Subthreshold Membrane Potential Oscillations Research Articles

Spinal nerve injury enhances subthreshold membrane potential oscillations in DRG neurons: relation to neuropathic pain.

Primary sensory neurons with myelinated axons were examined in vitro in excised whole lumbar dorsal root ganglia (DRGs) taken from adult rats up to 9 days after tight ligation and transection of the L(5) spinal nerve (Chung model of neuropathic pain). Properties of subthreshold membrane potential oscillations, and of repetitive spike discharge, were examined. About 5% of the DRG neurons sampled in control DRGs exhibited high-frequency, subthreshold sinusoidal oscillations in their membrane potential at rest (V(r)), and an additional 4.4% developed such oscillations on depolarization. Virtually all had noninflected action potentials (A(0) neurons). Amplitude and frequency of subthreshold oscillations were voltage sensitive. A(0) neurons with oscillations at V(r) appear to constitute a population distinct from A(0) neurons that oscillate only on depolarization. Axotomy triggered a significant increase in the proportion of neurons exhibiting subthreshold oscillations both at V(r) and on depolarization. This change occurred within a narrow time window 16-24 h postoperative. Axotomy also shifted the membrane potential at which oscillation amplitude was maximal to more negative (hyperpolarized) values, and lowered oscillation frequency at any given membrane potential. Most neurons that had oscillations at V(r), or that developed them on depolarization, began to fire repetitively when further depolarized. Spikes were triggered by the depolarizing phase of oscillatory sinusoids. Neurons that did not develop subthreshold oscillations never discharged repetitively and rarely fired more than a single spike or a short burst, on step depolarization. The most prominent spike waveform parameters distinguishing neurons capable of generating subthreshold oscillations, and hence repetitive firing, was their brief postspike afterhyperpolarization (AHP) and their low single-spike threshold. Neurons that oscillated at V(r) tended to have a more prolonged spike, with slower rise- and fall-time kinetics, and lower spike threshold, than cells that oscillated only on depolarization. The main effects of axotomy were to increase spike duration, slow rise- and fall-time kinetics, and reduce single-spike threshold. Tactile allodynia following spinal nerve injury is thought to result from central amplification ("central sensitization") of afferent signals entering the spinal cord from residual intact afferents. The central sensitization, in turn, is thought to be triggered and maintained in the Chung model by ectopic firing originating in the axotomized afferent neurons. Axotomy by spinal nerve injury enhances subthreshold membrane potential oscillations in DRG neurons, augments ectopic discharge, and hence precipitates neuropathic pain.

Serotonin suppresses subthreshold and suprathreshold oscillatory activity of rat inferior olivary neurones in vitro.

The effect of serotonin on membrane potential oscillations of inferior olivary neurones was studied in brainstem slices from 10- to 19-day-old rats. Serotonin at 50 and 5 microM induced a mean depolarization of 9.4 and 7.7 mV, respectively, that was preceded by a reversible suppression of subthreshold membrane potential oscillations. These effects were not changed by 1 microM tetrodotoxin and the suppression of subthreshold oscillations persisted after current-mediated restoration of resting potential. In spontaneously active neurones, serotonin abolished the rhythmicity of action potential firing without affecting spike frequency. Serotonin reduced the slope of the calcium-mediated rebound spike and both the duration and amplitude of the subsequent afterhyperpolarization. Serotonin also shifted the voltage dependence of the rebound spike to more negative values. Hyperpolarizing current pulses (200 ms) revealed that serotonin increased the pre-rectification and steady-state components of membrane resistance by 37 and 38 %, respectively, in 66 % of neurones, but decreased these parameters by 14 and 20% in the remaining cells. The serotonin effects were antagonized by 5 microM methysergide or 1-5 microM ketanserin and were mimicked by 10-20 microM dimethoxy-4-iodoamphetamine but not 10 microM 8-hydroxy-2-(di-N-propylamino)-tetralin. The data indicate that serotonin suppresses the rhythmic activity of olivary neurones via 5-HT2 receptors by inhibition of the T-type calcium current in combination with membrane depolarization due to activation of a cation current (Ih) and block of a resting K+ current (fast IK(ir)). This modulatory action of serotonin may account for the differential propensity of olivary neurones to fire rhythmically during different behavioural states in vivo.

Properties and role of I(h) in the pacing of subthreshold oscillations in entorhinal cortex layer II neurons.

