Calcium and small-conductance calcium-activated potassium channels in gonadotropin-releasing hormone neurons before, during, and after puberty

Highlights of neuroanatomical discoveries of the mammalian gonadotropin-releasing hormone system

The anatomy and morphology of gonadotropin-releasing hormone (GnRH) neurons makes them both a joy and a challenge to investigate. They are a highly unique population of neurons given their developmental migration into the brain from the olfactory placode, their relatively small number, their largely scattered distribution within the rostral forebrain, and, in some species, their highly varied individual anatomical characteristics. These unique features have posed technological hurdles to overcome and promoted fertile ground for the establishment and use of creative approaches. Historical and more contemporary discoveries defining GnRH neuron anatomy remain critical in shaping and challenging our views of GnRH neuron function in the regulation of reproductive function. We begin this review with a historical overview of anatomical discoveries and developing methodologies that have shaped our understanding of the reproductive axis. We then highlight significant discoveries across specific groups of mammalian species to address some of the important comparative aspects of GnRH neuroanatomy. Lastly, we touch on unresolved questions and opportunities for future neuroanatomical research on this fascinating and important population of neurons.

Heterogeneity in GnRH and kisspeptin neurons and their significance in vertebrate reproductive biology

Vertebrate reproduction is essentially controlled by the hypothalamus-pituitary-gonadal (HPG) axis, which is a central dogma of reproductive biology. Two major hypothalamic neuroendocrine cell groups containing gonadotropin-releasing hormone (GnRH) and kisspeptin are crucial for control of the HPG axis in vertebrates. GnRH and kisspeptin neurons exhibit high levels of heterogeneity including their cellular morphology, biochemistry, neurophysiology and functions. However, the molecular foundation underlying heterogeneities in GnRH and kisspeptin neurons remains unknown. More importantly, the biological and physiological significance of their heterogeneity in reproductive biology is poorly understood. In this review, we first describe the recent advances in the neuroendocrine functions of kisspeptin-GnRH pathways. We then view the recent emerging progress in the heterogeneity of GnRH and kisspeptin neurons using morphological and single-cell transcriptomic analyses. Finally, we discuss our views on the significance of functional heterogeneity of reproductive endocrine cells and their potential relevance to reproductive health.

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.

The Electrical Brain Activity in Men with Different Alpha-Rhythm Characteristics during Manual Movements Executed by the Subdominant Hand

Background

The ability to control motor actions and flexibly interact with the environment is considered one of the main components of the human brain executive functions. The spontaneous electroencephalogram (EEG) is among the physiological techniques making it possible to formulate a direct estimation of specific features of the activity of the human brain during manual movements.

Purpose

This study is devoted to an investigation of brain processes in men with a high or a low individual α-frequency determined during manual movements executed by the subdominant hand.

Methods

A test group consisting of 104 right-handed healthy men from the ages of 19 to 21 was divided into 2 groups in terms of the average magnitude of their individual α-frequency (ІαF) - groups with high (n = 53, IαF ≥10.04 Hz) and low (n = 51, IαF ≤10.04 Hz) values of ІαF. The power and coherence of the electrical activity of the cerebral cortex as well as the differences between the groups were evaluated by the testees during manual movements executed by the subdominant hand.

Results

Manual movements executed by the subdominant hand in response to the sensory signals are generally accompanied by the increased coherence of the EEG frequency components, especially, in the frontal, anterior temporal and central brain regions in men with different α-activity characteristics. Under these conditions, it has been found some electrogenesis power lowering in the cortical areas responsible for the sensory analysis, motor programming, sensory and motor information integration. Such changes have been combined with the local power increase of θ-, α1-oscillations in the frontal leads. Additionally, men with a low IαF were characterized by the local growth of α3-activity in the frontal areas of their cortex. Men from both groups also had the generalized increase in the capacity of the high-frequency β2- and γ-oscillations. Some higher power and coherence of the EEG frequency components have been registered in men with the low IαF in comparison with men having some high α-frequency.

Conclusion

The functional content of the established differences may generally reflect some relatively lower tone of the cortex activation in men with a low IαF and can be specifically compensated by some increased "intensity" and the redundancy of brain processes.

Electrical Activity of the Cerebral Cortex during the Low-Frequency Acoustic Stimulation in Women with Different Characteristics of Alpha Rhythm

BACKGROUND

Human functional capabilities, to a certain extent, depend on an individual's nervous system. The recording of the spontaneous electroencephalogram (EEG) makes it possible to estimate specific features of the nervous system, and in particular the activity of the human brain.

