10(-4)-10(-2) M) depolarized only two of ten neurons by less than 3 mV. Similarly, 5-hydroxytryptamine produced a small depolarization in only two of thirteen neurons. Perfusion of gamma-aminobutyrate (10(-5)-10(-2) M) resulted in an increase in input conductance that waned despite continued application and was associated with a depolarization (2-14 mV) in 44/50 neurons. In some neurons, gamma-aminobutyrate application enhanced their repetitive firing ability, possibly as a result of the increased oscillatory behavior of the membrane at certain depolarized potentials. The effects of gamma-aminobutyrate were blocked by the GABAA-receptor antagonist, bicuculline (10(-4) M) but were unaffected by the GABAB-receptor agonist, baclofen (10(-4) M).(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Ultrastructural changes in acetylcholinesterase activity in the deafferented spinal trigeminal nucleus

Acetylcholinesterase (AChE) activity was investigated in synaptic areas of the cat spinal trigeminal nucleus (pars interpolaris and pars caudalis) ipsilateral and contralateral to complete retrogasserian rhizotomy. Vibratome sections of tissue taken from animals of 1, 3, 6, 14, and 21 days survival were examined by electron microscopy following a histochemical reaction for AChE activity employing a method based on the Karnovsky-Roots technique for demonstrating reaction product. As degeneration progressed with survival time, enzymatic activity was initially reduced in synaptic clefts of injured afferent terminals and subsequently was enhanced throughout the extracellular space, including within synaptic clefts of possibly reinnervated sites. These changes in enzymatic activity with primary deafferentation are discussed in relation to the process of reinnervation, the development of neuronal hyperactivity, and possible noncholinergic functions of AChE.

Ultrastructure of degenerative changes following ricin application to feline dental pulps

The ultrastructure of degenerative changes within the ipsilateral trigeminal ganglion, and partes caudalis and interpolaris of the spinal trigeminal nucleus in the cat is described following the application of the potent toxin ricin to the tooth pulps of unilateral maxillary and mandibular posterior teeth, including the cuspids. Survival times ranged from 6 to 10 days. Typical changes identified within the ipsilateral trigeminal ganglion included myelin fragmentation and 'compartmentalization' of the axoplasm of medium-sized myelinated axons, while small myelinated and unmyelinated axons underwent a more variable response ranging from electron-lucent to electron-dense changes. The affected cell body was characterized by the presence of swollen, electron-lucent mitochondria, a reduction of cytoplasmic ribosomes and a filamentous hyperplasia. Other changes often included an eccentric nucleus and satellite cell proliferation. Degenerative changes often occurred in isolated elements surrounded by normal profiles, suggesting specificity of ricin within the trigeminal ganglion. Changes within brainstem axons showed both an electron-dense and a lucent, fragmenting type of axonal alteration. Terminal changes ranged from electron-dense to lucent and also included filamentous hyperplasia and 'hyperglycogenesis'. The altered axonal knobs contained round synaptic vesicles that were presynaptic to dendritic profiles and postsynaptic to terminals containing flattened synaptic vesicles. The above brainstem alterations were identified specifically in the following areas: ventrolateral, medial and dorsomedial pars interpolaris; the ventrolateral and mid-dorsal to dorsomedial areas of the marginalis and outer substantia gelatinosa layers of pars caudalis; and in ventral pockets corresponding to lamina V of the medullary dorsal horn. Dense alterations within terminals containing flattened synaptic vesicles that are typically presynaptic to primary afferents in these areas were rare findings, but along with vacuolization of dendritic profiles suggest a trans-synaptic effect possibly due to the exocytosis of ricin. The results are discussed in relation to different reports of dental projections and with regards to patterns of transganglionic degeneration.

Benzodiazepine binding increases in the superficial laminae of the trigeminal subnucleus caudalis following central rhizotomy

Benzodiazepine (BZ) binding is being studied in the spinal trigeminal nucleus of the cat 3 and 11 days following unilateral retrogasserian rhizotomy using in vitro autoradiography and computer-assisted densitometry. At 3 days following rhizotomy there is an increase in number and decrease in affinity of flunitrazepam binding sites in the superficial laminae of subnucleus caudalis of the spinal trigeminal complex. By 11 days, affinity remains below control values and binding site number shows an insignificant but detectable increase. There is no change in binding site characteristics in deeper laminae at either survival time. The results are discussed in relation to the physiological hyperactivity and synaptic changes which occur following such lesions and to other conditions of deafferentation.

