Selective modulation of voltage-gated calcium channels in the terminal nerve gonadotropin-releasing hormone neurons of a teleost, the dwarf gourami (Colisa lalia)

Multiple functions of non-hypophysiotropic gonadotropin releasing hormone neurons in vertebrates

Gonadotropin releasing hormone (GnRH) is a hypophysiotropic hormone that is generally thought to be important for reproduction. This hormone is produced by hypothalamic GnRH neurons and stimulates the secretion of gonadotropins. On the other hand, vertebrates also have non-hypophysiotropic GnRH peptides, which are produced by extrahypothalamic GnRH neurons. They are mainly located in the terminal nerve, midbrain tegmentum, trigeminal nerve, and spinal cord (sympathetic preganglionic nerves). In vertebrates, there are typically three gnrh paralogues (gnrh1, gnrh2, gnrh3). GnRH-expression in the non-hypophysiotropic neurons (gnrh1 or gnrh3 in the terminal nerve and the trigeminal nerve, gnrh2 in the midbrain tegmentum) occurs from the early developmental stages. Recent studies have suggested that non-hypophysiotropic GnRH neurons play various functional roles. Here, we summarize their anatomical/physiological properties and discuss their possible functions, focusing on studies in vertebrates. GnRH neurons in the terminal nerve show different spontaneous firing properties during the developmental stages. These neurons in adulthood show regular pacemaker firing, and it has been suggested that these neurons show neuromodulatory function related to the regulation of behavioral motivation, etc. In addition to their recognized role in neuromodulation in adult, in juvenile fish, these neurons, which show more frequent burst firing than in adults, are suggested to have novel functions. GnRH neurons in the midbrain tegmentum show regular pacemaker firing similar to that of the adult terminal nerve and are suggested to be involved in modulations of feeding (teleosts) or nutrition-related sexual behaviors (musk shrew). GnRH neurons in the trigeminal nerve are suggested to be involved in nociception and chemosensory avoidance, although the literature on their electrophysiological properties is limited. Sympathetic preganglionic cells in the spinal cord were first reported as peptidergic modulatory neurons releasing GnRH with a putative function in coordinating interaction between vasomotor and exocrine outflow in the sympathetic nervous system. The functional role of non-hypophysiotropic GnRH neurons may thus be in the global modulation of neural circuits in a manner dependent on internal conditions or the external environment.

Burst generation mediated by cholinergic input in terminal nerve-gonadotrophin releasing hormone neurones of the goldfish

Peptidergic neurones play a pivotal role in the neuromodulation of widespread areas in the nervous system. Generally, it has been accepted that the peptide release from these neurones is regulated by their firing activities. The terminal nerve (TN)-gonadotrophin releasing hormone (GnRH) neurones, which are one of the well-studied peptidergic neurones in vertebrate brains, are characterised by their spontaneous regular pacemaker activities, and GnRH has been suggested to modulate the sensory responsiveness of animals. Although many peptidergic neurones are known to exhibit burst firing activities when they release the peptides, TN-GnRH neurones show spontaneous burst firing activities only infrequently. Thus, it remains to be elucidated whether the TN-GnRH neurones show burst activities and, if so, how the mode switching between the regular pacemaking and bursting modes is regulated in these neurones. In this study, we found that only a single pulse electrical stimulation of the neuropil surrounding the TN-GnRH neurones reproducibly induces transient burst activities in TN-GnRH neurones. Our combined physiological and morphological data suggest that this phenomenon occurs following slow inhibitory postsynaptic potentials mediated by cholinergic terminals surrounding the TN-GnRH neurones. We also found that the activation of muscarinic acetylcholine receptors induces persistent opening of potassium channels, resulting in a long-lasting hyperpolarisation. This long hyperpolarisation induces sustained rebound depolarisation that has been suggested to be generated by a combination of persistent voltage-gated Na(+) channels and low-voltage-activated Ca(2+) channels. These new findings suggest a novel type of cholinergic regulation of burst activities in peptidergic neurones, which should contribute to the release of neuropeptides.

