Glutamatergic inputs contribute to phasic activity in vasopressin neurons

Role of the paraventricular nucleus of the hypothalamus in sleep–wake regulation

The paraventricular nucleus of the hypothalamus (PVH) acts as a cohesive functional unit that regulates neuroendocrine and autonomic function, complex behavior, and negative emotions after stress. However, how the PVH integrates arousal with these biological functions has only recently been explored. Clinical reports, combined with neurotoxic lesioning, immunochemistry, neuronal activity recordings, and the polysomnographic analyses of genetically modified animals, have revealed that the PVH is important for the control of wakefulness. Here, we review emerging anatomical and neural mechanisms for sleep–wake regulation in the PVH to support its essential role in the promotion and maintenance of wakefulness.

Opioid Receptor-Mediated Regulation of Neurotransmission in the Brain

Opioids mediate their effects via opioid receptors: mu, delta, and kappa. At the neuronal level, opioid receptors are generally inhibitory, presynaptically reducing neurotransmitter release and postsynaptically hyperpolarizing neurons. However, opioid receptor-mediated regulation of neuronal function and synaptic transmission is not uniform in expression pattern and mechanism across the brain. The localization of receptors within specific cell types and neurocircuits determine the effects that endogenous and exogenous opioids have on brain function. In this review we will explore the similarities and differences in opioid receptor-mediated regulation of neurotransmission across different brain regions. We discuss how future studies can consider potential cell-type, regional, and neural pathway-specific effects of opioid receptors in order to better understand how opioid receptors modulate brain function.

Mechanism and function of phasic firing in vasopressin-releasing magnocellular neurosecretory cells

Magnocellular neurosecretory cells that release vasopressin (MNC^VP ) from axon terminals in the neurohypophysis display a unique pattern of action potential firing termed phasic firing. Under basal conditions, only a small proportion of MNC^VP display spontaneous phasic firing. However, acute and chronic conditions that stimulate vasopressin release, such as hemorrhage and dehydration, greatly enhance the number of MNC^VP that fire phasically. Phasic firing optimizes VP neurosecretion at axon terminals by allowing action potential broadening to promote calcium-dependent frequency-facilitation, at the same time as preventing the secretory fatigue caused by spike inactivation that occurs during prolonged continuous stimulation. This review provides an update on our mechanistic understanding of these processes and highlights important gaps in our knowledge that must be addressed in future experiments.

Single-Cell RNA Sequencing Analysis of the Drosophila Larval Ventral Cord

Drosophila provides a powerful genetic system and an excellent model to study the development and function of the nervous system. The fly's small brain and complex behavior has been instrumental in mapping neuronal circuits and elucidating the neural basis of behavior. The fast pace of fly development and the wealth of genetic tools has enabled systematic studies on cell differentiation and fate specification, and has uncovered strategies for axon guidance and targeting. The accessibility of neuronal structures and the ability to edit and manipulate gene expression in selective cells and/or synaptic compartments has revealed mechanisms for synapse assembly and neuronal connectivity. Recent advances in single-cell RNA sequencing (scRNA-seq) have further enhanced our appreciation and understanding of neuronal diversity in a fly brain. However, due to the small size of the fly brain and its constituent cells, scRNA-seq methodologies require a few adaptations. Here, we describe a set of protocols optimized for scRNA-seq analysis of the Drosophila larval ventral nerve cord, starting from tissue dissection and cell dissociation to cDNA library preparation, sequencing, and data analysis. We apply this workflow to three separate samples and detail the technical challenges associated with successful application of scRNA-seq to studies on neuronal diversity. An accompanying article (Vicidomini, Nguyen, Choudhury, Brody, & Serpe, 2021) presents a custom multistage analysis pipeline that integrates modules contained in different R packages to ensure high-flexibility, high-quality RNA-seq data analysis. These protocols are developed for Drosophila larval ventral nerve cord, but could easily be adapted to other tissues and model organisms. © 2021 U.S. Government. Basic Protocol 1: Dissection of larval ventral nerve cords and preparation of single-cell suspensions Basic Protocol 2: Preparation and sequencing of single-cell transcriptome libraries Basic Protocol 3: Alignment of raw sequencing data to indexed genome and generation of count matrices.

