In vitro GnRH release from the isolated medial basal hypothalamus of the male guinea pig: evidence for the existence of two pools of releasable GnRH

Pyrethroid Insecticide Cypermethrin Accelerates Pubertal Onset in Male Mice via Disrupting Hypothalamic-Pituitary-Gonadal Axis

Pyrethroids, a class of insecticides that are widely used worldwide, have been identified as endocrine-disrupting chemicals (EDCs). Our recent epidemiological study reported on an association of increased pyrethroids exposure with elevated gonadotropins levels and earlier pubertal development in Chinese boys. In this study, we further investigated the effects of cypermethrin (CP), one of the most ubiquitous pyrethroid insecticides, on hypothalamic-pituitary-gonadal (HPG) axis and pubertal onset in male animal models. Early postnatal exposure to CP at environmentally relevant doses (0.5, 5, and 50 μg/kg CP) significantly accelerated the age of puberty onset in male mice. Administration of CP induced a dose-dependent increase in serum levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and testosterone in male mice. CP did not affect gonadotropin-releasing hormone (GnRH) gene expression in the hypothalamus, but CP at higher concentrations stimulated GnRH pulse frequency. CP could induce the secretion of LH and FSH, as well as the expression of gonadotropin subunit genes [chorionic gonadotropin α (CGα), LHβ, and FSHβ] in pituitary gonadotropes. CP stimulated testosterone production and the expression of steroidogenesis-related genes [steroidogenic acute regulatory (StAR) and Cytochrome p 450, family 11, subfamily A, polypeptide 1 (CYP11A1)] in testicular Leydig cells. The interference with hypothalamic sodium channels as well as calcium channels in pituitary gonadotropes and testicular Leydig cells was responsible for CP-induced HPG axis maturation. Our findings established in animal models provide further evidence for the biological plausibility of pyrethroid exposure as a potentially environmental contributor to earlier puberty in males.

Multitasking in Gonadotropin-Releasing Hormone Neuron Dendrites

Gonadotropin-releasing hormone (GnRH) neurons integrate synaptic information in their dendrites in order to precisely control GnRH secretion and hence fertility. Recent discoveries concerning the structure and function of GnRH neuron dendrites have shed new light on the control of GnRH neuron excitability and GnRH secretion. This work suggests that GnRH neurons have a unique projection to the median eminence that possesses both dendritic and axonal properties. We propose that this 'dendron' projection allows GnRH neurons to multitask and integrate information in ways that would not be possible in a classically envisioned axon projection.

Diurnal in vivo and rapid in vitro effects of estradiol on voltage-gated calcium channels in gonadotropin-releasing hormone neurons

A robust surge of gonadotropin-releasing hormone (GnRH) release triggers the luteinizing hormone surge that induces ovulation. The GnRH surge is attributable to estradiol feedback, but the mechanisms are incompletely understood. Voltage-gated calcium channels (VGCCs) regulate hormone release and neuronal excitability, and may be part of the surge-generating mechanism. We examined VGCCs of GnRH neurons in brain slices from a model exhibiting daily luteinizing hormone surges. Mice were ovariectomized (OVX), and a subset was treated with estradiol implants (OVX+E). OVX+E mice exhibit negative feedback in the A.M. and positive feedback in the P.M. GnRH neurons express prominent high-voltage-activated (HVA) and small low-voltage-activated (LVA) macroscopic (whole-cell) Ca currents (I(Ca)). LVA-mediated currents were not altered by estradiol or time of day. In contrast, in OVX+E mice, HVA-mediated currents varied with time of day; HVA currents in cells from OVX+E mice were lower than those in cells from OVX mice in the A.M. but were higher in the P.M. These changes were attributable to diurnal alternations in L- and N-type components. There were no diurnal changes in any aspect of HVA-mediated I(Ca) in OVX mice. Acute in vitro treatment of cells from OVX and OVX+E mice with estradiol rapidly increased HVA currents primarily through L- and R-type VGCCs by activating estrogen receptor beta and GPR30, respectively. These results suggest multiple mechanisms contribute to the overall feedback regulation of HVA-mediated I(Ca) by estradiol. In combination with changes in synaptic inputs to GnRH neurons, these intrinsic changes in GnRH neurons may play critical roles in estradiol feedback.

