Muscarinic modulation of TREK currents in mouse sympathetic superior cervical ganglion neurons

Muscarinic receptors play a key role in the control of neurotransmission in the autonomic ganglia, which has mainly been ascribed to the regulation of potassium M‐currents and voltage‐dependent calcium currents. Muscarinic agonists provoke depolarization of the membrane potential and a reduction in spike frequency adaptation in postganglionic neurons, effects that may be explained by M‐current inhibition. Here, we report the presence of a riluzole‐activated current (I_RIL) that flows through the TREK‐2 channels, and that is also inhibited by muscarinic agonists in neurons of the mouse superior cervical ganglion (mSCG). The muscarinic agonist oxotremorine‐M (Oxo‐M) inhibited the I_RIL by 50%, an effect that was abolished by pretreatment with atropine or pirenzepine, but was unaffected in the presence of himbacine. Moreover, these antagonists had similar effects on single‐channel TREK‐2 currents. I_RIL inhibition was unaffected by pretreatment with pertussis toxin. The protein kinase C blocker bisindolylmaleimide did not have an effect, and neither did the inositol triphosphate antagonist 2‐aminoethoxydiphenylborane. Nevertheless, the I_RIL was markedly attenuated by the phospholipase C (PLC) inhibitor ET‐18‐OCH3. Finally, the phosphatidylinositol‐3‐kinase/phosphatidylinositol‐4‐kinase inhibitor wortmannin strongly attenuated the I_RIL, whereas blocking phosphatidylinositol 4,5‐bisphosphate (PIP₂) depletion consistently prevented I_RIL inhibition by Oxo‐M. These results demonstrate that TREK‐2 currents in mSCG neurons are inhibited by muscarinic agonists that activate M1 muscarinic receptors, reducing PIP₂ levels via a PLC‐dependent pathway. The similarities between the signaling pathways regulating the I_RIL and the M‐current in the same neurons reflect an important role of this new pathway in the control of autonomic ganglia excitability.     

Development of resurgent and persistent sodium currents in mesencephalic trigeminal neurons

Sodium channels play multiple roles in the formation of neural membrane properties in mesencephalic trigeminal (Mes V) neurons and in other neural systems. Mes V neurons exhibit conditional robust high‐frequency spike discharges. As previously reported, resurgent and persistent sodium currents (I_NaR and I_NaP, respectively) may carry small currents at subthreshold voltages that contribute to generation of spike firing. These currents play an important role in maintaining and allowing high‐frequency spike discharge during a burst. In the present study, we investigated the developmental changes in tetrodotoxin‐sensitive I_NaR and I_NaP underlying high‐frequency spike discharges in Mes V neurons. Whole‐cell patch‐clamp recordings showed that both current densities increased one and a half times from postnatal day (P) 0–6 neurons to P7–14 neurons. Although these neurons do not exhibit subthreshold oscillations or burst discharges with high‐frequency firing, I_NaR and I_NaP do exist in Mes V neurons at P0–6. When the spike frequency at rheobase was examined in firing Mes V neurons, the developmental change in firing frequency among P7–14 neurons was significant. I_NaR and I_NaP density at ?40 mV also increased significantly among P7–14 neurons. The change to an increase in excitability in the P7–14 group could result from this quantitative change in I_NaP. In neurons older than P7 that exhibit repetitive firing, quantitative increases in I_NaR and I_NaP density may be major factors that facilitate and promote high‐frequency firing as a function of age in Mes V neurons.     

