GABAERGIC INTERNEURONS IN THE SOMATOSENSORY THALAMUS OF THE GUINEA-PIG - A LIGHT AND ULTRASTRUCTURAL IMMUNOCYTOCHEMICAL INVESTIGATION

This work was performed to confirm previous data reporting the presenc e of GABAergic interneurons in the ventrobasal complex of guinea-pig, and to investigate the intrinsic organization of this nucleus compared to that of thalamic nuclei lacking interneurons. Immunocytochemical e xperiments were performed on the thalamus of adult guinea-pigs perfuse d with mixed aldehydes using an anti-GABA serum. At light microscopy, the immunoreaction on floating Vibratome sections showed that GABAergi c neurons are present only in the reticular and lateral geniculate nuc lei and in the ventrobasal complex. Quantitative evaluation of their n umber indicated that they are 20 and 15% of the total neuronal populat ion in lateral geniculate nucleus and ventrobasal complex, respectivel y, while they are less than 1% in ventrolateral nucleus. At the ultras tructural level, the postembedding immunogold procedure showed the pre sence, in the ventrobasal complex, of GABA-labeled profiles involved i n complex synaptic arrangements similar to those found in carnivores a nd primates. Conversely. GABA-labeled terminals in thalamic nuclei dev oid of interneurons formed exclusively axo-dendritic or axe-somatic co ntacts, like in rats and mice. The present data suggest that GABAergic neurons in the ventrobasal complex of guinea-pigs give rise to functi onally important rearrangements of its intrinsic synaptic organization and that they represent the morphological basis for an intrinsic modu latory mechanism that is absent in other thalamic nuclei lacking inhib itory interneurons. The phylogenetic implications of these findings ar e also discussed in comparison to other animal species.