Intrinsic Membrane Conductances Research Articles

Mammalian octopus cells are direction selective to frequency sweeps by excitatory synaptic sequence detection

Octopus cells are remarkable projection neurons of the mammalian cochlear nucleus, with extremely fast membranes and wide-frequency tuning. They are considered prime examples of coincidence detectors but are poorly characterized invivo. We discover that octopus cells are selective to frequency sweep direction, a feature that is absent in their auditory nerve inputs. In vivo intracellular recordings reveal that direction selectivity does not derive from across-frequency coincidence detection but hinges on the amplitudes and activation sequence of auditory nerve inputs tuned to clusters of hot spot frequencies. A simple biophysical octopus cell model excited with real nerve spike trains recreates direction selectivity through interaction of intrinsic membrane conductances with the activation sequence of clustered excitatory inputs. We conclude that octopus cells are sequence detectors, sensitive to temporal patterns across cochlear frequency channels. The detection of sequences rather than coincidences is a much simpler but powerful operation to extract temporal information.

Location, location, location: the organization and roles of potassium channels in mammalian motoneurons.

The spatial and temporal balance of spinal α-motoneuron (αMN) intrinsic membrane conductances underlies the neural output of the final common pathway for motor commands. Although the complete set and precise localization of αMN K+ channels and their respective outward conductances remain unsettled, important K+ channel subtypes have now been documented, including Kv1, Kv2, Kv7, TASK, HCN and SK isoforms. Unique kinetics and gating parameters allow these channels to differentially shape and/or modify αMN firing properties, and recent immunohistochemical localization of K+ -channel complexes reveals a framework in which their spatial distribution and/or focal clustering within different surface membrane compartments is highly tuned to their physiological function. Moreover, highly evolved regulatory mechanisms enable specific channels to operate over variable levels of αMN activity and contribute to either state-dependent enhancement or diminution of firing. While recent data suggest an additional, non-conducting role for clustered Kv2.1 channels in the formation of endoplasmic reticulum-plasma membrane junctions postsynaptic to C-bouton synapses, electrophysiological evidence demonstrates that conducting Kv2.1 channels effectively regulate αMN firing, especially during periods of high activity in which the cholinergic C-boutons are engaged. Intense αMN activity or cell injury rapidly disrupts the clustered organization of Kv2.1 channels in αMNs and further impacts their physiological role. Thus, αMN K+ channels play a critical regulatory role in motor processing and are potential therapeutic targets for diseases affecting αMN excitability and motor output, including amyotrophic lateral sclerosis.

Synaptic Strengths Dominate Phasing of Motor Circuit: Intrinsic Conductances of Neuron Types Need Not Vary across Animals.

Identified neurons and the networks they compose produce stereotypical, albeit individually unique, activity across members of a species. We propose, for a motor circuit driven by a central pattern generator (CPG), that the uniqueness derives mainly from differences in synaptic strength rather than from differences in intrinsic membrane conductances. We studied a dataset of recordings from six leech (Hirudo sp.) heartbeat control networks, containing complete spiking activity patterns from inhibitory premotor interneurons, motor output spike patterns, and synaptic strength patterns to investigate the source of uniqueness. We used a conductance-based multicompartmental motor neuron model to construct a bilateral motor circuit model, and controlled it by playing recorded input spike trains from premotor interneurons to generate output inhibitory synaptic patterns similar to experimental measurements. By generating different synaptic conductance parameter sets of this circuit model, we found that relative premotor synaptic strengths impinging onto motor neurons must be different across individuals to produce animal-specific output burst phasing. Obtaining unique outputs from each individual's circuit model did not require different intrinsic ionic conductance parameters. Furthermore, changing intrinsic conductances failed to compensate for modified synaptic strength patterns. Thus, the pattern of synaptic strengths of motor neuron inputs is critical for the phasing of this motor circuit and can explain individual differences. When intrinsic conductances were allowed to vary, they exhibited the same conductance correlations across individuals, suggesting a motor neuron "type" required for proper network function. Our results are general and may translate to other systems and neuronal networks that control output phasing.

