Formation of new synaptic terminals in the somatosensory thalamus of the rat after lesions of the dorsal column nuclei

Bilateral transient changes in thalamic nucleus ventroposterior lateralis after thoracic hemisection in the rat

We made simultaneous bilateral recordings of unit activity in the nucleus ventroposterior lateralis (VPL) in intact rats and after acute and chronic left thoracic hemisection. We observed an immediate bilateral decline in multireceptive units, reflecting a loss of nociceptive input on the lesion side and a loss of low-threshold inputs contralaterally. Unit properties were restored to normal by 6 wk. Mean spontaneous discharge frequency remained unchanged in left VPL at all intervals. Right VPL displayed a substantial increase in spontaneous discharge frequency at 2 and 4 wk, returning to normal by 6 wk. Activity in left VPL driven by Pinch or Brush of the right limb was unchanged except for an immediate decrease in the response to Pinch, which was reversed by 2 wk despite persistent left hemisection. In right VPL, the response to Pinch or Brush of the left hindlimb was enhanced at 2 and 4 wk but returned to normal by 6 wk. Behaviorally, the same rats displayed increased sensitivity to mechanical stimulation of the left hindlimb, but, unlike VPL activity, there was no significant behavioral recovery. Bursting cells were also observed bilaterally in VPL, but this did not match the restriction of scratches to the hindlimb contralateral to the hemisection considered to be evidence for neuropathic pain. The novel findings include recovery of responsiveness to Pinch on the side ipsilateral to the hemisection despite the lack of spinothalamic input as well as failure for the thalamus contralateral to hemisection to maintain its elevated responsiveness.

Modulation of receptive field properties of thalamic somatosensory neurons by the depth of anesthesia

Modulation of receptive field properties of thalamic somatosensory neurons by the depth of anesthesia. The dominant frequency of electrocorticographic (ECoG) recordings was used to determine the depth of halothane or urethan anesthesia while recording extracellular single-unit responses from thalamic ventral posterior medial (VPM) neurons. A piezoelectric stimulator was used to deflect individual whiskers to assess the peak onset latency, magnitude, probability of response, and receptive field (RF) size. There was a predictable increase in the dominant ECoG frequency from deep stage IV to light stage III-1 anesthetic levels. There was no detectable frequency at stage IV, a 1- to 2-Hz dominant frequency at stage III-4, 3-4 Hz at stage III-3, 5-7 Hz at stage III-2, and a dual 6- and 10- to 13-Hz pattern at stage III-1. Reflexes and other physical signs showed a correlation with depth of anesthesia but exhibited too much overlap between stages to be used as a criterion for any single stage. RF size and peak onset latency of VPM neurons to whisker stimulations increased between stage III-4 and III-1. A dramatic increase in RF size and response latency occurred at the transition from stage III-3 (RF size approximately 2 whiskers, latency approximately 7 ms) to stage III-2 (RF size approximately 6 whiskers, latency approximately 11 ms). Response probability and magnitude decreased from stage III-4 to stage III-3 and III-2. No responses were ever evoked in VPM cells by vibrissa movement at stage IV. These changes in VPM responses as a function of anesthetic depth were seen only when the nucleus principalis (PrV) and nucleus interpolaris (SpVi) trigeminothalamic pathways were both intact. Eliminating SpVi inputs to VPM, either by cutting the primary trigeminal afferent fibers to SpVi or cutting axons projecting from SpVi to VPM, immediately reduced the RF size to fewer than three whiskers. In addition, the predictable changes in VPM response probability, response magnitude, and peak onset latency at different anesthetic depths were all absent after SpVi pathway interruption. We conclude that 1) the PrV input mediates the near "one-to-one" correspondence between a neuronal response in VPM and a single mystacial whisker, 2) in contrast, the SpVi input to VPM is primarily responsible for the RF properties of VPM neurons at light levels of anesthesia and presumably in the awake animal, and 3) alterations in VPM responses produced by changing the depth of anesthesia are due to its selective influence on the properties mediated by SpVi inputs at the level of the thalamus.

