The effects of intracerebroventricular infusion of prolactin on luteinizing hormone, testosterone and growth hormone secretion in male sheep

Kisspeptin cells in the ovine arcuate nucleus express prolactin receptor but not melatonin receptor

Melatonin is secreted at night by the pineal gland and governs the reproductive system in seasonal breeders, such as sheep. The mechanism by which melatonin regulates reproduction is not known. The circannual rhythmicity of other factors, including prolactin, is also regulated by photoperiod via changes in melatonin secretion. In sheep, plasma prolactin levels are higher in the nonbreeding season than the breeding season. Kisspeptin, synthesised by neurones in the ovine arcuate nucleus (ARC) and preoptic area, is a key regulator of reproduction through stimulation of gonadotrophin-releasing hormone secretion and its expression in the ARC is reduced during the nonbreeding season. We hypothesised that kisspeptin expression is directly, or indirectly, regulated by melatonin and/or prolactin. We first examined the expression of melatonin receptor (MTNR1A) in kisspeptin (Kiss1 mRNA) neurones in the ARC of ovariectomised (OVX) sheep using double-label in situ hybridisation. MTNR1A mRNA was not expressed by kisspeptin neurones, whereas strong expression was detected in the pars tuberalis. We then examined the expression of the long-form prolactin receptor (PRLR-L) in ARC kisspeptin neurones. In OVX ewes, approximately 60% of kisspeptin neurones expressed PRLR-L mRNA at similar levels in the breeding and nonbreeding seasons. We then aimed to determine whether prolactin treatment during the breeding season regulates kisspeptin expression in the ARC. Continuous central infusion of prolactin (20 μg/h for 7 days) in oestradiol-treated OVX sheep did not alter Kiss1 mRNA expression or luteinising hormone secretion, although it induced substantial phosphorylated signal transducer and activator of transcription 5-immunoreactive nuclei staining in the mediobasal hypothalamus. We conclude that the seasonal change in kisspeptin neurones cannot be regulated directly by melatonin, although it may be a result of changes in prolactin levels. Despite this, kisspeptin expression was unchanged after exogenous prolactin treatment in breeding season ewes.

Lactational exposure to atypical antipsychotic drugs disrupts the pituitary-testicular axis in mice neonates during post-natal development

Olanzapine (OLNZ) and risperidone (RISP), two widely prescribed drugs for post-partum psychosis, transfer through milk to the neonates. Hence, neonates are susceptible to their adverse side effects. In the present study, the pituitary-testicular axis of lactationally exposed mice neonates (PND 28) was examined to evaluate the reproductive adverse effects. Testicular histopathology, immunocytochemistry and morphometric analysis of pituitary PRL (prolactin) and LH (luteinizing hormone) cells and plasma hormonal (PRL, LH and testosterone) levels were the various end points studied. Significantly regressed testes, reduced seminiferous tubules with disrupted germ-cell alignment, spermatogonial exfoliation into the tubule lumens and sparse sperms in the lumens were observed. PRL-immunointensity and plasma levels were elevated, whereas immunoreactivity and plasma levels of LH were decreased. Plasma testosterone levels were also decreased. The hypogonadism thus observed might be mediated by drug-induced hyperprolactinemia, which further inhibited secretions of LH and testosterone. Age may be the factor which made the neonates vulnerable to the PRL elevation by OLNZ which otherwise causes transient elevation in adults and is considered safe. The adverse impact was persistent until adulthood with higher doses of both of the drugs as evident by the analysis of testicular weight, histology and hormonal profiles of post-pubertal mice (PND 63) lactationally exposed as neonates.

Prolactin regulation of testosterone secretion and testes growth in DLS rams at the onset of seasonal testicular recrudescence

Our objective was to test the hypothesis that prolactin (PRL) acts at both the pituitary and testis levels to regulate testosterone secretion in the adult ram. The focus was on the mid-regression to mid-redevelopment stages of a photoperiod-condensed ‘seasonal’ testicular cycle. DLS rams (six per group) were given daily s.c. injections of bromocriptine (4 mg) or vehicle during the entire period. Serum PRL concentration in control rams peaked at 103.4±22.1 ng/ml in late regression and then steadily declined (P<0.01) to 19.5±4.3 ng/ml, whereas PRL in treated rams was always ≤4.0 ng/ml. Suppression of PRL tended (P<0.10) to increase the amplitude of natural LH pulses (transition stages) or reduce the number of LH receptors in the testis (regressed stage), although neither change disturbed testosterone levels in peripheral blood. These subtle changes were accompanied by significant (P<0.05) alterations in the capability of the pituitary to release LH (85% more) and of the testes to secrete testosterone (20% less). These effects of PRL were unmasked when rams were given highly stimulative i.v. injections of GNRH (single 3 μg dose) and NIH-oLH-S24 (three 5 μg doses given 20 min apart) respectively. PRL insufficiency also appeared to slow down the ‘seasonal’ rise in FSH secretion and slightly delayed (2 weeks) the times when the testes began to grow and were first significantly (P<0.05) enlarged from the regressed state. We conclude that PRL is an important part of the intricate regulation of the pituitary–gonadal system in moderately seasonal DLS rams.