Sodium pentobarbitone enhances responses of thalamic relay neurones to GABA in rat brain slices

Microglia facilitate and stabilize the response to general anesthesia via modulating the neuronal network in a brain region-specific manner

General anesthesia leads to a loss of consciousness and an unrousable state in patients. Although general anesthetics are widely used in clinical practice, their underlying mechanisms remain elusive. The potential involvement of nonneuronal cells is unknown. Microglia are important immune cells in the central nervous system (CNS) that play critical roles in CNS function and dysfunction. We unintentionally observed delayed anesthesia induction and early anesthesia emergence in microglia-depleted mice. We found that microglial depletion differentially regulates neuronal activities by suppressing the neuronal network of anesthesia-activated brain regions and activating emergence-activated brain regions. Thus, microglia facilitate and stabilize the anesthesia status. This influence is not mediated by dendritic spine plasticity. Instead, it relies on the activation of microglial P2Y12 and subsequent calcium influx, which facilitates the general anesthesia response. Together, we elucidate the regulatory role of microglia in general anesthesia, extending our knowledge of how nonneuronal cells modulate neuronal activities.

GABA(A) receptor changes in acute allopregnanolone tolerance

To study acute tolerance, rats were anesthetized with interrupted i.v. allopregnanolone infusions where the "silent second" in the electroencephalogram (EEG) was the target. Animals were killed either directly at the first silent second or at the silent second level after 30 or 90 min of anaesthesia. Acute tolerance was demonstrated at 90 min of anaesthesia as earlier shown. In situ hybridization showed a decreased expression of the gamma-aminobutyric acid(A) (GABA(A)) receptor subunit alpha4mRNA amount in the thalamus ventral-posteriomedial nucleus of the tolerant rats. A parallel change in the abundance of the alpha4 subunit was detected with immunohistochemistry. The increase in maintenance dose rate (MDR) was significantly negatively correlated with the alpha4mRNA in the thalamus ventral-posteriomedial nucleus, and positively correlated with alpha2mRNA in different hippocampal subregions. There was also a positive relationship between the alpha1mRNA amounts in the different hippocampal subregions, with significant differences between groups. These changes in GABA(A) receptor subunits mRNA expression and protein (alpha4) might be of importance for the development of acute tolerance to allopregnanolone.

Selective GABA-receptor actions of amobarbital on thalamic neurons

1. We studied amobarbital's effects on membrane properties and currents, and electrically evoked inhibitory postsynaptic currents (IPSCs) mediated by gamma-aminobutyric acid (GABA) in rat thalamic slices. Using concentration-response relationships, we compared amobarbital's effects in nociceptive nuclei and non-nociceptive nucleus reticularis thalami (nRT). 2. Amobarbital decreased input resistance by activating GABA(A) receptors. Amobarbital produced a larger decrease in ventrobasal than nRT neurons. 3. Amobarbital depressed burst and tonic firing. Depression of burst firing was more effective, particularly in ventrobasal and intralaminar neurons. Depression was reversed by GABA(A) antagonists, and surmountable by increasing current injection, implicating a receptor-mediated shunt mechanism. 4. Amobarbital did not affect the tetrodotoxin-isolated low threshold Ca(2+) spike during GABA(A) blockade. Amobarbital reduced excitability without altering outward leak, or hyperpolarisation-activated inward currents. 5. Amobarbital increased mean conductance and burst duration of single GABA(A) channels. Consistent with this, amobarbital increased amplitude and decay time of IPSCs with distinct EC(50)s, implicating actions at two GABA(A) receptor sites. 6. Activation of GABA(A) receptors by low concentrations, fast IPSC amplitude modulation, and failure to affect intrinsic currents distinguished amobarbital's mechanism of action from previously characterised barbiturates. The selective actions of amobarbital on GABA(A) receptor may have relevance in explaining anaesthetic and analgesic uses.

