Short-term memory resists the depressant effect of the nonimmobilizer 1-2-dichlorohexafluorocyclobutane (2N) more than long-term memory

Enhancement of α5-containing γ-aminobutyric acid type A receptors by the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (F6) is abolished by the β3(N265M) mutation

BACKGROUND

Modulation of γ-aminobutyric acid type A receptors (GABAARs) by general anesthetics may contribute to their ability to produce amnesia. Receptors containing α5 subunits, which mediate tonic and slow synaptic inhibition, are co-localized with β3 and γ2 subunits in dendritic layers of the hippocampus and are sensitive to low (amnestic) concentrations of anesthetics. Because α5 and β3 subunits influence performance in hippocampus-dependent learning tasks in the presence and absence of general anesthetics, and the experimental inhaled drug 1,2-dichlorohexafluorocyclobutane (F6) impairs hippocampus-dependent learning, we hypothesized that F6 would modulate receptors that incorporate α5 and β3 subunits. We hypothesized further that the β3(N265M) mutation, which controls receptor modulation by general anesthetics, would similarly influence modulation by F6.

METHODS

Using whole-cell electrophysiologic recording techniques, we tested the effects of F6 at concentrations ranging from 4 to 16 μM on receptors expressed in human embryonic kidney 293 cells. We measured drug modulation of wild-type α5β3 and α5β3γ2L GABAARs and receptors harboring the β3(N265M) mutation. We also tested the effects of F6 on α1β2γ2L receptors, which were reported previously to be insensitive to this drug when expressed in Xenopus oocytes.

RESULTS

F6 enhanced the responses of wild-type α5β3γ2L but not α1β2γ2L receptors to low concentrations of GABA in a concentration-dependent manner. Receptors that incorporated the mutant β3(N265M) subunit were insensitive to F6. When applied together with a high concentration of GABA, F6 blocked currents through α5β3 but not α5β3γ2L receptors. F6 did not alter deactivation of α5β3γ2L receptors after brief high- concentration pulses of GABA.

CONCLUSIONS

The nonimmobilizer F6 modulates GABAARs in a manner that depends on subunit composition and mode of receptor activation by GABA, supporting a possible role for α5-containing receptors in suppression of learning and memory by F6. Furthermore, common structural requirements indicate that similar molecular mechanisms may be responsible for the enhancing effects of F6 and conventional general anesthetics.

Distinctive recruitment of endogenous sleep-promoting neurons by volatile anesthetics and a nonimmobilizer

BACKGROUND

Numerous studies demonstrate that anesthetic-induced unconsciousness is accompanied by activation of hypothalamic sleep-promoting neurons, which occurs through both pre- and postsynaptic mechanisms. However, the correlation between drug exposure, neuronal activation, and onset of hypnosis remains incompletely understood. Moreover, the degree to which anesthetics activate both endogenous populations of γ-aminobutyric acid (GABA)ergic sleep-promoting neurons within the ventrolateral preoptic (VLPO) and median preoptic nuclei remains unknown.

METHODS

Mice were exposed to oxygen, hypnotic doses of isoflurane or halothane, or 1,2-dichlorohexafluorocyclobutane (F6), a nonimmobilizer. Hypothalamic brain slices prepared from anesthetic-naive mice were also exposed to oxygen, volatile anesthetics, or F6 ex vivo, both in the presence and absence of tetrodotoxin. Double-label immunohistochemistry was performed to quantify the number of c-Fos-immunoreactive nuclei in the GABAergic subpopulation of neurons in the VLPO and the median preoptic areas to test the hypothesis that volatile anesthetics, but not nonimmobilizers, activate sleep-promoting neurons in both nuclei.

RESULTS

In vivo exposure to isoflurane and halothane doubled the fraction of active, c-Fos-expressing GABAergic neurons in the VLPO, whereas F6 failed to affect VLPO c-Fos expression. Both in the presence and absence of tetrodotoxin, isoflurane dose-dependently increased c-Fos expression in GABAergic neurons ex vivo, whereas F6 failed to alter expression. In GABAergic neurons of the median preoptic area, c-Fos expression increased with isoflurane and F6, but not with halothane exposure.

CONCLUSIONS

Anesthetic unconsciousness is not accompanied by global activation of all putative sleep-promoting neurons. However, within the VLPO hypnotic doses of volatile anesthetics, but not nonimmobilizers, activate putative sleep-promoting neurons, correlating with the appearance of the hypnotic state.

Isoflurane impairs odour discrimination learning in rats: differential effects on short- and long-term memory

BACKGROUND

Anaesthetics suppress the formation of lasting memories at concentrations that do not suppress perception, but it is unclear which elements of the complex cascade leading from a conscious experience to a lasting memory trace are disrupted. Experiments in conscious humans suggest that subhypnotic concentrations of anaesthetics impair consolidation or maintenance rather than acquisition of a representation (long-term more than short-term memory). We sought to test whether these agents similarly impair learning in rats.

