The firing patterns of spinal neurons: in situ patch‐clamp recordings reveal a key role for potassium currents

Neuron firing patterns underpin the detection and processing of stimuli, influence synaptic interactions, and contribute to the function of networks. To understand how intrinsic membrane properties determine firing patterns, we investigated the biophysical basis of single and repetitive firing in spinal neurons of hatchling Xenopus laevis tadpoles, a well‐understood vertebrate model; experiments were conducted in situ. Primary sensory Rohon–Beard (RB) neurons fire singly in response to depolarising current, and dorsolateral (DL) interneurons fire repetitively. RB neurons exhibited a large tetrodotoxin‐sensitive sodium current; in DL neurons, the sodium current density was significantly lower. High‐voltage‐activated calcium currents were similar in both neuron types. There was no evidence of persistent sodium currents, low‐voltage‐activated calcium currents, or hyperpolarisation‐activated currents. In RB neurons, the potassium current was dominated by a tetraethylammonium‐sensitive slow component (I_Ks); a fast component (I_Kf), sensitive to 4‐aminopyridine, predominated in DL neurons. Sequential current‐clamp and voltage‐clamp recordings in individual neurons suggest that high densities of I_Ks prevent repetitive firing; where I_Ks is small, I_Kf density determines the frequency of repetitive firing. Intermediate densities of I_Ks and I_Kf allow neurons to fire a few additional spikes on strong depolarisation; this property typifies a novel subset of RB neurons, and may activate escape responses. We discuss how this ensemble of currents and firing patterns underpins the operation of the Xenopus locomotor network, and suggest how simple mechanisms might underlie the similar firing patterns seen in the neurons of diverse species.     

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.     

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.     

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.     

Resonance properties of different neuronal populations in the immature mouse neocortex

In vivo recordings in the immature neocortex revealed spontaneous and sensory‐driven oscillatory activity from delta (0.5–4 Hz) to gamma (30–100 Hz) frequencies. In order to investigate whether the resonance properties of distinct neuronal populations in the immature neocortex contribute to these network oscillations, we performed whole‐cell patch‐clamp recordings from visually identified neurons in tangential and coronal neocortical slices from postnatal day (P)0–P7 C57Bl/6 mice. Subthreshold resonance was analysed by sinusoidal current injection of varying frequency. All Cajal–Retzius cells showed subthreshold resonance, with an average frequency of 2.6 ± 0.1 Hz (n = 60), which was massively reduced by ZD7288, a blocker of hyperpolarization‐activated cation currents. Approximately 65.6% (n = 61) of the supragranular pyramidal neurons showed subthreshold resonance, with an average frequency of 1.4 ± 0.1 Hz (n = 40). Application of Ni²⁺ suppressed subthreshold resonance, suggesting that low‐threshold calcium currents contribute to resonance in these neurons. Approximately 63.6% (n = 77) of the layer V pyramidal neurons showed subthreshold resonance, with an average frequency of 1.4 ± 0.2 Hz (n = 49), which was abolished by ZD7288. Only 44.1% (n = 59) of the subplate neurons showed subthreshold resonance, with an average frequency of 1.3 ± 0.2 Hz (n = 26) and a small resonance strength. In summary, these results demonstrate that neurons in all investigated layers show resonance behavior, with either hyperpolarization‐activated cation or low‐threshold calcium currents contributing to the subthreshold resonance. The observed resonance frequencies are in the range of slow activity patterns observed in the immature neocortex, suggesting that subthreshold resonance may support the generation of this activity.     