Characterization of voltage-activated ionic currents in the GnRH-containing terminalis nerve in transgenic zebrafish

The terminalis nerve (TN) is in a class of cranial nerves that plays important roles in animal development, physiology and behavior. Here, we report a study on the characterization of voltage-activated ionic currents in GnRH-containing TN cells in zebrafish. The experiments were performed using acutely dissociated TN cells from the transgenic zebrafish Tg (GnRH-3::GFP). In the transgenic zebrafish, the TN cells express GFP under the transcriptional control of the zebrafish GnRH-3 promoter. In all of the GnRH-containing TN cells examined, we recorded both low-voltage-activated (LVA) and high-voltage-activated (HVA) calcium current (I(Ca)). The characteristics of the I(Ca) were similar to those described in other zebrafish cell types. However, the distribution patterns of the currents in the GnRH-containing TN cells were different in comparison to the distribution of the currents in other cell types. In addition, we characterized TTX-sensitive sodium current (I(Na)) and 4AP-sensitive and TEA-resistant potassium current (I(K)). The characteristics of voltage-activated I(Na) and I(K) in the GnRH-containing TN cells were similar to those described in other zebrafish cell types. Together, the data from this study revealed the electrophysiological properties of the GnRH-containing TN cells, thereby providing insight on the regulatory mechanisms of TN-signaling in animal physiology.

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.

Excitatory action of GABA in the terminal nerve gonadotropin-releasing hormone neurons

The terminal nerve (TN)-gonadotropin-releasing hormone (GnRH) neurons have been suggested to function as a neuromodulatory system that regulates the motivational and arousal state of the animal and have served as a model system for the study of GnRH neuron physiology. To investigate the synaptic control of the TN-GnRH neurons, we analyzed electrophysiologically the effect of GABA on the TN-GnRH neurons. GABA generally hyperpolarizes most of the neurons in the adult brain by activating GABA(A) receptors while the activation of GABA(A) receptors depolarizes some specific neurons in the mature brain. Here we examined the GABA(A) receptor-mediated responses in the TN-GnRH neurons of adult teleost fish, the dwarf gourami, by means of gramicidin-perforated patch-clamp and cell-attached patch-clamp recordings. The reversal potential for the currents through GABA(A) receptors under the voltage clamp was depolarized relative to the resting membrane potential. GABA(A) receptor activation depolarized TN-GnRH neurons under the current clamp and had excitatory effect on their electrical activity, whereas the stronger GABA(A) receptor activation had bidirectional effect (excitatory-inhibitory). This excitatory effect is suggested to arise from high [Cl(-)](i) and was shown to be suppressed by bumetanide, the blocker of Cl(-)-accumulating sodium-potassium-2-chloride co-transporter (NKCC). The present results demonstrate that GABA(A) receptor activation induces excitation in TN-GnRH neurons, which may facilitate their neuromodulatory functions by increasing their spontaneous firing frequencies. The excitatory actions of GABA in the adult brain have recently been attracting much attention, and the easily accessible large TN-GnRH neurons should be a nice model system to analyze their physiological functions.

Coordinated synchronization in the electrically coupled network of terminal nerve gonadotropin-releasing hormone neurons as demonstrated by double patch-clamp study

The peptidergic neurons play important roles such as neuromodulatory and neuroendocrine functions in the central nervous system. However, our knowledge about the organization and the function of the peptidergic neuromodulator systems is still very poor. The terminal nerve GnRH peptidergic neurons of a teleost, the dwarf gourami (Colisa lalia), serve as an excellent model system for such study. The cell bodies are large and make up a tight cell cluster, and the easy access to the cell bodies on the ventral surface of the brain makes the electrophysiological measurements in a precisely controlled manner. Here we show direct evidence to demonstrate the electrical coupling and the synchronization of the neural firing activity among the terminal nerve GnRH neurons by using the double patch-clamp recording technique. The electrical coupling coefficient was strong enough (ranged from 0.083 to 0.370) to synchronize spontaneous firings of GnRH neurons in the cluster. A model, in which the firings in the cluster occur within a small time window (dozens of milliseconds), was verified by using the serial loose-seal extracellular patch-clamp recordings and the cross-correlogram analysis. The present findings provide several insights for understanding the physiological mechanisms and functional significance of synchronized activities in the peptidergic and/or aminergic neuromodulator system as well as in the peptidergic neuroendocrine cells.