Plasticity of GABA- and glutamate-containing terminals in the mouse thalamic ventrobasal complex deprived of vibrissal afferents: an immunogold-electron microscopic study

GABA and glutamate immunogold staining demonstrated that nerve cells of the thalamic ventrobasal complex (VB) of mice were positive exclusively for glutamate. None of the neuronal perikarya reacted the GABA antibody. By using alternate thin sections of the normal VB, it was also shown that large "specific" somatosensory and small corticothalamic terminals, both of which contained spherical synaptic vesicles, exhibited only glutamate-like immunoreactivity. A third axonal type, containing flat-ovoid synaptic vesicles, stained only for GABA. Seventy-five days after coagulation of the vibrissal follicles in newborn mice, a characteristic multiplication of GABA positive axon terminals was observed. In addition, it was demonstrated that, similarly to modified cortical endings (Hámori et al., J. Comp. Neurol. 254:166-183, '86), many GABA positive terminals appeared as specific afferent endings, replacing the missing "specific" vibrissal afferents. This finding shows a remarkable plasticity of inhibitory GABA axons during developmental synaptogenesis and provides further evidence that the size, location, and the type of attachment of presynaptic terminals are dependent on their postsynaptic target.

Focal magnetic coil stimulation reveals motor cortical system reorganized in humans after traumatic quadriplegia

A figure of '8' magnetic coil (MC) was used to stimulate focally the motor cortex of two adult, traumatic quadriplegics and three normal adults. The two patients were injured approximately 2 years previously and had intense physiotherapy, including biofeedback training of biceps and deltoid muscles, respectively, which were the most caudal muscles spared. The focal MC elicited compound motor action potentials (CMAPs) from these muscles from a much wider area of scalp than in the normal subjects. Latency of biceps and deltoid CMAPs were inversely related to CMAP amplitude. A reorganization of the motor cortical projection system is inferred, in which areas normally eliciting digit movements instead activate muscles in quadriplegics just above the spinal level. The reorganization applies also to the central sense of movement normally elicited by focal frontal cortex stimulation. Possible mechanisms of the reorganization and an implication for rehabilitation are discussed.

Sensory nerve crush and regeneration and the receptive fields and response properties of neurons in the primary somatosensory cerebral cortex of cats

Extracellular recordings were made of activity evoked in neurons of the forepaw focus of somatosensory cerebral cortex by electrical stimulation of each paw in control cats and cats that had undergone crush injury of all cutaneous sensory nerves to the contralateral forepaw 31 to 63 days previously. Neurons responding only to stimulation of the contralateral forepaw were classified as sa; neurons responding to stimulation of both forepaws were classified as sb; neurons responding to stimulation of both contralateral paws were classified as sc; and neurons responding to stimulation of at least three paws were classified as m. The ratio sa:sb:sc:m neurons was 46:3:0:0 in control cats and 104:15:3:26 in cats that had undergone nerve crush 1-2 months prior to study. sa neurons from experimental cats had depth distributions similar to those in controls and responded to contralateral forepaw stimulation with more spikes per discharge, longer latency, and higher threshold than sa neurons in control cats. m neurons from experimental cats were distributed deeper in the cortex than sa neurons, and, when compared to experimental sa neurons, they responded with longer latency and poorer frequency-following ability; however, the number of spikes per discharge and threshold were not significantly different. The appearance of wide-field neurons in this tissue may be explained in terms of strengthening of previously sub-threshold inputs to neurons in the somatosensory system. If the neurons in sensory cortex play a requisite role in cutaneous sensations and if changes similar to those reported here occur and persist in human cortex after nerve crush, then "complete" recovery of sensation in such patients may occur against a background of changed cortical neuronal responsiveness.

First month of development of fetal neurons transplanted as a cell suspension into the adult CNS

It has been demonstrated elsewhere that fetal thalamic tissue, when transplanted as a cell suspension into the excitotoxically neuron-depleted adult somatosensory thalamus, can grow, differentiate, and receive projections from host afferents. In the present study, we used the same paradigm to analyse the transplanted neurons during their morphogenesis, i.e. during the first month after transplantation. Using various anatomical criteria, at the light and electron microscope levels, we compared the development of transplanted neurons with the normal ontogeny of homologous neuronal populations. Confined solely to the mechanically lesioned area during implantation at seven days post-grafting, the transplant increased in size to occupy most of the previously neuron-depleted area by the third week after grafting. The final size of the transplant thus depended upon the size of the lesion. At seven days post-grafting, the neurons were small in size and the cellular density was high. At this immature stage few synaptic contacts were visible and the ultrastructure was characterized by large extracellular spaces. At 10 days post-grafting, the size of the neurons had increased and the cellular density had decreased. Both an extensive dendritic proliferation and a simultaneous active synaptogenesis could also be observed. All these events continued to evolve and during the third week the neuropil progressively acquired more mature ultrastructural characteristics. Synaptic contacts exhibiting characteristics comparable to those observed in the intact thalamus also became more numerous. At 20 days post-grafting, axonal myelination had started, the development of the graft apparently stopped and the various criteria had stabilized. Until that developmental stage, growth of grafted neurons compared to that of normal thalamic ones. At later stages, however, grafted neurons failed to grow larger and did not reach the size of the homologous population in the adult animal. It seems, therefore, that transplants of thalamic fetal neurons can be used as a tool with which to study thalamic neuronal development, within definable limits.

