A subthreshold persistent sodium current mediates bursting in rat subfornical organ neurones

Calcium-Sensing Receptor (CaSR), Its Impact on Inflammation and the Consequences on Cardiovascular Health

The calcium Sensing Receptor (CaSR) is a cell surface receptor belonging to the family of G-protein coupled receptors. CaSR is mainly expressed by parathyroid glands, kidneys, bone, skin, adipose tissue, the gut, the nervous system, and the cardiovascular system. The receptor, as its name implies is involved in sensing calcium fluctuations in the extracellular matrix of cells, thereby having a major impact on the mineral homeostasis in humans. Besides calcium ions, the receptor is also activated by other di- and tri-valent cations, polypeptides, polyamines, antibiotics, calcilytics and calcimimetics, which upon binding induce intracellular signaling pathways. Recent studies have demonstrated that CaSR influences a wide variety of cells and processes that are involved in inflammation, the cardiovascular system, such as vascular calcification, atherosclerosis, myocardial infarction, hypertension, and obesity. Therefore, in this review, the current understanding of the role that CaSR plays in inflammation and its consequences on the cardiovascular system will be highlighted.

Ionic mechanisms underlying tonic and burst firing behavior in subfornical organ neurons: a combined experimental and modeling study

Subfornical organ (SFO) neurons exhibit heterogeneity in current expression and spiking behavior, where the two major spiking phenotypes appear as tonic and burst firing. Insight into the mechanisms behind this heterogeneity is critical for understanding how the SFO, a sensory circumventricular organ, integrates and selectively influences physiological function. To integrate efficient methods for studying this heterogeneity, we built a single-compartment, Hodgkin-Huxley-type model of an SFO neuron that is parameterized by SFO-specific in vitro patch-clamp data. The model accounts for the membrane potential distribution and spike train variability of both tonic and burst firing SFO neurons. Analysis of model dynamics confirms that a persistent Na⁺ and Ca²⁺ currents are required for burst initiation and maintenance and suggests that a slow-activating K⁺ current may be responsible for burst termination in SFO neurons. Additionally, the model suggests that heterogeneity in current expression and subsequent influence on spike afterpotential underlie the behavioral differences between tonic and burst firing SFO neurons. Future use of this model in coordination with single neuron patch-clamp electrophysiology provides a platform for explaining and predicting the response of SFO neurons to various combinations of circulating signals, thus elucidating the mechanisms underlying physiological signal integration within the SFO. NEW & NOTEWORTHY Our understanding of how the subfornical organ (SFO) selectively influences autonomic nervous system function remains incomplete but theoretically results from the electrical responses of SFO neurons to physiologically important signals. We have built a computational model of SFO neurons, derived from and supported by experimental data, which explains how SFO neurons produce different electrical patterns. The model provides an efficient system to theoretically and experimentally explore how changes in the essential features of SFO neurons affect their electrical activity.

Conditional Bistability, a Generic Cellular Mnemonic Mechanism for Robust and Flexible Working Memory Computations

Persistent neural activity, the substrate of working memory, is thought to emerge from synaptic reverberation within recurrent networks. However, reverberation models do not robustly explain the fundamental dynamics of persistent activity, including high-spiking irregularity, large intertrial variability, and state transitions. While cellular bistability may contribute to persistent activity, its rigidity appears incompatible with persistent activity labile characteristics. Here, we unravel in a cellular model a form of spike-mediated conditional bistability that is robust and generic. and provides a rich repertoire of mnemonic computations. Under asynchronous synaptic inputs of the awakened state, conditional bistability generates spiking/bursting episodes, accounting for the irregularity, variability, and state transitions characterizing persistent activity. This mechanism has likely been overlooked because of the subthreshold input it requires, and we predict how to assess it experimentally. Our results suggest a reexamination of the role of intrinsic properties in the collective network dynamics responsible for flexible working memory.SIGNIFICANCE STATEMENT This study unravels a novel form of intrinsic neuronal property: conditional bistability. We show that, thanks to its conditional character, conditional bistability favors the emergence of flexible and robust forms of persistent activity in PFC neural networks, in opposition to previously studied classical forms of absolute bistability. Specifically, we demonstrate for the first time that conditional bistability (1) is a generic biophysical spike-dependent mechanism of layer V pyramidal neurons in the PFC and that (2) it accounts for essential neurodynamical features for the organization and flexibility of PFC persistent activity (the large irregularity and intertrial variability of the discharge and its organization under discrete stable states), which remain unexplained in a robust fashion by current models.

Firing and intrinsic properties of antennal lobe neurons in the Noctuid moth Agrotis ipsilon

