Thalamocortical projections were studied in adult cats using biotinylated dextran amines, wheat germ agglutinin conjugated to horseradish peroxidase, and autoradiography with tritiated leucine and/or proline. The input from 7 architectonically defined nuclei to 14 auditory cortical fields was characterized qualitatively and quantitatively. The principal results were that 1) every thalamic nucleus projected to more than 1 field (range, 4-14 fields; mean, 7 fields); 2) only the projection from the ventral division to some primary fields (primary auditory cortex and posterior auditory cortex) had a periodic, clustered distribution, whereas the input from other divisions to nonprimary areas was continuous; 3) layers III-V received >85% of the total axonal profiles; 4) in most experiments, five or more layers were labeled; 5) the projections to nonprimary auditory areas had many laterally oriented axons; 6) the heaviest input to layer I in all experiments was usually in its upper half, suggesting a sublaminar arrangement; 7) the largest axonal trunks (up to 6 microm in diameter) arose from the medial division and ended in layer Ia, where they ran laterally for long distances; 8) there were three projection patterns: type 1 had its peak in layers III-IV with little input to layer I, and it arose from the ventral division and the dorsal superficial, dorsal, and suprageniculate nuclei of the dorsal division; type 2 had heavy labeling in layer I and less in layers III-IV, arising from the dorsal division nuclei primarily, especially the caudal dorsal and deep dorsal nuclei; and type 3 was a trimodal concentration in layers I, III-IV, and VI that originated chiefly in the medial division and had the lowest density of labeling; and 9) the quantitative profiles with the three methods were very similar. The results suggest that the subdivisions of the auditory thalamus have consistent patterns of laminar distribution to different cortical areas, that an average of five or more layers receive significant input in a specific area, that a given thalamic nucleus can influence areas as far as 20 mm apart, that the first information to arrive at the cortex may reach layer I by virtue of the giant axons, and that several laminar patterns of auditory thalamocortical projection exist. The view that the auditory thalamus (and perhaps other thalamic nuclei) serves mainly a relay function underestimates its many modes for influencing the cortex on a laminar basis.

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.

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 ␥-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.

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.

A study of neurons and processes (puncta) immunolabeled by antibodies to ␥-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 µm 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.