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

Microcystin-leucine arginine causes brain injury and functional disorder in Lithobates catesbeianus tadpoles by oxidative stress and inflammation

Microcystin-leucine arginine (MC-LR) is a toxin commonly found in eutrophic waters worldwide, but its potential effects on amphibian brain toxicity and exposure mechanisms are unclear. In this study, Lithobates catesbeianus tadpoles were exposed to MC-LR for 30 days at realistic ambient concentrations (0, 0.5, and 2 µg/L) to reveal its effects on brain health. The MC-LR bioaccumulation in the brain increased in dependence on the concentration of MC-LR exposure. Exposure to 0.5 and 2 µg/L MC-LR resulted in a significant down-regulation of the expression of structural components of the blood-brain barrier (CLDN1), while the expression of genes associated with inflammation (NLRP3, TNF, IL-1β, and CXCL12) was significantly up-regulated with increased number of eosinophils. In the hippocampal and hypothalamic regions, the number of vacuolated neuropils increased with increasing MC-LR exposure concentration, while the expression of genes associated with neuronal development (LGALS1, CACNA2D2, and NLGN4X) and neurotransmitter transmission (SLC6A13 and AChE) was significantly down-regulated. Moreover, the levels of neurotransmitters (5-HT, glutamate, GABA, and ACh) were significantly reduced. These results provide strong evidence that MC-LR exposure at realistic ambient concentrations of 0.5 and 2 µg/L can break the blood-brain barrier and raise the accumulation of MC-LR in the brain tissue, causing structural damage and functional disorder to brain neurons. Further, based on transcriptomic and biochemical analysis, it was revealed that MC-LR exposure induces DNA damage through oxidative stress and may be an important pathway causing brain structural damage and functional disorder. Overall, this study demonstrates the significant effects of MC-LR on the brain tissue of amphibians, highlighting the sensitivity of amphibians to MC-LR.

Multiple gonadotropin-releasing hormone systems in non-mammalian vertebrates: Ontogeny, anatomy, and physiology

Three paralogous genes for gonadotropin-releasing hormone (GnRH; gnrh1, gnrh2, and gnrh3) and GnRH receptors exist in non-mammalian vertebrates. However, there are some vertebrate species in which one or two of these paralogous genes have become non-functional during evolution. The developmental migration of GnRH neurons in the brain is evolutionarily conserved in mammals, reptiles, birds, amphibians, and jawed teleost fish. The three GnRH paralogs have specific expression patterns in the brain and originate from multiple sites. In acanthopterygian teleosts (medaka, cichlid, etc.), the preoptic area (POA)-GnRH1 and terminal nerve (TN)-GnRH3 neuronal types originate from the olfactory regions. In other fish species (zebrafish, goldfish and salmon) with only two GnRH paralogs (GnRH2 and GnRH3), the TN- and POA-GnRH3 neuronal types share the same olfactory origin. However, the developmental origin of midbrain (MB)-GnRH2 neurons is debatable between mesencephalic or neural crest site. Each GnRH system has distinctive anatomical and physiological characteristics, and functions differently. The POA-GnRH1 neurons are hypophysiotropic in nature and function in the neuroendocrine control of reproduction. The non-hypophysiotropic GnRH2/GnRH3 neurons probably play neuromodulatory roles in metabolism (MB-GnRH2) and the control of motivational state for sexual behavior (TN-GnRH3).

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.