The antennal lobe (AL) of the Noctuid moth Agrotis ipsilon has emerged as an excellent model for studying olfactory processing and its plasticity in the central nervous system. Odor-evoked responses of AL neurons and input-to-output transformations involved in pheromone processing are well characterized in this species. However, the intrinsic electrical properties responsible of the firing of AL neurons are poorly known. To this end, patch-clamp recordings in current- and voltage-clamp mode from neurons located in the two main clusters of cell bodies in the ALs were combined with intracellular staining on A. ipsilon males. Staining indicated that the lateral cluster (LC) is composed of 85% of local neurons (LNs) and 15% of projection neurons (PNs). The medial cluster (MC) contains only PNs. Action potentials were readily recorded from the soma in LNs and PNs located in the LC but not from PNs in the MC where recordings showed small or no action potentials. In the LC, the spontaneous activity of about 20% of the LNs presented irregular bursts while being more regular in PNs. We also identified a small population of LNs lacking voltage-gated Na(+) currents and generating spikelets. We focused on the firing properties of LNs since in about 60% of LNs, but not in PNs, action potentials were followed by depolarizing afterpotentials (DAPs). These DAPs could generate a second action potential, so that the activity was composed of action potential doublets. DAPs depended on voltage, Ca(2+)-channels and possibly on Ca(2+)-activated non-specific cationic channels. During steady state current injection, DAPs occurred after each action potential and did not require high-frequency firing. The amplitude of DAPs increased when the interspike interval was small, typically within bursts, likely arising from a Ca(2+) build up. DAPs were more often found in bursting than in non-bursting LNs but do not support bursting activity. DAPs and spike doublets also occurred during odor-evoked activity suggesting that they can mediate olfactory integration in the AL.

Cellular actions of nesfatin-1 in the subfornical organ

Nesfatin-1, a centrally acting anorexigenic peptide, is produced in several brain areas involved in metabolic processes and has been implicated in the control of ingestive behaviours and cardiovascular functions. The present study aimed to determine whether the subfornical organ (SFO), a central nervous system (CNS) site that has been extensively implicated in the regulation of appetite and thirst, may represent a potential site for central actions of nesfatin-1. We first used the reverse transcriptase-polymerase chain reaction and were able to confirm the presence of mRNA for the nucleobindin-2 gene in the SFO. We then used whole-cell patch clamp recordings to investigate the influence of nesfatin-1 on the membrane potential of dissociated SFO neurones. A total of 80.3% (49 of 61) of neurones tested showed a response to nesfatin-1 (100 nm, 10 nm and 1 nm). Of these, 47.5% depolarised, with a mean depolarisation of 8.2 ± 0.9 mV (n = 29) and 32.8% hyperpolarised with a mean hyperpolarisation of -8.9 ± 1.2 mV (n = 20). Peak magnitudes were seen at a concentration of 1 nm nesfatin-1, whereas no effect was observed at 100 pm nesftain-1 (n = 3). Furthermore, voltage clamp ramp and step protocols revealed a nesfatin-1 induced activation of the delayed rectifier potassium conductance, IK . Pharmacological blockade of this conductance greatly reduced the magnitude and occurrence of the observed hyperpolarisations. The present study thus demonstrates that nesfatin-1 has the ability to influence the membrane potential of SFO neurones, and thus identifies the SFO as a potential site at which nesfatin-1 may act to regulate ingestive behaviour and cardiovascular control.

ENaC-expressing neurons in the sensory circumventricular organs become c-Fos activated following systemic sodium changes

The sensory circumventricular organs (CVOs) are specialized collections of neurons and glia that lie in the midline of the third and fourth ventricles of the brain, lack a blood-brain barrier, and function as chemosensors, sampling both the cerebrospinal fluid and plasma. These structures, which include the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), are sensitive to changes in sodium concentration but the cellular mechanisms involved remain unknown. Epithelial sodium channel (ENaC)-expressing neurons of the CVOs may be involved in this process. Here we demonstrate with immunohistochemical and in situ hybridization methods that ENaC-expressing neurons are densely concentrated in the sensory CVOs. These neurons become c-Fos activated, a marker for neuronal activity, after various manipulations of peripheral levels of sodium including systemic injections with hypertonic saline, dietary sodium deprivation, and sodium repletion after prolonged sodium deprivation. The increases seen c-Fos activity in the CVOs were correlated with parallel increases in plasma sodium levels. Since ENaCs play a central role in sodium reabsorption in kidney and other epithelia, we present a hypothesis here suggesting that these channels may also serve a related function in the CVOs. ENaCs could be a significant factor in modulating CVO neuronal activity by controlling the magnitude of sodium permeability in neurons. Hence, some of the same circulating hormones controlling ENaC expression in kidney, such as angiotensin II and atrial natriuretic peptide, may coordinate ENaC expression in sensory CVO neurons and could potentially orchestrate sodium appetite, osmoregulation, and vasomotor sympathetic drive.

Roles of the calcium sensing receptor in the central nervous system

The calcium sensing receptor (CaSR) is expressed by subpopulations of neuronal and glial cells throughout the brain and is activated by extracellular calcium [Formula: see text] . During development, the CaSR regulates neuronal cell growth and migration as well as oligodendroglial maturation and function. Emerging evidence suggests that in nerve terminals, CaSR is implicated in synaptic plasticity and neurotransmission. In this review, we analyze the roles attributed to CaSR in regulating diverse brain functions, including central regulation of body fluid composition and blood pressure. We also discuss the potential relevance of Ca(2+)-sensing in brain by other family C G protein-coupled receptors. Finally, evidence that the CaSR contributes to the pathogenesis of various brain disorders raises the possibility that pharmacological modulators of the CaSR may have therapeutic benefit.