Pentobarbital modulates intrinsic and GABA-receptor conductances in thalamocortical inhibition

We investigated interactions of an anesthetic barbiturate, pentobarbital, with non-ligand gated channels and identified inhibitory synaptic transmission in thalamic neurons. Using whole cell voltage-clamp, current-clamp and single channel recording techniques in rat ventrobasal neurons of slices and dispersed preparations, we determined the mechanisms of pentobarbital actions on ionic currents and inhibitory postsynaptic currents (IPSCs), mediated by aminobutyric acid (GABA). We investigated pentobarbital effects on intrinsic currents using hyperpolarizing voltage commands from rest and tetrodotoxin blockade of action potentials. At concentrations near 8 microM, pentobarbital increased input conductance and induced net outward current, I(PB), at potentials near action potential threshold. The reversal potential of I(PB) was -75 mV, implicating K+ and other ions. Cs+ (3 mM) which inhibits both K+ currents and inward rectifier (Ih), completely blocked IPB, whereas the selective Ih blocker, ZD-7288 (25 microM), or Ba2+ (2 mM) which suppresses only K+ currents, reduced IPB. Pentobarbital inhibited the Ih, consistent with a ZD-7288-induced shift in reversal potential for IPB toward K+ equilibrium potential. Pentobarbital increased the inward K+ rectifier, IKir, and leak current, Ileak. We determined the susceptibility of IPSCs, evoked by reticular stimulation, to antagonism by bicuculline, picrotoxinin and 2-hydroxysaclofen and identified their receptor subclass components. At EC50 = 53 microM, pentobarbital increased the duration of IPSCs. The prolonged IPSC duration during pentobarbital was attributable to enhanced open probability of GABAA channels, because combined with GABA, pentobarbital application increased mean channel open time without affecting channel conductance. At concentrations up to 100 microM, pentobarbital did not directly activate GABAA receptors. The concentration-response relationships for pentobarbital effects on the intrinsic currents and IPSCs overlapped, implying multiple sites of action and possible redundancy in anesthetic mechanisms. This is the first study to show that an i.v. anesthetic modulates the intrinsic currents, Ih, IKir, and Ileak, as well as IPSC time course in the same neurons. These effects likely underlie inhibition in thalamocortical neurons during pentobarbital anesthesia.

Pentobarbital depressant effects are independent of GABA receptors in auditory thalamic neurons

Pentobarbital, a general anesthetic, has received extensive study for its ability to potentiate inhibition at GABA(A) subtype of receptors for GABA. Using whole cell current-clamp techniques and bath applications, we determined the effects of pentobarbital and GABA receptor antagonists on the membrane properties and tonic or burst firing of medial geniculate neurons in thalamic slices. Pentobarbital (0.01-200 microM) induced depressant effects in 50 of 66 neurons (76%). Pentobarbital hyperpolarized neurons by a mean of 3 mV and decreased the number of action potentials in tonic firing, evoked by current pulse injection from near the resting potential. Pentobarbital also decreased burst firing or low threshold Ca(2+)-spikes, evoked by current pulse injection into neurons at potentials hyperpolarized from rest. The blockade of tonic and burst firing, as well as low threshold Ca(2+)-spikes, was surmountable by increasing the amplitude of input current. The GABA(A) receptor antagonists, bicuculline (100 microM) and picrotoxinin (50-100 microM), did not block the depressant effects of pentobarbital (10 microM). The GABA(B) receptor antagonist, saclofen (200 microM), and GABA(C) receptor antagonist, (1,2,3,6-tetrahydropyridine-4-yl)methylphosphinate (10-50 microM), did not significantly alter the depressant effects. Pentobarbital produced excitatory effects (0.1-50 microM) on 11 neurons (17%) but had no effects on 5 neurons (7%). The excitation consisted of approximately 3 mV depolarization, increased tonic and burst firing and the rate of rise and amplitude of low threshold Ca(2+) spikes. These effects were associated with a increase in input resistance. In contrast, the depressant effects of pentobarbital correlated to a decreased input resistance measured with hyperpolarizing current pulse injection (IC(50) = 7.8 microM). Pentobarbital reduced Na(+)-dependent rectification on depolarization and lowered the slope resistance over a wide voltage range. Tetrodotoxin eliminated both Na(+)-dependent rectification and the pentobarbital-induced decrease in membrane resistance at depolarized voltages in two-thirds of the neurons. The pentobarbital-induced decrease in membrane resistance at voltages hyperpolarized from rest was not evident during co-application with Cs(+), known to block the hyperpolarization-activated rectifiers. In summary, the pentobarbital acted at low concentrations to depress thalamocortical neurons. The depression resulted from decreased rectification on depolarization, which no longer boosted potentials over threshold, and an increased conductance that shunted spike generation. The depressant effects of pentobarbital did not involve known types of GABA receptor interactions.

