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)-cont aining constituents of the anterodorsal (AD) and anteroventral (AV) thalami c nuclei in adult rats. We used a polyclonal antibody to GABA and a postemb edding immunogold detection method in animals in which the cortical project ion neurons of these nuclei had been labelled by retrograde transport of ch olera toxin/horseradish peroxidase (HRP) injected into the retrosplenial gr anular 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 homoge neous population of axon terminals with Gray type-2 (symmetric) synaptic co ntacts corresponding to F-axon terminals; and (2) small-medium sized myelin ated axons scattered individually or in small groups within the neuropil wh ich may be their parent axons. These axons and terminals may originate from the ipsilateral thalamic reticular nucleus; others may arise from the basa l forebrain or brainstem. The GABA-immunopositive terminals comprised appro ximately 16% of all axon terminal profiles in AD and 12% in AV, a significa nt difference. However, because the immunoreactive axon terminals in AD wer e significantly larger than those in AV (1.09+/-0.47 mu m(2) vs 0.90+/-0.43 mu m(2)) and would therefore be encountered more frequently, it is not pos sible 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 t he following frequency distribution (AD/AV, no significant difference): som ata 5%/7%; large dendrites (greater than or equal to 1.5 mu m) 14%/9%; medi um dendrites (1.00-1.49 mu m) 35%/45% and small dendrites (<1 mu m) 46%/40% . Despite evidence from previous studies, we found no evidence in this stud y for the presence of GABAergic interneurons or for GABA-containing project ion neurons in AD or AV.