Terminal nerve gonadotrophin-releasing hormone (GnRH) neurones express multiple GnRH receptors in a teleost, the dwarf gourami (Colisa lalia)

Gonadotophin-releasing hormone (GnRH) peptide released from the terminal nerve (TN)-GnRH neurones of the dwarf gourami primarily modifies the electrical properties of various neurones, including the TN-GnRH neurones themselves. However, our knowledge on the expression of GnRH receptors (GnRHRs) in the TN-GnRH neurones is still limited. Here, we used the single-cell reverse transcriptase-polymerase chain reaction after whole-cell patch-clamp recording to study the distribution of various GnRHR types expressed in the individual TN-GnRH neurones. We found that TN-GnRH neurones express two of the three types of GnRHRs cloned in the dwarf gourami: GnRHR1-2 and -R2, but not -R1-1. Furthermore, in agreement with our previous findings, all TN-GnRH neurones contained mRNAs of salmon GnRH but not chicken GnRH-II.

Ion channels and their neural functions: contribution to general problems from studies of brains in non-mammalian species

Ion channels and their neural functions have often been studied in the brains of mammals. However, the brains of some teleost fish species have special features that, at first glance, appear to be atypical and peculiar to that species alone. These teleost fish will contribute considerably to the understanding of general features of the ion channels and their neural functions. We have been interested in the neural mechanisms underlying the adaptive and flexible response of animals to changing environments and have been studying gonadotropin-releasing hormone (GnRH) peptidergic neuron systems, which we think are central for controlling such biologically adaptive responses. We have also been interested in a pretectal nucleus, corpus glomerulosum, which is tentatively regarded to play important roles in the organization of visually-guided feeding behaviors. In both systems, we found that certain types of apparently 'non-typical' Na(+) channels play important roles in neurobiological functions. Here we discuss how the study of functions of these apparently non-typical ion channels might contribute to our understanding of neural functions of vertebrate brains in general.

Voltage-gated currents of tilapia prolactin cells

The first recordings of neuron-like electrical activity from endocrine cells were made from fish pituitary cells. However, patch-clamping studies have predominantly utilized mammalian preparations. This study used whole-cell patch-clamping to characterize voltage-gated ionic currents of anterior pituitary cells of Oreochromis mossambicus in primary culture. Due to their importance for control of hormone secretion we emphasize analysis of calcium currents (I(Ca)), including using peptide toxins diagnostic for mammalian neuronal Ca(2+) channel types. These appear not to have been previously tested on fish endocrine cells. In balanced salines, inward currents consisted of a rapid TTX-sensitive sodium current and a smaller, slower I(Ca); there followed outward potassium currents dominated by delayed, sustained TEA-sensitive K(+) current. About half of cells tested from a holding potential (V(h)) of -90 mV showed early transient K(+) current; most cells showed a small Ca(2+)-mediated outward current. I-V plots of isolated I(Ca) with 15 mM [Ca(2+)](o) showed peak currents (up to 20 pA/pF from V(h) -90 mV) at approximately +10 mV, with approximately 60% I(Ca) for V(h) -50 mV and approximately 30% remaining at V(h) -30 mV. Plots of normalized conductance vs. voltage at several V(h)s were nearly superimposable. Well-sustained I(Ca) with predominantly Ca(2+)-dependent inactivation and inhibition of approximately 30% of total I(Ca) by nifedipine or nimodipine suggests participation of L-type channels. Each of the peptide toxins (omega-conotoxin GVIA, omega-agatoxin IVA, SNX482) alone blocked 36-54% of I(Ca). Inhibition by any of these toxins was additive to inhibition by nifedipine. Combinations of the toxins failed to produce additive effects. I(Ca) of up to 30% of total remained with any combination of inhibitors, but 0.1mM cadmium blocked all I(Ca) rapidly and reversibly. We did not find differences among cells of differing size and hormone content. Thus, I(Ca) is carried by high voltage-activated Ca(2+) channels of at least three types, but the molecular types may differ from those characterized from mammalian neurons.

