Immunoelectron microscopic study of gamma-aminobutyric acid inputs to identified thalamocortical projection neurons in the anterior thalamus of the rat.

We have carried out a semi-quantitative ultrastructural study to determine the characteristics and distribution of gamma-aminobutyric acid (GABA)-containing constituents of the anterodorsal (AD) and anteroventral (AV) thalamic nuclei in adult rats. We used a polyclonal antibody to GABA and a postembedding immunogold detection method in animals in which the cortical projection neurons of these nuclei had been labelled by retrograde transport of cholera toxin/horseradish peroxidase (HRP) injected into the retrosplenial granular cortex. Two types of GABA-immunopositive structures were identified, with gold particle densities 4-40 times higher than the highest densities over blood-vessel lumens and areas of empty resin: (1) an apparently homogeneous population of axon terminals with Gray type-2 (symmetric) synaptic contacts corresponding to F-axon terminals; and (2) small-medium sized myelinated axons scattered individually or in small groups within the neuropil which may be their parent axons. These axons and terminals may originate from the ipsilateral thalamic reticular nucleus; others may arise from the basal forebrain or brainstem. The GABA-immunopositive terminals comprised approximately 16% of all axon terminal profiles in AD and 12% in AV, a significant difference. However, because the immunoreactive axon terminals in AD were significantly larger than those in AV (1.09+/-0.47 microm2 vs 0.90+/-0.43 microm2) and would therefore be encountered more frequently, it is not possible to conclude that the GABAergic innervation of AD is heavier than that of AV. The GABA-positive terminals established synaptic contacts with cell bodies and dendrites of all sizes (some of which were HRP-labelled) with the following frequency distribution (AD/AV, no significant difference): somata 5%/7%; large dendrites (> or = 1.5 microm) 14%/9%; medium dendrites (1.00-1.49 microm) 35%/45% and small dendrites (< 1 microm) 46%/40%. Despite evidence from previous studies, we found no evidence in this study for the presence of GABAergic interneurons or for GABA-containing projection neurons in AD or AV.