Rat GnRH neurons exhibit large conductance voltage- and Ca2+-Activated K+ (BK) currents and express BK channel mRNAs

Estradiol elicits distinct firing patterns in arcuate nucleus kisspeptin neurons of females through altering ion channel conductances

Hypothalamic kisspeptin (Kiss1) neurons are vital for pubertal development and reproduction. Arcuate nucleus Kiss1 (Kiss1ARH) neurons are responsible for the pulsatile release of Gonadotropin-releasing Hormone (GnRH). In females, the behavior of Kiss1ARH neurons, expressing Kiss1, Neurokinin B (NKB), and Dynorphin (Dyn), varies throughout the ovarian cycle. Studies indicate that 17β-estradiol (E2) reduces peptide expression but increases Vglut2 mRNA and glutamate neurotransmission in these neurons, suggesting a shift from peptidergic to glutamatergic signaling. To investigate this shift, we combined transcriptomics, electrophysiology, and mathematical modeling. Our results demonstrate that E2 treatment upregulates the mRNA expression of voltage-activated calcium channels, elevating the whole-cell calcium current and that contribute to high-frequency burst firing. Additionally, E2 treatment decreased the mRNA levels of Canonical Transient Receptor Potential (TPRC) 5 and G protein-coupled K+ (GIRK) channels. When TRPC5 channels in Kiss1ARH neurons were deleted using CRISPR, the slow excitatory postsynaptic potential (sEPSP) was eliminated. Our data enabled us to formulate a biophysically realistic mathematical model of the Kiss1ARH neuron, suggesting that E2 modifies ionic conductances in Kiss1ARH neurons, enabling the transition from high frequency synchronous firing through NKB-driven activation of TRPC5 channels to a short bursting mode facilitating glutamate release. In a low E2 milieu, synchronous firing of Kiss1ARH neurons drives pulsatile release of GnRH, while the transition to burst firing with high, preovulatory levels of E2 would facilitate the GnRH surge through its glutamatergic synaptic connection to preoptic Kiss1 neurons.

Vasoactive intestinal peptide excites GnRH neurons via KCa3.1, a potential player in the slow afterhyperpolarization current

Vasoactive intestinal peptide (VIP) is an important component of the suprachiasmatic nucleus (SCN) which relays circadian information to neuronal populations, including GnRH neurons. Human and animal studies have shown an impact of disrupted daily rhythms (chronic shift work, temporal food restriction, clock gene disruption) on both male and female reproduction and fertility. To date, how VIP modulates GnRH neurons remains unknown. Calcium imaging and electrophysiology on primary GnRH neurons in explants and adult mouse brain slice, respectively, were used to address this question. We found VIP excites GnRH neurons via the VIP receptor, VPAC2. The downstream signaling pathway uses both Gs protein/adenylyl cyclase/protein kinase A (PKA) and phospholipase C/phosphatidylinositol 4,5-bisphosphate (PIP2) depletion. Furthermore, we identified a UCL2077-sensitive target, likely contributing to the slow afterhyperpolarization current (IAHP), as the PKA and PIP2 depletion target, and the KCa3.1 channel as a specific target. Thus, VIP/VPAC2 provides an example of Gs protein-coupled receptor-triggered excitation in GnRH neurons, modulating GnRH neurons likely via the slow IAHP. The possible identification of KCa3.1 in the GnRH neuron slow IAHP may provide a new therapeutical target for fertility treatments.

Epigenomic and transcriptomic analyses reveal early activation of the HPG axis in in vitro-produced male dairy calves

