Altered stimulus representation in rat auditory cortex is not causal for loss of consciousness under general anaesthesia.

BACKGROUND: Current concepts suggest that impaired representation of information in cortical networks contributes to loss of consciousness under anaesthesia. We tested this idea in rat auditory cortex using information theory analysis of multiunit responses recorded under three anaesthetic agents with different molecular targets: isoflurane, propofol, and dexmedetomidine. We reasoned that if changes in the representation of sensory stimuli are causal for loss of consciousness, they should occur regardless of the specific anaesthetic agent. METHODS: Spiking responses were recorded with chronically implanted microwire arrays in response to acoustic stimuli incorporating varied temporal and spectral dynamics. Experiments consisted of four drug conditions: awake (pre-drug), sedation (i.e. intact righting reflex), loss of consciousness (a dose just sufficient to cause loss of righting reflex), and recovery. Measures of firing rate, spike timing, and mutual information were analysed as a function of drug condition. RESULTS: All three drugs decreased spontaneous and evoked spiking activity and modulated spike timing. However, changes in mutual information were inconsistent with altered stimulus representation being causal for loss of consciousness. First, direction of change in mutual information was agent-specific, increasing under dexmedetomidine and decreasing under isoflurane and propofol. Second, mutual information did not decrease at the transition between sedation and LOC for any agent. Changes in mutual information under anaesthesia correlated strongly with changes in precision and reliability of spike timing, consistent with the importance of temporal stimulus features in driving auditory cortical activity. CONCLUSIONS: The primary sensory cortex is not the locus for changes in representation of information causal for loss of consciousness under anaesthesia.

Propofol-induced unresponsiveness is associated with impaired feedforward connectivity in cortical hierarchy.

BACKGROUND: Impaired consciousness has been associated with impaired cortical signal propagation after transcranial magnetic stimulation (TMS). We hypothesised that the reduced current propagation under propofol-induced unresponsiveness is associated with changes in both feedforward and feedback connectivity across the cortical hierarchy. METHODS: Eight subjects underwent left occipital TMS coupled with high-density EEG recordings during wakefulness and propofol-induced unconsciousness. Spectral analysis was applied to responses recorded from sensors overlying six hierarchical cortical sources involved in visual processing. Dynamic causal modelling (DCM) of induced time-frequency responses and evoked response potentials were used to investigate propofol's effects on connectivity between regions. RESULTS: Sensor space analysis demonstrated that propofol reduced both induced and evoked power after TMS in occipital, parietal, and frontal electrodes. Bayesian model selection supported a DCM with hierarchical feedforward and feedback connections. DCM of induced EEG responses revealed that the primary effect of propofol was impaired feedforward responses in cross-frequency theta/alpha-gamma coupling and within frequency theta coupling (F contrast, family-wise error corrected P<0.05). An exploratory analysis (thresholded at uncorrected P<0.001) also suggested that propofol impaired feedforward and feedback beta band coupling. Post hoc analyses showed impairments in all feedforward connections and one feedback connection from parietal to occipital cortex. DCM of the evoked response potential showed impaired feedforward connectivity between left-sided occipital and parietal cortex (T contrast P=0.004, Bonferroni corrected). CONCLUSIONS: Propofol-induced loss of consciousness is associated with impaired hierarchical feedforward connectivity assessed by EEG after occipital TMS.

Disruption of cortical network activity by the general anaesthetic isoflurane.

BACKGROUND: Actions of general anaesthetics on activity in the cortico-thalamic network likely contribute to loss of consciousness and disconnection from the environment. Previously, we showed that the general anaesthetic isoflurane preferentially suppresses cortically evoked synaptic responses compared with thalamically evoked synaptic responses, but how this differential sensitivity translates into changes in network activity is unclear. METHODS: We investigated isoflurane disruption of spontaneous and stimulus-induced cortical network activity using multichannel recordings in murine auditory thalamo-cortical brain slices. RESULTS: Under control conditions, afferent stimulation elicited short latency, presumably monosynaptically driven, spiking responses, as well as long latency network bursts that propagated horizontally through the cortex. Isoflurane (0.05-0.6 mM) suppressed spiking activity overall, but had a far greater effect on network bursts than on early spiking responses. At isoflurane concentrations >0.3 mM, network bursts were almost entirely blocked, even with increased stimulation intensity and in response to paired (thalamo-cortical + cortical layer 1) stimulation, while early spiking responses were <50% blocked. Isoflurane increased the threshold for eliciting bursts, decreased their propagation speed and prevented layer 1 afferents from facilitating burst induction by thalamo-cortical afferents. CONCLUSIONS: Disruption of horizontal activity spread and of layer 1 facilitation of thalamo-cortical responses likely contribute to the mechanism by which suppression of cortical feedback connections disrupts sensory awareness under anaesthesia.