Effects of steroid hormones on spermatogenesis and GnRH release in male Leopard frogs, Rana pipiens

Several lines of evidence suggest reproduction in the ranid frogs is potently regulated by the gonadal steroids, in particular 5alpha-dihydrotestosterone (DHT) and 17beta-estradiol (E(2)), and a non-gonadal steroid, the stress hormone corticosterone (Cort). Little is known about how these steroid hormones act upon the GnRH system to regulate the downstream reproductive events. We address these gaps in our knowledge by investigating the effects of Cort, E(2), and DHT administration on the in vitro release of GnRH and on the spermatogenesis of adult male leopard frog, Rana pipiens. R. pipiens were implanted for 20 days with silastic capsules containing cholesterol (Ch; control), Cort, E(2), or DHT. Upon sacrifice, acute hypothalamic explants were cultured and measured for GnRH release, and testes processed for histological analysis. Although only E(2) implant significantly reduced the gonadosomatic index, all three steroid hormones altered spermatogenesis. Cort modestly but significantly reduced the presence of spermatids. The effects of E(2) and DHT were both stimulatory and inhibitory, depending on the stage of spermatogenesis. None of the steroid hormones altered baseline GnRH release. Interestingly, only E(2) significantly stimulated veratridine-induced GnRH release, suggesting E(2) treatment increased the releasable pool of GnRH and/or enhance the excitability of GnRH neurons. In sum, this is the first study to report the direct measurement of GnRH secretion in a poikilothermic tetrapod. Our results revealed potent but sometimes paradoxical effects of steroid hormones, especially E(2), on the reproductive regulation of the male R. pipiens.

Role of Na+ and Ca2+ channels in the preoptic LH surge generating mechanism in proestrous rats

We studied whether Na+ and Ca2+ channels are involved in the neural mechanism responsible for the surge of gonadotropin-releasing hormone (GnRH) in proestrous rats. In experiment 1, female rats in proestrus were i.p. injected at 1345 h with pentobarbital sodium (35 mg/kg) to block spontaneous surge of LH and electrical stimulation was applied between 1400 and 1600 h to the preoptic area (POA) together with POA injection of 0.5 microl saline containing the Na+ channel blocker tetrodotoxin (TTX) at a concentration of 1 microM, 2 microM, or 5 microM. Since 5 microM TTX completely blocked the increase in serum LH concentrations evoked by the POA stimulation, we used this concentration in experiment 2 to observe the TTX effect on the spontaneous LH surge. In experiment 2, bilateral injections of 1.5 microl of 5 microM TTX at 1430 h in the POA in proestrous rats postponed the peak time and reduced the peak level of the LH surge. In experiment 3, bilateral injections of 1.5 microl of 5 microM L-type Ca2+ channel blocker nifedipine at 1430 h in the POA completely blocked the LH surge. Since the cell bodies of GnRH neurons are primarily concentrated in the POA in rats, these results suggest that both voltage-sensitive Na+ channels and Ca2+ channels contribute to the generation of action potentials at GnRH cell bodies for the surge release of GnRH.

Guinea-pig gonadotropin-releasing hormone: immunoreactivity and biological activity

The DNA sequence of the encoding gene predicts a unique structure for guinea-pig gonadotropin-releasing hormone (GnRH). We assessed the immunoreactivity of synthetic mammalian GnRH, of a synthetic peptide with predicted guinea-pig GnRH structure, and of extracts from rat and guinea-pig hypothalami, using two different RIA systems. Whereas immunoreactivity of mammalian and guinea-pig GnRH was rather similar when using an antiserum with conformational specificity for mammalian GnRH (Root RR-5 antiserum), binding of both peptides to an antiserum with sequential specificity (Kelch R-13 antiserum) was markedly different. The findings for GnRH extracted from rat and guinea-pig hypothalami were similar to those for the corresponding synthetic peptides. Assessment of in-vivo biological activity of synthetic mammalian and guinea-pig GnRH in the intact male guinea-pig showed that both peptides stimulate LH secretion dose-dependently, the response to mammalian GnRH being slightly greater at low dose. This study confirms that the GnRH expressed in the brain of the adult guinea-pig differs from mammalian GnRH and indicates that mammalian and guinea-pig GnRH display conformational similarity and that both can stimulate guinea-pig luteinizing hormone secretion.