Muscarine increases cation conductance and decreases potassium conductance in rat locus coeruleus neurones

Orexin-A directly depolarizes dorsomedial hypothalamic neurons, including those innervating the rostral ventrolateral medulla

The dorsomedial hypothalamus (DMH) receives dense orexinergic innervation. Intra-DMH application of orexins increases arterial pressure and heart rate in rats. We studied the effects of orexin-A on DMH neurons, including those innervating the medullary cardiovascular center, the rostral ventrolateral medulla (RVLM), by using whole-cell recordings in brain slices. In the presence of tetrodotoxin, orexin-A (30-1000 nM) depolarized 56% of DMH neurons (EC50 82.4 ± 4.4 nM). Under voltage-clamp recording, orexin-A (300 nM) induced three types of responses characterized by different current-voltage relationships, namely unchanged, increased, and decreased slope conductance in 68%, 14%, and 18% of orexin-A-responsive neurons, respectively. The reversal potential of the decreased-conductance response was near the equilibrium potential of K+ and became more positive in a high-K+ solution, suggesting that K+ conductance blockade is the underlying mechanism. In a low-Na+ solution, unchanged-, increased-, and decreased-conductance responses were observed in 56%, 11%, and 33% of orexin-A-responsive neurons, respectively, implying that a non-selective cation current (NSCC) underlies orexin-A-induced responses in a small population of DMH neurons. KBR-7943 (70 μM), an inhibitor of Na+-Ca2+ exchanger (NCX), suppressed orexin-A-induced depolarization in 7 of 10 neurons. In the presence of KBR-7943, the majority of orexin-A-responsive neurons exhibited decreased-conductance responses. These findings suggest that NCX activation may underlie orexin-A-induced depolarization in the majority of orexin-responsive DMH neurons. Of 19 RVLM-projecting DMH neurons identified by retrograde labeling, 17 (90%) were orexin-A responsive. In conclusion, orexin-A directly excited over half of DMH neurons, including those innervating the RVLM, through decreasing K+ conductance, activating NCX, and/or increasing NSCC.

Cholinergic modulation of CRH and non-CRH neurons in Barrington's nucleus of the mouse

Corticotropin-releasing hormone (CRH) is expressed in Barrington's nucleus (BarN), which plays an essential role in the regulation of micturition. To control the neural activities of BarN, glutamatergic and GABAergic inputs from multiple sources have been demonstrated; however, it is not clear how modulatory neurotransmitters affect the activity of BarN neurons. We have employed knock-in mice, CRH-expressing neurons of which are labeled with a modified yellow fluorescent protein (Venus). Using whole cell patch-clamp recordings, we examined the responses of Venus-expressing (putative CRH-expressing) neurons in BarN (Bar^CRH), as well as non-CRH-expressing neurons (Bar^CRH-negative), following bath application of cholinergic agonists. According to the present study, the activity of Bar^CRH neurons could be modulated by dual cholinergic mechanisms. First, they are inhibited by a muscarinic receptor-mediated mechanism, most likely through the M2 subclass of muscarinic receptors. Second, Bar^CRH neurons are excited by a nicotinic receptor-mediated mechanism. Bar^CRH-negative neurons also responded to cholinergic agents. Choline transporter-immunoreactive nerve terminals were observed in close proximity to the neurites, as well as the somata of Bar^CRH. The present results suggest that BarN neurons are capable of responding to cholinergic input.NEW & NOTEWORTHY This study investigates the effects of bath-applied cholinergic agonists on Barrington's nucleus (BarN) neurons in vitro. They were either excitatory, through nicotinic receptors, or inhibitory, through muscarinic receptors. Putative corticotropin-releasing hormone (CRH)-expressing neurons in BarN, as well as putative non-CRH-expressing neurons, responded to cholinergic agonists.

Delta glutamate receptor conductance drives excitation of mouse dorsal raphe neurons

The dorsal raphe nucleus is the predominant source of central serotonin, where neuronal activity regulates complex emotional behaviors. Action potential firing of serotonin dorsal raphe neurons is driven via α1-adrenergic receptors (α1-AR) activation. Despite this crucial role, the ion channels responsible for α1-AR-mediated depolarization are unknown. Here, we show in mouse brain slices that α1-AR-mediated excitatory synaptic transmission is mediated by the ionotropic glutamate receptor homolog cation channel, delta glutamate receptor 1 (GluD1). GluD1R-channels are constitutively active under basal conditions carrying tonic inward current and synaptic activation of α1-ARs augments tonic GluD1R-channel current. Further, loss of dorsal raphe GluD1R-channels produces an anxiogenic phenotype. Thus, GluD1R-channels are responsible for α1-AR-dependent induction of persistent pacemaker-type firing of dorsal raphe neurons and regulate dorsal raphe-related behavior. Given the widespread distribution of these channels, ion channel function of GluD1R as a regulator of neuronal excitability is proposed to be widespread in the nervous system.

Decrease of inhibitory synaptic currents of locus coeruleus neurons via orexin type 1 receptors in the context of naloxone-induced morphine withdrawal

Acute opioid withdrawal syndrome is a series of neurological symptoms caused by the abrupt cessation of the chronic administration of opioids such as morphine. The locus coeruleus (LC) in the brain stem receives a dense projection of orexinergic fibers from the hypothalamus and is a candidate site for the expression of the somatic aspects of morphine withdrawal. Previous studies have shown that orexin-A contributes to the behavioral symptoms of naloxone-induced morphine withdrawal, partly by reducing the activity of GABAergic neurons, suggesting that orexin-A may negatively modulate fast GABAergic neurotransmission during morphine withdrawal. We used whole-cell patch-clamp recordings of LC neurons in brainstem slices to investigate the effect of orexin-A on bicuculline-sensitive GABAergic inhibitory postsynaptic currents (IPSCs) during naloxone-induced morphine withdrawal. Male Wistar rats (P14-P21) were given morphine (20 mg/kg, i.p.) daily for seven consecutive days to create dependency on the drug. The application of naloxone (1 µM) to brain slices of morphine-treated rats reduced the amplitude of evoked IPSCs (eIPSCs) as well as spontaneous IPSCs (sIPSCs) frequency but did not change sIPSCs amplitude. Orexin-A (100 nM) significantly enhanced the suppressive effect of naloxone on eIPSCs amplitude and sIPSCs frequency but had no effect on the presence of the orexin type 1 receptor (OX1R) antagonist, SB-334867. Orexin-A alone had no significant effect on eIPSCs and sIPSCs in the absence of naloxone. In summary, our results show that orexin-A, via OX1R, potentiates the suppressive effect of naloxone on GABAergic IPSCs of LC neurons in morphine-treated rats. We conclude that orexins may have a critical role in regulating GABAergic neurotransmission to LC neurons during naloxone-induced morphine withdrawal.

Cellular tolerance at the µ-opioid receptor is phosphorylation dependent

Phosphorylation of the μ-opioid receptor (MOR) is known as a key step in desensitization and internalization but the role in the development of long-term tolerance at the cellular level is not known. Viral expression of wild type (exWT) and mutant MORs, where all phosphorylation sites on the C-terminus (Total Phosphorylation Deficient (TPD)) were mutated to alanine, were examined in locus coeruleus neurons in a MOR knockout rat. Both receptors activated potassium conductance similar to endogenous receptors in wild type animals. The exWT receptors, like endogenous receptors, acutely desensitized, internalized and, after chronic morphine treatment, displayed signs of tolerance. However, TPD receptors did not desensitize or internalize with agonist treatment. In addition the TPD receptors did not develop cellular tolerance following chronic morphine treatment. Thus C-terminal phosphorylation is necessary for the expression of acute desensitization, trafficking and one sign of long-term tolerance to morphine at the cellular level.

Regulation of neural ion channels by muscarinic receptors

The excitable behaviour of neurons is determined by the activity of their endogenous membrane ion channels. Since muscarinic receptors are not themselves ion channels, the acute effects of muscarinic receptor stimulation on neuronal function are governed by the effects of the receptors on these endogenous neuronal ion channels. This review considers some principles and factors determining the interaction between subtypes and classes of muscarinic receptors with neuronal ion channels, and summarizes the effects of muscarinic receptor stimulation on a number of different channels, the mechanisms of receptor - channel transduction and their direct consequences for neuronal activity. Ion channels considered include potassium channels (voltage-gated, inward rectifier and calcium activated), voltage-gated calcium channels, cation channels and chloride channels. This article is part of the Special Issue entitled 'Neuropharmacology on Muscarinic Receptors'.