Ih Equalizes Membrane Input Resistance in a Heterogeneous Population of Fusiform Neurons in the Dorsal Cochlear Nucleus.

In a neuronal population, several combinations of its ionic conductances are used to attain a specific firing phenotype. Some neurons present heterogeneity in their firing, generally produced by expression of a specific conductance, but how additional conductances vary along in order to homeostatically regulate membrane excitability is less known. Dorsal cochlear nucleus principal neurons, fusiform neurons, display heterogeneous spontaneous action potential activity and thus represent an appropriate model to study the role of different conductances in establishing firing heterogeneity. Particularly, fusiform neurons are divided into quiet, with no spontaneous firing, or active neurons, presenting spontaneous, regular firing. These modes are determined by the expression levels of an intrinsic membrane conductance, an inwardly rectifying potassium current (IKir). In this work, we tested whether other subthreshold conductances vary homeostatically to maintain membrane excitability constant across the two subtypes. We found that Ih expression covaries specifically with IKir in order to maintain membrane resistance constant. The impact of Ih on membrane resistance is dependent on the level of IKir expression, being much smaller in quiet neurons with bigger IKir, but Ih variations are not relevant for creating the quiet and active phenotypes. Finally, we demonstrate that the individual proportion of each conductance, and not their absolute conductance, is relevant for determining the neuronal firing mode. We conclude that in fusiform neurons the variations of their different subthreshold conductances are limited to specific conductances in order to create firing heterogeneity and maintain membrane homeostasis.

Altered membrane properties and firing patterns of external globus pallidus neurons in the R6/2 mouse model of Huntington's disease.

In mouse models of Huntington's disease (HD), striatal neuron properties are significantly altered. These alterations predict changes in striatal output regions. However, little is known about alterations in those regions. The present study examines changes in passive and active membrane properties of neurons in the external globus pallidus (GPe), the first relay station of the indirect pathway, in the R6/2 mouse model of juvenile HD at presymptomatic (1 month) and symptomatic (2 month) stages. In GPe, two principal types of neurons can be distinguished based on firing properties and the presence (type A) or absence (type B) of Ih currents. In symptomatic animals (2 month), cell membrane capacitance and input resistance of type A neurons were increased compared with controls. In addition, action potential afterhyperpolarization amplitude was reduced. Although the spontaneous firing rate of GPe neurons was not different between control and R6/2 mice, the number of spikes evoked by depolarizing current pulses was significantly reduced in symptomatic R6/2 animals. In addition, these changes were accompanied by altered firing patterns evidenced by increased interspike interval variation and increased number of bursts. Blockade of GABAA receptors facilitated bursting activity in R6/2 mice but not in control littermates. Thus, alterations in firing patterns could be caused by changes in intrinsic membrane conductances and modulated by synaptic inputs. © 2016 Wiley Periodicals, Inc.

Glutamatergic synaptic currents of nigral dopaminergic neurons follow a postnatal developmental sequence

