The lateral and medial median eminence: distribution of dopamine, norepinephrine, and luteinizing hormone-releasing hormone and the effect of prolactin on catecholamine turnover.

In the adult male rat, we have succeeded in microdissecting the median eminence into a LHRH-rich lateral region (MEl) and a LHRH-poor medial region (MEm). Dissected in this manner, the MEm has a 4.8-fold lower LHRH concentration and a 1.5-fold higher DA concentration than the MEl. The concentration of norepinephrine (NE) is not different in the two regions. Estimates of the rate constants for dopamine (DA) and NE decay after synthesis inhibition revealed no significant differences between the MEl and MEm kinetics for either amine. Hyperprolactinemia, produced by ovine PRL administration, resulted in marked increases in DA turnover rates in both the MEm (2.7-fold) and the MEl (4.7-fold). These effects of PRL were specific to the tuberoinfundibular DA neurons, as DA turnover was unaffected in nigrostriatal, mesolimbic, and incertohypothalamic DA neurons, and no changes were observed in NE turnover in any study area. The results support the involvement of PRL in the short loop feedback regulation of its own secretion in the MEm. The data further demonstrate a hypothalamic mechanism in the MEl by which hyperprolactinemia could inhibit LHRH release.