Dopaminergic and adrenergic effects on plasma LH levels in rats with hypothalamic islands.

Abstract

In female rats hypothalamic islands were prepared by making a cut around the mediobasal hypothalamus. Three weeks later the rats were ovariectomized. Two weeks after ovariectomy, part of the animals showed elevated blood LH levels with oscillations, although the mean LH levels were significantly lower than in sham-operated ovariectomized animals. There was a significant correlation between the occurrence of elevated blood LH levels after ovariectomy and the inclusion of at least part of the suprachiasmatic nucleus within the islands. Nor-adrenaline content of the hypothalamus was decreased significantly, but there was no significant change in hypothalamic dopamine content. Injection of doses of apomorphine, ranging form 2.5 microgram/kg to 25 mg/kg sc into island-bearing and sham-operated animals caused a dose-dependent decrease of plasma LH. However, the magnitude of the decrease was smaller in rats with hypothalamic islands than in controls. Even the highest doses of apomorphine did not decrease plasma LH further than to about 100 ng of LH-RP-1/ml. Phenoxybenzamine (10 and 40 mg/kg ip) and phentolamine (20 mg/kg ip) caused a decrease of plasma LH in ovariectomized control rats, but in ovariectomized rats with hypothalamic islands these drugs caused an increase. Clonidine (100 microgram/kg sc followed by 100 microgram/kg ip) had no effect in ovariectomized control rats, but in ovariectomized rats with a hypothalamic island an increase was induced. These results may have been caused by an increased sensitivity of hypothalamic alpha-receptors resulting from severing afferent noradrenergic fibres.