Zebrafish Olfacto-Retinal Centrifugal Axon Projection and Distribution: Effects of Gonadotropin-Releasing Hormone and Dopaminergic Signaling

The terminalis neurons (TNs) have been described in teleost species. In zebrafish, the TNs are located in the olfactory bulb. The TNs synthesize and release gonadotropin-releasing hormone (GnRH) as one of the major neurotransmitters. The TNs project axons to many brain areas, which include the neural retina. In the retina, the TN axons synapse with dopaminergic interplexiform cells (DA-IPCs) and retinal ganglion cells (RGCs). In this research, we examine the role of GnRH and dopaminergic signaling in TN axon projection to the retina using the transgenic zebrafish Tg(GnRH-3::GFP). While the TNs developed at 34 h postfertilization (hpf), the first TN axons were not detected in the retina until 48-50 hpf, when the first DA-IPCs were differentiated. In developing embryos, inhibition of retinal GnRH signaling pathways severely interrupted the projection of TN axons to the retina. However, inhibition of retinal dopaminergic signaling produced little effect on TN axon projection. In adult retinas, inactivation of GnRH receptors disrupted the patterns of TN axon distribution, and depletion of DA-IPCs abolished the TN axons. When DA-IPCs regenerated, the TN axons reappeared. Together, the data suggest that in developing zebrafish retinas GnRH signaling is required for TN axon projection, whereas in adult retinas activation of GnRH and dopaminergic signaling transduction is required for normal distribution of the TN axons.

Electrical synapses connect a network of gonadotropin releasing hormone neurons in a cichlid fish

Initiating and regulating vertebrate reproduction requires pulsatile release of gonadotropin-releasing hormone (GnRH1) from the hypothalamus. Coordinated GnRH1 release, not simply elevated absolute levels, effects the release of pituitary gonadotropins that drive steroid production in the gonads. However, the mechanisms underlying synchronization of GnRH1 neurons are unknown. Control of synchronicity by gap junctions between GnRH1 neurons has been proposed but not previously found. We recorded simultaneously from pairs of transgenically labeled GnRH1 neurons in adult male Astatotilapia burtoni cichlid fish. We report that GnRH1 neurons are strongly and uniformly interconnected by electrical synapses that can drive spiking in connected cells and can be reversibly blocked by meclofenamic acid. Our results suggest that electrical synapses could promote coordinated spike firing in a cellular assemblage of GnRH1 neurons to produce the pulsatile output necessary for activation of the pituitary and reproduction.

Neurobiological study of fish brains gives insights into the nature of gonadotropin-releasing hormone 1-3 neurons

Accumulating evidence suggests that up to three different molecular species of GnRH peptides encoded by different paralogs of gnrh genes are expressed by anatomically distinct groups of GnRH neurons in the brain of one vertebrate species. They are called gnrh1, gnrh2, and gnrh3. Recent evidence from molecular, anatomical, and physiological experiments strongly suggests that each GnRH system functions differently. Here, we review recent advancement in the functional studies of the three different GnRH neuron systems, mainly focusing on the electrophysiological analysis of the GnRH-green fluorescent protein (GFP) transgenic animals. The introduction of GFP-transgenic animals for the electrophysiological analysis of GnRH neurons greatly advanced our knowledge on their anatomy and electrophysiology, especially of gnrh1 neurons, which has long defied detailed electrophysiological analysis of single neurons because of their small size and scattered distribution. Based on the results of recent studies, we propose that different electrophysiological properties, especially the spontaneous patterns of electrical activities and their time-dependent changes, and the axonal projections characterize the different functions of GnRH1-3 neurons; GnRH1 neurons act as hypophysiotropic neuroendocrine regulators, and GnRH2 and GnRH3 neurons act as neuromodulators in wide areas of the brain.

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 GFP-tagged GnRH-containing terminalis neurons in transgenic zebrafish