Conveyance of cortical pacing for parkinsonian tremor-like hyperkinetic behavior by subthalamic dysrhythmia

Parkinson's disease is characterized by both hypokinetic and hyperkinetic symptoms. While increased subthalamic burst discharges have a direct causal relationship with the hypokinetic manifestations (e.g., rigidity and bradykinesia), the origin of the hyperkinetic symptoms (e.g., resting tremor and propulsive gait) has remained obscure. Neuronal burst discharges are presumed to be autonomous or less responsive to synaptic input, thereby interrupting the information flow. We, however, demonstrate that subthalamic burst discharges are dependent on cortical glutamatergic synaptic input, which is enhanced by A-type K+ channel inhibition. Excessive top-down-triggered subthalamic burst discharges then drive highly correlative activities bottom-up in the motor cortices and skeletal muscles. This leads to hyperkinetic behaviors such as tremors, which are effectively ameliorated by inhibition of cortico-subthalamic AMPAergic synaptic transmission. We conclude that subthalamic burst discharges play an imperative role in cortico-subcortical information relay, and they critically contribute to the pathogenesis of both hypokinetic and hyperkinetic parkinsonian symptoms.

NMDA receptor-dependent signalling pathways regulate arginine vasopressin expression in the paraventricular nucleus of the rat

The antidiuretic hormone arginine vasopressin (AVP) regulates water homeostasis, blood pressure and a range of stress responses. It is synthesized in the hypothalamus and released from the posterior pituitary into the general circulation upon a range of stimuli. While the mechanisms leading to AVP secretion have been widely investigated, the molecular mechanisms regulating AVP gene expression are mostly unclear. Here we investigated the neurotransmitters and signal transduction pathways that activate AVP gene expression in the paraventricular nucleus (PVN) of the rat using acute brain slices and quantitative real-time PCR. We show that stimulation with l-glutamate robustly induced AVP gene expression in acute hypothalamic brain slices containing the PVN. More specifically, we show that AVP transcription was stimulated by NMDA. Using pharmacological treatments, our data further reveal that the activation of ERK1/2 (PD184352), CaMKII (KN-62) and PI3K (LY294002; 740 Y-P) is involved in the NMDA-induced AVP gene expression in the PVN. Together, this study identifies NMDA-mediated cell signalling pathways that regulate AVP gene expression in the rat PVN.

P2X2 Receptor Expression and Function Is Upregulated in the Rat Supraoptic Nucleus Stimulated Through Refeeding After Fasting

Magnocellular neurons in the supraoptic nucleus (SON), which synthesize and release arginine vasopressin (AVP) and oxytocin (OT), express several subtypes of ATP-stimulated purinergic P2X receptors (P2XR) that modulate neuronal activity as well as neurotransmitter and hormone release. However, the physiological impact of this modulation is not well understood. Here, we tested a hypothesis that P2XRs play a role in the sustained release of hormones from SON neurons stimulated through fasting/refeeding. We studied the effect of 2 h of refeeding after 48 h of fasting on P2XR and P2YR mRNA expression and ATP-induced presynaptic and postsynaptic responses in the SON of 30-day-old rats. Quantitative real-time PCR revealed that the expression of P2X2R and AVP mRNA was upregulated, whereas P2X4R, P2X7R, P2Y2R, and OT mRNA levels were not significantly changed and P2Y1R mRNA expression was decreased. Whole-cell patch clamp recordings performed on isolated rat brain slices showed that the amplitude of the ATP-stimulated somatic current and the ATP-induced increases in the frequency of spontaneous GABAergic inhibitory postsynaptic currents were significantly higher in SON neurons from fasted/refed rats than in SON neurons from normally fed rats. No evidence was found for changes in the presynaptic effect of ATP in SON neurons not expressing somatic P2XRs. These results suggest that the increased activity of SON neurons synthesizing AVP is associated with enhanced expression of P2X2Rs on neuronal cell bodies and their GABAergic presynaptic nerve terminals.

A Nitric Oxide-Dependent Presynaptic LTP at Glutamatergic Synapses of the PVN Magnocellular Neurosecretory Cells in vitro in Rats

The magnocellular neurosecretory cells (MNCs) of the hypothalamic paraventricular nucleus (PVN) integrate incoming signals to secrete oxytocin (OT), and vasopressin (VP) from their nerve terminals in the posterior pituitary gland. In the absence of gamma-aminobutyric acid A (GABA_A) and cannabinoids 1 (CB1) receptor activity, we used whole-cell patch-clamp recording, single-cell reverse transcription-multiplex polymerase chain reaction (SC-RT-mPCR), biocytin histochemistry and pharmacological methods to examine the mechanism of high frequency stimulus (HFS, 100 Hz)-induced long-term potentiation (LTP) at glutamatergic synapses in the PVN MNCs of juvenile male rats. Our results showed that HFS-induced LTP at glutamatergic synapses was accompanied by a decrease in the paired-pulse ratio (PPR) of the PVN MNCs. In these MNCs, HFS-induced LTP persisted in the presence of a group 1 metabotropic glutamate receptor (mGluR1) antagonist; however, it was abolished by an N-methyl-D-aspartic acid (NMDA) receptor blocker. Notably, HFS-induced LTP in the PVN MNCs was completely prevented by a nitric oxide synthase (NOS) inhibitor. The application of an NO donor not only induced the LTP of excitatory glutamatergic inputs in the PVN MNCs, but also occluded the HFS-induced LTP in these MNCs. Moreover, HFS-induced LTP in the PVN MNCs was also abolished by a specific protein kinase A (PKA) inhibitor, KT5720. SC-RT-mPCR analysis revealed that 64.5% (62/96) of MNCs expressed OT mRNA. Our results indicate that a HFS can induce an NMDA receptor and NO cascades dependent on presynaptic glutamatergic LTP in the PVN MNCs via a PKA signaling pathway.