A review of recent observations concerning the synaptic organization of the basilar pontine nuclei

Ultrastructural studies are described that have identified in the basilar pontine nuclei (BPN), the synaptic boutons formed by the corticopontine, cerebellopontine, tectopontine, and dorsal column nuclei-pontine afferent projection systems. In addition, immunocytochemical studies visualized neuronal somata, dendrites, and synaptic boutons that contain immunoreactivity for GABA or the synthesizing enzyme glutamic acid decarboxylase (GAD). Based upon differences in the mode of degeneration and postsynaptic locus of degenerative synaptic boutons in the BPN, it is suggested that two types of cortical neurons and three classes of deep cerebellar nuclear cells project to the BPN. For similar reasons, it appears that two types of neurons in the dorsal column nuclei project to the BPN while only one type of afferent synaptic bouton takes origin from the superior colliculus. Furthermore it appears that the population of BPN neurons projecting to the paramedian lobule receives convergent inputs from the cutaneous periphery and the corresponding region of sensorimotor cortex. Studies employing GAD immunohistochemistry indicate that GABA-ergic neurons and axon terminals are present in the BPN and thus support the suggestion that a local inhibitory interneuron is present within the BPN. Taken together these observations suggest that basilar pontine neurons might play a more active role in the integration of various types of information destined for the cerebellar cortex than has previously been recognized.

Lack of central sprouting of primary afferent fibers after ricin deafferentation

A new deafferentation technique, the application of ricin to peripheral nerves, was used to test for collateral sprouting of undamaged primary afferent fibers within the adult mammalian spinal cord dorsal horn. The right sciatic nerves in rats were injected with ricin 14 to 57 days prior to bilateral labelling of dorsal rootlets with horseradish peroxidase. To equate the number of surviving dorsal root fibers on the two sides, the left sciatic nerves were injected 5 days prior to labelling. In each animal, horseradish peroxidase was applied to a bilateral pair of lumbar or low thoracic dorsal rootlets 18 hours prior to sacrifice to test for sprouting by labelling primary afferent fibers and terminals in the right (experimental) and left (control) dorsal horns. Although there is overlap of degenerated and intact primary afferent fields in this preparation, a postulated precondition for sprouting (Murray and Goldberger: J. Neurosci. 6:3205-3217, '86), we found no evidence for sprouting of undamaged, myelinated afferent fibers in the experimental dorsal horns. The pattern of labelling was symmetrical in all animals, and the density of labelling was not consistently greater on the experimental side. These results support the conclusions of Rodin et al. (J. Comp. Neurol. 215:187-198, '83) and Rodin and Kruger (Somatosens. Res. 2:171-192, '84), who also found no sprouting in the rat's dorsal horn after surgical deafferentation, and do not support the assertion that the difference between the results of those studies and earlier studies in cats was due to a lack of overlap of degenerated and intact dorsal roots in the rat.

Fine structure of the dorsal part of the nucleus submedius of the rat thalamus: an anatomical study with reference to possible pain pathways

The dorsal portion of the nucleus submedius of the rat thalamus receives spinal and trigeminal projections which may convey noxious inputs. The present study was undertaken to analyse the fine structure of the nucleus with particular reference to a possible trigemino-thalamo-prefrontal cortical pathway relaying in nucleus submedius. Presynaptic terminals in the dorsal portion of the nucleus submedius were classified into three broad categories usually observed in thalamic nuclei: "small round", "flat" and "large round" types. Axonal tracing using either anterograde transport of horseradish peroxidase or degeneration methods indicated that some "small round" terminals originate from the pre-frontal cortex. Some "large round" terminals were labelled from the trigeminal subnucleus caudalis. These "large round" terminals exhibited distinct morphological features when compared with trigeminal terminals in other thalamic nuclei. In particular they made synaptic contacts predominantly with dendritic protrusions and were surrounded by multilamellate astrocytic processes. Double-labelling experiments were performed by means of the combined retrograde transport of horseradish peroxidase and Wallerian degradation techniques. Terminals degrading after lesion of the trigeminal subnucleus caudalis contacted submedius neurons labelled retrogradely from the prefrontal cortex. These observations demonstrate the existence of a direct monosynaptically relayed pathway from subnucleus caudalis to prefrontal cortex which relays in the dorsal part of nucleus submedius.

