Projections of auditory cortex to the medial geniculate body of the cat.

The corticofugal projection from 12 auditory cortical fields onto the medial geniculate body was investigated in adult cats by using wheat germ agglutinin conjugated to horseradish peroxidase or biotinylated dextran amines. The chief goals were to determine the degree of divergence from single cortical fields, the pattern of convergence from several fields onto a single nucleus, the extent of reciprocal relations between corticothalamic and thalamocortical connections, and to contrast and compare the patterns of auditory corticogeniculate projections with corticofugal input to the inferior colliculus. The main findings were that (1) single areas showed a wide range of divergence, projecting to as few as 5, and to as many as 15, thalamic nuclei; (2) most nuclei received projections from approximately five cortical areas, whereas others were the target of as few as three areas; (3) there was global corticothalamic-thalamocortical reciprocity in every experiment, and there were also significant instances of nonreciprocal projections, with the corticothalamic input often more extensive; (4) the corticothalamic projection was far stronger and more divergent than the corticocollicular projection from the same areas, suggesting that the thalamus and the inferior colliculus receive differential degrees of corticofugal control; (5) cochleotopically organized areas had fewer corticothalamic projections than fields in which tonotopy was not a primary feature; and (6) all corticothalamic projections were topographic, focal, and clustered, indicating that areas with limited cochleotopic organization still have some internal spatial arrangement. The areas with the most divergent corticothalamic projections were polysensory regions in the posterior ectosylvian gyrus. The projection patterns were indistinguishable for the two tracers. These findings suggest that every auditory thalamic nucleus is under some degree of descending control. Many of the projections preserve the relations between cochleotopically organized thalamic and auditory areas, and suggest topographic relations between nontonotopic areas and nuclei. The collective size of the corticothalamic system suggests that both lemniscal and extralemniscal auditory thalamic nuclei receive significant corticofugal input.

GABAergic organization of the cat medial geniculate body.

A study of neurons and processes (puncta) immunolabeled by antibodies to gamma-aminobutyric acid (GABA) or glutamic acid decarboxylase was undertaken in the medial geniculate body of the adult cat. The proportion and types of GABAergic cells were determined with high resolution methods, including postembbedding immunocytochemistry on semithin plastic sections. A second goal was to draw parallels and differences between the auditory thalamus and other thalamic nuclei. Finally, the types of GABAergic puncta and their concentration in the three major subdivisions of the medial geniculate body were analyzed. The results were that (1) each division had many GABAergic neurons, averaging approximately 26% of the neuronal population; (2) the ventral division had the highest proportion of these cells (33%), the medial division the fewest (18%), and the dorsal division was intermediate (26%); (3) there was a gradient in the proportion of GABAergic neurons, i.e., the ventral and medial division values increased caudorostrally, whereas the value in the dorsal division declined; (4) the predominant GABAergic cell type in each division was a small neuron with a soma approximately 10-12 microm in diameter; (5) a small population of much larger GABAergic neurons was present mainly in the dorsal division; (6) in addition to the fine, granular puncta in each division, a type of giant GABAergic puncta was found only in the dorsal division nuclei. The results obtained with the two antibodies were essentially identical. These findings suggest a structural basis for qualitative differences in the distribution of GABAergic processing within the medial geniculate complex. The GABAergic arrangement in the ventral division was stereotyped, with only one type of putative GABAergic interneuron, and the puncta were correspondingly homogeneous. In contrast, the dorsal division had two types of GABAergic neurons, and the giant GABAergic puncta represent a new substrate for inhibitory interactions. The medial division also had more than one type of GABAergic neuron and a slightly lower concentration of puncta. These qualitative and quantitative distinctions suggest a morphologic basis for possible differences in inhibitory processing among medial geniculate body subdivisions.

Two systems of giant axon terminals in the cat medial geniculate body: convergence of cortical and GABAergic inputs.