Glial regulation of neuronal function: from synapse to systems physiology

Classically, glia have been regarded as non-excitable cells that provide nourishment and physical scaffolding for neurones. However, it is now generally accepted that glia are active participants in brain function that can modulate neuronal communication via several mechanisms. Investigations of anatomical plasticity in the magnocellular neuroendocrine system of the hypothalamic paraventricular and supraoptic nuclei led the way in the development of much of our understanding of glial regulation of neuronal activity. In this review, we provide an overview of glial regulation of magnocellular neurone activity from a historical perspective of the development of our knowledge of the morphological changes that are evident in the paraventricular and supraoptic nuclei. We also focus on recent data from the authors' laboratories presented at the 9th World Congress on Neurohypophysial Hormones that have contributed to our understanding of the multiple mechanisms by which glia modulate the activity of neurones, including: gliotransmitter modulation of synaptic transmission; trans-synaptic modulation by glial neurotransmitter transporter regulation of neurotransmitter spillover; and glial neurotransmitter transporter modulation of excitability by regulation of ambient neurotransmitter levels and their action on extrasynaptic receptors. The magnocellular neuroendocrine system secretes oxytocin and vasopressin from the posterior pituitary gland to control birth, lactation and body fluid balance, and we finally speculate as to whether glial regulation of individual magnocellular neurones might co-ordinate population activity to respond appropriately to altered physiological circumstances.

The calcium-sensing receptor: a molecular perspective

Compelling evidence of a cell surface receptor sensitive to extracellular calcium was observed as early as the 1980s and was finally realized in 1993 when the calcium-sensing receptor (CaR) was cloned from bovine parathyroid tissue. Initial studies relating to the CaR focused on its key role in extracellular calcium homeostasis, but as the amount of information about the receptor grew it became evident that it was involved in many biological processes unrelated to calcium homeostasis. The CaR responds to a diverse array of stimuli extending well beyond that merely of calcium, and these stimuli can lead to the initiation of a wide variety of intracellular signaling pathways that in turn are able to regulate a diverse range of biological processes. It has been through the examination of the molecular characteristics of the CaR that we now have an understanding of how this single receptor is able to convert extracellular messages into specific cellular responses. Recent CaR-related reviews have focused on specific aspects of the receptor, generally in the context of the CaR's role in physiology and pathophysiology. This review will provide a comprehensive exploration of the different aspects of the receptor, including its structure, stimuli, signalling, interacting protein partners, and tissue expression patterns, and will relate their impact on the functionality of the CaR from a molecular perspective.

Diverse roles of extracellular calcium-sensing receptor in the central nervous system

The G-protein-coupled calcium-sensing receptor (CaSR), upon activation by Ca(2+) or other physiologically relevant polycationic molecules, performs diverse functions in the brain. The CaSR is widely expressed in the central nervous system (CNS) and is characterized by a robust increase in its expression during postnatal brain development over adult levels throughout the CNS. Developmental increases in CaSR levels in brain correlate with myelinogenesis. Indeed, neural stem cells differentiating to the oligodendrocyte lineage exhibit the highest CaSR expression compared with those differentiating to astrocytic or neuronal lineages. In adult CNS, CaSR has broad relevance in maintaining local ionic homeostasis. CaSR shares an evolutionary relationship with the metabotropic glutamate receptor and forms heteromeric complexes with the type B-aminobutyric acid receptor subunits that affects its cell surface expression, activation, signaling, and functions. In normal physiology as well as in pathologic conditions, CaSR is activated by signals arising from mineral ions, amino acids, polyamines, glutathione, and amyloid-beta in conjunction with Ca(2+) and other divalent cationic ligands. CaSR activation regulates membrane excitability of neurons and glia and affects myelination, olfactory and gustatory signal integration, axonal and dendritic growth, and gonadotrophin-releasing hormonal-neuronal migration. Insofar as the CaSR is a clinically important therapeutic target for parathyroid disorders, development of its agonists or antagonists as therapeutics for CNS disorder could be a major breakthrough.

Astrocyte-mediated distributed plasticity at hypothalamic glutamate synapses

Afferent activity can induce fast, feed-forward changes in synaptic efficacy that are synapse specific. Using combined electrophysiology, caged molecule photolysis, and Ca(2+) imaging, we describe a plasticity in which the recruitment of astrocytes in response to afferent activity causes a fast and feed-forward, yet distributed increase in the amplitude of quantal synaptic currents at multiple glutamate synapses on magnocellular neurosecretory cells in the hypothalamic paraventricular nucleus. The plasticity is largely multiplicative, consistent with a proportional increase or "scaling" in the strength of all synapses on the neuron. This effect requires a metabotropic glutamate receptor-mediated rise in Ca(2+) in the astrocyte processes surrounding the neuron and the release of the gliotransmitter ATP, which acts on postsynaptic purinergic receptors. These data provide evidence for a form of distributed synaptic plasticity that is feed-forward, expressed quickly, and mediated by the synaptic activation of neighboring astrocytes.

Extracellular calcium as an integrator of tissue function

The past several decades of research into calcium signaling have focused on intracellular calcium (Ca(i)(2+)), revealing both exquisite spatial and dynamic control of this potent second messenger. Our understanding of Ca(i)(2+) signaling has benefited from the evolution of cell culture methods, development of high affinity fluorescent calcium indicators (both membrane-permeant small molecules and genetically encoded proteins), and high-resolution fluorescence microscopy. As our understanding of single cell calcium dynamics has increased, translational efforts have attempted to push calcium signaling studies back into tissues, organs and whole animals. Emerging results from these more complicated, diffusion-limited systems have begun to define a role for extracellular calcium (Ca(o)(2+)) as an agonist, spurred by the cloning and characterization of a G protein-coupled receptor activated by Ca(o)(2+) (the calcium sensing receptor, CaR). Here, we review the current state-of-the art for measurement of Ca(o)(2+) fluctuations, and the evidence that fluctuations in Ca(o)(2+) can act as primary signals regulating cell function. Current results suggest that Ca(o)(2+) in bone and epidermis may act as a chemotactic homing signal, targeting cells to the appropriate tissue locations prior to initiation of the differentiation program. Ca(i)(2+) signaling-mediated Ca(o)(2+) fluctuations in interstitial spaces may integrate cell signaling responses in multicellular networks through activation of CaR. Appreciation of the importance of Ca(o)(2+) fluctuations in coordinating cell function will likely spur identification of additional, niche-specific Ca(2+) sensors, and provide unique insights into the regulation of multicellular signaling networks.