Various subsets of brain neurons express a hyperpolarization-activated inward current (I(h)) that has been shown to be instrumental in pacing oscillatory activity at both a single-cell and a network level. A characteristic feature of the stellate cells (SCs) of entorhinal cortex (EC) layer II, those neurons giving rise to the main component of the perforant path input to the hippocampal formation, is their ability to generate persistent, Na(+)-dependent rhythmic subthreshold membrane potential oscillations, which are thought to be instrumental in implementing theta rhythmicity in the entorhinal-hippocampal network. The SCs also display a robust time-dependent inward rectification in the hyperpolarizing direction that may contribute to the generation of these oscillations. We performed whole cell recordings of SCs in in vitro slices to investigate the specific biophysical and pharmacological properties of the current underlying this inward rectification and to clarify its potential role in the genesis of the subthreshold oscillations. In voltage-clamp conditions, hyperpolarizing voltage steps evoked a slow, noninactivating inward current, which also deactivated slowly on depolarization. This current was identified as I(h) because it was resistant to extracellular Ba(2+), sensitive to Cs(+), completely and selectively abolished by ZD7288, and carried by both Na(+) and K(+) ions. I(h) in the SCs had an activation threshold and reversal potential at approximately -45 and -20 mV, respectively. Its half-activation voltage was -77 mV. Importantly, bath perfusion with ZD7288, but not Ba(2+), gradually and completely abolished the subthreshold oscillations, thus directly implicating I(h) in their generation. Using experimentally derived biophysical parameters for I(h) and the low-threshold persistent Na(+) current (I(NaP)) present in the SCs, a simplified model of these neurons was constructed and their subthreshold electroresponsiveness simulated. This indicated that the interplay between I(NaP) and I(h) can sustain persistent subthreshold oscillations in SCs. I(NaP) and I(h) operate in a "push-pull" fashion where the delay in the activation/deactivation of I(h) gives rise to the oscillatory process.

Morphological and electrophysiological characteristics of layer V neurons of the rat medial entorhinal cortex

This study aimed to characterize the morphological and electrophysiological properties of neurons in layer V of the entorhinal cortex in the rat brain. Using the in vitro slice preparation and sharp electrode techniques, we recorded from layer V neurons located in the medial entorhinal cortex. Recorded cells were also labeled with biocytin. Based on morphological criteria, layer V of the entorhinal cortex is comprised of three categories of neurons: pyramidal cells, horizontal cells, and polymorphic cells. Horizontal cells could be easily distinguished from the pyramidal cells because the bulk of their dendritic plexus extended horizontally within layer V. Polymorphic cells vary in size and shape. Interestingly, they typically do not have apical dendrites, and some of them have dendrites that extend into the subiculum. Based on electrophysiological criteria alone, it was not possible to unequivocally distinguish the morphological cell types because they were somewhat heterogeneous with respect to several parameters including inward rectification, spike-frequency adaptation, and intrinsic oscillations. Nevertheless, although most horizontal cells displayed time-dependent inward rectification, most pyramidal cells displayed fast inward rectification exclusively. None of the entorhinal cortex layer V cells displayed oscillatory activity like that of neocortical layer V "bursting" cells, although neurons from all groups displayed rhythmic subthreshold membrane potential oscillations. In summary, we have found that layer V of the rat medial entorhinal cortex consists of three morphologically distinct neuronal subtypes that cannot be clearly distinguished from each other by traditional electrophysiological measures.

Responses of neurones of the rat suprachiasmatic nucleus to retinal illumination under photopic and scotopic conditions.

1. We have examined the responses of neurones in the suprachiasmatic nuclei (SCN) of the rat to retinal illumination under photopic and scotopic conditions to identify the types of photoreceptor input to these nuclei. 2. The majority of visually responsive SCN neurones studied under dark adaptation received rod input (48 of 52, 92 %). The action spectrum conformed to the sensitivity of rhodopsin, with maximal sensitivity at around 505 nm. 3. When also studied under light adaptation, most visually responsive SCN neurones (20 out of 26, 77 %) responded to input from cones. The action spectra conformed to the spectrum of green cone opsin, with a main sensitivity peak at 510 nm and a significant secondary peak in the near-ultraviolet region of the spectrum. 4. The frequency of spontaneous activity was typically low under scotopic conditions (range 0.2-17.2 Hz) and higher under photopic conditions (range 0.6-40 Hz) for any given neurone. The most common response under scotopic conditions was an 'on-excitation' (32 of 48, 62.5 %), which changed under photopic conditions to an on-excitation followed by a more prominent off-inhibition. 5. Responses also changed due to endogenous ultradian cycles. Depending on the phase, responses could be altogether absent and even reverted from excitation to inhibition on opposite phases of a cycle. Ultradian cycles had a circadian dependence and were most common at around the light phase:dark phase (L:D) and D:L transition points of the circadian cycle. 6. Under photopic conditions, SCN neurones showed rhythmic electrical activity, with a preferred firing interval that had a value between 18 and 39 ms. This rhythmic activity was probably the result of endogenous subthreshold membrane potential oscillations. 7. In conclusion, light acting either via rod or cone pathways could have powerful, opposing actions on SCN neurones. These actions were state dependent. The presence of these neuronal responses suggests a role for rod and cone photoreceptors in SCN function.