PURPOSE

This study is aimed at investigating the brain in women with a high or a low individual α-frequency (ІαF), determined in a quiescent state during low-frequency acoustic stimulation.

METHODS

The study group consisted of 113 right-handed healthy women in the age group 19-21 years. The study group was divided into 2 categories in terms of the average magnitude of ІαF - groups with high (n = 59, IαF ≥10.25 Hz) and low (n = 54, IαF ≤10.25 Hz) values of ІαF. The power and coherence of electrical activity of the cerebral cortex as well as the differences between the groups were evaluated in a quiescent state during low-frequency acoustic stimulation.

RESULTS

In case of low-frequency acoustic stimulation, a high rate of EEG θ-, β2-power, γ-waves in the cortex, α1-oscillations - in frontal areas were registered in women with a high α-frequency, while the power α2-, α3- and β1-activity became lower in the cortex. Women with a low α-frequency had a decrease of the cerebral cortex α-θ-, α- and β-activity. This was recorded as more generalized, and the expression of γ-waves was more localized. All the participants had increased values of coherence of the EEG frequency components in the cortex. Some overexertion on the right side was detected in the right hemisphere posterior structures. Some local reduction of the coherence of θ-, α1-, α3-oscillations was found in women with a high α-frequency and α1-activity among the study group members with a low α-frequency in the frontal and anterior temporal areas.

CONCLUSION

The establishment of common factors in the study group is an important step towards defining clear prognostic criteria for the physiological capabilities of people based on the congenital features of brain function.

Control of puberty onset and fertility by gonadotropin-releasing hormone neurons

The gonadotropin-releasing hormone (GnRH) neuronal network generates pulse and surge modes of gonadotropin secretion critical for puberty and fertility. The arcuate nucleus kisspeptin neurons that innervate the projections of GnRH neurons in and around their neurosecretory zone are key components of the pulse generator in all mammals. By contrast, kisspeptin neurons located in the preoptic area project to GnRH neuron cell bodies and proximal dendrites and are involved in surge generation in female rodents (and possibly other species). The hypothalamic-pituitary-gonadal axis develops embryonically but, apart from short periods of activation immediately after birth, remains suppressed through a combination of gonadal and non-gonadal mechanisms. At puberty onset, the pulse generator reactivates, probably owing to progressive stimulatory influences on GnRH neurons from glial and neurotransmitter signalling, and the re-emergence of stimulatory arcuate kisspeptin input. In females, the development of pulsatile gonadotropin secretion enables final maturation of the surge generator that ultimately triggers the first ovulation. Representation of the GnRH neuronal network as a series of interlocking functional modules could help conceptualization of its functioning in different species. Insights into pulse and surge generation are expected to aid development of therapeutic strategies ameliorating pubertal disorders and infertility in the clinic.

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.

Protein kinase CK2 contributes to diminished small conductance Ca2+-activated K+ channel activity of hypothalamic pre-sympathetic neurons in hypertension

Small conductance calcium-activated K(+) (SK) channels regulate neuronal excitability. However, little is known about changes in SK channel activity of pre-sympathetic neurons in the hypothalamic paraventricular nucleus (PVN) in essential hypertension. SK channels, calmodulin, and casein kinase II (CK2) form a molecular complex. Because CK2 is up-regulated in the PVN in spontaneously hypertensive rats (SHRs), we hypothesized that CK2 increases calmodulin phosphorylation and contributes to diminished SK channel activity in PVN pre-sympathetic neurons in SHRs. Perforated whole-cell recordings were performed on retrogradely labeled spinally projecting PVN neurons in Wistar-Kyoto (WKY) rats and SHRs. Blocking SK channels with apamin significantly increased the firing rate of PVN neurons in WKY rats but not in SHRs. CK2 inhibition restored the stimulatory effect of apamin on the firing activity of PVN neurons in SHRs. Furthermore, apamin-sensitive SK currents and depolarization-induced medium after-hyperpolarization potentials of PVN neurons were significantly larger in WKY rats than in SHRs. CK2 inhibition significantly increased the SK channel current and medium after-depolarization potential of PVN neurons in SHRs. In addition, CK2-mediated calmodulin phosphorylation level in the PVN was significantly higher in SHRs than in WKY rats. Although SK3 was detected in the PVN, its expression level did not differ significantly between SHRs and WKY rats. Our findings suggest that CK2-mediated calmodulin phosphorylation is increased and contributes to diminished SK channel function of PVN pre-sympathetic neurons in SHRs. This information advances our understanding of the mechanisms underlying hyperactivity of PVN pre-sympathetic neurons and increased sympathetic vasomotor tone in hypertension. Small conductance calcium-activated K(+) (SK) channels, calmodulin, and protein kinase CK2 form a molecular complex and regulate neuronal excitability. Our study suggests that augmented CK2 activity in hypertension can increase calmodulin (CaM) phosphorylation, which leads to diminished SK channel function in pre-sympathetic neurons. Diminished SK channel activity plays a role in hyperactivity of pre-sympathetic neurons in the hypothalamus in hypertension.