In cattle, several calves born after IVP ("in vitro" embryo production) present similar birthweight to those generated after MOET (multiple ovulation and embryo transfer). However, the underlying molecular patterns in organs involved in the developmental process are unknown and could indicate physiological programming. The objectives of this study were: (1) to compare epigenomic and transcriptomic modifications in the hypothalamus, pituitary, gonadal and adrenal organs between 3 months old ovum pick-up-IVP and MOET male calves (n = 4 per group) and (2) to use blood epigenomic data to proxy methylation of the inner organs. Extracted gDNA and RNA were sequenced through whole-genome bisulfite sequencing and RNA sequencing, respectively. Next, bioinformatic analyses determined differentially methylated cytosines (DMC) and differentially expressed genes (DEG) (FDR < 0.05) in IVP versus MOET samples and the KEGG pathways that were overrepresented by genes associated with DMC or DEG (FDR < 0.1). Pathways related to hypothalamus, pituitary, gonadal (HPG) axis activation (GnRH secretion in the hypothalamus, GnRH signaling in the pituitary, and steroidogenesis in the testicle) were enriched in IVP calves. Modeling the effect of the methylation levels and the group on the expression of all the genes involved in these pathways confirmed their upregulation in HPG organs in IVP calves. The application of the DIABLO method allowed the identification of 15 epigenetic and five transcriptomic biomarkers, which were able to predict the embryo origin using the epigenomic data from the blood. In conclusion, the use of an integrated epigenomic-transcriptomic approach suggested an early activation of the HPG axis in male IVP calves compared to MOET counterparts, and the identification of potential biomarkers allowed the use of blood samples to proxy methylation levels of the relevant internal organs.

Proestrus Differentially Regulates Expression of Ion Channel and Calcium Homeostasis Genes in GnRH Neurons of Mice

In proestrus, the changing gonadal hormone milieu alters the physiological properties of GnRH neurons and contributes to the development of the GnRH surge. We hypothesized that proestrus also influences the expression of different ion channel genes in mouse GnRH neurons. Therefore, we performed gene expression profiling of GnRH neurons collected from intact, proestrous and metestrous GnRH-GFP transgenic mice, respectively. Proestrus changed the expression of 37 ion channel and 8 calcium homeostasis-regulating genes. Voltage-gated sodium channels responded with upregulation of three alpha subunits (Scn2a1, Scn3a, and Scn9a). Within the voltage-gated potassium channel class, Kcna1, Kcnd3, Kcnh3, and Kcnq2 were upregulated, while others (Kcna4, Kcnc3, Kcnd2, and Kcng1) underwent downregulation. Proestrus also had impact on inwardly rectifying potassium channel subunits manifested in enhanced expression of Kcnj9 and Kcnj10 genes, whereas Kcnj1, Kcnj11, and Kcnj12 subunit genes were downregulated. The two-pore domain potassium channels also showed differential expression with upregulation of Kcnk1 and reduced expression of three subunit genes (Kcnk7, Kcnk12, and Kcnk16). Changes in expression of chloride channels involved both the voltage-gated (Clcn3 and Clcn6) and the intracellular (Clic1) subtypes. Regarding the pore-forming alpha-1 subunits of voltage-gated calcium channels, two (Cacna1b and Cacna1h) were upregulated, while Cacna1g showed downregulation. The ancillary subunits were also differentially regulated (Cacna2d1, Cacna2d2, Cacnb1, Cacnb3, Cacnb4, Cacng5, Cacng6, and Cacng8). In addition, ryanodine receptor 1 (Ryr1) gene was downregulated, while a transient receptor potential cation channel (Trpm3) gene showed enhanced expression. Genes encoding proteins regulating the intracellular calcium homeostasis were also influenced (Calb1, Hpca, Hpcal1, Hpcal4, Cabp7, Cab 39l, and Cib2). The differential expression of genes coding for ion channel proteins in GnRH neurons at late proestrus indicates that the altering hormone milieu contributes to remodeling of different kinds of ion channels of GnRH neurons, which might be a prerequisite of enhanced cellular activity of GnRH neurons and the subsequent surge release of the neurohormone.