The spontaneous activity pattern of adult dopaminergic (DA) neurons of the substantia nigra pars compacta (SNc) results from interactions between intrinsic membrane conductances and afferent inputs. In adult SNc DA neurons, low-frequency tonic background activity is generated by intrinsic pacemaker mechanisms, whereas burst generation depends on intact synaptic inputs in particular the glutamatergic ones. Tonic DA release in the striatum during pacemaking is required to maintain motor activity, and burst firing evokes phasic DA release, necessary for cue-dependent learning tasks. However, it is still unknown how the firing properties of SNc DA neurons mature during postnatal development before reaching the adult state. We studied the postnatal developmental profile of spontaneous and evoked AMPA and NMDA (N-Methyl-D-aspartic acid) receptor-mediated excitatory postsynaptic currents (EPSCs) in SNc DA neurons in brain slices from immature (postnatal days P4–P10) and young adult (P30–P50) tyrosine hydroxylase (TH)-green fluorescent protein mice. We found that somato-dendritic fields of SNc DA neurons are already mature at P4–P10. In contrast, spontaneous glutamatergic EPSCs show a developmental sequence. Spontaneous NMDA EPSCs in particular are larger and more frequent in immature SNc DA neurons than in young adult ones and have a bursty pattern. They are mediated by GluN2B and GluN2D subunit-containing NMDA receptors. The latter generate long-lasting, DQP 1105-sensitive, spontaneous EPSCs, which are transiently recorded during this early period. Due to high NMDA activity, immature SNc DA neurons generate large and long lasting NMDA receptor-dependent (APV-sensitive) bursts in response to the stimulation of the subthalamic nucleus. We conclude that the transient high NMDA activity allows calcium influx into the dendrites of developing SNc DA neurons.

Spontaneous Inhibitory Synaptic Currents Mediated by a G Protein-Coupled Receptor

G protein-coupled receptors (GPCRs) affect many physiological processes by modulating both intrinsic membrane conductances and synaptic transmission. This study describes spontaneous miniature inhibitory postsynaptic currents mediated by vesicular dopamine release acting locally on metabotropic D2 receptors leading to the activation of a G protein-coupled inwardly rectifying potassium conductance. Thus, individual exocytotic events result in spontaneous GPCR-mediated transmission, similar to synaptic activation of classical ligand-gated ion channels.

Effect of intrinsic membrane conductances on Phase Resetting Curves in a conductance-based neuron model

It is well known that the magnitude and dynamics of a neuron’s membrane conductances influence the nature of its response to synaptic input [1]. However, the effect of specific membrane conductance values on the neuron’s response to perturbations (such as inhibitory synaptic input) has not yet been rigorously examined. In this work, we use a conductance-based model of a spontaneously bursting pyloric neuron of the crustacean stomatogastric ganglion (STG) to examine the relationship between intrinsic membrane conductances and neuronal output in response to perturbations. We describe the effect of these perturbations using a Phase Resetting Curve (PRC), which indicates the magnitude of the delay or advancement of a periodically bursting neuron’s trajectory along its limit cycle as a function of the perturbation’s timing [2]. The present study utilizes an existing database of single-compartment conductance-based model neurons [3] to determine the effects of specific membrane conductances on the shape of the PRC. The database consists of about 1.7 million model neurons, each of which was obtained by independently varying eight maximal conductances over six equally spaced possible values. PRCs for each of the bursting model neurons were obtained by simulating 1000 nS of inhibitory synaptic input at different phases of the rhythmic activity of each model neuron and measuring the resulting period change relative to the free-running burst period [3]. For all regularly bursting neurons with analyzable PRCs, we examined the effect of systematically varying each conductance upon specific features of the PRC, including the minimum delay, maximum delay, and neutral phase point. Increasing the leak conductance resulted in a decrease in magnitude of the minimum and maximum values of the PRC, indicating that the PRC was shifted toward smaller delays. We further saw that certain conductance pairs, including the hyperpolarization-activated and leak conductances, exhibit a correlation in model neuron populations with tightly constrained PRC attributes, suggesting that this conductance pair may aid in preserving PRC shape. Preliminary results also suggest that the effects of certain membrane conductances upon PRC shape are influenced by the magnitude of the synaptic input.