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.

A morphometric study of the effects of maturation and aging on synaptic patterns in the spinal trigeminal nucleus of the cat

Morphometric methods have been used to study the synaptic and terminal patterns in cat trigeminal nucleus, pars interpolaris, during development and aging. Ages 1, 3, 6, 11, 16, 21, 27, 110, 600 days and 8 and 11 years were studied. Both proportions and densities (number per unit area) of certain terminals and synapses showed significant changes with age. Axoaxonic synapses especially showed two major periods of increase (3-6 days and 21-27 days). The values of most parameters increased in the 21-27 day period to peak levels and then decreased gradually with age. The results indicate two separate critical synaptogenic periods of development and a loss of synaptic elements in aging. Factors contributing to these changes are discussed as is the potential for plasticity in the different afferents at each period.

Ultrastructure of transganglionic HRP transport in cat trigeminal system

The ultrastructure of transganglionic transport of horseradish peroxidase (HRP) from the inferior alveolar (IA) nerve to the brainstem is being studied in the cat. The IA nerve was soaked in an HRP solution and following a two-day survival the animal was perfused transcardially with a paraformaldehyde-glutaraldehyde solution. The tissue was immediately dissected and postfixed for 1-3 h in perfusate. Sections of 75 micron thickness were cut with a Vibratome and reacted utilizing tetramethyl benzidine (TMB) as the chromagen. Optimum results for electron microscopy were obtained by osmication in a pH 6.0, 1% osmium tetroxide solution for 45 min at 45 degrees C, followed by rapid dehydration and embedment in Epon. The resulting HRP-TMB reaction product was characterized and identified ultrastructurally in ganglion cells, peripheral and central axons and in brainstem terminals. The HRP-TMB reaction product varied in density but had consistent crystalline-like laminations of a repeating unit and characterized by a membrane 4-5 nm in diameter. Some of the HRP-TMB reaction product found in terminals and axons was below the limit of resolution of the light microscope.

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

The fine structural organization of the principal sensory trigeminal nucleus was compared with that of the spinal trigeminal nucleus (subnuclei oralis, interpolaris, and the deep layers of caudalis) in adult albino rats. Direct comparisons indicate similarities between all of the subdivisions of the brainstem trigeminal complex both in the major morphological classes of neurons present and in basic patterns of synaptic connections. Major differences between the several subdivisions occur in the relative numbers and distribution of the different cell types. The spinal trigeminal nucleus is distinguished by more numerous large (22-40 micron) polygonal neurons which give rise to long straight primary dendrites. Both the perikaryal surface and the thick primary dendrites of many of these cells are densely innervated by synaptic terminals. Especially large cells of this type are a prominent feature of subnucleus oralis. By contrast, the principal sensory nucleus is distinguished by its high density of small to medium-sized (8-20 micron) round or ovoid neurons. These smaller neurons tend to receive a sparse axosomatic innervation. In addition to these differences the spinal trigeminal neuropil is distinguished by the striking manner in which it is broken up by large rostrocaudally oriented bundles of myelinated axons. Proximal dendrites of polygonal and fusiform neurons often wrap around these large axon bundles. Morphologically heterogeneous populations of synaptic terminals with round vesicles (R terminals) and terminals with predominantly flattened vesicles (F terminals) occur in all of the subdivisions of the trigeminal complex. Both types of terminal make primarily axodendritic synapses, but both also make axosomatic synapses, and axospinous synapses with somatic as well as dendritic spines. In addition, axoaxonic synaptic contacts from F terminals onto large R terminals are seen in all subdivisions. Convincing examples of presynaptic dendrites were not observed in any of the brainstem subdivisions. Synaptic glomeruli, characteristic groupings of dendrites and synaptic terminals, are found throughout the brainstem trigeminal complex. The dendritic elements in these glomeruli tend to be small-diameter dendrites, spines, and large, spinelike appendages. Within the glomerulus these elements are postsynaptic to a single large R terminal and may also be postsynaptic to smaller F terminals. In addition, axoaxonic synaptic contacts from the F terminals onto the R terminal are a consistent feature of trigeminal synaptic glomeruli.(ABSTRACT TRUNCATED AT 400 WORDS)

