Not Just an Anchor: The Human Filum Terminale Contains Stretch Sensitive and Nociceptive Nerve Endings and Responds to Electrical Stimulation With Paraspinal Muscle Activation.

BACKGROUND: Neural components of the fibrous filum terminale (FT) are well known but are considered as embryonic remnants without functionality. OBJECTIVE: To investigate the ultrastructure of human FT specimens for sensory nerve endings and record paraspinal muscle activity on electrostimulation of the FT. METHODS: We prospectively investigated a cohort of 53 patients who underwent excision of the FT for the treatment of tethered cord syndrome. Surgical FT specimens were investigated by light and transmission electron microscopy. Intraoperative electrophysiological routine monitoring was extended by recording paraspinal muscles above and below the laminotomy level. RESULTS: Light microscopy revealed tiny peripheral nerves piercing the pia mater of the FT and entering its fibrous core. Transmission electron microscopy unveiled within the fibrous core of the FT myelinated nerve structures in 8 of the 53 patients and unmyelinated ones in 10 of the 53 patients. Both nerve endings encapsulated in fibrous tissue or unencapsulated nonmyelinated Schwann cell nerve bundles, that is, Remak cells, were found. Those nerve endings resembled mechanoreceptor and nociceptive receptor structures found in human skin, muscle tendons, and skeletal ligaments. Specifically, we found Ruffini mechanoreceptors and in addition nerve endings which resembled nociceptive glioneural structures of the skin. Bipolar electrostimulation of the FT was associated with paraspinal muscle activity above and below the spinal segment at which the FT was stimulated. CONCLUSION: Morphological and electrophysiological results indicate the presence of functional sensory nerve endings in the FT. Like other spine ligaments, the FT may serve as a proprioceptive element but may also contribute to back pain in spine disorders.

On the classification of pathways in the auditory midbrain, thalamus, and cortex.

Auditory forebrain pathways exhibit several morphological and physiological properties that underlie their specific neurobiological roles in auditory processing. Anatomically, such projections can be distinguished by their terminal size, arborization patterns, and postsynaptic dendritic locations. These structural features correlate with several postsynaptic physiological properties, such as EPSP amplitude, short-term plasticity, and postsynaptic receptor types. Altogether, these synaptic properties segregate into two main classes that are associated with either primarily information-bearing (Class 1) or modulatory (Class 2) roles, and have been used to delineate the principle routes of information flow through the auditory midbrain, thalamus, and cortex. Moreover, these synaptic properties engender as yet unexplored issues regarding the neuronal processing of auditory information, such as the convergent integration and long-term plasticity of auditory forebrain inputs.

Contralateral corticothalamic projections from MI whisker cortex: potential route for modulating hemispheric interactions.

Rat whisking behavior is characterized by high amounts of bilateral coordination in which whisker movements on both sides of the face are linked. To elucidate the neural substrate that might mediate this bilateral coordination, neuronal tracers were used to characterize the bilateral distribution of corticothalamic projections from primary motor (MI) cortex. Some rats received tracers in the MI whisker region, whereas others received tracers in the MI forepaw region. The MI whisker region projects bilaterally to the anteromedial (AM), ventromedial (VM), and ventrolateral (VL) nuclei, and to parts of the intralaminar nuclei. By contrast, the MI forepaw region sends virtually no projections to the contralateral thalamus. Consistent with these findings, bilateral injections of different tracers into the MI whisker region of each hemisphere produced tracer overlap on both sides of the thalamus. Furthermore, MI whisker projections to the contralateral thalamus terminate in close proximity to the thalamocortical neurons that project to the MI whisker region of that contralateral hemisphere. The terminal endings of the contralateral corticothalamic projections contain small synaptic varicosities and other features that resemble the modulator pathways described for other corticothalamic projection systems. In addition, tracer injections into AM, VM, and VL revealed dense clusters of labeled neurons in layer VI of the medial agranular (Agm) zone, which corresponds to the MI whisker region. These results suggest that projections from the MI whisker region to the contralateral thalamus may modulate the callosal interactions that are presumed to play a role in coordinating bilateral whisking behavior.

