Changes in Hypothalamic NE and DA Turnover Resulting from Steroid-Induced LH and Prolactin Surges in Ovariectomized Rats1

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Abstract
Treatment of ovariectomized, estrogen primed rats with progesterone (P) resulted in a LH and prolactin (PRL) surge. A 2-fold increase in anterior hypothalamic fragment (AHF) norepinephrine (NE) turnover was observed by 4 h after P treatment and returned to control (Pre-P) levels by 6 and 9 h after P administration. AHF dopamine (DA) turnover decreased by 6 and 9 h after P treatment. No change in posterior hypothalamic fragment (PHF) catecholamine turnover occurred after P administration. The increased AHF-NE turnover preceded the LH surge and occurred at the peak of serum PRL. The decrease in AHF-DA turnover occurred when both LH and PRL were elevated. Treatment with alpha-methyl-para-tyrosine (αmpt) at the time of P administration completely blocked the subsequent P-induced LH release observed in control animals. Alpha-mpt treatment 1 h before blood collection decreased serum LH in animals killed at 4 and 6 h, but not at 0 and 2 h after P administration. Sustained treatment with the DA agonist, piribedil mesylate (1 mg/kg), did not affect peak levels of LU but blocked the PRL rise, indicating that the decrease in DA turnover after P administration is not essential for mediating the subsequent LU surge, but may be involved in the P-induced PRL surge. It is concluded that the transient increase in AHF-NE turnover is necessary for the P-induced increase in serum LH, whereas a decrease in hypothalamic DA turnover may be involved in the P-induced surge of PRL.