Dystrophic axons in the nucleus gracilis of the normal rat containing cholecystokinin-like immunoreactivity. Light- and electron-microscopic observations

Cholecystokinin immunoreactive (CCKI) axons in the nucleus gracilis of the normal rat were studied by light and electron microscopy using the peroxidase-antiperoxidase (PAP) immunocytochemical method. Immunoreactivity was found in both normal and dystrophic axons at various ages. Slightly enlarged CCKI varicosities were first observed in 2-month-old rats, gradually increased in number (max. at 21 months) and in diameter (up to 20 micron at 21 months), and then decreased (at 27 and 36 months). Dilatated CCKI fibers continuous with dystrophic varicosities also tended to increase proportionally in diameter. The dystrophic CCKI axonal profiles, first identified at the light-microscopic level and then observed under electron microscopy, had consistently characteristic features. In addition, numerous dystrophic nonreactive axonal profiles had morphological features differing from those of dystrophic reactive ones at advanced ages. These findings suggest that CCK may be partly involved in the formation of dystrophic axons in the nucleus gracilis.

Noradrenergic hyperinnervation of the trigeminal sensory nuclei

Administration of 6-hydroxydopamine to neonatal rats results in a permanent increase in the norepinephrine content in several brainstem areas. To assess the physiological effects of this hyperinnervation, we studied the noradrenergic inhibition of transmission of sensory information through the principal sensory and rostral spinal trigeminal nuclei. Unit activity produced by tactile stimulation of the face was recorded extracellularly from trigeminal sensory neurons in normal and hyperinnervated rats. The noradrenergic neurons projecting to the trigeminal sensory nuclei (locus coeruleus and the region of the lateral lemniscus) were stimulated 40 ms prior to delivery of a tactile stimulus to the face, producing complete inhibition. The interstimulus interval was then increased in 100 ms increments until the sensory response returned to control values. Compared with controls, the duration of inhibition was 30% longer in hyperinnervated rats and 25% shorter in rats depleted of catecholamines with reserpine and alpha-methyl-p-tyrosine. While the beta-adrenergic blocker, propranolol, had no effect on the duration of inhibition in normal animals, the mean latency of response to tactile stimulation was decreased from 15.3 to 10.4 ms. Propranolol given to hyperinnervated rats decreased the latency of the response to tactile stimulation from 15.1 to 9.1 ms and decreased the duration of inhibition by 40% compared with untreated hyperinnervated rats, suggesting an alteration in numbers or sensitivity of beta-receptors. Since the drug treatment never eliminated the inhibition due to locus coeruleus stimulation, there is also a non-noradrenergic component. We conclude from these observations that noradrenergic hyperinnervation is not completely counteracted by receptor down regulation.

Long-term effect of mossy fiber degeneration in the rat

Mossy fiber-deafferentated rats (20) were permitted to survive from 34 to to 120 days and subsequently examined following Golgi-Cox preparation or after processing for electron microscopy. The primary response to mossy fiber deafferentation was transneuronal degeneration of the granule cell system. Morphological evidence is provided that suggests that the mossy fiber varicosity plays an important role in the fragmentation and removal of the granule cell digitiform dendrite. Computer-assisted image analysis of Golgi-impregnated Purkinje cells indicated significant losses in both smooth branch and spiny branchlet numbers following loss of the mossy fiber input. Ultrastructural examination revealed that a secondary transneuronal degeneration occurred within the dendritic arborization of both Purkinje cells and molecular layer interneurons. Although an overall reduction in the number of dendritic spines occurred along the terminal branchlets following mossy fiber deafferentation, several of the existing spines underwent marked changes in length, with some elongating to more than twice their size. By increasing the length of their spines, denervated Purkinje cells may acquire new synaptic contacts.