Ultradian calcium rhythms in the paraventricular nucleus and subparaventricular zone in the hypothalamus

Despite that the various functions in mammals fluctuate in the ultradian fashion, the origin and mechanism of the rhythm are largely unknown. In this study, we found synchronous ultradian calcium rhythms in the hypothalamic paraventricular nucleus (PVN), subparaventricular zone (SPZ), and suprachiasmatic nucleus (SCN). The ultradian rhythms were originated from the SPZ-PVN region and transmitted to the SCN. Neurochemical interventions revealed that the glutamatergic mechanism is critical for generation and a tetrodotoxin-sensitive neural network for synchrony of the ultradian rhythm. The GABAergic system could have a role in refining the circadian output signals. The study provides the first clue to understand the loci and mechanism of ultradian rhythm in the hypothalamus.

The suprachiasmatic nucleus (SCN), the master circadian clock in mammals, sends major output signals to the subparaventricular zone (SPZ) and further to the paraventricular nucleus (PVN), the neural mechanism of which is largely unknown. In this study, the intracellular calcium levels were measured continuously in cultured hypothalamic slices containing the PVN, SPZ, and SCN. We detected ultradian calcium rhythms in both the SPZ-PVN and SCN regions with periods of 0.5–4.0 hours, the frequency of which depended on the local circadian rhythm in the SPZ-PVN region. The ultradian rhythms were synchronous in the entire SPZ-PVN region and a part of the SCN. Because the ultradian rhythms were not detected in the SCN-only slice, the origin of ultradian rhythm is the SPZ-PVN region. In association with an ultradian bout, a rapid increase of intracellular calcium in a millisecond order was detected, the frequency of which determined the amplitude of an ultradian bout. The synchronous ultradian rhythms were desynchronized and depressed by a sodium channel blocker tetrodotoxin, suggesting that a tetrodotoxin-sensitive network is involved in synchrony of the ultradian bouts. In contrast, the ultradian rhythm is abolished by glutamate receptor blockers, indicating the critical role of glutamatergic mechanism in ultradian rhythm generation, while a GABA_A receptor blocker increased the frequency of ultradian rhythm and modified the circadian rhythm in the SCN. A GABAergic network may refine the circadian output signals. The present study provides a clue to unraveling the loci and network mechanisms of the ultradian rhythm.

Ultrastructural characterization of tumor necrosis factor alpha receptor type 1 distribution in the hypothalamic paraventricular nucleus of the mouse

The immune/inflammatory signaling molecule tumor necrosis factor α (TNFα) is an important mediator of both constitutive and plastic signaling in the brain. In particular, TNFα is implicated in physiological processes, including fever, energy balance, and autonomic function, known to involve the hypothalamic paraventricular nucleus (PVN). Many critical actions of TNFα are transduced by the TNFα type 1 receptor (TNFR1), whose activation has been shown to potently modulate classical neural signaling. There is, however, little known about the cellular sites of action for TNFR1 in the PVN. In the present study, high-resolution electron microscopic immunocytochemistry was used to demonstrate the ultrastructural distribution of TNFR1 in the PVN. Labeling for TNFR1 was found in somata and dendrites, and to a lesser extent in axon terminals and glia in the PVN. In dendritic profiles, TNFR1 was mainly present in the cytoplasm, and in association with presumably functional sites on the plasma membrane. Dendritic profiles expressing TNFR1 were contacted by axon terminals, which formed non-synaptic appositions, as well as excitatory-type and inhibitory-type synaptic specializations. A smaller population of TNFR1-labeled axon terminals making non-synaptic appositions, and to a lesser extent synaptic contacts, with unlabeled dendrites was also identified. These findings indicate that TNFR1 is structurally positioned to modulate postsynaptic signaling in the PVN, suggesting a mechanism whereby TNFR1 activation contributes to cardiovascular and other autonomic functions.