Enhancement of μ-opioid receptor desensitization by orexin-A in rat locus coeruleus neurons

Opioids have always been used in clinical practice for pain management. However, development of tolerance to their effects following long term administration, seriously restricts further clinical use of these drugs. In this regard, μ-opioid receptor (MOR) desensitization, as an initial step in development of opioid tolerance, is of particular significance. Previous studies support the involvement of orexinergic system in development of opioid tolerance. Locus coeruleus (LC) nucleus has been shown to modulate pain and development of tolerance. Opioid receptors (particularly μ) are densely expressed within the LC. Moreover, it receives widespread orexinergic inputs and orexin type 1 receptors (OX1Rs) are also highly expressed in this brain region. In the present study, the effect of orexin-A (OXA) on met-enkephalin (ME)-induced MOR desensitization was investigated in locus coeruleus neurons of male Wistar rats (2-3weeks of age). ME (30μM), as a potent MOR agonist, was applied for 10min and the outward K⁺ current was recorded using whole cell patch clamp recording. The percentage of decrease in ME-induced K⁺ current was considered as the degree of MOR desensitization. Results indicated that OXA (100nM) enhances ME-induced MOR desensitization via affecting OX1Rs in rat locus coeruleus neurons and this effect is mediated by a protein kinase C dependent mechanism within the LC. The activity of orexinergic system might potentiate the signaling pathways underlying opioid-induced receptor desensitization.

Persistent sodium current drives conditional pacemaking in CA1 pyramidal neurons under muscarinic stimulation

Hippocampal CA1 pyramidal neurons are normally quiescent but can fire spontaneously when stimulated by muscarinic agonists. In brain slice recordings from mouse CA1 pyramidal neurons, we examined the ionic basis of this activity using interleaved current-clamp and voltage-clamp experiments. Both in control and after muscarinic stimulation, the steady-state current-voltage curve was dominated by inward TTX-sensitive persistent sodium current (I(NaP)) that activated near -75 mV and increased steeply with depolarization. In control, total membrane current was net outward (hyperpolarizing) near -70 mV so that cells had a stable resting potential. Muscarinic stimulation activated a small nonselective cation current so that total membrane current near -70 mV shifted to become barely net inward (depolarizing). The small depolarization triggers regenerative activation of I(NaP), which then depolarizes the cell from -70 mV to spike threshold. We quantified the relative contributions of I(NaP), hyperpolarization-activated cation current (I(h)), and calcium current to pacemaking by using the cell's own firing as a voltage command along with specific blockers. TTX-sensitive sodium current was substantial throughout the entire interspike interval, increasing as the membrane potential approached threshold, while both Ih and calcium current were minimal. Thus, spontaneous activity is driven primarily by activation of I(NaP) in a positive feedback loop starting near -70 mV and providing increasing inward current to threshold. These results show that the pacemaking "engine" from I(NaP) is an inherent property of CA1 pyramidal neurons that can be engaged or disengaged by small shifts in net membrane current near -70 mV, as by muscarinic stimulation.

Gastrin-releasing peptide acts via postsynaptic BB2 receptors to modulate inward rectifier K+ and TRPV1-like conductances in rat paraventricular thalamic neurons

Gastrin-releasing peptide (GRP) is a bombesin-like peptide with a widespread distribution in mammalian CNS, where it has a role in food intake, circadian rhythm generation, fear memory, itch sensation and sexual behaviour. While it has been established that GRP predominantly excites neurons, details of the membrane mechanism involved in this action remain largely undefined. We used perforated patch clamp recording in acute brain slice preparations to investigate GRP-affected receptors and ionic conductances in neurons of the rat paraventricular thalamic nucleus (PVT). PVT is a component of the midline and intralaminar thalamus that participates in arousal, motivational drives and stress responses, and exhibits a prominence of GRP-like immunoreactive fibres. Exposure of PVT neurons to low nanomolar concentrations of GRP induced sustained TTX-resistant membrane depolarizations that could trigger rhythmic burst discharges or tonic firing. Membrane current analyses in voltage clamp revealed an underlying postsynaptic bombesin type 2 receptor-mediated inward current that resulted from the simultaneous suppression of a Ba(2+)-sensitive inward rectifier K(+) conductance and activation of a non-selective cation conductance with biophysical and pharmacological properties reminiscent of transient receptor potential vanilloid (TRPV) 1. A role for a TRPV1-like conductance was further implied by a significant suppressant influence of a TRPV1 antagonist on GRP-induced membrane depolarization and rhythmic burst or tonic firing. The results provide a detailed picture of the receptor and ionic conductances that are involved in GRP's excitatory action in midline thalamus.

Reversed synaptic effects of hypocretin and NPY mediated by excitatory GABA-dependent synaptic activity in developing MCH neurons

In mature neurons, GABA is the primary inhibitory neurotransmitter. In contrast, in developing neurons, GABA exerts excitatory actions, and in some neurons GABA-mediated excitatory synaptic activity is more prevalent than glutamate-mediated excitation. Hypothalamic neuropeptides that modulate cognitive arousal and energy homeostasis, hypocretin/orexin and neuropeptide Y (NPY), evoked reversed effects on synaptic actions that were dependent on presynaptic GABA release onto melanin-concentrating hormone (MCH) neurons. MCH neurons were identified by selective green fluorescent protein (GFP) expression in transgenic mice. In adults, hypocretin increased GABA release leading to reduced excitation. In contrast, in the developing brain as studied here with analysis of miniature excitatory postsynaptic currents, paired-pulse ratios, and evoked potentials, hypocretin acted presynaptically to enhance the excitatory actions of GABA. The ability of hypocretin to enhance GABA release increases inhibition in adult neurons but paradoxically enhances excitation in developing MCH neurons. In contrast, NPY attenuation of GABA release reduced inhibition in mature neurons but enhanced inhibition during development by attenuating GABA excitation. Both hypocretin and NPY also evoked direct actions on developing MCH neurons. Hypocretin excited MCH cells by activating a sodium-calcium exchanger and by reducing potassium currents; NPY reduced activity by increasing an inwardly rectifying potassium current. These data for the first time show that both hypocretin and NPY receptors are functional presynaptically during early postnatal hypothalamic development and that both neuropeptides modulate GABA actions during development with a valence of enhanced excitation or inhibition opposite to that of the adult state, potentially allowing neuropeptide modulation of use-dependent synapse stabilization.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Sodium leak channels in neuronal excitability and rhythmic behaviors

Extracellular K⁺, Na⁺, and Ca²⁺ ions all influence the resting membrane potential of the neuron. However, the mechanisms by which extracellular Na⁺ and Ca²⁺ regulate basal neuronal excitability are not well understood. Recent findings suggest that NALCN, in association with UNC79 and UNC80, contributes a basal Na⁺ leak conductance in neurons. Mutations in Nalcn, Unc79, or Unc80 lead to severe phenotypes that include neonatal lethality and disruption in rhythmic behaviors. This review discusses the properties of the NALCN complex, its regulation, and its contribution to neuronal function and animal behavior.

Genetic inactivation of Kcnj16 identifies Kir5.1 as an important determinant of neuronal PCO2/pH sensitivity

The molecular identity of ion channels which confer PCO(2)/pH sensitivity in the brain is unclear. Heteromeric Kir4.1/Kir5.1 channels are highly sensitive to inhibition by intracellular pH and are widely expressed in several brainstem nuclei involved in cardiorespiratory control, including the locus coeruleus. This has therefore led to a proposed role for these channels in neuronal CO(2) chemosensitivity. To examine this, we generated mutant mice lacking the Kir5.1 (Kcnj16) gene. We show that although locus coeruleus neurons from Kcnj16((+/+)) mice rapidly respond to cytoplasmic alkalinization and acidification, those from Kcnj16((-/-)) mice display a dramatically reduced and delayed response. These results identify Kir5.1 as an important determinant of PCO(2)/pH sensitivity in locus coeruleus neurons and suggest that Kir5.1 may be involved in the response to hypercapnic acidosis.

Conserved role of unc-79 in ethanol responses in lightweight mutant mice

The mechanisms by which ethanol and inhaled anesthetics influence the nervous system are poorly understood. Here we describe the positional cloning and characterization of a new mouse mutation isolated in an N-ethyl-N-nitrosourea (ENU) forward mutagenesis screen for animals with enhanced locomotor activity. This allele, Lightweight (Lwt), disrupts the homolog of the Caenorhabditis elegans (C. elegans) unc-79 gene. While Lwt/Lwt homozygotes are perinatal lethal, Lightweight heterozygotes are dramatically hypersensitive to acute ethanol exposure. Experiments in C. elegans demonstrate a conserved hypersensitivity to ethanol in unc-79 mutants and extend this observation to the related unc-80 mutant and nca-1;nca-2 double mutants. Lightweight heterozygotes also exhibit an altered response to the anesthetic isoflurane, reminiscent of unc-79 invertebrate mutant phenotypes. Consistent with our initial mapping results, Lightweight heterozygotes are mildly hyperactive when exposed to a novel environment and are smaller than wild-type animals. In addition, Lightweight heterozygotes exhibit increased food consumption yet have a leaner body composition. Interestingly, Lightweight heterozygotes voluntarily consume more ethanol than wild-type littermates. The acute hypersensitivity to and increased voluntary consumption of ethanol observed in Lightweight heterozygous mice in combination with the observed hypersensitivity to ethanol in C. elegans unc-79, unc-80, and nca-1;nca-2 double mutants suggests a novel conserved pathway that might influence alcohol-related behaviors in humans.