Nitric oxide as intracellular modulator: internal production of NO increases neuronal excitability via modulation of several ionic conductances

Nitric oxide (NO) has been shown to regulate neuronal excitability in the nervous system, but little is known as to whether NO, which is synthesized in certain neurons, also serves functional roles within NO‐producing neurons themselves. We investigated this possibility by using a nitric oxide synthase (NOS)‐expressing neuron, and studied the role of intrinsic NO production on neuronal firing properties in single‐cell culture. B5 neurons of the pond snail Helisoma trivolvis fire spontaneous action potentials (APs), but once the intrinsic activity of NOS was inhibited, neurons became hyperpolarized and were unable to fire evoked APs. These striking long‐term effects could be attributed to intrinsic NO acting on three types of conductances, a persistent sodium current (I_NaP), voltage‐gated Ca currents (I_Ca) and small‐conductance calcium‐activated potassium (SK) channels. We show that NOS inhibitors 7‐nitroindazole and S‐methyl‐l ‐thiocitrulline resulted in a decrease in I_NaP, and that their hyperpolarizing and inhibiting effects on spontaneous spiking were mimicked by the inhibitor of I_NaP, riluzole. Moreover, inhibition of NOS, soluble guanylate cyclase (sGC) or protein kinase G (PKG) attenuated I_Ca, and blocked spontaneous and depolarization‐induced spiking, suggesting that intrinsic NO controlled I_Ca via the sGC/PKG pathway. The SK channel inhibitor apamin partially prevented the hyperpolarization observed after inhibition of NOS, suggesting a downregulation of SK channels by intrinsic NO. Taken together, we describe a novel mechanism by which neurons utilize their self‐produced NO as an intrinsic modulator of neuronal excitability. In B5 neurons, intrinsic NO production is necessary to maintain spontaneous tonic and evoked spiking activity.     

Subthreshold delta‐frequency resonance in thalamic reticular neurons

The thalamic reticular nucleus (nRt) is an assembly of GABAergic projection neurons that participate in the generation of brain rhythms during synchronous sleep and absence epilepsy. NRt cells receive inhibitory and excitatory synaptic inputs, and are endowed with an intricate set of intrinsic conductances. However, little is known about how intrinsic and synaptic properties interact to generate rhythmic discharges in these neurons. In order to better understand this interaction, I studied the subthreshold responses of nRt cells to time‐varying inputs. Patch‐clamp recordings were performed in acute slices of rat thalamus (postnatal days 12–21). Sinusoidal current waveforms of linearly changing frequencies were injected into the soma, and the resulting voltage oscillations were recorded. At the resting membrane potential, the impedance profile showed a characteristic resonance at 1.7 Hz. The relative strength of the resonance was 1.2, and increased with membrane hyperpolarization. Small suprathreshold current injections led to preferred spike generation at the resonance frequency. Bath application of ZD7288 or Cs⁺, inhibitors of the hyperpolarization‐activated cation current (I_h), transformed the resonance into low‐pass behaviour, whereas the T‐channel blockers mibefradil and Ni²⁺ decreased the strength of the resonance. It is concluded that nRt cells have an I_h‐mediated intrinsic frequency preference in the subthreshold voltage range that favours action potential generation in the delta‐frequency band.     

Loss of β1 accessory Na+ channel subunits causes failure of carbamazepine,but not of lacosamide,in blocking high‐frequency firing via differential effects on persistent Na+ currents

Purpose: In chronic epilepsy, a substantial proportion of up to 30% of patients remain refractory to antiepileptic drugs (AEDs). An understanding of the mechanisms of pharmacoresistance requires precise knowledge of how AEDs interact with their targets. Many commonly used AEDs act on the transient and/or the persistent components of the voltage‐gated Na⁺ current (I_NaT and I_NaP, respectively). Lacosamide (LCM) is a novel AED with a unique mode of action in that it selectively enhances slow inactivation of fast transient Na⁺ channels. Given that functional loss of accessory Na⁺ channel subunits is a feature of a number of neurologic disorders, including epilepsy, we examined the effects of LCM versus carbamazepine (CBZ) on the persistent Na⁺ current (I_NaP), in the presence and absence of accessory subunits within the channel complex. Methods: Using patch‐clamp recordings in intact hippocampal CA1 neurons of Scn1b null mice, I_NaP was recorded using slow voltage ramps. Application of 100 μm CBZ or 300 μm LCM reduced the maximal I_NaP conductance in both wild‐type and control mice. Key Findings: As shown previously by our group in Scn1b null mice, CBZ induced a paradoxical increase of I_NaP conductance in the subthreshold voltage range, resulting in an ineffective block of repetitive firing in Scn1b null neurons. In contrast, LCM did not exhibit such a paradoxical increase, and accordingly maintained efficacy in blocking repetitive firing in Scn1b null mice. Significance: These results suggest that the novel anticonvulsant LCM maintains activity in the presence of impaired Na⁺ channel β₁ subunit expression and thus may offer an improved efficacy profile compared with CBZ in diseases associated with an impaired expression of β sub‐units as observed in epilepsy.     