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

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

Respiration-modulated membrane potential and chemosensitivity of locus coeruleus neurones in the in vitro brainstem-spinal cord of the neonatal rat.

1. The activity of locus coeruleus (LC) neurones (n = 126) was examined in whole-cell (conventional and amphotericin B-perforated patch) recordings, and the relationship of this activity to the respiratory discharge recorded on the C4 or C5 phrenic nerve roots was determined at different CO2 concentrations (2 and 8 %; bath pH 7. 8 and 7.2) in the in vitro brainstem-spinal cord preparation of the neonatal rat (1-5 days old). 2. In most neurones (n = 105) ongoing activity was modulated at respiratory frequency. Typically, this consisted of a phase of depolarization and increased discharge frequency synchronous with the phrenic burst, followed by a phase of hyperpolarization and inhibition of discharge (n = 94 of 105). The incidence of respiratory modulation decreased from 91 % on P1 to 57 % on P5. 3. Bath application of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5 microM) or the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV; 100 microM) abolished both phases of respiratory modulation. The hyperpolarizing phase alone was abolished by the adrenoceptor antagonists idazoxan (5 microM) or phentolamine (0.8 microM). These results indicate that excitatory amino acid pathways are involved in the transmission of both the excitatory and inhibitory components and that the latter involves in addition an alpha2-adrenoceptor-mediated pathway. 4. Increasing the CO2 concentration from 2 to 8 % resulted in a shortening of expiratory duration and weakening or loss of respiratory-phased inhibition; this was accompanied by depolarization, increased discharge frequency and, in those neurones where they were initially present (60 %), an increase in the frequency of subthreshold membrane potential oscillations. The depolarizing response was retained in the presence of tetrodotoxin (TTX, 0.2-1.0 microM). 5. These results indicate that in this neonatal preparation LC neurones form part of the synaptically connected brainstem respiratory network, and that the LC constitutes a site of CO2- or pH-dependent chemoreception.

Properties of γ‐frequency oscillations initiated by propagating population bursts in retrohippocampal regions of rat brain slices

1. In the hippocampal formation in vivo, brief periods of gamma-frequency activity follow population bursts called sharp waves. The approximately 200 Hz activity of the sharp wave itself may serve to enhance synaptic connections and the approximately 40 Hz gamma activity has been offered as a mechanism for solving the 'binding' problem. We describe epochs of gamma-frequency activity which follow population spikes evoked by low frequency repetitive extracellular stimuli in retrohippocampal neurons of horizontal rat brain slices. 2. gamma-Frequency activity recorded intracellularly from deep layer neurons of entorhinal cortex, presubiculum and parasubiculum consisted of one action potential correlated with each of the three to five gamma cycles recorded with a proximate field potential electrode. A minority of cells exhibited only sub-threshold gamma-frequency membrane potential oscillations (ranging from 5 to 10 mV). No cells fired more than one spike per gamma cycle under any conditions. 3. The range of synchrony varied from individual cells which showed gamma-frequency firing without corresponding oscillations in close field recordings to field potential recordings of oscillations which were well correlated across regions. The lead or lag between any two retrohippocampal regions was in the direction of the conduction delay for the primary population spike, but typically was less, and approached zero milliseconds for some cycles in most cells. The level of synchrony was stable for particular stimulating conditions (intensity, stimulation rate, stimulus location). 4. The duration of the period of gamma activity had the duration of a slow depolarizing potential which was mediated by NMDA receptor activation. NMDA receptor antagonists or low concentrations of AMPA receptor antagonists reduced the duration of, or completely abolished the slow potential, thereby eliminating the gamma portion of the evoked response. 5. gamma-Frequency firing was eliminated by the GABAA receptor antagonist picrotoxin but small (< 5 mV) membrane potential oscillations remained after focal picrotoxin applications, and these exhibited the voltage dependence of EPSPs. Bath application of thiopental lowered the frequency of gamma oscillations, confirming the involvement of GABAA receptors. 6. The GABAB receptor antagonist 2-hydroxy-saclofen appeared to enhance the gamma activity by increasing the duration of the gamma epoch and increasing the amplitude of individual gamma cycles in field potential recordings. These saclofen-induced cycles were, however, less well synchronized across regions. 7. We show that synchronous gamma (40-100 Hz) activity follows population bursts by deep layer retrohippocampal neurons in undrugged slices from rat brain. Responses from medial entorhinal, parasubicular or presubicular cells were not distinguishable. These events can be initiated by a propagating population spike. We suggest that an NMDA receptor mediated depolarization enables the network of deep layer retrohippocampal neurons to oscillate by providing a sustained excitation, the duration of which determines the duration of the gamma episode. gamma-Frequency firing is primarily the result of GABAA receptor dependent inhibition during this period of sustained depolarization. Recurrent excitation appears to be inconsequential for principal cell firing, but may contribute to interneuron firing.