Interactions between distinct GABA(A) circuits in hippocampus.

Synchronous activity among synaptically connected interneurons is thought to organize temporal patterns such as gamma and theta rhythms in cortical circuits. Interactions between distinct interneuron circuits may underlie more complex patterns, such as nested rhythms. Here, we demonstrate such an interaction between two groups of CA1 interneurons, GABA(A,slow) and GABA(A,fast) cells, that may contribute to theta and gamma rhythms, respectively. Stratum lacunosum-moleculare (SL-M) stimuli that activate GABA(A,slow) inhibitory postsynaptic currents (IPSCs) in pyramidal cells simultaneously depress the rate and amplitude of spontaneous GABA(A,fast) IPSCs for several hundred milliseconds. This suppression has a similar pharmacological profile to GABA(A,slow) IPSCs, and SL-M stimuli elicit GABA(A,slow) IPSCs in interneurons. We conclude that GABA(A,slow) cells inhibit both pyramidal cells and GABA(A,fast) interneurons and postulate that this interaction contributes to nested theta/gamma rhythms in hippocampus.

Kinetic differences between synaptic and extrasynaptic GABA(A) receptors in CA1 pyramidal cells.

GABA(A)-mediated IPSCs typically decay more rapidly than receptors in excised patches in response to brief pulses of applied GABA. We have investigated the source of this discrepancy in CA1 pyramidal neurons. IPSCs in these cells decayed rapidly, with a weighted time constant tau(Decay) of approximately 18 msec (24 degrees C), whereas excised and nucleated patch responses to brief pulses of GABA (2 msec, 1 mM) decayed more than three times as slowly (tau(Decay), approximately 63 msec). This discrepancy was not caused by differences between synaptic and exogenous transmitter transients because (1) there was no dependence of tau(Decay) on pulse duration for pulses of 0.6-4 msec, (2) responses to GABA at concentrations as low as 10 microM were still slower to decay (tau(Decay), approximately 41 msec) than IPSCs, and (3) responses of excised patches to synaptically released GABA had decay times similar to brief pulse responses. These data indicate that the receptors mediating synaptic versus brief pulse responses have different intrinsic properties. However, synaptic receptors were not altered by the patch excision process, because fast, spontaneous IPSCs could still be recorded in nucleated patches. Elevated calcium selectively modulated patch responses to GABA pulses, with no effect on IPSCs recorded in nucleated patches, demonstrating the presence of two receptor populations that are differentially regulated by intracellular second messengers. We conclude that two receptor populations with distinct kinetics coexist in CA1 pyramidal cells: slow extrasynaptic receptors that dominate the responses of excised patches to exogenous GABA applications and fast synaptic receptors that generate rapid IPSCs.

Intracellular recordings from neurobiotin-labeled cells in brain slices of the rat medial nucleus of the trapezoid body.

Principal cells in the medial nucleus of the trapezoid body (MNTB) are believed to be critical components in the circuit subserving sound localization. These cells, located in the superior olivary complex, convert excitatory inputs, arriving from the contralateral cochlear nucleus by way of large somatic synapses (the calyces of Held), to inhibitory projections onto principal cells in the ipsilateral lateral superior olive (LSO). We have characterized a population of cells in the rat MNTB using intracellular recording and labeling techniques in a brain slice preparation. MNTB principal cells had spherical or ellipsoid somata that gave rise to single large-diameter dendrites, which branched extensively and often extended beyond the borders of MNTB. Commonly observed axonal projection targets included LSO, the superior paraolivary nucleus, and the medial superior olive, and occasionally the lateral nucleus of the trapezoid body. The projections of individual MNTB cells showed an orderly topography that is consistent with the known tonotopic maps of the nuclei. In response to current injection, principal cells exhibited several nonlinearities, including rectification for depolarizing currents and a "sag" in the membrane potential for hyperpolarizing currents. Superthreshold depolarizing currents elicited transient firing behavior. Application of the potassium channel blocker 4-aminopyridine reduced or eliminated the rectification in the current-voltage relationships and caused depolarizing currents to elicit repetitive firing. Stimulation of afferent inputs elicited short-latency spikes, presumably driven by calyceal synaptic inputs; long-latency, presumably polysynaptic, EPSPs; and short- and long-latency IPSPs. The duration of synaptic events was strongly dependent on membrane potential, and this effect was probably due to the intrinsic membrane properties of the cell. In all cases tested, EPSPs were blocked by CNQX or DNQX, and IPSPs were blocked by strychnine. Two injected non-principal cells differed from principal cells in their morphologies and physiological characteristics.