METHODS

We used operant conditioning in rats to examine the effect of isoflurane on acquisition compared with long-term (24 h) memory of non-aversive olfactory memories using two different odour discrimination tasks. Rats learned the 'valences' of odour pairs presented either separately (task A) or simultaneously (task B), under control conditions and under isoflurane inhalation. In a separate set of experiments, we tested the ability of the animals to recall a learning set that had been acquired 24 h previously.

RESULTS

Under 0.4% isoflurane inhalation, the average number of trials required to reach criterion performance (18 correct responses in 20 successive trials) increased from 21.9 to 43.5 (P<0.05) and 24.2 to 54.4 (P<0.05) for tasks A and B, respectively. Under 0.3% isoflurane inhalation, only task B was impaired (from 24.2 to 31.5 trials, P<0.05). Recall at 24 h was dose-dependently impaired or prevented by isoflurane for both tasks.

CONCLUSIONS

Isoflurane interfered with long-term memory of odour valence without preventing its acquisition. This paradigm may serve as a non-aversive animal model of conscious amnesia.

How we recall (or don't): the hippocampal memory machine and anesthetic amnesia

PURPOSE

The hippocampal formation occupies a central position for the processing of sensory input into learned, remembered, and consciously retrievable information. The mechanisms by which anesthetic drugs interfere with these processes are now emerging. We review the current understanding of the role of the hippocampal formation in the generation of memory traces and how anesthetics might interfere with its function.

CLINICAL FEATURES

Intraoperative amnesia is a desired endpoint of general anesthesia from the perspective of both the patient and the practitioner. "Intraoperative awareness with recall" can result when learning and memory do occur. In addition, anesthetics are capable of inducing a state of "conscious amnesia" that can provide insight into the workings of the brain and might be useful clinically.

CONCLUSIONS

Anesthesiologists routinely induce the most fascinating pharmacologic effects in existence, the reversible interference of anesthetics with higher cognitive functions. Understanding how the drugs in our custody exert their effects should be our contribution to mankind's universal knowledge base.

Slowing of the hippocampal θ rhythm correlates with anesthetic-induced amnesia

BACKGROUND

Temporary, antegrade amnesia is one of the core desirable endpoints of general anesthesia. Multiple lines of evidence support a role for the hippocampal θ rhythm, a synchronized rhythmic oscillation of field potentials at 4-12 Hz, in memory formation. Previous studies have revealed a disruption of the θ rhythm at surgical levels of anesthesia. We hypothesized that θ-rhythm modulation would also occur at subhypnotic but amnestic concentrations. Therefore, we examined the effect of three inhaled agents on properties of the θ rhythm considered critical for the formation of hippocampus-dependent memories.

METHODS

We studied the effects of halothane and nitrous oxide, two agents known to modulate different molecular targets (GABAergic [γ-aminobutyric acid] vs. non-GABAergic, respectively) and isoflurane (GABAergic and non-GABAergic targets) on fear-conditioned learning and θ oscillations in freely behaving rats.

RESULTS

All three anesthetics slowed θ peak frequency in proportion to their inhibition of fear conditioning (by 1, 0.7, and 0.5 Hz for 0.32% isoflurane, 60% N2O, and 0.24% halothane, respectively). Anesthetics inconsistently affected other characteristics of θ oscillations.

CONCLUSIONS

At subhypnotic amnestic concentrations, θ-oscillation frequency was the parameter most consistently affected by these three anesthetics. These results are consistent with the hypothesis that modulation of the θ rhythm contributes to anesthetic-induced amnesia.

Selective activation of G-protein coupled receptors by volatile anesthetics

Ion channels and ionotropic neurotransmitter receptors have long been investigated as the principle targets of inhaled volatile anesthetics (VAs), but emerging evidence suggests that G-protein coupled receptors (GPCRs) might also directly interact with VAs. To survey the extent of interaction between VAs and diverse GPCRs, we have turned to the 1000+ member family of olfactory receptors (ORs), taking advantage of their unique expression pattern of a single OR per neuron. Through optical imaging and electrophysiological recordings, we show that different VAs trigger the normal transduction cascade in distinct subsets of cells in a dose-dependant manner. Together with evidence of antagonism by odorants, this selective activation strongly implicates a direct action of VAs upon particular olfactory receptors. The finding that VAs stimulate nearly 8% of olfactory GPCRs suggests that probing related Class A GPCRs may reveal a pool of VA targets whose altered signaling contributes to anesthetic effects.