Rearrangement of neuronal responses in the trigeminal system of the rat following peripheral nerve section

The infraorbital nerve was cut in either neonatal (on day 0) or adult (day 60) rats and the peripheral regeneration prevented. After 60 days either anatomical or electrophysiological techniques were used to study the peripheral nerve, trigeminal nucleus and somatosensory cortex. In neonatally sectioned animals the number of myelinated fibres surviving, at 60 days, in the peripheral nerve proximal to the lesion was 11% compared with 100% survival after adult nerve section. This reduction in surviving nerve fibres in neonatally lesioned animals was associated with a significant reduction in cross-sectional area of all trigeminal nuclei (principalis, oralis, interpolaris and caudalis) of 18-29%. No significant change in area was present in animals sectioned as adults. Neonatally lesioned animals also showed a reduction of approximately 20% in the number of cells visible in cross-sections of all trigeminal nuclei. Animals sectioned as neonates showed marked plasticity at all nuclei in the trigeminal complex as well as in the cortex. Deafferented cells responded to new peripheral receptive fields so that the somatotopic organization of these cells was modified. Such cells are referred to throughout as 'reactivated' cells. However, in animals sectioned as adults no evidence of plasticity could be detected in the trigeminal nuclei. Only very limited reactivation was apparent in the cortex, so that the majority of deafferented cells remained unresponsive at both sites. A detailed comparison was made of twenty-three reactivated cells and twenty-five normal cells from nucleus principalis of animals with nerve section on day 0. The reactivated cells commonly showed larger, more complex receptive fields, longer latencies and lower following frequencies, although stimulus thresholds were similar. Thus reactivated cells showed more convergence and poorer synaptic security than normal cells. However, stimulation of the contralateral thalamus produced similar responses from both groups of cells, suggesting that not all inputs to reactivated cells were modified. The time course of the reactivation of cells in nucleus caudalis from animals lesioned on day 0 was followed over 30 days. No acute effect, for up to 24 h, was detected. However, somatotopic reorganization had started by day 7, proceeded rapidly between days 7 and 14, and was completed by day 21.

Optic axons ignore denervated foreign territory in goldfish tectum

Ultrastructural morphometry was used to test firstly, whether regenerating optic axons in goldfish tectum will form connections in adjacent denervated foreign territory and secondly, whether intact optic axons will collaterally sprout to innervate this territory. The stratum fibrosum marginale of the tectum was partially denervated by removing the torus longitudinalis and cutting the tectal commissure. Optic axons do not normally synapse in this layer. Thirty days after partial denervation and optic tract section the numbers of normal synapses in a micron 2 column through the stratum fibrosum marginale reached a minimum and then started to increase so that by 159-173 days after the operation they had returned to control levels. Optic terminals, recognised by their degeneration after optic nerve section, did not contribute to these increasing numbers of synapses. Instead, optic axons preferentially reinnervated their normal tectal layers. Similarly, there was no evidence of collateral sprouting of intact optic axons into the partially denervated stratum fibrosum marginale and numbers of normal synapses in the layer also returned to control levels. These probably arose from collateral sprouting of terminals remaining after the partial denervation, raising the possibility that optic axons were prevented from synapsing there by the rapid occupation of vacant sites by other axons. However, delaying the partial denervation with respect to the tract section did not alter the result. These results support the idea of a specific affinity between optic axons and their postsynaptic targets.

An examination of intraspinal sprouting in dorsal root axons with the tracer horseradish peroxidase

Postdeafferentation reorganization in the central terminal fields of spared dorsal root axons was evaluated by examining the intraspinal distribution of horseradish peroxidase-labeled sciatic nerve afferent fibers at various intervals following the removal of several lumbar dorsal root ganglia. The sciatic projection to the spinal cord, as determined by the pattern and density of intraspinal reaction product, was remarkably stable following the ganglionectomies. For as long as 3 months later, there was no evidence that sciatic afferent fibers had formed anomalous connections either with new spinal segments or in denervated areas within normal segments of entry. These findings cast doubt upon the existence of anatomic reorganization within the spinal cord following its partial deafferentation and suggest that physiological processes other than new axonal growth underlie observations such as postdenervation alterations in the response properties of dorsal horn neurons and the recovery of behavioral function.