Magnocellular Neurons and Posterior Pituitary Function

The posterior pituitary gland secretes oxytocin and vasopressin (the antidiuretic hormone) into the blood system. Oxytocin is required for normal delivery of the young and for delivery of milk to the young during lactation. Vasopressin increases water reabsorption in the kidney to maintain body fluid balance and causes vasoconstriction to increase blood pressure. Oxytocin and vasopressin secretion occurs from the axon terminals of magnocellular neurons whose cell bodies are principally found in the hypothalamic supraoptic nucleus and paraventricular nucleus. The physiological functions of oxytocin and vasopressin depend on their secretion, which is principally determined by the pattern of action potentials initiated at the cell bodies. Appropriate secretion of oxytocin and vasopressin to meet the challenges of changing physiological conditions relies mainly on integration of afferent information on reproductive, osmotic, and cardiovascular status with local regulation of magnocellular neurons by glia as well as intrinsic regulation by the magnocellular neurons themselves. This review focuses on the control of magnocellular neuron activity with a particular emphasis on their regulation by reproductive function, body fluid balance, and cardiovascular status. © 2016 American Physiological Society. Compr Physiol 6:1701-1741, 2016.

Electrophysiology of Hypothalamic Magnocellular Neurons In vitro: A Rhythmic Drive in Organotypic Cultures and Acute Slices

Hypothalamic neurohormones are released in a pulsatile manner. The mechanisms of this pulsatility remain poorly understood and several hypotheses are available, depending upon the neuroendocrine system considered. Among these systems, hypothalamo-neurohypophyseal magnocellular neurons have been early-considered models, as they typically display an electrical activity consisting of bursts of action potentials that is optimal for the release of boluses of the neurohormones oxytocin and vasopressin. The cellular mechanisms underlying this bursting behavior have been studied in vitro, using either acute slices of the adult hypothalamus, or organotypic cultures of neonatal hypothalamic tissue. We have recently proposed, from experiments in organotypic cultures, that specific central pattern generator networks, upstream of magnocellular neurons, determine their bursting activity. Here, we have tested whether a similar hypothesis can be derived from in vitro experiments in acute slices of the adult hypothalamus. To this aim we have screened our electrophysiological recordings of the magnocellular neurons, previously obtained from acute slices, with an analysis of autocorrelation of action potentials to detect a rhythmic drive as we recently did for organotypic cultures. This confirmed that the bursting behavior of magnocellular neurons is governed by central pattern generator networks whose rhythmic drive, and thus probably integrity, is however less satisfactorily preserved in the acute slices from adult brains.

Asynchronous presynaptic glutamate release enhances neuronal excitability during the post-spike refractory period

KEY POINTS

Many excitatory synapses in the brain release glutamate with both synchronous and asynchronous components. Immediately following an action potential, neurons display a reduced excitability due to the post-spike afterhyperpolarization (AHP). This gives rise to a relative refractory period. When an action potential is evoked by glutamate synaptic input possessing asynchronous release, the delayed glutamate release events act to depolarize the neuron during the AHP and overcome the relative refractory period. These results demonstrate a new role for asynchronous release in regulating post-spike excitability and the relative refractory period in central neurons.

ABSTRACT

Post-spike afterhyperpolarizations (AHPs) functionally inhibit neuronal excitability for tens to hundreds of milliseconds following each action potential. This imposes a relative refractory period during which synaptic excitation is less effective at evoking spikes. Here we asked whether some synapses have mechanisms in place that allow them to overcome the AHP and drive spiking in target cells during this period of reduced excitability. We examined glutamate synapses onto oxytocin and vasopressin neurons in the paraventricular nucleus of the hypothalamus. These synapses can display pronounced asynchronous glutamate release following a single presynaptic spike, with the time course of release being similar to that of the post-spike AHP. To test whether asynchronous release is more effective at overcoming the relative refractory period, we evoked a single action potential with either a brief synchronous depolarization or an asynchronous potential and then assessed excitability at multiple time points following the spike. Neurons receiving asynchronous depolarizing synaptic inputs had a shorter relative refractory period than those receiving synchronous depolarizations. Our data demonstrate that synapses releasing glutamate in an asynchronous and delayed manner are ideally adapted to counter the AHP. By effectively overcoming the relative refractory period, the kinetics of excitatory synaptic input can play an important role in controlling post-spike excitability.