Subthreshold oscillations of membrane potential of rat subfornical organ neurons

Previous work has demonstrated that subfornical organ neurons exhibit spontaneous action potentials interspersed with periods of membrane potential oscillation. We used whole cell patch clamp recording to investigate properties of these oscillations. The amplitude, but not the frequency, of the oscillations exhibited voltage dependence. The oscillations were unaffected by application of blockers for gamma-amino-n-butyric acid type A (GABA(A)) and glutamate receptors, nor were they affected by application of Cd2+ to block Ca2+ channels, or Cs+ to block hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels. The oscillations, however, were abolished by the application of tetrodotoxin, indicating a role for voltage-gated Na+ channels. Voltage clamp experiments demonstrated that the activation of persistent Na+ current, but not transient Na+ current, matched with the voltage dependence of activation of the oscillation.

Integration of asynchronously released quanta prolongs the postsynaptic spike window

Classically, the release of glutamate in response to a presynaptic action potential causes a brief increase in postsynaptic excitability. Previous reports indicate that at some central synapses, a single action potential can elicit multiple, asynchronous release events. This raises the possibility that the temporal dynamics of neurotransmitter release may determine the duration of altered postsynaptic excitability. In response to physiological challenges, the magnocellular neurosecretory cells (MNCs) in the paraventricular nucleus of the hypothalamus (PVN) exhibit robust and prolonged increases in neuronal activity. Although the postsynaptic conductances that may facilitate this form of activity have been investigated thoroughly, the role of presynaptic release has been largely overlooked. Because the specific patterns of activity generated by MNCs require the activation of excitatory synaptic inputs, we sought to characterize the release dynamics at these synapses and determine whether they contribute to prolonged excitability in these cells. We obtained whole-cell recordings from MNCs in brain slices of postnatal day 21-44 rats. Stimulation of glutamatergic inputs elicited large and prolonged postsynaptic events that resulted from the summation of multiple, asynchronously released quanta. Asynchronous release was selectively inhibited by the slow calcium buffer EGTA-AM and potentiated by brief high-frequency stimulus trains. These trains caused a prolonged increase in postsynaptic spike activity that could also be eliminated by EGTA-AM. Our results demonstrate that glutamatergic terminals in PVN exhibit asynchronous release, which is important in generating large postsynaptic depolarizations and prolonged spiking in response to brief, high-frequency bursts of presynaptic activity.

Glial Nax Channels Control Lactate Signaling to Neurons for Brain [Na+] Sensing

Sodium (Na) homeostasis is crucial for life, and Na levels in body fluids are constantly monitored in the brain. The subfornical organ (SFO) is the center of the sensing responsible for the control of salt-intake behavior, where Na(x) channels are expressed in specific glial cells as the Na-level sensor. Here, we show direct interaction between Na(x) channels and alpha subunits of Na(+)/K(+)-ATPase, which brings about Na-dependent activation of the metabolic state of the glial cells. The metabolic enhancement leading to extensive lactate production was observed in the SFO of wild-type mice, but not of the Na(x)-knockout mice. Furthermore, lactate, as well as Na, stimulated the activity of GABAergic neurons in the SFO. These results suggest that the information on a physiological increase of the Na level in body fluids sensed by Na(x) in glial cells is transmitted to neurons by lactate as a mediator to regulate neural activities of the SFO.

Effects of adrenalectomy on the excitability of neurosecretory parvocellular neurones in the hypothalamic paraventricular nucleus

Glucocorticoids are well known to inhibit the release of hypophysiotrophic hormones from neurones originating in the paraventricular nucleus (PVN), but the cellular mechanisms of the inhibition are not well understood. Here, we examined the effects of adrenalectomy (ADX) on the spontaneous firing activity in the neurosecretory parvocellular PVN neurones of rat brain slices. The neurones were identified by injecting a retrograde dye into the pituitary stalk and classified according to their electrophysiological properties. The intranuclear distribution, electrophysiological properties, and hypophysiotrophic hormone phenotype of the labelled type II PVN neurones were similar to neurosecretory parvocellular PVN neurones. In the neurones of sham-operated rats under the cell-attached recording mode, we observed three spontaneous activity patterns: tonic regular (24%), tonic irregular (36%), and silent (40%). Noradrenaline (100 microM) induced an excitatory or an inhibitory effect on the spontaneous activity. Noradrenergic excitation was blocked by prazosin (2 microM, alpha(1)-adrenoceptor antagonist), and mimicked by phenylephrine (100 microM, alpha(1)-adrenoceptor agonist), whereas noradrenergic inhibition was blocked by yohimbine (2 microM, alpha(2)-adrenoceptor antagonist) and mimicked by clonidine (50 microM, alpha(2)-adrenoceptor agonist). In the neurones of ADX rats, we found burst firing in 35% of neurones tested and an increase in the frequency of spontaneous firing. The burst firing was not observed in the neurones of the sham-operated rats. ADX caused a 1.7-fold increase in the proportion of neurones showing the noradrenergic excitation. Supplementation of the ADX rats with corticosterone (10 mg pellet) reversed the ADX-induced burst firing, and the potentiation of noradrenergic excitation. In summary, our results show that removal of corticosterone by ADX can elevate the neuronal excitability by increasing the spontaneous firing rate and by potentiating the alpha(1)-adrenoceptor-mediated noradrenergic excitation, and it can facilitate hormone release by inducing burst firing. Our results provide new insight to the cellular mechanisms of the feedback inhibition by glucocorticoids in the neurosecretory parvocellular neurones of the PVN.