Differential contributions of Ca2+‐activated K+ channels and Na+/K+‐ATPases to the generation of the slow afterhyperpolarization in CA1 pyramidal cells

In many types of CNS neurons, repetitive spiking produces a slow afterhyperpolarization (sAHP), providing sustained, intrinsically generated negative feedback to neuronal excitation. Changes in the sAHP have been implicated in learning behaviors, in cognitive decline in aging, and in epileptogenesis. Despite its importance in brain function, the mechanisms generating the sAHP are still controversial. Here we have addressed the roles of M‐type K⁺ current (I_M), Ca²⁺‐gated K⁺ currents (I_Ca(K)'s) and Na⁺/K⁺‐ATPases (NKAs) current to sAHP generation in adult rat CA1 pyramidal cells maintained at near‐physiological temperature (35 °C). No evidence for I_M contribution to the sAHP was found in these neurons. Both I_Ca(K)'s and NKA current contributed to sAHP generation, the latter being the predominant generator of the sAHP, particularly when evoked with short trains of spikes. Of the different NKA isoenzymes, α₁‐NKA played the key role, endowing the sAHP a steep voltage‐dependence. Thus normal and pathological changes in α₁‐NKA expression or function may affect cognitive processes by modulating the inhibitory efficacy of the sAHP.     

In vitro assessment of the effect of methylene blue on voltage-gated sodium channels and action potentials in rat hippocampal CA1 pyramidal neurons

Methylene blue (MB) is a vital dye to allow better visualization and marker of parathyroid glands. The compound causes a toxic encephalopathy in clinical observations and some neuronal adverse effects in experimental studies. Of neurotoxic effects, reduced field excitatory postsynaptic potentials (fEPSPs) in hippocampal slice cultures and apoptosis induced in neurons by MB, suggest that MB may affect electrophysiological properties in neurons. Consequently, studies were undertaken to characterize the effects of MB on voltage-gated sodium currents (I_Na) in hippocampal CA1 neurons. MB was tested at a clinically-relevant concentration (10 μM), of which as a surgical marker of the parathyroid glands, and other concentrations (0.25 μM, 1 μM, and 100 μM). The results showed that MB reduced the amplitude of I_Na at the concentrations of 10 μM and 100 μM. No significant changes were found with the other two concentrations of MB. 10 μM of MB did not produce a shift in the activation–voltage curve of I_Na but produced a hyperpolarizing shift in the inactivation–voltage curve of I_Na and delayed the recovery of I_Na from inactivation. Action potential (AP) properties and the pattern of repetitive firing were examined using whole-cell current-clamp recordings. Peak amplitude, overshoot and maximum velocity of depolarization (V_max) of the evoked single AP decreased in the presence of the 10 μM MB solution. The rate of repetitive firing also decreased. The results suggest MB as a surgical marker of the parathyroid glands, may cause sodium channel inhibition on neurons in the nervous system.     