Effect of season on neuropeptide Y and galanin within the hypothalamus of the ewe in relation to plasma luteinizing hormone concentrations and the breeding season: an immunohistochemical analysis.

Within the hypothalamus, neurones that express neuropeptide Y (NPY) and galanin have been implicated in the regulation of gonadotropin-releasing hormone (GnRH) and gonadotropin secretion. We aimed to determine the extent to which the expression of these two neuronal systems is linked to the seasonal reproductive cycle, and the effect of chronic oestrogen treatment. Immunohistochemical analysis was used to examine changes between the breeding season and anestrus in ovariectomized (OVX) ewes with or without oestrogen treatment (s.c. implants for 2 weeks). Serial blood sampling established plasma luteinizing hormone (LH) profiles, and the ewes were subsequently killed and the brains perfused for immunohistochemistry. In OVX ewes, the amplitude of LH pulses was greater in the nonbreeding season than in the breeding season. Oestrogen treatment caused a marked reduction in plasma LH concentrations during anestrus, but not in the breeding season. The number of cells in the arcuate nucleus/median eminence region (ARC-ME) that stained for NPY was lower in ewes killed in anestrus (September) than in ewes killed in the breeding season (May), but there was no seasonal change in the number of galanin-stained cells. Within season, oestrogen treatment did not affect NPY- or galanin-cell number. There was no effect of season or oestrogen on the area of varicose fibres/terminals for either peptide in the ARC-ME, but galanin immunostaining was more intense during the breeding season. We conclude that the amount of NPY in cell bodies of the ARC-ME is lower in ewes in the nonbreeding season; this could reflect a steroid-independent effect of photoperiod. We also conclude that the long-term negative-feedback effect of oestrogen on GnRH/LH secretion does not appear to be mediated by NPY- or galanin-containing neurones in the ewe.

Hypothalamopontine projections in the rat: anterograde axonal transport studies utilizing light and electron microscopy.

Projections to the basilar pontine nuclei (BPN) from a variety of hypothalamic nuclei were traced in the rat utilizing the anterograde transport of biotinylated dextran amine. Light microscopy revealed that the lateral hypothalamic area (LH), the posterior hypothalamic area (PH), and the medial and lateral mammillary nuclei (MMN and LMN) are the four major hypothalamic nuclei that give rise to labeled fibers and terminals reaching the rostral medial and dorsomedial BPN subdivisions. Hypothalamopontine fibers extended caudally through the pontine tegmentum dorsal to the nucleus reticularis tegmenti pontis and then coursed ventrally from the main descending bundle toward the ipsilateral basilar pontine gray. Some hypothalamopontine fibers crossed the midline in the tegmental area just dorsal to the pontine gray to terminate in the contralateral BPN. Electron microscopy revealed that the ultrastructural features of synaptic boutons formed by axons arising in the LH, PH, MMN, and LMN are similar to one another. All labeled hypothalamopontine axon terminals contained round synaptic vesicles and formed asymmetric synaptic junctions with dendritic shafts as well as dendritic appendages, and occasionally with neuronal somata. Some labeled boutons formed the central axon terminal in a glomerular synaptic complex. In summary, the present findings indicate that the hypothalamus projects predominantly to the rostral medial and dorsomedial portions of the BPN which, in turn, provide input to the paraflocculus and vermis of the cerebellum. Since the hypothalamic projection zones in the BPN also receive cerebral cortical input, including limbic-related cortex, the hypothalamopontine system might serve to integrate autonomic or limbic-related functions with movement or somatic motor-related activity. Alternatively, since the cerebellum also receives direct input from the hypothalamus, the BPN may function to provide additional somatic and visceral inputs that are used by the cerebellum to perform the integrative function.

Location of muscarinic type 2 receptors within the synaptic circuitry of the cat visual thalamus.