The thalamus plays a critical role in processing sensory information that involves interactions between extrinsic connections and intrinsic circuitry. Little is known regarding how these different systems might interact. We found an unexpected nuclear convergence of two types of giant axon terminals, each of which must have independent origins, in the dorsal division of the cat medial geniculate body. The first class of giant terminal was labeled after injections of biotinylated dextran amines (BDA) in seven auditory cortical areas. A second type was found in sections immunostained for gamma-aminobutyric acid (GABA); these endings had the same nuclear distribution, and they were numerous. The origin of this GABAergic terminal is unknown. The giant corticothalamic terminals were presumably those described in prior accounts using different tracers (Rouiller and de Ribaupierre [1990] Neurosci. Lett. 208:29-35; Ojima [1994] Cerebral Cortex 6:646-663), but with BDA they are labeled more fully. Clusters of such endings were often linked, and hundreds may occur in a single section. Their boutons formed a substantial proportion of the corticothalamic population. Other types of corticogeniculate axon terminals were also labeled, including two kinds that are much smaller and that match closely the classical descriptions of corticothalamic axons. The giant GABAergic endings were found in all dorsal division nuclei and in thalamic visual nuclei such as the lateral posterior nucleus. Like the giant cortical endings, the giant GABAergic terminals often encircled large, pale, immunonegative profiles that may be dendritic. This implies a close spatial, and perhaps a close functional, relationship between the populations of giant axon terminals. Insofar as physiological studies found that pharmacological inactivation of rat somatic sensory cortex suppresses peripheral information transmission through the posterior thalamus, corticofugal input may be essential for normal processing (Diamond et al. [1992] J. Comp. Neurol. 319:66-84). Our findings suggest that the giant corticothalamic endings could play an important role in descending control. Perhaps they are counterbalanced by a GABAergic system and affect thalamic oscillations implicated in shifts in vigilance and attention.

Neural architecture of the rat medial geniculate body.

The rat medial geniculate body was subdivided using Nissl preparations to establish nuclear boundaries, with Golgi-Cox impregnations to identify projection and local circuit neurons, and in fiber stained material to delineate the fiber tracts and their distribution. Three divisions were recognized (ventral, dorsal and medial): the first two had subdivisions. The ventral division had lateral and medial parts. The main cell type had bushy tufted dendrites which, with the afferent axons, formed fibrodendritic laminae oriented from dorso-lateral to ventro-medial; such laminae were not as regular medially, in the ovoid nucleus. The dorsal division contained several nuclei (dorsal superficial, dorsal, deep dorsal, suprageniculate, and ventrolateral) and neurons with radiating or bushy dendrites; the nuclear subdivisions differed in the concentration of one cell type or another, and in packing density. A laminar organization was present only in the dorsal superficial nucleus. Medial division neurons were heterogeneous in size and shape, ranging from tiny cells to magnocellular neurons; the various cell types intermingled. so that no further subdivision could be made. This parcellation scheme was consistent with, and supported by, the findings from plastic embedded or fiber stained material. There were very few small neurons with locally ramifying axons and which could perform an intrinsic role like that of Golgi type II cells. Their rarity was consistent with the small number of such profiles in plastic embedded or Nissl material and the few GABAergic medial geniculate body neurons seen in prior immunocytochemical work. While similar neuronal types and nuclear subdivisions are recognized in the rat and cat, there may be major interspecific differences with regard to interneuronal organization in the auditory thalamus whose functional correlates are unknown.

Origins of medial geniculate body projections to physiologically defined zones of rat primary auditory cortex.

Medial geniculate body neurons projecting to physiologically identified subregions of rat primary auditory cortex (area 41, Te1) were labeled with horseradish peroxidase in adult rats. The goals were to determine the type(s) of projection neuron and the spatial arrangement of these cells with respect to thalamic subdivisions. Maps of best frequency were made with single neuron or unit cluster extracellular recording at depths of 500-800 microm, which correspond to layers III-IV in Nissl preparations. Tracer injections were made in different cortical isofrequency regions (2, 11, 22, or 38 kHz, respectively). Labeled neurons were plotted on representative sections upon which the architectonic subdivisions were drawn independently. Most of the cells of origin lay in the ventral division in every experiment. Injections at low frequencies labeled bands of neurons laterally in the ventral division; progressively more rostral deposits at higher frequencies labeled bands or clusters more medially in the ventral division, and through most of its caudo-rostral extent. Medial division labeling was variable. Labeled cells were always in the lateral half of the nucleus and were often scattered. There were few labeled cells in the dorsal division. Seven types of thalamocortical neuron were identified: ventral division cells had a tufted branching pattern, while medial division neurons have heterogeneous shapes and sizes and were larger. Dorsal division neurons had a radiate branching pattern. The size range of labeled neurons spanned that of Nissl stained neuronal somata. Area 41 may receive two types of thalamic projection: ventral division input is strongly convergent, highly topographic, spatially focal, and restricted to one type of neuron only, while the medial division projection is more divergent, coarsely topographical, involves multiple cortical areas, and has several varieties of projection neuron. Despite species differences in local circuitry, many facets of thalamocortical organization are conserved in phylogeny.

