Study of the effect of 26RF- and 43RF-amides on Testosterone and Prolactin secretion in the adult male rhesus monkey (Macaca mulatta)

Abstract

RF-amides (RFa), a superfamily of evolutionary-conserved neuropeptides, are expressed in both invertebrates and vertebrates. While some endocrine functions have been attributed to these peptides in lower vertebrates and few mammalian models, not much is known about their actions in primates. Therefore, the present study was designed to examine the effects of peripheral administration of two recently cloned human RFa peptides, 26RFa and 43RFa, on testosterone and prolactin secretion in the adult male adult male rhesus monkey (Macaca mulatta). For control purposes, a scrambled sequence of 26RFa (Sc-26RFa) and normal saline (1 ml) were injected. Three different doses of 26RFa and 43RFa (19-nmol, 38-nmol and 76-nmol) and a single dose (38-nmol) of Sc-26RFa were tested. A set of four chair-restraint habituated monkeys was used. Comparison of post-treatment T levels with respective pre levels showed that none of the doses of both 26RFa and 43RFa changed T release. Similarly, Sc-26RFa and saline administration also did not affect T levels. In contrast, all doses of 26RFa and 43RFa significantly (P < 0.05) stimulated prolactin secretion. 43RFa dose dependently increased prolactin secretion while dose dependency was not observed for 26RFa. Saline and Sc-26RFa injection had no effect on prolactin concentrations. Thus, present study demonstrated that peripheral administration of 26RFa and 43RFa, in the doses tested, have no effect on T secretion, suggesting possible selective lack of their neuroendocrine role in controlling hypothalamic–pituitary–gonadal axis in the adult male primates. The prominent stimulation of prolactin suggests a neuroendocrine role of RFa peptides in regulation of prolactin release in primates.

Introduction

Adenohypophysis serves as a relay center between the brain and peripheral endocrine organs, and hence it play critical role in the homeostatic regulation of such vital processes as metabolism, growth, reproduction and behavior [29]. The adenohypophysis is functionally linked to the hypothalamus, a critical neuroendocrine regulator area of the brain [4], [23]. The hypothalamus secretes a number of peptides that reach the adenohypophysis via hypophysial portal vein and affect its functioning [9], [23]. In addition to classical hypothalamic releasing and inhibiting factors, adenohypophysis function is also regulated, directly or indirectly, by a number of other hypothalamic peptides.

In recent decades, a large series of peptides with a distinctive Arg–Phe–NH₂ motif at the C-terminus has been identified, and are grouped as the RF-amide (RFa) peptide superfamily [5]. In mammals, major RFa peptides are the neuropeptides FF and AF, prolactin-releasing peptide (PrRP), metastin and other kisspeptins encoded by the Kiss1 gene, as well as RF-related peptides, RFRP-1 and RFRP-3 [10], [11], [12], [14], [19], [30], [37]. More recently, a 26-amino acids RFa peptide, first isolated in the frog and named 26RFa, has also been cloned in some mammalian species [6]. This peptide is the ligand for a previously orphan G protein coupled receptor, GPR103. The GPR103 couples to Gi/o and Gq signaling pathway and it leads to rise in intracellular calcium concentrations and decrease of cAMP production in CHO-GPR103 transfected cells [8]. The N-elongated form of 26RFa, 43RFa, has the same efficacy for binding and activation of GPR103 [8]. Within the brain, 26RFa appears to be mainly expressed in different hypothalamic nuclei, such as the ventromedial hypothalamic nucleus and lateral hypothalamus [8]. However, 26RFa and GPR103 genes expression has also been shown in other brain areas and peripheral tissues [13].

Various physiological roles have been assigned to these peptides in various species [18], [20], [36]. Recently, some preliminary evidences showed the effect of 26RFa and 43RFa on gonadotropin and prolactin (PRL) secretion in rats [18]. In adult cyclic female rat, both central and peripheral administration of 26RFa and 43RFa stimulated gonadotropin secretion. In adult male rats, central as well as peripheral administration of 26RFa inhibited PRL secretion while had no effect on the gonadotropin release [18]. Later, these original findings were confirmed by the study of Patel and colleagues [20], who observed that central injection of 43RFa stimulated LH release, while peripheral administration had no effect. Until now, to the best of our knowledge, in non-rodent mammals especially in primates, the neuroendocrine role of 26RFa and 43RFa is not known at all. Therefore, we considered it relevant to explore the potential role of 26RFa and 43RFa in the regulation of the HPG axis and PRL secretion in the adult male rhesus monkey, a representative higher primate. Due to unavailability of proper facilities for measurements of the rhesus monkey LH, plasma testosterone concentration was measured as terminal marker of the HPG axis.