Volatile anaesthetics inhibit a cyclic AMP-dependent sodium-potassium current in cultured sensory neurones of bullfrog

HCN Channels: New Therapeutic Targets for Pain Treatment

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are highly regulated proteins which respond to different cellular stimuli. The HCN currents (I_h) mediated by HCN1 and HCN2 drive the repetitive firing in nociceptive neurons. The role of HCN channels in pain has been widely investigated as targets for the development of new therapeutic drugs, but the comprehensive design of HCN channel modulators has been restricted due to the lack of crystallographic data. The three-dimensional structure of the human HCN1 channel was recently reported, opening new possibilities for the rational design of highly-selective HCN modulators. In this review, we discuss the structural and functional properties of HCN channels, their pharmacological inhibitors, and the potential strategies for designing new drugs to block the HCN channel function associated with pain perception.

The Role of Dendritic Signaling in the Anesthetic Suppression of Consciousness

Despite considerable progress in the identification of the molecular targets of general anesthetics, it remains unclear how these drugs affect the brain at the systems level to suppress consciousness. According to recent proposals, anesthetics may achieve this feat by interfering with corticocortical top–down processes, that is, by interrupting information flow from association to early sensory cortices. Such a view entails two immediate questions. First, at which anatomical site, and by virtue of which physiological mechanism, do anesthetics interfere with top–down signals? Second, why does a breakdown of top–down signaling cause unconsciousness? While an answer to the first question can be gleaned from emerging neurophysiological evidence on dendritic signaling in cortical pyramidal neurons, a response to the second is offered by increasingly popular theoretical frameworks that place the element of prediction at the heart of conscious perception.

The electrocortical effects of enflurane: experiment and theory

BACKGROUND

High concentrations of enflurane will induce a characteristic electroencephalogram pattern consisting of periods of suppression alternating with large short paroxysmal epileptiform discharges (PEDs). In this study, we compared a theoretical computer model of this activity with real local field potential (LFP) data obtained from anesthetized rats.

METHODS

After implantation of a high-density 8 x 8 electrode array in the visual cortex, the patterns of LFP and multiunit spike activity were recorded in rats during 0.5, 1.0, 1.5, and 2.0 minimum alveolar anesthetic concentration (MAC) enflurane anesthesia. These recordings were compared with computer simulations from a mean field model of neocortical dynamics. The neuronal effect of increasing enflurane concentration was simulated by prolonging the decay time constant of the inhibitory postsynaptic potential (IPSP). The amplitude of the excitatory postsynaptic potential (EPSP) was modulated, inverse to the neocortical firing rate.

RESULTS

In the anesthetized rats, increasing enflurane concentrations consistently caused the appearance of suppression pattern (>1.5 MAC) in the LFP recordings. The mean rate of multiunit spike activity decreased from 2.54/s (0.5 MAC) to 0.19/s (2.0 MAC). At high MAC, the majority of the multiunit action potential events became synchronous with the PED. In the theoretical model, prolongation of the IPSP decay time and activity-dependent EPSP modulation resulted in output that was similar in morphology to that obtained from the experimental data. The propensity for rhythmic seizure-like activity in the model could be determined by analysis of the eigenvalues of the equations.

CONCLUSION

It is possible to use a mean field theory of neocortical dynamics to replicate the PED pattern observed in LFPs in rats under enflurane anesthesia. This pattern requires a combination of a moderately increased total area under the IPSP, prolonged IPSP decay time, and also activity-dependent modulation of EPSP amplitude.

Propofol block of I(h) contributes to the suppression of neuronal excitability and rhythmic burst firing in thalamocortical neurons

Although the depressant effects of the general anesthetic propofol on thalamocortical relay neurons clearly involve gamma-aminobutyric acid (GABA)(A) receptors, other mechanisms may be involved. The hyperpolarization-activated cation current (I(h)) regulates excitability and rhythmic firing in thalamocortical relay neurons in the ventrobasal (VB) complex of the thalamus. Here we investigated the effects of propofol on I(h)-related function in vitro and in vivo. In whole-cell current-clamp recordings from VB neurons in mouse (P23-35) brain slices, propofol markedly reduced the voltage sag and low-threshold rebound excitation that are characteristic of the activation of I(h). In whole-cell voltage-clamp recordings, propofol suppressed the I(h) conductance and slowed the kinetics of activation. The block of I(h) by propofol was associated with decreased regularity and frequency of delta-oscillations in VB neurons. The principal source of the I(h) current in these neurons is the hyperpolarization-activated cyclic nucleotide-gated (HCN) type 2 channel. In human embryonic kidney (HEK)293 cells expressing recombinant mouse HCN2 channels, propofol decreased I(h) and slowed the rate of channel activation. We also investigated whether propofol might have persistent effects on thalamic excitability in the mouse. Three hours following an injection of propofol sufficient to produce loss-of-righting reflex in mice (P35), I(h) was decreased, and this was accompanied by a corresponding decrease in HCN2 and HCN4 immunoreactivity in thalamocortical neurons in vivo. These results suggest that suppression of I(h) may contribute to the inhibition of thalamocortical activity during propofol anesthesia. Longer-term effects represent a novel form of propofol-mediated regulation of I(h).

