PARVALBUMIN IMMUNOREACTIVITY IN THE THALAMUS OF GUINEA-PIG - LIGHT AND ELECTRON-MICROSCOPIC CORRELATION WITH GAMMA-AMINOBUTYRIC-ACID IMMUNOREACTIVITY

The relationship of the calcium binding protein parvalbumin (PV) with gamma-aminobutyric acidergic (GABAergic) neurons differs within differ ent thalamic nuclei and animal species. In this study, the distributio n of PV and GABA throughout the thalamus of the guinea pig was investi gated at the light microscopic level by using immunoperoxidase methods . Intense PV labelling was found in all the GABAergic neurons of the r eticular nucleus and in scattered GABAergic neurons in the anteroventr al nucleus, whereas GABAergic interneurons in the ventrobasal and late ral geniculate nuclei were not PV labelled. At the electron microscopi c level, preembedding immunoperoxidase for PV was combined with postem bedding immunogold for GABA. In the ventrobasal nucleus, four types of profiles were recognized: 1) terminals with flattened vesicles and fo rming symmetric synapses, which were labelled with both PV and GABA an d could therefore be identified as afferents from the reticular nucleu s; 2) boutons morphologically similar to presynaptic dendrites of inte rneurons, labelled only with GABA; 3) large terminals with round vesic les and asymmetric synapses, labelled only with PV, which contacted GA BAergic presynaptic dendrites in glomerular arrangements and resembled ascending excitatory afferents; and 4) terminals unlabelled by either antiserum. In the ventrobasal nucleus of the guinea pig a double immu nocytochemical labelling permits therefore the differentiation of two populations of GABAergic vesicle-containing profiles, i.e., the termin als originating from reticular nucleus (that are double labelled) and the presynaptic dendrites originating from interneurons (that are GABA -labelled only). The possibility to differentiate GABAergic inputs fro m the reticular nucleus and from interneurons can shed light to the fu nctional interpretation of synaptic circuits in thalamic sensory nucle i. (C) 1994 Wiley-Liss, Inc.