Heteromeric Canonical Transient Receptor Potential 1 and 4 Channels Play a Critical Role in Epileptiform Burst Firing and Seizure-Induced Neurodegeneration

Canonical transient receptor potential channels (TRPCs) are receptor-operated cation channels that are activated in response to phospholipase C signaling. Although TRPC1 is ubiquitously expressed in the brain, TRPC4 expression is the most restrictive, with the highest expression level limited to the lateral septum. The subunit composition of neuronal TRPC channels remains uncertain because of conflicting data from recombinant expression systems. Here we report that the large depolarizing plateau potential that underlies the epileptiform burst firing induced by metabotropic glutamate receptor agonists in lateral septal neurons was completely abolished in TRPC1/4 double-knockout mice, and was abolished in 74% of lateral septal neurons in TRPC1 knockout mice. Furthermore, neuronal cell death in the lateral septum and the cornu ammonis 1 region of hippocampus after pilocarpine-induced severe seizures was significantly ameliorated in TRPC1/4 double-knockout mice. Our data suggest that both TRPC1 and TRPC4 are essential for an intrinsic membrane conductance mediating the plateau potential in lateral septal neurons, possibly as heteromeric channels. Moreover, excitotoxic neuronal cell death, an underlying process for many neurological diseases, is not mediated merely by ionotropic glutamate receptors but also by heteromeric TRPC channels activated by metabotropic glutamate receptors. TRPC channels could be an unsuspected but critical molecular target for clinical intervention for excitotoxicity.

Analyzing how neuronal parameters influence network activity

A half-center oscillator (HCO) is a common circuit building block of central pattern generator (CPG) networks that produce rhythmic motor patterns in animals. Rhythmic activity in the heartbeat CPG of the leech is based on alternating bursting in two pairs of inhibitory interneurons that make reciprocal spike-mediated and graded synapses across the ganglionic midline. Our study here constitutes the next step toward a full investigation of how intrinsic membrane and synaptic parameters affect the electrical activity of a half-center oscillator (HCO) model and how different parameter regimes influence stability and modulatability of the HCO model’s output. In our previous study [2], we constructed an efficient relational database table with the resulting characteristics of Hill et al.’s [1] HCO single-compartment conductance-based model. The model consists of two reciprocally inhibitory neurons and replicates the electrical activity of the oscillator interneurons of the leech heartbeat CPG under a variety of experimental conditions. To systematically explore the parameter space of this model, we used a brute-force approach. We varied a set of eight selected parameters (maximal conductances of intrinsic and synaptic currents) in all combinations resulting in a parameter space of 10,485,760 simulated models (10,321,920 HCO models and 163,840 corresponding isolated neuron models). After performing all the simulations, we built a SQL database table for their firing characteristics [3,4], which we can use to ask fundamental questions about the activity of HCOs. We began our analysis by classifying these HCO and isolated neuron model simulations by their activity characteristics: models showing the same electrical activity are segregated to the same group. The HCOs were split into ten groups labeled: spiking, silent, asymmetric activity, plateau, irregular spikes, asymmetric bursting, one burst, irregular period, unbalanced, and functional; the isolated neuron models were split into seven groups: spiking, silent, bistable, plateau, irregular spikes, irregular period, and regular bursting[2]. By querying the database, we compared the activity characteristics of the groups of our simulated HCO models with those of our simulated isolated neuron models and found that regularly bursting neurons compose only a small minority of functional HCO models (419 models); the vast majority was composed of spiking neurons (32,568 models). This finding indicates that, within the parameter space considered, spiking neurons are highly prevalent, suggesting that this mode of activity is robust, and that regular bursting neurons are rare, suggesting that this state requires a delicate balance of intrinsic membrane conductances (parameters) and that they may not be robust to parameter changes. Also, our data suggest that regular bursting isolated neurons form functional HCOs that are more robust to variations in synaptic parameters than those formed by spiking isolated neurons. We will now use the entire database to explore in depth the interaction of parameters that lead to the different activity groups we have identified. We will be particularly interested in parameter changes which correspond to known neuromodulations such as the modulation of h current by myomodulin [5].

