Morphometric and Myelin Basic Protein Expression Changes in Arcuate Nucleus Kisspeptin Neurons Underlie Activation of Hypothalamic Pituitary Gonadal-axis in Monkeys (Macaca Mulatta) during the Breeding Season

ABSTRACT

Introduction

Kisspeptin is involved in the hypothalamic pituitary gonadal-axis’ seasonal regulation in rodents and sheep. Studies of kisspeptin signaling in regulating the transition between breeding and nonbreeding seasons have focused on kisspeptin expression, myelin basic protein (MBP) expression around kisspeptin-ir cells, and quantifying the synaptic connections between kisspeptin and gonadotropin-releasing hormone (GnRH) neurons in various animal models; however, the role of kisspeptin in regulating the seasonal breeding of primates has not been explored yet.

Objective

This study investigated changes in kisspeptin signaling during breeding and a non-breeding season in a non-human primate model, the rhesus monkey.

Methods

Three adult male monkeys (n = 3) from the breeding season and two monkeys (n = 2) from the non-breeding season were used in this study. After measuring the testicular volume and collecting a single blood sample, all animals were humanely euthanized under controlled conditions, and their hypothalami were collected and processed. Two 20 µm thick hypothalamic sections (mediobasal hypothalamus) from each animal were processed for kisspeptin-MBP and kisspeptin-GnRH immunohistochemistry (IHC). One section from each animal was used as a primary antibody omitted control to check the nonspecific binding in each IHC.

Results

Compared to the non-breeding season, plasma testosterone levels and testicular volumes were significantly higher in monkeys during the breeding season. Furthermore, compared to the non-breeding season, increased kisspeptin expression and a higher number of synaptic contacts between kisspeptin fibers and GnRH cell bodies were observed in the arcuate nucleus of the breeding season monkeys. In contrast, enlarged kisspeptin soma and higher MBP expression were observed in non-breeding monkeys.

Conclusion

Our results indicated enhanced kisspeptin signaling in primate hypothalamus during the breeding season. These findings support the idea that kisspeptin acts as a mediator for the seasonal regulation of the reproductive axis in higher primates.

KEYWORDS:

• Kisspeptin
• GnRH
• myelination
• seasonal reproduction
• rhesus macaques

Acknowledgments

The authors are highly grateful to Dr. Ali Nawaz of the Department of Zoology, Quaid-i-Azam University, Islamabad, Pakistan, for his help in the animal collection.

Availability of Data and Materials

All the data is provided within the manuscript.

Disclosure Statement

No potential conflict of interest was reported by the author(s).

Ethics Approval Consent To Participate

All experimental protocols were carried out according to the guidelines of the Research Ethics Committee of the Quaid-I Azam University, Pakistan.

Additional information

Funding

This study was funded by the Higher Education Commission (HEC) of Pakistan and the University Research Fund (URF) of the Quaid-i-Azam University, Islamabad, Pakistan.