The subfornical organ: a central target for circulating feeding signals

The mechanisms through which circulating ghrelin relays hunger signals to the CNS are not yet fully understood. In this study, we have examined the potential role of the subfornical organ (SFO), a circumventricular structure that lacks the normal blood-brain barrier, as a CNS site in which ghrelin acts to influence the hypothalamic centers controlling food intake. We report that ghrelin increased intracellular calcium concentrations in 28% (12 of 43) of dissociated SFO neurons and that the SFO expresses mRNA for the growth hormone secretagogue receptor. Whole-cell patch recordings from SFO neurons demonstrated that in 29% (9 of 31) of neurons tested ghrelin induced a mean depolarization of 7.4 +/- 0.69 mV, accompanied by an increase in action potential frequency. Voltage-clamp recordings revealed that ghrelin activates a putative nonselective cationic conductance. Previous reports that the satiety signal amylin exerts similar excitatory effects on SFO neurons led us to examine whether these two peptides influence different subpopulations of SFO neurons. Concentration-dependent depolarizing effects of amylin were observed in 59% (28 of 47) of SFO neurons (mean depolarization, 8.32 +/- 0.60 mV). In contrast to ghrelin, voltage-clamp recordings suggest that amylin influences a voltage-dependent current activated at depolarized potentials. We tested single SFO neurons with both peptides and identified cells responsive only to ghrelin (n = 9) and only to amylin (n = 7) but no cells that responded to both peptides. These data support a role for the SFO as a center at which ghrelin and amylin may influence separate subpopulations of neurons to influence the hypothalamic regulation of feeding.

Persistent Sodium Currents in Mesencephalic V Neurons Participate in Burst Generation and Control of Membrane Excitability

The functional and biophysical properties of a persistent sodium current ( INaP) previously proposed to participate in the generation of subthreshold oscillations and burst discharge in mesencephalic trigeminal sensory neurons (Mes V) were investigated in brain stem slices (rats, p7–p12) using whole cell patch-clamp methods. INaPactivated around −76 mV and peaked at −48 mV, with V1/2of −58.7 mV. Ramp voltage-clamp protocols showed that INaPundergoes time- as well as voltage-dependent inactivation and recovery from inactivation in the range of several seconds (τonset= 2.04 s, τrecov= 2.21 s). Riluzole (≤5 μM) substantially reduced INaP, membrane resonance, postinhibitory rebound (PIR), and subthreshold oscillations, and completely blocked bursting, but produced modest effects on the fast transient Na+current ( INaT). Before complete cessation, burst cycle duration was increased substantially, while modest and inconsistent changes in burst duration were observed. The properties of the INaTwere obtained and revealed that the amplitude and voltage dependence of the resulting “window current” were not consistent with those of the observed INaPrecorded in the same neurons. This suggests an additional mechanism for the origin of INaP. A neuronal model was constructed using Hodgkin-Huxley parameters obtained experimentally for Na+and K+currents that simulated the experimentally observed membrane resonance, subthreshold oscillations, bursting, and PIR. Alterations in the model gNaPparameters indicate that INaPis critical for control of subthreshold and suprathreshold Mes V neuron membrane excitability and burst generation.

INaP underlies intrinsic spiking and rhythm generation in networks of cultured rat spinal cord neurons

We have shown previously that rhythm generation in disinhibited spinal networks is based on intrinsic spiking, network recruitment and a network refractory period following the bursts. This refractory period is based mainly on electrogenic Na/K pump activity. In the present work, we have investigated the role of the persistent sodium current (INaP) in the generation of bursting using patch-clamp and multielectrode array recordings. We detected INaP exclusively in the intrinsic spiking cells. The blockade of INaP by riluzole suppressed the bursting by silencing the intrinsic spiking cells and suppressing network recruitment. The blockade of the persistent sodium current produced a hyperpolarization of the membrane potential of the intrinsic spiking cells, but had no effect on non-spiking cells. We also investigated the involvement of the hyperpolarization-activated cationic current (I(h)) in the rhythmic activity. The bath application of ZD7288, a specific I(h) antagonist, slowed down the rate of the bursts by increasing the interburst intervals. I(h) was present in approximately 70% of the cells, both in the intrinsic spiking cells as well as in the non-spiking cells. We also found both kinds of cells in which I(h) was not detected. In summary, in disinhibited spinal cord cultures, a persistent sodium current underlies intrinsic spiking, which, via recurrent excitation, generates the bursting activity. The hyperpolarization-activated cationic current contributes to intrinsic spiking and modulates the burst frequency.

