The effects of alpha-melanocyte-stimulating hormone (alpha-MSH) on basal and stimulated of prolactin were studied. Intraventricular (i.v.t.) injection of 2 microgram of alpha-MSH in ovariectomized (OVX) rats produced a 50% depression of plasma prolactin levels within 10 min, whereas 100 ng of alpha-MSH or saline had no effect. 2 microgram of alpha-MSH (i.v.t.) produced a more dramatic and sustained depression of prolactin levels if initial prolactin concentrations were elevated. This dose of alpha-MSH was also effective in lowering plasma prolactin levels in OVX rats, primed 72 h earlier with 50 microgram of estradiol benzoate (s.c.), although this depression was of lower relative magnitude than in the unprimed OVX animals. The intravenous injection of a dopamine receptor blocker, spiroperidol (0.1 mg/kg), prior to (i.v.t.) alpha-MSH blocked the inhibitory effect of alpha-MSH on prolactin release. In vitro, alpha-MSH in doses ranging from 0.5 to 5 microgram did not alter prolactin release from hemipituitaries of male or OVX female rates and from dispersed pituitary cells from female rats. These results suggest that alpha-MSH can modulate prolactin release possibly via the hypothalamic dopaminergic system, and that this modulation may be of greater significance during stress.
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Khorram et al. (2008) studied this question.