Involvement of catecholamines in eliciting LH peaks in 15-day-old female rats: effects of treatment with prolactin.

In the serum of 15-day-old female rats, treated either with alpha-methyl-p-tyrosine (alpha-MPT) or saline, serum LH levels were determined. From the brains of all animals the medial preoptic areas (MPO) and the anterior and posterior mediobasal hypothalami (AMBH and PMBH) were punched out and stored frozen in perchloric acid. In the punches of those animals with very high or basal LH levels catecholamine concentrations were measured radioenzymatically and on the basis of the degree of catecholamine depletion after alpha-MPT the turnover rates of dopamine (DA) and norepinephrine (NE) were calculated. NE turnover in the MPO was significantly higher in the animals with high LH levels as compared to those with low LH values. In the PMBH an inverse correlation between LH levels and NE turnover rates could be demonstrated. NE turnover in the AMBH, and DA turnover in all 3 structures did not correlate with serum LH levels. Serum FSH and prolactin levels were higher in the animals with high LH levels. These changes are similar to those observed in adult proestrous rats and it is concluded that the LH peaks in 15-day-old animals reflect a positive feedback action of estradiol. In a second experiment prolactin was administered to determine if it could prevent the occurrence of these LH peaks. Ovine prolactin (2 X 0.5 micrograms/g body weight) injected from day 8--15 suppressed serum LH levels to almost undetectable values. Catecholamine turnover measurements in these animals revealed increased DA turnover in the MPO, AMBH and PMBH, whereas NE turnover was not affected. These results indicate that DA turnover of the incertohypothalamic and of the tuberoinfundibular DA neurons is accelerated by prolactin and that an increased DA turnover in one of these systems is responsible for inhibition of LH release.