Bidirectional control of spike timing by GABAA receptor-mediated inhibition during theta oscillation in CA1 pyramidal neurons

Precisely controlled spike times relative to theta-frequency network oscillations play an important role in hippocampal memory processing. Here we study how inhibitory synaptic input during theta oscillation contributes to the control of spike timing. Using whole-cell patch-clamp recordings from CA1 pyramidal cells in vitro with dynamic clamp to simulate theta-frequency oscillation (5 Hz), we show that gamma-aminobutyric acid-A (GABA(A)) receptor-mediated inhibitory postsynaptic potentials (IPSPs) can not only delay but also advance the postsynaptic spike depending on the timing of the inhibition relative to the oscillation. Spike time advancement with IPSP was abolished by the h-channel blocker ZD7288 (10 microM), suggesting that IPSPs can interact with intrinsic membrane conductances to yield bidirectional control of spike timing.

Reliable neuromodulation from circuits with variable underlying structure

Recent work argues that similar network performance can result from highly variable sets of network parameters, raising the question of whether neuromodulation can be reliable across individuals with networks with different sets of synaptic strengths and intrinsic membrane conductances. To address this question, we used the dynamic clamp to construct 2-cell reciprocally inhibitory networks from gastric mill (GM) neurons of the crab stomatogastric ganglion. When the strength of the artificial inhibitory synapses (g(syn)) and the conductance of an artificial I(h) (g(h)) were varied with the dynamic clamp, a variety of network behaviors resulted, including regions of stable alternating bursting. Maps of network output as a function of g(syn) and g(h) were constructed in normal saline and again in the presence of serotonin or oxotremorine. Both serotonin and oxotremorine depolarize and excite isolated individual GM neurons, but by different cellular mechanisms. Serotonin and oxotremorine each increased the size of the parameter regions that supported alternating bursting, and, on average, increased burst frequency. Nonetheless, in both cases some parameter sets within the sample space deviated from the mean population response and decreased in frequency. These data provide insight into why pharmacological treatments that work in most individuals can generate anomalous actions in a few individuals, and they have implications for understanding the evolution of nervous systems.

Glutamatergic synaptic inputs and ICAN: the basis for an emergent property underlying respiratory rhythm generation?

