Analysis of estradiol-independent and -dependent endogenous opioid peptide suppression of pulsatile LH release between the mornings of diestrus 2 and proestrus in the rat estrous cycle

The aim of this study was to analyze possible estradiol (E2)-independent and -dependent endogenous opioid peptide (EOP) suppression of pulsatile LH release between the mornings of diestrus 2 (D2) and proestrus by examining the LH response to naloxone infusions in the presence or absence of proestrous levels of E2. Pulsatile LH secretion remained unchanged between D2 and proestrus but mean blood LH levels, pulse amplitude and frequency increased within 24 hr following ovariectomy on D2. This increase was due in large part to the loss of E2 negative feedback, since restoration of physiological proestrous E2 levels returned LH pulse frequency to proestrous a.m. levels and greatly reduced pulse amplitude. In ovariectomized rats lacking E2 negative feedback, continuous infusion of the EOP receptor antagonist naloxone (0.5 and 2 mg/kg/hr) caused a further increase in pulse amplitude and frequency. This naloxone-induced increment in pulsatile LH release was exerted via centrally located EOP receptors since naloxone did not alter pituitary responsiveness to LHRH, and its stimulatory action on pulsatile release was diminished by simultaneous infusion with morphine. Naloxone also increased pulsatile LH release in E2-treated animals. The naloxone-induced increments in LH pulse amplitude were the same in the presence or absence of E2 negative feedback. Moreover, the increments in amplitude produced by naloxone in E2-treated rats were significantly less than those resulting from the combination of ovariectomy plus naloxone infusion in empty capsule-implanted rats. These data indicated that naloxone infusion in E2-implanted animals blocked an E2-independent EOP suppression of this parameter of pulsatile release.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ascorbate-induced release of LHRH: Noradrenergic and opioid modulation

Ascorbic acid caused a calcium-dependent release of luteinizing hormone-releasing hormone (LHRH) from the rat mediobasal hypothalamus in vitro. The ascorbate-induced release of LHRH was effectively blocked by the adrenergic alpha-receptor blocker phentolamine, but not by the beta-receptor blocker propranolol. The effect of ascorbate was also suppressed by the mu receptor selective opiate agonist sufentanil. These results suggest that the effects of ascorbic acid on the in vitro release of LHRH are mediated by endogenous norepinephrine. Moreover, our results are consistent with the hypothesis that the release of LHRH in the rat is regulated by the hypothalamic noradrenergic system, which is in turn modulated in some, as yet undetermined, fashion by opioid peptides.     

Type II glucocorticoid receptors in the ovine hypothalamus: distribution,influence of estrogen and absence of co-localization with GnRH

There is a strong association between the stress-induced increase in cortisol secretion and perturbation of the neuroendocrine reproductive axis. Previous studies implicate a neural target for glucocorticoids and it is possible that cortisol may act directly on gonadotropin releasing hormone (GnRH) neurons and, thus, luteinizing hormone release, through type II glucocorticoid receptors (GRs). In this study we investigated the effect of estradiol on GR immunoreactivity and determined whether GnRH neurons contain GRs. GRs were dispersed throughout most diencephalic structures but were most concentrated within the medial preoptic area and arcuate nucleus. GR cell numbers were significantly higher in these two areas in ewes pre-treated only with progesterone compared to ewes pre-treated with estradiol plus progesterone; there was no variation in the paraventricular nucleus between groups. No colocalization between GnRH and GRs was observed at any level of the brain. These results suggest that estrogen may down-regulate GRs and glucocorticoids do not act directly on GnRH neurons in the ewe.     

Electrical properties of kisspeptin neurons and their regulation of GnRH neurons

Kisspeptin neurons are critical components of the neuronal network controlling the activity of the gonadotropin-releasing hormone (GnRH) neurons. A variety of genetically-manipulated mouse models have recently facilitated the study of the electrical activity of the two principal kisspeptin neuron populations located in the rostral periventricular area of the third ventricle (RP3V) and arcuate nucleus (ARN) in acute brain slices. We discuss here the mechanisms and pathways through which kisspeptin neurons regulate GnRH neuron activity. We then examine the different kisspeptin-green fluorescent protein mouse models being used for kisspeptin electrophysiology and the data obtained to date for RP3V and ARN kisspeptin neurons. In light of these new observations on the spontaneous firing rates, intrinsic membrane properties, and neurotransmitter regulation of kisspeptin neurons, we speculate on the physiological roles of the different kisspeptin populations.     

