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

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