High sensitivity of spontaneous spike frequency to sodium leak current in a Lymnaea pacemaker neuron

The spontaneous rhythmic firing of action potentials in pacemaker neurons depends on the biophysical properties of voltage‐gated ion channels and background leak currents. The background leak current includes a large K⁺ and a small Na⁺ component. We previously reported that a Na⁺‐leak current via U‐type channels is required to generate spontaneous action potential firing in the identified respiratory pacemaker neuron, RPeD1, in the freshwater pond snail Lymnaea stagnalis. We further investigated the functional significance of the background Na⁺ current in rhythmic spiking of RPeD1 neurons. Whole‐cell patch‐clamp recording and computational modeling approaches were carried out in isolated RPeD1 neurons. The whole‐cell current of the major ion channel components in RPeD1 neurons were characterized, and a conductance‐based computational model of the rhythmic pacemaker activity was simulated with the experimental measurements. We found that the spiking rate is more sensitive to changes in the Na⁺ leak current as compared to the K⁺ leak current, suggesting a robust function of Na⁺ leak current in regulating spontaneous neuronal firing activity. Our study provides new insight into our current understanding of the role of Na⁺ leak current in intrinsic properties of pacemaker neurons.     

Potassium currents and excitability in second-order auditory and vestibular neurons

Potassium channels are involved in the control of neuronal excitability by fixing the membrane potential, shaping the action potential, and setting firing rates. Recently, attention has been focused on identifying the factors influencing excitability in second-order auditory and vestibular neurons. Located in the brainstem, second-order auditory and vestibular neurons are sites for convergence of inputs from first-order auditory or vestibular ganglionic cells with other sensory systems and also motor areas. Typically, second-order auditory neurons exhibit two distinct firing patterns in response to depolarization: tonic, with a repetitive firing of action potentials, and phasic, characterized by only one or a few action potentials. In contrast, all mature vestibular second-order neurons fire tonically on depolarization. Already, certain fundamental roles have emerged for potassium currents in these neurons. In mature auditory and vestibular neurons, I_K, the delayed rectifier, is required for the fast repolarization of action potentials. In tonically firing auditory neurons, I_A, the transient outward rectifier, defines the discharge pattern. I_DS, a delayed rectifier-like current distinguished by its low threshold of activation, is found in phasically firing auditory and some developing vestibular neurons where it limits firing to one or a few spikes, and also may contribute to forming short-duration excitatory postsynaptic potential (EPSPs). Also, I_DS sets the threshold for action potential generation rather high, which may prevent spontaneous discharge in phasically firing cells. During development, there is a gradual acquisition and loss of some potassium conductances, suggesting developmental regulation. As there are similarities in membrane properties of second-order auditory and vestibular neurons, investigations on firing pattern and its underlying mechanisms in one system should help to uncover fundamental properties of the other. J. Neurosci. Res. 53:511–520, 1998. © 1998 Wiley-Liss, Inc.     

Long‐term high‐intensity sound stimulation inhibits h current (Ih) in CA1 pyramidal neurons

Afferent neurotransmission to hippocampal pyramidal cells can lead to long‐term changes to their intrinsic membrane properties and affect many ion currents. One of the most plastic neuronal currents is the hyperpolarization‐activated cationic current (I_h), which changes in CA1 pyramidal cells in response to many types of physiological and pathological processes, including auditory stimulation. Recently, we demonstrated that long‐term potentiation (LTP) in rat hippocampal Schaffer‐CA1 synapses is depressed by high‐intensity sound stimulation. Here, we investigated whether a long‐term high‐intensity sound stimulation could affect intrinsic membrane properties of rat CA1 pyramidal neurons. Our results showed that I_h is depressed by long‐term high‐intensity sound exposure (1 min of 110 dB sound, applied two times per day for 10 days). This resulted in a decreased resting membrane potential, increased membrane input resistance and time constant, and decreased action potential threshold. In addition, CA1 pyramidal neurons from sound‐exposed animals fired more action potentials than neurons from control animals; however, this effect was not caused by a decreased I_h. On the other hand, a single episode (1 min) of 110 dB sound stimulation which also inhibits hippocampal LTP did not affect I_h and firing in pyramidal neurons, suggesting that effects on I_h are long‐term responses to high‐intensity sound exposure. Our results show that prolonged exposure to high‐intensity sound affects intrinsic membrane properties of hippocampal pyramidal neurons, mainly by decreasing the amplitude of I_h.     