Subsets of gonadotropin-releasing hormone (GnRH) neurons are activated during a steroid-induced luteinizing hormone surge and mating in mice: a combined retrograde tracing double immunohistochemical study

The decapeptide gonadotropin-releasing hormone (GnRH) plays a pivotal role in reproduction and is synthesized by GnRH-producing cell bodies in the basal forebrain. Experiments were designed to investigate whether GnRH cells projecting outside the blood brain barrier or those projecting within the brain are activated during the steroid-induced LH surge or mating in female mice. Retrograde uptake of intraperitoneally administered fluorogold (FG) by GnRH cells and double immunostaining for GnRH and Fos was employed for this purpose. The number of GnRH cells with FG uptake was comparable among the surged, mated and control mice. However, the number of Fos-positive GnRH cells was significantly higher in the steroid-induced LH surge group than in the mated mice. The number of Fos+FG-positive GnRH cells was higher and the number of FG-only GnRH cells was lower in mice with a steroid-induced LH surge as compared with the mated mice. This suggests the existence of a subgroup of GnRH cells projecting outside the blood-brain barrier activated during the steroid-induced LH surge but not during mating. The activation of similar proportions of GnRH cells without FG uptake in both the mated and the surge group indicate that nonneuroendocrine GnRH cells are not silent but can be activated by both mating and steroid hormones. Thus, functional subgroups may exist within the GnRH system with considerable overlap in the input to these cells.     

Central Nervous System Sites of Action of an Enkephalin Analogue, (D-Met2, Pro5)-Enkephalinamide, and Naloxone on the Secretion of Five Anterior Pituitary Hormones of Male Rats

Central nervous system sites of action of opioid peptides on pituitary hormone secretion were investigated. One nmol of an enkephalin analogue, (D-Met², Pro⁵)-enkephalinamide, and 10 nmol of the opiate antagonist naloxone were injected into ten different regions of the brain of conscious male rats and their effect on the release of five anterior pituitary hormones tested. The injections were made through a special injection cannula which was inserted into the brain through a guide cannula fixed on the skull and implanted into the brain 5 to 7 days earlier. Both compounds injected into the medial septum, medial preoptic area and hypothalamic paraventricular nucleus affected prolactin, growth hormone and luteinizing hormone (LH) secretion. The enkephalin analogue stimulated prolactin and growth hormone and inhibited LH release. Naloxone induced the opposite effect. Drugs given into the hypothalamic ventromedial nucleus caused changes in plasma prolactin and growth hormone levels. Enkephalinamide increased and naloxone decreased plasma concentrations of both hormones. Administration of the compounds into the dorsal raphe area resulted in alterations of prolactin and LH release, the analogue caused elevation of prolactin and inhibition of LH release, whereas the opiate antagonist resulted in opposite changes. Only an LH response was obtained from the hypothalamic dorsomedial nucleus and a growth hormone response from the central amygdala. Also in these cases the enkephalin analogue decreased LH and elevated growth hormone plasma levels, and naloxone brought about a rise in LH and a diminution of growth hormone concentration. None of the regions were effective in inducing a clear-cut adrenocorticotrophin or follicle-stimulating hormone response. The parietal cortex, medial amygdala and the dentate gyrus were entirely ineffective sites. The findings suggest that in the brain there are multiple sites of action of opioids on pituitary trophic hormone secretion and the effective sites are not identical in terms of pituitary hormone response.     