Calcium-sensing receptor in the brain

Following its cloning through an homology-based method from a rat striatal library, the calcium-sensing receptor (CaR) has been localized in the brains of adult and developing rats by immunocytochemistry and in situ hybridization with CaR-specific antibodies and cDNA probes, respectively. The receptor resides in numerous regions of the brain at widely varying levels. The highest levels are present within the subfornical organ (SFO) and the olfactory bulbs. Substantial levels of expression are also evident within the hippocampus, striatum, cingulate cortex, cerebellum, ependymal zones of the cerebral ventricles, and perivascular nerves around cerebral arteries. There are abundant levels of CaR expression within the SFO, an important hypothalamic thirst center, suggesting that it participates in the central control of systemic fluid and electrolyte balance. Therefore, while mineral ion homeostasis is not often considered to have central regulatory elements (i.e. in the brain), there are perhaps more complex relationships than recognized previously among the system governing mineral ion homeostasis and other homeostatic systems known to exhibit prominent neuroendocrine elements (i.e. water homeostasis). Furthermore, the expression of the CaR in all three types of glial cells indicates potential roles in the maintenance of local ionic homeostasis as well as in disease processes such as glioma.

GABAB receptor modulation of rapid inhibitory and excitatory neurotransmission from subfornical organ and other afferents to median preoptic nucleus neurons

Cardiovascular and behavioral responses to circulating angiotensin require intact connectivity along the upper lamina terminalis joining the subfornical organ (SFO) with the median preoptic nucleus (MnPO). Whole cell patch-clamp recordings in sagittal rat brain slice preparations revealed that 28/40 MnPO neurons responded to electrical stimulation of SFO efferents with bicuculline-sensitive GABA(A) receptor-mediated inhibition and glutamate-mediated postsynaptic excitation involving AMPA and N-methyl-d-aspartate (NMDA) receptor subtypes, blockable with 2,3-dioxo-6nitro-1, 2,3,4-tetrahydrobenzo [f] quinoxaline-7-sulfoamide disodium (NBQX) and d-2-amino-4-phosphonovaleric acid (d-APV), respectively. Bath applications of baclofen induced a concentration-dependent (0.3-10 microM) reduction in these SFO-evoked postsynaptic currents, attenuation of SFO-evoked paired-pulse depression, and reduction in frequency (but not amplitude) of miniature postsynaptic currents, consistent with an action at presynaptic GABA(B) receptors. Baclofen's effects on miniature currents lacked sensitivity to barium, omega-conotoxin GVIA, and cadmium. Acting at postsynaptic GABA(B) receptors, baclofen hyperpolarized a majority of MnPO neurons by increasing a G protein-coupled inwardly rectifying potassium conductance and suppressing an N-type high-voltage-activated calcium conductance. The latter contributed to reduction in action potential afterhyperpolarization and enhanced cell firing and spike frequency adaptation when tested with a depolarizing stimulus. All baclofen-induced effects were blockable with CGP52432. CGP52432 alone had no significant effect on SFO-evoked postsynaptic current amplitudes or paired-pulse ratios, but did induce an increase in miniature inhibitory postsynaptic current (mIPSC) frequency in 2/4 cells tested, indicating that ambient levels of GABA could activate presynaptic GABA(B) receptors on undefined inputs. These observations indicate that MnPO neurons receive both a GABAergic and glutamatergic innervation from SFO. Both forms of rapid neurotransmission are subject to modulation via pre- and postsynaptic GABA(B) receptors.

Ionic mechanisms of burst firing in dissociated Purkinje neurons

Cerebellar Purkinje neurons have intrinsic membrane properties that favor burst firing, seen not only during complex spikes elicited by climbing fiber input but also with direct electrical stimulation of cell bodies. We examined the ionic conductances that underlie all-or-none burst firing elicited in acutely dissociated mouse Purkinje neurons by short depolarizing current injections. Blocking voltage-dependent calcium entry by cadmium or replacement of external calcium by magnesium enhanced burst firing, but it was blocked by cobalt replacement of calcium, probably reflecting block of sodium channels. In voltage-clamp experiments, we used the burst waveform of each cell as a voltage command and used ionic substitutions and pharmacological manipulations to isolate tetrodotoxin (TTX)-sensitive sodium current, P-type and T-type calcium current, hyperpolarization-activated cation current (Ih), voltage-activated potassium current, large-conductance calcium-activated potassium current, and small-conductance calcium-activated potassium (SK) current. Measured near the middle of the first interspike interval, TTX-sensitive sodium current carried the largest inward current, and T-type calcium current was also substantial. Current through P-type channels was large immediately after a spike but decayed rapidly. These inward currents were opposed by substantial components of voltage-dependent and calcium-dependent potassium current. Termination of the burst is caused partly by decay of sodium current, together with a progressive buildup of SK current after the first interspike interval. Although burst firing depends on the net balance between multiple large currents flowing after a spike, it is surprisingly robust, probably reflecting complex interactions between the exact voltage waveform and voltage and calcium dependence of the various currents.

