SOME ASPECTS OF THE NEUROENDOCRINE REGULATION OF GROWTH HORMONE SECRETION IN AN INFRA-HUMAN PRIMATE

Abstract

The secretion of growth hormone (GH) from the anterior pituitary is under the dual control of two hypothalamic factors, the stimulatory GH-releasing hormone (GHRH) and the inhibitory somatostatin (SS). The synthesis and secretion of these two neurohormones is influenced by a variety of neuromodulators, neuropeptides and neurotransmitters, including glutamate and aspartate. N-methyl-D-aspartate (NMDA) and N-methyl-D,L-aspartate (NMA) are excitatory amino acid analogues of aspartate which bind to a specific receptor in the brain known as NMDA receptor. In both rats and primates, the activation of these receptors has been shown to stimulate the secretion of GH via a hypothalamic release of GHRH. The present study attempts to examine the role of NMDA receptor in the central regulation of GH secretion during specific stages of pubertal development of the male rhesus monkey (Macaca mulatta). In addition, a possible modulation of GH secretion by testosterone (T) at both the pituitary and the hypothalamic levels has been assessed in this primate species. The involvement of NMDA receptor in the regulation of GH secretion during different seasons of the year has also been investigated. Infantile, prepubertal, peripubertal and adult intact or chronically orchidectomized monkeys were used in this investigation. NMA (15 mg/kg BW), GHRH (0-25 ).!g/kg BW) or normal saline was infused through a teflon cannula implanted in the saphenous vein. Blood samples were collected 20-60 min before and 40-120 min after the injection of the drug or the neurohormone at 15-20 min intervals. NMA or GHRH was dissolved in normal saline immediately before use and passed through a 0.22 ).lm filter at the time of injection. All bleedings were carried out under ketamine hydrochloride anesthesia (initial dose 5 mg/kg BW, im followed by 2.5 mg/kg at 30 min intervals). The plasma levels of GH, T and estradiol (E2) were determined by using specific assay systems. In the first set of experiments, the hypothalamic-somatotrope activity under basal conditions as well as the sensitivity ofNMDA receptor to NMA stimulation in infantile, prepubertal, peripubertal and adult monkeys were studied. The mean basal plasma concentrations of GH 1ll the four groups of animals showed marked age-related differences. The levels of GH were found to be higher in infantile and peripubertal monkeys as compared to those of prepubelial and adult animals. A single iv injection of NMA (15 mg/kg BW) produced differential effects on GH secretion during specific stages of postnatal development depending upon the level of GH secretion under basal conditions. Whereas NMA had no demonstrable effect on GH secretion in infantile and peripubertal animals in which the basal GH levels were high, it produced pronounced effects on GH secretion in prepubertal and adult monkeys having low baseline GH concentrations. In another set of experiments, the role of T in modulation of GH secretion at the level of pituitary was assessed by measuring the GHRH-stimulated GH secretion in intact, orchidectomized and T (testosterone enanthate: 125 mg/week, im for four weeks) treated orchidectomized monkeys. Initially, GH response to graded doses of GHRH (0-25 ~ g/kg BW) was determined using 3 intact and 3 orchidectomized adult monkeys for obtaining an optimal dose to be used in the subsequent experiment. In the following experiment, GH response to two successive inj ections of GHRH (25 ~ g/kg BW) at 3 h intervals was studied in intact, orchidect.omized and T -replaced orchidectomized monkeys. The administration of GHRH resulted in a significant stimulation of GH secretion at all the doses tested and in both the groups studied. In both intact and orchidectomized animals, the greatest response was observed at 6.25 ~g/kg and no further increase was noted with the higher doses of GHRH. Furthermore, the responses to GHRH were not significantly different between intact and orchidectomized monkeys at any of the dose levels. The administration of two successive injections of GHRH at 3 h intervals elicited significant rise in GH secretion in response to both the injections in all the three groups of animals, but the response to the second GHRH administration was significantly lower as compared to the first GHRH challenge. In the third set of experiments, the role of T in influencing the secretion of GH at the level of hypothalamus was studied by measuring the NMA-induced GH secretion in intact, orchidectomized, and T (testosterone enanthate: 125 mg/week, im for five weeks) substituted orchidectomized monkeys. A single iv injection ofNMA (15 mg/kg BW) to intact sexually mature monkeys resulted in a rapid and large increase in GH circulating levels reaching a peak within 10 min of the injection, whereas a similar dose regimen of NMA failed to induce a significant rise in peripheral concentrations of GH in orchidectomized animals. Testosterone replacement in orchidectomized monkeys reestablished the GH responsiveness to NMA stimulation. In the final set of experiments, seasonal variations in body weight, testicular size, GH and T secretion of six adult male rhesus monkeys were examined in a study spanning a period of one year. The role of NMDA receptor in modulation of the seasonal activity of the hypothalamic-pituitary unit was also assessed by measuring the responsiveness of GH and T to a bolus intravenous injection ofNMA at alternate months. Whereas the body weight recorded over a period of one year failed to demonstrate any significant change, the testicular volume underwent a marked seasonal variation. Testicular regression was observed from Apr to Jun (non-breeding season) and testicular recrudescence from Oct to Dec (breeding-season). Mean basal plasma GH concentrations remained unchanged in different seasons, although significant individual variations in GH secretion were evident. Basal concentrations of plasma T, on the other hand, showed marked variations tlu-oughout the year. High T concentrations were recorded in fall and winter, whereas relatively low plasma concentrations of the hormone were observed in spring and summer. Administration of NMA led to a significant 111crease 111 mean plasma GH concentrations over basal levels, reaching peak values within 20 min of the injection in most of the animals. GH response to NMA stimulation was significantly higher in spring-summer than in autumn-winter. Likewise, most of the animals, when challenged with NMA, exhibited a significant increase in mean plasma T concentrations over basal levels, reaching peak values within 40 min of the injection. The response to NMA was markedly greater during the non-breeding season (lun-Aug) than the breeding season (Oct-Dec), suggesting a season dependent response ofT secretion to NMA stimulation. In conclusion, the present study suggests that the glutamatergic component of the control system that governs GH secretion by utilizing NMDA receptor may play an impOltant role in regulation of age-related changes in the secretion of GH in the male rhesus monkey. Elevated circulating levels of GH during the infantile and peripubertal stages could be due to high NMDA receptor mediated neurotransmission. Conversely, low peripheral concentrations of GH in prepubertal and adult monkeys could be due to lower NMDA receptor mediated inputs to GHRH neurons. The present investigation also suggests that T does not affect the sensitivity or the secretory capacity of the pituitary somatotropes, but stimulates the secretion of GH by increasing the amount of GHRH secreted by the hypothalamic neurons. The current data also provide evidence that NMDA receptor system controlling GH secretion is influenced by season-related alterations and that activation of NMDA receptor may constitute a pmt of the mechanism underlying the onset of seasonal breeding activity in the male rhesus monkey.