A unified model for two modes of bursting in GnRH neurons

Gonadotropin-releasing hormone (GnRH) neurons exhibit at least two intrinsic modes of action potential burst firing, referred to as parabolic and irregular bursting. Parabolic bursting is characterized by a slow wave in membrane potential that can underlie periodic clusters of action potentials with increased interspike interval at the beginning and at the end of each cluster. Irregular bursting is characterized by clusters of action potentials that are separated by varying durations of interburst intervals and a relatively stable baseline potential. Based on recent studies of isolated ionic currents, a stochastic Hodgkin-Huxley (HH)-like model for the GnRH neuron is developed to reproduce each mode of burst firing with an appropriate set of conductances. Model outcomes for bursting are in agreement with the experimental recordings in terms of interburst interval, interspike interval, active phase duration, and other quantitative properties specific to each mode of bursting. The model also shows similar outcomes in membrane potential to those seen experimentally when tetrodotoxin (TTX) is used to block action potentials during bursting, and when estradiol transitions cells exhibiting slow oscillations to irregular bursting mode in vitro. Based on the parameter values used to reproduce each mode of bursting, the model suggests that GnRH neurons can switch between the two through changes in the maximum conductance of certain ionic currents, notably the slow inward Ca(2+) current I s, and the Ca(2+) -activated K(+) current I KCa. Bifurcation analysis of the model shows that both modes of bursting are similar from a dynamical systems perspective despite differences in burst characteristics.

Kisspeptin and Gonadotropin-Releasing Hormone Neuronal Excitability: Molecular Mechanisms Driven by 17β-Estradiol

Kisspeptin is a neuropeptide that signals via a Gαq-coupled receptor, GPR54, in gonadotropin-releasing hormone (GnRH) neurons and is essential for pubertal maturation and fertility. Kisspeptin depolarizes and excites GnRH neurons primarily through the activation of canonical transient receptor potential (TRPC) channels and the inhibition of K+ channels. The gonadal steroid 17β-estradiol (E2) upregulates not only kisspeptin (Kiss1) mRNA but also increases the excitability of the rostral forebrain Kiss1 neurons. In addition, a primary postsynaptic action of E2 on GnRH neurons is to upregulate the expression of channel transcripts that orchestrate the downstream signaling of kisspeptin in GnRH neurons. These include not only TRPC4 channels but also low-voltage-activated T-type calcium channels and high-voltage-activated L-, N- and R-type calcium channel transcripts. Moreover, E2 has direct membrane-initiated actions to alter the excitability of GnRH neurons by enhancing ATP-sensitive potassium channel activity, which is critical for maintaining GnRH neurons in a hyperpolarized state for the recruitment of T-type calcium channels that are important for burst firing. Therefore, E2 modulates the excitability of GnRH neurons as well as of Kiss1 neurons by altering the expression and/or function of ion channels; moreover, kisspeptin provides critical excitatory input to GnRH neurons to facilitate burst firing activity and peptide release.

Ion channels and information processing in GnRH neuron dendrites

Recent findings indicate that a majority of action potentials originate from dendrites of GnRH neurons. This localization of the dendrite as the principle site of action potential initiation has sparked considerable interest in the nature of ionic channels throughout GnRH neurons. This paper will review the ionic conductances described within GnRH neurons and their implications for physiological output, such as sensitivity to steroids and diurnal state. To date, a majority of information regarding ionic conductances in GnRH neurons pertains to somata and the first 50–100 µm of dendrite length. Thus, unraveling the tapestry created by the nature and distribution of dendritic conductances in GnRH neurons lies at the forefront of understanding the control of reproductive hormone secretion.

Identified GnRH neuron electrophysiology: a decade of study

Over the past decade, the existence of transgenic mouse models in which reporter genes are expressed under the control of the gonadotropin-releasing hormone (GnRH) promoter has made possible the electrophysiological study of these cells. Here, we review the intrinsic and synaptic properties of these cells that have been revealed by these approaches, with a particular regard to burst generation. Advances in our understanding of neuromodulation of GnRH neurons and synchronization of this network are also discussed.