Auditory connections and neurochemistry of the sagulum.

We studied the cytoarchitecture, neurochemical organization, and connections of the sagulum. The goal was to clarify its role in midbrain, lateral tegmental, and thalamic auditory processing. On cytoarchitectonic grounds, ventrolateral (parvocellular) and dorsomedial (magnocellular) subdivisions were recognized. The patterns of immunostaining for gamma-aminobutyric acid (GABA) and glycine were distinct. Approximately 5-10% of the neurons were GABAergic, and more than one type was identified; GABAergic axon terminals were abundant in number and varied in form. Glycinergic neurons were much rarer, < 1% of the population, and glycinergic axon terminals were correspondingly sparse. Wheat germ agglutinin conjugated to horseradish peroxidase was used for purposes of connectional mapping, and biotinylated dextran amines revealed the structure of corticosagular axons. All nine cortical areas injected project to the ipsilateral sagulum. Five (areas AI, AII, SF, EPD, and Te) had heavier projections than the others. Areas AI and AII projected throughout the rostrocaudal sagulum. Labeling from AI was moderate in density and concentrated in the central sagulum, whereas the input from AII was heavier and ended more laterally. Suprasylvian fringe input was light, especially caudally, and was chiefly in the central sagulum. The projection from the dorsal region of the posterior ectosylvian gyrus was comparatively stronger and was in the dorsolateral sagulum. Finally, the temporal cortex sent axons to the most lateral sagulum, spanning the dorsoventral extent, whereas insular cortex axons ended diffusely in the dorsolateral sagulum. Corticofugal axons ranged from fine boutons en passant to larger globular terminals. The sagulum may represent the earliest significant opportunity in the ascending auditory pathway for corticofugal modulation. The most extensive input arises from the polymodal association areas. The sagulum then projects divergently to the dorsal cortex of the inferior colliculus and the dorsal division of the medial geniculate body. The projection from the dorsal division of the auditory thalamus to nonprimary auditory cortex completes this circuit between the forebrain and the midbrain and represents a nexus in the ascending and descending auditory systems. Such circuits could play a critical role in auditory-motor adjustments to sound.

Auditory cortical projections to the cat inferior colliculus.

The projection from 11 auditory cortical areas onto the subdivisions of the inferior colliculus was studied in adult cats by using two different anterograde tracers to label cortico-collicular (CC) axon terminals. The main results were that: 1) a significant CC projection arose from every field; 2) the principal inferior collicular targets were the dorsal cortex, lateral nucleus, caudal cortex, and intercollicular tegmentum, with only a sparse projection to the central nucleus; 3) the input was usually bilateral, with the ipsilateral side by far the most heavily labeled, and the contralateral projection was a symmetrical subset of the ipsilateral input; 4) the CC system is both divergent and convergent, with single cortical areas projecting to six or more collicular subdivisions, and each auditory midbrain subdivision receiving a convergent projection from two to ten cortical areas; 5) cortical areas devoid of tonotopic organization have topographic projections to collicular target nuclei; 6) the heaviest CC projection terminated in the caudal half of the inferior colliculus; and finally, 7) the relative strength of the cortico-collicular labeling was far less than that of the corresponding corticothalamic projection in the same experiments. The CC system is strategically placed to influence both descending and ascending pathways arising in the inferior colliculus. Nuclei that participate in the premotor system, like the inferior collicular subdivisions that project to the pons, receive substantial corticofugal input. Both the dorsal (pericentral) and the lateral (external) nuclei of the inferior colliculus project to parts of the medial geniculate body whose closest auditory affiliations are with non-tonotopic cortical regions involved in higher order auditory perception. The cortico-collicular system may link brainstem and colliculo-thalamic circuits to coordinate premotor and perceptual aspects of hearing.

Postembedding immunocytochemistry of large sections of brain tissue: an improved flat embedding technique.

