Ovarian steroids influence the activity of neuroendocrine dopaminergic neurons

The secretion of prolactin (PRL) from the anterior lobe (AL) of the pituita ry gland is tonically inhibited by dopamine (DA) of hypothalamic origin. Wh ile ovarian steroids play a role in the regulation of the secretion of PRL, their effect on all three populations of hypothalamic neuroendocrine dopam inergic neurons is not fully understood. In this study we describe the effe cts of ovarian steroids on regulation of the release of DA from tuberoinfun dibular dopaminergic (TIDA), tuberohypophyseal dopaminergic (THDA) and peri ventricular-hypophyseal dopaminergic (PHDA) neurons. Adult female rats were bilaterally ovariectomized (OVX) and, 10 days following ovariectomy (day 0 ), injected with corn oil (vehicle), estrogen, or estrogen plus progesteron e (day 1). Animals were sacrificed every 2 h from 09.00 to 21.00 h by rapid decapitation. Trunk blood was collected and the concentration of PRL in se rum was determined by radioimmunoassay. The median eminence (ME) and the AL , intermediate (IL) and neural (NL) lobes of the pituitary gland were disse cted and the concentration of DA and DOPAC in each was measured by HPLC-EC. OVX rats presented small but significant increases in the secretion of PRL at 15.00 and 17.00 h. Replacement of estrogen or estrogen plus progesteron e increased the basal concentration of PRL. Moreover, injection of estrogen only, or estrogen plus progesterone increased the concentration of PRL in serum at 15.00 h through 19.00 h, respectively, followed by a decrease to b aseline thereafter. The turnover of DA in the ME and NL of OVX rats increas ed at 13.00 and returned to low levels. Turnover of DA in the IL of OVX rat s increased in the morning by 11.00 h and remained elevated before decreasi ng by 17.00 h. The turnover of DA in the ME, IL and NL of OVX rats increase d by 19.00 h. Injection of estrogen advanced the increase of TIDA activity by 2 h in the ME compared to OVX rats. Moreover, administration of estrogen suppressed the activity of THDA and PHDA neurons in the afternoon compared to OVX rats. In estrogen plus progesterone-treated rats, the activity of h ypothalamic neuroendocrine dopaminergic neurons terminating in the ME, IL, and NL was inhibited prior to the increase in the secretion of PRL. The con centration of DA in the AL diminished prior to the estrogen-induced increas e of PRL. Administration of progesterone, in concert with estrogen, delayed the increase of PRL in serum and the decrease of DA in the AL, compared to estrogen-treated rats, by 4 h. These data suggest a major role for ovarian steroids in controlling increases in the secretion of PRL by not only stim ulating PRL release from lactotrophs, but also by inhibiting the activity o f all three populations of hypothalamic neuroendocrine DAergic neurons. (C) 2000 Elsevier Science B.V. All rights reserved.