Alternative transcripts of the GABA(A) receptor epsilon subunit in human and rat

Activation of the Rat α1β2ε GABAA Receptor by Orthosteric and Allosteric Agonists

GABA_A receptors are a major contributor to fast inhibitory neurotransmission in the brain. The receptors are activated upon binding the transmitter GABA or allosteric agonists including a number of GABAergic anesthetics and neurosteroids. Functional receptors can be formed by various combinations of the nineteen GABA_A subunits cloned to date. GABA_A receptors containing the ε subunit exhibit a significant degree of constitutive activity and have been suggested to be unresponsive to allosteric agents. In this study, we have characterized the functional properties of the rat α1β2ε GABA_A receptor. We confirm that the α1β2ε receptor exhibits a higher level of constitutive activity than typical of GABA_A receptors and show that it is inefficaciously activated by the transmitter and the allosteric agonists propofol, pentobarbital, and allopregnanolone. Manipulations intended to alter ε subunit expression and receptor stoichiometry were largely without effect on receptor properties including sensitivity to GABA and allosteric agonists. Surprisingly, amino acid substitutions at the conserved 9' and 6' positions in the second transmembrane (TM2) domain in the ε subunit did not elicit the expected functional effects of increased constitutive activity and resistance to the channel blocker picrotoxin, respectively. We tested the accessibility of TM2 residues mutated to cysteine using the cysteine-modifying reagent 4-(hydroxymercuri)benzoic acid and found a unique pattern of water-accessible residues in the ε subunit.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Urinary Sex Steroid and Glucocorticoid Hormones Are Associated With Muscle Mass and Strength in Healthy Adults

CONTEXT

Sex steroid hormones exhibit anabolic effects whereas a deficiency engenders sarcopenia. Moreover, supraphysiological levels of glucocorticoids promote skeletal muscle atrophy, whereas physiologic levels of glucocorticoids may improve muscle performance.

OBJECTIVE

To study the relationship between both groups of steroid hormones at a physiological range with skeletal muscle mass and function in the general population.

DESIGN

Cross-sectional analysis of the associations between urinary excreted androgens, estrogens, glucocorticoids, and steroid hormone metabolite ratios with lean mass and handgrip strength in a population-based cohort.

SETTING

Three centers in Switzerland including 1128 participants.

MEASURES

Urinary steroid hormone metabolite excretion by gas chromatography-mass spectrometry, lean mass by bioimpedance analysis, and isometric handgrip strength by dynamometry.

RESULTS

For lean mass a strong positive association was found with 11β-OH-androsterone and with most glucocorticoids. Androsterone showed a positive association in middle-aged and older adults. Estriol showed a positive association only in men. For handgrip strength, strong positive associations with androgens were found in middle-aged and older adults, whereas positive associations were found with cortisol metabolites in young to middle-aged adults.

CONCLUSIONS

Sex steroids and glucocorticoids are strongly positively associated with skeletal muscle mass and strength in the upper limbs. The associations with muscle strength appear to be independent of muscle mass. Steroid hormones exert age-specific anabolic effects on lean mass and handgrip strength. Deficits in physical performance of aged muscles may be attenuated by androgens, whereas glucocorticoids in a physiological range increase skeletal muscle mass at all ages, as well as muscle strength in particular in younger adults.

Bicuculline- and neurosteroid-sensitive tonic chloride current in rat hypoglossal motoneurons and atypical dual effect of SR95531