Peptide neurotransmitters activate a cation channel complex of NALCN and UNC-80

Several neurotransmitters act through G-protein-coupled receptors to evoke a 'slow' excitation of neurons. These include peptides, such as substance P and neurotensin, as well as acetylcholine and noradrenaline. Unlike the fast (approximately millisecond) ionotropic actions of small-molecule neurotransmitters, the slow excitation is not well understood at the molecular level, but can be mainly attributed to suppressing K(+) currents and/or activating a non-selective cation channel. The molecular identity of this cation channel has yet to be determined; similarly, how the channel is activated and its relative contribution to neuronal excitability induced by the neuropeptides are unknown. Here we show that, in the mouse hippocampal and ventral tegmental area neurons, substance P and neurotensin activate a channel complex containing NALCN and a large previously unknown protein UNC-80. The activation by substance P through TACR1 (a G-protein-coupled receptor for substance P) occurs by means of a unique mechanism: it does not require G-protein activation but is dependent on Src family kinases. These findings identify NALCN as the cation channel activated by substance P receptor, and suggest that UNC-80 and Src family kinases, rather than a G protein, are involved in the coupling from receptor to channel.

Kisspeptin excites gonadotropin-releasing hormone neurons through a phospholipase C/calcium-dependent pathway regulating multiple ion channels

The present study used perforated-patch electrophysiology and calcium imaging in GnRH transgenic mouse lines to determine the mechanisms underlying the potent excitatory effects of kisspeptin upon GnRH neurons in the acute brain slice preparation. Kisspeptin (100 nm) depolarized (6 +/- 1 mV) and/or evoked an 87 +/- 4% increase in firing rate of 75% of adult GnRH neurons (n = 51). No sex differences were found. Analyses of input resistance and current-voltage curves indicated that a heterogeneous closure of potassium channels and opening of nonselective cation (NSC) channels was involved in kisspeptin's depolarizing response. Pharmacological pretreatment with either barium, a potassium channel blocker, or flufenamic acid, an NSC channel antagonist, reduced the percentage of responding GnRH neurons from 75 to 40% (P < 0.05). Pretreatment with both barium and flufenamic acid reduced the response rate to 17% (P < 0.05). To examine the intracellular signaling cascade involved, GnRH neurons were treated with antagonists of phospholipase C (PLC), inositol-trisphosphate receptors (IP3R), and ERK1/2 before kisspeptin exposure. PLC and IP3R antagonism reduced the percentage of responding GnRH neurons from 80 to 15 and 7%, respectively (P < 0.001). Real-time calcium imaging showed that kisspeptin evoked an approximately 10% increase in intracellular calcium levels in GnRH neurons that was followed by a decrease and return to pretest calcium levels. Additional experiments indicated that mechanisms intrinsic to the GnRH neuron are responsible for their prolonged depolarizing response to kisspeptin. These studies indicate that kisspeptin activates G protein-coupled receptor 54 (GPR54) to initiate a PLC-IP3R-calcium cascade that modulates both potassium and NSC channels to initiate depolarization in GnRH neurons.

ACh-induced depolarization in inner ear artery is generated by activation of a TRP-like non-selective cation conductance and inactivation of a potassium conductance

Adequate cochlear blood supply by the spiral modiolar artery (SMA) is critical for normal hearing. ACh may play a role in neuroregulation of the SMA but several key issues including its membrane action mechanisms remain poorly understood. Besides its well-known endothelium-dependent hyperpolarizing action, ACh can induce a depolarization in vascular cells. Using intracellular and whole-cell recording techniques on cells in guinea pig in vitro SMA, we studied the ionic mechanism underlying the ACh-depolarization and found that: (1) ACh induced a DAMP-sensitive depolarization when intermediate conductance KCa channels were blocked by charybdotoxin or nitrendipine. The ACh-depolarization was associated with a decrease in input resistance (R(input)) in high membrane potential (V(m)) ( approximately -40 mV) cells but with no change or an increase in R input in low Vm ( approximately -75 mV) cells. ACh-depolarization was attenuated by background membrane depolarization from approximately -70 mV in the majority of cells; (2) ACh-induced inward current in smooth muscle cells embedded in a SMA segment often showed a U-shaped I/V curve, the reversal potential of its two arms being near EK and 0 mV, respectively; (3) ACh-depolarization was reduced by low Na+, zero K+ or 20mM K+ bath solutions; (4) ACh-depolarization was inhibited by La3+ in all cells tested, by 4-AP and flufenamic acid in low Vm cells, but was not sensitive to Cd2+, Ni2+, nifedipine, niflumic acid, DIDS, IAA94, linopirdine or amiloride. We conclude that ACh-induced vascular depolarization was generated mainly by activation of a TRP-like non-selective cation channel and by inactivation of an inward rectifier K+ channel.

Multiple conductances are modulated by 5-HT receptor subtypes in rat subthalamic nucleus neurons

Firing patterns of subthalamic nucleus (STN) neurons influence normal and abnormal movements. The STN expresses multiple 5-HT receptor subtypes that may regulate neuronal excitability. We used whole-cell patch-clamp recordings to characterize 5-HT receptor-mediated effects on membrane currents in STN neurons in rat brain slices. In 80 STN neurons under voltage-clamp (-70 mV), 5-HT (30 microM) evoked inward currents in 64%, outward currents in 17%, and biphasic currents in 19%. 5-HT-induced outward current was caused by an increased K(+) conductance (1.4+/-0.2 nS) and was blocked by the 5-HT(1A) antagonist WAY 100135. The 5-HT-evoked inward current, which was blocked by antagonists at 5-HT(2C) and/or 5-HT(4) receptors, had two types of current-voltage (I-V) relations. Currents associated with the type 1 I-V relation showed negative slope conductance at potentials <-110 mV and were occluded by Ba(2+). In contrast, the type 2 I-V relation appeared linear and had positive slope conductance (0.64+/-0.11 nS). Type 2 inward currents were Ba(2+)-insensitive, and the reversal potential of -19 mV suggests a mixed cation conductance. In STN neurons in which 5-HT evoked inward currents, 5-HT potentiated burst firing induced by N-methyl-d-aspartate (NMDA). But in neurons in which 5-HT evoked outward current, 5-HT slowed NMDA-dependent burst firing. We conclude that 5-HT receptor subtypes can differentially regulate firing pattern by modulating multiple conductances in STN neurons.

Cholinergic modulation of periaqueductal grey neurons: Does it contribute to epileptogenesis after organophosphorus nerve agent intoxication?

Previous work has shown that a single focal microinjection of the unselective cholinergic agonist, carbachol, into the periaqueductal grey (PAG) of the midbrain is sufficient to induce forebrain seizures in rats. In order to determine the cholinergic mechanisms underlying epileptogenesis at the cellular and network level of the PAG, we performed whole-cell recordings from rat PAG neurons in vitro and examined how the activation of muscarinic and nicotinic receptors modulates cellular excitability and synaptic responses. Stimulation of muscarinic receptors produced either a pirenzepine-sensitive depolarization (40% of PAG neurons), or a gallamine-sensitive hyperpolarization (20%), suggesting the involvement of M1 and M2 receptors, respectively. In the remaining neurons (40%), no change was observed. Voltage-clamp recordings showed that muscarinic depolarization resulted from the inhibition of a resting K(+) current, in part accompanied by simultaneous activation of a presumed non-selective cation current. Muscarinic hyperpolarization was caused by the activation of a G protein-coupled, inwardly rectifying K(+) current. Stimulation of muscarinic receptors enhanced the frequency of spontaneous inhibitory postsynaptic currents (IPSCs), but strongly suppressed evoked IPSCs. In addition, nicotine almost doubled the frequency of miniature IPSCs. Based on our findings and the network properties of the PAG, we advance a scenario in which excessive stimulation of cholinergic receptors would substantially contribute to generalized seizures after organophosphorus nerve agent poisoning.

D2-like dopamine receptors depolarize dorsal raphe serotonin neurons through the activation of nonselective cationic conductance

The dorsal raphe (DR) receives a prominent dopamine (DA) input that has been suggested to play a key role in the regulation of central serotoninergic transmission. DA is known to directly depolarize DR serotonin neurons, but the underlying mechanisms are not well understood. Here, we show that activation of D2-like dopamine receptors on DR 5-HT neurons elicits a membrane depolarization and an inward current associated with an increase in membrane conductance. The DA-induced inward current (I(DA)) exhibits a linear I-V relationship and reverses polarity at around -15 mV, suggesting the involvement of a mixed cationic conductance. Consistent with this notion, lowering the extracellular concentration of sodium reduces the amplitude of I(DA) and induces a negative shift of its reversal potential to approximately -45 mV. This current is abolished by inhibiting G-protein function with GDPbetaS. Examination of the downstream signaling mechanisms reveals that activation of the nonselective cation current requires the stimulation of phospholipase C but not an increase in intracellular calcium. Thus, pharmacological inhibition of phospholipase C reduces the amplitude of I(DA). In contrast, buffering intracellular calcium has no effect on the amplitude of I(DA). Bath application of transient receptor potential (TRP) channels blockers, 2-aminoethoxydiphenyl borate and SKF96365 [1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole], strongly inhibits I(DA) amplitude, suggesting the involvement of TRP-like conductance. These results reveal previously unsuspected mechanism by which D2-like DA receptors induce membrane depolarization and enhance the excitability of DR 5-HT neurons.

