STRESS-INDUCED SUPPRESSION OF THE PROLACTIN AFTERNOON SURGE IN OVARIECTOMIZED, ESTROGEN-TREATED RATS AND THE NOCTURNAL SURGE IN PSEUDOPREGNANT RATS ARE ACCOMPANIED BY AN INCREASE IN MEDIAN-EMINENCE DIHYDROXYPHENYLACETIC ACID CONCENTRATIONS

Experiments were performed to determine whether the suppression of prolactin (PRL) surges during restraint was accompanied by changes in the activity of tuberoinfundibular dopamine (TIDA) neurons in the median eminence. Animals were either ovariectomized and estrogentreated (OVX-PEP) or cervically stimulated to induce pseudopregnancy (PSP). Restraint stress was administered by tying the hind legs together with plastic-coated bell wire. Animals were decapitated following 15 or 30 min of restraint stress or immediately after removal from the animal room (control) when PRL levels were basal (10.00 h), at the peak of the afternoon PRL surge in OVX-PEP animals (17.00 h) or the nocturnal PRL surge in PSP animals (05.00 h). Median eminence dopamine (DA) and dihydroxyphenylacetic acid (DOPAC) were significantly decreased in control rats at 17.00 h when compared to control rats at 10.00 h (103.1 ±3.7 vs. 85.8 ± 3.3 and 11.4 ±0.4 vs. 7.1 ±0.4 pg/µg protein, respectively) and plasma PRL was markedly elevated. Restraint stress at 10.00 h resulted in a significant increase in serum PRL, but this increase was not accompanied by a change in DA or DOPAC when compared to control animals (103.1 ±3.7 vs. 107.9±4.8 and 11.4±0.4 vs. 10.4±0.6 pg/µg protein, respectively). The restraint stress-induced decrease in the afternoon PRL surge at 17.00 h and in the nocturnal PRL surge at 05.00 h were accompanied by an increase in DA and DOPAC (85.8 ± 3.3 vs. 91.2 ± 3.8 and 7.1 ± 0.4 vs. 8.6 ± 0.7 pg/g protein, respectively, for the afternoon surge, and 91.4 ±2.8 vs. 102.2 ±3.7 and 8.2 ±0.8 vs. 13.5 ±0.9 pg/µg protein, respectively, for the nocturnal surge); the increase in DA in the afternoon PRL surge, however, was not statistically significant. These results indicate that suppression of the afternoon estrogen-induced PRL surge or the nocturnal PRL surge of PSP during stress is accompanied by an increase in the activity of TIDA neurons in the median eminence. Furthermore, these results suggest that the response of TIDA neurons to stress may be dependent upon their activity at the time of stress. © 1990 S. Karger AG, Basel.