Interaction of oestradiol, alpha-melanotrophin and noradrenaline within the ventromedial nucleus in the control of female sexual behaviour.

A possible inter-relationship between oestrogen, alpha-melanotrophin (alpha MSH) and NA in the ventromedial nucleus (VMN) has been studied in ovariectomised-adrenalectomised rats primed with a low dose of oestradiol benzoate (2 or 5 micrograms OB), which induces receptivity in approximately half the rats. Priming with OB increased NA turnover in the VMN (as assessed by the decline in NA concentration 1 h after 250 mg/kg alpha-methyl tyrosine alpha MT) and also enhanced the release of NA from basal medial hypothalamic fragments in vitro. This occurred whether the rats were receptive or non-receptive. Injection of 20 micrograms/rat alpha MSH, 4 h before autopsy in OB-primed rats, reduced NA turnover in the VMN but only in the receptive animals. alpha MSH had no effect on NA content in the VMN, but prevented the decline normally induced by the alpha MT, indicating an inhibition of release. Application of 1 microgram/ml alpha MSH to incubated hypothalamic fragments enhanced release of NA in the tissue obtained from untreated controls and the OB-non-receptive group, but had no effect on the tissue of the OB-receptive animals. Perhaps NA release had already occurred in vivo in the latter group. alpha MSH (100 ng/side/rat) and NA (20, 200 and 2,000 ng/side/rat) were injected into the VMN of ovariectomised-adrenalectomised rats primed with 1 microgram OB. Both agents stimulated lordosis in non-receptive animals with a peak activity at 60 min.(ABSTRACT TRUNCATED AT 250 WORDS)