tipE regulates Na+-dependent repetitive firing in Drosophila neurons

The tipE gene, originally identified by a temperature-sensitive paralytic mutation in Drosophila, encodes a transmembrane protein that dramatically influences sodium channel expression in Xenopus oocytes. There is evidence that tipE also modulates sodium channel expression in the fly; however, its role in regulating neuronal excitability remains unclear. Here we report that the majority of neurons in both wild-type and tipE mutant (tipE-) embryo cultures fire sodium-dependent action potentials in response to depolarizing current injection. However, the percentage of tipE- neurons capable of firing repetitively during a sustained depolarization is significantly reduced. Expression of a tipE+ transgene, in tipE- neurons, restores repetitive firing to wild-type levels. Analysis of underlying currents reveals a slower rate of repolarization-dependent recovery of voltage-gated sodium currents during repeated activation in tipE- neurons. This phenotype is also rescued by expression of the tipE+ transgene. These data demonstrate that tipE regulates sodium-dependent repetitive firing and recovery of sodium currents during repeated activation. Furthermore, the duration of the interstimulus interval necessary to fire a second full-sized action potential is significantly longer in single- versus multiple-spiking transgenic neurons, suggesting that a slow rate of recovery of sodium currents contributes to the decrease in repetitive firing in tipE- neurons.     

A-type K+ current dominates somatic excitability of delayed firing neurons in rat substantia gelatinosa

Substantia gelatinosa neurons display three main types of intrinsic firing behavior: tonic, adapting, and delayed onset. Here, voltage‐gated currents expressed by delayed firing neurons were studied in nucleated patches obtained in spinal cord slices of 3–5 weeks‐old rats. Inward Na⁺ current was negligible under these conditions and was usually occluded by superposition of much larger outward currents. Two kinds of outward currents were found, an A‐type (K_A) and delayed rectifier (K_DR) potassium currents. K_A activated rapidly (<1.5 ms at >?20 mV) and operated at subthreshold membrane potentials; voltages of steady‐state half‐maximal activation and inactivation were ?38.7 and ?87.2 mV, respectively. Inactivation was biexponential with a dominant fast component (～90%, time constant ～8 ms). K_DR activated more slowly (<8 ms at >?20 mV), half‐maximal activation was ?23.6 mV, and decayed mono‐exponentially with a time constant 70–110 ms. Maximal amplitudes of K_A were almost 10‐times larger than those of K_DR, their respective densities were 8.5 and 0.97 μS μm^?2. Tetraethylammonium, 5 mM, blocked K_DR but not K_A, whereas both currents were depressed by 5 mM 4‐aminopyridine. In current‐clamp recordings, 4‐action potential but not tetraethylammonium abolished firing delay suggesting the causative role of K_A. Thus, the predominance of fast K_A over other somatic currents is a distinctive feature of delayed firing neurons among all other types of substantia gelatinosa neurons and likely explains the appearance of their typical firing delay. Synapse, 2011. © 2010 Wiley‐Liss, Inc.     

Functional characterization of the H-current in SCN neurons in subjective day and night: a whole-cell patch-clamp study in acutely prepared brain slices

Neurons of the rat suprachiasmatic nucleus (SCN) exhibit a circadian rhythm in spontaneous firing rate. In this whole-cell patch-clamp study in slices, we examined the possibility that H-current (I_H) contributes to the spontaneous firing rate of SCN neurons. Most of our experiments were performed during the subjective day, because this is the time epoch during which one would expect the largest excitatory effect of I_H if it were to fluctuate in a circadian rhythm. Current-clamp experiments showed that blockade of I_H by Cs⁺ (1 mM) did not influence the spontaneous firing rate and resting membrane potential. Voltage-clamp experiments revealed that I_H, when activated at the resting membrane potential, is probably too small in magnitude and too slow in activation to make a significant contribution to the spontaneous firing rate. Both results suggest that I_H does not significantly contribute to the spontaneous firing of SCN neurons. In addition, we investigated whether the kinetics and voltage dependence of I_H were modulated in a circadian manner. However, no substantial day–night differences in I_H were found. We conclude that I_H, as recorded in whole-cell mode, does not contribute significantly to spontaneous firing in most SCN neurons and that this current, is more likely to be involved in `rescuing' SCN neurons from large and long-lasting hyperpolarizations by depolarizing the membrane.     