Nonoverlapping thalamocortical connections to normal and deprived primary somatosensory cortex for similar forelimb receptive fields in chronic spinal cats

The fluorescent dye retrograde tracing technique, using fast blue in combination with fluorogold, was used to examine thalamocortical projections from the ventrobasal complex to primary somatosensory cortex in chronic spinal cats that sustained T12 cord transection at 2 weeks of age. Following cord transection at this age, it has been shown that forelimb afferents can excite the deprived hindlimb projection zone, in addition to the region of somatosensory cortex that they normally occupy (McKinley et al., 1987). These two regions of cortex are separated by over 10 mm, thus facilitating the determination of whether the forelimb representation in "hindlimb cortex" is derived from the sector of the ventrobasal complex of the thalamus representing the forelimb, hindlimb, or both. Injections of the two dyes into separate regions of the cortex that were excited by the same peripheral forelimb receptive fields produced single labeling of two nonoverlapping clusters of thalamic neurons. This finding suggests that the projections for these two areas are independent and distinct, and indicates that altered thalamocortical projections do not contribute the critical component underlying reorganizational changes observed at the cortical level after spinal cord transection. It is hypothesized that the degree of reorganization required to achieve the magnitude of change observed in the cortex must occur below the level of the thalamocortical relay.

The differential synaptic organization of the spinal and lemniscal projections to the ventrobasal complex of the rat thalamus. Evidence for convergence of the two systems upon single thalamic neurons

The synaptic organization of terminals originating either from the spinal cord (spinothalamic) or from the dorsal column nuclei (lemniscal) was investigated in the ventrobasal complex of the rat thalamus. Wheatgerm agglutinin conjugated to horseradish peroxidase was used as an anterogradely transported axonal tracer, using benzidine dihydrochloride as a chromogen for the identification by electron microscopy of spinal and lemniscal projections to the ventrobasal thalamus. A double anterograde tract tracing strategy, based labeling by wheatgerm agglutinin conjugated to horseradish peroxidase of spinal terminals and simultaneous visualization of lemniscal terminals identified by Wallerian degeneration induced by lesion of the neurons of origin in the dorsal column nuclei, was used to compare the postsynaptic elements contacted by the two pathways and to look for a possible convergence of the two pathways onto single thalamic neurons. Spinal and lemniscal terminals are large (2-2.5 microns mean average diameter) terminals containing several mitochondria and numerous rounded vesicles. A quantitative analysis of the mean average diameters of the terminals revealed that one could not differentiate between synapses formed by the two pathways on a morphological basis. Terminals of the two pathways make asymmetrical contacts (Gray type I) with dendrites of varying diameter, dendritic protrusions, and cell somata. A quantitative analysis of the least diameter of the postsynaptic elements demonstrates projections of the two systems to different, partially overlapping regions of thalamic neurons. Lemniscal terminals originating from the dorsal column nuclei frequently contact cell somata; axosomatic spinothalamic contacts are uncommon. In addition, lemniscal projections tend to contact more proximal dendrites than do spinal projections, and this differential synaptic organization is statistically significant. From a functional point of view, this differential synaptic organization might indicate that lemniscal inputs have greater influence than spinal inputs in affecting the activity of thalamic neurons. Labeled spinothalamic terminals contact the same dendritic profile as do degenerating lemniscal terminals in about 10% of single sections. Because the present study did not include a complete reconstruction of ventrobasal complex neurons of the thalamus or even regions of dendritic arbors, the degree of convergence is likely to be significantly underestimated. These findings indicate that the anatomical basis exists for an interaction between nociceptive and non-nociceptive somesthetic systems at the level of single ventrobasal neurons of the thalamus of the rat.