A method for osmicating, dehydrating, and flat-embedding large slabs of brain tissue in epoxy resin is presented. This permits the production of semithin sections for postembedding immunocytochemistry that are far larger than can be obtained with other embedding approaches. Vibratomed slabs, 50-200 microns thick and as large as 6 x 8 mm are embedded in a 'soft' Araldite epoxy. The slabs are laminated onto the flat surface of a pre-cast epoxy slide. After polymerization, the tissue can be studied on the slide as a whole mount to view osmicated fiber tracts or, in experimental tract tracing studies, to locate retrogradely labeled cells before semithin sections are cut. The rigidity of the epoxy slide ensures that the slabs remain flat and are easily removed and mounted for resectioning. Semithin sections are cut using 6 mm wide glass knives or a 6 mm wide diamond knife and are mounted singly or in serial pairs and immunostained using conventional etching and immunoperoxidase techniques. The relative softness of the epoxy permits dozens of semithin sections to be cut from large blocks without appreciably degrading a glass knife edge. After further polymerization the embedment is also compatible with electron microscopy.

GABAergic feedforward projections from the inferior colliculus to the medial geniculate body.

A novel and robust projection from gamma-aminobutyric acid-containing (GABAergic) inferior colliculus neurons to the media] geniculate body (MGB) was discovered in the cat using axoplasmic transport methods combined with immunocytochemistry. This input travels with the classical inferior colliculus projection to the MGB, and it is a direct ascending GABAergic pathway to the sensory thalamus that may be inhibitory. This bilateral projection constitutes 10-30% of the neurons in the auditory tectothalamic system. Studies by others have shown that comparable input to the corresponding thalamic visual or somesthetic nuclei is absent. This suggests that monosynaptic inhibition or disinhibition is a prominent feature in the MGB and that differences in neural circuitry distinguish it from its thalamic visual and somesthetic counterparts.

Evolution of GABAergic circuitry in the mammalian medial geniculate body.

Many features in the mammalian sensory thalamus, such as the types of neurons, their connections, or their neurotransmitters, are conserved in evolution. We found a wide range in the proportion of gamma-aminobutyric acidergic (GABAergic) neurons in the medial geniculate body, from <1% (bat and rat) to 25% or more (cat and monkey). In the bat, some medial geniculate body subdivisions have no GABAergic cells. Species-specific variation also occurs in the somesthetic ventrobasal complex. In contrast, the lateral geniculate body of the visual system has about the same proportion of GABAergic cells in many species. In the central auditory pathway, only the medial geniculate body shows this arrangement; the relative number of GABAergic cells in the inferior colliculus and auditory cortex is similar in each species. The range in the proportion of GABAergic neurons suggests that there are comparative differences in the neural circuitry for thalamic inhibition. We conclude that the number of GABAergic neurons in thalamic sensory nuclei may have evolved independently or divergently in phylogeny. Perhaps these adaptations reflect neurobehavioral requirements for more complex, less stereotyped processing, as in speech-like communication.

Postembedding immunocytochemistry of large sections of brain tissue: an improved flat-embedding technique.

A method for osmicating, dehydrating, and flat-embedding large slabs of brain tissue in epoxy resin is presented. This permits the production of semithin sections for postembedding immunocytochemistry that are far larger than can be obtained with other embedding approaches. Vibratomed slabs, 50-200 microns thick and as large as 6 x 8 mm are embedded in a 'soft' Araldite epoxy. The slabs are laminated onto the flat surface of a pre-cast epoxy slide. After polymerization, the tissue can be studied on the slide as a whole mount to view osmicated fiber tracts, or in experimental tract-tracing studies, to locate retrogradely labeled cells before semithin sections are cut. The rigidity of the epoxy slide ensures that the slabs remain flat and are easily removed and mounted for resectioning. Semithin sections are cut using 8 mm wide glass knives or a 6 mm wide diamond knife and are mounted singly or in serial pairs and immunostained using conventional etching and immunoperoxidase techniques. The relative softness of the epoxy permits dozens of semithin sections to be cut from large blocks without appreciably degrading a glass knife edge. After further polymerization the embedment is also compatible with electron microscopy.

GABA and glycine in the central auditory system of the mustache bat: structural substrates for inhibitory neuronal organization.