Emerging molecular mechanisms of general anesthetic action

General anesthetics are essential to modern medicine, and yet a detailed understanding of their mechanisms of action is lacking. General anesthetics were once believed to be "drugs without receptors" but this view has been largely abandoned. During the past decade significant progress in our understanding of the mechanisms of general anesthetic action at the molecular, cellular and neural systems levels has been made. Different molecular targets in various regions of the nervous system are involved in the multiple components of anesthetic action, and these targets can vary between specific anesthetics. Neurotransmitter-gated ion channels, particularly receptors for GABA and glutamate, are modulated by most anesthetics, at both synaptic and extrasynaptic sites, and additional ion channels and receptors are also being recognized as important targets for general anesthetics. In this article, these developments, which have important implications for the development of more-selective anesthetics, are reviewed in the context of recent advances in ion channel structure and function.

Impairment of hyperpolarization-activated, cyclic nucleotide-gated channel function by the intravenous general anesthetic propofol

Propofol (2,6-diisopropylphenol) is a widely used intravenous general anesthetic, which has been reported to produce bradycardia in patients at concentrations associated with profound sedation and loss of consciousness. Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels conduct a monovalent cationic current I(h) (also known as I(q) or I(f)) that contributes to autorhythmicity in both the brain and heart. Here we studied the effects of propofol on recombinant HCN1, HCN2, and HCN4 channels and found that the drug inhibits and slows activation of all three channels at clinically relevant concentrations. In oocyte expression studies, HCN1 channel activation was most sensitive to slowing by propofol (EC(50) values of 5.6 +/- 1.0 microM for fast component and 31.5 +/- 7.5 microM for slow component). HCN1 channels also showed a marked propofol-induced hyperpolarizing shift in the voltage dependence of activation (EC(50) of 6.7 +/- 1.0 microM) and accelerated deactivation (EC(50) of 4.5 +/- 0.9 microM). Furthermore, propofol reduced heart rate in an isolated guinea pig heart preparation over the same range of concentrations. These data suggest that propofol modulation of HCN channel gating is an important molecular mechanism that can contribute to the depression of central nervous system function and also lead to bradyarrhythmias in patients receiving propofol during surgical anesthesia.

Role of the hyperpolarization-activated cation current (Ih) in pacemaker activity in area postrema neurons of rat brain slices

To clarify the functional properties of the hyperpolarization-activated cation current (Ih) as a pacemaker current in area postrema neurons, whole-cell recordings were made in visually identified cells in rat brain slices. The activation of Ih was identified in approximately 62 % of area postrema neurons tested. The cells displaying Ih showed a depolarizing "sag" in responses to hyperpolarizing current injection in current-clamp mode. The reversal potential for the Ih was -36 mV, and this was shown to depend on the external concentration of Na+ and K+ ions. Extracellular Cs+ ions (2 mM) and ZD7288 (100 microM), a potent selective Ih channel antagonist, blocked Ih and induced a membrane potential hyperpolarization, suggesting the sustained activation of Ih near the resting potential and a contribution from Ih to membrane potential maintenance at more depolarized levels. In contrast, extracellular Ba2+ ions caused a depolarization of the membrane potential, suggesting the blockade of inward rectifier K+ currents. ZD7288 decreased the spontaneous discharge rate by prolonging the slow depolarization between two spikes, with minimal effect on the amplitude of the afterhyperpolarization or action potential waveforms. Ih stabilized the latency of rebound action potentials. Ih was weakly activated by external 8-bromoadenosine 3',5' cyclic monophosphate (1 mM) or forskolin (50-100 microM), indicating that the Ih channel subtypes in area postrema cells could be modulated by intracellular cAMP. Our findings indicate that Ih contributes to the subthreshold membrane and firing properties of rat area postrema neurons and may regulate their resting membrane potential and firing patterns.

