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 estrogen-treated (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 vs. 7.1 +/- 0.4 pg/micrograms 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/micrograms protein, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)