α-Melanocyte-stimulating hormone (αMSH) is produced in the brain mainly in the neurones of the arcuate nucleus. αMSH acts centrally in behaviours such as feeding, stretching-yawning reflex and sexual behaviour via MC3 and MC4 receptors. These central actions are remarkably similar to those mediated by central oxytocin. Projections between the arcuate nucleus and the supraoptic nucleus (SON) have been described and MC4 receptor mRNA is expressed in the SON (Mountjoy et al. 1994). Consequently, we investigated the modulatory effect of αMSH on oxytocin neurone activity and oxytocin secretion.

Firstly, urethane-anaesthetised Sprague-Dawley rats were fitted with femoral artery and I.C.V. cannulae. Blood samples were collected every 5 min after I.C.V. injection of MC4 agonist (400 ng/2 µl) or vehicle. In a second experiment, rats were implanted with jugular vein and I.C.V. cannulae under brief halothane-anaesthesia. After two days, blood samples were taken 10 min before, and 10, 15 and 90 min after I.C.V. injection of αMSH(1 µg/5 µl) or vehicle. Since cholecystokinin (CCK) increases oxytocin secretion and Fos expression in oxytocin neurones, other rats were given I.V. injection of CCK (20 µg kg^-1) or vehicle as controls. Rats were given an overdose of pentobarbitone and perfused transcardially 90 min after drug injection. Free-floating brain sections were processed for Fos immunocytochemistry and for Fos/oxytocin double immunocytochemistry. Oxytocin was measured in unextracted plasma samples by specific RIA.

In anaesthetised rats, MC4 agonist decreased plasma oxytocin concentration (n = 7), while vehicle had no effect (n = 5; One-way RM ANOVA P < 0.05). In conscious rats, αMSH (n = 7) had no significant effect on oxytocin secretion compared to vehicle (n = 6), whereas rats injected I.V. with CCK showed a 180 % increase in oxytocin secretion (n = 7; P < 0.05). In the same rats, αMSH increased Fos expression in the SON (mean ± S.E.M.; 110 ± 19 Fos-positive cells per section; n = 7) significantly more than either vehicle (18 ± 6; n = 6; Mann-Whitney test P < 0.05) or CCK (55 ± 11 Fos-positive cells per section; n = 8; P < 0.05). Double immunocytochemistry confirmed the increased Fos expression in oxytocin neurones after αMSH (52 ± 6% Fos-positive oxytocin cells/section) compared to CCK (37 ± 5% Fos-positive oxytocin cells/ section; P < 0.05).

Thus central administration of αMSH induces strong Fos expression in oxytocin neurones but decreases oxytocin secretion from the pituitary gland. This illustrates that Fos induction does not necessarily reflect excitation of the neurones. These studies confirm the modulatory effect of αMSH on oxytocin neurones via MC4 receptors.

This work was supported by Merck & Co. C.C. is supported by a MRC studentship.