Metabotropic glutamate receptor-mediated excitation and inhibition of sympathetic preganglionic neurones

The effects of metabotropic glutamate receptor (mGluR) subtype selective compounds on the excitability of sympathetic preganglionic neurones (SPNs) were investigated. Non-selective mGluR agonists (1 S,3 R)-aminocyclopentane-1,3-dicarboxylic acid and (2 S,1′ S,2′ S)-2-(carboxycyclopropyl)glycine, induced dose-dependent depolarisations in 96 and 75% of SPNs, respectively and hyperpolarisations in 2 and 21% of SPNs. Both agonists could induce subthreshold membrane potential oscillations in previously non-oscillating SPNs and either increased or reduced the frequency of spontaneously occurring oscillations. A selective group I mGluR agonist, 3,5-dihydroxyphenylglycine, depolarised all SPNs tested, induced oscillations in membrane potential of otherwise non-oscillating SPNs and increased the frequency of spontaneous oscillations. Agonists with selectivity for group II mGluRs (1 S, 3 S)-aminocyclopentane-1,3-dicarboxylic acid and ( S)-4-carboxy-3-hydroxy-phenylglycine (( S)-4C3HPG) did not induce depolarising responses. However ( S)-4C3HPG induced hyperpolarising responses associated with a reduction in the frequency of spontaneous oscillations in two of six SPNs tested. Depolarising and hyperpolarising responses were maintained in the presence of tetrodotoxin indicating a direct action of the agonists upon SPNs. In individual SPNs responses of opposite polarity could be induced from the same initial membrane potential using different agonists, indicating that the opposing responses involved different ionic mechanisms. The broad spectrum mGluR antagonist ( S)- α-methyl-4-carboxyphenylglycine and the selective group I mGluR antagonist ( S)-4-carboxyphenylglycine reversibly depressed mGluR agonist induced depolarisations. These results indicate that SPNs express two mGluR populations with opposing actions on neuronal excitability: group I mGluRs depolarise SPNs and can drive oscillatory membrane potential activity; a minority of SPNs express group II mGluRs which mediate membrane hyperpolarisations and reduce the frequency of membrane potential oscillations.

Autoactivity of A5 neurons: role of subthreshold oscillations and persistent Na+ current.

A5 noradrenergic neurons play a key role in autonomic regulation, nociception, and respiration. The purpose of the present experiments was to characterize some of the intrinsic properties of A5 cells in vitro. Whole cell recordings were obtained from 85 spinally projecting neurons of the ventrolateral pons of neonate rats. Immunohistochemistry showed that 60% of the ventrolateral pontine cells were noradrenergic. Eighty percent of A5 neurons were spontaneously active (0.1-5.5 spikes/s). Their discharge rate was unchanged by a mixture of synaptic blockers that eliminated postsynaptic potentials (PSPs). The nonnoradrenergic cells could not be distinguished from A5 cells on the basis of discharge rate, action potential duration, inward rectification, input resistance, or accommodation. A5 cells displayed subthreshold irregular oscillations of the membrane potential (main frequency component 0.5-2 Hz). These oscillations were unchanged in the presence of low external Ca(2+)-high Mg2+ and were very reduced by hyperpolarizing the cells below -65 mV. The oscillations were partially attenuated by 1 microM tetrodotoxin (TTX) and were eliminated by reducing external Na+ (27 mM). Stepping the membrane potential from -65 to -50 mV for 200 ms revealed the presence of a transient and a persistent inward current that were both blocked by 0.1 microM TTX or by extracellular Na+ reduction. In conclusion, most A5 neurons are spontaneously active in vitro. They display irregular subthreshold membrane potential oscillations generated by voltage-activated conductances that include a persistent TTX-sensitive Na+ current. Most of the activity of A5 cells appears due to intrinsic properties rather than to synaptic inputs.