A cholinergic projection from the parabrachial region (PBR) of the brainstem to the visual thalamus has been studied in great detail during the past 20 years. A number of physiological studies have demonstrated that this projection causes a dramatic change in thalamic activity during the transition from sleep to wakefulness. Additionally, the PBR may mediate more subtle changes in thalamic activity as attentional levels fluctuate during the waking state. The synaptic circuitry underlying these events has been identified in the cat thalamus. However, there is currently no anatomical information regarding the distribution of cholinergic receptors in relation to this circuitry. To begin to understand how the PBR projection modulates thalamic activity, we used immunocytochemical techniques to examine the distribution of muscarinic type 2 (M2) receptors in the visual thalamus of the cat. The distribution of M2 receptors correlates well with previous reports of the distribution of cholinergic terminals in the visual thalamus. At the light microscopic level, dense M2 staining was seen in the neuropil of the dorsal lateral geniculate nucleus (dLGN) and pulvinar nucleus and in somata and proximal dendrites of cells in the thalamic reticular nucleus (TRN). In the dLGN and pulvinar nucleus, we quantitatively analyzed the distribution of M2 receptors using electron microscopy. Postembedding immunocytochemistry for gamma aminobutyric acid (GABA) was used to determine whether M2 receptors are present on interneurons or thalamocortical cells. In particular, we examined the distribution of M2 receptors with respect to the known sites of PBR terminations. The dendrites of both thalamocortical cells and interneurons were stained for the M2 receptors in both the glomerular and extraglomerular neuropil. However, the densest staining was found in glomerular GABAergic profiles that displayed the morphology associated with interneuron dendritic terminals (F2 profiles). Our data suggest that M2 receptors play an important role both in blocking thalamic spindle oscillations and in increasing the efficacy of signal transmission during increased attentional states.

The "orphan" Na+/Cl(-)-dependent transporter, Rxt1, is primarily localized within nerve endings of cortical origin in the rat striatum.

Previous studies have shown that the striatum expresses very low levels of Na+/Cl(-)-dependent "orphan" transporter Rxt1 transcripts but contains high levels of protein. This study investigated the origin of Rxt1 expression in rat striatum. Striatal Rxt1 contents assessed by immunocytochemistry or western blotting were found to be significantly reduced after corticostriatal denervation but not after striatal or thalamic lesion with kainic acid or selective 6-hydroxydopamine-induced nigrostriatal deafferentation. Corticostriatal neurons retrogradely labeled by intrastriatal fluorogold injections were shown to express Rxt1 mRNA. Combination of anterograde biotin-dextran amine labeling of the corticostriatal pathway with Rxt1 immunogold detection at the ultrastructural level demonstrated the presence of Rxt1 in about one-third of the corticostriatal synaptic terminals and in numerous unidentified synaptic terminals. All the Rxt1-positive terminals formed asymmetrical contacts on spines. These data provide evidence that striatal Rxt1 immunoreactivity is mainly of extrinsic origin and more specifically associated with the corticostriatal pathway. Rxt1 appears as a selective presynaptic marker of synapses formed by presumably excitatory amino acid afferents, but it segregates a subclass of these synapses, thereby revealing a functional heterogeneity among excitatory amino acid systems.

Arborisation and termination of single motor thalamocortical axons in the rat.

The aim of this study was to examine the arborisations and terminations of individual thalamocortical axons in the motor system of the rat. Small, extracellular injections of an anterograde tracer (dextran-biotin) were made into the ventrolateral (VL) or ventral posterolateral (VPL) thalamic nuclei to label thalamocortical projections. Eleven motor axons and one somatosensory axon were reconstructed through serial sections just rostral from the injection site to their terminations in sensorimotor cortex. The smallest arbor arising from a single motor axon extended approximately 0.9 mm rostrocaudal and 0.9 mm mediolateral, the largest extended 3.9 mm rostrocaudal and 1.0 mm mediolateral. In some cases, two distinct plexuses of terminals were formed by an axon. In addition, motor axons formed terminals in cortical layer V only or in layers I, III, and V. By contrast (and in keeping with previous reports), the somatosensory axon formed a single plexus of terminals in layer IV of the cortex that extended approximately 0.3 mm rostrocaudal and 0.4 mm mediolateral. It is concluded that individual motor thalamocortical neurones are in a position to influence much more widespread cortical regions than somatosensory thalamocortical neurones.