Mechanism of anesthesia revealed by shunting actions of isoflurane on thalamocortical neurons

By using thalamic brain slices from juvenile rats and the whole cell recording technique, we determined the effects of aqueous applications of the anesthetic isoflurane (IFL) on tonic and burst firing activities of ventrobasal relay neurons. At concentrations equivalent to those used for in vivo anesthesia, IFL induced a hyperpolarization and increased membrane conductance in a reversible and concentration-dependent manner (ionic mechanism detailed in companion paper). The increased conductance short-circuited the effectiveness of depolarizing pulses and was the main cause for inhibition of tonic firing of action potentials. Despite the IFL-induced hyperpolarization, which theoretically should have promoted bursting, the shunt blocked the low-threshold Ca2+ spike (LTS) and associated burst firing of action potentials as well as the high-threshold Ca2+ spike (HTS). Increasing the amplitude of either the depolarizing test pulse or hyperpolarizing prepulse or increasing the duration of the hyperpolarizing prepulse partially reversed the blockade of the LTS burst. In voltage-clamp experiments on the T-type Ca2+ current, which produces the LTS, IFL decreased the spatial distribution of imposed voltages and hence impaired the activation of spatially distant T channels. Although IFL may have increased a dendritic leak conductance or decreased dendritic Ca2+ currents, the somatic shunt appeared to block initiation of the LTS and HTS as well as their electrotonic propogation to the axon hillock. In summary, IFL hyperpolarized thalamocortical neurons and shunted voltage-dependent Na+ and Ca2+ currents. Considering the importance of the thalamus in relaying different sensory modalities (i.e., somatosensation, audition, and vision) and motor information as well as the corticothalamocortical loops in mediating consciousness, the shunted firing activities of thalamocortical neurons would be instrumental for the production of anesthesia in vivo.

Antiepileptic drug cellular mechanisms of action: where does lamotrigine fit in?

Current frontline antiepileptic drugs tend to fall into several cellular mechanistic categories, and these categories often correlate with the clinical spectrum of action of the various antiepileptic drugs. Many antiepileptic drugs effective in control of partial and generalized tonic-clonic seizures are use- and voltage-dependent blockers of sodium channels. This mechanism selectively dampens pathologic activation of sodium channels, without interacting with normal sodium channel function. Examples include phenytoin, carbamazepine, valproic acid, and lamotrigine. Many antiepileptic drugs effective in control of generalized absence seizures block low threshold calcium currents. Low threshold calcium channels are present in high densities in thalamic neurons, and these channels trigger regenerative bursts that drive normal and pathologic thalamocortical rhythms, including the spike wave discharges of absence seizures. Examples include ethosuximide, trimethadione, and methsuximide. Several antiepileptic drugs that have varying clinical actions interact with the gamma-amino-butyric acid (GABA)ergic system. Diazepam and clonazepam selectively augment function of a subset of GABAA receptors, and these drugs are broad-spectrum antiepileptic drugs. In contrast, barbiturates augment function of all types of GABAA receptors, and are ineffective in control of generalized absence seizures, but effective in control of many other seizure types. Tiagabine and vigabatrin enhance cerebrospinal levels of GABA by interfering with reuptake and degradation of GABA, respectively. These antiepileptic drugs are effective in partial seizures. Lamotrigine is effective against both partial and generalized seizures, including generalized absence seizures. Its sole documented cellular mechanism of action is sodium channel block, a mechanism shared by phenytoin and carbamazepine. These drugs are ineffective against absence seizures. Consequently, unless there are unique aspects to the sodium channel block by lamotrigine, it seems unlikely that this mechanism alone could explain its broad clinical efficacy. Therefore, lamotrigine may have as yet uncharacterized cellular actions, which could combine with its sodium channel blocking actions, to account for its broad clinical efficacy.