Suppression of the hyperpolarization-activated inward current contributes to the inhibitory actions of propofol on rat CA1 and CA3 pyramidal neurons

Intracellular and field potential recordings were taken from the hippocampal CA1 and CA3 neurons in rat brain slices to investigate the effects of 2,6 di-isopropylphenol (propofol) on the neuronal excitability during GABA(A)-C1 channel blockade by picrotoxin (100 microM). Propofol produced a membrane hyperpolarization and an inhibition of the magnitude of the 'voltage sag' that was mediated by the activation of a hyperpolarization-activated inward current (I(H)). Propofol (>100 microM) decreased the spontaneous discharge rate of epileptiform burst responses in CA1 neurons up to 38+/-6% of the control level. Propofol also markedly reduced the duration of both spontaneous and evoked epileptiform burst responses. A propofol-induced decrease in the spontaneous discharge rate in CA3 neurons was coincident with that in CA1 neurons. The effects of propofol on the membrane potential and spontaneous discharge rate but not on the duration of burst responses were duplicated by ZD7288 (potent selective antagonist for I(H) channels), indicating that the blockade of I(H) significantly contributes to reduction of cell's excitability. The present study suggests that various actions including suppressive effects on I(H) contribute to the anesthetic and anti-convulsant properties of propofol.

Propofol suppresses a hyperpolarization-activated inward current in rat hippocampal CA1 neurons

We examined the effect of propofol and thiopental, intravenous anesthetics, on the hyperpolarization-activated inward current (I(H)), whose functional role on the neuronal activity has been evaluated. Whole-cell recordings of I(H) evoked by hyperpolarizing step pulses were taken from hippocampal CA1 neurons in rat brain slices. Propofol reduced I(H) current in a dose-dependent manner. However, thiopental had no significant effect on the activation of I(H). According to the functional role of I(H), the suppression of I(H) should result in a reduction of neuronal activity. We suggest that the effectiveness of propofol as an anticonvulsant or an antiemetic is associated with the blockade of the I(H) channel.

Multiple ionic mechanisms mediate inhibition of rat motoneurones by inhalation anaesthetics

1. We studied the effects of inhalation anaesthetics on the membrane properties of hypoglossal motoneurones in a neonatal rat brainstem slice preparation. 2. In current clamp, halothane caused a membrane hyperpolarization that was invariably associated with decreased input resistance; in voltage clamp, halothane induced an outward current and increased input conductance. Qualitatively similar results were obtained with isoflurane and sevoflurane. 3. The halothane current reversed near the predicted K+ equilibrium potential (EK) and was reduced in elevated extracellular K+ and in the presence of Ba2+ (2 mM). Moreover, the Ba2+-sensitive component of halothane current was linear and reversed near EK. The halothane current was not sensitive to glibenclamide or thyrotropin-releasing hormone (TRH). Therefore, the halothane current was mediated, in part, by activation of a Ba2+-sensitive K+ current distinct from the ATP- and neurotransmitter-sensitive K+ currents in hypoglossal motoneurones. 4. Halothane also inhibited Ih, a hyperpolarization-activated cationic current; this was primarily due to a decrease in the absolute amount of current, although halothane also caused a small, but statistically significant, shift in the voltage dependence of Ih activation. Extracellular Cs+ (3 mM) blocked Ih and a component of halothane-sensitive current with properties reminiscent of Ih. 5. A small component of halothane current, resistant to Ba2+ and Cs+, was observed in TTX-containing solutions at potentials depolarized to approximately -70 mV. Partial Na+ substitution by N-methyl-D-glucamine completely abolished this residual current, indicating that halothane also inhibited a TTX-resistant Na+ current active near rest potentials. 6. Thus, halothane activates a Ba2+-sensitive, relatively voltage-independent K+ current and inhibits both Ih and a TTX-insensitive persistent Na+ current in hypoglossal motoneurones. These effects of halothane decrease motoneuronal excitability and may contribute to the immobilization that accompanies inhalation anaesthesia.