Relationships between striatin-containing neurons and cortical or thalamic afferent fibres in the rat striatum. An ultrastructural study by dual labelling.

Striatin, a recently isolated rat brain calmodulin-binding protein belonging to the WD-repeat protein family, is thought to be part of a calcium signal transduction pathway presumably specific to excitatory synapses, at least in the striatum. This study was aimed to specify the cellular and subcellular localization of striatin, and to determine the possible synaptic relationships between the two main excitatory afferent pathways, arising from the cerebral cortex and the thalamus, and the striatin-containing elements, in the rat striatum. Anterograde tract-tracing by means of biotinylated dextran amine injection in the frontoparietal cerebral cortex or the parafascicular nucleus of the thalamus was combined with immunogold detection of striatin. Striatin-immunoreactivity was confined to the neuronal somatodendritic compartment, including spines. Whereas 90-95% of the striatal neurons were striatin-positive, only about 50% of the sections of dendritic spines engaged in asymmetrical synaptic contacts exhibited striatin labelling. Among the sections of striatin-immunopositive dendritic spines, the number of immunogold particles ranged from one to more than seven, indicating an heterogeneity of the spine labelling. Moreover, within each class of spines presenting at least two silver-gold particles, the distribution of the particles varied from a clear-cut alignment under the postsynaptic densities (24-33% of spines) to a location distant from the synaptic area. In the cell bodies and dendrites, striatin labelling was usually not associated with the cytoplasmic membrane nor with the postsynaptic densities. In the striatum ipsilateral to the tracer injections, only 34.8% of the synaptic contacts formed by corticostriatal afferents involved striatin-positive elements (slightly labelled dendritic spines), whereas 56.7% of the synaptic contacts formed by thalamostriatal boutons were made on striatin-positive targets (mostly dendrites). In both cases, striatin labelling was usually not associated with the postsynaptic density. Most of the immunoreactive dendritic spines were in contact with unidentified afferents. These data reveal that striatin is expressed in the vast majority of the cell bodies of striatal spiny neurons, but is heterogeneously distributed among the dendritic spines of those neurons. Data also indicate a preferential relationship between striatin-containing structures and afferents from the parafascicular thalamic nucleus with respect to the frontoparietal cerebral cortex. But, at the dendritic spine level, striatin may be involved in signal transduction mechanisms involving as yet unidentified excitatory afferents to striatal neurons.

Specific neurochemical derangements of brain projecting neurons in apolipoprotein E-deficient mice.

Apolipoprotein E (apoE)-deficient mice provide a useful system for studying the role of apoE in neuronal maintenance and repair. Previous studies revealed specific memory impairments in these mice that are associated with presynaptic derangements in projecting forebrain cholinergic neurons. In the present study we examined whether dopaminergic, noradrenergic, and serotonergic projecting pathways of apoE-deficient mice are also affected and investigated the mechanisms that render them susceptible. The densities of nerve terminals of forebrain cholinergic projections were monitored histochemically by measurements of acetylcholinesterase activity, whereas those of the dopaminergic nigrostriatal pathway, the noradrenergic locus coeruleus cortical projection, and the raphe-cortical serotonergic tract were measured autoradiographically using radioligands that bind specifically to the respective presynaptic transporters of these neuronal tracts. The results obtained revealed that synaptic densities of cholinergic, noradrenergic, and serotonergic projections in specific brain regions of apoE-deficient mice are markedly lower than those of controls. Furthermore, the extent of presynaptic derangement within each of these tracts was found to be more pronounced the further away the nerve terminal is from its cell body. In contrast, the nerve terminal density of the dopaminergic neurons that project from the substantia nigra to the striatum was unaffected and was similar to that of the controls. The rank order of these presynaptic derangements at comparable distances from the respective cell bodies was found to be septohippocampal cholinergic > nucleus basalis cholinergic > locus coeruleus adrenergic > raphe serotonergic > nigrostriatal dopaminergic, which interestingly is similar to that observed in Alzheimer's disease. These results suggest that two complementary factors determine the susceptibility of brain projecting neurons to apoE deficiency: pathway-specific differences and the distance of the nerve terminals from their cell body.