The mammalian respiratory control system in the brainstem produces a robust rhythm that drives respiratory muscles and maintains blood gas homeostasis from birth to death. Unfortunately, there is a great deal that is still not known with respect to how key cellular and synaptic mechanisms contribute to this fundamental rhythm. Among the models currently being debated (Fig. 1), the pacemaker-network hypothesis proposes that some respiratory neurons have intrinsic membrane conductances that cause the membrane potential to spontaneously oscillate (i.e. pacemaker properties). Synaptic inputs modulate pacemaker neuron excitability, but ultimately the breathing rhythm is derived from pacemaker properties (Smith et al. 2000; Ramirez & Viemari, 2005). In contrast, the network hypothesis proposes that the breathing rhythm is due to neurons that are reciprocally coupled by inhibitory synaptic connections, thereby establishing a straightforward oscillatory neural network (Richter et al. 1992). Finally, the group-pacemaker hypothesis proposes that inspiratory bursts of action potentials are an emergent property of non-pacemaker neurons that are interconnected by chemical and electrotonic excitatory synaptic interactions; pacemaker neurons are not required (Rekling & Feldman, 1998; Feldman & Del Negro, 2006). The pacemaker-network and network hypotheses incorporate the interaction between synaptic inputs and intrinsic membrane conductances into their models, but these interactions are at the heart of the group-pacemaker model. Figure 1 Conceptual models for respiratory rhythm generation Shortly after the group-pacemaker model was first proposed (Rekling & Feldman, 1998), Del Negro et al. (2002) tested the hypothesis that one type of pacemaker neuron was necessary for rhythm generation by pharmacologically blocking persistent sodium (INaP) currents with riluzole in rhythmically active medullary slices. Since respiratory burst frequency was not altered in their experiments, the authors concluded that pacemaker neurons with INaP were not required for rhythm generation. However, other laboratories obtained the opposite results with similar experiments (e.g. Ramirez & Viemari, 2005). In addition, a separate class of pacemaker neurons that express Ca2+-activated non-specific cation (ICAN) currents may also contribute to rhythm generation (Ramirez & Viemari, 2005). To test whether both types of respiratory pacemaker neurons are necessary for rhythm generation, Del Negro et al. (2005) blocked INaP with riluzole and ICAN with flufenamic acid. Although the rhythm was initially abolished under these conditions, rhythmic activity could be restored by increasing network excitability with substance P application. Again, the authors concluded that respiratory pacemaker neurons (at least the types that are known to exist so far) are not required for rhythm generation, and that rhythm generation must therefore be an emergent property of the network. In this issue of The Journal of Physiology, Pace et al. (2007) extend this line of research by investigating potential mechanisms underlying the group-pacemaker hypothesis at the cellular level. With whole-cell recordings of inspiratory neurons in rhythmically active medullary slices, Pace et al. (2007) show that large inspiratory drive potentials (i.e. 10–30 mV depolarizations occurring during the inspiratory phase) are due to activation of ICAN. They also show that ICAN activation requires activation of NMDA or group I metabotropic glutamate (mGluR1) receptors. The authors conclude that glutamatergic excitatory synaptic inputs are required to evoke the ICAN-dependent inspiratory drive potentials, rather than ICAN alone being the main source of inspiratory drive potentials (as suggested by the pacemaker hypothesis). The importance of this study is that, once again, it demonstrates the plausibility of the group-pacemaker hypothesis for respiratory rhythm generation. Does this mean that pacemaker neurons are not involved in rhythm generation? Of course not! Other investigators suggest that pacemaker neurons may still play a critical role in rhythm generation (Ramirez et al. 2007). Also, the respiratory control system is multifunctional, state-dependent, highly modulated, and capable of significant plasticity. Thus, the relative contribution of pacemaker properties and synaptic inputs to rhythm generation is likely to be very dynamic. During normal quiet breathing, the respiratory rhythm may be predominantly due to inhibitory synaptic transmission (network hypothesis) and glutamatergic–ICAN interactions (group-pacemaker hypothesis). During hypoxia, however, the respiratory control network may shift to a different operating mode that is more dependent on pacemaker properties and produce gasping behaviour (Paton et al. 2006). To an outside observer, the debate regarding the cellular and synaptic mechanisms underlying respiratory rhythm generation may appear to be an exercise in hair-splitting semantics. However, understanding precisely how the brain produces the breathing rhythm is not only a fascinating biological problem, but it may also lead to critical insights for treating respiratory-related diseases and pathological conditions, such as sleep-disordered breathing, spinal cord injury, sudden infant death syndrome, Rett syndrome and congenital hypoventilation.

Information encoding and computation with spikes and bursts

Neurons compute and communicate by transforming synaptic input patterns into output spike trains. The nature of this transformation depends crucially on the properties of voltage-gated conductances in neuronal membranes. These intrinsic membrane conductances can enable neurons to generate different spike patterns including brief, high-frequency bursts that are commonly observed in a variety of brain regions. Here we examine how the membrane conductances that generate bursts affect neural computation and encoding. We simulated a bursting neuron model driven by random current input signal and superposed noise. We consider two issues: the timing reliability of different spike patterns and the computation performed by the neuron. Statistical analysis of the simulated spike trains shows that the timing of bursts is much more precise than the timing of single spikes. Furthermore, the number of spikes per burst is highly robust to noise. Next we considered the computation performed by the neuron: how different features of the input current are mapped into specific output spike patterns. Dimensional reduction and statistical classification techniques were used to determine the stimulus features triggering different firing patterns. Our main result is that spikes, and bursts of different durations, code for different stimulus features, which can be quantified without a priori assumptions about those features. These findings lead us to propose that the biophysical mechanisms of spike generation enables individual neurons to encode different stimulus features into distinct spike patterns.