Postsynaptic actions of substance P on rat periaqueductal grey neurons in vitro

The postsynaptic actions of substance P on rat midbrain periaqueductal grey (PAG) neurons were examined using whole-cell patch-clamp recordings in brain slices. Substance P produced an inward current in a subpopulation (60%) of PAG neurons. The substance P induced current was concentration dependent (EC50=27 nM) and was reduced by the NK1, NK2 and NK3 antagonists L-732,138 (20 microM), GR 159897 (3 microM) and SB 218795 (3 microM). The selective NK1, NK2 and NK3 agonists [Sar9,Met(O2)11]-Substance P (100 nM), GR 64349 (300-500 nM) and senktide (300 nM) also produced inward currents in subpopulations of neurons. A greater proportion of substance P-sensitive neurons (70%) than substance P-insensitive neurons (31%) responded to the mu/delta opioid agonist met-enkephalin (10 microM). Substance P reduced the outward current produced by met-enkephalin. The reversal potential of the substance P induced current varied from -5 mV to below -140 mV in the absence of met-enkephalin, and was -105 mV in the presence of met-enkephalin. These results indicate that substance P acts via NK1, NK2 and NK3 receptors to excite subpopulations of opioid-sensitive and insensitive PAG neurons by increasing a non-selective cation conductance and by reducing a K+ current. In addition, substance P has anti-opioid actions that are largely mediated by a reduction in the opioid induced K+ current.

Corticotropin-releasing hormone directly activates noradrenergic neurons of the locus ceruleus recorded in vitro

The neuropeptide corticotropin-releasing hormone (CRH) activates locus ceruleus (LC) neurons, thereby increasing norepinephrine levels throughout the CNS. Despite anatomical and physiological evidence for CRH innervation of the LC, the mechanism of CRH-evoked activation of LC neurons is unknown. Moreover, given the apparent absence of mRNA for CRH receptors in LC neurons, the exact location of action of CRH within the cerulear region is debated. Using in vitro intracellular recordings from rat brainstem, we examined whether CRH exerts a direct effect on LC neurons and which ionic currents are likely affected by CRH. We demonstrate that CRH dose-dependently increases the firing rate of LC neurons through a direct (TTX- and cadmium-insensitive) mechanism by decreasing a potassium conductance. The CRH-evoked activation of LC neurons is, at least in part, mediated by CRH1 receptors and a cAMP-dependent second messenger system. These data provide additional support that CRH functions as an excitatory neurotransmitter in the LC and the hypothesis that dysfunction of the CRH peptidergic and noradrenergic systems observed in patients with mood and anxiety disorders are functionally related.

Glucagon-like peptide 1 excites hypocretin/orexin neurons by direct and indirect mechanisms: implications for viscera-mediated arousal

Glucagon-like peptide 1 (GLP-1) is produced by neurons in the caudal brainstem that receive sensory information from the gut and project to several hypothalamic regions involved in arousal, interoceptive stress, and energy homeostasis. GLP-1 axons and receptors have been detected in the lateral hypothalamus, where hypocretin neurons are found. The electrophysiological actions of GLP-1 in the CNS have not been studied. Here, we explored the GLP-1 effects on GFP (green fluorescent protein)-expressing hypocretin neurons in mouse hypothalamic slices. GLP-1 receptor agonists depolarized hypocretin neurons and increased their spike frequency; the antagonist exendin (9-39) blocked this depolarization. Direct GLP-1 agonist actions on membrane potential were abolished by choline substitution for extracellular Na+, and dependent on intracellular GDP, suggesting that they were mediated by sodium-dependent conductances in a G-protein-dependent manner. In voltage clamp, the GLP-1 agonist Exn4 (exendin-4) induced an inward current that reversed near -28 mV and persisted in nominally Ca2+-free extracellular solution, consistent with a nonselective cationic conductance. GLP-1 decreased afterhyperpolarization currents. GLP-1 agonists enhanced the frequency of miniature and spontaneous EPSCs with no effect on their amplitude, suggesting presynaptic modulation of glutamate axons innervating hypocretin neurons. Paraventricular hypothalamic neurons were also directly excited by GLP-1 agonists. In contrast, GLP-1 agonists had no detectable effect on neurons that synthesize melanin-concentrating hormone (MCH). Together, our results show that GLP-1 agonists modulate the activity of hypocretin, but not MCH, neurons in the lateral hypothalamus, suggesting a role for GLP-1 in the excitation of the hypothalamic arousal system possibly initiated by activation by viscera sensory input.

ATP excites interneurons and astrocytes to increase synaptic inhibition in neuronal networks

We investigated the role of extracellular ATP at astrocytes and inhibitory GABAergic interneurons in the stratum radiatum area of the mouse hippocampus. We show that exogenously applied ATP increased astrocyte intracellular Ca2+ levels and depolarized all calbindinand calretinin-positive interneurons in the stratum radiatum region of mouse hippocampus, leading to action potential firing and enhanced synaptic inhibition onto the postsynaptic targets of interneurons. Electrophysiological, pharmacological, and immunostaining studies suggested that the effect of ATP on interneurons was mediated by P2Y1 receptors, and that the depolarization of interneurons was caused by the concomitant reduction and activation of potassium and nonselective cationic conductances, respectively. Electrical stimulation of the Schaffer collaterals and perforant path, as well as local stimulation within the stratum radiatum, evoked increases in intracellular Ca2+ in astrocytes. Facilitation of GABAergic IPSCs onto interneurons also occurred during electrical stimulation. Both the stimulation-evoked increases in astrocyte Ca2+ levels and facilitation of GABAergic IPSCs were sensitive to antagonists of P2Y1 receptors and mimicked by exogenous P2Y1 receptor agonists, suggesting that endogenously released ATP can activate P2Y receptors on both astrocytes and interneurons. Overall, our data are consistent with the hypothesis that ATP released from neurons and astrocytes acts on P2Y1 receptors to excite interneurons, resulting in increased synaptic inhibition within intact hippocampal circuits.

GABA Transporter Currents Activated by Protein Kinase A Excite Midbrain Neurons during Opioid Withdrawal

Adaptations in neurons of the midbrain periaqueductal gray (PAG) induced by chronic morphine treatment mediate expression of many signs of opioid withdrawal. The abnormally elevated action potential rate of opioid-sensitive PAG neurons is a likely cellular mechanism for withdrawal expression. We report here that opioid withdrawal in vitro induced an opioid-sensitive cation current that was mediated by the GABA transporter-1 (GAT-1) and required activation of protein kinase A (PKA) for its expression. Inhibition of GAT-1 or PKA also prevented withdrawal-induced hyperexcitation of PAG neurons. Our findings indicate that GAT-1 currents can directly increase the action potential rates of neurons and that GAT-1 may be a target for therapy to alleviate opioid-withdrawal symptoms.

Protein kinase C activation enhances morphine-induced rapid desensitization of mu-opioid receptors in mature rat locus ceruleus neurons

Recent studies have shown that morphine, in contrast to other agonists at the mu-opioid receptor, causes very little rapid mu-opioid receptor desensitization or internalization in adult rat mammalian neurons, raising important questions about how morphine tolerance is induced. Here we show that morphine can indeed cause marked rapid desensitization of mu-opioid receptors in mature rat locus ceruleus neurons when protein kinase C is also activated. Thus, activation of Gq-coupled M3 muscarinic receptors or application of a phorbol ester enhanced the desensitization of the mu-opioid receptor-evoked potassium current in rat locus ceruleus neurons. The enhancement of desensitization was reversible by the protein kinase C inhibitors chelerythrine and 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)-maleimide (GF109203X) and resulted from an effect at the level of the mu-opioid receptor rather than the potassium channel. This is the first finding that morphine can induce rapid mu-opioid receptor desensitization in adult rat neurons, and because reduced protein kinase C activity in vivo attenuates morphine tolerance, we propose that G-protein coupled receptor cross-talk and the level of protein kinase C activity may play critical roles in the desensitization of the mu-opioid receptor and could underlie the development of morphine tolerance.

Hypocretin/orexin innervation and excitation of identified septohippocampal cholinergic neurons

Hypothalamic fibers containing the wake-promoting peptides, hypocretins (Hcrts) or orexins, provide a dense innervation to the medial septum-diagonal band of Broca (MSDB), a sleep-associated brain region that has been suggested to show intense axonal degeneration in canine narcoleptics. The MSDB, via its cholinergic and GABAergic projections to the hippocampus, controls the hippocampal theta rhythm and associated learning and memory functions. Neurons of the MSDB express very high levels of the Hcrt receptor 2, which is mutated in canine narcoleptics. In the present study, we investigated the electrophysiological effects of Hcrt peptides on septohippocampal cholinergic neurons that were identified in living brain slices of the MSDB using a selective fluorescent marker. Hcrt activation of septohippocampal cholinergic neurons was reversible, reproducible, and concentration dependent and mediated via a direct postsynaptic mechanism. Both Hcrt1 and Hcrt2 activated septohippocampal cholinergic neurons with similar EC(50) values. The Hcrt effect was dependent on external Na(+), reduced by external Ba(2+), and also reduced in recordings with CsCl-containing electrodes, suggesting a dual underlying ionic mechanism that involved inhibition of a K(+) current, presumably an inward rectifier, and a Na(+)-dependent component. The Na(+) component was dependent on internal Ca(2+), blocked by replacing external Na(+) with Li(+), and also blocked by bath-applied Ni(2+) and KB-R7943, suggesting involvement of the Na(+)-Ca(2+) exchanger. Using double-immunolabeling studies at light and ultrastructural levels, we also provide definitive evidence for a hypocretin innervation of cholinergic neurons. Thus Hcrt effects within the septum should increase hippocampal acetylcholine release and thereby promote hippocampal arousal.