Cortical, thalamic, and amygdaloid projections of rat temporal cortex.

The cortical, thalamic, and amygdaloid connections of the rodent temporal cortices were investigated by using the anterograde transport of iontophoretically injected biocytin. Injections into area Te1 labeled axons and terminals in the ventral regions of the dorsal and ventral subnuclei of the medial geniculate complex, area Te3, the rostrodorsal part of area Te2, and the ventrocaudal caudate putamen. No amygdaloid labeling was observed. Thalamic projections from Te2 targeted the lateral posterior nucleus, the dorsal part of the dorsal subnucleus of the medial geniculate complex, and the peripeduncular nucleus. Corticocortical projections mainly terminated in the dorsal perirhinal cortex, but moderately dense projections were observed in medial and lateral peristriate cortex, and only light projections were observed to Te1 and Te3. Projections to these isocortical regions terminated in layers I and VI. Amygdaloid projections targeted the ventromedial subdivision of the lateral nucleus and the adjacent part of the anterior basolateral nucleus. Area Te3 was observed to project to the ventrolateral parts of the dorsal and ventral subnuclei of the medial geniculate complex, the dorsal perirhinal cortex, rostral Te2, and Te1. In the amygdala, labeled fibers and terminals were concentrated in the dorsolateral subdivision of the lateral nucleus. These data confirm that areas Te1 and Te3 are hierarchically organized cortical areas connected with auditory relay nuclei in the thalamus. Area Te2, in contrast, appears to be weakly connected with Te1 and Te3 but is heavily connected with the peristriate cortex and tectorecipient thalamic nuclei. Te2 appears to be a visually related cortical area. The data also indicate that projections from Te2 and Te3 target different subregions of the lateral nucleus and that Te2, but not Te3, projects to the basolateral nucleus.

Presynaptic terminals in hyaline cells of normal and overstimulated chick inner ears.

Hyaline cells are non-sensory epithelial cells of the vibrating part of the basilar membrane of chicks; they receive an extensive efferent innervation. Although these anatomical features suggest roles in auditory transduction, very little is known about the function of these cells. One possible way to understand function is by lesion experiments. We used synapsin-specific antibodies to study changes that occur in the pattern of efferent innervation in hyaline cells after lesion of the sensory epithelium induced by acoustic overstimulation. We found only small changes in hyaline cells after such trauma. These included a small increase in size and a small decrease in density of nerve terminals on hyaline cells. This suggests that hyaline cells and their nerve terminals are less susceptible to acoustic trauma than hair cells. Using neurofilament-specific antibodies we found little or no trauma-induced change in the density of nerve fibres that cross the basilar papilla and reach the hyaline cell region. This finding suggested that trauma to the hair cells does not necessarily lead to changes in the efferent fibres that cross the papilla and extend into the hyaline cell region. Using the trauma and the morphological parameters studied here, it appears that a moderate lesion in the hair cell region in the avian inner ear does not influence the hyaline cells or their innervation.

Kappa opioid receptor-like immunoreactivity is present in substance P-containing subcortical afferents in guinea pig dentate gyrus.