Flufenamic acid modulates multiple currents in gonadotropin-releasing hormone neurons

Reproduction in mammals is dependent upon the appropriate neurosecretion of gonadotropin-releasing hormone (GnRH), yet the endogenous generation of activity underlying GnRH secretion remains poorly understood. We have demonstrated that the depolarizing afterpotential (DAP), which modulates bursting activity, is reduced in isolated GnRH neurons from aged animals. Calcium-activated non-specific cation (CAN) channels contribute to the DAP in other vertebrate neurosecretory cells. We used the CAN channel blocker flufenamic acid (FFA) to examine the contribution of CAN channels to the DAP in GnRH neurons during aging. Recordings were performed on isolated fluorescent GnRH neurons from young, middle-aged and aged female mice. Flufenamic acid inhibited spontaneous activity, but significantly increased the DAP in neurons from young and middle-aged animals. Apamin did not significantly potentiate the DAP, but did reduce the effects of FFA, suggesting that the increased DAP is partially due to blockade of apamin-sensitive SK channels. Flufenamic acid increased the current underlying the DAP (I(ADP)) and decreased the preceding fast outward current (I(OUT)) at all ages. These current responses were not affected by apamin, but TEA evoked similar changes. Thus, a potassium current, likely mediated through BK channels, contributes to the fast AHP and appears to offset the DAP; this current is sensitive to FFA, but insensitive to age. The effect of FFA on the DAP, but not I(ADP), is diminished in aged animals, possibly reflecting an age-related modulation of the apamin-sensitive SK channel. Future studies will examine the expression of SK channels during the aging process in GnRH neurons.

Voltage-gated Ca2+ channel mRNAs and T-type Ca2+ currents in rat gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons play a pivotal role in the neuroendocrine regulation of reproduction. We have previously reported that rat GnRH neurons exhibit voltage-gated Ca(2+) currents. In this study, oligo-cell RT-PCR was carried out to identify subtypes of the alpha(1) subunit of voltage-gated Ca(2+) channels in adult rat GnRH neurons. GnRH neurons expressed mRNAs for all five types of voltage-gated Ca(2+) channels. For T-type Ca(2+) channels, alpha(1H) was dominantly expressed in GnRH neurons. Electrophysiological analysis in acute slice preparations revealed that GnRH neurons from adult rats exhibited T-type Ca(2+) currents with fast inactivation kinetics (~20 ms at -30 mV) and a time constant of recovery from inactivation of ~0.6 s. These results indicate that rat GnRH neurons express subtypes of the alpha(1) subunit for all five types of voltage-gated Ca(2+) channel, and that alpha(1H) was the dominant subtype in T-type Ca(2+) channels.

Ca2+ channels and Ca2+-activated K+ channels in adult rat gonadotrophin-releasing hormone neurones

Gonadotrophin-releasing hormone (GnRH) neurones represent the final output neurones in the neuroendocrine system for the control of reproduction. To understand the reproductive neuroendocrine system, an investigation of the intrinsic and extrinsic properties of GnRH neurones is essential. In this review, we focus on the intrinsic properties and summarise our recent findings of ion channels expressed in rat GnRH neurones. Rat GnRH neurones express all four types of high voltage-activated Ca(2+) channel (L, N, P/Q, R) and the low voltage-activated Ca(2+) channel (T). GnRH neurones also express two types of Ca(2+)-activated K(+) [K(Ca)] channel: the small conductance Ca(2+)-activated K(+) (SK) channel and the large conductance Ca(2+)- and voltage-activated K(+) (BK) channel. The activities of these Ca(2+) and K(Ca) channels regulate cell excitability and cellular calcium homeostasis.

Gonadal steroid action and brain sex differentiation in the rat

Gonadal steroids that establish sexually dimorphic characteristics of brain morphology and physiology act at a particular stage of ontogeny. Testosterone secreted by the testes during late gestational and neonatal periods causes significant brain sexual dimorphism in the rat. This results in both sex-specific behaviour and endocrinology in adults. Sexual differentiation may be due to neurogenesis, migration or survival. Each mechanism appears to be uniquely regulated in a site-specific manner. Thus, the volume of an aggregate of neurones in the rat medial preoptic area (POA), termed the sexually dimorphic nucleus of the POA (SDN-POA), is larger in males than in females. The anteroventral periventricular nucleus (AVPV) is packed with neurones containing oestrogen receptor (ER)beta in female rats but, in males, ERbeta-positive neurones scatter into the more lateral portion of the POA. POA neurones are born up to embryonic days 16-17 and not after parturition. Therefore, neurogenesis is unlikely to contribute to the larger SDN-POA in males. DNA microarray analysis for oestrogen-responsive genes and western blotting demonstrated site-specific regulation of apoptosis- and migration-related genes in the SDN-POA and AVPV.