Distinct forms of cholinergic modulation in parallel thalamic sensory pathways

Mammalian thalamus is a critical site where early perception of sensorimotor signals is dynamically regulated by acetylcholine in a behavioral state-dependent manner. In this study, we examined how synaptic transmission is modulated by acetylcholine in auditory thalamus where sensory relay neurons form parallel lemniscal and nonlemniscal pathways. The former mediates tonotopic relay of acoustic signals, whereas the latter is involved in detecting and transmitting auditory cues of behavioral relevance. We report here that activation of cholinergic muscarinic receptors had opposite membrane effects on these parallel synaptic pathways. In lemniscal neurons, muscarine induced a sustained membrane depolarization and tonic firing by closing a linear K(+) conductance. In contrast, in nonlemniscal neurons, muscarine evoked a membrane hyperpolarization by opening a voltage-independent K(+) conductance. Depending on the level of membrane hyperpolarization and the strength of local synaptic input, nonlemniscal neurons were either suppressed or selectively engaged in detecting and transmitting synchronized synaptic input by firing a high-frequency spike burst. Immunohistochemical and Western blotting experiments showed that nonlemniscal neurons predominantly expressed M2 muscarinic receptors, whereas lemniscal cells had a significantly higher level of M1 receptors. Our data indicate that cholinergic modulation in the thalamus is pathway-specific. Enhanced cholinergic tone during behavioral arousal or attention may render synaptic transmission in nonlemniscal thalamus highly sensitive to the context of local synaptic activities.

Muscarinic receptors involved in the subthreshold cholinergic actions of neostriatal spiny neurons

Administration of the peptide MT-1 (48 nM), a selective agonist of muscarinic M(1)-type receptors, mimicked the subthreshold actions of muscarine (1 microM) on neostriatal neurons, i.e., it produced a reduction in subthreshold inward rectification leading to an enhancement in input resistance (R(N)) and evoked discharge. In all recorded cells, MT-1 effects remained in the presence of the specific peptidergic antagonist of the M(4)-type receptor, MT-3 (10 nM), but were blocked by the specific M(1)-type receptor antagonist MT-7 (5 nM). These results suggest that most muscarinic facilitatory actions in the subthreshold voltage range occur through M(1)-type receptors. However, in a fraction of cells (40%) muscarine produced an excitability enhancement not blocked by MT-7. This additional facilitatory action, not present when using MT-1, was blocked by MT-3, suggesting it was mediated by M(4)-type receptor activation. This facilitation could not be blocked by Cs(+), TTX, or Cd(2+), but only by a reduction in extracellular sodium. This result is the first evidence that M(4)-type receptor activation enhances a cationic inward current in a fraction of neostriatal projection neurons.

Hypocretins (orexins) regulate serotonin neurons in the dorsal raphe nucleus by excitatory direct and inhibitory indirect actions

The hypocretins (hcrt1 and hcrt2) are expressed by a discrete population of hypothalamic neurons projecting to many regions of the CNS, including the dorsal raphe nucleus (DRN), where serotonin (5-HT) neurons are concentrated. In this study, we investigated responses to hcrts in 216 physiologically identified 5-HT and non-5-HT neurons of the DRN using intracellular and whole-cell recording in rat brain slices. Hcrt1 and hcrt2 induced similar amplitude and dose-dependent inward currents in most 5-HT neurons tested (EC50, approximately 250 nm). This inward current was not blocked by the fast Na+ channel blocker TTX or in a Ca2+-free solution, indicating a direct postsynaptic action. The hcrt-induced inward current reversed near -18 mV and was primarily dependent on external Na+ but not on external or internal Ca2+, features typical of Na+/K+ nonselective cation channels. At higher concentrations, hcrts also increased spontaneous postsynaptic currents in 5-HT neurons (EC50, approximately 450-600 nm), which were TTX-sensitive and mostly blocked by the GABA(A) antagonist bicuculline, indicating increased impulse flow in local GABA interneurons. Accordingly, hcrts were found to increase the basal firing of presumptive GABA interneurons. Immunolabeling showed that hcrt fibers projected to both 5-HT and GABA neurons in the DRN. We conclude that hcrts act directly to excite 5-HT neurons primarily via a TTX-insensitive, Na+/K+ nonselective cation current, and indirectly to activate local inhibitory GABA inputs to 5-HT cells. The greater potency of hcrts in direct excitation compared with indirect inhibition suggests a negative feedback function for the latter at higher levels of hcrt activity.

Hypocretins (orexins) regulate serotonin neurons in the dorsal raphe nucleus by excitatory direct and inhibitory indirect actions

The hypocretins (hcrt1 and hcrt2) are expressed by a discrete population of hypothalamic neurons projecting to many regions of the CNS, including the dorsal raphe nucleus (DRN), where serotonin (5-HT) neurons are concentrated. In this study, we investigated responses to hcrts in 216 physiologically identified 5-HT and non-5-HT neurons of the DRN using intracellular and whole-cell recording in rat brain slices. Hcrt1 and hcrt2 induced similar amplitude and dose-dependent inward currents in most 5-HT neurons tested (EC50, approximately 250 nm). This inward current was not blocked by the fast Na+ channel blocker TTX or in a Ca2+-free solution, indicating a direct postsynaptic action. The hcrt-induced inward current reversed near -18 mV and was primarily dependent on external Na+ but not on external or internal Ca2+, features typical of Na+/K+ nonselective cation channels. At higher concentrations, hcrts also increased spontaneous postsynaptic currents in 5-HT neurons (EC50, approximately 450-600 nm), which were TTX-sensitive and mostly blocked by the GABA(A) antagonist bicuculline, indicating increased impulse flow in local GABA interneurons. Accordingly, hcrts were found to increase the basal firing of presumptive GABA interneurons. Immunolabeling showed that hcrt fibers projected to both 5-HT and GABA neurons in the DRN. We conclude that hcrts act directly to excite 5-HT neurons primarily via a TTX-insensitive, Na+/K+ nonselective cation current, and indirectly to activate local inhibitory GABA inputs to 5-HT cells. The greater potency of hcrts in direct excitation compared with indirect inhibition suggests a negative feedback function for the latter at higher levels of hcrt activity.

Muscarinic activation of a cation current and associated current noise in entorhinal-cortex layer-II neurons

The effects of muscarinic stimulation on the membrane potential and current of in situ rat entorhinal-cortex layer-II principal neurons were analyzed using the whole cell, patch-clamp technique. In current-clamp experiments, application of carbachol (CCh) induced a slowly developing, prolonged depolarization initially accompanied by a slight decrease or no significant change in input resistance. By contrast, in a later phase of the depolarization input resistance appeared consistently increased. To elucidate the ionic bases of these effects, voltage-clamp experiments were then carried out. In recordings performed in nearly physiological ionic conditions at the holding potential of -60 mV, CCh application promoted the slow development of an inward current deflection consistently associated with a prominent increase in current noise. Similarly to voltage responses to CCh, this inward-current induction was abolished by the muscarinic antagonist, atropine. Current-voltage relationships derived by applying ramp voltage protocols during the different phases of the CCh-induced inward-current deflection revealed the early induction of an inward current that manifested a linear current/voltage relationship in the subthreshold range and the longer-lasting block of an outward K(+) current. The latter current could be blocked by 1 mM extracellular Ba(2+), which allowed us to study the CCh-induced inward current (I(CCh)) in isolation. The extrapolated reversal potential of the isolated I(CCh) was approximately 0 mV and was not modified by complete substitution of intrapipette K(+) with Cs(+). Moreover, the extrapolated I(CCh) reversal shifted to approximately -20 mV on removal of 50% extracellular Na(+). These results are consistent with I(CCh) being a nonspecific cation current. Finally, noise analysis of I(CCh) returned an estimated conductance of the underlying channels of approximately 13.5 pS. We conclude that the depolarizing effect of muscarinic stimuli on entorhinal-cortex layer-II principal neurons depends on both the block of a K(+) conductance and the activation of a "noisy" nonspecific cation current. We suggest that the membrane current fluctuations brought about by I(CCh) channel noise may facilitate the "theta" oscillatory dynamics of these neurons and enhance firing reliability and synchronization.