We have previously shown that kappa opioid receptor-like immunoreactivity (KT-LI) is present in axons and terminals in the granule cell layer and inner molecular layer of the guinea pig dentate gyrus. The distribution and ultrastructural appearance of processes with KT-LI were similar to those of the substance P (SP)-containing afferents which arise from the supramammillary region of the hypothalamus (SUM) and enter the hippocampal formation through the fimbria-fornix. The objective of the present study was to determine whether the terminals with KT-LI are likely to be SUM afferents. To accomplish this we 1) compared the intensity of KT- and SP-immunolabeling in the dentate gyrus ipsilateral and contralateral to a unilateral fornix transection and 2) used dual-labeling electron microscopy to determine whether terminals with KT-LI colocalize SP-LI in the dentate gyrus. Light microscopic examination of the dentate gyrus demonstrated that KT-LI and SP-LI were in thin processes with overlapping distributions in strata granulosum and moleculare. Following fornix transection, both KT-LI and SP-LI were dramatically reduced in these regions of the dentate gyrus ipsilateral to the transection, consistent with an SUM origin. By electron microscopy, most (71%) terminals with KT-LI also contained detectable SP-LI in single-section analysis. Many dual-labeled terminals formed thick asymmetric synaptic contacts with large dendritic shafts (2-5 microns) or granule cell perikarya, and a smaller proportion contacted dendritic spines; these characteristics resembled those of identified SUM afferents in other species. The demonstrations that 1) KT-LI colocalizes with SP-LI in a morphologically distinctive population of axon terminals and 2) most of the processes with KT-LI enter through the fimbria-fornix suggest that kappa opioid receptors are present in the SUM projection to the dentate gyrus.

Distribution of GABAergic elements postsynaptic to ventroposteromedial thalamic projections in layer IV of rat barrel cortex.

The spatial synaptic pattern formed by boutons, originating in the ventroposteromedial thalamic nucleus, with GABAergic neurons in the rat barrel cortex was mapped. The aim was to shed light on the structural basis by which inhibitory circuits may be activated at the first stage of cortical information processing. The thalamic afferent projection was labelled by anterograde transport of Phaseolus vulgaris leucoagglutinin (PHA-L), whereas the GABAergic targets in layer IV of the rat barrel cortex was visualized by postembedding GABA immunogold-labelling or by pre-embedding parvalbumin immunocytochemistry. In the first set of experiments, we mapped barrels, contained in single ultrathin sections, by means of a computer-controlled electron microscope stage in their entire layer IV representation. From a total of 1199 asymmetric PHA-L-labelled synapses, only 98 were on GABAergic elements, mainly on dendritic shafts. This corresponded to 8.2% of all synapses counted. These synapses on GABAergic targets were essentially homogeneously distributed without a reliable relationship to barrel subdivisions, i.e., hollow versus wall; or layer IVa versus layer IVb. In the second part of the study, we demonstrated that parvalbumin-containing neurons represent the major GABAergic cell type targetted by thalamic afferents in layer IV of the barrel cortex, since all parvalbumin-positive cells investigated received multiple synaptic contacts (up to eight synapses per neuron) from the ventroposteromedial thalamic nucleus. These results imply that interneurons responsible for perisomatic inhibition (basket and chandelier cells known to contain parvalbumin) are likely to be strongly excited by thalamic afferents, despite the relatively low proportion of thalamic synapses on GABAergic elements compared to spines of principal cells, and participate in the early stages of cortical sensory information processing.

Thalamic collaterals of corticostriatal axons: their termination field and synaptic targets in cats.

Branched cortical projections to the thalamus and striatum were investigated in cats by injecting the retrograde-anterograde tracer biotinylated-dextran amine (BDA) into the caudate nucleus. These injections gave rise to plexuses of labeled fibers and varicosities in widespread thalamic territories. For instance, the lateroposterior nucleus and pulvinar (LP-PUL) mostly contained thick axons that contributed clusters of large-sized varicosities, each forming multiple asymmetric synapses, usually with vesicle-filled dendrites. In contrast, the intralaminar nuclei mostly contained thin axonal segments that emitted small en passant varicosities that formed single asymmetric synapses with spines. Because the caudate nucleus does not project to the thalamus, this labeling had to arise from a neuronal population with branching axons to both structures. Previous findings pointed to three possible sources: brainstem monoaminergic cells, intralaminar thalamic neurons, and corticostriatal cells. The first candidate could be ruled out because monoaminergic neurons contribute small-sized terminals that usually lack membrane specializations. The second possibility was discarded because retrograde tracer injections into the LP-PUL did not give rise to retrograde labeling in the intralaminar nuclear complex but to massive retrograde labeling in deep layers of cortical areas 5 and 7. Therefore, we concluded that the thalamic anterograde labeling originated from corticostriatal neurons, with axons branching to the thalamus. In keeping with this conclusion, Phaseolus vulgaris-leucoagglutinin (PHA-L) injections into cortical areas 5-7 labeled a group of thick corticothalamic fibers that ended in clusters of large boutons in the LP-PUL. These PHA-L-positive terminals were indistinguishable from those labeled after injections of BDA into the caudate nucleus, but they were easy to distinguish from the typical corticothalamic fibers. These findings indicate that the cerebral cortex could coordinate the activity of the striatum and the thalamus via a rich axonal network that collateralizes to both structures. The extent and synaptic organization of this branched projection impose a revision of the traditional scheme of thalamic connectivity.

