This work was performed to confirm previous data reporting the presence 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 experiments were performed on the thalamus of adult guinea-pigs perfused with mixed aldehydes using an anti-GABA serum. At light microscopy, the immunoreaction on floating Vibratome sections showed that GABAergic neurons are present only in the reticular and lateral geniculate nuclei and in the ventrobasal complex. Quantitative evaluation of their number indicated that they are 20 and 15% of the total neuronal population in lateral geniculate nucleus and ventrobasal complex, respectively, while they are less than 1% in ventrolateral nucleus. At the ultrastructural level, the postembedding immunogold procedure showed the presence, in the ventrobasal complex, of GABA-labeled profiles involved in complex synaptic arrangements similar to those found in carnivores and primates. Conversely. GABA-labeled terminals in thalamic nuclei devoid of interneurons formed exclusively axo-dendritic or axo-somatic contacts, like in rats and mice. The present data suggest that GABAergic neurons in the ventrobasal complex of guinea-pigs give rise to functionally important rearrangements of its intrinsic synaptic organization and that they represent the morphological basis for an intrinsic modulatory mechanism that is absent in other thalamic nuclei lacking inhibitory interneurons. The phylogenetic implications of these findings are also discussed in comparison to other animal species.
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