Gonadotrophin-releasing hormone (GnRH) release in marmosets II: pulsatile release of GnRH and pituitary gonadotrophin in adult females

Unlike other mammals, including rodents, Old World primates and humans, common marmosets and probably all other New World primates synthesise and release chorionic gonadotrophin (CG), and not luteinising hormone (LH) from pituitary gonadotrophs. However, little is known about the physiological dynamics of gonadotrophin-releasing hormone (GnRH)-regulated CG release from gonadotrophs and whether such CG release has pulsatile release characteristics similar to those of LH in other mammalian species. Consequently, we performed a series of in vivo and in vitro studies in ovariectomised laboratory rats and female marmosets to compare GnRH-induced pituitary LH and CG release characteristics, respectively. Exogenous GnRH stimulated a slower onset of release of marmoset pituitary CG, both in vivo and in vitro, and induced an approximately 400% greater increase in the duration of marmoset pituitary CG release compared to that for rat LH. Not surprisingly, hypothalamic pulsatile release of GnRH in vivo was not obviously concordant with endogenous episodic changes in circulating levels of CG in marmosets, in contrast to the clear concordance observed between in vivo GnRH and LH release previously demonstrated in rats and other mammals. Pituitary CG release in marmosets thus demonstrates considerable divergence from the timely hypothalamic GnRH-regulated LH release in other female mammals, implying potentially different physiological dynamics in gonadotrophin regulation of marmoset ovarian function.     

Facilitation or inhibition of the oestradiol-induced gonadotrophin surge in the immature female rat by progesterone: effects on pituitary responsiveness to gonadotrophin-releasing hormone (GnRH), GnRH self-priming and pituitary mRNAs for the progesterone receptor A and B isoforms

Progesterone can either facilitate or inhibit the oestradiol (E(2))-induced gonadotrophin surge. We have previously developed immature female rat models to characterise and investigate the mechanisms of progesterone inhibition or facilitation. The aim of the present study was to determine the role of pituitary responsiveness to gonadotrophin-releasing hormone (GnRH) and GnRH self-priming under conditions of progesterone-facilitation and progesterone-inhibition, and whether the underlying mechanisms reflect changes in mRNAs encoding the A and B isoforms of the progesterone receptor (PR) in the pituitary gland. Pituitary responsiveness to GnRH, determined by measuring the luteinising hormone (LH) response to one i.v. injection of GnRH, was decreased by 60-80% (P < 0.001) in the progesterone-inhibition model. GnRH self-priming, estimated as the increment in the LH response to the second of two injections of GnRH separated by 60 min, was also significantly reduced (P < 0.05) in this model. In the progesterone-facilitation model, the LH response to GnRH injection was increased 2.5-3-fold (P < 0.05), an effect suppressed by the progesterone receptor antagonist, mifepristone. Progesterone-facilitation of LH release and increased pituitary responsiveness to GnRH were blocked by sheep anti-GnRH serum injected i.v. immediately after insertion of progesterone implants. The PR-B mRNA isoform, measured by solution hybridisation/RNase protection assay, was the predominant form in the pituitary of the immature female rat. PR-B was increased by E(2) and decreased by progesterone in both models. Thus, in immature female rats, progesterone-inhibition of the E(2)-induced LH surge is due to significant reduction in pituitary responsiveness to GnRH as well as in the magnitude of GnRH self-priming. Progesterone-facilitation of the E(2)-induced LH surge is due to increased pituitary responsiveness to GnRH, which is mediated by PR, and depends on endogenous GnRH release. The differences between progesterone-facilitation and progesterone-inhibition are not due to differences in regulation of pituitary PR-B mRNA.     

