Adult female Sprague-Dawley rats were ovariectomized and 2 weeks later were either hypophysectomized, sham hypophysectomized, or received no further surgical manipulation. Two weeks later, noradrenaline (NA) levels in the median eminence of hypophysectomized (HYPEX) rats were significantly elevated over those observed in sham-operated or control rats, while dopamine (DA) and epinephrine levels in this brain region of HYPEX rats were significantly lower than those observed in the other two groups. The concentration of NA in the remaining hypothalamus or in the telencephalon was not affected by hypophysectomy. The elevation of serum PRL by the implantation of two anterior pituitary homografts under the kidney capsule or the daily administration of 250 μg ovine PRL reduced the elevation of NA in the median eminence of HYPEX rats in the former case to a level significantly below that in controls and in the latter case to a level not significantly different from that observed in control rats. The greater effect of the homografts on median eminence NA probably reflects the continuously elevated levels of PRL in these rats. The rate of decline of NA in the median eminence after the administration of U- 14, 624 [1-phenyl-3-(2-thiazoyl)2-thiourea], a DA β-hydroxylase inhibitor, was significantly reduced in HYPEX compared to control rats, which suggests that the turnover of NA is reduced in the median eminence of HYPEX rats. By comparison ±-methyl- para-tyrosine treatment produced little effect on NA levels in the median eminence of either HYPEX or control rats. DA turnover in the median eminence was reduced in HYPEX compared to control rats. The levels of NA were markedly reduced in the median eminence of ovariectomized rats in which PRL was elevated by the implantation of two anterior pituitary homografts under the kidney capsule for 1 week, and there was evidence for a decrease in NA turnover in these animals after U-14, 624 administration. Collectively, these data are consistent with an inverse relationship between median eminence NA and circulating PRL levels; the physiological significance of this relationship is not yet established.
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