Postnatal expression of melanocortin-3 receptor in rat diencephalon and mesencephalon.

In situ hybridization was applied to examine the postnatal expression of melanocortin-3 (MC-3) receptor mRNA in the rat brain. Very weak and limited signals were seen in the hypothalamus on postnatal day 0 (P0) and in the dorsal lateral thalamus on P4. A marked increase was noted in several regions of the diencephalon and mesencephalon on P7. The highest levels were reached on P21, which was the time when an adult-like pattern was established. On P21, intense signals were seen in the ventromedial nucleus and the arcuate nucleus of the tuberal hypothalamus, the habenular nucleus of the epithalamus and the ventral tegmental area. [125I] Nle4, D-Phe7-alpha-MSH showed overlapping, but wider labelling of melanocortin receptors, that followed a similar developmental course. alpha-MSH-like immunoreactivity was seen widely in the forebrain and midbrain from P14. In contrast to the staining of alpha-MSH in neurons and their process, gamma 2-MSH-like immunoreactivity was detected strongly in the blood vessels. The neuronal localization of MC-3 receptor mRNA suggests that this receptor may mediate the neurotropic actions of melanocortin peptides in the developing brain.