'Free' postsynaptic-like densities in normal adult brain: their occurrence, distribution, structure and association with subsurface cisterns

Samples of cerebral cortex (parietal and occipital) and thalamic nuclei (ventrobasal, posterolateral, dorsal lateral geniculate) from normal, adult, aldehyde perfusion fixed mice and rats were examined by electron microscopy for the presence of free postsynaptic-like densities (FPSDs). FPSDs are plaques of intracellular paramembranous electron-dense material, ultrastructurally indistinguishable from postsynaptic densities, but not aligned with a presynaptic specialization. In a systematic survey of the neuropil around 6000 neuronal perikarya, 250 FPSDs were encountered. Almost all of these were within dendritic spines and shafts and about 90% of them were apposed by a neuronal perikaryon, the remainder by a dendritic shaft. In every case a subsurface cistern (SSC) was present in the cell body or dendrite apposed to the FPSD, and was flattened along the extent of the FPSD. In none of the material were the FPSDs associated, even remotely, with degenerating elements, suggesting that they are not formed by degeneration of presynaptic boutons. The incidence of FPSD-SSC complexes was higher in thalamus than in cerebral cortex which, together with previous observations indicating their absence from normal cerebellar cortex, suggests significant regional variations in distribution. It is suggested that FPSDs might represent synaptic precursors perhaps induced to form as a response to loss (possibly age-dependent loss) of synaptic contacts on a neuron and that the SSCs are somehow involved in maintaining the FPSDs and/or preparing them for innervation by adjacent axon terminals to form new synaptic contacts.

Plasticity of the parallel fiber-Purkinje cell synapse by spine takeover and new synapse formation in the adult rat

Alteration in synaptic connectivity between Purkinje cell spines and parallel fibers of the cerebellum were studied following partial deafferentation of Purkinje cells in the the adult rat. Transection of parallel fibers by two lesions placed at a 1 mm interval on the folial crest were used to produce degeneration of these afferents. Ultrastructural analysis of synapses on Purkinje cell spines revealed degeneration with vacating of postsynaptic sites within 6 h. Reactive synaptogenesis as takeover of Purkinje cell spines by formation of new synapses from remaining parallel fibers occurred even before degenerating parallel fibers had vacated postsynaptic sites. This was accompanied by a marked increase in the number of dual innervations by reactive parallel fibers within one day. Some vacated postsynaptic sites were lost as indicated by a reduction in the number of synapses and others may have been taken over by newly formed synapses on spines. In addition, new synapses formed between the shafts of Purkinje cell branchlets and parallel fibers. Sprouting of parallel fibers occurred as small extensions without tubules while Purkinje cell spines reacted by forming elongated and multiple heads which contacted different parallel fibers. After 5 days degenerating boutons were rarely found. Enlarged spine heads were each capped by a proportionally enlarged parallel fiber bouton and joined by an elongated synaptic junction to parallel fibers. Some parallel fiber boutons were greatly enlarged and capped numerous profiles of spines. This study shows that formation of new pre- and postsynaptic sites takes precedence over reoccupation of original contacts and that multiple synapses on individual spines are being eliminated to give rise to single contacts with boutons. This elimination resulted in enlargement of synaptic contact areas between Purkinje cell spines and parallel fibers by taking over postsynaptic sites from some vacated and eliminated boutons.

Synaptic proliferation in the auditory cortex of the young adult rat following callosal lesions

The long-term effects of partial deafferentation in the neocortex of adult rats were studied in four-month old rats in which the corpus callosum had been completely sectioned when they were one-month old. Quantitative light microscopy was used to identify morphological changes in the auditory cortex resulting from the loss of established callosal connections. Particular attention was directed at those cortical layers known to receive the heaviest callosal projection (layers II and III) and at neurons known to be postsynaptic to callosal afferents (layer V pyramidal neurons). The comparative analysis of both semithin plastic sections and Golgi-impregnated material from long-term, callosally-lesioned rats and age-matched control animals reveals no differences in the overall cortical thickness, the thickness of cortical layers, the numbers of neurons or the density of spines along apical dendrites of layer V pyramidal neurons. However, as a result of the callosal lesion, large diameter apical dendrites are significantly thinner in the callosally deafferented cortex and there is a small increase in the number of neuroglial cells in the deeper cortical layers. To determine whether another system of afferents to the auditory cortex spreads into the deafferented callosal domain, geniculate lesions were made in long-term, callosally-lesioned animals and age-matched controls. The terminal projection patterns of thalamic afferents were compared using the Fink-Heimer technique and quantitative electron microscopy. Normally in the auditory cortex there is only a small region of overlap between the terminal projection fields of callosal afferents and thalamic afferents, the latter projecting chiefly to layer IV and low layer III. However, three months after callosal lesions, thalamic axons had proliferated superficially into part of the callosal domain. Furthermore, in the normal auditory cortex after geniculate lesions, there were three rostrocaudally oriented bands of relatively dense thalamocortical terminal degeneration separated by regions of less dense degeneration. In the doubly lesioned animals these bands of degeneration were less distinct due to a proliferation of thalamic axons into the regions characterized by sparse projections.