Kisspeptin inhibits a slow afterhyperpolarization current via protein kinase C and reduces spike frequency adaptation in GnRH neurons

Kisspeptin signaling via its cognate receptor G protein-coupled receptor 54 (GPR54) in gonadotropin-releasing hormone (GnRH) neurons plays a critical role in regulating pituitary secretion of luteinizing hormone and thus reproductive function. GPR54 is G(q)-coupled to activation of phospholipase C and multiple second messenger signaling pathways. Previous studies have shown that kisspeptin potently depolarizes GnRH neurons through the activation of canonical transient receptor potential channels and inhibition of inwardly rectifying K(+) channels to generate sustained firing. Since the initial studies showing that kisspeptin has prolonged effects, the question has been why is there very little spike frequency adaption during sustained firing? Presently, we have discovered that kisspeptin reduces spike frequency adaptation and prolongs firing via the inhibition of a calcium-activated slow afterhyperpolarization current (I(sAHP)). GnRH neurons expressed two distinct I(sAHP), a kisspeptin-sensitive and an apamin-sensitive I(sAHP). Essentially, kisspeptin inhibited 50% of the I(sAHP) and apamin inhibited the other 50% of the current. Furthermore, the kisspeptin-mediated inhibition of I(sAHP) was abrogated by the protein kinase C (PKC) inhibitor calphostin C, and the PKC activator phorbol 12,13-dibutyrate mimicked and occluded any further effects of kisspeptin on I(sAHP). The protein kinase A (PKA) inhibitors H-89 and the Rp diastereomer of adenosine 3',5'-cyclic monophosphorothioate had no effect on the kisspeptin-mediated inhibition but were able to abrogate the inhibitory effects of forskolin on the I(sAHP), suggesting that PKA is not involved. Therefore, in addition to increasing the firing rate through an overt depolarization, kisspeptin can also facilitate sustained firing through inhibiting an apamin-insensitive I(sAHP) in GnRH neurons via a PKC.

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.

mRNA expression of ion channels in GnRH neurons: subtype-specific regulation by 17β-estradiol

Burst firing of neurons optimizes neurotransmitter release. GnRH neurons exhibit burst firing activity and T-type calcium channels, which are vital for burst firing activity, are regulated by 17β-estradiol (E2) in GnRH neurons. To further elucidate ion channel expression and E2 regulation during positive and negative feedback on GnRH neurosecretion, we used single cell RT-PCR and real-time qPCR to quantify channel mRNA expression in GnRH neurons. GFP-GnRH neurons expressed numerous ion channels important for burst firing activity. E2-treatment sufficient to induce an LH surge increased mRNA expression of HCN1 channels, which underlie the pacemaker current, the calcium-permeable Ca(V)1.3, Ca(V)2.2, Ca(V)2.3 channels, and TRPC4 channels, which mediate the kisspeptin excitatory response. E2 also decreased mRNA expression of SK3 channels underlying the medium AHP current. Therefore, E2 exerts fundamental changes in ion channel expression in GnRH neurons, to prime them to respond to incoming stimuli with increased excitability at the time of the surge.

Simulated GABA synaptic input and L-type calcium channels form functional microdomains in hypothalamic gonadotropin-releasing hormone neurons

