Neuropharmacological analysis of the control of LH secretion in gonadectomized male and female rats: altered hypothalamic responses to inhibitory neurotransmitters in long-term castrated rats

The control of LHRH and LH by neurotransmitters and neuromodulators such as the endogenous opioid peptides is essentially the same in intact adult male and female rats: adrenergic and dopaminergic agonists stimulate LH release and opioid agonists inhibit it. Several weeks after gonadectomy, however, the contribution of the endogenous ligands of adrenergic, dopaminergic and opioidergic receptors to the control of LHRH is altered. A detailed pharmacological analysis in long-term ovariectomized females confirmed previous reports that adrenergic and dopaminergic agonists still enhance secretion of LHRH and LH and opioid receptor agonists still suppress it. A similar investigation in long-term castrated males also confirmed previous reports that opioid agonists fail to block LH secretion. In addition, we have found that while adrenergic and dopaminergic agonists cause increases in serum concentrations of LH, adrenoreceptor and dopamine receptor antagonists do not inhibit LH release in long-term castrates. Furthermore, the opioid antagonist naloxone does not raise serum LH levels in either sex after long-term gonadectomy. These observations therefore imply reduced opioidergic, dopaminergic and adrenergic transmission, in relation to LHRH release, after long-term castration. In addition, opioid receptor activity (assessed by responsiveness to an opioid receptor agonist) of female rats is maintained, whereas that of male rats is lost, after long-term gonadectomy.     

Effects of anterior hypothalamic deafferentation and neonatal monosodium-L-glutamate treatment on pulsatile LH secretion in the castrated rat

Previous research has shown that electrolytic lesions of the anterior arcuate nuclei combined with anterior hypothalamic deafferentation (AHD) blocked pulsatile luteinizing hormone (LH) release in the ovariectomized rat, but that neither lesion was sufficient by itself. This led to the hypothesis that two separate neural pathways were capable of effecting pulsatile LH secretion, which was tested in another way in the present study. Newborn rats were injected with monosodium glutamate (MSG), a treatment which damages the arcuate nuclei, or NaCl of the same osmolarity as a control. As adults, the rats were gonadectomized and 2 weeks later, AHD or sham AHD was performed. One week after that, the rats were catheterized, and small blood samples were taken every 5 min for 3 h. Plasma was assayed for LH by radioimmunoassay. Neither MSG alone, AHD alone, nor the combination affected the number of animals with pulsatile LH patterns. In female rats, MSG treatment slightly decreased pulse frequency, while AHD depressed mean LH levels in male rats. No differences were seen in LH pulse amplitude. These results suggest either that MSG spares those neurons of the arcuate nuclei which are important for LH secretion, that electrolytic lesions of the arcuate nuclei destroy fibres of passage which are important for LH release, or that functional reorganization of the systems controlling LH release occurs following neonatal MSG treatment.     

The role of LH in the autofeedback inhibition of LH and FSH secretion in ovariectomized rats

This study examined the role of exogenous LH in the autofeedback regulation of LH and FSH release in ovariectomized rats. The rats were implanted with third ventricular cannulae three weeks after ovariectomy and fitted with silastic jugular cannulae one week later. Baseline hormone levels were established on the day of experimentation in conscious, unrestrained animals. Thereafter, experimental animals received intraventricularly (IVT) either a 9 ug or 20 ug dose of a purified preparation of human (h)LH that did not crossreact in our rLH RIA. In response to 20 ug, but not 9 ug of LH, plasma levels of both LH and FSH were significantly reduced during the following one hour period compared to values in controls receiving buffer IVT. Administration of ovine (o)LH (6 ug, IVT), a preparation which crossreacts in the rLH RIA, significantly elevated plasma levels of detectable LH during the experimental period indicating that LH reaches the blood stream from the third ventricle and, thus, may effect endogenous hormone release at either the pituitary or hypothalamic levels. However, in animals preinjected with 9 or 20 ug of hLH IVT one hour earlier the surge of both LH and FSH in response to LHRH (25 ng iv) was not different from that in the buffer-injected controls receiving LHRH which indicates that pituitary responsiveness was not suppressed by the effective dose of hLH. The results of this study suggest that the inhibitory shortloop feedback of LH on endogenous LH and FSH secretion in ovariectomized rats occurs at the level of the hypothalamus.     