Cerebellar-responsive neurons in the thalamic ventroanterior-ventrolateral complex of rats: in vivo electrophysiology

In vivo intracellular recordings were obtained from identified thalamocortical neurons in the ventroanterior-ventrolateral complex in urethane-anesthetized rats. This thalamic nucleus has few interneurons. Neurons that responded to cerebellar stimulation were injected intracellularly with horseradish peroxidase or biocytin and examined with light and electron microscopy (see companion paper). Intrinsic membrane properties and voltage-dependent rhythmic activity of cerebellar-responsive ventroanterior-ventrolateral neurons were similar to those described previously for thalamic neurons. Thus, in addition to conventional "fast" Na(+)-dependent spikes, rat ventroanterior-ventrolateral neurons had "slow" Ca(2+)-mediated low-threshold spikes and membrane conductances that supported rhythmic oscillations. Two modes of spontaneous activity were observed: (i) a tonic firing pattern that consisted of irregularly occurring fast spikes that predominated when the membrane potential was more positive than about -60 mV, and (ii) a rhythmic firing pattern, observed when the membrane potential was more negative than about -65 mV, composed of periodic (4-8 Hz) membrane hyperpolarizations and ramp depolarizations that often produced a low-threshold spike and a burst of fast spikes. In some neurons, spontaneous fast prepotentials were also observed, often with a relatively constant rate (up to 70 Hz). Cerebellar stimulation elicited excitatory postsynaptic potentials that in some cases appeared to be all-or-none and were similar in form to fast prepotentials. Stimulation of ipsilateral motor cortex elicited a short-latency antidromic response followed by a monosynaptic excitatory postsynaptic potential, which had a slower rise time than excitatory postsynaptic potentials evoked from cerebellum, suggesting that cortical inputs were electrotonically distal to cerebellar inputs. In the presence of moderate membrane hyperpolarization, the cortically evoked excitatory postsynaptic potential was followed by a long-lasting hyperpolarization (100-400 ms duration), a rebound depolarization and one or two cycles resembling spontaneous rhythmic activity. Membrane conductance was increased during the initial component of the long hyperpolarization, much of which was probably due to an inhibitory postsynaptic potential. In contrast, membrane conductance was unchanged or slightly decreased during the latter three-quarters of the long hyperpolarization. The amplitude of this component of the long hyperpolarization usually decreased when the membrane was hyperpolarized with intracellular current injection. Thus, both disfacilitation and an inhibitory postsynaptic potential may have contributed to the latter portion of the cortically-evoked long hyperpolarization. The cortically-evoked inhibitory postsynaptic potentials likely originated predominantly from feedforward activation of GABAergic neurons in the thalamic reticular nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Intrinsic and synaptically generated delta (1-4 Hz) rhythms in dorsal lateral geniculate neurons and their modulation by light-induced fast (30-70 Hz) events

Thalamocortical neurons of cat dorsal lateral geniculate nucleus were recorded under urethane anesthesia. Neurons were identified by antidromic invasion from the internal capsule and by orthodromic stimulation from the optic chiasm or light stimuli. An intrinsic oscillation within the frequency of sleep delta waves (1-4 Hz) was induced by hyperpolarizing current pulses triggering a rhythmic sequence of low-threshold spikes alternating with after hyperpolarizing potentials. The increased propensity to oscillation after blockage of inputs arising in the retina indicates that afferent synaptic drives interfere with the intrinsic oscillation of lateral geniculate cells. The relatively rare occurrence of this type of oscillation in impaled neurons, as compared with extracellular recordings in the same nucleus or to intracellular recordings in other dorsal thalamic nuclei, suggests that the interplay between the two intrinsic currents generating delta oscillation is particularly critical in lateral geniculate cells. Another type of delta oscillation was characterized by excitatory postsynaptic potentials which gave rise to action potentials or to low-threshold spikes at more depolarized or hyperpolarized levels, respectively. It is suggested that this rhythm reflects synaptic coupling by intranuclear recurrent axonal collaterals. Light stimulation induced fast (30-70 Hz) excitatory events that were blocked after lidocaine injections into the eye. In all tested cells, changes in the ambient luminosity of the experimental room blocked the intrinsic as well as the synaptic oscillation within the delta frequency. In some cells, this suppressing effect was associated with depolarization and increased firing rate. These results demonstrate different types of sleep delta oscillations in visual thalamic neurons and show that they are modulated not only by brainstem regulatory systems, but also by specific drives along the visual channel.