The terminalis nerve (TN) has been described in all vertebrate species, in which it plays important roles in animal behavior and physiology. In teleost fish, the TN is located in the olfactory bulb and in its nerve tract. Here, we report a study on the characterization of the TN cell development, axon projection and physiology in zebrafish (Danio rerio). We have generated several lines of transgenic zebrafish [Tg (GnRH-3::GFP)] that express GFP in the TN cells. The transgenes are expressed under the transcriptional control of the zebrafish GnRH-3 promoter. During development, the first GFP-positive TN cell was identified at approximately 34 h post-fertilization (hpf). By 38 hpf, several clusters of TN cells were identified in the olfactory bulb and olfactory nerve tract. In the olfactory bulb, the TN cells projected axons caudally. In the forebrain, some of the TN axons extended caudally, but most crossed the midline of the brain at the commissural anterior. In the midbrain, some of the TN axons extended dorsally towards the tectum, whereas other axons extended caudally, or extended ventrally to the optic nerve where they entered the neural retina. We also examined the cell membrane property of the TN cells. Using patch-clamp techniques, we recorded spontaneous and evoked action potentials from isolated TN cells. We examined the expression of glutamate receptors in the TN cells. The data shed light on the mechanisms of TN function in the nervous system and in the regulation of animal physiology.

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.

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.

Electrophysiological characteristics of gonadotrophin-releasing hormone 1-3 neurones: insights from a study of fish brains

The vertebrate gonadotrophin-releasing hormone (GnRH) neurones are considered to consist of one group of hypothalamic neuroendocrine and two groups of extrahypothalamic neuromodulatory GnRH neurones, and each group of neurones expresses different molecular species of GnRH peptide. Different GnRH peptides are produced by one of the three paralogous GnRH genes, gnrh1, gnrh2 and gnrh3, which are considered to have originated from gene duplications. All three GnRH systems are well developed in teleost brains. By taking advantage of this, and especially the use of GnRH-green fluorescent protein transgenic fish, the anatomical and electrophysiological properties of all three types of GnRH neurones can now be studied. The hypophysiotropic GnRH1 neurones in the preoptic area show episodic spontaneous electrical activities, whereas the extrahypothalamic GnRH2 neurones in the midbrain and GnRH3 neurones in the terminal nerve show regular intrinsic pacemaker activities. It is suggested that these different electrophysiological properties are related to their different functions (i.e. GnRH1 neurones act as hypophysiotropic neuroendocrine regulators and GnRH2 and GnRH3 neurones act as neuromodulators). The present review focuses on recent electrophysiological analyses of GnRH3 neurones, which have revealed the excitatory GABAergic and the inhibitory FMRFamide-like peptidergic regulations acting upon them, as well as gap junctional electrotonic coupling.

Regular pacemaker activity characterizes gonadotropin-releasing hormone 2 neurons recorded from green fluorescent protein-transgenic medaka

GnRH2 is a molecule conserved from fish to humans, suggesting its important functions. However, recent studies have shown that GnRH2 neurons project widely in the brain but not to the pituitary, which suggests their functions other than stimulation of gonadotropin secretion. In contrast to the wealth of knowledge in GnRH1 and GnRH3 neuronal systems, the GnRH2 neuronal system remains to be studied, and there has been no single cell approach so far, partly because of the lack of GnRH2 system in rodents. Here, we generated GnRH2-green fluorescent protein (GFP) transgenic medaka for the first single cell electrophysiological recording from GnRH2 neurons in vertebrates. Whole-cell and on-cell patch clamp analyses revealed their regular pacemaker activities that are intrinsic to the GnRH2 neurons. Pacemaker activities of GnRH2 neurons were not peculiar to medaka because dwarf gourami GnRH2 neurons also showed similar pacemaker activities. By comparing with spontaneous action currents from GFP-expressing GnRH1 and GnRH3 neurons in the adult transgenic medaka, which were already in our hands, we have demonstrated that GnRH2 neurons show pacemaker activity similar to nonhypophysiotropic GnRH3 neurons but not to hypophysiotropic GnRH1 neurons. Thus, by taking advantage of medaka brain, which has all three GnRH neuronal systems with different axonal projection patterns and thus different functions, we have gained insights into the close relationship between the pattern of spontaneous electrical activity and the functions of the three. Moreover, the three types of GnRH-GFP transgenic medaka will provide useful models for studying multifunctional GnRH systems in future.