Ionic mechanisms of burst firing in dissociated Purkinje neurons

Cerebellar Purkinje neurons have intrinsic membrane properties that favor burst firing, seen not only during complex spikes elicited by climbing fiber input but also with direct electrical stimulation of cell bodies. We examined the ionic conductances that underlie all-or-none burst firing elicited in acutely dissociated mouse Purkinje neurons by short depolarizing current injections. Blocking voltage-dependent calcium entry by cadmium or replacement of external calcium by magnesium enhanced burst firing, but it was blocked by cobalt replacement of calcium, probably reflecting block of sodium channels. In voltage-clamp experiments, we used the burst waveform of each cell as a voltage command and used ionic substitutions and pharmacological manipulations to isolate tetrodotoxin (TTX)-sensitive sodium current, P-type and T-type calcium current, hyperpolarization-activated cation current (Ih), voltage-activated potassium current, large-conductance calcium-activated potassium current, and small-conductance calcium-activated potassium (SK) current. Measured near the middle of the first interspike interval, TTX-sensitive sodium current carried the largest inward current, and T-type calcium current was also substantial. Current through P-type channels was large immediately after a spike but decayed rapidly. These inward currents were opposed by substantial components of voltage-dependent and calcium-dependent potassium current. Termination of the burst is caused partly by decay of sodium current, together with a progressive buildup of SK current after the first interspike interval. Although burst firing depends on the net balance between multiple large currents flowing after a spike, it is surprisingly robust, probably reflecting complex interactions between the exact voltage waveform and voltage and calcium dependence of the various currents.

Persistent Na+ current modifies burst discharge by regulating conditional backpropagation of dendritic spikes

The estimation and detection of stimuli by sensory neurons is affected by factors that govern a transition from tonic to burst mode and the frequency characteristics of burst output. Pyramidal cells in the electrosensory lobe of weakly electric fish generate spike bursts for the purpose of stimulus detection. Spike bursts are generated during repetitive discharge when a frequency-dependent broadening of dendritic spikes increases current flow from dendrite to soma to potentiate a somatic depolarizing afterpotential (DAP). The DAP eventually triggers a somatic spike doublet with an interspike interval that falls inside the dendritic refractory period, blocking spike backpropagiation and the DAP. Repetition of this process gives rise to a rhythmic dendritic spike failure, termed conditional backpropagation, that converts cell output from tonic to burst discharge. Through in vitro recordings and compartmental modeling we show that burst frequency is regulated by the rate of DAP potentiation during a burst, which determines the time required to discharge the spike doublet that blocks backpropagation. DAP potentiation is magnified through a positive feedback process when an increase in dendritic spike duration activates persistent sodium current (I(NaP)). I(NaP) further promotes a slow depolarization that induces a shift from tonic to burst discharge over time. The results are consistent with a dynamical systems analysis that shows that the threshold separating tonic and burst discharge can be represented as a saddle-node bifurcation. The interaction between dendritic K(+) current and I(NaP) provides a physiological explanation for a variable time scale of bursting dynamics characteristic of such a bifurcation.

Flufenamic acid blocks depolarizing afterpotentials and phasic firing in rat supraoptic neurones

Depolarizing afterpotentials (DAPs) that follow action potentials in magnocellular neurosecretory cells (MNCs) are thought to underlie the generation of phasic firing, a pattern that optimizes vasopressin release from the neurohypophysis. Previous work has suggested that the DAP may result from the Ca(2+)-dependent reduction of a resting K(+) conductance. Here we examined the effects of flufenamic acid (FFA), a blocker of Ca(2+)-dependent non-selective cation (CAN) channels, on DAPs and phasic firing using intracellular recordings from supraoptic MNCs in superfused explants of rat hypothalamus. Application of FFA, but not solvent (0.1 % DMSO), reversibly inhibited (IC(50) = 13.8 microM; R = 0.97) DAPs and phasic firing with a similar time course, but had no significant effects (P > 0.05) on membrane potential, spike threshold and input resistance, nor on the frequency and amplitude of spontaneous synaptic potentials. Moreover, FFA did not affect (P > 0.05) the amplitude, duration, undershoot, or frequency-dependent broadening of action potentials elicited during the spike trains used to evoke DAPs. These findings suggest that FFA inhibits the DAP by directly blocking the channels responsible for its production, rather than by interfering with Ca(2+) influx. They also support a role for DAPs in the generation of phasic firing in MNCs. Finally, the absence of a depolarization and increased membrane resistance upon application of FFA suggests that the DAP in MNCs may not be due to the inhibition of resting K(+) current, but to the activation of CAN channels.

Subfornical organ disconnection and Fos-like immunoreactivity in hypothalamic nuclei after intragastric hypertonic saline

The subfornical organ (SFO) may act as a sodium- or osmoreceptor that drives hypothalamic and other nuclei to secrete vasopressin and to elicit drinking. However, in response to mild doses of hypertonic saline, Fos-like immunoreactivity (Fos-ir) is absent in the SFO whereas it is well expressed in the hypothalamic supraoptic (SON) and paraventricular (PVN) nuclei. This suggests that the hypothalamus may be activated in advance of the SFO. In this study, the fibers connecting the SFO and hypothalamus were disconnected by a wire knife cut so that Fos-ir could be examined in both the SFO and hypothalamus after an intragastric (ig) load of 0.5% of body weight of 0.6 M NaCl. Compared with Fos-ir in isotonic-loaded rats, Fos-ir after the hypertonic load was not significantly elevated in the SFO or median preoptic nucleus in sham-cut or knife-cut rats and was only slightly elevated in the OVLT in sham-cut rats. However, the hypertonic load in sham-cut rats greatly elevated Fos-ir in the SON and in the entire PVN, but this expression was reduced significantly by 30-50% in knife-cut rats. Thus, the connectivity between SFO and the hypothalamus is critical for the full expression of Fos-ir in the hypothalamus during moderate ig hypertonic saline loading even when the SFO itself does not yet express Fos-ir.