Subcellular Distribution of Persistent Sodium Conductance in Cortical Pyramidal Neurons

Cortical pyramidal neurons possess a persistent Na⁺ current (I_NaP), which, in contrast to the larger transient current, does not undergo rapid inactivation. Although relatively quite small, I_NaP is active at subthreshold voltages and therefore plays an important role in neuronal input–output processing. The subcellular distribution of channels responsible for I_NaP and the mechanisms that render them persistent are not known. Using high-speed fluorescence Na⁺ imaging and whole-cell recordings in brain slices obtained from mice of either sex, we reconstructed the I_NaP elicited by slow voltage ramps in soma and processes of cortical pyramidal neurons. We found that in all neuronal compartments, the relationship between persistent Na⁺ conductance and membrane voltage has the shape of a Boltzmann function. Although the density of channels underlying I_NaP was about twofold lower in the axon initial segment (AIS) than in the soma, the axonal channels were activated by ∼10 mV less depolarization than were somatic channels. This difference in voltage dependence explains why, at functionally critical subthreshold voltages, most I_NaP originates in the AIS. Finally, we show that endogenous polyamines constrain I_NaP availability in both somatodendritic and axonal compartments of nondialyzed cortical neurons.SIGNIFICANCE STATEMENT The most salient characteristic of neuronal sodium channels is fast inactivation. However, a fraction of the sodium current does not inactivate. In cortical neurons, persistent current (I_NaP) plays a prominent role in many important functions. Its subcellular distribution and generation mechanisms are, however, elusive. Using high-speed fluorescence Na⁺ imaging and electrical recordings, we reconstructed the I_NaP in soma and processes of cortical pyramidal neurons. We found that at near-threshold voltages I_NaP originates predominately from the axon, because of the distinctive voltage dependence of the underlying channels and not because of their high density. Finally, we show that the presence of endogenous polyamines significantly constrains I_NaP availability in all compartments of nondialyzed cortical 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.     

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.     

Postnatal development of temporal integration,spike timing and spike threshold regulation by a dendrotoxin‐sensitive K+ current in rat CA1 hippocampal cells

Spike timing and network synchronization are important for plasticity, development and maturation of brain circuits. Spike delays and timing can be strongly modulated by a low‐threshold, slowly inactivating, voltage‐gated potassium current called D‐current (I_D). I_D can delay the onset of spiking, cause temporal integration of multiple inputs, and regulate spike threshold and network synchrony. Recent data indicate that I_D can also undergo activity‐dependent, homeostatic regulation. Therefore, we have studied the postnatal development of I_D‐dependent mechanisms in CA1 pyramidal cells in hippocampal slices from young rats (P7–27), using somatic whole‐cell recordings. At P21–27, these neurons showed long spike delays and pronounced temporal integration in response to a series of brief depolarizing current pulses or a single long pulse, whereas younger cells (P7–20) showed shorter discharge delays and weak temporal integration, although the spike threshold became increasingly negative with maturation. Application of α‐dendrotoxin (α‐DTX), which blocks I_D, reduced the spiking latency and temporal integration most strongly in mature cells, while shifting the spike threshold most strongly in a depolarizing direction in these cells. Voltage‐clamp analysis revealed an α‐DTX‐sensitive outward current (I_D) that increased in amplitude during development. In contrast to P21–23, I_D in the youngest group (P7–9) showed smaller peri‐threshold amplitude. This may explain why long discharge delays and robust temporal integration only appear later, 3 weeks postnatally. We conclude that I_D properties and I_D‐dependent functions develop postnatally in rat CA1 pyramidal cells, and I_D may modulate network activity and plasticity through its effects on synaptic integration, spike threshold, timing and synchrony.     

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.     

Sodium and calcium mechanisms of rhythmic bursting in excitatory neural networks of the pre‐Bötzinger complex: a computational modelling study

