Kisspeptin and the regulation of the hypothalamic-pituitary-gonadal axis in the rhesus monkey (Macaca mulatta)

Abstract

The present article reviews recent studies of monkeys and, in some cases, humans that have been conducted to examine the role of kisspeptin-GPR54 signaling in the regulation of the hypothalamic-pituitary-gonadal axis in higher primates. This area of peptide biology was initiated in 2003 by the discovery that loss of function mutations of GPR54 in man were associated with hypogonadotropic hypogonadism and absent or delayed puberty. Puberty in the monkey, an experimental model commonly used to study this fundamental developmental stage, is first described. This is followed by a review of the role of kisspeptin in the regulation of the postnatal ontogeny of GnRH pulsatility. The roles of kisspeptin in GnRH pulse generation and in the feedback loops governing gonadotropin secretion in primates are then discussed. A brief section on kisspeptin-GPR54 signaling at the pituitary and gonadal levels is also included. The review concludes with a discussion of the phenomenon of GPR54 downregulation by continuous exposure to kisspeptin and its therapeutic implications.

Figure 1

The up-down-up pattern of GnRH pulse generator activity during postnatal development in agonadal male (stippled area) and female (closed data points ± error bars) rhesus monkeys as reflected by circulating mean LH (top panel) and FSH (bottom panel) concentrations from birth to 142-166 weeks of age. Note, in females the intensity and duration of the prepubertal hiatus in the secretion of FSH, and LH to a lesser extent, is truncated in comparison to males. Redrawn with permission from [21].

Figure 2

LH discharges induced precociously in agonadal juvenile male monkeys (20-24 months of age) by an intermittent iv infusion of GnRH that mimics a castrate adult hypophysiotropic drive to the gonadotrophs (open arrows on Day 1) and by brief infusions of h-kisspeptin-10 every h (closed arrows) initiated on Day 1 immediately following termination of GnRH priming and maintained for 48 h in a crossover design (kisspeptin-10, closed data points; vehicle, open data points). LH responses are shown for the first three kisspeptin-10 or vehicle pulses on Day 1, three such pulses on Day 2 and the last two pulses on Day 3. GnRH priming (open arrows) was reinitiated after completion of kisspeptin-10 or vehicle administration on Day 3. Reprinted with permission from [20].

Figure 3

Human kisspeptin-10 induced LH discharges induced in GnRH primed agonadal juvenile male monkeys by brief iv infusions every h for 48 h shown in Figure 1 were completely blocked by treatment with a GnRH receptor antagonist initiated immediately after the first pulse of kisspeptin-10 (black data points) at 1100 h on Day 1. The open data points show kisspeptin-10 induced LH pulses during injection of the antagonist vehicle. LH responses are shown for the first three kisspeptin-10 or vehicle pulses on Day 1, three such pulses on Day 2 and the last two pulses on Day 3. GnRH priming (open arrows) was reinitiated after completion of kisspeptin-10 or vehicle administration on Day 3. Reprinted with permission from [20].

Figure 4

The top panel shows a parasagittal schematic of the region of the arcuate nucleus destroyed by a radiofrequency lesion placed on day 0 in an ovariectomized rhesus monkey. While this lesion preserved circulating prolactin concentrations (top panel) and pituitary response to a continuous infusion of GnRH (LHRH) on day 22, spontaneous and estradiol induced LH and FSH secretion were abolished. Reprinted with permission from [19].

Figure 5

The distribution of KiSS1 mRNA expressing neurons as revealed by in situ hybridization in a representative sagittal hypothalamic section from a premenopausal (left) and postmenopausal (right) woman. KiSS1 expressing perikarya are concentrated only in the infundibular nucleus (INF), also known as the arcuate nucleus (see Figure 4) of the MBH. Only the occasional KiSS1 mRNA expressing neuron was observed in the medial preoptic area (MPOA). KiSS1 was upregulated in the postmenopausal women. Each small black dot represents one KiSS1 expressing neuron. ac, anterior commissure; fx, fornix; INF, infundibular nucleus; MB, mammillary body; oc, optic chiasm; PH, posterior hypothalamus. Scale bar, 2 mm. Reprinted with permission from [28].

Figure 6

A confocal double fluorescence projection showing the relationship between kisspeptin containing perikarya (green Alexa Flor 488) in the arcuate nucleus (ARC) and GnRH cell bodies (red, Cy3) in the ventral hypothalamic tract (VHT) in a coronal hypothalamic section taken at the level of the median eminence (ME) of a male rhesus monkey. At this level, the median eminence is richly innervated with GnRH axons, while kisspeptin axons are found predominately at the boundary of the arcuate nucleus and median eminence. 3V, third ventricle. Scale bar, 100 μm. The primary antibody for kisspeptin was GQ2 (2), kindly provided by Dr Stephen Bloom and used at a dilution of 1:120,000. The primary antibody for GnRH was LR1 kindly provided by Dr. Robert Benoit (Montreal General Hospital, Canada) and previously validated [6]. It was used at a dilution of 1:100,000. Reprinted with permission from [24].

Figure 7

Effect of continuous administration of h-kisspeptin-10 (closed data points) or vehicle (open data points) initiated at 1000h on day 1 and maintained for 98 h (shaded horizontal bar) on LH release in GnRH primed agonadal juvenile male rhesus monkeys. White arrow indicates iv injection of the last GnRH priming pulse at 0800 on day1 and the black arrow indicates iv injection of a bolus of kisspeptin-10 1 h later. Reprinted with permission for [29].

Figure 8

Interrogation of the GnRH neuronal network-gonadotroph axis in GnRH primed agonadal juvenile male rhesus monkeys, during the last 3 h of the 48 h continuous infusion (shaded horizontal bar) of h-kisspeptin-10 (closed data points) or vehicle (open data points) shown in Figure 7, with single sequential boluses of kisspeptin-10 (black arrow), NMDA (gray arrow) and GnRH (white arrow). * Infusion of kisspeptin-10 significantly different (P<0.05) from preinjection mean. Reprinted with permission for [29].