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

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 G alpha i1/2, G alpha i3, G alpha o and G alpha s proteins before and after dopaminergic innervation. G alpha i3 increased through gestational day 20, and then remained level to postnatal day 6. At this time, coinciding with the induction of D2short expression, G alpha i3 immunoreactive intensity increased markedly, possibly indicating co-regulation of these proteins. On postnatal day 6, G alpha s immunoreactive intensity increased in some, but not all, melanotropes. The resulting heterogeneity in Gs expression persisted in the adult. G alpha i1/2 immunoreactivity did not change and G alpha o was detected only subsequent to the event of innervation. Thus, dopamine released from axons and acting through D2 receptor stimulation could increase G alpha i3 immunoreactivity and decrease G alpha s immunoreactive intensity in some melanotropes.