Dual actions of volatile anesthetics on GABA(A) IPSCs: dissociation of blocking and prolonging effects.

BACKGROUND: Volatile agents alter inhibitory postsynaptic currents (IPSCs) at clinically relevant concentrations, an action that is thought to make an important contribution to their behavioral effects. The authors investigated the mechanisms underlying these effects by evaluating the concentration dependence of modulation by enflurane, isoflurane, and halothane of IPSCs in rat hippocampal slices. METHODS: Action potential-independent gamma-aminobutyric acid(A) IPSCs (miniature IPSCs [mIPSCs]) were recorded from CA1 pyramidal neurons. The effects on mIPSC amplitude were used to distinguish between presynaptic (altered release) and postsynaptic (altered receptor response) actions of volatile agents. The concentration dependence of blocking and prolonging actions was compared among the volatile agents to determine whether a single modulatory process could account for both effects. RESULTS: The application of volatile anesthetics prolonged the decay and reduced the amplitude of mIPSCs in a dose-dependent manner. The effects on decay time for isoflurane and enflurane could not be distinguished. However, the blocking effect of enflurane was significantly greater than that of isoflurane at all concentrations. Despite the blocking effect, the net action of these agents was enhanced inhibition, because charge transfer was always significantly greater than control. Isoflurane, and to a lesser extent enflurane and halothane, caused a picrotoxin-sensitive increase in baseline noise. Moderate increases in mIPSC frequency were also observed for all agents. CONCLUSIONS: These results show that enflurane, isoflurane, and halothane reduce IPSC amplitude through a direct postsynaptic action. Furthermore, the concentration dependence of the actions of the agents reveals a dissociation between the effects on the amplitude and the time course of IPSCs, suggesting that distinct mechanisms underlie the two actions.

Regularity analysis in a compartmental model of chopper units in the anteroventral cochlear nucleus.

1. We investigate the discharge patterns of chopper units in the anteroventral cochlear nucleus (AVCN) by developing an equivalent cylinder compartmental model of AVCN stellate cells, which are the sources of the chopper response pattern. The model consists of a passive dendritic tree connected to somatic and axonal compartments with voltage-sensitive channels. Synaptic inputs to the model are simulated auditory nerve fiber responses to best-frequency tones. 2. We adjust the anatomic and electrical parameters of the model to agree with available intracellular data from stellate cells in the AVCN of the mouse and the cat and compare the response of the model to injected current with responses recorded in vitro. The model shows approximately linear current-voltage characteristics for small hyperpolarizing currents. The model's input resistance and the time course of its response to hyperpolarizing current applied at the soma are comparable with those measured from stellate cells in vitro. In response to sustained depolarizing current, the model fires repetitively with nearly perfect regularity, a property also observed in vitro. 3. Auditory nerve inputs to the cell are modeled as deadtime-modified Poisson processes with a multiexponential adaptation in the Poisson rate. We are able to adjust the number, rate, and location of excitatory and inhibitory inputs to the model and succeed in simulating chopper response patterns seen in vivo. 4. Chopper units exhibit a variety of regularity and adaptation patterns in response to tone stimuli. Physiological data from brain slice experiments and experiments in vivo imply that this heterogeneity is primarily due to differences in input configurations. By systematically varying the number and position of excitatory and inhibitory inputs, we can simulate a range of chopper response patterns. 5. We quantify the regularity of the model's response using the coefficient of variation (CV) of the interspike interval. We find that the CV decreases, i.e., the regularity increases, as the number of converging inputs or their distance from the soma increases. The regularity of the output is more sensitive to the number of converging inputs than to their location on the dendritic tree. The statistics of the first spike latency (FSL) are also sensitive to the configuration of excitatory inputs. The mean and minimum FSL are more sensitive to the electrotonic distance of the inputs from the soma than to the number of inputs, whereas the standard deviation of the FSL is highly dependent on the number of converging inputs and is nearly independent of their location.(ABSTRACT TRUNCATED AT 400 WORDS)

Hyperpolarization-activated cation current (Ih) in neurons of the medial nucleus of the trapezoid body: voltage-clamp analysis and enhancement by norepinephrine and cAMP suggest a modulatory mechanism in the auditory brain stem.