The role of the beta-adrenergic system in the function of calcium-regulating glands

The examination of 27 patients with coronary heart disease and experiments on 42 rabbits showed that intravenous administration of beta-adrenostimulant isoprenaline enhanced the blood level of parathormone and depressed that of calcitonin whereas administration of any beta-adrenoblocker showed contrary results. The patterns of adrenergic system effects on function of the calcium-regulating glands were discussed.     

Anaesthesia with alphaxalone plus alphadolone acetate decreases serum concentrations of LH in castrated rats

Alphaxalone is considered the anaesthetic of choice in neuroendocrine reproductive studies in female rats, since it appears to have little, if any, effect on release of gonadotrophin-releasing hormone. There has been less study of the effects of this anaesthetic on the male reproductive neuroendocrine axis, however. Accordingly, the time-dependent effects of alphaxalone, as well as of urethane and ketamine, on the increased levels of LH in castrated rats were determined. Each anaesthetic was administered i.p. and each depressed LH levels significantly compared with those in castrated unanaesthetized rats killed by decapitation (controls). The effect of the anaesthetics was noted 15 min after administration and persisted at 30 and 60 min in animals anaesthetized with alphaxalone and urethane. Only in ketamine-anaesthetized animals did serum concentrations of LH finally rise to concentrations not significantly different from those in control rats. Thus alphaxalone, though useful in female neuroendocrine studies, is as profoundly disruptive as other anaesthetics on the male rat hypothalamic-pituitary reproductive unit.     

Effect of leptin on LH levels and hypothalamic release of GnRH. Its relationship with the hypothalamic neurotransmitter amino acids system in adult male rats

The objective of the present paper was to determine the effect of leptin on the reproductive axis in adult male rats, as well as the hypothalamic mechanisms involved in this effect. For this purpose, we studied the in vivo effect of leptin in adult male rats on serum LH levels, and the in vitro effect on hypothalamic GnRH and amino acid neurotrasmitter release. For in vivo experiments, animals were injected i.p. with leptin at a dose of 30, 100 and 300 microg/kg. In the in vitro experiments, hypothalamic samples were incubated for 60 min in Earle's medium with leptin: 10(-9), 10(-10) and 10(-12) M for GnRH determination, and 10(-10) M for amino acids evaluation. Finally, we studied the effect of the lowest effective leptin dose on plasma LH levels in peripubertal male rats to compare the effect between this group and adults. Leptin induces significant decreases of serum LH levels with the different studied doses (p < 0.01 vs. control) in adult male rats, while in peripubertal male rats, it induced a significant (p < 0.01 vs. control) increment in serum LH levels. On the other hand, in vitro leptin in adult male rats, significantly decreases GnRH release as well as the hypothalamic release of glutamate (GLU). In contrast, leptin increased the GABA release by this hypothalamus in these animals. These results indicate that leptin has an inhibitory effect on the GnRH-LH axis in adult male rats and this effect appears to be connected with an inhibition of hypothalamic release of GLU (the excitatory amino acid) and a stimulatory effect on GABA release (the inhibitory amino acid). On the other hand, in peripubertal male rats, leptin showed a stimulatory effect.     

Activation of sexual behaviour in castrated rats: the role of oestradiol

Sexual behaviour was induced in castrated male rats with oestradiol-17 beta- or testosterone-filled constant-release implants. Testosterone-induced sexual behaviour was unaffected by treatment with the 5 alpha-reductase inhibitor 17 beta-N,N-diethylcarbamoyl-4-aza-5 alpha-androstan-3-one (4-MA; 16.7 mg/day) but treatment with the aromatization inhibitor 1,4,6-androstatriene-3,17-dione (ATD; 10 mg/day) prevented testosterone from inducing the behaviour. Sexual behaviour could be activated in castrated rats treated with testosterone plus ATD by treatment with 4-MA or with implants filled with a low dose of oestradiol. Lordosis behaviour induced in ovariectomized rats with testosterone-filled implants and progesterone was blocked by ATD treatment and could not be activated with 4-MA but oestradiol implants restored the display of lordosis in the testosterone plus ATD-treated females. 4-MA inhibited the in-vitro formation of [14C]5 alpha-dihydrotestosterone from [14C]testosterone by combined preoptic and hypothalamic tissue at all doses tested and a high dose of oestradiol exerted a similar effect. The results suggest that androgen aromatization is required for testosterone-activated female sexual behaviour but not for testosterone-activated male sexual behaviour. It is suggested that oestradiol normally acts to control the sexual behaviour of male rats by modifying neural androgen metabolism.     