Information encoding and computation with spikes and bursts

Neurons compute and communicate by transforming synaptic input patterns into output spike trains. The nature of this transformation depends crucially on the properties of voltage-gated conductances in neuronal membranes. These intrinsic membrane conductances can enable neurons to generate different spike patterns including brief, high-frequency bursts that are commonly observed in a variety of brain regions. Here we examine how the membrane conductances that generate bursts affect neural computation and encoding. We simulated a bursting neuron model driven by random current input signal and superposed noise. We consider two issues: the timing reliability of different spike patterns and the computation performed by the neuron. Statistical analysis of the simulated spike trains shows that the timing of bursts is much more precise than the timing of single spikes. Furthermore, the number of spikes per burst is highly robust to noise. Next we considered the computation performed by the neuron: how different features of the input current are mapped into specific output spike patterns. Dimensional reduction and statistical classification techniques were used to determine the stimulus features triggering different firing patterns. Our main result is that spikes, and bursts of different durations, code for different stimulus features, which can be quantified without a priori assumptions about those features. These findings lead us to propose that the biophysical mechanisms of spike generation enables individual neurons to encode different stimulus features into distinct spike patterns.

Active and passive membrane properties of rat sympathetic preganglionic neurones innervating the adrenal medulla.

The intravascular release of adrenal catecholamines is a fundamental homeostatic process mediated via thoracolumbar spinal sympathetic preganglionic neurones (AD-SPN). To understand mechanisms regulating their excitability, whole-cell patch-clamp recordings were obtained from 54 retrogradely labelled neonatal rat AD-SPN. Passive membrane properties included a mean resting membrane potential, input resistance and time constant of -62 +/- 6 mV, 410 +/- 241 MOmega and 104 +/- 53 ms, respectively. AD-SPN were homogeneous with respect to their active membrane properties. These active conductances included transient outward rectification, observed as a delayed return to rest at the offset of the membrane response to hyperpolarising current pulses, with two components: a fast 4-AP-sensitive component (A-type conductance), contributing to the after-hyperpolarisation (AHP) and spike repolarisation; a slower prolonged Ba(2+)-sensitive component (D-like conductance). All AD-SPN expressed a Ba(2+)-sensitive instantaneous inwardly rectifying conductance activated at membrane potentials more negative than around -80 mV. A potassium-mediated, voltage-dependent sustained outward rectification activated at membrane potentials between -35 and -15 mV featured an atypical pharmacology with a component blocked by quinine, reduced by low extracellular pH and arachidonic acid, but lacking sensitivity to Ba(2+), TEA and intracellular Cs(+). This quinine-sensitive outward rectification contributes to spike repolarisation. Following block of potassium conductances by Cs(+) loading, AD-SPN revealed the capability for autorhythmicity and burst firing, mediated by a T-type Ca(2+) conductance. These data suggest the output capability is dynamic and diverse, and that the range of intrinsic membrane conductances expressed endow AD-SPN with the ability to generate differential and complex patterns of activity. The diversity of intrinsic membrane properties expressed by AD-SPN may be key determinants of neurotransmitter release from SPN innervating the adrenal medulla. However, factors other than active membrane conductances of AD-SPN must ultimately regulate the differential ratio of noradrenaline (NA) versus adrenaline (A) release secreted in response to various physiological and environmental demands.

Intrinsic excitability changes in vestibular nucleus neurons after unilateral deafferentation