Electroresponsiveness of medial entorhinal cortex layer III neurons in vitro

The entorhinal cortex funnels sensory information from the entire cortical mantle into the hippocampal formation via the perforant path. A major component of this pathway originates from the stellate cells in layer II and terminates on the dentate granule cells to activate the hippocampal trisynaptic circuit. In addition, there is also a significant, albeit less characterized, component of the perforant path that originates in entorhinal layer III pyramidal cells and terminates directly in area CA1. As a step in understanding the functional role of this monosynaptic component of the perforant path, we undertook the electrophysiological characterization of entorhinal layer III neurons in an in vitro rat brain slice preparation using intracellular recording techniques with sharp micropipettes and under current-clamp conditions. Cells were also intracellularly injected with biocytin to assess their pyramidal cell morphology. Layer III pyramidal cells did not display either the rhythmic subthreshold membrane potential oscillations nor spike-cluster discharge that characterizes the spiny stellate cells from layer II. In contrast, layer III pyramidal cells displayed a robust tendency towards spontaneous activity in the form of regular tonic discharge. Analysis of the voltage–current relations also demonstrated, in these neurons, a rather linear membrane voltage behaviour in the subthreshold range with the exception of pronounced inward rectification in the depolarizing direction. Depolarizing inward rectification was unaffected by Ca 2+-conductance block but was abolished by voltage-gated Na +-conductance block with tetrodotoxin, suggesting that a persistent Na +-conductance provides much of the inward current sustaining tonic discharge. In addition, in the presence of tetrodotoxin, an intermediate threshold (∼−50 mV) Ca 2+-dependent rebound potential was also observed which could constitute another pacemaker mechanism. A high-threshold Ca 2+-conductance was also found to contribute to the action potential as judged by the decrease in spike duration towards the peak observed during Ca 2+-conductance block. On the other hand, Ca 2+-conductance block increased spike duration at the base and abolished the monophasic spike afterhyperpolarization. Analysis of the input–output relations revealed firing properties similar to those of regularly spiking neocortical cells. Current–pulse driven spike trains displayed moderate adaptation and were followed by a Ca 2+-dependent slow afterhyperpolarization. In summary, the intrinsic electroresponsiveness of entorhinal layer III pyramidal cells suggest that these neurons may perform a rather high-fidelity transfer function of incoming neocortical sensory information directly to the CA1 hippocampal subfield. The pronounced excitability of layer III cells, due to both Na + and Ca 2+ conductances, may also be related to their tendency towards degeneration in epilepsy.

Actions of the anaesthetic Saffan on rat sympathetic preganglionic neurones in vitro

1. Whole-cell patch-clamp recordings were used to investigate the effects of the anaesthetic Saffan on the electrophysiological properties of sympathetic preganglionic neurones (SPNs) in rat spinal cord slices. 2. Saffan (1-54 microM) abolished or reduced the frequency of spontaneous action potential firing and abolished spontaneous, sub-threshold membrane potential oscillations. Saffan caused dose-dependent decreases in input resistance and depending upon the initial resting membrane potential, either a depolarization, a hyperpolarization or no change in membrane potential. 3. Responses to Saffan were blocked by the GABAA receptor antagonists bicuculline (5-20 microM) and picrotoxin (20 microM), but not by the glycine receptor antagonist strychnine (20 microM) indicating that they were mediated by GABAA receptors. 4. Changes in the properties of SPN action potentials were also observed. In the presence of Saffan the amplitude and duration of the action potential after-hyperpolarization were reduced and larger depolarizations were required in order to evoke trains of action potentials. 5. To examine the effects of Saffan on electrotonic coupling between SPNs, experiments were performed with the Na+ channel blocker QX-314 in the intracellular solution and antidromic oscillations were evoked by ventral root stimulation. Saffan failed to abolish antidromic oscillations, but reduced their amplitude and duration. This indicates that the abolition of spontaneous membrane potential oscillations was not a direct effect on the coupling between SPNs, but was a result of the abolition of spontaneous activity by Saffan. 6. The responses to Saffan occurred within the plasma concentration range of Saffan during anaesthesia, suggesting that the electrophysiological properties of SPNs may be altered during anaesthesia with Saffan. This would be expected to lead to changes in sympathetic tone and in the integration of sympathetic output.