Fine structural organization of spinothalamic and trigeminothalamic lamina I terminations in the nucleus submedius of the cat.

We examined lamina I trigemino- and spinothalamic tract (TSTT) terminals labeled with Phaseolus vulgaris leucoagglutinin in the nucleus submedius (Sm), a nociceptive relay in the cat's thalamus. Volume-rendered (three-dimensional) reconstructions of ten lamina I TSTT terminals identified with light and electron microscopy were built from serial ultrathin sections by computer, which enabled the overall structures of the terminal complexes to be characterized in detail. Two fundamentally different terminations were observed: compact clusters of numerous boutons, which predominate in the dense focus of a lamina I terminal field in the Sm, and boutons-of-passage, which are present throughout the terminal field and predominate in its periphery. Reconstructions of cluster terminations reveal that all boutons of each cluster make synaptic contact with protrusions and branch points on a single dendrite and involve presynaptic dendrites (PSDs) in triadic arrangements, providing a basis for the secure relay of sensory information. In contrast, reconstructions show that boutons-of-passage are generally characterized by simple contacts with PSDs, indicating an ascending inhibitory lamina I influence. These different synaptic arrangements are consistent with physiological evidence indicating that the morphologically distinct nociceptive-specific and thermoreceptive-(cold)-specific lamina I TSTT neurons terminate differently within the Sm. Thus, a suitable structural substrate exists in the cat's Sm for the inhibitory effect of cold on nociception, a behavioral and physiological phenomenon of fundamental significance. We conclude that the Sm is more than a simple relay for nociception, and that it may be an integrative comparator of ascending modality-selective information that arrives from neurons in lamina I.

Individual axon morphology and thalamocortical topography in developing rat somatosensory cortex.

The morphology of individual thalamocortical axons in developing rat primary somatosensory cortex was studied using lipophilic tracers. Anterograde labeling with lipophilic dyes demonstrated a topographical organization of thalamocortical projections exiting the thalamus as early as embryonic day (E) 16; retrograde labeling studies demonstrated topography of these projections as they reached the cortex as early as E18. At E17, axons course tangentially within the intermediate zone and turn or branch near the deepest layer of cortex (layer VIb), suggesting the presence of guidance cues in this region. Axons appear to grow and branch progressively within layers VIb and VIa during the following days; axons in the intermediate zone may give rise to radially directed branches. Individual axons appear to grow steadily and progressively into the cortex, with the leading front of axons at the transition zone between the cortical plate (CP) and the differentiating cortical layers. At birth (P0), thalamocortical axons extend radially through layers VIa and V and emit branches within these layers; some axons reach the CP. By P1, layer IV has begun to differentiate and axons begin to form a few simple branches in the vicinity of the layer IV cells. Over the ensuing week, axons generate more branches within layer IV, but the tangential extent of individual axon arbors does not exceed the width of a barrel. By P7, individual axons overlap within barrel clusters, and individual axons span the width of a cluster. These observations indicate that thalamic afferents develop by progressive growth of arbors that remain spatially restricted, rather than by overbranching and retracting arbors.

Immunohistochemical localization of calcitonin gene-related peptide in the terete nucleus of the rat hypothalamus.