Two synergistic plastic mechanisms have recently been identified in rat medial vestibular nucleus (MVN) neurons during ‘vestibular compensation’, the behavioral recovery that follows damage to the vestibular receptors or nerve of one inner ear. Ipsi-lesional MVN neurons develop a significant increase in their intrinsic excitability, and a marked decrease in the functional efficacy of GABA A and GABA B receptors, within 4 h of unilateral vestibular deafferentation. These mechanisms presumably counteract the disfacilitation and excessive commissural inhibition of the ipsi-lesional cells after deafferentation, and thus promote the recovery of resting activity. In this study, we investigated the intrinsic membrane properties and spike firing characteristics of rostral ipsi-lesional MVN neurons in slices from animals that underwent vestibular compensation for either 24–72 h or 7–10 days. Significant changes were observed in the spontaneous in vitro discharge rate, resting membrane potentials and voltage-activated membrane conductances of type B cells, but not type A cells. There was a significant increase in the number of type B LTS cells compared to normal. These findings indicate that during vestibular compensation marked changes occur in ion channel expression and function selectively in type B MVN neurons. These changes are appropriate to increase the responsiveness of type B cells both to their own intrinsic pacemaker-like membrane conductances and excitatory synaptic inputs. Together with the downregulation of inhibitory receptor efficacy, this increased intrinsic excitability may be sufficient to restore the resting discharge of the deafferented neurons in vivo. These results therefore provide further evidence for synaptic and neuronal plasticity in ipsi-lesional MVN neurons during vestibular compensation.

Response Properties of Baroreceptive NTS Neurons

Neurons in the nucleus of the solitary tract (NTS) responding to activation of arterial baroreceptors were recorded intracellularly using patch pipettes in an in situ arterially perfused working heart-brain stem preparation of rat. Seven of 15 (i.e., 46%) of NTS neurons showed adaptive (nonlinear) excitatory synaptic response patterns during baroreceptor stimulation followed by an "evoked hyperpolarization." This evoked hyperpolarization was stimulus intensity dependent and capable of shunting out a subsequent baroreceptor input. We suggest that this adaptive response behavior may be mediated, in part, by calcium-dependent potassium currents (IKCa) since neurons showed spike frequency adaptation during step depolarizations and an after-hyperpolarization after repetitive firing. Furthermore, in in vivo anesthetized rats, NTS microinjections of either charybdotoxin (225 fmol) or apamin (4.5 pmol) to block IKCa increased the baroreceptor reflex gain. Our data purport that the responsiveness of baroreceptive NTS neurons can be regulated by intrinsic membrane conductances such as IKCa. Modulation of such conductances during either physiological (exercise) or pathophysiological (essential hypertension) conditions may lead to changes in both the operating point and gain of the baroreceptor reflex.

Persistently modified h-channels after complex febrile seizures convert the seizure-induced enhancement of inhibition to hyperexcitability

Febrile seizures are the most common type of developmental seizures, affecting up to 5% of children. Experimental complex febrile seizures involving the immature rat hippocampus led to a persistent lowering of seizure threshold despite an upregulation of inhibition. Here we provide a mechanistic resolution to this paradox by showing that, in the hippocampus of rats that had febrile seizures, the long-lasting enhancement of the widely expressed intrinsic membrane conductance Ih converts the potentiated synaptic inhibition to hyperexcitability in a frequency-dependent manner. The altered gain of this molecular inhibition-excitation converter reveals a new mechanism for controlling the balance of excitation-inhibition in the limbic system. In addition, here we show for the first time that h-channels are modified in a human neurological disease paradigm.

Intracellular recording of lamina X neurons in a horizontal slice preparation of rat lumbar spinal cord

A horizontal slice preparation of postnatal rat lumbar spinal cord has been developed which allows correlative observations of the morphology, electrophysiology, and receptor pharmacology of lamina X neurons. These slices better maintain afferent input and somatodendritic morphology and are amenable to subsequent immunohistochemical processing. Stable intracellular recordings obtained from postnatal day 14–45 animals reveal that a number of different intrinsic membrane conductances contribute to the regulation of excitability in lamina X neurons. In addition, lamina X neurons possess inhibitory GABAergic as well as excitatory glutamate and cholecystokinin receptors. This preparation will be useful in future studies designed to characterize developmental changes in the intrinsic membrane properties, synaptic profiles and neuropeptide responsiveness of lamina X neurons in the rat. Such a characterization is important given that lamina X represents a unique sexually dimorphic region that is a convergence site for somatic and visceral afferent inputs, which includes nociceptive information.