Inward rectifier K+ channel Kir2.3 is localized at the postsynaptic membrane of excitatory synapses

Classical inwardly rectifying K+ channels (Kir2.0) are responsible for maintaining the resting membrane potential near the K+ equilibrium potential in various cells, including neurons. Although Kir2.3 is known to be expressed abundantly in the forebrain, its precise localization has not been identified. Using an antibody specific to Kir2.3, we examined the subcellular localization of Kir2.3 in mouse brain. Kir2.3 immunoreactivity was detected in a granular pattern in restricted areas of the brain, including the olfactory bulb (OB). Immunoelectron microscopy of the OB revealed that Kir2.3 immunoreactivity was specifically clustered on the postsynaptic membrane of asymmetric synapses between granule cells and mitral/tufted cells. The immunoprecipitants for Kir2.3 obtained from brain contained PSD-95 and chapsyn-110, PDZ domain-containing anchoring proteins. In vitro binding assay further revealed that the COOH-terminal end of Kir2.3 is responsible for the association with these anchoring proteins. Therefore, the Kir channel may be involved in formation of the resting membrane potential of the spines and, thus, would affect the response of N-methyl-D-aspartic acid receptor channels at the excitatory postsynaptic membrane.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Expression of mRNA and functional alpha(1)-adrenoceptors that suppress the GIRK conductance in adult rat locus coeruleus neurons

1. Locus coeruleus neurons in adult rats express binding sites and mRNA for alpha(1)-adrenoceptors even though the depolarizing effect of alpha(1)-adrenoceptor agonists on neonatal neurons disappears during development. 2. In this study intracellular microelectrodes were used to record from locus coeruleus neurons in brain slices of adult rats and reverse transcription-polymerase chain reaction (RT - PCR) was used to investigate the mRNA expression of alpha(1)- and alpha(2)-adrenoceptors in juvenile and adult rats. 3. The alpha(1)-adrenoceptor agonist phenylephrine had no effect on the membrane conductance of locus coeruleus neurons (V(hold) -60 mV) but decreased the G protein coupled, inward rectifier potassium (GIRK) conductance induced by alpha(2)-adrenoceptor or mu-opioid agonists. The GIRK conductance induced by noradrenaline was increased in amplitude when alpha(1)-adrenoceptors were blocked with prazosin. 4. RT - PCR of total cellular RNA isolated from microdissected locus coeruleus tissue demonstrated strong mRNA expression of alpha(1a)-, alpha(1b)- and alpha(1d)-adrenoceptors in both juvenile and adult rats. However, only mRNA transcripts for the alpha(1b)-adrenoceptors were consistently detected in cytoplasmic samples taken from single locus coeruleus neurons of juvenile rats, suggesting that this subtype may be responsible for the physiological effects seen in juvenile rats. 5. Juvenile and adult locus coeruleus tissue expressed mRNA for the alpha(2a)- and alpha(2c)-adrenoceptors while the alpha(2b)-adrenoceptor was only weakly expressed in juveniles and was not detected in adults. 6. The results of this study show that alpha(1)-adrenoceptors expressed in adult locus coeruleus neurons function to suppress the GIRK conductance that is activated by mu-opioid and alpha(2)-adrenoceptors.

Mechanisms of orexin-induced depolarizations in rat dorsal motor nucleus of vagus neurones in vitro

1. Whole-cell patch-clamp recordings were made from neurones of the dorsal motor nucleus of the vagus (DMNV), including Fluoro-gold-labelled parasympathetic preganglionic neurones (PPNs), in slices of the rat medulla. In the latter case, rats had received an I.P. injection of Fluoro-gold solution (10 microg) 2-3 days earlier. 2. Superfusion of orexin A or B (10-300 nM) caused a slow depolarization in approximately 30% of the DMNV neurones, including PPNs. Orexin-induced depolarizations, which persisted in TTX (0.5 microM)-containing Krebs solution, were reduced by 70% in a low-Na+ (26 mM) Krebs solution, indicating the involvement of Na+ ions. A significant change in orexin-induced depolarizations was not obtained in either a high-K+ (7 mM) or Cd2+ (100 microM) Krebs solution. 3. Inclusion of the hydrolysis-resistant guanine nucleotide GDP-beta-S in the patch solution significantly reduced the orexin A- or B-induced depolarizations. 4. Under whole-cell voltage-clamp conditions, the orexin-induced inward current declined with hyperpolarization, but did not reverse polarity in the potential range between -120 and 0 mV. In low-Na+ solution, the orexin-induced current was reduced, and the I-V curve reversed polarity at about -105 mV; the response was further reduced and the reversal potential shifted to -90 mV in a low-Na+, high-K+ Krebs solution. 5. It is concluded that the peptides orexin A and B, acting on orexin receptors, which are GTP-binding-protein coupled, are excitatory to DMNV neurones. In addition, more than one conductance, which may include a non-selective cation conductance and a K+ conductance, appears to be involved in the orexin-induced depolarization.

D2-like dopamine receptor activation excites rat dorsal raphe 5-HT neurons in vitro

The aim of the present study was to investigate the effect of dopamine (DA) on the excitability of dorsal raphe nucleus (DRN) 5-hydroxytryptamine (5-HT) neurons using the patch-clamp technique in brain slices. Bath application of DA (1-300 microM) produced a concentration-dependent membrane depolarization in all 5-HT neurons examined. This effect persisted in the presence of tetrodotoxin (TTX; 1 microM) and low extracellular calcium. Moreover, blockade of ionotropic glutamate receptors with 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 2-amino-5-phosphonopentanoic acid (AP5) did not prevent DA-induced depolarization, indicating that it was mediated by a direct effect of DA on 5-HT neurons. The DA-induced depolarization was not antagonized by selective alpha1-adrenergic receptor antagonists, prazosin and WB 4101, but by a nonselective DA receptor antagonist, haloperidol. In addition, the selective D2-like receptor agonist quinpirole and antagonist sulpiride mimicked and blocked DA-induced depolarization, respectively. These results indicate that DA-induced membrane depolarization in DRN 5-HT neurons is mediated by the activation of D2-like DA receptors. The DA-induced membrane depolarization and inward current were associated with an increase in membrane conductance. Examination of the current-voltage (I-V) relationship for the DA-induced inward current revealed that the amplitude of the current increased with membrane hyperpolarization and reversed polarity at a potential near -15 mV. These data suggest that DA-induced depolarization in DRN 5-HT neurons is not mediated by a decrease in potassium conductance, but most likely by the activation of a nonselective cation current.

Muscarinic modulation of GABAergic transmission to neurons in the rat subfornical organ

Cholinergic actions on subfornical organ (SFO) neurons in rat slice preparations were studied by using whole cell voltage- and current-clamp recordings. In the voltage-clamp recordings, carbachol and muscarine decreased the frequency of GABAergic inhibitory postsynaptic currents (IPSCs) in a dose-dependent manner, with no effect on the amplitudes or the time constants of miniature IPSCs. Meanwhile, carbachol did not influence the amplitude of the outward currents induced by GABA. Furthermore, carbachol and muscarine also elicited inward currents in a TTX-containing solution. From the current-voltage relationship, the reversal potential was estimated to be −7.1 mV. These carbachol-induced responses were antagonized by atropine. In the current-clamp recordings, carbachol depolarized the membrane with increased frequency of action potentials. These observations suggest that acetylcholine suppresses GABA release through muscarinic receptors located on the presynaptic terminals. Acetylcholine also directly affects the postsynaptic membrane through muscarinic receptors, by opening nonselective cation channels. A combination of these presynaptic and postsynaptic actions may enhance activation of SFO neurons by acetylcholine.

Acetylcholine modulates respiratory pattern: effects mediated by M3-like receptors in preBötzinger complex inspiratory neurons

Perturbations of cholinergic neurotransmission in the brain stem affect respiratory motor pattern both in vivo and in vitro; the underlying cellular mechanisms are unclear. Using a medullary slice preparation from neonatal rat that spontaneously generates respiratory rhythm, we patch-clamped inspiratory neurons in the preBötzinger complex (preBötC), the hypothesized site for respiratory rhythm generation, and simultaneously recorded respiratory-related motor output from the hypoglossal nerve (XIIn). Most (88%) of the inspiratory neurons tested responded to local application of acetylcholine (ACh) or carbachol (CCh) or bath application of muscarine. Bath application of 50 microM muscarine increased the frequency, amplitude, and duration of XIIn inspiratory bursts. At the cellular level, muscarine induced a tonic inward current, increased the duration, and decreased the amplitude of the phasic inspiratory inward currents in preBötC inspiratory neurons recorded under voltage clamp at -60 mV. Muscarine also induced seizure-like activity evident during expiratory periods in XIIn activity; these effects were blocked by atropine. In the presence of tetrodotoxin (TTX), local ejection of 2 mM CCh or ACh onto preBötC inspiratory neurons induced an inward current along with an increase in membrane conductance under voltage clamp and induced a depolarization under current clamp. This response was blocked by atropine in a concentration-dependent manner. Bath application of 1 microM pirenzepine, 10 microM gallamine, or 10 microM himbacine had little effect on the CCh-induced current, whereas 10 microM 4-diphenylacetoxy-N-methylpiperidine methiodide blocked the current. The current-voltage (I-V) relationship of the CCh-induced response was linear in the range of -110 to -20 mV and reversed at -11.4 mV. Similar responses were found in both pacemaker and nonpacemaker inspiratory neurons. The response to CCh was unaffected when patch electrodes contained a high concentration of EGTA (11 mM) or bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (10 mM). The response to CCh was reduced greatly by substitution of 128 mM Tris-Cl for NaCl in the bath solution; the I-V curve shifted to the left and the reversal potential shifted to -47 mV. Lowering extracellular Cl(-) concentration from 140 to 70 mM had no effect on the reversal potential. These results suggest that in preBötC inspiratory neurons, ACh acts on M3-like ACh receptors on the postsynaptic neurons to open a channel permeable to Na(+) and K(+) that is not Ca(2+) dependent. This inward cation current plays a major role in depolarizing preBötC inspiratory neurons, including pacemakers, that may account for the ACh-induced increase in the frequency of respiratory motor output observed at the systems/behavioral level.

