G-protein expression in melanotropes changes coincident with innervation of the developing rat pituitary intermediate lobe

Abstract

The two isoforms of the dopamine D2 receptor, the D2short and the D2long differ in a 29 amino acid insert in the third cytoplasmic loop with which G proteins interact. We have previously reported that in rat melanotropes, expression of D2short increases markedly at the end of the first postnatal week which is concurrent with innervation of the intermediate lobe. Using immunohistochemistry, this study examined expression of Gαi1/2, Gαi3, Gαo and Gαs proteins before and after dopaminergic innervation. Gαi3 increased through gestational day 20, and then remained level to postnatal day 6. At this time, coinciding with the induction of D2short expression, Gαi3 immunoreactive intensity increased markedly, possibly indicating co-regulation of these receptors. On postnatal day 6, Gαs immunoreactive intensity increased in some, but not all, melanotropes. The resulting heterogeneity in Gs expression persisted in the adult. Gαi1/2 immunoreactivity did not change and Gαo was detected only subsequent to the event of innervation. Thus, dopamine released from axons and acting through D2 receptor stimulation could increase Gαi3 immunoreactivity and decrease Gαs immunoreactive intensity in some melanotropes.