Electrophysiological properties and their modulation by norepinephrine in the ambiguus neurons of the guinea pig

Electrophysiological properties of guinea pig ambiguus (AMB) neurons were studied in a brainstem slice preparation. During subthreshold depolarization AMB neurons displayed an early slow depolarization and a late outward rectification both of which were blocked by replacing Ca²⁺ with Co²⁺ in the extracellular solution. AMB neurons showed hyperpolarizing inward rectification which was blocked by extracellular Cs⁺ and is likely caused by the activation of Ih. In 58% (n = 49) of AMB neurons spike firing was restricted to the early phase of a long-lasting depolarizing current injection (phasic firing). The remaining AMB neurons showed repetitive firing throughout the depolarization (tonic firing). A Ca²⁺-mediated K⁺ current (I_K(Ca)) caused an afterhyperpolarization that followed both single and repetitive spike firing. I_K(Ca) also controlled the firing pattern in both types of firing, especially in the phasic firing. Norepinephrine (NE) blocked both the hyperpolarizing inward rectification and the Ca²⁺-dependent AHP. These effects of NE were antagonized by propranolol. It is proposed that the blockade of I_K(Ca) and Ih contribute to the improvement of the ‘signal-to-noise ratio’ by NE in AMB neurons.     

Interaction between Low Voltage-activated Currents in Reticular Thalamic Neurons in a Rat Model of Absence Epilepsy

A transient potassium (K⁺) outward current (I_A) contributes to the distinctive patterns of low-threshold spike firing observed in various classes of thalamic neurons through a functional interaction with a calcium (Ca²⁺)-mediated inward current (I_T). The present study was undertaken to investigate the properties of transient K⁺ currents and their interaction with I_T in neurons of the reticular thalamic nucleus, and to compare these properties in reticular thalamic nucleus neurons from a rat model of absence epilepsy, designated the Genetic Absence Epilepsy Rat from Strasbourg (GAERS), with those from a Non-epileptic Control strain (NEC). This comparative approach appeared to be particularly important in view of the recent finding of a selective increase in I_T in reticular thalamic nucleus neurons from GAERS. Neurons were acutely isolated from the reticular thalamic nucleus through enzymatic procedures, and identified by morphological and immunocytochemical criteria. Ionic currents were analysed using whole-cell patch-clamp techniques. Transient K⁺ currents in reticular thalamic nucleus neurons with properties indicative of I_A activated at ～?55 mV (with half-activation at ?27 and ?33 mV in NEC and GAERS respectively), declined rapidly with a voltage-dependent time constant (τ= 4 ms at +45 mV), were 50% steady-state-inactivated at ?81 and ?86 mV in the two strains of rats respectively, and rapidly recovered from inactivation with a monoexponential time course (τ= 31 and 37 ms respectively). No significant differences in I_A properties or densities were found between reticular thalamic nucleus neurons from GAERS and NEC rats. Analysis of the interaction between I_A and I_T indicated a shift in the balance between the two opposing membrane conductances towards the generation of a low-voltage-activated inward current in reticular thalamic nucleus neurons from GAERS compared with NEC, and a lack of I_A to functionally compensate for this shift, which in turn may contribute to pathological forms of low-threshold spike firing characterizing spike-and-wave discharges.     

The mechanism of spontaneous firing in histamine neurons.