Time course of reactive synaptogenesis in the subcortical somatosensory system

These experiments were designed to determine when synaptogenesis begins in the adult rat ventral posterolateral nucleus of the thalamus following lesions of the dorsal column nuclei. Given the relatively uncomplicated structure of the neuropil in the ventral posterolateral nucleus of the rat, the specificity of reactive synaptogenesis of the lemniscal input and the effect of the loss of lemniscal terminals on terminals from other sources could be determined. By use of morphometric analysis of electron micrographs, the numerical density of the 3 terminal types in the neuropil was determined at a series of postlesion survival times ranging from 12 hours to 50 days. Synaptogenesis began about 30 days after the lesions of the dorsal column nuclei and was complete by 50 days. The slow onset of synaptogenesis was in response to a loss of the lemniscal terminals, which account for only 3% of the total number of synapses in the ventral posterolateral nucleus. The low level of synaptogenesis early in the recovery process differs from the recovery seen in other central nervous system sites, which show an early rapid increase in synapses in response to much greater denervation. The loss of lemniscal terminals has relatively little effect on the numerical density or distribution of the terminals of other types. The new terminals that are formed come both from axons that originate from the undamaged portion of the dorsal column nuclei and from axons originating in the spinal cord.

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.

Origin and ultrastructural identification of dorsal column nuclear synaptic terminals in the basilar pontine gray of rats

The ultrastructural characteristics of HRP-WGA-labeled or degenerating axon terminals arising from neurons in the dorsal column nuclei (DCN) were identified within the contralateral basilar pontine nuclei (BPN) following unilateral HRP-WGA injections or ablations of the DCN. The cells of origin of these projections were also identified through the application of the retrograde tracer HRP-WGA. Two groups of degenerating DCN-pontine terminals were identified. Both formed asymmetrical synaptic contacts with dendritic shafts and/or dendritic appendages of pontine neurons. One group of degenerating terminals contained small, round synaptic vesicles, while the other exhibited a mixture of dense core and pleomorphic vesicles. The former group, which clearly represented the majority of degenerating terminals observed, was interpreted to progress from an early filamentous form of degeneration to a later electron-dense variety and to originate from dorsally located DCN cells distributed primarily at the level of and just caudal to the area postrema. Other DCN-labeled neurons were more ventrally located and were postulated to give rise to those degenerative boutons that contained a mixture of dense core and pleomorphic-shaped vesicles. The present study also identified the cells of origin of two additional projections to the basilar pons: one from cells in the external cuneate nucleus and another from neurons of the medullary reticular formation.

Plasticity in the olfactory cortex: age-dependent effects of deafferentation

In order to assess the role of input-target interactions in the development of olfactory cortex, the primary afferent fibers from the olfactory bulb to the superficial part of layer I of the cortex (layer Ia) were removed in developing and mature rats. After survival periods that vary from a few days to 2-6 months, changes were assessed in (1) the radial thickness of layer I, (2) the laminar distribution of intracortical associational fibers, which normally terminate in a deep part of layer I (layer Ib), and (3) the distribution of glia in layer I. The findings indicate that the lamination of fibers within layer I is not intrinsically prespecified, but gradually becomes "set" during the first month after birth. If the fibers from the olfactory bulb are removed, the dendrites of cortical cells are capable of accepting inputs from other fiber systems, depending on the maturational state of the dendrites and the ingrowing axons. Development of the abnormal inputs is associated with relatively normal dendritic growth, whereas lack of adequate input results in dendritic atrophy. Thus, after neonatal bulb ablation, the intracortical fibers occupy both superficial and deep parts of layer I, and a normal synaptic density is established throughout the layer. Layer I also develops to nearly its normal adult thickness, although the high density of glia that normally characterizes layer Ia is not apparent. With bulb ablation at progressively older ages (from postnatal day (P-) 3 to 21), the cortical associational fibers show progressively less extension into the denervated layer Ia. Layer I continues to grow, but not to the same extent as after P-1 ablations. In these experiments the glia distribution resembles the pattern present at the time of denervation. After adult olfactory bulb ablation, the long intracortical fibers extend very little into layer Ia, which undergoes pronounced shrinkage and becomes filled with a high concentration of glia. However, partial reinnervation of layer Ia is accomplished by the proliferation of a normally sparse native fiber system, which has been identified only with the Timm method. These results are interpreted as evidence that the normal development of lamination of afferent fibers to the olfactory cortex depends on axodendritic interaction during development.