1. Principal cells in the medial nucleus of the trapezoid body (MNTB) are part of a circuit in the superior olivary complex (SOC) that processes binaural information important for sound localization. MNTB cells have two voltage-dependent currents active near rest that contribute to these cells' highly nonlinear membrane properties and shape their responses to synaptic input. One of these currents, a low-threshold, 4-aminopyridine (4-AP)-sensitive K+ current, has been studied previously under current clamp. Using the single-electrode voltage-clamp technique, we have investigated the other of these currents, a hyperpolarization-activated, mixed cation current (Ih), in brain slices of the rat SOC. 2. Ih is responsible for a prominent "sag" in the voltage response to a steady hyperpolarizing current recorded under current clamp in MNTB cells. In voltage-clamp recordings, hyperpolarizing voltage steps from the resting potential elicited a large inward current that activated and deactivated with biexponential kinetics. Activation time constants were voltage dependent, with tau 1 and tau 2 = 246 and 1620 ms at -75 mV and 107 and 560 ms at -100 mV. 3. Ih was blocked by 1-5 mM cesium and had a reversal potential of -43 mV. Steady-state activation curves derived from tail currents yielded a half-activation voltage of -75.7 mV and slope factor of 5.7 mV, corresponding to < 10% activation of Ih at rest. 4. Application of norepinephrine (15-20 microM) or 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) (1 mM) caused a depolarizing shift in the steady-state activation curve and decreased the activation time constants. The shift in the activation curve resulted in a large increase in the activation of Ih at rest, an inward shift in the holding current, and an increase in the resting membrane conductance. In current-clamp recordings, this increase in the resting activation level of Ih resulted in membrane depolarization of 2-3 mV in the absence of 4-AP, and 5-10 mV in the presence of 4-AP, an increase in the input conductance, and a reduction in the voltage sag in response to hyperpolarizing currents. 5. The resulting change in the resting point of MNTB cells exposed to norepinephrine or 8-Br-cAMP is likely to alter the responses of these cells to synaptic input, both via the direct effect on the resting membrane conductance and by changing the activation of the low-threshold, 4-AP-sensitive potassium current.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+- and voltage-dependent inactivation of Ca2+ channels in nerve terminals of the neurohypophysis.

Ca2+ channel inactivation was investigated in neurohypophysial nerve terminals by using patch-clamp techniques. The contribution of intracellular Ca2+ to inactivation was evaluated by replacing Ca2+ with Ba2+ or by including BAPTA in the internal recording solution. Ca2+ channel inactivation during depolarizing pulses was primarily voltage-dependent. A contribution of intracellular Ca2+ was revealed by comparing steady-state inactivation of Ca2+ channels with Ca2+ current and with intracellular [Ca2+]. However, this contribution was small compared to that of voltage. In contrast to voltage-gated Ca2+ channels in other preparations, in the neurohypophysis Ba2+ substitution or intracellular BAPTA increased the speed of inactivation while reducing the steady-state level of inactivation. Ca2+ channel recovery from inactivation was studied by using a paired-pulse protocol. The rate of Ca2+ channel recovery from inactivation at negative potentials was increased dramatically by Ba2+ substitution or intracellular BAPTA, indicating that intracellular Ca2+ inhibits recovery. Stimulation with trains of brief pulses designed to mimic physiological bursts of electrical activity showed that Ca2+ channel inactivation was much greater with 20 Hz trains than with 14 Hz trains. Inactivation induced by 20 Hz trains was reduced by intracellular BAPTA, suggesting an important role for Ca2+-dependent inactivation during physiologically relevant forms of electrical activity. Inhibitors of calmodulin and calcineurin had no effect on Ca2+ channel inactivation, arguing against a mechanism of inactivation involving these Ca2+-dependent proteins. The inactivation behavior described here, in which voltage effects on Ca2+ channel inactivation predominate at positive potentials and Ca2+ effects predominate at negative potentials, may be relevant to the regulation of neuropeptide release.