The neural mechanisms generating rhythmic bursting activity in the mammalian brainstem, particularly in the pre‐Bötzinger complex (pre‐BötC), which is involved in respiratory rhythm generation, and in the spinal cord (e.g. locomotor rhythmic activity) that persist after blockade of synaptic inhibition remain poorly understood. Experimental studies in rodent medullary slices containing the pre‐BötC identified two mechanisms that could potentially contribute to the generation of rhythmic bursting: one based on the persistent Na⁺ current (I_NaP), and the other involving the voltage‐gated Ca²⁺ current (I_Ca) and the Ca²⁺‐activated nonspecific cation current (I_CAN), activated by intracellular Ca²⁺ accumulated from extracellular and intracellular sources. However, the involvement and relative roles of these mechanisms in rhythmic bursting are still under debate. In this theoretical/modelling study, we investigated Na⁺‐dependent and Ca²⁺‐dependent bursting generated in single cells and heterogeneous populations of synaptically interconnected excitatory neurons with I_NaP and I_Ca randomly distributed within populations. We analysed the possible roles of network connections, ionotropic and metabotropic synaptic mechanisms, intracellular Ca²⁺ release, and the Na⁺/K⁺ pump in rhythmic bursting generated under different conditions. We show that a heterogeneous population of excitatory neurons can operate in different oscillatory regimes with bursting dependent on I_NaP and/or I_CAN, or independent of both. We demonstrate that the operating bursting mechanism may depend on neuronal excitation, synaptic interactions within the network, and the relative expression of particular ionic currents. The existence of multiple oscillatory regimes and their state dependence demonstrated in our models may explain different rhythmic activities observed in the pre‐BötC and other brainstem/spinal cord circuits under different experimental conditions.     

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.     

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.     

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.     

Coordinated Postnatal Maturation of Striatal Cholinergic Interneurons and Dopamine Release Dynamics in Mice

Dynamic changes in motor abilities and motivated behaviors occur during the juvenile and adolescent periods. The striatum is a subcortical nucleus critical to action selection, motor learning, and reward processing. Its tonically active cholinergic interneuron (ChI) is an integral regulator of the synaptic activity of other striatal neurons, as well as afferent axonal projections of midbrain dopamine (DA) neurons; however, little is known about its development. Here, we report that ChI spontaneous activity increases during postnatal development of male and female mice, concomitant with a decreased afterhyperpolarization (AHP). We characterized the postnatal development of four currents that contribute to the spontaneous firing rate of ChIs, including I_SK, I_A, I_h, and I_NaP. We demonstrated that the developmental increase in I_NaP drives increased ChI firing rates during the postnatal period and can be reversed by the I_NaP inhibitor, ranolazine. We next addressed whether immature cholinergic signaling may lead to functional differences in DA release during the juvenile period. In the adult striatum, nicotinic acetylcholine receptors (nAChRs) prevent linear summation of DA release in response to trains of high-frequency stimuli. We show that, in contrast, during the second postnatal week, DA release linearly sums with trains of high-frequency stimuli. Consistently, nAChR antagonists exert little effect on dopamine release at postnatal day (P)10, but enhance the summation of evoked DA release in mice older than postnatal day P28. Together, these results reveal that postnatal maturation of ChI activity is due primarily to enhanced I_NaP and identify an interaction between developing cholinergic signaling and DA neurotransmission in the juvenile striatum.SIGNIFICANCE STATEMENT Motor skills and motivated behavior develop rapidly in juvenile rodents. Recent work has highlighted processes that contribute to the postnatal maturation of striatal principal neurons during development. The functional development of the striatal cholinergic interneuron (ChI), however, has been unexplored. In this study, we tracked the ontogeny of ChI activity and cellular morphology, as well as the developmental trajectory of specific conductances that contribute to the activity of these cells. We further report a link between cholinergic signaling and dopamine (DA) release, revealing a change in the frequency-dependence of DA release during the early postnatal period that is mediated by cholinergic signaling. This study provides evidence that striatal microcircuits are dynamic during the postnatal period and that they undergo coordinated maturation.     