Hypothalamic gonadotropin-releasing hormone (GnRH) neurons integrate the multiple internal and external cues that regulate sexual reproduction. In contrast to other neurons that exhibit extensive dendritic arbors, GnRH neurons usually have a single dendrite with relatively little branching. This largely precludes the integration strategy in which a single dendritic branch serves as a unit of integration. In the present study, we identify a gradient in L-type calcium channels in dendrites of mouse GnRH neurons and its interaction with GABAergic and glutamatergic inputs. Higher levels of L-type calcium channels are in somata/proximal dendrites (i.e., 0-26 μm) and distal dendrites (∼130 μm dendrite length), but intervening midlengths of dendrite (∼27-130 μm) have reduced L-type calcium channels. Using uncaging of GABA, there is a decreasing GABAergic influence along the dendrite and the impact of GABA(A) receptors is dependent on activation of L-type calcium channels. This results in amplification of proximal GABAergic signals and attenuation of distal dendritic signals. Most interestingly, the intervening dendritic regions create a filter through which only relatively high-amplitude, low-frequency GABAergic signaling to dendrites elicits action potentials. The findings of the present study suggest that GnRH dendrites adopt an integration strategy whereby segments of single nonbranching GnRH dendrites create functional microdomains and thus serve as units of integration.

Kisspeptin-10 inhibits cell migration in vitro via a receptor-GSK3 beta-FAK feedback loop in HTR8SVneo cells

Kisspeptin inhibits cancer cell metastasis and placental trophoblast cell migration. Kisspeptin gene expression in the placenta and circulating kisspeptin levels change during normal pregnancy and they are altered in preeclampsia. We therefore assessed the effect of kisspeptin-10 on the in vitro migration of a human placental cell line derived from first trimester extravillious trophoblasts (HTR8SVneo). HTR8SVneo cells specifically bound 125I-Kisspeptin-10 but kisspeptin-10 did not induce inositol phosphate production. Cell migration was inhibited by kisspeptin-10 with a maximal inhibition at 100nM. The signaling pathways involved in inhibition of cell migration were examined. Treatment with kisspeptin-10 elicited phosphorylation of GSK3 beta at Ser9 (which inhibits activity), with a 3-fold increase at 5 min. Transient phosphorylation of ERK1/2 and p38MAPK peaked at 10min. Phosphorylation of focal adhesion kinase (FAK) at Tyr925 increased 3-fold at 10 min. Inhibition of GSK3 beta correlated with release of beta-catenin into the cytoplasm. These signaling events were differentially blocked by inhibitors of G(q/11), Src, EGFR, PI(3)K, PKC and MEK. The data suggest that kisspeptin/GPR54 EGF-receptor transactivation leads to phosphorylation of ERK1/2, causing activation of p90rsk which in turn inhibits GSK3 beta via Ser9 phosphorylation. Inactivation of GSK3 beta results in release of beta-catenin into the cytoplasm, affecting cell-cell adhesion and Tyr925 phosphorylation of FAK, which increases phosphorylation of ERK1/2 via RAS/Raf-1 creating a feedback loop to enhance the effects on migration. These findings indicate that kisspeptin-10 inhibits the migration of human placental trophoblast-derived HTR8SVneo cells by stimulating complex ERK1/2-p90rsk-GSK3 beta-FAK feedback interactions.

Kisspeptin inhibits high-voltage activated Ca2+ channels in GnRH neurons via multiple Ca2+ influx and release pathways

Kisspeptin plays an important role in puberty and subsequent fertility by activating its receptor, G-protein-coupled receptor 54 (GPR54), and increasing cytoplasmic free Ca(2+) concentration ([Ca(2+)](i)) and gonadotropin-releasing hormone (GnRH) secretion in GnRH neurons. Yet the mechanism by which kisspeptin increases [Ca(2+)](i) in GnRH neurons remains to be fully elucidated. In other neurons, voltage-gated Ca(2+) channel (VGCC) activity has been shown to be inversely related to [Ca(2+)](i). We used whole-cell patch-clamp recording to examine the effects of kisspeptin-10 (KP-10) on VGCC activity evoked by step depolarizations in GnRH neurons in brain slices from pubertal male GnRH-green fluorescent protein transgenic mice. Prolonged (>30 s) KP-10 application inhibited Ca(2+) currents. The GPR54 antagonist peptide 234, chelation of intracellular Ca(2+) by 1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid, substitution of Ba(2+) for Ca(2+), the calmodulin antagonists calmidazolium and trifluoperazine, the phospholipase C inhibitor edelfosine, the canonical transient receptor potential (TRPC) channel and inositol 1,4,5-trisphosphate receptor (IP(3)R) antagonist 2-APB, the TRPC channel antagonist BTP2 and the endoplasmic reticulum Ca(2+)-ATPase blocker cyclopiazonic acid each prevented inhibition. The IP(3)R antagonists caffeine (10 µM), heparin and intracellular 2-APB prevented inhibition to a lesser extent. The ryanodine receptor (RyR) antagonists ryanodine and dantrolene prevented inhibition, and the RyR agonist caffeine (30 mM) mimicked the effects of KP-10 on Ca(2+) currents. Our results suggest that kisspeptin induces Ca(2+) influx through TRPC channels and Ca(2+) release via IP(3)Rs and RyRs, and that this is followed by Ca(2+)/CaM-dependent inhibition of VGCCs.