Gene structures, biochemical characterization and distribution of rat melatonin receptors

G-protein coupled receptors for the pineal hormone melatonin have been partially cloned from rats. However, insufficient information about their cDNA sequences has hindered studies of their distribution and physiological responses to melatonin using rats as an animal model. We have cloned cDNAs of two rat membrane melatonin receptor subtypes, melatonin receptor 1a (MT1) and melatonin receptor 1b (MT2), using a rapid amplification of cDNA end (RACE) method. The rat MT1 and MT2 cDNAs encode proteins of 353 and 364 amino acids, respectively, and show 78-93% identities with the human and mouse counterparts. Stable expression of either rat MT1 or MT2 in NIH3T3 cells resulted in high affinity 2-[(125)I]-iodomelatonin ((125)I-Mel) binding (K (d) = 73.2 +/- 9.0 and 73.7 +/- 2.9 pM, respectively), and exhibited a similar rank order of inhibition of specific (125)I-Mel binding by five ligands (2-iodomelatonin > melatonin > 6-hydroxymelatonin > luzindole > N-acetyl-5-hydroxytryptamine). RT-PCR analysis showed that MT1 is highly expressed in the hypothalamus, lung, kidney, adrenal gland, stomach, and ovary, while MT2 is highly expressed in the hippocampus, kidney, and ovary. We also performed multi-cell RT-PCR to examine the expression of mRNAs encoding MT1 and MT2 in adult GnRH neurons. MT1 was weakly expressed in male GnRH neurons, and was less expressed in the female neurons. MT2 expression was undetectable in GnRH neurons from either sex. This study delineates the gene structures, fundamental properties, and distribution of both rat melatonin receptor subtypes, and may offer opportunities to assess the physiological significance of melatonin in rats.

Kisspeptin excites gonadotropin-releasing hormone neurons through a phospholipase C/calcium-dependent pathway regulating multiple ion channels

The present study used perforated-patch electrophysiology and calcium imaging in GnRH transgenic mouse lines to determine the mechanisms underlying the potent excitatory effects of kisspeptin upon GnRH neurons in the acute brain slice preparation. Kisspeptin (100 nm) depolarized (6 +/- 1 mV) and/or evoked an 87 +/- 4% increase in firing rate of 75% of adult GnRH neurons (n = 51). No sex differences were found. Analyses of input resistance and current-voltage curves indicated that a heterogeneous closure of potassium channels and opening of nonselective cation (NSC) channels was involved in kisspeptin's depolarizing response. Pharmacological pretreatment with either barium, a potassium channel blocker, or flufenamic acid, an NSC channel antagonist, reduced the percentage of responding GnRH neurons from 75 to 40% (P < 0.05). Pretreatment with both barium and flufenamic acid reduced the response rate to 17% (P < 0.05). To examine the intracellular signaling cascade involved, GnRH neurons were treated with antagonists of phospholipase C (PLC), inositol-trisphosphate receptors (IP3R), and ERK1/2 before kisspeptin exposure. PLC and IP3R antagonism reduced the percentage of responding GnRH neurons from 80 to 15 and 7%, respectively (P < 0.001). Real-time calcium imaging showed that kisspeptin evoked an approximately 10% increase in intracellular calcium levels in GnRH neurons that was followed by a decrease and return to pretest calcium levels. Additional experiments indicated that mechanisms intrinsic to the GnRH neuron are responsible for their prolonged depolarizing response to kisspeptin. These studies indicate that kisspeptin activates G protein-coupled receptor 54 (GPR54) to initiate a PLC-IP3R-calcium cascade that modulates both potassium and NSC channels to initiate depolarization in GnRH neurons.