Physiological roles of thyrotrophin-releasing hormone and vasoactive intestinal peptide on the pulsatile secretory patterns of prolactin in pituitary-grafted female rats

Abstract

Much is known about the fact that thyrotrophin-releasing hormone (TRH) and vasoactive intestinal peptide (VIP) stimulate prolactin secretion but areas of uncertainty remain. This work was undertaken to describe the effects of TRH and VIP on the pulsatile secretion pattern of prolactin, in adult sham-operated and pituitary-grafted hyperprolactinaemic female rats. Two pulses of TRH (1 microgram/rat) or one pulse of VIP (20 micrograms/rat) were given 60 or 120 min after the period of blood sampling. Pituitary grafting increased the mean values of prolactin, absolute amplitude and duration of the peaks and decreased their frequency, compared with control animals. In sham-operated rats, TRH elevated prolactin levels by increasing the absolute and relative amplitudes and duration of the pulses, along with a decrease in their frequency. No priming effects of TRH were observed in this study. Hyperprolactinaemia blunted TRH effects on the pulsatile secretion pattern of prolactin. In sham-operated rats, VIP administration increased the absolute and relative amplitudes of the prolactin peaks. None of the other parameters studied were changed. In pituitary-grafted animals, VIP administration increased the absolute and relative amplitudes of the prolactin peaks but to a lesser extent compared with controls. These data suggest that TRH and VIP affect prolactin pulsatility differentially. The effects of TRH and VIP were blunted to some extent by exposure to previously elevated circulating prolactin levels.

Effects of the dopamine agonist cabergoline on the pulsatile and TRH-induced secretion of prolactin, LH, and testosterone in male beagle dogs

“…The sharp rise of prolactin concentrations measured in coincidence with the TSH rise 20 min after intravenous TRH injection shows the likewise stimulation of thyrotrophic and lactotrophic pituitary cells and confirms that TRH is a potent PRF in the dog [33] as described in other species (sheep [14]; rat [17]). Suppression of this TRH-mediated stimulation of prolactin release by cabergoline suggests dopaminergic suppression of the sensitivity of the lactotrophs towards TRH.…”

“…Only a few years ago, a specific prolactin-release promoting peptide has been identified and characterised in the hypothalamus [11,12]. In addition, several other prolactin-stimulating factors have been reported, such as serotonin [13] and thyrotrophin-releasing hormone (TRH) [14][15][16][17][18]. In a recent study the strong prolactin stimulating effect of TRH was also demonstrated in cattle in vitro and in vivo when compared with the prolactin release induced by a bovine posterior pituitary extract and prolactin releasing peptide [19].…”

Reductions in circulating anabolic hormones induced by sustained sleep deprivation in rats

“…PRL secretion exhibits low-amplitude pulsatility (40,42), and therefore measurements integrated over time better reflect basal PRL secretion than do singular determinations. Blood was sampled six times at 10-min intervals over a 50-min period, based on data by Lopez et al (42) and Lafuente et al (40) that a PRL pulse lasts 16 -25 min and would be expected to occur every 40 min.…”

“…PRL challenge tests by intravenous TRH bolus injection were completed one time during the baseline period and one time per week during the experimental period in the experimental subgroup. Blood was sampled immediately before and at 10, 20, and 40 min after intravenous administration of TRH (10 g/kg), based on previous studies (40,48). The total volume of blood drawn was 2.1 ml, with volume and RBC replacement.…”

“…In our analysis, we used three times the CV of the radioactive counts per minute, which ranged from 4.4 to 5.8%, as the constant in the algorithm signifying a significant change from previous and subsequent values across all values within that particular assay. Characterization of GH and PRL secretion were based on the following parameters given by Lafuente et al (40): 1) minimum and maximum values observed; 2) integrative value (average value of the samples collected during the period of observation); 3) absolute amplitude (difference in concentration between peak and minimum values); and 4) relative amplitude (ratio of absolute amplitude to the minimum value). Pulses of GH detected by ULTRA additionally were assessed for frequency in those instances in which two peaks were captured during the 6-h sampling period.…”