17β-oestradiol regulation of gonadotrophin-releasing hormone neuronal excitability

17β-Oestradiol (E(2)) is essential for cyclical gonadotrophin-releasing hormone (GnRH) neuronal activity and secretion. In particular, E(2) increases the excitability of GnRH neurones during the afternoon of pro-oestrus in the rodent, which is associated with increased synthesis and secretion of GnRH. It is well established that E(2) regulates the activity of GnRH neurones through both presynaptic and postsynaptic mechanisms. E(2) significantly modulates the mRNA expression of numerous ion channels in GnRH neurones and alters the associated endogenous conductances, including potassium (K(ATP) , A-type) currents and low-voltage T-type and high-voltage L-type calcium currents. Notably, K(ATP) channels are critical for maintaining GnRH neurones in a hyperpolarised state for recruiting the T-type calcium channels, which are important for burst firing in GnRH neurones. In addition, there are other critical channels contributing to burst firing pattern, including the small conductance Ca(2+) -activated K(+) channels that may be modulated by E(2) . Despite these advances, the cellular mechanisms underlying the cyclical GnRH neuronal activity and GnRH release are largely unknown. Ultimately, the ensemble of both pre- and postsynaptic targets of the actions of E(2) will dictate the excitability and activity pattern of GnRH neurones.

Understanding calcium homeostasis in postnatal gonadotropin-releasing hormone neurons using cell-specific Pericam transgenics

The gonadotropin-releasing hormone (GnRH) neurons are the key output cells of a complex neuronal network controlling fertility in mammals. To examine calcium homeostasis in postnatal GnRH neurons, we generated a transgenic mouse line in which the genetically encodable calcium indicator ratiometric Pericam (rPericam) was targeted to the GnRH neurons. This mouse model enabled real-time imaging of calcium concentrations in GnRH neurons in the acute brain slice preparation. Investigations in GnRH-rPericam mice revealed that GnRH neurons exhibited spontaneous, long-duration (~8s) calcium transients. Dual electrical-calcium recordings revealed that the calcium transients were correlated perfectly with burst firing in GnRH neurons and that calcium transients in GnRH neurons regulated two calcium-activated potassium channels that, in turn, determined burst firing dynamics in these cells. Curiously, the occurrence of calcium transients in GnRH neurons across puberty or through the estrous cycle did not correlate well with the assumption that GnRH neuron burst firing was contributory to changing patterns of pulsatile GnRH release at these times. The GnRH-rPericam mouse was also valuable in determining differential mechanisms of GABA and glutamate control of calcium levels in GnRH neurons as well as effects of G-protein-coupled receptors for GnRH and kisspeptin. The simultaneous measurement of calcium levels in multiple GnRH neurons was hampered by variable rPericam fluorescence in different GnRH neurons. Nevertheless, in the multiple recordings that were achieved no evidence was found for synchronous calcium transients. Together, these observations show the great utility of transgenic targeting strategies for investigating the roles of calcium with specified neuronal cell types.

Regulation of endogenous conductances in GnRH neurons by estrogens

17β-estradiol (E2) regulates the activity of the gonadotropin-releasing hormone (GnRH) neurons through both presynaptic and postsynaptic mechanisms, and this ovarian steroid hormone is essential for cyclical GnRH neuronal activity and secretion. E2 has significant actions to modulate the mRNA expression of numerous ion channels in GnRH neurons and/or to enhance (suppress) endogenous conductances (currents) including potassium (K(ATP), A-type) and calcium low voltage T-type and high voltage L-type currents. Also, it is well documented that E2 can alter the excitability of GnRH neurons via direct action, but the intracellular signaling cascades mediating these actions are not well understood. As an example, K(ATP) channels are critical ion channels needed for maintaining GnRH neurons in a hyperpolarized state for recruiting T-type calcium channels that are important for burst firing in GnRH neurons. E2 modulates the activity of K(ATP) channels via a membrane-initiated signaling pathway in GnRH neurons. Obviously there are other channels, including the small conductance activated K(+) (SK) channels, that maybe modulated by this signaling pathway, but the ensemble of mER-, ERα-, and ERβ-mediated effects both pre- and post-synaptic will ultimately dictate the excitability of GnRH neurons.