Neonatal testosterone suppresses a neuroendocrine pulse generator required for reproduction

The pituitary gland releases hormones in a pulsatile fashion guaranteeing signalling efficiency. The determinants of pulsatility are poorly circumscribed. Here we show in magnocellular hypothalamo-neurohypophyseal oxytocin (OT) neurons that the bursting activity underlying the neurohormonal pulses necessary for parturition and the milk-ejection reflex is entirely driven by a female-specific central pattern generator (CPG). Surprisingly, this CPG is active in both male and female neonates, but is inactivated in males after the first week of life. CPG activity can be restored in males by orchidectomy or silenced in females by exogenous testosterone. This steroid effect is aromatase and caspase dependent, and is mediated via oestrogen receptor-α. This indicates the apoptosis of the CPG network during hypothalamic sexual differentiation, explaining why OT neurons do not burst in adult males. This supports the view that stereotypic neuroendocrine pulsatility is governed by CPGs, some of which are subjected to gender-specific perinatal programming.

Effects of reciprocal inhibitory coupling in model neurons

Central pattern generators are neuron networks that produce vital rhythmic motor outputs such as those observed in mastication, walking and breathing. Their activity patterns depend on the tuning of their intrinsic ionic conductances, their synaptic interconnectivity and entrainment by extrinsic neurons. The influence of two commonly found synaptic connectivities--reciprocal inhibition and electrical coupling--are investigated here using a neuron model with subthreshold oscillation capability, in different firing and entrainment regimes. We study the dynamics displayed by a network of a pair of neurons with various firing regimes, coupled by either (i) only reciprocal inhibition or by (ii) electrical coupling first and then reciprocal inhibition. In both scenarios a range of coupling strengths for the reciprocal inhibition is tested, and in general the neuron with the lower firing rate stops spiking for strong enough inhibitory coupling, while the faster neuron remains active. However, in scenario (ii) the originally slower neuron stops spiking at weaker inhibitory coupling strength, suggesting that the electrical coupling introduces an element of instability to the two-neuron network.

Physiological regulation of magnocellular neurosecretory cell activity: integration of intrinsic, local and afferent mechanisms

The hypothalamic supraoptic and paraventricular nuclei contain magnocellular neurosecretory cells (MNCs) that project to the posterior pituitary gland where they secrete either oxytocin or vasopressin (the antidiuretic hormone) into the circulation. Oxytocin is important for delivery at birth and is essential for milk ejection during suckling. Vasopressin primarily promotes water reabsorption in the kidney to maintain body fluid balance, but also increases vasoconstriction. The profile of oxytocin and vasopressin secretion is principally determined by the pattern of action potentials initiated at the cell bodies. Although it has long been known that the activity of MNCs depends upon afferent inputs that relay information on reproductive, osmotic and cardiovascular status, it has recently become clear that activity depends critically on local regulation by glial cells, as well as intrinsic regulation by the MNCs themselves. Here, we provide an overview of recent advances in our understanding of how intrinsic and local extrinsic mechanisms integrate with afferent inputs to generate appropriate physiological regulation of oxytocin and vasopressin MNC activity.

GABAergic inhibition through synergistic astrocytic neuronal interaction transiently decreases vasopressin neuronal activity during hypoosmotic challenge

The neuropeptide vasopressin is crucial to mammalian osmotic regulation. Local hypoosmotic challenge transiently decreases and then increases vasopressin secretion. To investigate mechanisms underlying this transient response, we examined the effects of hypoosmotic challenge on the electrical activity of rat hypothalamic supraoptic nucleus (SON) vasopressin neurons using patch-clamp recordings. We found that 5 min exposure of hypothalamic slices to hypoosmotic solution transiently increased inhibitory postsynaptic current (IPSC) frequency and reduced the firing rate of vasopressin neurons. Recovery occurred by 10 min of exposure, even though the osmolality remained low. The γ-aminobutyric acid (GABA)A receptor blocker, gabazine, blocked the IPSCs and the hypoosmotic suppression of firing. The gliotoxin l-aminoadipic acid blocked the increase in IPSC frequency at 5 min and the recovery of firing at 10 min, indicating astrocytic involvement in hypoosmotic modulation of vasopressin neuronal activity. Moreover, β-alanine, an osmolyte of astrocytes and GABA transporter (GAT) inhibitor, blocked the increase in IPSC frequency at 5 min of hypoosmotic challenge. Confocal microscopy of immunostained SON sections revealed that astrocytes and magnocellular neurons both showed positive staining of vesicular GATs (VGAT). Hypoosmotic stimulation in vivo reduced the number of VGAT-expressing neurons, and increased co-localisation and molecular association of VGAT with glial fibrillary acidic protein that increased significantly by 10 min. By 30 min, neuronal VGAT labelling was partially restored, and astrocytic VGAT was relocated to the ventral portion while it decreased in the somatic zone of the SON. Thus, synergistic astrocytic and neuronal GABAergic inhibition could ensure that vasopressin neuron firing is only transiently suppressed under hypoosmotic conditions.

