Peripubertal Changes in the Nature of the GnRH Response to Alpha-Adrenoceptor Stimulation in vitro and their Modulation by Testosterone

Abstract

Adrenergic mechanisms have been widely implicated in the regulation of GnRH secretion in adult rats but their role in young animals, in which the activity of the GnRH neurones is minimal, is unclear. These experiments were done to examine the effects of α-adrenoceptor stimulation on the secretion in vitro of GnRH by hypothalami from immature and adult male rats. The α₁-adrenoceptor agonist, phenylephrine (10^–9–10^–7M), stimulated release of GnRH from hypothalami from adult (200 g) and peripubertal (150 g) rats but inhibited markedly the secretion of the releasing factor from the limited stores available in hypothalami from immature (50 or 100 g) rats. The stimulatory and inhibitory responses to phenylephrine, evident in adult and younger rats respectively, were concentration-dependent and antagonized readily by the selective αι-adrenoceptor antagonist, alfuzosin (10^–6M), but not by the β-adrenoceptor antagonist, propranolol (10^–6M). Hypothalami from 14-day castrated adult rats, in which the serum LH was elevated and hypothalamic GnRH content reduced, responded to α₁-adrenoceptor stimulation in vitro, like those from immature rats, with a marked reduction in GnRH release. In contrast, hypothalami from corresponding castrates bearing testosterone implants, which maintained the hypothalamic GnRH content and serum LH and testosterone concentrations at levels similar to those of intact controls, exhibited the normal ‘adult’ response to phenylephrine. Studies utilizing ³H-prazosin indicated that the number (B_max) of hypothalamic α₁-adrenoceptor binding sites increases at puberty but that receptor affinity (K_D) is unchanged. A significant increase in β-adrenoceptor binding sites was also apparent in the adult but the density of α₂-adrenoceptors was unchanged. The data accentuate the importance of adrenergic mechanisms in the regulation of GnRH secretion and raise the possibility that alterations in the nature of the GnRH response to α₁-adrenoceptor stimulation may be a contributory factor in the initiation of puberty.

Glucagon-like peptide-1 stimulates luteinizing hormone-releasing hormone secretion in a rodent hypothalamic neuronal cell line.

“…Static incubation of whole hypothalami. The static incubation system used was a modification of the method previously described (29). Adult male Wistar rats (200-220 g) were maintained in groups of five animals per cage as above.…”

In vivo and in vitro Studies on the Opioidergic Control of the Secretion of Gonadotrophin-Releasing Hormone and Luteinizing Hormone in Sexually Immature and Adult Male Rats

“…Our finding that the ability of naloxone to initiate the release of GnRH in vitro is effectively blocked by alfuzosin, an ai-adrenoceptor an tagonist which readily blocks the GnRH response to «i-adrenoceptor stimulation [25], is not in accord with this view. To the contrary, it supports the many lines of evidence which suggest that the p-receptor-mediated opioid inhibition of the secretion of the releasing hormone involves an intermediary catecholaminergic mechanism.…”

“…Alternatively an inhibitory input to the GnRH neurones which effectively prevents the secretory response to naloxone in the intact imma-ture male rat in vivo may be absent in our in vitro preparation. In this context it is interesting to note that several groups have claimed that in the prepubertal male rat in vivo the excitatory noradrenergic drive to the GnRH neurones in overridden by a second catechoiaminergic mechanism which depresses the se cretion of the releasing hormone [25,26]. In addition, gonadal factors may also play a role, for castrated immature rats, unlike their intact counterparts, exhibit a rise in serum LH when chal lenged with naloxone [23].…”

“…Although in the adult rat the tonic inhibitory influence of the opioids on GnRH secretion is apparent in vivo in both the male and the female, in the immature animal, in which the GnRH neurones are only weakly active, it is normally evident only in the female [2,[19][20][21][22][23], Thus, prepubertal female rats respond to the administration of either opioid receptor antagonists or opioid antibodies with a pronounced hypersecretion of LH [ 19,21,24], The male, by contrast, is unresponsive to pharmacolog ical blockade of opioidergic transmission until it approaches the peripubertal period [20,22,23], The reason for this sexual dimorphism is unclear but it may be associated with a delay in the development of the opioid neurone/receptor complex and/or with the existence in the prepubertal male of other more powerful inhibitory drives to the GnRH neurones [25,26],…”