Histaminergic neurons project to virtually the whole central nervous system and display regular firing related to behavioral state. Electrophysiological studies of histaminergic neurons show that these neurons fire in a beating pacemaker pattern, which is intrinsic to individual neurons. Onset of an action potential occurs as the result of a slow depolarizing potential, which consists of voltage dependent calcium current(s) and non-inactivating sodium current. The calcium component is a voltage-dependent current activated by the return to threshold following the afterhyperpolarization (AHP) while the sodium current appears to be persistent. The action potential is followed by an AHP, which limits firing rate. The AHP is due to two potassium currents, one voltage-, the other calcium-dependent; it determines the amount of voltage-dependent currents available for activation. We show original results indicating that calcium current can be activated during AHP-like ramps and that the amount of calcium current near threshold is strongly dependent on the membrane potential and on the size of the AHP. The amount of calcium entering during the action potential will determine the duration of the AHP and thus, the firing rate.     

The hyperpolarization‐activated non‐specific cation current (Ih) adjusts the membrane properties,excitability, and activity pattern of the giant cells in the rat dorsal cochlear nucleus

Giant cells of the cochlear nucleus are thought to integrate multimodal sensory inputs and participate in monaural sound source localization. Our aim was to explore the significance of a hyperpolarization‐activated current in determining the activity of giant neurones in slices prepared from 10 to 14‐day‐old rats. When subjected to hyperpolarizing stimuli, giant cells produced a 4‐(N‐ethyl‐N‐phenylamino)‐1,2‐dimethyl‐6‐(methylamino) pyridinium chloride (ZD7288)‐sensitive inward current with a reversal potential and half‐activation voltage of –36 and –88 mV, respectively. Consequently, the current was identified as the hyperpolarization‐activated non‐specific cationic current (I_h). At the resting membrane potential, 3.5% of the maximum I_h conductance was available. Immunohistochemistry experiments suggested that hyperpolarization‐activated, cyclic nucleotide‐gated, cation non‐selective (HCN)1, HCN2, and HCN4 subunits contribute to the assembly of the functional channels. Inhibition of I_h hyperpolarized the membrane by 6 mV and impeded spontaneous firing. The frequencies of spontaneous inhibitory and excitatory postsynaptic currents reaching the giant cell bodies were reduced but no significant change was observed when evoked postsynaptic currents were recorded. Giant cells are affected by biphasic postsynaptic currents consisting of an excitatory and a subsequent inhibitory component. Inhibition of I_h reduced the frequency of these biphasic events by 65% and increased the decay time constants of the inhibitory component. We conclude that I_h adjusts the resting membrane potential, contributes to spontaneous action potential firing, and may participate in the dendritic integration of the synaptic inputs of the giant neurones. Because its amplitude was higher in young than in adult rats, I_h of the giant cells may be especially important during the postnatal maturation of the auditory system.     

Circadian regulation of mouse suprachiasmatic nuclei neuronal states shapes responses to orexin

Our knowledge of how circadian and homeostatic brain circuits interact to temporally organize physiology and behavior is limited. Progress has been made with the determination that lateral hypothalamic orexin (OXA) neurons control arousal and appetitive states, while suprachiasmatic nuclei (SCN) neurons function as the master circadian clock. During the day, SCN neurons exhibit heterogeneity in spontaneous resting membrane potential (RMP), with some neurons becoming severely depolarized (hyperexcited) and ceasing to fire action potentials (APs), while other neurons rest at moderate RMP and fire APs. Intriguingly, the day phase is when the SCN clock is most readily influenced by arousal, but it is unclear if and how heterogeneity in the excitability state of SCN neurons shapes their response to arousal signals, such as OXA. In whole‐cell recordings we show that during the day OXA recruits GABA‐GABA_A receptor signaling to suppress the RMP of hyperexcited silent as well as moderately hyperpolarized AP‐firing SCN neurons. In the AP‐firing neurons, OXA hyperpolarized and silenced these SCN cells, while in the hyperexcited silent neurons OXA suppressed the RMP of these cells and evoked either AP‐firing, depolarized low‐amplitude membrane oscillations, or continued silence at a reduced RMP. These results demonstrate how the resting state of SCN neurons determines their response to OXA, and illustrate that the inhibitory action of this neurochemical correlate of arousal can trigger paradoxical AP firing.     