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.

Calcium dynamics in gonadotropin-releasing hormone neurons

The gonadotropin-releasing hormone (GnRH) neurons represent the key output cells of the neuronal network controlling fertility. Intracellular calcium ion concentration ([Ca(2+)](i)) is likely to be a key signaling tool used by GnRH neurons to regulate and co-ordinate multiple cell processes. This review examines the dynamics and control of [Ca(2+)](i) in GT1 cells, embryonic GnRH neurons in the nasal placode culture, and adult GnRH neurons in the acute brain slice preparation. GnRH neurons at all stages of development display spontaneous [Ca(2+)](i) transients driven, primarily, by their burst firing. However, the intracellular mechanisms generating [Ca(2+)](i) transients, and the control of [Ca(2+)](i) by neurotransmitters, varies markedly across the different developmental stages. The functional roles of [Ca(2+)](i) transients are beginning to be unraveled with one key action being that of regulating the dynamics of GnRH neuron burst firing.

Two slow calcium-activated afterhyperpolarization currents control burst firing dynamics in gonadotropin-releasing hormone neurons

Gonadotropin-releasing hormone (GnRH) neurons release GnRH in a pulsatile manner to control fertility in all mammals. The mechanisms underlying burst firing in GnRH neurons, thought to contribute to pulsatile GnRH release, are not yet understood. Using minimally invasive, dual electrical-calcium recordings in acute brain slices from GnRH-Pericam transgenic mice, we find that the soma/proximal dendrites of GnRH neurons exhibit long-duration (approximately 10 s) calcium transients that are perfectly synchronized with their burst firing. These transients were found to be generated by calcium entry through voltage-dependent L-type calcium channels that was amplified by inositol-1,4,5-trisphosphate receptor-dependent store mechanisms. Perforated-patch current- and voltage-clamp electrophysiology coupled with mathematical modeling approaches revealed that these broad calcium transients act to control two slow afterhyperpolarization currents (sI(AHP)) in GnRH neurons: a quick-activating apamin-sensitive sI(AHP) that regulates both intraburst and interburst dynamics, and a slow-onset UCL2077-sensitive sI(AHP) that regulates only interburst dynamics. These observations highlight a unique interplay between electrical activity, calcium dynamics, and multiple calcium-regulated sI(AHP)s critical for shaping GnRH neuron burst firing.

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.

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.

Differential regulation of gonadotropin-releasing hormone neuron activity and membrane properties by acutely applied estradiol: dependence on dose and estrogen receptor subtype

Gonadotropin-releasing hormone (GnRH) neurons are critical to controlling fertility. In vivo, estradiol can inhibit or stimulate GnRH release depending on concentration and physiological state. We examined rapid, nongenomic effects of estradiol. Whole-cell recordings were made of GnRH neurons in brain slices from ovariectomized mice with ionotropic GABA and glutamate receptors blocked. Estradiol was bath applied and measurements completed within 15 min. Estradiol from high physiological (preovulatory) concentrations (100 pm) to 100 nm enhanced action potential firing, reduced afterhyperpolarizing potential (AHP) and increased slow afterdepolarization amplitudes (ADP), and reduced I(AHP) and enhanced I(ADP). The reduction of I(AHP) was occluded by previous blockade of calcium-activated potassium channels. These effects were mimicked by an estrogen receptor (ER) beta-specific agonist and were blocked by the classical receptor antagonist ICI182780. ERalpha or GPR30 agonists had no effect. The acute stimulatory effect of high physiological estradiol on firing rate was dependent on signaling via protein kinase A. In contrast, low physiological levels of estradiol (10 pm) did not affect intrinsic properties. Without blockade of ionotropic GABA and glutamate receptors, however, 10 pm estradiol reduced firing of GnRH neurons; this was mimicked by an ERalpha agonist. ERalpha agonists reduced the frequency of GABA transmission to GnRH neurons; GABA can excite to these cells. In contrast, ERbeta agonists increased GABA transmission and postsynaptic response. These data suggest rapid intrinsic and network modulation of GnRH neurons by estradiol is dependent on both dose and receptor subtype. In cooperation with genomic actions, nongenomic effects may play a role in feedback regulation of GnRH secretion.

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.

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.