Use-dependent block of Ih in mouse dorsal root ganglion neurons by sinus node inhibitors

1. The sinus node inhibitors UL FS 49 and DK-AH 269 reduce heart rate by slowing diastolic depolarization rate in the sino-atrial (SA) node, which might originate from the use-dependent blockade of a hyperpolarization-activated current If. A hyperpolarization-activated current Ih, which is present in many types of neurons, is similar to If. We studied the effects of these drugs on Ih in cultured mouse dorsal root ganglion (DRG) neurons. 2. With the whole-cell patch-clamp technique use-dependent block of Ih was observed. The steady-state block following a voltage-clamp pulse train (1-s steps from -38 to -108 mV applied at 0.5 Hz) was dependent on drug concentration and showed an apparent Kd of 0.1 and 0.79 microM with DK AH 269 and UL-FS 49 respectively. 3. The rate of block increased linearly with drug concentration. The rate of recovery from block was, however, much slower compared to cardiac tissue. 4. There was no significant effect of UL-FS 49 on the activation curve. 5. At high concentrations of UL-FS 49 a clear association of the drug with the open channel was observed. 6. When the cell was stimulated at a frequency of 3 Hz, a distinct hyperpolarization was observed in the presence of extracellular Cs+ or when Ih was blocked with UL-FS 49, but not in the absence of Cs+ and UL-FS 49. 7. These results indicate that Ih protects the cell against hyperpolarizations and subsequent inexcitability. The action of the drugs on the hyperpolarization-activated current in cardiac and neuronal tissue show some similarities; however, some pronounced differences indicate that different subtypes of the channel might exist.

Inhibition by wortmannin of M-current in bullfrog sympathetic neurones

1. The actions of wortmannin, an inhibitor of myosin light chain kinase (MLCK), on M-type potassium current of dissociated bullfrog sympathetic neurones have been examined. 2. The amplitude of M-current was measured by whole cell recordings from cells pretreated with wortmannin (0.01-10 microM) or the wortmannin vehicle, dimethylsulphoxide (0.0001-0.1 vol%), for 30 min. Internal (recording pipette) solutions having three different pCa values (6, 7 and 8) were used for the measurements. 3. Irrespective of the pCa, M-current was not detectable when the cells were pretreated with 10 microM wortmannin. Wortmannin, 3 microM, produced 85-95% inhibition of the M-current. Pretreatment with 10-30 nM wortmannin was without effect on M-current. 4. The M-current inhibition by wortmannin at concentrations of 0.1-1 microM depended on the pCa of the internal solution. Inhibition occurred only when the calcium-rich (pCa = 6) internal solution was used. 5. Pre-treatment of the cells with wortmannin (10 microM) did not affect rapidly-inactivating A-type or delayed rectifier-type potassium currents not did it alter inwardly rectifying sodium-potassium current (IH). 6. These observations show that M-current inhibition by wortmannin has two pharmacological profiles. One is calcium-dependent and occurs at lower concentrations (0.1-1 microM), and is attributed to inhibition of MLCK by wortmannin. At higher concentrations (3-10 microM), wortmannin has an additional, calcium-independent action, inhibiting the M-current by an unknown mechanism.

Rhythmogenesis, amplitude modulation, and multiplexing in a cortical architecture

In a network of excitatory and inhibitory neurons, hyperpolarization-activated inward currents can help to produce population rhythms in which individual cells participate sparsely and randomly. A shift in the activation curve of such a current changes the fraction of the cells participating in any given cycle of the population rhythm, thus changing the amplitude of the field potential. Furthermore, the frequency of the population rhythm remains relatively fixed over a substantial range of amplitudes, allowing the population rhythm to play a separate processing role from that of the individual components.

Cortical and thalamic cellular correlates of electroencephalographic burst-suppression