Effects of morphine withdrawal on the membrane properties of medium spiny neurons in the nucleus accumbens shell

Medium spiny neurons (MSNs) in the nucleus accumbens (NAc) undergo persistent alterations in their biological and physiological characteristics upon exposure to drugs of abuse. Previous studies demonstrated that the biochemical, morphological, and intrinsic physiological properties of MSNs are heterogeneous and provided new insights into the physiological and molecular roles of individual MSNs in addictive behaviors. However, it remains unclear whether MSNs in the NAc shell (NAcSh), an important region for mediating behavioral sensitization, are electrophysiologically heterogeneous and how such heterogeneity is relevant to neuroadaptation associated with drug addiction. Here, the membrane properties, i.e., the intrinsic excitability and spike adaptation, of MSNs in the NAcSh from saline- or morphine-treated rats were investigated in vitro by whole-cell recording. In saline-treated rats, three distinct cell types were identified by their membrane properties: type I neurons showed high levels of intrinsic excitability and rapid spike adaptation; type II neurons showed moderate levels of intrinsic excitability and relatively slow spike frequency adaptation; type III neurons showed low levels of intrinsic excitability and putative strong spike adaptation. MSNs in rats undergoing withdrawal from chronic morphine treatment (10–14 days after the last injection) also exhibited the typical firing behaviors of these three types of neurons. However, the membrane properties of the MSNs were differentially altered after withdrawal. There was an enhancement in intrinsic excitability in type II MSNs and a promotion of spike adaptation in type I MSNs. The apamin-sensitive afterhyperpolarization current (I_AHP) and the apamin-insensitive I_AHP of the NAcSh MSNs were attenuated after chronic morphine withdrawal. These findings suggest that individual MSNs in the NAcSh manifest unique electrophysiological properties, which might contribute to psychostimulant-induced neuroadaptation.     

New rules governing synaptic plasticity in core nucleus accumbens medium spiny neurons

The nucleus accumbens is a forebrain region responsible for drug reward and goal‐directed behaviors. It has long been believed that drugs of abuse exert their addictive properties on behavior by altering the strength of synaptic communication over long periods of time. To date, attempts at understanding the relationship between drugs of abuse and synaptic plasticity have relied on the high‐frequency long‐term potentiation model of T.V. Bliss & T. Lømo [(1973) Journal of Physiology, 232, 331–356]. We examined synaptic plasticity using spike‐timing‐dependent plasticity, a stimulation paradigm that reflects more closely the in vivo firing patterns of mouse core nucleus accumbens medium spiny neurons and their afferents. In contrast to other brain regions, the same stimulation paradigm evoked bidirectional long‐term plasticity. The magnitude of spike‐timing‐dependent long‐term potentiation (tLTP) changed with the delay between action potentials and excitatory post‐synaptic potentials, and frequency, whereas that of spike‐timing‐dependent long‐term depression (tLTD) remained unchanged. We showed that tLTP depended on N‐methyl‐d ‐aspartate receptors, whereas tLTD relied on action potentials. Importantly, the intracellular calcium signaling pathways mobilised during tLTP and tLTD were different. Thus, calcium‐induced calcium release underlies tLTD but not tLTP. Finally, we found that the firing pattern of a subset of medium spiny neurons was strongly inhibited by dopamine receptor agonists. Surprisingly, these neurons were exclusively associated with tLTP but not with tLTD. Taken together, these data point to the existence of two subgroups of medium spiny neurons with distinct properties, each displaying unique abilities to undergo synaptic plasticity.     

Dynamical response properties of neocortical neurons to conductance‐driven time‐varying inputs

Ensembles of cortical neurons can track fast‐varying inputs and relay them in their spike trains, far beyond the cut‐off imposed by membrane passive electrical properties and mean firing rates. Initially explored in silico and later demonstrated experimentally, investigating how neurons respond to sinusoidally modulated stimuli provides a deeper insight into spike initiation mechanisms and information processing than conventional F–I curve methodologies. Besides net membrane currents, physiological synaptic inputs can also induce a stimulus‐dependent modulation of the total membrane conductance, which is not reproduced by standard current‐clamp protocols. Here, we investigated whether rat cortical neurons can track fast temporal modulations over a noisy conductance background. We also determined input–output transfer properties over a range of conditions, including: distinct presynaptic activation rates, postsynaptic firing rates and variability and type of temporal modulations. We found a very broad signal transfer bandwidth across all conditions, similar large cut‐off frequencies and power‐law attenuations of fast‐varying inputs. At slow and intermediate input modulations, the response gain decreased for increasing output mean firing rates. The gain also decreased significantly for increasing intensities of background synaptic activity, thus generalising earlier studies on F‐I curves. We also found a direct correlation between the action potentials' onset rapidness and the neuronal bandwidth. Our novel results extend previous investigations of dynamical response properties to non‐stationary and conductance‐driven conditions, and provide computational neuroscientists with a novel set of observations that models must capture when aiming to replicate cortical cellular excitability.     