Ultrastructural analysis of the terminals of various somatosensory pathways in the ventrobasal complex of the rat thalamus: an electron-microscopic study using wheatgerm agglutinin conjugated to horseradish peroxidase as an axonal tracer

We used wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) as an anterograde tracer to label the terminals of the lemniscal, spinothalamic, and trigeminothalamic pathways in the ventrobasal complex of the rat thalamus (VB). The use of benzidine dihydrochloride (BDHC) as the chromogen allowed us to view the labeled profiles with the electron microscope and permitted us to compare the morphology of the terminals from the various pathways. We found that all the labeled somatosensory pathways terminate in the VB in the form of large terminals that contain round synaptic vesicles and make numerous asymmetrical synaptic contacts, usually with dendritic protrusions and proximal dendrites. The present results demonstrate that pathways conveying noxious and non-noxious somatosensory information terminate upon thalamic neurons with synaptic terminals having similar morphological features.

Recovery from partial deafferentation increases 2-deoxyglucose uptake in distant spinal segments

2-Deoxy[14C]glucose autoradiography was used to study the responsiveness of the partially deafferented rat spinal cord to electrical stimulation of low-threshold afferent fibers. Unilateral extradural dorsal rhizotomies were carried out at L3 to S2, sparing L5. Postoperative sensory deficits were appropriate to the extent of the lesion. Acute, 7 day, and 14 to 20 day postrhizotomy animals and unoperated controls were anesthetized prior to isotope injection and electrical stimulation of Ia fibers in the L5 root. Quantitative densitometry was carried out on enlarged autoradiographs, subdividing the spinal gray matter into laminar divisions drawn from the corresponding stained sections. Optical densities from stimulated and unstimulated sides were compared using paired t tests for each experimental group at each lumbosacral segment (L1 to S2) and at T13 for the day 14 to 20 animals. This procedure provided an objective basis for statistical comparisons between homologous areas even where the differences in density were small. Unoperated animals showed activation in 11 of 23 dorsal horn zones extending to L2 and never involving the base of the dorsal horn (lamina V). Acute and day 7 groups did not appreciably differ from the control group except for activation of lamina V within the L4 segment. In the most delayed group, 18 dorsal horn regions were activated, extending to L1 with an additional zone in T13 . Lamina V contained significant labeling in three segments. In no group was there increased labeling of the ventral horn. The results are interpreted as showing that stimulus-related, neural activity increases after a 2-week delay in regions of spinal cord distant from the normal zone of significant metabolic change. This increase in neural activity during recovery is discussed in relation to time-dependent electrophysiologic, structural, and metabolic responses to deafferentation. The longitudinal spread of dorsal horn activation by preserved afferent fibers in the spared root may facilitate more effective central transmission of sensory information.

Trigeminal afferents to the diencephalon in the rat

Trigemino-diencephalic connections were studied in the rat using wheat-germ agglutinin conjugated to horseradish peroxidase as an anterogradely transported axonal tracer. Injection of the tracer into the subnucleus principalis produced two foci of dense labelling: one ventromedial: and one dorsal within the medial part of the ventrobasal complex. Other diencephalic structures containing granules of reaction product were the medial part of the medial geniculate body, the ventral area of the zona incerta and the nucleus lateralis posterior, pars lateralis. Injection of the tracer into the subnucleus interpolaris labelled the same structures, but less densely. After an injection into the subnucleus caudalis, labelling was observed in the same thalamic areas, although projections to the zona incerta or the lateralis posterior were not consistent. Additional labelling was observed in the subfascicular area of the mesodiencephalic junction, the nucleus submedius and the intralaminar nuclei centralis medialis and lateralis. In those cases of injection into the subnuclei principalis and interpolaris, all observed thalamic sites of projection were contralateral to the injection site. Following injection into the subnucleus caudalis, projections toward lateral thalamic structures were contralateral, but the nucleus submedius and the intralaminar nuclei exhibited bilateral labelling. Using high magnification (1250 X) with bright-field illumination, an analysis of the morphology of some terminal arborizations was attempted. Despite some technical limitations, the analysis indicated that in the ventrobasal complex, some terminal ramifications of axons originating from the three trigeminal subnuclei under study arborize so as to encompass a rounded area, the diameter of which could be as large as 100 microns, thereby resembling the classically described "bushy arbors". Such arborizations could not be distinguished in the axons projecting to the medial part of the medial geniculate body. In this latter nucleus, the terminals appeared to arise from a stem fiber as short side branches at approximately right angles to the parent stem axon. In the other areas where afferent terminal labelling was observed, the density of the network of the labelled fibers often complicated the analysis of morphological features. However, arborizations such as those observed in the ventrobasal complex or the medial geniculate nucleus could not be distinguished.(ABSTRACT TRUNCATED AT 400 WORDS)