The distribution and morphology of neurons and axonal endings (puncta) immunostained with antibodies to gamma-aminobutyric acid (GABA) and glycine (Gly) were analyzed in auditory brainstem, thalamic, and cortical centers in the mustache bat. The goals of the study were (1) to compare and contrast the location of GABAergic and glycinergic neurons and puncta, (2) to determine whether nuclei containing immunoreactive neurons likewise have a similar concentration of puncta, (3) to assess the uniformity of immunostaining within a nucleus and to consider regional differences that were related to or independent of cytoarchitecture, and (4) to compare the patterns recognized in this bat with those in other mammals. There are nine major conclusions. (1) Glycinergic immunostaining is most pronounced in the hindbrain. (2) In the forebrain, GABA alone is present. (3) Some nuclei have GABAergic or glycinergic neurons exclusively; a few have neither. (4) Although there is sometimes a close relationship between the relative number of immunopositive neurons and the density of the puncta, just as often there is no particular correlation between them; this reflects the fact that many GABAergic and glycinergic neurons project beyond their nucleus of origin. (5) Even nuclei devoid of or with few GABAergic or glycinergic neurons contain relatively abundant numbers of puncta; some neurons receive axosomatic terminals of each type. (6) In a few nuclei there are physiological subregions with specific local patterns of immunostaining. (7) The patterns of immunostaining resemble those in other mammals; the principal exceptions are in nuclei that, in the bat, are hypertrophied (such as those of the lateral lemniscus) and in the medial geniculate body. (8) Cellular colocalization of GABA and Gly is specific to only a few nuclei. (9) GABA and glutamic acid decarboxylase (GAD) immunostaining have virtually identical distributions in each nucleus. Several implications follow. First, the arrangements of GABA and Gly in the central auditory system represent all possible patterns, ranging from mutually exclusive to overlapping within a nucleus to convergence of both types of synaptic endings on single neurons. Second, although both transmitters are present in the hindbrain, glycine appears to be dominant, and it is often associated with circuitry in which precise temporal control of aspects of neuronal discharge is critical. Third, the auditory system, especially at or below the level of the midbrain, contains significant numbers of GABAergic or glycinergic projection neurons. The latter feature distinguishes it from the central visual and somatic sensory pathways.

Projections of physiologically defined subdivisions of the inferior colliculus in the mustached bat: targets in the medial geniculate body and extrathalamic nuclei.

This study examined the output of the central nucleus of the inferior colliculus to the medial geniculate body and other parts of the nervous system in the mustached bat (Pteronotus parnellii). Small deposits of anterograde tracers (horseradish peroxidase, [3H]leucine, Phaseolus vulgaris leucoagglutinin, wheat germ agglutinin conjugated to horseradish peroxidase, or biocytin) were made at physiologically defined sites in the central nucleus representing major components of the bat's echolocation signal. The topography, frequency specificity, and axonal morphology of these outputs were studied. The medial geniculate body was a major target of inferior collicular neurons, with three distinct input patterns. The projection to the ventral division was tonotopically organized, but had a relatively sparse contribution from neurons representing frequency modulated components of the biosonar pulse. The second input was to the rostral medial geniculate body, in which projections from inferior collicular neurons representing constant frequency sonar components were separated from those representing frequency modulated components. A third input was to the suprageniculate nucleus, which received strong, topographically arranged projections. Inputs to the dorsal nucleus and medial division were also observed. Extrathalamic regions receiving input included the pontine gray, external nucleus of the inferior colliculus, pericollicular tegmentum, nucleus of the brachium of the inferior colliculus, and pretectum. These central nucleus projections differed in organization and the structure of axon terminals, suggesting different physiological influences on their target nuclei. These results demonstrate that the central nucleus has divergent projections to various sensory and premotor nuclei, besides its well-established projection to the medial geniculate body.

Patterns of GABAergic immunoreactivity define subdivisions of the mustached bat's medial geniculate body.