Hyposmolality differentially and spatiotemporally modulates levels of glutamine synthetase and serine racemase in rat supraoptic nucleus

Prolonged hyposmotic challenge (HOC) has a dual effect on vasopressin (VP) secretion [Yagil and Sladek (1990) Am J Physiol 258(2 Pt 2):R492-R500]. We describe an electrophysiological correlate of this phenomenon, whereby in vitro HOC transiently reduced the firing activity of VP neurons within the supraoptic nucleus of brain slices, which was followed by a rebound increase of their activity; this was paralleled by changes in the level of proteins relevant to astroglia-neuronal interactions. Hence, in vitro HOC transiently (at 5 min) increased the level of astrocyte-specific glial fibrillary acidic protein (GFAP), which then declined to control or base level (at 20 min); this was blocked by the gliotoxin L-aminoadipic acid, but not by tetanus toxin, which was used to inhibit neurotransmission. Similarly, in vivo HOC led to changes in GFAP level, which after an early increase (10 min) returned to normal (30 min). Immunoassays revealed that neuronal, but not astrocytic, expression of serine racemase (SR) was increased at the late stage of HOC in vivo, whereas at an early stage there was a transient increase in level of the astrocyte-specific glutamine synthetase (GS). Furthermore, there was an increased molecular association between GFAP and GS at 10 min, whereas SR increased its association with the neuronal nuclear antigen NeuN at 30 min. These results suggest that the dual effect of HOC on VP neuronal secretion/activity could be related to metabolic/signaling changes in astrocytes (glutamate-glutamine conversion) and neurons (D-serine synthesis/ammonia production), which may account for the rebound in VP neuronal activity, presumably by promoting the activation of neuronal glutamate receptors.

Burst firing in gonadotrophin-releasing hormone neurones does not require ionotrophic GABA or glutamate receptor activation

Burst firing is a feature of many neuroendocrine cell types, including the hypothalamic gonadotrophin-releasing hormone (GnRH) neurones that control fertility. The role of intrinsic and extrinsic influences in generating GnRH neurone burst firing is presently unclear. In the present study, we investigated the role of fast amino acid transmission in burst firing by examining the effects of receptor antagonists on bursting displayed by green fluorescent protein GnRH neurones in sagittal brain slices prepared from adult male mice. Blockade of AMPA and NMDA glutamate receptors with a cocktail of CNQX and AP5 was found to have no effects on burst firing in GnRH neurones. The frequency of bursts, dynamics of individual bursts, or percentage of firing clustered in bursts was not altered. Similarly, GABA(A) receptor antagonists bicuculline and picrotoxin had no effects upon burst firing in GnRH neurones. To examine the importance of both glutamate and GABA ionotrophic signalling, a cocktail including picrotoxin, CNQX and AP5 was used but, again, this was found to have no effects on GnRH neurone burst firing. To further question the impact of endogenous amino acid release on burst firing, electrical activation of anteroventral periventricular nuclei GABA/glutamate inputs to GnRH neurones was undertaken and found to have no impact on burst firing. Taken together, these observations indicate that bursting in GnRH neurones is not dependent upon acute ionotrophic GABA and glutamate signalling and suggest that extrinsic inputs to GnRH neurones acting through AMPA, NMDA and GABA(A) receptors are unlikely to be required for burst initiation in these cells.

Kainate receptor-induced retrograde inhibition of glutamatergic transmission in vasopressin neurons

Presynaptic kainate receptors (KARs) exert a modulatory action on transmitter release. We here report that applications of agonists of GluK1-containing KARs in the rat supraoptic nucleus has an opposite action on glutamatergic transmission according to the phenotype of the postsynaptic neuron. Whereas glutamate release was facilitated in oxytocin (OT) neurons, it was inhibited in vasopressin (VP) cells. Interestingly, an antagonist of GluK1-containing KARs caused an inhibition of glutamate release in both OT and VP neurons, revealing the existence of tonically activated presynaptic KARs that are positively coupled to transmitter release. We thus postulated that the inhibition of glutamate release observed with exogenous applications of GluK1 agonists on VP neurons could be indirect. In agreement with this hypothesis, we first showed that functional GluK1-containing KARs were present postsynaptically on VP neurons but not on OT cells. We next showed that the inhibitory effect induced by exogenous GluK1 receptor agonist was compromised when BAPTA was added in the recording pipette to buffer intracellular Ca2+ and block the release of a putative retrograde messenger. Under these conditions, GluK1-containing KAR agonist facilitates glutamatergic transmission in VP neurons in a manner similar to that observed for OT neurons and that resulted from the activation of presynaptic GluK1 receptors. GluK1-mediated inhibition of glutamate release in VP neurons was also blocked by a κ-opioid receptor antagonist. These findings suggest that activation of postsynaptic GluK1-containing KARs on VP neurons leads to the release of dynorphin, which in turn acts on presynaptic κ-opioid receptors to inhibit glutamate release.