The structural organization of the ventrobasal complex of the rat as revealed by the analysis of physiologically characterized neurons injected intracellularly with horseradish peroxidase

The ventrobasal complex (VB) of the rat thalamus contains neurons responding to non-noxious somatic stimuli as well as neurons driven exclusively by noxious stimuli. This study presents a comparison of morphological features of these two kinds of neurons. Thirteen neurons electrophysiologically characterized were impaled with the micropipette used for the recordings and intracellularly injected with horseradish peroxidase. After revealing the marker and preparation for electron microscopic procedures, 3 out of the 13 neurons were carefully studied using both the light and the electron microscope. VB neurons are stellate cells with a central rounded cell body and 6 to 10 primary dendrites which branch rapidly, giving a 'tufted' appearance. Dendrites of all orders present various types of protrusions. At the electron microscope level, 3 main kinds of synaptic profiles were observed contacting the injected neurons: small terminals with round vesicles which make asymmetrical contacts with distal dendrites; medium-sized terminals with flattened vesicles which make symmetrical contacts with dendrites of all orders and the soma; and large terminals with round vesicles which make asymmetrical contacts with primary dendrites and the soma. This study failed to reveal obvious morphological differences between functionally different VB neurons. In addition, it showed that their synaptology was apparently equivalent.

Reticularis thalami afferents to the ventrobasal complex of the rat thalamus: an electron microscope study

Afferents from the nucleus reticularis thalami (RT) to the thalamic ventrobasal complex were studied in the rat by looking for degenerating terminals after selective neurotoxic lesion of RT using injections of kainic acid. Several lines of evidence are presented indicating that RT afferents terminate in the VB by F type (Gray type II) terminals and that F type terminals in the VB all depend of RT neurons.

The structural organization of the ventral posterolateral nucleus in the rat

The structural plan of the ventral posterolateral nucleus (VLP) in the rat was analyzed by using a variety of techniques to study the pattern of distribution of the ascending afferent fibers and the synaptology of the neuropil within this somatosensory relay nucleus. Golgi stains, Fink-Heimer methods, HRP labeling methods, and electron microscopy were all used in the analysis. The neurons in VPL are aligned in rostrocaudal and dorsoventral rows that are roughly parallel to the curvature of the external medullary lamina (EML) and curve partially around the rostral pole of the ventral posteromedial nucleus (VPM). Golgi-impregnated sections reveal that the dendritic trees of the VPL neurons conform in general to the laminar pattern of VPL. Thick proximal dendrites extend about 25 mu m from the cell bodies. Most proximal dendrites are aligned with the laminae of VPL but the distal dendrites spread over many laminae within VPL. The inputs from the dorsal column nuclei (DCN) end only on proximal dendrites as large, round-vesicle terminals. About 20--25% of the small round-vesicle terminals originate in the cerebral cortex and synapse only on the distal dendrites. The third type of synapse contains many flattened vesicles and is of unknown origin. No serial synapses or vesicle-containing dendrites were observed. Input from the spinal cord projects to two segregated zones which are transitional between the ventral lateral nucleus (VL) and VPL rostrally and between the posterior thalamic complex (PO) and VPL caudally. Each transition zone contains neurons characteristic of both VPL and the adjacent region. Ascending afferent projections were demonstrated by the antegrade transport of HRP following injections into the ventral mesencephalon and by Fink-Heimer stains of degeneration resulting from small lesions of the DCN. Both methods indicate that large-caliber axons course parallel to each other and give off collaterals that diverge to widespread areas of the VPL. The widespread terminal fields that result do not conform to the laminar pattern of the nucleus. Small punctate lesions of DCN result in sparse degeneration that is also widespread in VPL. Structures which appear to be clusters of terminal arborizations of the ascending afferent input were also observed in VPL. These results suggest that somatotopy and modality separation in VPL may be determined either by intrinsic and/or corticothalamic morphological relationships and not by precise topographical ordering of sensory input.