Evidence for a stimulatory beta-adrenergic component in the release of the ovulatory LH surge in pro-oestrous rats

The effect on ovulation of intraventricular infusions of noradrenaline, adrenaline and various pharmacological agents acting on the adrenergic receptor subtypes were investigated in cyclic female rats on the day of pro-oestrus. The inhibitory effects on ovulation of the different infusions were monitored by administering the drugs before 11.00 h (several hours before the critical period for the ovulatory LH surge). In experiments designed to show how the drugs under investigation might stimulate ovulation, pentobarbitone sodium (35 mg/kg) was given at 14.30 h; this anaesthetic inhibits ovulation and its effects can be overcome by substances that advance the preovulatory LH surge. Noradrenaline (an alpha-agonist) stimulated ovulation when administered on the morning of pro-oestrus to rats injected with pentobarbitone early in the afternoon of the same day. Phenoxybenzamine and phentolamine (non-selective alpha-antagonists) and clonidine (a selective alpha 2-agonist) all inhibited ovulation when infused on the morning of pro-oestrus. Yohimbine (a moderately selective alpha 2-antagonist) neither stimulated nor inhibited ovulation. Both isoprenaline (a non-selective beta-agonist) and fenoterol (a selective beta 2-agonist) stimulated ovulation in pentobarbitone-treated rats when administered on the morning of pro-oestrus and fenoterol was also able to overcome the pentobarbitone block when infused later in the afternoon. Propranolol (a non-selective beta-antagonist) and metoprolol (a selective beta 1-antagonist) were stimulatory only when administered in the afternoon. Adrenaline (both an alpha- and beta-agonist), prenalterol (a selective beta 1-agonist), atenolol (a selective beta 1-antagonist) and ICI 118,551 (a selective beta 2-antagonist) neither stimulated nor inhibited ovulation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Distribution of hypothalamic cholinergic and adrenergic system controlling hypophyseal-adrenal system function

It has been shown on rat experimental models that the cholinergic systems, participating in hypothalamic regulation of the hypophyseal adrenocortical function, are localized in the posterior hypothalamus whereas alpha-adrenergic systems in the posterior and anterior hypothalamus, and beta-adrenergic systems in the ventromedial hypothalamus.     

Effect of electrical stimulation of the dorsomedial hypothalamic nucleus on pulsatile LH release in ovariectomized rats

The present experiments were carried out to examine the influence of electrical stimulation of the dorsomedial hypothalamic nucleus (DMH) on pulsatile LH release in ovariectomized rats, and the possible involvement of serotonin (5HT) in mediating any observed effects. Unanesthetized animals were bled continuously through jugular vein cannulae for an initial 1 1/2 h control period. Rats were then stimulated for two 1-hour periods, separated by a 45-min nonstimulation period. DMH stimulation suppressed pulsatile LH release and decreased mean blood LH levels during both stimulation periods in control rats as well as animals in which 5HT synthesis was inhibited by p-chlorophenylalanine or 5HT receptors were blocked by metergoline. The decrease in mean blood LH levels in all groups was due solely to an increase in the LH interpulse interval. LH pulses that did occur during the stimulation-induced suppression did not show a decreased LH pulse amplitude. Rather, in control and metergoline-treated animals this parameter increased during stimulation, suggesting a buildup of readily releasable LHRH and/or pituitary LH. Lastly, estradiol benzoate suppressed pulsatile LH secretion but did not reverse the inhibitory LH response to DMH stimulation. These experiments indicate that the DMH, because of neurons originating within this nucleus and/or fibers passing through, may be a brain region that influences pulsatile LH release in a suppressive manner. This effect is exerted solely on the periodicity of the process and not through a 5HT-mediated mechanism.     

Correlation between LH secretion in castrated rats with cellular proliferation and synthesis of DNA in the anterior pituitary gland

Testosterone metabolism in the brain and pituitary and cloacal glands of male and female Japanese quail was studied in vitro during sexual maturation (from 1 day to 5 weeks after hatching). The production of 5 alpha-dihydrotestosterone in the hyperstriatum and cloacal gland and that of androstenedione in the cloacal gland of males was highest at 1 day after hatching, which could be related to the peak of plasma androgens previously demonstrated in neonatal quail. 5 beta-Reductase activity was very high in the brain, but not the pituitary or cloacal glands of young chicks and decreased markedly, especially in the hypothalamus, during sexual maturation. As 5 beta-reduced metabolites of testosterone are inactive androgens, it is suggested that the decrease of 5 beta-reductase activity with age corresponds to a potentiation of the effects of testosterone at the level of the brain.