The anatomy and the spatial distribution of neurons and axonal endings (puncta) immunoreactive for glutamic acid decarboxylase (GAD) or gamma-aminobutyric acid (GABA) were studied in the medial geniculate body of the mustached bat (Pteronotus parnellii). The principal findings are that: 1) most GABAergic neurons are present in the dorsal and ventral divisions with few, if any, in the medial division; 2) only a small fraction, about 1% or less, of auditory thalamic neurons are immunopositive; 3) the density of immunoreactive puncta is independent on the number of GABAergic neurons in the thalamic divisions, with the ventral division having the largest number/unit area, the medial division about 75% of this value, and the dorsal division only about 50%; and 4) the form of the puncta was unique to each division, those in the ventral division being medium-sized and comparatively simple, those in the medial division predominantly large, coarse, and complex, while dorsal division ending were finer and more delicate. These patterns recapitulate, with some significant exceptions, those found in the rat and cat. The puncta could originate from several sources; while many may arise from intrinsic GABAergic Golgi type II local circuit neurons, these cells may not be the only or even the principal source. Thus, the dorsal division contains comparatively many immunopositive cells though fewer puncta than might be expected if the bulk of these were to arise from auditory thalamic interneurons. This suggests that other, extrinsic sources, such as the thalamic reticular nucleus, may be the source of such endings. A second point is that the form and density of the puncta is regionally specific within the medial geniculate complex. These local patterns might have a significant and regionally specific role in controlling the differential excitability of auditory thalamic neurons. The distribution of presumptive synaptic endings also has implications for the number and arrangement of glomeruli or synaptic nests. Thus, these circuit elements, which are common to the thalamic nuclei in other species, might play an important role in local synaptic circuits between different types of cells. If so, then the structural variations embodied in these patterns could subserve functional arrangements that differ among species. Such patterns might reflect concomitant physiological differences in the organization of local circuits within the microchiropteran medial geniculate body.

Detection of somatic DNA recombination in the transgenic mouse brain.

A DNA construct containing the bacterial beta-galactosidase gene (lacZ) was used to study somatic DNA recombination in the transgenic mouse brain. Recombination-positive areas of the adult brain were stained blue with X-gal, a substrate of beta-galactosidase. Blue-colored cells appeared soon after birth, and continued to emerge in postnatal tissue. Staining was prominent in sensory as opposed to motor regions of the brain, and was present in more than 70 discrete areas of the nervous system. The possibility of DNA rearrangement is discussed with respect to the development of the central nervous system.

Populations of GABAergic neurons and axons in layer I of rat auditory cortex.

Neurons and axon terminals (puncta) immunostained by an antibody against glutamic acid decarboxylase were studied in layer I of adult rats in architectonically identified area 41 of auditory cortex. The borders of area 41 and the laminar subdivisions of cortex were established in normal material and in other studies of cortical connections. Vibratomed or frozen sections were immunostained. The objectives of the study were to classify the types of (i) glutamic acid decarboxylase-positive neurons and (ii) puncta, and (iii) to examine their spatial distribution within layer I. Control sections were devoid of specific immunostaining. More than 90% of layer I cells are glutamic acid decarboxylase-positive. Four types of neuron were identified in Golgi material, including small neurons with stellate dendritic fields, horizontal cells with laterally projecting arbors, medium-sized neurons with stellate, widely ramifying dendritic fields, and large neurons with broad dendritic fields spanning the depth of layer I or branching laterally. In the glutamic acid decarboxylase material, examples with a somatodendritic shape matching each of these types were found. The average somatic diameter of glutamic acid decarboxylase-positive neurons (mean = 59 microns2, S.D. = 21 microns2) suggests that the small and medium-sized neurons predominate. Glutamic acid decarboxylase-positive neurons occur throughout the depth of layer I, but are far more numerous in the deeper half (68% in layer Ib) than in the superficial part (32% in layer Ia). Glutamic acid decarboxylase-positive neurons form small clusters of three to five cells across the cortical surface, with a range of 0-9/100 microns across the cortex. Most glutamic acid decarboxylase-positive neuronal perikarya were intensely immunostained, and the dendrites of the medium-sized and large neurons were traced as far as 50-75 microns beyond their initial branching point. Glutamic acid decarboxylase-positive puncta also had variable shapes. Both small, fine puncta (less than 0.5 micron in diameter) and larger, coarser ones (greater than 1.5 micron in diameter) were present, though the former were much more common. In traverses from the pia to the layer II border, the puncta average about 40/100 microns2 (range: 20-80), and the shape of individual pia--layer II traverses is multipeaked, often with a slight trough at congruent to 80 microns depth, then rising slowly in number toward layer II.(ABSTRACT TRUNCATED AT 400 WORDS)

Anatomy of glutamic acid decarboxylase immunoreactive neurons and axons in the rat medial geniculate body.