An electron microscopic study of the afferent connections of the lateral reticular nucleus of the cat

The mode and pattern of termination of the afferents to the lateral reticular nucleus (LRN) of the cat were examined at the cellular level through the ultrastructural localization of induced degeneration. Examination of the LRN following hemicordotomy at the fifth and sixth cervical levels revealed that most of the degenerating terminals were in contact with intermediate and distal dendrites, and that most of these degenerating terminals were small and contained round vesicles. Fewer degenerating terminals were observed on the somata and proximal dendrites after spinal hemisection, and most of these terminals were large and contained round vesicles. Following lesions of the pericruciate cortex, small degenerating terminals were occasionally observed making contact onto intermediate and distal dendrites. Degenerating rubral terminals were observed synapsing on somata, somatic and dendritic spines, proximal dendrites and most commonly on intermediate and distal dendrites following lesioning of the red nucleus. The degenerating axosomatic rubro-LRN terminals belonged to the large, round-vesicle terminal population, while those degenerating terminals contacting intermediate and distal dendrites belonged to the small, round-vesicle population. Small, degenerating terminals were occasionally seen following lesions of the fastigial nucleus, and they made synaptic contact mainly onto intermediate and distal dendrites and dendritic spines. The present ultrastructural observations taken together with the convergence pattern of LRN afferents and the available electrophysiological data on inputs to the LRN suggest an extensive integration of converging impulses from two or more afferent sources to the rostral LRN neurons. The results of this study therefore support the veiw that the rostral LRN functions as a comparator of command signals from the motor cortex and red nucleus and feedback signals from the spinal cord and cerebellum during ongoing movement.

Study of axonal dystrophy. II Dystrophy and atrophy of the presynaptic boutons: a dual pathology

In succession to the previous quantitative work, a qualitative study has been carried out on the nature of a dual pathology affecting presynaptic boutons in the posterior tract nuclei of ageing rats. Based on the morphology of dystrophic boutons in early stage, it is concluded that the initial and therefore essential characteristic of dystrophic process is an abnormal increase of normal axonal components within the presynaptic boutons, and that various abnormal substructures of spheroids hitherto reported in the literature are probably the results of their secondary metamorphosis. The dystrophic process within the posterior tract nuclei is a selective one, involving presynaptic boutons and preterminal axons only of the posterior tract fibres. Comparison of the frequency of early dystrophic boutons and of fully grown-up spheroids indicates that a small percentage of boutons deriving from posterior tract fibres become dystrophic and of these dystrophic boutons only a small percentage again continue to develop unto large spheroids, throughout lifespan of the animals. On the other hand, in search of a morphological counterpart for the age-related decrease of volume ratio of presynaptic boutons to the neuropil, some dubious atrophic changes were also found in presynaptic boutons, which could have been easily missed from observation if studied qualitatively alone. Accordingly, no less numerous boutons other than dystrophic ones are supposed to atrophy 'independently' and to disappear 'silently' during the same period. The dystrophic and the atrophic changes involve different boutons (of different or the same terminal axons) within the same gray matter. This dual pathology of boutons needs further elucidation of its neurocytopathological as well as neurobiological background in the future.

Morphometrical synaptology of Clarke cells and of distal dendrites in the nucleus dorsalis: an electron microscopic study in the cat

The fine structural synaptology of large Clarke cells in L3 has been investigated from a morphometrical point of view in both normal and adult cats which received horseradish peroxidase (HRP) injections in the cerebellum. This marking method made it possible to distinguish small or distal dendrites of large Clarke cells from those of interneurons and the marginal cells of Clarke's column. A total of 1036 boutons was observed on the perikarya of 21 large Clarke cells; 81.9% (848/1036) were small-sized boutons, the cross-sectional areas of which ranged between 0.3 and 2.9 sq. micrometer, while 18.1% (186/1036) were giant boutons ranging between 3.0 and 8.0 sq. micrometer. From 1075 boutons on 17 primary dendrites of Clarke cells, 72.4% (778/1075) were small-sized boutons and 27.6% (297/1075) were giant boutons. From 1679 boutons contacting 366 distal or small HRP-labeled dendrites, 89.9% (1507/1679) were small boutons and 10.1% were giant boutons. The giant boutons were more frequently located on the proximal dendrites than on the cell bodies or small distal dendrites of Clarke cells. The proportion of S- and F-type boutons was different in 3 parts of large Clarke cells. F-type boutons were more frequent on soma (55.0% 570/1036) and primary dendrites (59.4%, 635/1075). S-type boutons outnumbered the F-type on small or distal dendrites (62.6%, 1952/1679). The S/F ratio seemed to increase from the cell body toward the distal dendrites. The results suggest that Clarke cells receive predominantly small S-type boutons since the total receptive area of the dendrites is supposed to exceed that of the cell body.