Synapse formation after injury in the adult rat brain: preferential reinnervation of denervated fimbrial sites by axons of the contralateral fimbria

The dorsolateral quadrant of the lateral septal nucleus receives projections from both the ipsilateral and the contralateral fimbria. In the adult rat the effect of fimbrial lesions on synapse formation has been studied by a quantitative electron microscopic analysis of the various types of synapses present, using electron-dense degeneration to identify fimbrial fibre terminals. In this area, the fimbrial axons from both sides together account for about 30% of the total number of synapses and they terminate mainly on dendritic spines. The ipsilateral fimbria forms twice as many synapses as the contralateral fimbria. When one fimbria is cut and time left for the degeneration to be removed, the numbers of synapses are restored to normal levels and the remaining fimbria acquires, on both sides of the septum, a number of synapses equal to the sum of the two individual fimbria, This suggests that the axons of the surviving fimbria have completely reinnervated the denervated postsynaptic sites formerly occupied by the cut fimbria of the other side, effectively excluding non-fimbrial axon terminals, even though the latter constitute the majority (70%) of the synaptic terminals in the region. When both fimbria are cut the numbers of synapses are once again restored to normal levels. However, since there are now no fimbrial axons left, the denervated fimbrial postsynaptic sites must this time have been reinnervated by non-fimbrial axons. Reinnervation by non-fimbrial axons is numerically equally effective in reclaiming the denervated sites, although when compared to the reinnervation by fimbrial axons, the removal of degenerating terminals is somewhat slower, and among the reinnervating terminals there is a much higher incidence of axon terminals making more than one synaptic contact in the plane of section. Thus, fimbrial axons, when present, have the ability to exclude the reinnervation of denervated fimbrial sites by non-fimbrial axons, despite the fact that the latter are both more numerous and also clearly capable of reinnervating those sites when no fimbrial axons are present. Two possible mechanisms are discussed: a spatial preference based on the geometrical arrangements in the neuropil, and a temporal preference based on the relative rates of response of the fimbrial vs the non-fimbrial axons.

Synapse formation after injury in the adult rat brain: failure of fimbrial axons to reinnervate the bed nucleus of the stria terminalis

Selectivity in the reinnervation of denervated postsynaptic sites in the adult rat septal nuclei has been studied by both light and electron microscopic degeneration techniques after lesions of the fimbria and stria terminalis. In the mid-rostrocaudal septum the ventral border of the lateral septal nucleus is coextensive with the dorsal border of the strial bed nucleus. In the normal rat, fimbrial axons establish synapses throughout the lateral septal nucleus of the same side, and also in the dorsal part of the lateral septal nucleus on the opposite side. The stria terminalis establishes synapses in the ipsilateral but not in the contralateral bed nucleus at this level. Both the fimbria and the stria terminalis were completely severed on the left side, and after adequate survival for the removal of all degeneration, the distribution of the remaining fimbria was plotted. Interesting changes were found on the side contralateral to the second lesion, where the fimbria both increases the number of its synaptic terminals within its proper contralateral territory (the dorsal part of the lateral septal nucleus) and also extends its distribution into the ventral part of the lateral septal nucleus--the territory normally reserved for the ipsilateral fimbria. Although completely surrounding the strial bed nucleus, fimbrial axons fail to invade the bed nucleus, and fimbrial terminals are therefore unable to reinnervate denervated strial postsynaptic sites. Since there are no obvious structural barriers between the neuropil of the lateral septal nucleus and that of the strial bed nucleus it is suggested that this failure is most likely to be due either to some biochemical incompatibility between fimbrial axons and strial postsynaptic sites, or to the fact that the fimbrial axons are denied access because some other (unidentified) axonal system forms new presynaptic terminals which effectively pre-empt the sites in the strial bed nucleus.

A combined horseradish peroxidase and Golgi study on the afferent connections of the ventrobasal complex of the thalamus in the cat

Afferent connections to the ventrobasal complex (VB) of the thalamus were studied by means of retrograde transport of horseradish peroxidase (HRP) and by the Golgi-method. After HRP-injection into the VB, peroxidase-positive cells were observed contralaterally in the dorsal column nuclei (DCN), in the trigeminal nuclei and in the lateral cervical nucleus (LCN), and ipsilaterally in the somatosensoty I (SI) and II (SII) cortical areas. Labeled cells of different shape and size were compared with neurons impregnated by the Golgi-technique. On tn size and shape even within one region and that they correspond to the relay or efferent neurons observed in the Golgi-material.