Endothelin-mediated calcium responses in supraoptic nucleus astrocytes influence magnocellular neurosecretory firing activity

In addition to their peripheral vasoactive effects, accumulating evidence supports an important role for endothelins (ETs) in the regulation of the hypothalamic magnocellular neurosecretory system, which produces and releases the neurohormones vasopressin (VP) and oxytocin (OT). Still, the precise cellular substrates, loci and mechanisms underlying the actions of ETs on the magnocellular system are poorly understood. In the present study, we combined patch-clamp electrophysiology, confocal Ca(2+) imaging and immunohistochemistry to study the actions of ETs on supraoptic nucleus (SON) magnocellular neurosecretory neurones and astrocytes. Our studies show that ET-1 evoked rises in [Ca(2+) ](i) levels in SON astrocytes (but not neurones), an effect largely mediated by the activation of ET(B) receptors and mobilisation of thapsigargin-sensitive Ca(2+) stores. The presence of ET(B) receptors in SON astrocytes was also verified immunohistochemically. ET(B) receptor activation either increased (75%) or decreased (25%) SON firing activity, both in VP and putative OT neurones, and these effects were prevented when slices were preincubated in glutamate receptor blockers or nitric oxide synthase blockers, respectively. Moreover, ET(B) -mediated effects in SON neurones were also prevented by a gliotoxin compound, and when changes in [Ca(2+) ](i) were prevented with bath-applied BAPTA-AM or thapsigargin. Conversely, intracellular Ca(2+) chelation in the recorded SON neurones failed to block ET(B) -mediated effects. In summary, our results indicate that ET(B) receptor activation in SON astrocytes induces the mobilisation of [Ca(2+) ](i) , likely resulting in the activation of glutamate and nitric oxide signalling pathways, evoking in turn excitatory and inhibitory SON neuronal responses, respectively. Taken together, our study supports an important role for astrocytes in mediating the actions of ETs on the magnocellular neurosecretory system.

Architecture of the hypothalamo-posthypophyseal complex is controlled by monoamines

The hypothalamo-neurohypophyseal system displays significant plasticity when subjected to physiological stimuli, such as dehydration, parturition, or lactation. This plasticity arises at the neurochemical and electrophysiological levels but also at a structural level. Several studies have demonstrated the role of monoaminergic afferents in controlling neurochemical and electrophysiological plasticity of the supraoptic nucleus (SON) and of the neurohypophysis (NH), but little is known about how the changes in structural plasticity are triggered. We used Tg8 mice, disrupted for the monoamine oxidase A gene, to study monamine involvement in the architecture of the SON and of the NH. SON astrocytes in Tg8 mice displayed an active status, characterized by an increase in S100β expression and a significant decrease in vimentin expression, with no modification in glial fibrillary acidic protein (GFAP) levels. Astrocytes showed a decrease in glutamate dehydrogenase (GDH) levels, whereas glutamine synthetase (GS) levels remained constant, suggesting a reduction in astrocyte glutamate catabolism. Tenascin C and polysialic acid-neural cell adhesion molecule (PSA-NCAM) expressions were also elevated in the SON of Tg8 mice, suggesting an increased capacity for structural remodelling in the SON. In the NH, similar date were obtained with a stability in GFAP expression and an increase in PSA-NCAM immunostaining. These results establish monoamine (serotonin and noradrenaline) involvement in SON and NH structural arrangement. Monoamines therefore appear to be crucial for the coordination of the neurochemical and structural aspects of neuroendocrine plasticity, allowing the hypothalamo-neurohypopyseal system to respond appropriately when stimulated.

Facilitation of glutamate and GABA release by P2X receptor activation in supraoptic neurons from freshly isolated rat brain slices