Synaptic fine structure and the termination of corticospinal fibers in the lateral basal region of the cat spinal cord

The lateral basal region (LBR) of the spinal cord gray matter (Rexed's laminae IV-VII) by physiologic and anatomic criteria is the major terminal zone for the corticospinal (CS) tract in the cat. The neurons in this area are medium-sized with abundant spines on their dendrites. Axon terminals on the dendrites and somata of these neurons form synapses easily classified as asymmetric with spheroid vesicles and symmetric with flattened vesicles. There are rare exceptions to this. In a systematic count of terminals, 82% have spheroid and 18% flattened vesicles. The majority of all terminals are on dendrites (84.9%) and a minority on somata (14.1%). Less than 1% are axoaxonic. Degeneration of the corticospinal tract was produced by transecting one hemisphere of our experimental cats. Its termination in the lower cervical cord was studied for 17 hours to 7 days after surgery. Vesicle depletion and clumping and dense polymembranous inclusions were the most common forms of degeneration. Filamentous proliferation in the terminals was also prominent; dark degeneration, however, was infrequent. The percent of degenerating CS terminals in the LBR was the following: 17 hours - 2.2%, 36 hours - 4.19%, 2 days - 10.3%, 3 days - 8.4%, 4 days - 6.9%, 7 days - 10.25%; 84.8% of degenerating CS terminals were axodendritic and 15.2% axosomatic.

Relative numbers of several types of synaptic connections in the substantia gelatinosa of the cat spinal cord

The relative numbers of axo-dendritic, axo-axonic, dendro-axonic and dendro-dendritic synapses were determined by classifying and recording all such specialized contacts in sample areas of the substantia gelatinosa. The samples were taken from segments L1-L5 of the cat spinal cord. In the glomerular complexes 97% of all synapses were recorded as axo-dentritic. In substantia gelatinosa deprived of glomerular complexes by dorsal root section, 96.5% were axo-dendritic. The remainders were about equally divided between axo-axonic, dendro-dendritic and dendro-axonic synapses.

Terminal proliferation and synaptogenesis following partial deafferentation: the reinnervation of the inner molecular layer of the dentate gyrus following removal of its commissural afferents

The inner one-third of the dendritic region of the dentate gyrus granule cells in adult rats receives projections primarily from the commissural fibers of the contralateral hippocampus and the associational fibers of the ipsilateral hippocampus. At two to four days following the complete removal of the contralateral hippocampus, approximately 25% of the terminals in the inner molecular layer are observed degenerating. This provides an excellent model system to investigate possible terminal proliferation induced by deafferentation since (1) the experimental lesion is easily reproducible, (2) no retrograde reactions occur in the granule cells as a direct result of the lesion, (3) no shrinkage is detected in this region following commissural deafferentation, (4) the same dendritic region can be relocated precisely in each animal, and (5) the synaptic counts are highly consistent between animals. Results from this study and from previous investigations demonstrate that the commissural projection is contained within a 0-80 mu zone directly above the granule cell layer; Complete photomontages of this zone were taken, but only the 40-80 mu zone was quantified for neuronal and glial changes in three normal, five 2- to 4-day, and five 50- to 75-day postlesion animals. The average synaptic count dropped to 64% of control values by 2 to 4 days, returned to 97% by 50- to 75 days postlesion, The number of terminals showing multiple synaptic contacts increased slightly in the long-term animals. Measurements of average terminal area showed no change between the short- and long-term survival groups. These results indicate that this dendritic region is reinnervated following partial deafferentation and that the reinnervation is due primarily to the formation of new terminals rather than the expansion of pre-existing terminals.