Mechanism of Anesthesia Revealed by Shunting Actions of Isoflurane on Thalamocortical Neurons

Abstract

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.

The Role of Dendritic Signaling in the Anesthetic Suppression of Consciousness

“…115 This observation appears to be at odds with the data from human electroencephalography mentioned just before. In further support of the thalamus as a target of anesthetics, halothane and isoflurane have been shown to hyperpolarize thalamocortical neurons in vitro, 116,117 and GABA agonists injected into the intralaminar thalamic nuclei cause a loss of the righting reflex (considered a behavioral correlate of a loss of consciousness) in rats. 118 Moreover, Alkire's group recently demonstrated that rats under general anesthesia can be awakened by injections of nicotine into the intralaminar nuclei 119 or by the infusion of an antibody against a voltagegated potassium channel in the same location.…”

Sleep and general anesthesia

“…64 A study that concluded that cortical changes in the EEG preceded changes in thalamic local field potentials at loss of consciousness supports this view but, interestingly, a recent study concluded the opposite during sleep onset. 65 Alternatively, anesthetics are known to hyperpolarize thalamocortical neurons, [66][67][68][69][70] and this could lead directly to the delta oscillations and spindle activity that are observed during anesthesia. Since the pattern of cortical deactivation that is seen during anesthesia resembles that of natural sleep, it seems more plausible to us that anesthetic effects at the level of the thalamus, or on the arousal pathways that modulate thalamic activity, are responsible for coordinating the overall network response.…”

Pentobarbital Depressant Effects Are Independent of GABA Receptors in Auditory Thalamic Neurons

“…Using previously described procedures (Ries and Puil 1999;Tennigkeit et al 1996), coronal slices (approximately 300 m) containing the ventral portion of the medial geniculate body (MGB) were prepared from brain tissue of anesthetized Sprague-Dawley rats (13-15 days). Most recordings were made in neurons from P14 rats.…”

“…The junction potential of Ϫ11 mV was subtracted from all membrane potentials, e.g., a recorded resting potential of Ϫ55 mV corresponds to an actual potential of Ϫ66 mV (see Ries and Puil 1999;Zhang and Krnjevic 1993). The data were analyzed with pClamp 8 (Clampfit, Axon Instruments) or Prism GraphPad software (v. 2.0, San Diego, CA).…”

“…This depression occurs in the same range of concentrations that potentiate GABA action. A prominent effect of pentobarbital is to increase K ϩ -conductance that shunts the current required for excitation (Nicoll and Madison 1982;O'Beirne et al 1987;Ries and Puil 1999;Sirois et al 1998). Pentobarbital also inhibits Ca 2ϩ -currents (Ffrench-Mullen et al 1993;Werz and Macdonald 1985) and suppresses Ca 2ϩ -dependent transmitter release (Weakly 1969).…”