Parvalbumin and GABA in the developing somatosensory thalamus of the rat: an immunocytochemical ultrastructural correlation

The calcium binding protein parvalbumin (PV) is widely distributed in the m ammalian nervous system and its relationship with GABAergic neurons differs within thalamic nuclei and animal species. In the rat somatosensory thalam us PV immunoreactive (ir) neurons were found only in the GABAergic reticula r thalamic nucleus (RT), while a dense PVir neuropil is present in the vent robasal complex (VB). In this study the distribution and relationship of PV and GABA were investigated in RT and VB during postnatal development at el ectron microscopic level. The pre-embedding immunoperoxidase detection of P V was combined with the post-embedding immunogold localization of GABA. In RT, at all developmental ages, neuronal cell bodies, dendrites and rare axo nal terminals were both PVir and GABAir. In VB during the first postnatal w eek several small vesicle-containing profiles were double-labelled and some of them were identifiable as synaptic terminals. From postnatal day 7 (P7) to P9 the medial part of VB was more intensely PVir than the lateral one a nd some differences in the sequence of maturation of PVir terminals were no ted between these two VB subdivisions. Single-labelled PVir profiles were f irst observed at P8, whereas single-labelled PVir terminals appeared at P12 and at P15 they became more frequent and larger, showing the typical morph ology of ascending afferents described in adult VB. These results demonstra te the late expression of PV and acquisition of adult morphology in ascendi ng terminals of rat VB during postnatal development in comparison with the innervation arising from the GABAergic RT.