Muscarinic modulation of the oscillatory and repetitive firing properties of entorhinal cortex layer II neurons.

Neurons in layer II of the entorhinal cortex (EC) are key elements in the temporal lobe memory system because they integrate and transfer into the hippocampal formation convergent sensory input from the entire cortical mantle. EC layer II also receives a profuse cholinergic innervation from the basal forebrain that promotes oscillatory dynamics in the EC network and may also implement memory function. To understand the cellular basis of cholinergic actions in EC, we investigated by intracellular recording in an in vitro rat brain slice preparation the muscarinic modulation of the electroresponsive properties of the two distinct classes of medial EC layer II projection neurons, the stellate cells (SCs) and non-SCs. In both SCs and non-SCs, muscarinic receptor activation with carbachol (CCh, 10-50 microM) caused atropine-sensitive (300 nM) membrane depolarization. In SCs, the CCh-induced membrane depolarization was associated with subthreshold membrane potential oscillations and "spike cluster" discharge, which are typically expressed by these cells on depolarization. CCh, however, caused a decrease of the dominant frequency of the membrane potential oscillations from 9.2 +/- 1.1 (SD) Hz to 6.3 +/- 1.1 Hz, as well as a decrease of the intracluster firing frequency from 18.1 +/- 1.7 Hz to 13.6 +/- 1.3 Hz. In addition, spike cluster discharge was less robust, and the cells tended to shift into tonic firing during CCh. In contrast to SCs, in non-SCs, CCh drastically affected firing behavior by promoting the development of voltage-dependent, long-duration (1-5 s) slow bursts of action potentials that could repeat rhythmically at slow frequencies (0.2-0.5 Hz). Concomitantly, the slow afterhyperpolarization (sAHP) was replaced by long-lasting plateau postdepolarizations. In both SCs and non-SCs, CCh also produced conspicuous changes on the action potential waveform and its afterpotentials. Notably, CCh significantly decreased spike amplitude and rate of rise, which suggests muscarinic modulation of a voltage-dependent Na+ conductance. Finally, we also observed that whereas CCh abolished the sAHP in both SCs and non-SCs, the membrane-permeant analogues of adenosine 3',5'-cyclic monophosphate, 8-(4-chlorophenylthio)-adenosine-cyclic monophosphate and 8-bromo-adenosine-cyclic-monophosphate, abolished the sAHP in SCs but not in non-SCs. The data demonstrate that cholinergic modulation further differentiates the intrinsic electroresponsiveness of SCs and non-SCs, and add support to the presence of two parallel processing systems in medial EC layer II that could thereby differentially influence their hippocampal targets. The results also indicate an important role for the cholinergic system in tuning the oscillatory dynamics of entorhinal neurons.

Subthreshold oscillations and resonant behavior: two manifestations of the same mechanism

The ability to generate subthreshold membrane potential oscillations in neurons from the inferior olive nucleus has been attributed to the electrical properties of these neurons, as well as to the properties of the network. In the present in vitro study we quantitatively characterized both intrinsic membrane and network properties that are directly involved in the oscillatory activity of olivary neurons in the guinea-pig. We also implemented an alternating current analysis to explore the resonance behavior of these neurons and to compare the resonant properties with the properties of the oscillatory activity. Spectral analysis, used for the quantitative characterization of the oscillatory activity under various experimental conditions, revealed that the pattern of the oscillatory activity is network specific rather than cell specific. These results are in agreement with the hypothesis that the oscillatory activity of olivary neurons is generated by a network of electrically coupled neurons. Using alternating current analysis, we found that impedance–frequency curves of olivary neurons demonstrate a peak impedance (resonance) at a frequency between 3 and 10 Hz, which corresponds to the frequency of the spontaneous oscillations. Like the spontaneous oscillations, this peak is tetrodotoxin insensitive, unaffected by K + channel blockers and almost completely blocked in the presence of Ni 2+ in the physiological solution. Increasing the temperature increases the resonance frequency, as well as the frequency of the spontaneous oscillations. These results show that the resonant behavior of individual neurons is the basis of the oscillatory behavior of the network and that resonance can serve as a lumped parameter which encodes the oscillatory tendency of a neuron.