Dendritic action potential initiation in hypothalamic gonadotropin-releasing hormone neurons

It is dogma that action potentials are initiated at the soma/axon hillock of neurons. However, dendrites often exhibit conductances necessary for spike generation and represent functionally independent processing compartments within neurons. GnRH neurons provide an interesting neuronal phenotype with simple, relatively unbranched, unipolar or bipolar dendrites of extensive lengths (>1000 microm) covered in spines. These neurons control fertility and must integrate a variety of internal homeostatic and external environmental cues. We used imaging, electrophysiological, and modeling studies to understand how they integrate and process information along dendrites. Simultaneous recordings from distal dendrites and somata of individual GnRH neurons indicate distal dendrites are the primary site of spike initiation in these cells. Compartmental modeling indicates that sites of spike initiation depend upon location of excitatory input and dendrite geometry. Together, these studies demonstrate a novel pattern of spike generation in mammalian neurons and indicate that afferent inputs within distal dendritic microdomains directly initiate action potentials.

Small-conductance calcium-activated potassium channels control excitability and firing dynamics in gonadotropin-releasing hormone (GnRH) neurons

The cellular mechanisms determining the firing patterns of GnRH neurons are presently under intense investigation. In this study, we used GnRH-green fluorescent protein transgenic mice and perforated-patch electrophysiology to examine the role of small conductance calcium-activated potassium (SK) channels in determining the electrical excitability and burst-firing characteristics of adult GnRH neurons. After establishing an appropriate protocol for examining the afterhyperpolarization potential (AHP) currents in GnRH neurons, the highly selective SK channel blocker apamin was used to demonstrate that all GnRH neurons express functional SK channels (35.7 +/- 2.7 pA, mean decay time constant = 2167 msec, apamin IC(50) = 9.6 nm) and that this channel underlies approximately 90% of the AHP in these cells. Current-clamp experiments showed that apamin-sensitive SK channels were tonically active in the majority (74%) of GnRH neurons, with apamin (100 nm) administration resulting in a mean 6.9 +/- 0.5 mV membrane depolarization. Apamin also elevated the firing rate of GnRH neurons, including increased burst frequency and duration in spontaneously bursting cells as well as the ability of GnRH neurons to fire action potentials in response to current injection. In GnRH neurons activated by current injection, apamin significantly enhanced the amplitude of the afterdepolarization potential after a single action potential and eliminated spike frequency adaptation. Together, these studies show that apamin-sensitive SK channels play a key role in restraining GnRH neuron excitability. Through direct modulation of the AHP and indirect actions on the afterdepolarization potential, the SK channel exerts a powerful tonic influence upon the firing dynamics of GnRH neurons.

Synaptic integration in hypothalamic gonadotropin releasing hormone (GnRH) neurons

The impact of the A-type GABA (GABA-A) receptor in gonadotropin releasing hormone (GnRH) neurons is controversial. In adult GnRH neurons, the GABA-A receptor conductance has been reported to either hyperpolarize or depolarize GnRH neurons. Regardless of whether GABA is inhibitory or excitatory in GnRH neurons, GABAergic input would be integrated with post-synaptic potentials generated by other synaptic inputs. We used dynamic current clamping and compartmental computer modeling to examine the integration of AMPA-type glutamatergic input and GABA-mediated input in both the hyperpolarizing (inhibitory) and depolarizing (excitatory) modes in GnRH neurons from transgenic mice (Mus Musculus) generated on a C57BL6 background. In both living and model neurons, action potentials were most likely a few ms after a maximum in AMPA conductance coincided with a minimum in inhibitory GABA. Excitatory GABA interacted differently with AMPA, with spikes most likely, in both dynamic clamping of living neurons and in model neurons, when a maximum in AMPA coincided with the decay from peak of a maximum in GABA. Distributing synapses along the dendrite maximized the temporal relationship between AMPA and GABA conductances and therefore, the potential for spiking. Thus, these two dominant neurotransmitters could interact in multiple frames to generate action potentials in GnRH neurons.