The supraoptic nuclei (SON), the hypothalamic release site of vasopressin and oxytocin, receive a non-glutamatergic, excitatory input from the caudal medulla that uses noradrenaline and ATP as neurotransmitters. Here, we studied the actions of extracellular ATP on SON neurons in hypothalamic slices isolated from the brains of 16- to 24-day-old rats. Whole-cell current clamp recordings performed 1-6 h after isolation showed that exogenous ATP application increased the frequency of action potentials and induced the depolarization of resting membranes. Voltage clamp recordings showed that ATP increased the frequency of GABAergic or glutamatergic spontaneous synaptic currents without changing their amplitude and evoked inward current (126±13 pA) in about 80% of SON neurons. The application of ATPγS and 2MeSATP mimicked the effects of ATP, but 2MeSADP, 2MeSAMP and αβmeATP had no effect. The P2X7 receptor agonist, BzATP, did not induce an inward current, but it increased intracellular calcium concentration in non-neuronal SON cells in slices. Suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) inhibited ATP-induced currents, whereas pH 6.5 and ivermectin, a specific allosteric modulator of the P2X4 receptor, potentiated ATP-induced currents. The P2Y1-selective antagonist, 2'-deoxy-N⁶-methyladenosine 3',5'-bisphosphate tetrasodium salt (MRS 2179), had no effect on ATP-induced responses. Quantitative real-time PCR showed that P2X2>P2X7>P2X4 purinergic receptor mRNAs were expressed in the SON tissue, but the levels of P2X1, P2X3, P2X5, P2X6, P2Y1, P2Y2 and P2Y12 mRNA were minor. These results show that SON neurons express functional presynaptic and extrasynaptic P2X2 and P2X4 receptors that modulate glutamate and GABA release and control the electrical excitability of SON neurons.

NMDA receptor subunit expression in the supraoptic nucleus of adult rats: dominance of NR2B and NR2D

The supraoptic nucleus (SON) of the hypothalamus contains magnocellular neurosecretory neurons (MNC) which synthesize and release the peptide hormones vasopressin and oxytocin. Glutamate is a prominent excitatory neurotransmitter in the SON and regulates MNC excitability. NMDA receptors (NMDAR), a type of ionotropic glutamate receptor, mediate synaptic plasticity of MNCs and are necessary for characteristic burst firing patterns which serve to maximize hormone release. NMDARs are di- or tri-heteromeric complexes of NR1 and NR2 subunits. Receptor properties depend on NR2 subunit composition and variable splicing of NR1. We investigated the expression profile of NR1 and NR2 subunits in the SON at the mRNA and protein levels plus protein expression of NR1 splice variants in control and salt-loaded adult rats. There was robust mRNA expression of all subunits, with NR2D levels being the highest. At the protein level, NR1, NR2B, and NR2D were robustly expressed, while NR2A was weakly expressed. NR2C protein was not detected with either of the two antibodies tested. All four NR1 splice variant cassettes (N1, C1, C2, C2') were detected in the SON, although NR1 N1 expression was too low for accurate analysis. Three days of salt-loading did not alter mRNA, protein, or splice variant expression of NMDAR subunits in the SON. Robust NR2D protein expression has not been previously shown in MNCs and is uncommon in the adult brain. Although the functional significance of this unusual expression profile is unknown, it may contribute to important physiological characteristics of SON neurons, such as burst firing and resistance to excitotoxicity.

Morphological plasticity of the rat supraoptic nucleus--cellular consequences

The supraoptic nuclei of the hypothalamus display a remarkable anatomical plasticity during lactation, parturition and chronic dehydration, conditions associated with massive neurohypophysial hormone secretion. This structural remodeling is characterized by a pronounced reduction of the astrocytic coverage of oxytocin neurons, resulting in an increase in the number and extent of directly juxtaposed neuronal surfaces. Although the exact role played by such an anatomical remodeling in the physiology of the hypothalamo-neurohypophysial system is still unknown, several findings obtained over the last decade indicate that synaptic and extrasynaptic transmissions are impacted by these structural changes. We review these data and try to extrapolate how such changes at the cellular level might affect the overall activity of the system. One repercussion of the retraction of glial processes is the accumulation of glutamate in the extracellular space. This build-up of glutamate causes an increased activation of pre-synaptic metabotropic glutamate receptors, which are negatively coupled to neurotransmitter release, and a switch in the mode of action of pre-synaptic kainate receptors that control GABA release. Finally, the range of action of substances released from astrocytes and acting on adjacent magnocellular neurons is also affected during the anatomical remodeling. It thus appears that the structural plasticity of the hypothalamic magnocellular nuclei strongly affects neuron-glial interactions and, as a consequence, induces significant changes in synaptic and extrasynaptic transmission.

Glutamatergic synaptic transmission in neuroendocrine cells: Basic principles and mechanisms of plasticity

Glutamate synapses drive the output of neuroendocrine cells in the hypothalamus, but until recently, relatively little was known about the fundamental properties of transmission at these synapses. Here we review recent advances in the understanding of glutamate signals in magnocellular neurosecretory cells (MNCs) in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus that serve as the last step in synaptic integration before neurohormone release. While these synapses exhibit many similarities with other glutamate synapses described throughout the brain, they also exhibit a number of unique properties that are particularly well suited to the physiology of this system and will be discussed here. In addition, a number of recent studies begin to provide insights into new forms of synaptic plasticity that may be common in other brain regions, but in these cells, may serve important adaptive roles.