Intracellular evidence for incompatibility between spindle and delta oscillations in thalamocortical neurons of cat

Recent studies have revealed that the thalamus does not only generate spindle oscillations (7-14 Hz), but that it also participates in the genesis of a slower (less than 4 Hz) rhythm within the frequency range of delta waves on the electroencephalogram. In thalamic cells, delta is an intrinsic oscillation consisting of low-threshold spikes alternating with afterhyperpolarizing potentials. It is known from electroencephalographic recordings in humans and animals that slow or delta waves prevail during late sleep stages, whereas spindle oscillations are characteristic for the early stages of sleep. We studied the dependence of spindles and delta oscillations on membrane potential, as well as the effects of spindles on delta oscillations, in thalamocortical neurons of cats under urethane anesthesia and in cerveau isolé preparations (low collicular transections). Spindles appeared at membrane potentials between -55 and -65 mV, whereas delta oscillations occurred by bringing the membrane potential between -68 and -90 mV. Spindles either evoked by cortical stimulation or occurring spontaneously in cerveau isolé preparations prevented delta oscillations. This effect was probably due to the increase in membrane conductance associated with spindles. Barbiturates also blocked delta activity in thalamocortical neurons, probably through the same mechanism. A certain degree of incompatibility between spindles and delta rhythms in thalamocortical cells may explain the prevalence of these two types of oscillations during different stages of sleep with synchronization of the electroencephalogram.

Electrophysiology of a slow (0.5-4 Hz) intrinsic oscillation of cat thalamocortical neurones in vivo

1. Electrophysiologically identified thalamocortical neurones have been intra- and extracellularly recorded in acutely prepared cats, under different anaesthetic conditions. 2. A slow (0.5-4 Hz) membrane potential oscillation was observed in thalamocortical cells recorded in motor, sensory, associational and intralaminar thalamic nuclei. The oscillation consisted of rhythmic low-threshold spikes alternating with after-hyperpolarizations. 3. About 80% of the neurones with intact cortical connections were set into the slow oscillatory mode by bringing their membrane potential to between -68 and -90 mV. The oscillation did not depend upon the occurrence of fast action potentials and did not outlast the imposed hyperpolarization. 4. Anatomical or functional disconnection from related cortical areas resulted in a membrane potential hyperpolarization of about 9 mV and in the occurrence of spontaneous slow oscillations in virtually all recorded neurones. The intrinsic nature of the phenomenon was supported by the lack of rhythmic postsynaptic potentials as the cells were prevented from oscillating by outward current injection. 5. In contrast with other thalamic nuclei, the slow oscillation has not been observed in anterior thalamic neurones despite their having similar basic electrophysiological properties. 6. Barbiturate administration suppressed the slow oscillatory mode, an effect accompanied by a decrease in the membrane input resistance. 7. Multiunit recordings of spontaneously oscillating cells showed epochs characterized by phase-related firing. This synchronous discharge was paralleled by a clear-cut build-up of field potentials in the frequency range of electroencephalogram slow or delta waves. 8. These results demonstrate that the majority of thalamocortical neurones are endowed with electrophysiological properties allowing them to oscillate at 0.5-4 Hz, if they have a membrane potential more negative than -65 mV and a high input resistance. Such a condition is physiologically achieved in the deepest stages of electroencephalogram-synchronized sleep, as a result of brain stem-thalamic as well as cortico-thalamic deafferentation. We postulate a thalamic contribution in the genesis of electroencephalogram delta waves during slow wave sleep, once independently oscillating thalamocortical cells become in phase.