This is a study of the form, density, and distribution of glutamic acid decarboxylase (GAD) immunoreactive neurons and puncta (axon terminals) in the adult rat medial geniculate complex. GAD-positive elements were stained by either the peroxidase-antiperoxidase or avidin-biotin procedures. Thalamic architectonic subdivisions were defined independently in Golgi, Nissl, plastic-embedded semi-thin, and fiber-stained preparations, and from investigations of medial geniculate connectivity. GAD-positive neurons represent only approximately 1% of medial geniculate neurons. They occur in the three major medial geniculate subdivisions (ventral, dorsal, and medial). There is variability between subdivisions in the form and number of such neurons, and among the puncta. In the ventral division, immunopositive somata may have sparsely branched dendrites as long as 300-400 microns and capped with varicose expansions or bouton-like sprays of appendages. These closely appose the somata or primary dendrites of other cells; the axons of these GAD-positive neurons are also immunostained. In the dorsal division there are fewer GAD-positive neurons and their structure is different. Their dendrites are rarely immunoreactive for more than 100-150 microns; nor can their immunostained axons be traced very far. In the medial division the number of GAD-positive neurons, considering the relatively small size of this division, was high. These neurons rarely have immunostained dendrites, and more than one type of neuron is immunoreactive. The average somatic diameter of GAD-positive neurons is about 60% of that of non-immunostained cells in semi-thin material; however, the range of somatic area and the dendritic variability of these neurons suggest that cells representing more than one population are immunopositive and include all but the largest neurons. The puncta also show regional differences. Small (0.5-2 microns in diameter), medium (2-3 microns), or large (greater than 3 microns) puncta occur. In the ventral division, the predominantly medium-sized puncta are about four times as numerous on a unit/area basis than in the dorsal division, where they are far smaller and more delicate; medial division puncta are as numerous as those in the ventral division, but are much larger and coarser, and may form perisomatic arrangements. Controls were devoid of specific immunostaining.(ABSTRACT TRUNCATED AT 400 WORDS)

Patterns of reciprocity in auditory thalamocortical and corticothalamic connections: study with horseradish peroxidase and autoradiographic methods in the rat medial geniculate body.

The patterns of reciprocity between retrogradely labeled thalamocortical cells of origin and anterogradely projecting corticothalamic axon terminals were studied in the subdivisions of the adult rat medial geniculate body following auditory cortical injections of mixtures of horseradish peroxidase and [3H]leucine. The labeling produced by each method was examined independently, both qualitatively and quantitatively, in adjacent series of tetramethylbenzidine-processed sections and in autoradiographs after 24-96 hour survivals. The distribution and number of labeled cells and axon terminals were assessed separately for each method and compared systematically throughout the rostro-caudal extent of the medial geniculate complex. The principal finding was that zones containing many retrogradely labeled neuronal somata are not completely coextensive with areas of heavy terminal labeling within the medial geniculate body, although there is a gross congruence of thalamocortical-corticothalamic projections. Conversely, we found many zones of autoradiographic silver grains without retrogradely labeled somata in the adjacent sections; in general, the autoradiographic zones of non-reciprocity were more extensive and marked than were retrograde zones of non-reciprocity. The rat medial geniculate complex could be subdivided on the basis of its neuronal organization, cytoarchitecture, fiber architecture, and thalamocortical and corticothalamic connections into three major parts: the ventral, dorsal, and medial divisions. This pattern of organization was comparable, though not identical, to that of the corresponding subdivisions in the cat medial geniculate body (Winer: Adv. Anat. Embryol. Cell Biol. 86:1-98, '85). While the retrograde labeling appeared to mark many of the different types of neurons in each of the three divisions, there were distinct local and quantitative and qualitative differences in the distribution of autoradiographic terminal labeling. The ventral division received the heaviest cortical input, the medial division the least labeling, while the dorsal division was intermediate. Thus, corticogeniculate projections to the ventral division often produced values 20-100 times above background (absolute values: 2,001-10,000 silver grains/14,400 micron2; background: less than 100 silver grains/14,400 micron2); the same projection to the dorsal division usually resulted in grain counts no more than 5-20 times above background (501-2,000/14,400 micron2), while in the medial division the number of silver grains rarely exceeded two to five times the background (201-500/14,400 micron2).(ABSTRACT TRUNCATED AT 400 WORDS)