Fasting reduces KiSS-1 expression in the anteroventral periventricular nucleus (AVPV): effects of fasting on the expression of KiSS-1 and neuropeptide Y in the AVPV or arcuate nucleus of female rats

Changes in metabolic state, such as those induced by fasting, have profound effects on reproduction. In rats, the time-course over which fasting inhibits luteinising hormone (LH) release is reduced to 48 h by the presence of oestradiol-17beta (E(2)). Hypothalamic kisspeptin plays a key role in mediating the actions of E(2) on gonadotrophin-releasing hormone (GnRH) neurones, and thereby promotes LH release. KiSS-1-expressing neurones are found in the anteroventral periventricular nucleus (AVPV) and arcuate nucleus (ARC). Extensive evidence implicates the AVPV in GnRH release and the ARC in energy balance. The latter nucleus also contains neurones that express neuropeptide Y (NPY), an orexigenic peptide implicated in GnRH control. To elucidate the involvement of kisspeptin and/or NPY in hypothalamic responses to fasting, their expression was quantified by in situ hybridisation histochemistry in ovariectomised rats, with or without E(2) replacement, before and after 48 h of fasting. In the presence of E(2), but not in its absence, the fasting suppressed plasma LH. In the AVPV, the low level of KiSS-1 expression found in the absence of E(2) was unaffected by fasting. By contrast, the elevated level found in the presence of E(2) was suppressed by fasting. Independent of E(2), fasting had no effect on KiSS-1 expression in the ARC, but increased NPY expression at that site. The present study has identified the AVPV as a site at which KiSS-1 expression can be influenced by fasting. The results suggest that inhibition of KiSS-1 expression in the AVPV may be a significant factor in restraining the gonadotrophic axis in response to negative energy balance in the presence of oestrogen. The extent to which the concurrent rise in NPY expression in the ARC may contribute to the suppression of LH release by influencing AVPV kisspeptin neurones, directly or indirectly, or by actions independent of kisspeptin, remains to be established.     

Seasonal Plasticity in the Peptide Neuronal Systems: Potential Roles of Gonadotrophin‐Releasing Hormone,Gonadotrophin‐Inhibiting Hormone,Neuropeptide Y and Vasoactive Intestinal Peptide in the Regulation of the Reproductive Axis in Subtropical Indian Weaver Birds

Two experiments examined the expression of gonadotrophin‐releasing and inhibiting hormones (GnRH‐I, GnRH‐II and GnIH), neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) in subtropical Indian weaver birds, which demonstrate relative photorefractoriness. Experiment 1 measured peptide expression levels in the form of immunoreactive (‐IR) cells, percentage cell area and cell optical density in the preoptic area (GnRH‐I), midbrain (GnRH‐II), paraventricular nucleus (GnIH), mediobasal hypothalamus [dorsomedial hypothalamus (DMH), infundibular complex (INc), NPY and VIP] and lateral septal organ (VIP) during the progressive, breeding, regressive and nonbreeding phases of the annual reproductive cycle. GnRH‐I was decreased in the nonbreeding and VIP was increased in INc in the breeding and regressive states. GnRH‐II and NPY levels did not differ between the testicular phases. Double‐labelled immunohistochemistry (IHC) revealed a close association between the GnRH/GnIH, GnRH/NPY, GnRH/VIP and GnIH/NPY peptide systems, implicating them interacting and playing roles in the reproductive regulation in weaver birds. Experiment 2 further measured these peptide levels in the middle of day and night in weaver birds that were maintained under short days (8 : 16 h light /dark cycle; photosensitive), exposed to ten long days (16 : 8 h light /dark cycle; photostimulated) or maintained for approximately 2 years on a 16 : 8 h light /dark cycle (photorefractory). Reproductively immature testes in these groups precluded the possible effect of an enhanced gonadal feedback on the hypothalamic peptide expression. There were group differences in the GnRH‐I (not GnRH‐II), GnIH, NPY and VIP immunoreactivity, albeit with variations in immunoreactivity measures in the present study. These results, which are consistent with those reported in birds with relative photorefractoriness, show the distribution and possibly a complex interaction of key neuropeptides in the regulation of the annual reproductive cycle in Indian weaver birds.     