The topographical distribution of calcitonin gene-related peptide (CGRP) containing neuronal elements in the posterior hypothalamus has been re-examined by means of indirect labeled immunohistochemistry in the rat. In contrast to previous findings, no CGRP-like immunoreactive neuronal perikarya were present in the premamillary nuclei, but they form a small group of cells, which is identical to the recently described terete nucleus. In coronal sections, this small and rounded cluster of intermingled CGRP-immunopositive perikarya and fibers is situated in the lateral hypothalamus at the premamillary level, ventrolateral to the fornix. Perikarya, dendrites, presynaptic terminals, as well as nonmyelinated and some myelinated axons were labeled by CGRP-like immunoreactive material within and in the immediate vicinity of the terete nucleus. On some of the CGRP-positive neurons, neuropeptide Y-immunoreactive axon terminals establish synaptic contacts.

Axon terminals immunolabeled for dopamine or tyrosine hydroxylase synapse on GABA-immunoreactive dendrites in rat and monkey cortex.

Dopamine afferents to the cortex regulate the excitability of pyramidal neurons via a direct synaptic input. However, it has not been established whether dopamine also modulates pyramidal cell activity indirectly through synapses on gamma-aminobutyric acid (GABA) interneurons, and whether such inputs differ across cortical regions and species. We sought to address these issues by an immunocytochemical electron microscopic approach that combined peroxidase staining for dopamine or tyrosine hydroxylase (TH) with a pre-embedding gold-silver marker for GABA. In the deep layers of the rat prefrontal cortex and in the superficial layers of the monkey prefrontal and primary motor cortices, terminal varicosities immunoreactive for dopamine or TH formed primarily thin, symmetric synapses on distal dendrites. Both GABA-immunoreactive dendrites as well as unlabeled spines and dendrites were contacted by dopamine- or TH-immunoreactive terminals. Synaptic specializations were detected at some, but not all of these contacts. The relative frequency of these appositional and synaptic contacts did not appear to differ between the rat and monkey prefrontal cortex, or between the monkey prefrontal and motor cortices. Across regions and species, labeled and unlabeled targets of dopamine- or TH-positive terminals received additional synaptic input from unlabeled, and occasionally GABA-immunoreactive terminals. Close appositions between dopamine- or TH-immunoreactive and GABA-positive terminals were observed only rarely. These findings indicate that dopamine afferents provide direct synaptic inputs to GABA local circuit neurons in a consistent fashion across cortical regions and species. Thus, dopamine's cellular actions involve direct as well as modulatory effects on both GABA interneurons and pyramidal projection neurons.

Morphology of physiologically identified retinal X and Y axons in the cat's thalamus and midbrain as revealed by intraaxonal injection of biocytin.

Prior morphological studies of individual retinal X and Y axon arbors based on intraaxonal labeling with horseradish peroxidase have been limited by restricted diffusion or transport of the label. We used biocytin instead as the intraaxonal label, and this completely delineated each of our six X and 14 Y axons, including both thalamic and midbrain arbors. Arbors in the lateral geniculate nucleus appeared generally as has been well documented previously. Interestingly, all of the labeled axons projected a branch beyond thalamus to the midbrain. Each X axon formed a terminal arbor in the pretectum, but none continued to the superior colliculus. In contrast, 11 of 14 Y axons innervated both the pretectum and the superior colliculus, one innervated only the pretectum, and two innervated only the superior colliculus. Two of the Y axons were quite unusual in that their receptive fields were located well into the hemifield ipsilateral with respect to the hemisphere into which they were injected. These axons exhibited remarkable arbors in the lateral geniculate nucleus, diffusely innervating the C-laminae and medial interlaminar nucleus, but, unlike all other X and Y arbors, they did not innervate the A-laminae at all. In addition to these qualitative observations, we analyzed a number of quantitative features of these axons in terms of numbers and distributions of terminal boutons. We found that Y arbors contained more boutons than did X arbors in both thalamus and midbrain. Also, for axons with receptive fields in the contralateral hemifield (all X and all but two Y axons), 90-95% of their boutons terminated in the lateral geniculate nucleus; the other two Y axons had more of their arbors located in midbrain.