IMMUNOCYTOCHEMICAL AND ULTRASTRUCTURAL-STUDY OF THE RAT PERIRETICULARTHALAMIC NUCLEUS DURING POSTNATAL-DEVELOPMENT

The perireticular thalamic nucleus (PRT) consists of scattered neurons that are located in the internal capsule adjacent to the gamma aminob utyric acid (GABA)-immunoreactive (ir) reticular thalamic nucleus (RT) and whose number decreases during development. The common feature of PRT neurons in different species is the immunoreactivity for the calci um binding protein parvalbumin (PV), which is also expressed by RT cel ls. In this study, we analyzed, at the light and electron microscopic level, the distribution and morphology of PV-ir neurons and their rela tionship with GABA in adult and developing rats. We found that the ros trocaudal distribution and the morphology of PV-ir neurons of the PRT were different at each stage of postnatal development examined. The ad ult configuration of the PV-ir population in the PRT was achieved at p ostnatal day 21. With electron microscopy, the developing PRT was obse rved to contain PV-ir neuronal cell bodies and dendrites contacted by several PV-negative synaptic terminals, some of which were GABA-ir, wh ereas the adult PRT contained also large PV-ir boutons, generally GABA -ir. Very few GABA-ir neurons were found in the PRT region and only du ring the first postnatal week, thus indicating that the PV-ir neurons of PRT represent a distinct population from those of RT. Our results d emonstrate a morphological, neurochemical, and ultrastructural complex ity of the PRT not only during development, but also in adulthood. The se findings provide new data supporting the suggested roles of the PRT during postnatal development, and may indicate that in adult life it can play other so far unknown functions. (C) 1998 Wiley-Liss, Inc.