Effects of hypertonia on voltage-gated ion currents in freshly isolated hippocampal neurons, and on synaptic currents in neurons in hippocampal slices

We studied the effects of hypertonia on voltage-gated currents of freshly isolated hippocampal CA1 neurons, using open pipette whole-cell as well as gramicidin-perforated patch-clamp recording. Extracellular osmolarity (π_o) was raised by adding mannitol (50 or 100 mmol/l) to the bathing solution. Hypertonia depressed voltage-gated sodium, potassium and calcium currents in all trials. The threshold activation voltage of the currents did not change during hypertonic depression, but maximal activation of Ca²⁺ current shifted to a more negative potential, suggesting stronger depression of high- compared to low-voltage activated currents. During 30 min high π_o treatment (recorded with open pipette), the depression reached maximum in 10–15 min of exposure. The depression of the computed transient component of the K⁺ current recorded by open pipette was statistically not significant. Following hypertonic treatment recovery of the I_Na, the sustained I_K and sustained I_Ca were incomplete compared to control cells maintained in normal solution for an equal length of time. In hippocampal tissue slices hypertonia (+25, +50 and +100 mmol/l fructose) reversibly depressed excitatory postsynaptic currents (EPSCs). We conclude that the shutdown of membrane ion currents by elevated π_o is not selective, but the degree of the suppression varies among current types. Raising π_o in human patients, possibly combined with mild artificial acidosis, may be useful in the prevention and treatment of acute crises associated with excessive excitation or depolarization of neurons.     

Cocaine disinhibits dopamine neurons in the ventral tegmental area via use‐dependent blockade of GABA neuron voltage‐sensitive sodium channels

The aim of this study was to evaluate the effects of cocaine on γ‐aminobutyric acid (GABA) and dopamine (DA) neurons in the ventral tegmental area (VTA). Utilizing single‐unit recordings in vivo, microelectrophoretic administration of DA enhanced the firing rate of VTA GABA neurons via D2/D3 DA receptor activation. Lower doses of intravenous cocaine (0.25–0.5 mg/kg), or the DA transporter (DAT) blocker methamphetamine, enhanced VTA GABA neuron firing rate via D2/D3 receptor activation. Higher doses of cocaine (1.0–2.0 mg/kg) inhibited their firing rate, which was not sensitive to the D2/D3 antagonist eticlopride. The voltage‐sensitive sodium channel (VSSC) blocker lidocaine inhibited the firing rate of VTA GABA neurons at all doses tested (0.25–2.0 mg/kg). Cocaine or lidocaine reduced VTA GABA neuron spike discharges induced by stimulation of the internal capsule (ICPSDs) at dose levels 0.25–2 mg/kg (IC₅₀ 1.2 mg/kg). There was no effect of DA or methamphetamine on ICPSDs, or of DA antagonists on cocaine inhibition of ICPSDs. In VTA GABA neurons in vitro, cocaine reduced (IC₅₀ 13 μm ) current‐evoked spikes and TTX‐sensitive sodium currents in a use‐dependent manner. In VTA DA neurons, cocaine reduced IPSCs (IC₅₀ 13 μm ), increased IPSC paired‐pulse facilitation and decreased spontaneous IPSC frequency, without affecting miniature IPSC frequency or amplitude. These findings suggest that cocaine acts on GABA neurons to reduce activity‐dependent GABA release on DA neurons in the VTA, and that cocaine’s use‐dependent blockade of VTA GABA neuron VSSCs may synergize with its DAT inhibiting properties to enhance mesolimbic DA transmission implicated in cocaine reinforcement.