Primary culture of the isolated terminal nerve-gonadotrophin-releasing hormone neurones derived from adult teleost (dwarf gourami, Colisa lalia) brain for the study of peptide release mechanisms

Terminal nerve (TN)-gonadotrophin-releasing hormone (GnRH) neurones are suggested to release GnRH peptides from widely-branched neural processes and the somatodendritic regions, depending on their firing activities. The released GnRH may exert its neuromodulatory actions on GnRH receptors located on various target neurones. The electrophysiological and morphological characteristics of TN-GnRH neurones, which are shared with other peptidergic neurones of vertebrate brains, are thought to represent general features of neuromodulatory and ⁄ or neurosecretory neurones. To address questions concerning the ways in which the electrical activities of peptidergic (TN-GnRH) neuronal somata affect GnRH release from different neuronal compartments, we established a primary culture system of TN-GnRH neurones, which will facilitate simultaneous recordings of various physiological signals from different compartments of a single TN-GnRH neurone cultured in a flat plane. The whole brain of an adult freshwater teleost, the dwarf gourami, was dissected out. The TN-GnRH neurones were then isolated and plated on a coverslip in culture medium. The isolated TN-GnRH neurones could be cultured for up to 2 weeks. In culture, the neurones grew both axon- and dendrite-like neurites, and these processes were phenotypically similar to those found in situ. Unlike the neurones in situ, the cultured neurones had somewhat depolarised resting membrane potentials and showed no spontaneous discharge, which, however, should not be considered to comprise unhealthy culture conditions. Instead, they showed subthreshold spontaneous membrane potential oscillations and could be induced to fire in phasic or tonic patterns. In addition, stimulus-induced exocytotic events could be demonstrated in the soma and neurites using a fluorescent dye, FM1-43. Thus, the present isolated culture of TN-GnRH neurones will open up a wide range of possibilities for studying cellular mechanism of exocytosis, generation of spontaneous firing activity, and neurite outgrowth in peptidergic neurones.

Three types of gonadotrophin-releasing hormone neurones and steroid-sensitive sexually dimorphic kisspeptin neurones in teleosts

In general, the gonadotrophin-releasing hormone (GnRH) neuronal systems of vertebrates consist of one group of hypothalamic neuroendocrine and one or two group(s) of extrahypothalamic neuromodulatory GnRH neurones. By taking advantage of the brains of dwarf gourami and GnRH-green fluorescent protein transgenic medaka, the spontaneous electrical activities of all three different types of GnRH neurones have now been characterised. The hypophysiotrophic preoptic area-gnrh1 neurones show irregular and episodic spontaneous electrical activities, whereas extrahypothalamic midbrain gnrh2 and terminal nerve-gnrh3 neurones show regular pacemaker potentials. It is suggested that these spontaneous electrical activities are related to their different functions as neuroendocrine hormones (gnrh1 neurones) and neuromodulators (gnrh2 and gnrh3 neurones). On the other hand, recent evidence strongly suggests that the GnRH neurones are regulated by another class of peptidergic neurones, the kisspeptin neurones. The gene encoding kisspeptin (kiss1 gene) has been cloned, and the anatomical distribution of kiss1 mRNA expressing neurones (kiss1 neurones) has recently been studied in brains of several fish species. In medaka, two kiss1 neuronal populations in hypothalamic areas, called the nucleus ventral tuberis (NVT) and nucleus posterioris periventricularis (NPPv), have been identified. The NVT kiss1 neurones are sexually dimorphic in number (male >> female) under breeding conditions and are sensitive to ovarian oestrogens, whereas the NPPv kiss1 neurones are neither sexually dimorphic, nor sensitive to steroids. The steroid-sensitive changes in kiss1 mRNA expression in the NVT occur physiologically, closely linked to the reproductive state. As in the mammalian counterpart, the medaka kiss1 neuronal system is suggested to be involved in the central regulation of reproductive functions. Interestingly, kiss2, another gene paralogous to kiss1, has been cloned in some fish species. The possible regulation of reproduction and other unknown functions by kiss1 and kiss2 neurones may be the focus of future studies.