Rhythmic firing of medial septum non-cholinergic neurons.

The presence of theta rhythm (5-10 Hz) in the hippocampus has been shown to enable long-term potentiation, a synaptic mechanism which has been proposed to underlie learning and memory. Medial septum cholinergic and GABAergic neurons that project to the hippocampus have been hypothesized to play conjointly a major role in the genesis of this rhythm. Building upon previous studies that have established the electrophysiological criteria for distinguishing cholinergic and non-cholinergic neurons in this area, it is demonstrated here that medial septum non-cholinergic neurons, putatively GABAergic, have the ability to discharge in rhythmic clusters of action potentials occurring at frequencies ranging from 1 to 8 Hz. Within the clusters, the firing frequency of action potentials varied between 13 and 57 Hz in a voltage-dependent manner. In addition, small voltage-dependent subthreshold membrane potential oscillations (16-54 Hz) were observed between clusters. Both subthreshold oscillations and clusters were eliminated by tetrodotoxin at 1 microM. These results indicate that non-cholinergic medial septum neurons could convey to the hippocampus not only theta but also higher frequency rhythmicity in the beta-gamma range (20-60 Hz).

Dissection and reduction of a modeled bursting neuron.

An 11-variable Hodgkin-Huxley type model of a bursting neuron was investigated using numerical bifurcation analysis and computer simulations. The results were applied to develop a reduced model of the underlying subthreshold oscillations (slow-wave) in membrane potential. Two different low-order models were developed: one 3-variable model, which mimicked the slow-wave of the full model in the absence of action potentials and a second 4-variable model, which included expressions accounting for the perturbational effects of action potentials on the slow-wave. The 4-variable model predicted more accurately the activity mode (bursting, beating, or silence) in response to application of extrinsic stimulus current or modulatory agents. The 4-variable model also possessed a phase-response curve that was very similar to that of the original 11-variable model. The results suggest that low-order models of bursting cells that do not consider the effects of action potentials may erroneously predict modes of activity and transient responses of the full model on which the reductions are based. These results also show that it is possible to develop low-order models that retain many of the characteristics of the activity of the higher-order system.

Subthreshold membrane-potential oscillations in immature rat CA3 hippocampal neurones.

Subthreshold membrane potential oscillations (MPOs) were recorded intracellularly in 31 of 43 (>70%) immature CA3 hippocampal neurones (from 3-17 days postnatally). MPOs (3-5 mV, 3-15 Hz) occurred at resting membrane potential (RMP) in 20 of 31 neurones, or following depolarization (11 of 31 neurones); with sufficient depolarization spontaneous action potentials (APs) were generated from the positive-going phase of MPOs. In all cells, MPOs were blocked by steady membrane hyperpolarization. Tetrodotoxin abolished MPOs (n = 4); Co(2+) markedly reduced them (n = 3), and tetraethylammonium, added in the presence of TTX, revealed lower frequency oscillatory activity (n = 2). We conclude that subthreshold MPOs in immature hippocampus, possibly linked to theta rhythm generation and memory acquisition, depend on voltage-dependent Na+ electrogenesis and they might be additionally controlled by Ca(2+) and K+ conductances.

Subthreshold oscillations of the membrane potential: a functional synchronizing and timing device.

1. Subthreshold membrane potential oscillations have been observed in different types of CNS neurons. In this in vitro study, we examined the possible role of these oscillations by analyzing the responses of neurons from the inferior olivary nucleus to a combined stimulation of sine wave and synaptic potentials. 2. A nonlinear summation of the sine wave and the synaptic potential occurred in olivary neurons; a superlinear summation occurred when the synaptic potential was elicited at the trough of the sine wave or during the rising phase. On the other hand, a less than linear summation occurred when the synaptic potentials were evoked during the falling phase of the wave. 3. Significant changes in the delay of the synaptic responses were observed. As a result of these changes, the maximum amplitude of the response occurred at the peak of the sine wave, regardless of the exact time of stimulation. The output of the neuron was therefore synchronized with the sine wave and depended only partly on the input phase. 4. These data demonstrate that neurons from the inferior olivary nucleus are capable of operating as accurate synchronizing devices. Moreover, by affecting the delay line, they act as a logic gate that ensures that the information will be added to the system only at given times.