Modeling the excitability of mammalian nerve fibers: influence of afterpotentials on the recovery cycle

Human nerve fibers exhibit a distinct pattern of threshold fluctuation following a single action potential known as the recovery cycle. We developed geometrically and electrically accurate models of mammalian motor nerve fibers to gain insight into the biophysical mechanisms that underlie the changes in axonal excitability and regulate the recovery cycle. The models developed in this study incorporated a double cable structure, with explicit representation of the nodes of Ranvier, paranodal, and internodal sections of the axon as well as a finite impedance myelin sheath. These models were able to reproduce a wide range of experimental data on the excitation properties of mammalian myelinated nerve fibers. The combination of an accurate representation of the ion channels at the node (based on experimental studies of human, cat, and rat) and matching the geometry of the paranode, internode, and myelin to measured morphology (necessitating the double cable representation) were needed to match the model behavior to the experimental data. Following an action potential, the models generated both depolarizing (DAP) and hyperpolarizing (AHP) afterpotentials. The model results support the hypothesis that both active (persistent Na(+) channel activation) and passive (discharging of the internodal axolemma through the paranodal seal) mechanisms contributed to the DAP, while the AHP was generated solely through active (slow K(+) channel activation) mechanisms. The recovery cycle of the fiber was dependent on the DAP and AHP, as well as the time constant of activation and inactivation of the fast Na(+) conductance. We propose that experimentally documented differences in the action potential shape, strength-duration relationship, and the recovery cycle of motor and sensory nerve fibers can be attributed to kinetic differences in their nodal Na(+) conductances.

Selective potentiation of N-type calcium channels by angiotensin II in rat subfornical organ neurones

1. Here we have characterized Ca(2+) currents in rat subfornical organ neurones, and their modulation by angiotensin II. Currents were of the high voltage-activated (HNA) subtype, as the threshold for activation was near -30 mV (mid-point potential (V(50)) of activation -14 mV). Using Ba(2+) as the charge carrier, little inactivation was observed, and it occurred only at depolarized potentials (V(50) of inactivation -12 mV). More inactivation was observed using Ca(2+) as the charge carrier, indicating that Ca(2+)-dependent inactivation plays a role in regulating Ca(2+) channel function in subfornical organ (SFO) neurones. 2. The net Ba(2+) current could be blocked by Cd(2+) (EC(50) 1.6 microM), confirming that currents are of the HVA variety. By using selective antagonists, we identified the presence of both L- and N-type channels; 20 microM nifedipine blocked 22 +/- 1 % of the current, while omega-conotoxin GVIA blocked 65 +/- 7 %, indicating that these currents make up the net current through Ca(2+) channels. 3. Angiotensin II potentiated the inward current throughout the range of activation. Using depolarizing voltage ramps, 1 nM angiotensin potentiated the peak current by 14 +/- 5 %. We then used selective blockade of the HVA component currents; 20 microM nifedipine failed to prevent the potentiation by angiotensin II (12 +/- 5 %), while blocking N-type channels with omega-conotoxin GVIA blocked the facilitation by ANG (2.3 +/- 2 %). Losartan (1 microM) prevented the actions of ANG on the inward current (1.6 +/- 1 %), indicating that the selective effects of ANG on N-type channels in SFO neurones are mediated by AT(1) receptors.

Peptidergic excitation of supraoptic nucleus neurons: involvement of stretch-inactivated cation channels

Although the primary stimulus regulating vasopressin (VP) release is a change in systemic osmolality, other physiological parameters are known to affect VP secretion or modulate the osmotic control over its release. Neuropeptides feature prominently in afferents underlying the central regulation of the VP-releasing magnocellular neurosecretory cells (MNCs). Although little is yet known of the circumstances under which peptides are released onto MNCs, previous studies have shown that a common response profile to exogenous peptide application is a slow excitation that seems to result from the activation of a nonselective cation conductance. In this paper we review the basis for the excitatory effects of angiotensin II, cholecystokinin, and neurotensin in MNCs acutely isolated from the supraoptic nucleus of adult rats. Saturating concentrations of these three peptides evoked nonadditive increases in macroscopic cation conductance. During single-channel recordings Ang II, CCK, and NT caused kinetically identical increases in the probability of opening of 35-pS nonselective cation channels. Patches containing only one channel further revealed that the activity of single channels could be regulated by separate applications of all three peptides. Peptide-stimulated channels were also found to be inactivated by increases in membrane stretch and to be blocked by low concentrations of gadolinium (Gd(3+)). It is concluded that many excitatory peptides depolarize MNCs by stimulating the stretch-inactivated cation channels underlying osmoreception. Convergent regulation of these channels provides a potentially powerful mechanism for integrating signals derived from the various afferents involved in the regulation of MNCs.

Spontaneously active GABAergic interneurons in the subfornical organ of rat slice preparations

Inhibitory postsynaptic currents (IPSCs) were recorded from subfornical organ (SFO) neurons in slice preparations of rats, using whole-cell voltage clamp techniques. Some SFO neurons showed bimodal distributions in amplitude with the large and small IPSCs. The large IPSCs vanished in the tetrodotoxin perfusion medium, but the small did not. Both sizes of the IPSCs were completely abolished by application of bicuculline and picrotoxin. Further subpopulation of SFO neurons with the bimodal distributions showed intermittent bursts of the large IPSCs. Immunohistochemical approach revealed existence of gamma-aminobutyric acid (GABA)-immunoreactive neurons and axons in the SFO. These suggest that spontaneously-active and intermittently-burst-firing GABA interneurons affect other SFO neurons in slice preparations of rats.