The forgotten effects of thyrotropin-releasing hormone: Metabolic functions and medical applications

Thyrotropin-releasing hormone (TRH) causes a variety of thyroidal and non-thyroidal effects, the best known being the feedback regulation of thyroid hormone levels. This was employed in the TRH stimulation test, which is currently little used. The role of TRH as a cancer biomarker is minor, but exaggerated responses to TSH and prolactin levels in breast cancer led to the hypothesis of a potential role for TRH in the pathogenesis of this disease. TRH is a rapidly degraded peptide with multiple targets, limiting its suitability as a biomarker and drug candidate. Although some studies reported efficacy in neural diseases (depression, spinal cord injury, amyotrophic lateral sclerosis, etc.), therapeutic use of TRH is presently restricted to spinocerebellar degenerative disease. Regulation of TRH production in the hypothalamus, patterns of expression of TRH and its receptor in the body, its role in energy metabolism and in prolactin secretion are addressed in this review.     

The regulation of LHRH action at the pituitary and gonadal receptor level: a review

Luteinizing hormone releasing-hormone (LHRH) and its highly active agonists are under clinical investigation for the control of reproductive function and for suppression of hormone dependent tumours. The regulation of LHRH action by pituitary receptors and expression of the biological LHRH effect by gonadotropin release and activation of steroid biosynthesis are discussed in this context. Pituitary LHRH receptors are controlled by autoregulation via endogenous LHRH secretion. The gonadal response to LHRH stimulation is regulated by LH action on receptors for LH, prolactin and FSH. Pituitary and gonadal inhibition are achieved by different mechanisms. Continuous exposure to LHRH blocks gonadotropin release and reduces pituitary LH/FSH content, whereas inhibition of steroid biosynthesis requires daily LH release to maintain receptor down-regulation. Pituitary enzymes involved in LHRH degradation at the receptor site are required for terminating hormone action, but their role in modulating hormonal responsiveness is secondary to receptor regulation. Direct gonadal effects of LHRH are exerted in the presence of gonadotropins by modulating the gonadotropin effect, e.g. in hypophysectomized animals. The presence of specific receptors for LHRH agonists in ovarian and testicular tissue suggests local control mechanisms for gonadotropin activation of steroid biosynthesis.     

The microtubular apparatus of cerebellar purkinje cell dendrites during postnatal development of the rat: The density and cold-stability of microtubules increase with age and are sensitive to thyroid hormone deficiency

A quantitative ultrastructural study of microtubules in Purkinje cell dendrites of normal and hypothyroid developing rats was performed after fixation either at room or at low temperature (4°C). In normal animals, the density of microtubules and their fold-stability increased with age, more especially during the period of intense dendritic growth. Thyroid deficiency delayed the appearance of microtubules and still more the acquisition of their fold-stability. These effects might explain the defects in Purkinje cell dendritic growth and branching observed in hypothyroid animals.     

The Critical Period for Experience-dependent Plasticity in a System of Binocular Visual Connections in Xenopus laevis: Its Temporal Profile and Relation to Normal Developmental Requirements

A commissural system of 'intertectal' connections in Xenopus mediates the registration of binocular visual maps at the midbrain optic tectum. Following surgical eye rotation in larval animals, the system can completely alter its pattern of connectivity to restore binocular visual registration at the tectum. This experimentally induced plasticity is known to require visual experience and thought to be subject to an age-related restriction: eye rotation in adult animals is reported to provoke no subsequent intertectal alteration. In this paper we describe the detailed age-dependence of this plasticity. One eye was rotated in 238 animals of various developmental stages between mid-larval and adult life. At each age, different animals received rotations of different sizes, ranging from 20 to 180^°. The pattern of intertectal connectivity was mapped electrophysiologically 1–2 years postoperatively. A 'critical' period was defined around the time of metamorphosis: the vast majority of animals receiving a rotation in larval life (up to -2 weeks before metamorphic climax) showed altered intertectal connections, whereas none of the animals operated upon at 3 months or more postmetamorphosis displayed the plasticity. At intervening ages, altered intertectal connections were found only in response to progressively smaller eye rotations. The profile of this critical period was further shown to mirror temporal features of the changes in eye position that occur in Xenopus as natural consequences of head growth, and which themselves impose a normal developmental requirement for intertectal plasticity. We conclude that the capacity of the Xenopus intertectal system for plasticity in response to abnormal experience undergoes a progressive age-dependent decline, and that the profile of this decline is delimited by normal requirements.