Endotoxin-Induced Inflammation Suppresses the Effect of Melatonin on the Release of LH from the Ovine Pars Tuberalis Explants-Ex Vivo Study.

Karolina Wojtulewicz,Andrzej Przemysław Herman,Dorota Tomaszewska-Zaremba

doi:10.3390/molecules22111933

Abstract

The secretion of the hormone melatonin reliably reflects environmental light conditions. Among numerous actions, in seasonal breeders, melatonin may regulate the secretion of the gonadotropins acting via its corresponding receptors occurring in the Pars Tuberalis (PT). However, it was previously found that the secretory activity of the pituitary may be dependent on the immune status of the animal. Therefore, this study was designed to determine the role of melatonin in the modulation of luteinizing hormone (LH) secretion from the PT explants collected from saline- and endotoxin-treated ewes in the follicular phase of the oestrous cycle. Twelve Blackhead ewes were sacrificed 3 h after injection with lipopolysaccharide (LPS; 400 ng/kg) or saline, and the PTs were collected. Each PT was cut into 4 explants, which were then divided into 4 groups: I, incubated with ‘pure’ medium 199; II, treated with gonadotropin-releasing hormone (GnRH) (100 pg/mL); III, treated with melatonin (10 nmol/mL); and IV, incubated with GnRH and melatonin. Melatonin reduced (p < 0.05) GnRH-induced secretion of LH only in the PT from saline-treated ewes. Explants collected from LPS-treated ewes were characterized by lower (p < 0.05) GnRH-dependent response in LH release. It was also found that inflammation reduced the gene expression of the GnRH receptor and the MT1 melatonin receptors in the PT. Therefore, it was shown that inflammation affects the melatonin action on LH secretion from the PT, which may be one of the mechanisms via which immune/inflammatory challenges disturb reproduction processes in animals.

Highlights

Melatonin (N-acetyl-5-methoxytryptamine) is a hormone synthesized by pinealocytes in the pineal gland
Melatonin treatments reduced (p < 0.05) the stimulatory effect of gonadotropin-releasing hormone (GnRH) on luteinizing hormone (LH) release only in explants collected from saline-treated ewes
This result confirms the previously published data showing that melatonin may be involved in the suppression of LH release from ovine Pars Tuberalis (PT) [21]

Summary

Introduction

Melatonin (N-acetyl-5-methoxytryptamine) is a hormone synthesized by pinealocytes in the pineal gland. Nocturnal concentration of melatonin and the duration of its release depend on the photoperiodic conditions. Melatonin is secreted in the highest concentrations and for the longest period in the winter during the short-day photoperiod, whereas the lowest concentrations and the shortest duration of melatonin are found in the summer during the long-day photoperiod. The melatonin signal is considered to be involved in the regulation of reproduction [2]. The duration of elevated melatonin levels over the long spring–summer days provides the signal to synchronize the circannual rhythm of reproductive neuroendocrine activity. The shortening days between the summer solstice and the autumn equinox are the critical signal involved in timing the end of reproductive activity in mid-winter, which contributes to ensuring the proper duration of the breeding season [3]

Methods

Results

Conclusion