Gonadotropin-Releasing Hormone and Receptor Distributions in the Visual Processing Regions of Four Coral Reef Fishes

Gonadotropin-releasing hormone (GnRH) is widely distributed in the brain of fishes where it may function as a neuromodulator of sensory processing and behavior. Immunocytochemical and neuronal label experiments were conducted on species from four families of coral reef fishes (Chaetodontidae, butterflyfish; Pomacentridae, damselfish; Gobiidae, goby; and Labridae, wrasse) to assess conservation of GnRH targets in the visual processing retina and brain. In all species, GnRH-immunoreactive (-ir) axons from the terminal nerve project principally to the boundary between the inner plexiform (IPL) and inner nuclear (INL) layers of the retina, and are less prominent in the optic nerve, ganglion cell, IPL and INL. However, the density of GnRH innervation within the retina differed among fish species with highest concentrations in the damselfish and butterflyfish and lowest in the goby and wrasse. Experiments also show that GnRH receptors are associated with GnRH-ir axons within the fish retina primarily at the IPL-INL boundary, the region of light-dark adaptation and image processing of contrast, motion or color. GnRH-ir axons overlapped central projections of retinal ganglion cell axons primarily within the stratum album centrale and stratum griseum centrale of the tectum in all species, and were concentrated in several diencephalic visual processing centers. GnRH receptors are also localized to diencephalic visual centers and the stratum griseum periventriculare of the tectum, where motion perception and coordination of motor behavioral responses in three-dimensional space occur. This work demonstrates that the basic neural substrates for peptide-sensory convergence are conserved at multiple processing levels in the visual system of several reef fishes. Species differences in GnRH innervation to the retina and GnRH receptor distributions may be related to phylogeny, their use of vision in natural behaviors, or possibly binding properties of the antibodies. Future studies are needed to characterize the exact GnRH variants and receptor types found in these species so that possible functional consequences of GnRH influence on vision can be defined.

Gonadotropin-releasing hormone induces actin cytoskeleton remodeling and affects cell migration in a cell-type-specific manner in TSU-Pr1 and DU145 cells

GnRH was first identified as the hypothalamic decapeptide that promotes gonadotropin release from pituitary gonadotropes. Thereafter, direct stimulatory and inhibitory effects of GnRH on cell proliferation were demonstrated in a number of types of primary cultured cells and established cell lines. Recently, the effects of GnRH on cell attachment, cytoskeleton remodeling, and cell migration have also been reported. Thus, the effects of GnRH on various cell activities are of great interest among researchers who study the actions of GnRH. In this study, we demonstrated that GnRH induces actin cytoskeleton remodeling and affects cell migration using two human prostatic carcinoma cell lines, TSU-Pr1 and DU145. In TSU-Pr1, GnRH-I and -II induced the filopodia formation of the cells and promoted cell migration, whereas in DU145, GnRH-I and -II induced the formation of the cells with stress fiber and inhibited cell migration. In our previous studies, we reported the stimulatory and inhibitory effects of GnRH on the cell proliferation of TSU-Pr1 and DU145 cells. This study provides the first evidence for the effects of GnRH on actin cytoskeleton remodeling and cell migration of cells in which cell proliferation was affected by GnRH at the same time. Moreover, we also demonstrated that the same human GnRH receptor subtype, human type I GnRH receptor, is essential for the effects of GnRH-I and -II on actin cytoskeleton remodeling and cell migration in both TSU-Pr1 and DU145 cells using the technique of gene knock-down by RNA interference.