Persistence of low hypothalamic dopaminergic activity after removal of chronic estrogen treatment

The purpose of this study was to determine whether inhibition of tuberoinfundibular dopaminergic (TIDA) neuron function which occurs during chronic estrogen administration persists after removal of the estrogen. Ovariectomized (OVX) Fischer 344 (F344) rats were implanted for 4 weeks with a Silastic capsule containing estradiol-17 beta (E2) and controls with an empty capsule for 4 weeks. Other rats which received E2 for 4 weeks had the capsule removed and experiments performed 4 weeks later. At the end of 4 weeks of E2 treatment, anterior pituitary (AP) weight was increased sixfold, serum prolactin (PRL) 65-fold, and AP DNA content fivefold over OVX control rats. Four weeks after removal of E2, AP weight, serum PRL, and AP DNA content declined, but remained significantly above OVX control values. At the end of 4 weeks of E2 treatment and after E2 withdrawal, release of [3H]dopamine (DA) from median eminence (ME) tissue superfused in vitro was lower than from ME of OVX control rats although [3H]DA accumulation was not significantly different among the treatment groups. Administration of apomorphine (APO), a dopamine agonist, significantly reduced plasma prolactin levels in OVX control rats, in rats at the end of 4 weeks E2 treatment, and in rats after 4 weeks of E2 withdrawal. Injection of haloperidol (HALO) produced similar increases in plasma PRL/estimated PRL-cell DNA in OVX controls, at the end of E2 treatment or after E2 withdrawal. However, injection of morphine (MOR), a drug which increases the release of PRL by inhibiting hypothalamic dopaminergic activity, resulted in a rise in plasma PRL/estimated PRL-cell DNA in OVX control rats that was significantly greater compared to rats at the end of E2 treatment or after E2 withdrawal. Since rats treated with E2 released less [3H]DA from ME tissue in vitro, and were less responsive to MOR, it can be that animals treated for 4 weeks with E2 show a decreased ability to release DA from TIDA neurons which persists even after termination of E2 treatment. These results suggest that chronic high circulating E2 levels result in a depression of TIDA neuronal activity which is sustained after E2 is removed.