The maturation of the calcium binding proteins calbindin-D28k (CB) and parvalbumin (PV) during the first 3 postnatal weeks was studied in the rat thalamus using immunohistochemistry. These two proteins display a non-homogeneous distribution in the adult thalamus. In the rat, CB is mainly localized in the neurons and neuropil of the thalamic midline, intralaminar, and ventromedial nuclei, as well as in the posterior complex. At birth, CB-immunoreactive cell bodies were evident in thalamic midline structures, and especially in the nucleus reuniens. The number of thalamic CB-positive cell bodies, as well as the intensity of the neuropil immunostaining, increased progressively in the first postnatal weeks. This quantitative increase was first apparent in the midline structures and then in the other thalamic territories which are CB-positive in adulthood, and followed a mediolateral gradient. The mature pattern was achieved by the end of the third postnatal week. In the adult rat thalamus the neurons of the reticular nucleus display PV-immunostaining and PV-positive fibers densely innervate most of dorsal thalamic domains. PV-immunoreactivity was clearly evident at birth in the cell bodies of the reticular nucleus. The density of PV-containing fibers increased progressively after birth in the dorsal thalamus, with a lateromedial gradient. At the end of the third postnatal week the ventroposterior (VP) complex appeared heavily innervated by PV-positive fibers, whose density in more medial structures was still lower than in the adult thalamus. A transient hyperinnervation of PV-immunoreactive fibers, displaying a dishomogeneous organization in distinct segments, was observed in VP, and especially in the ventroposteromedial nucleus, during the second postnatal week. Altogether these findings indicate that the maturation of CB and PV requires postnatally a relatively prolonged period of time. The possible involvement of these proteins in different functional aspects of thalamic neuronal maturation is discussed.