The synaptic basis of GABAA,slow.

Although two kinetically distinct evoked GABAA responses (GABAA,fast and GABAA,slow) have been observed in CA1 pyramidal neurons, studies of spontaneous IPSCs (sIPSCs) in these neurons have reported only a single population of events that resemble GABAA,fast in their rise and decay kinetics. The absence of slow sIPSCs calls into question the synaptic basis of GABAA,slow. We present evidence here that both evoked responses are synaptic in origin, because two classes of minimally evoked, spontaneous and miniature IPSCs exist that correspond to GABAA,fast and GABAA,slow. Slow sIPSCs occur infrequently, suggesting that the cells underlying these events have a low spontaneous firing rate, unlike the cells giving rise to fast sIPSCs. Like evoked GABAA,fast and GABAA,slow, fast and slow sIPSCs are modulated differentially by furosemide, a subtype-specific GABAA antagonist. Furosemide blocks fast IPSCs by acting directly on the postsynaptic receptors, because it reduces the amplitude of both miniature IPSCs and the responses of excised patches to applied GABA. Thus, in the hippocampus, parallel inhibitory circuits are composed of separate populations of interneurons that contact anatomically segregated and pharmacologically distinct postsynaptic receptors.

Layer-specific properties of the transient K current (IA) in piriform cortex.

Piriform cortex in the rat is highly susceptible to induction of epileptiform activity. Experiments in vivo and in vitro indicate that this activity originates in endopiriform nucleus (EN). In slices, EN neurons are more excitable than layer II (LII) pyramidal cells, with more positive resting potentials and lower spike thresholds. We investigated potassium currents in EN and LII to evaluate their contribution to these differences in excitability. Whole-cell currents were recorded from identified cells in brain slices. A rapidly inactivating outward current (IA) had distinct properties in LII (IA,LII) versus EN (IA,EN). The peak amplitude of IA,EN was 45% smaller than IA,LII, and the kinetics of activation and inactivation was significantly slower for IA,EN. The midpoint of steady-state inactivation was hyperpolarized by 10 mV for IA,EN versus IA,LII, whereas activation was similar in the two cell groups. Other voltage-dependent potassium currents were indistinguishable between EN and LII. Simulations using a compartmental model of LII cells argue that different cellular distributions of IA channels in EN versus LII cells cannot account for these differences. Thus, at least some of the differences are intrinsic to the channels themselves. Current-clamp simulations suggest that the differences between IA,LII and IA,EN can account for the observed difference in resting potentials between the two cell groups. Simulations show that this difference in resting potential leads to longer first spike latencies in response to depolarizing stimuli. Thus, these differences in the properties of IA could make EN more susceptible to induction and expression of epileptiform activity.

Networks of interneurons with fast and slow gamma-aminobutyric acid type A (GABAA) kinetics provide substrate for mixed gamma-theta rhythm.

During active exploration, hippocampal neurons exhibit nested rhythmic activity at theta ( approximately 8 Hz) and gamma ( approximately 40 Hz) frequencies. Gamma rhythms may be generated locally by interactions within a class of interneurons mediating fast GABA(A) (GABA(A,fast)) inhibitory postsynaptic currents (IPSCs), whereas theta rhythms traditionally are thought to be imposed extrinsically. However, the hippocampus contains slow biophysical mechanisms that may contribute to the theta rhythm, either as a resonance activated by extrinsic input or as a purely local phenomenon. For example, region CA1 of the hippocampus contains a slower class of GABA(A) (GABA(A,slow)) synapses, believed to be generated by a distinct group of interneurons. Recent evidence indicates that these GABA(A,slow) interneurons project to the GABA(A, fast) interneurons that contribute to hippocampal gamma rhythms. Here, we use biophysically based simulations to explore the possible ramifications of interneuronal circuits containing separate classes of GABA(A,fast) and GABA(A,slow) interneurons. Simulated interneuronal networks with fast and slow synaptic kinetics can generate mixed theta-gamma rhythmicity under restricted conditions, including strong connections among each population, weaker connections between the two populations, and homogeneity of cellular properties and drive. Under a broader range of conditions, including heterogeneity, the networks can amplify and resynchronize phasic responses to weak phase-dispersed external drive at theta frequencies to either GABA(A,slow) or GABA(A,fast) cells. GABA(A, slow) synapses are necessary for this process of amplification and resynchronization.