Activation of A-type gamma-amino butyric acid receptors excites gonadotrophin-releasing hormone neurones isolated from adult rats

Gonadotrophin-releasing hormone (GnRH) neurones represent the final output neurones in the neuroendocrine control of reproduction, and gamma-amino butyric acid (GABA) is one of the major players in the regulation of GnRH neurones. GABA inhibits a large proportion of brain neurones in adult animals by acting on A-type GABA receptors (GABA(A)Rs). Two contradictory reports on the action of GABA in the GnRH neurones of adult mice have been published. DeFazio et al. (Mol Endocrinol 2002; 16: 2872) demonstrated that activation of GABA(A)Rs excites the GnRH neurones of adult mice, whereas Han et al. (Endocrinology 2002; 143: 1459) showed that the response to GABA on GnRH neurones switches from depolarisation to hyperpolarisation around puberty in female mice. Therefore, we examined the reversal potential of GABA(A)R currents by means of perforated patch-clamp recording with gramicidin in overnight-cultured GnRH neurones isolated from adult GnRH-enhanced green fluorescent protein transgenic rats. The reversal potential was -26 +/- 1.4 mV (mean +/- SEM, n = 42) in GnRH neurones, whereas it was -57 +/- 2.7 mV (n = 34) in unidentified neurones, and GABA depolarised the GnRH neurones in current-clamp condition. The GABA(A)R currents in rat GnRH neurones were augmented by neurosteroids, allopregnanolone and 3 alpha,21-dihydroxy-5 alpha-pregnan-20-one, at submicromolar concentrations. In addition, the expression patterns of GABA(A)R subunit mRNAs were determined by multi-cell reverse transcription-polymerase chain reaction, which revealed that the alpha2, beta 3, gamma 1 and gamma 2 subunits were dominant and the alpha 6 and gamma 3 subunits were negative in rat GnRH neurones. These results indicate that GABA(A)Rs in the soma of rat GnRH neurones are comprised mainly of alpha2, beta 3 and gamma 1 or gamma 2 subunits and that they are sensitive to neurosteroids; moreover, they suggest that activation of these receptors depolarises GnRH neurones. Thus, GABA and neurosteroids influence the electrical activity of GnRH neurones.

17beta-estradiol at physiological concentrations augments Ca(2+) -activated K+ currents via estrogen receptor beta in the gonadotropin-releasing hormone neuronal cell line GT1-7

Estrogens play essential roles in the neuroendocrine control of reproduction. In the present study, we focused on the effects of 17beta-estradiol (E2) on the K(+) currents that regulate neuronal cell excitability and carried out perforated patch-clamp experiments with the GnRH-secreting neuronal cell line GT1-7. We revealed that a 3-d incubation with E2 at physiological concentrations (100 pm to 1 nm) augmented Ca(2+)-activated K(+) [K(Ca)] currents without influencing Ca(2+)-insensitive voltage-gated K(+) currents in GT1-7 cells. Acute application of E2 (1 nm) had no effect on the either type of K(+) current. The augmentation was completely blocked by an estrogen receptor (ER) antagonist, ICI-182,780. An ERbeta-selective agonist, 2,3-bis(4-hydroxyphenyl)-propionitrile, augmented the K(Ca) currents, although an ERalpha-selective agonist, 4,4',4''-[4-propyl-(1H)-pyrazole-1,3,5-triyl]tris-phenol, had no effect. Knockdown of ERbeta by means of RNA interference blocked the effect of E2 on the K(Ca) currents. Furthermore, semiquantitative RT-PCR analysis revealed that the levels of BK channel subunit mRNAs for alpha and beta4 were significantly increased by incubating cells with 300 pm E2 for 3 d. In conclusion, E2 at physiological concentrations augments K(Ca) currents through ERbeta in the GT1-7 GnRH neuronal cell line and increases the expression of the BK channel subunit mRNAs, alpha and beta4.

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

Gonadotropin-releasing hormone (GnRH) neurons form the final common pathway for the central regulation of reproduction. As in other neurons, the discharge pattern of action potentials is important for these neurons to function properly. Therefore it is important to elucidate the expression patterns of various types of ion channels in these neurons because they determine cell excitability. To date, voltage-gated Ca2+ channels and SK channels have been reported to be expressed in rat GnRH neurons. In this study, we focused on K+ channels and analyzed their expression in primary cultured GnRH neurons, prepared from GnRH-EGFP transgenic rats, by means of perforated patch-clamp recordings. GnRH neurons exhibited delayed-rectifier K+ currents and large conductance voltage- and Ca2+-activated K+ (BK) currents. Moreover, multicell RT-PCR (reverse transcriptase-polymerase chain reaction) experiments revealed the expression of BK channel mRNAs (alpha, beta1, beta2, and beta4). The results show the presence of delayed-rectifier K+ currents and BK currents besides previously reported slow afterhyperpolarization currents. These currents control the action potential repolarization and probably also the firing pattern, thereby regulating the cell excitability of GnRH neurons.