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

Abstract

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 ± 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/µg 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 ΗDA neurons to stress may be dependent upon their activity at the time of stress.

Comparison of the Effects of Ether and Restraint Stress on the Activity of Tuberoinfundibular Dopaminergic Neurons in Female and Male Rats

“…Furthermore, by virtue of its ability to inhibit DA synthesis, NSD 1015 decreases the amount of DA released into the hypophysial portal blood [22], and under these conditions the concurrent measurement of changes in prolactin secretion and tuberoinfundibular DA neuronal activity during stress cannot be made in the same animal. To circumvent these problems the activity of tuberoinfundibular DA neurons was estimated by measuring the concentrations of the DA metabo lite DOPAC in the median eminence in the absence of any drug pretreatment [11,23,24]. As shown in figure 2, 2 min ether exposure followed by 10, 20 or 30 min of supine restraint pro duced a sustained increase in serum prolactin concentrations which was accompanied by a marked decrease in DOPAC con centrations in the median eminence of female rats.…”

“…In the absence of preexposure to ether, 'milder' forms of physical restraint such as confinement in a cylindrical tube or hindlimb immobilization [24] increase prolactin secretion, but fail to decrease the activity of tuberoinfundibular DA neurons. In contrast, more rigorous forms of physical restraint, such as immobilization, suppress the activity of tuberoinfundibular DA neurons.…”

Effect of social isolation on 24-h pattern of stress hormones and leptin in rats

“…With few exceptions (Grosvenor et al, 1965;Morishige and Rothchild, 1974;Morehead et al, 1990), increased plasma levels of prolactin are associated with acute exposure to a stressful stimulus (Matt et al, 1983;Huhman et al, 1991;Dijkstra et al, 1992). Although prolactin reliably increases in response to acute stress, prolactin responses to stressors often become inhibited over continuous exposure.…”

Ovarian Steroids But Not the Locus Coeruleus Regulate Stress‐Induced Prolactin Secretion in Female Rats

“…Thus, norepinephrine seems not to regulate stress‐induced prolactin secretion. Alternatively, there is evidence suggesting that regulation of stress‐induced prolactin release may involve hypothalamic dopamine (45, 46) and some PRFs such as vasoactive intestinal peptide (47, 48) and salsolinol (49).…”