Locus coeruleus neurones in vitro: pH-sensitive oscillations of membrane potential in an electrically coupled network

The response to hypercapnic acidosis (2-8% CO2, bath pH 7.8-7.2) was examined in whole cell recordings from neonatal (P1 to P5) rat Locus coeruleus (LC) neurones in the in vitro brainstem-spinal cord preparation exposed to low Ca2+ (0.2 mM)-high Mg2+ (5 mM). This medium suppressed chemical synaptic transmission and resulted in a pattern of subthreshold oscillations of membrane potential and rhythmic burst discharge which was synchronized throughout the network. The oscillation was suppressed, and the discharge of individual neurones desynchronized, by the gap junction uncoupler, carbenoxolone, indicating that in low Ca2+-high Mg2+ LC neurones form an electrically coupled network. Switching from 2 to 8% CO2 decreased the oscillation amplitude and increased its frequency. The oscillation was suppressed by external Cd2+ and by TTX. but persisted during injection into the cell soma of QX-314. We conclude that in LC neurones acidosis increases the frequency of a Ca2+- and Na+-dependent dendritic oscillator which is synchronized by gap junction coupling throughout the network. This coupling is retained during acidosis.

Muscarinic receptor activity has multiple effects on the resting membrane potentials of CA1 hippocampal interneurons

Inhibitory interneurons appear to be an important target for the muscarinic actions of cholinergic inputs to the hippocampus. We investigated the effect of muscarinic receptor activity on the membrane potential (V(m)) and currents of rat hippocampal CA1 interneurons using whole-cell recording from visually identified CA1 interneurons. The predominant response observed was a muscarinic depolarization that was detected in interneurons from all layers of CA1. This depolarization was mediated by at least two mechanisms: a reduction in a potassium current and a mechanism that depended on extracellular sodium. Other interneurons responded to muscarinic agonists with a hyperpolarization or a biphasic response (hyperpolarization followed by depolarization). Hyperpolarizations and biphasic responses were found in all layers of CA1 but more frequently in stratum radiatum and stratum lacunosum moleculare. Muscarinic hyperpolarization was caused by the activation of a barium- and cesium-sensitive inwardly rectifying potassium channel. A small number of interneurons, primarily in or bordering the stratum pyramidale, produced slow membrane potential (0.04 Hz) oscillations. Many interneurons did not respond to muscarinic activity at all; half of these were in the stratum oriens. There was no strong correlation between any changes in V(m) response to muscarine and morphology, as determined by reconstruction of the interneurons. It was not possible to predict the morphology or the layer distribution of an interneuron based on the type of muscarinic membrane potential response it had. This lack of correlation between muscarinic function and morphology implies a greater complexity of interneuron function than has been realized previously.

Muscarinic receptor activity has multiple effects on the resting membrane potentials of CA1 hippocampal interneurons

Inhibitory interneurons appear to be an important target for the muscarinic actions of cholinergic inputs to the hippocampus. We investigated the effect of muscarinic receptor activity on the membrane potential (V(m)) and currents of rat hippocampal CA1 interneurons using whole-cell recording from visually identified CA1 interneurons. The predominant response observed was a muscarinic depolarization that was detected in interneurons from all layers of CA1. This depolarization was mediated by at least two mechanisms: a reduction in a potassium current and a mechanism that depended on extracellular sodium. Other interneurons responded to muscarinic agonists with a hyperpolarization or a biphasic response (hyperpolarization followed by depolarization). Hyperpolarizations and biphasic responses were found in all layers of CA1 but more frequently in stratum radiatum and stratum lacunosum moleculare. Muscarinic hyperpolarization was caused by the activation of a barium- and cesium-sensitive inwardly rectifying potassium channel. A small number of interneurons, primarily in or bordering the stratum pyramidale, produced slow membrane potential (0.04 Hz) oscillations. Many interneurons did not respond to muscarinic activity at all; half of these were in the stratum oriens. There was no strong correlation between any changes in V(m) response to muscarine and morphology, as determined by reconstruction of the interneurons. It was not possible to predict the morphology or the layer distribution of an interneuron based on the type of muscarinic membrane potential response it had. This lack of correlation between muscarinic function and morphology implies a greater complexity of interneuron function than has been realized previously.

Ionic mechanisms underlying excitatory effects of serotonin on embryonic rat motoneurons in long-term culture

The actions of serotonin were investigated on motoneurons isolated from embryonic day 14 rat spinal cord and enriched by metrizamide density gradient centrifugation. Trophic support was provided by a spinal cord glial monolayer, ciliary neurotrophic factor and heat-inactivated serum. Cultures were maintained for 17-83 days and investigated using whole-cell patch-clamp recording. Serotonin evoked slow depolarizations (6.2+/-0.7 or 9.3+/-1.3 mV in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione and strychnine, EC50 8.2 nM), which were reversibly blocked by 0.1 microM ketanserin. Serotonin generated synaptic potentials in motoneurons, lowered the threshold for repetitive firing and changed the slope of the current intensity-firing frequency relationship. The inward current evoked by serotonin (-147+/-15.2 pA) was ascribed to a complex ionic mechanism, which varied amongst neurons in the sampled population. It was due to closure of barium-sensitive potassium channels, effects on Ih and increase in a separate mixed cation current which comprised both transient voltage-sensitive and sustained components. We conclude that serotonergic responses develop in motoneurons cultured under these conditions in the absence of serotonergic input, sensory neurons or many interneurons.

5-HT modulates multiple conductances in immature rat rostral ventrolateral medulla neurones in vitro

1. Whole-cell patch-clamp recordings were made from rostral ventrolateral medulla (RVLM) neurones of brainstem slices from 8- to 12-day-old rats. In the presence of tetrodotoxin (0.5 microM), 5-HT (50 microM) elicited an outward current (I5-HT,outward) (10/44 neurones) associated with an increase in membrane conductance, and an inward current (I5-HT,inward) (29/44 neurones) accompanied by a decrease or no significant change in membrane conductance. 2. The steady-state I-V relationship of I5-HT,outward showed an inward rectification; the 5-HT-induced current, which reversed at -87.9 +/- 3.0 mV, was suppressed by 0.1 mM Ba2+. 3. Two types of steady-state I-V relationship for I5-HT,inward were noted: type I I5-HT,inward was characterized by a significant decrease in membrane conductance and reversed at a potential close to or negative to the theoretical K+ equilibrium potential (EK), -94 mV, in 8/17 neurones; type II I5-HT,inward was not associated with a significant change in membrane conductance and was relatively independent of membrane potential. 4. Both type I and type II I5-HT,inward were significantly reduced in a low [Na+]o solution. In this solution, I5-HT,inward decreased with hyperpolarization and had a linear steady-state I-V relationship with a reversal potential of approximately -110 mV. The reversal potential of type I I5-HT,inward shifted to about -80 mV as the [K+]o was increased from 3.1 to 7.0 mM in low [Na+]o solution. The type II I5-HT,inward did not reverse at the estimated EK in the same solution. 5. While not affected by externally applied Cs+ (1 mM), I5-HT,inward was significantly smaller in RVLM neurones patched with Cs+-containing electrodes; the current reversed at -11.9 +/- 6.4 mV in 8/15 responsive neurones. 6. It may be concluded that in rat RVLM neurones 5-HT increases an inwardly rectifying K+ conductance which may underlie the I5-HT, outward and that a combination of varying degrees of K+ conductance decrease and a Cs+-insensitive, non-selective cation conductance increase may account for the two types of conductance change associated with I5-HT,inward.

