Hyperprolactinemia Alters the Frequency and Amplitude of Pulsatile Luteinizing Hormone Secretion in the Ovariectomized Rat

Abstract

Studies were undertaken to (1) examine the effects of hyperprolactinemia on the frequency and amplitude of pulses of LH, and (2) determine if changes in pituitary sensitivity to LHRH were involved in the prolactin-induced suppression of LH secretion. Rats were bilaterally ovariectomized (day 0). Ovine prolactin (4 mg/kg body weight, subcutaneously) or vehicle was administered every 8 h beginning at 09.00 h on day 4 after ovariectomy and continuing until 09.00 h on day 6. On day 6, between 07.00 and 09.00 h all animals received a right atrial cannula, using ether anesthesia. In experiment I blood samples were taken at 10-min intervals beginning at 12.00 h on day 6, for a total of 180 min. To test the effect of hyperprolactinemia on pituitary responsiveness (experiment II) animals received an intravenous injection of LHRH (25 ng/100 g body weight) after the 180-min and again after the 240-min sample. Blood was drawn every 10 min for a total of 300 min. Serum was assayed for LH. Hyperprolactinemia altered the pattern of pulsatile secretion of LH. Treatment with ovine prolactin produced a decrease in both the frequency and amplitude of the LH pulses compared to values found in control animals. However, no differences in pituitary responsiveness between hyperprolactinemic and control animals were found at the dose of LHRH given. Thus, the prolactin-induced suppression of pulsatile secretion of LH was not apparently a result of alterations in the sensitivity of the pituitary to LHRH. From these studies we suggest that hyperprolactinemia directly affects a hypothalamic site which ultimately alters the LHRH pulse generator, thereby changing the secretion of LHRH.

Ovarian and PGF2α responses to stimulation of endogenous PRL pulses during the estrous cycle in mares

“…Secretion of PRL has been reported to inhibit LH secretion in several species [24], especially in rats and humans [25,26]. The combined effect of PRL and dopamine inhibits LH, but PRL alone does not.…”

Quantitation of prolactin receptor mRNA in the maternal rat brain during pregnancy and lactation

“…Similarly, mRNA for PRL-R has been detected in the brain using both RT-PCR (Chiu et al 1992, Sugiyama et al 1994, Pi & Grattan 1998b, 1999b and in situ hybridization (Chiu & Wise 1994, Bakowska & Morrell 1997, Mann & Bridges 2002. Prolactin has been implicated in the regulation of a variety of brain functions, including the suppression of adrenocorticotrophin secretion during the stress response (Torner et al 2001), an increase in feeding and appetite (Sauve & Woodside 1996, 2000, and the suppression of fertility (Cohen-Becker et al 1986), as well as the well-characterized short-loop negative feedback regulation of its own secretion by stimulating tuberoinfundibular dopamine (TIDA) neuronal activity (Freeman et al 2000). In addition, prolactin is known to be involved in further brain functions during lactation, such as the release of oxytocin for the milk ejection reflex (Parker et al 1991, Ghosh & Sladek 1995 and induction of maternal behaviour (Bridges 1994).…”

Effects of Estradiol on the Prolactin Surges and the Feedback Mechanisms of Gonadotropins in Pseudopregnant Rats

“…It has been thought that the N surge of PRL is essential for maintaining PSP and is stored in the hypo thalamus as memory for 6 days [4, 71. Gorospe and Freeman [10] have reported that at the end of PSP, estradiol plays an important role for terminating PSP in the condition of proge sterone to be decreased.Generally, a reverse relationship is observ ed between the secretion of gonadotropins and PRL in reproductive phenomenon [2,14], such as PSP, pregnancy and lactation, where PRL is secreted dominantly, whereas luteinizing hormone (LH) and follicle stimulating hormone (FSH) remain a basale level [2,19, 201. In these stage PRL probably stimulates progesterone secretion from the copora lutea, and proges terone suppresses the secretions of LH and FSH through the hypothalamus and/or pituitary.…”

“…Generally, a reverse relationship is observ ed between the secretion of gonadotropins and PRL in reproductive phenomenon [2,14], such as PSP, pregnancy and lactation, where PRL is secreted dominantly, whereas luteinizing hormone (LH) and follicle stimulating hormone (FSH) remain a basale level [2,19, 201. In these stage PRL probably stimulates progesterone secretion from the copora lutea, and proges terone suppresses the secretions of LH and FSH through the hypothalamus and/or pituitary.…”