Dye-coupling among neurons of the rat locus coeruleus during postnatal development

Simultaneous recordings from pairs of locus coeruleus neurons in neonatal rat brain slices previously demonstrated synchronous, subthreshold oscillations of membrane potential (rats < 24 days old) and electrotonic-coupling between 40% of pairs of neurons from rats less than 10 days old. 3 In the present study, slices from 1–21 day-old rats were stained with avidin-HRP—diaminobenzidine only if a single neuron per slice was impaled for longer than 10 min using an electrode containing biocytin. In slices from rats less than one week old, multiple stained neurons (3.8 ± 0.6 neurons/slice) were observed in 10 of 11 slices. Apparent contacts between stained neurons were observed at varying distances along dendrites. In rats older than one week significantly fewer multiple stained neurons were observed (three of 20 slices). The proportion of neurons displaying spontaneous subthreshold oscillations of membrane potential decreased with age, and the frequencies of subthreshold oscillations of membrane potential and entrained action potentials increased with age. The presence of multiple stained neurons was not correlated with the occurrence of subthreshold oscillations, cell input resistance, or the number of coupled neurons predicted from the shape of electrotonic potentials. In recordings from neurons displaying subthreshold oscillations, input resistance was lower and the number of coupled neurons predicted from electrotonic potentials was greater than in those without oscillations. These results suggest that low resistance pathways are common between locus coeruleus neurons in brain slices from rats younger than about one week old, consistent with previous electrotonic-coupling studies. Coupling between neurons may be partly responsible for synchronous oscillations, and could contribute to the widespread trophic role of noradrenergic neurons during early development.

Ionic mechanisms for the subthreshold oscillations and differential electroresponsiveness of medial entorhinal cortex layer II neurons

1. Layer II of the medial entorhinal cortex is composed of two electrophysiologically and morphologically distinct types of projection neurons: stellate cells (SCs), which are distinguished by rhythmic subthreshold oscillatory activity, and non-SCs. The ionic mechanisms underlying their differential electroresponsiveness, particularly in the subthreshold range of membrane potentials, were investigated in an "in vitro" slice preparation. 2. In both SCs and non-SCs, the apparent membrane input resistance was markedly voltage dependent, respectively decreasing or increasing at hyperpolarized or subthreshold depolarized potential levels. Thus the neurons displayed inward rectification in the hyperpolarizing and depolarizing range. 3. In the depolarizing range, inward rectification was blocked by tetrodotoxin (TTX, 1 microM) in both types of neurons and thus shown to depend on the presence of a persistent low-threshold Na+ conductance (gNap). However, in the presence of TTX, pronounced outward rectification became manifest in the subthreshold depolarizing range of membrane potentials (positive to -60 mV) in the SCs but not in the non-SCs. 4. The rhythmic subthreshold membrane potential oscillations that were present only in the SCs were abolished by TTX and not by Ca2+ conductance block with Cd2+ or Co2+. Subthreshold oscillations thus rely on the activation of voltage-gated Na+, and not Ca2+, conductances. The Ca2+ conductance block also had no effect on the subthreshold outward rectification. 5. Prominent time-dependent inward rectification in the hyperpolarizing range in the SCs persisted after Na(+)- and Ca2+ conductance block. This rectification was not affected by Ba2+ (1 mM), but was blocked by Cs+ (1-4 mM). Therefore, it is most probably generated by a hyperpolarization-activated cationic current (Q-like current). However, the Q-like current appears to play no major role in the generation of subthreshold rhythmic membrane potential oscillations, because these persisted in the presence of Cs+. 6. On the other hand, in the SCs, the fast, sustained, outward rectification that strongly developed (after Na+ conductance block) at the oscillatory voltage level was not affected by Cs+ but was blocked by Ba2+ (1 mM). Barium was also effective in blocking the subthreshold membrane potential oscillations. 7. In the non-SCs, which do not generate subthreshold rhythmic membrane potential oscillations or manifest subthreshold outward rectification in TTX, Ca2+ conductance block abolished spike repolarization and caused the development of long-lasting Na(+)-dependent plateau potentials at a high suprathreshold voltage level. At this level, where prominent delayed rectification is present, the Na+ plateaus sustained rhythmic membrane potential oscillations.(ABSTRACT TRUNCATED AT 400 WORDS)