This experimental study on anesthetized cats used intracellular recordings of cortical, thalamocortical and reticular thalamic neurons (n = 54), as well as multi-site extracellular recordings (n = 36), to investigate the cellular correlates of EEG burst-suppression patterns, defined as alternating wave bursts and periods of electrical silence. Burst-suppression was elicited by the administration of the same or other anesthetic agents upon the background of an already synchronized EEG activity. About 95% of cortical cells entered burst-suppression, in close time-relation with EEG activity, displaying sequences of phasic depolarizing events associated with bursts of EEG waves and an electrical silence of the neuronal membrane during flat EEG epochs. The membrane potential (Vm) hyperpolarized by approximately 10 mV prior to any EEG change and the slow rhythms reflecting deep stages of anesthesia progressively disorganized with transition to burst-suppression. During flat EEG epochs, the apparent input resistance (tested through short hyperpolarizing current pulses) decreased (range 12-60%) and neuronal responsiveness to orthodromic volleys (tested by thalamic and cortical evoked excitatory postsynaptic potentials) was dramatically reduced. It is proposed that the decreased input resistance is mainly due to an increase in K+ conductances. At variance with cortical neurons, only 60-70% of thalamic cells ceased firing before overt EEG burst-suppression and were completely silent during flat periods of EEG activity. The remaining 30-40% of thalamic cells discharged rhythmic (1-4 Hz) spike bursts during periods of EEG silence. This rhythm, within the frequency range of delta waves, is generated in thalamic cells by the interplay between two of their intrinsic currents at critical levels of Vm hyperpolarization. However, with the deepening of burst-suppression, when silent EEG periods became longer than 30 sec, thalamic cells also ceased firing. The assumption that full-blown burst-suppression is achieved through virtually complete disconnection in brain circuits implicated in the genesis of the EEG is corroborated by the revival of normal cellular and EEG activities after volleys setting into action thalamic and cortical networks.

Chemosensitivity of C-cells in bullfrog dorsal root ganglia to substance P and adenosine 5'-triphosphate

Dissociated bullfrog dorsal root ganglion cells were voltage clamped in the whole-cell configuration. In small C-cells having 20 microns as averaged diameter, substance-P (0.1-1 microM) inhibited an M-type potassium current while ATP (1-10 microM) activated a sodium-potassium current. In large A-cells (approximately 65 microns in diameter) in which ATP has been shown to inhibit M-current, substance P (0.1-1 microM) also inhibited this potassium current without activating the sodium-potassium current. Results provided evidence for the distinction between A- and C-cells in terms of their chemosensitivity.

Intracellular ATP changes the voltage-dependence of delayed rectifier potassium current in bullfrog primary afferent neurons

Dissociated bullfrog dorsal root ganglion cells were voltage-clamped in the whole-cell configuration to study the steady-state activation and inactivation curves for a delayed rectifier potassium current. The 50%-activation of the current occurred at +15 mV when measured with ATP (5 mM) in the pipette solution as opposed to -11 mV with 5'-adenylylimidodiphosphate (AMP-PNP, 5 mM) and -15 mV with adenosine 5'-O-(3-thiotriphosphate) (5 mM). The 50%-inactivation of the current occurred at -6 mV with ATP but at -31 mM with AMP-PNP. The results suggest that intracellular ATP modulates voltage-dependence of the delayed rectifier in amphibian afferent neurons.

Halothane-induced hyperpolarization and depression of postsynaptic potentials of guinea pig thalamic neurons in vitro

Intralaminar thalamic nuclei have been considered to be a component of the non-specific sensory system which is involved in physiological functions related to consciousness and pain sensation. The effect of halothane on membrane potentials and synaptic properties of neurons of the parafascicular (Pf) nucleus in guinea pig brain slices was investigated using intracellular recording methods. Halothane at concentrations of 0.4-1.0 mM, which are in the range of clinical concentrations, produced hyperpolarizations of 2-8 mV in approximately 50% of the cells. The halothane-induced hyperpolarization was nullified at a membrane potential close to the K+ equilibrium potential. The amplitude of the hyperpolarization was dependent on the external K+ concentration, and was decreased by either Ba2+, or 4-aminopyridine, or intracellular injection of Cs+. All these results indicate that the hyperpolarization was due to an increase in K+ conductance. Halothane at clinical concentrations depressed both excitatory and inhibitory postsynaptic potentials in a concentration-dependent manner. On the other hand hyperpolarizing responses to exogenous gamma-aminobutyric acid (GABA) in the presence of bicuculline were suppressed by halothane, but depolarizing responses to L-glutamate were not altered. The results indicate that the depressant action of the anesthetic on the excitatory postsynaptic potential (EPSP) may occur presynaptically, whereas the blocking action on the inhibitory postsynaptic potential (IPSP) may occur postsynaptically.

Morphological and electrophysiological properties of C-cells in bullfrog dorsal root ganglia

Dissociated bullfrog dorsal root ganglion cells were voltage-clamped in the whole-cell configuration. Small spheroidal C-cells had a mean diameter of 14-30 microns and shared about 10% of the total population of the cells. The C-cells were characterized by a prominent calcium-activated potassium current underlying a hyperpolarization following the action potential. In contrast, a hyperpolarization-activated cationic inward rectifier was missing in all C-cells tested. These properties were completely different from those which have been observed for large spheroidal A-cells.