Cholinergic modulation of neostriatal output: a functional antagonism between different types of muscarinic receptors

It is demonstrated that acetylcholine released from cholinergic interneurons modulates the excitability of neostriatal projection neurons. Physostigmine and neostigmine increase input resistance (RN) and enhance evoked discharge of spiny projection neurons in a manner similar to muscarine. Muscarinic RN increase occurs in the whole subthreshold voltage range (-100 to -45 mV), remains in the presence of TTX and Cd2+, and can be blocked by the relatively selective M1,4 muscarinic receptor antagonist pirenzepine but not by M2 or M3 selective antagonists. Cs+ occludes muscarinic effects at potentials more negative than -80 mV. A Na+ reduction in the bath occludes muscarinic effects at potentials more positive than -70 mV. Thus, muscarinic effects involve different ionic conductances: inward rectifying and cationic. The relatively selective M2 receptor antagonist AF-DX 116 does not block muscarinic effects on the projection neuron but, surprisingly, has the ability to mimic agonistic actions increasing RN and firing. Both effects are blocked by pirenzepine. HPLC measurements of acetylcholine demonstrate that AF-DX 116 but not pirenzepine greatly increases endogenous acetylcholine release in brain slices. Therefore, the effects of the M2 antagonist on the projection neurons were attributable to autoreceptor block on cholinergic interneurons. These experiments show distinct opposite functions of muscarinic M1- and M2-type receptors in neostriatal output, i.e., the firing of projection neurons. The results suggest that the use of more selective antimuscarinics may be more profitable for the treatment of motor deficits.

Cholinergic modulation of neostriatal output: a functional antagonism between different types of muscarinic receptors

It is demonstrated that acetylcholine released from cholinergic interneurons modulates the excitability of neostriatal projection neurons. Physostigmine and neostigmine increase input resistance (RN) and enhance evoked discharge of spiny projection neurons in a manner similar to muscarine. Muscarinic RN increase occurs in the whole subthreshold voltage range (-100 to -45 mV), remains in the presence of TTX and Cd2+, and can be blocked by the relatively selective M1,4 muscarinic receptor antagonist pirenzepine but not by M2 or M3 selective antagonists. Cs+ occludes muscarinic effects at potentials more negative than -80 mV. A Na+ reduction in the bath occludes muscarinic effects at potentials more positive than -70 mV. Thus, muscarinic effects involve different ionic conductances: inward rectifying and cationic. The relatively selective M2 receptor antagonist AF-DX 116 does not block muscarinic effects on the projection neuron but, surprisingly, has the ability to mimic agonistic actions increasing RN and firing. Both effects are blocked by pirenzepine. HPLC measurements of acetylcholine demonstrate that AF-DX 116 but not pirenzepine greatly increases endogenous acetylcholine release in brain slices. Therefore, the effects of the M2 antagonist on the projection neurons were attributable to autoreceptor block on cholinergic interneurons. These experiments show distinct opposite functions of muscarinic M1- and M2-type receptors in neostriatal output, i.e., the firing of projection neurons. The results suggest that the use of more selective antimuscarinics may be more profitable for the treatment of motor deficits.

Muscarinic activation of a non-selective cationic conductance in pyramidal neurons in rat basolateral amygdala

In the present study, a cationic membrane conductance activated by the acetylcholine agonist carbachol was characterized in vitro in neurons of the basolateral amygdala. Extracellular perfusion of the K+ channel blockers Ba2+ and Cs+ or loading of cells with cesium acetate did not affect the carbachol-induced depolarization. Similarly, superfusion with low-Ca2+ solution plus Ba2+ and intracellular EGTA did not affect the carbachol-induced depolarization, suggesting a Ca2+-independent mechanism. On the other hand, the carbachol-induced depolarization was highly sensitive to changes in extracellular K+ or Na+. When the K+ concentration in the perfusion medium was increased from 4.7 to 10 mM, the response to carbachol increased in amplitude. In contrast, lowering the extracellular Na+ concentration from 143.2 to 29 mM abolished the response in a reversible manner. Results of coapplication of carbachol and atropine, pirenzepine or gallamine indicate that the carbachol-induced depolarization was mediated by muscarinic cholinergic receptors, but not the muscarinic receptor subtypes M1, M2 or M4, specifically. These data indicate that, in addition to the previously described reduction of a time- and voltage-independent K+ current (IKleak), a voltage- and time-dependent K+ current (IM), a slow Ca2+-activated K+ current (sIahp) and the activation of a hyperpolarization-activated inward rectifier K+ current (IQ), carbachol activated a Ca2+-independent non-selective cationic conductance that was highly sensitive to extracellular K+ and Na+ concentrations.

Molecular and pharmacological analysis of cyclic nucleotide-gated channel function in the central nervous system

Most functional studies of cyclic nucleotide-gated (CNG) channels have been confined to photoreceptors and olfactory epithelium, in which CNG channels are abundant and easy to study. The widespread distribution of CNG channels in tissues throughout the body has only recently been recognized and the functions of this channel family in many of these tissues remain largely unknown. The molecular biological and pharmacological properties of the CNG channel family are summarized in order to put in context studies aimed at probing CNG channel functions in these tissues using pharmacological and genetic methods. Compounds have now been identified that are useful in distinguishing CNG channel activated pathways from cAMP/cGMP dependent-protein kinases or other pathways. The ways in which these interact with CNG channels are understood and this knowledge is leading to the identification of more potent and more specific CNG channel subtype-specific agonists or antagonists. Recent molecular and genetic analyses have identified novel roles of CNG channels in neuronal development and plasticity in both invertebrates and vertebrates. Targeting CNG channels via specific drugs and genetic manipulation (such as knockout mice) will permit better understanding of the role of CNG channels in both basic and higher orders of brain function.

Vasopressin-induced currents in rat neonatal spinal lateral horn neurons are G-protein mediated and involve two conductances

Arginine vasopressin (AVP) receptors are expressed early in the developing spinal cord. To characterize AVP-induced conductances in lower thoracic sympathetic preganglionic (SPN) and other lateral horn neurons, we used patch-clamp recording techniques in neonatal (11-21 days) rat spinal cord slices. Most (90%) of 273 neurons, including all 68 SPNs, responded to AVP with membrane depolarization and/or a V1 receptor-mediated, dose-dependent (0.01-1.0 microM) and tetrodotoxin (TTX)-resistant inward current. A role for G-proteins was indicated by persistence of this inward current after intracellular dialysis with GTP-gamma-S or GMP-PNP, its marked reduction with GDP-beta-S, and significant reduction, but not abolition, after preincubation with pertussis toxin or in the presence of N-ethylmaleimide. Analysis of individual current-voltage (I-V) relationships in 57 cells indicated the presence of two different membrane conductances. In 21 cells, net AVP-induced currents reversed around -103 mV, reflecting reduction in one or more barium-sensitive potassium conductances; in 12 cells, net AVP-induced current reversed around -40 mV and was not significantly sensitive to several potassium channel blockers including barium, tetraethylammonium chloride (TEA), 4-aminopyridine (4AP), cesium, or glibenclamide, suggesting increase in a nonselective cationic conductance that was separate from Ih; in 24 cells where I-V lines shifted in parallel, AVP-induced inward currents were significantly greater and probably involved both conductances. These data indicate that SPNs and a majority of unidentified neonatal lateral horn neurons possess functional G-protein-coupled V1-type vasopressin receptors. The wide distribution of AVP receptors in neonatal spinal lateral column cells suggests a role that may extend beyond involvement in regulation of autonomic nervous system function.

Characterization of carbachol-induced rhythmic bursting discharges in neurons from guinea pig lateral septum slices

A brain slice preparation from guinea pigs and intracellular recording techniques were used to examine the effects of carbachol application on the three classes (A, B, and C) of neurons (n = 68, 40 of class A, 12 of class B, 16 of class C) within the mediolateral part of the lateral septum (LSml). Bath application of carbachol elicited a sustained depolarization associated with an increase in membrane input resistance, action-potential firing and triggered rhythmic bursting discharges in 59% of recorded neurons. According to the configuration of these bursts, LSml neurons were classified into type I, II, and III neurons with reference to their response to carbachol. The frequency of spontaneous bursts was increased by depolarization caused by applied DC current in the three types of neurons. Bursts in type II and III neurons were voltage and dose dependent. These dependences were responsible for a continuum of variation in carbachol responses in these two types of neurons. As the neuron depolarized in the presence of carbachol, spontaneous action potentials increased in frequency and slow afterdepolarizing potentials (sADPs) appeared and preceded the occurrence of the first burst. These sADPs from adjacent action potentials appeared to progressively increase to initiate a burst. In the presence of carbachol, sADPs and bursts were also observable after action potentials evoked by depolarizing current pulses at the resting membrane potential (RMP) in LSml neurons. Evoked sADPs and bursts were associated with an apparent increase in input conductance. Application of low Na+ medium blocked both the sADP and bursts. Application of zero Ca2+ medium either 1) blocked completely the generation of sADPs and bursts (n = 16), or 2) did not block bursts (n = 14). Evoked sADPs and bursts were blocked by tetraethylammonium but were resistant to external Cs+. The results indicate that the activation of cholinergic receptors does not differentially affect the three classes of LSml neurons. The responses to carbachol in type II and III neurons form a continuum of variation, whereas these of type I neurons constitute a discrete entity. The selective cholinergic induction of a sADP, and the progressive activation of these sADPs in LSml neurons are thought to be responsible for the onset of the three types of rhythmic bursting discharges. We propose that sADPs and bursts induced by carbachol are generated by a cationic conductance largely permeable to Na+. In a subpopulation of LSml neurons (n = 16), the bursts are dependent on the presence of Ca2+ in the medium.