Enhanced Synaptic Inhibition Disrupts the Efferent Code of Cerebellar Purkinje Neurons in Leaner Cav2.1 Ca2+ Channel Mutant Mice

Cerebellar Purkinje cells (PCs) encode afferent information in the rate and temporal structure of their spike trains. Both spontaneous firing in these neurons and its modulation by synaptic inputs depend on Ca²⁺ current carried by Ca_v2.1 (P/Q) type channels. Previous studies have described how loss-of-function Ca_v2.1 mutations affect intrinsic excitability and excitatory transmission in PCs. This study examines the effects of the leaner mutation on fast GABAergic transmission and its modulation of spontaneous firing in PCs. The leaner mutation enhances spontaneous synaptic inhibition of PCs, leading to transitory reductions in PC firing rate and increased spike rate variability. Enhanced inhibition is paralleled by an increase in the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) measured under voltage clamp. These differences are abolished by tetrodotoxin, implicating effects of the mutation on spike-induced GABA release. Elevated sIPSC frequency in leaner PCs is not accompanied by increased mean firing rate in molecular layer interneurons, but IPSCs evoked in PCs by direct stimulation of these neurons exhibit larger amplitude, slower decay rate, and a higher burst probability compared to wild-type PCs. Ca²⁺ release from internal stores appears to be required for enhanced inhibition since differences in sIPSC frequency and amplitude in leaner and wild-type PCs are abolished by thapsigargin, an ER Ca²⁺ pump inhibitor. These findings represent the first account of the functional consequences of a loss-of-function P/Q channel mutation on PC firing properties through altered GABAergic transmission. Gain in synaptic inhibition shown here would compromise the fidelity of information coding in these neurons and may contribute to impaired cerebellar function resulting from loss-of function mutations in the Ca_V2.1 channel gene.     

Mechanisms and functional impact of Group I metabotropic glutamate receptor modulation of excitability in mouse MNTB neurons

We examined effects of Group I metabotropic glutamate receptors on the excitability of mouse medial nucleus of the trapezoid body (MNTB) neurons. The selective agonist, S-3,5-dihydroxyphenylglycine (DHPG), evoked a dose-dependent depolarization of the resting potential, increased membrane resistance, increased sag depolarization, and promoted rebound action potential firing. Under voltage-clamp, DHPG evoked an inward current, referred to as I_DHPG, which was developmentally stable through postnatal day P56. I_DHPG had low temperature dependence in the range 25–34°C, consistent with a channel mechanism. However, the I-V relationship took the form of an inverted U that did not reverse at the calculated Nernst potential for K⁺ or Cl⁻. Thus, it is likely that more than one ion type contributes to I_DHPG and the mix may be voltage dependent. I_DHPG was resistant to the Na⁺ channel blockers tetrodotoxin and amiloride, and to inhibitors of iGluR (CNQX and MK801). I_DHPG was inhibited 21% by Ba²⁺ (500 μM), 60% by ZD7288 (100 μM) and 73% when the two antagonists were applied together, suggesting that KIR channels and HCN channels contribute to the current. Voltage clamp measurements of I_H indicated a small (6%) increase in G_max by DHPG with no change in the voltage dependence. DHPG reduced action potential rheobase and reduced the number of post-synaptic AP failures during high frequency stimulation of the calyx of Held. Thus, activation of post-synaptic Group I mGlu receptors modifies the excitability of MNTB neurons and contributes to the reliability of high frequency firing in this auditory relay nucleus.