Hypoglossal motoneurons (HMs) are known to be under 'permanent' bicuculline-sensitive inhibition and to show 'transient' synaptic γ-aminobutyric acid (GABA)(A) and glycine inhibitory responses. The present paper describes a permanent bicuculline-sensitive current that should contribute to their tonic inhibition. This current was recorded in brainstem slices superfused without any exogenous agonist and remained detectable with tetrodotoxin. It could also be blocked by the other GABA(A) antagonists picrotoxin (PTX) and 2-(3-carboxypropyl)-3-amino-6-(4 methoxyphenyl)pyridazinium bromide) (SR95531; gabazine), but persisted in the presence of a specific blocker of α5-containing GABA(A) receptors. Addition of 2 μm 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol hydrochloride (THIP), known to preferentially activate GABA(A) receptors devoid of a γ-subunit, induced a sustained anionic current that could be further enhanced by neurosteroids such as allopregnanolone (100 nm). Thus, HMs show a tonic inhibitory current carried by extrasynaptic γ-free GABA(A) receptors, highly sensitive to neurosteroids. A second result was obtained by using SR95531 at concentrations sufficiently high to rapidly block the tonic current above the chloride equilibrium potential (E(C) (l)). Surprisingly, below E(C) (l) , SR95531 (10-40 μm) activated a sustained inward current, associated with a conductance increase, and resistant to bicuculline or PTX (100 μm). Similarly, after blockade of the bicuculline-sensitive current, SR95531 activated an outward current above E(C) (l). The bicuculline-resistant anionic current activated by SR95531 could be blocked by a GABA(C) receptor antagonist. Thus, two types of inhibitory GABA receptors, belonging to the GABA(A) and GABA(C) families, are able to show a sustained activity in HMs and provide promising targets for neuroprotection under overexcitatory situations known to easily damage these particularly fragile neurons.

Inhibitory transmission in locus coeruleus neurons expressing GABAA receptor epsilon subunit has a number of unique properties

Fast inhibitory synaptic transmission in the brain relies on ionotropic GABA(A) receptors (GABA(A)R). Eighteen genes code for GABA(A)R subunits, but little is known about the epsilon subunit. Our aim was to identify the synaptic transmission properties displayed by native receptors incorporating epsilon. Immunogold localization detected epsilon at synaptic sites on locus coeruleus (LC) neurons. In situ hybridization revealed prominent signals from epsilon, and mRNAs, some low beta1 and beta3 signals, and no gamma signal. Using in vivo extracellular and in vitro patch-clamp recordings in LC, we established that neuron firing rates, GABA-activated currents, and mIPSC charge were insensitive to the benzodiazepine flunitrazepam (FLU), in agreement with the characteristics of recombinant receptors including an epsilon subunit. Surprisingly, LC provided binding sites for benzodiazepines, and GABA-induced currents were potentiated by diazepam (DZP) in the micromolar range. A number of GABA(A)R ligands significantly potentiated GABA-induced currents, and zinc ions were only active at concentrations above 1 muM, further indicating that receptors were not composed of only alpha and beta subunits, but included an epsilon subunit. In contrast to recombinant receptors including an epsilon subunit, GABA(A)R in LC showed no agonist-independent opening. Finally, we determined that mIPSCs, as well as ensemble currents induced by ultra-fast GABA application, exhibited surprisingly slow rise times. Our work thus defines the signature of native GABA(A)R with a subunit composition including epsilon: differential sensitivity to FLU and DZP and slow rise time of currents. We further propose that alpha(3,) beta(1/3,) and epsilon subunits compose GABA(A)R in LC.

Expression of GABA(A) receptor alpha3-, theta-, and epsilon-subunit mRNAs during rat CNS development and immunolocalization of the epsilon subunit in developing postnatal spinal cord

Ionotropic GABA(A) receptors are heteromeric structures composed of a combination of five from at least 16 different subunits. Subunit genes are expressed in distinct cell types at specific times during development. The most abundant native GABA(A) receptors consist of alpha1-, beta2-, and gamma2-subunits that are co-expressed in numerous brain areas. alpha3-, theta-, And epsilon-subunits are clustered on the X chromosome and show striking overlapping expression patterns throughout the adult rat brain. To establish whether these subunits are temporally and spatially co-expressed, we used in situ hybridization to analyze their expression throughout rat development from embryonic stage E14 to postnatal stage P12. Each transcript exhibited a unique or a shared regional and temporal developmental expression profile. The thalamic expression pattern evolved from a restricted expression of epsilon and theta transcripts before birth, to a theta and alpha3 expression at birth, and finally to a grouped epsilon, theta and alpha3 expression postpartum. However, strong similarities occurred, such as a grouped expression of the three subunits within the hypothalamus, tegmentum and pontine nuclei throughout the developmental process. At early stages of development (E17), epsilon and theta appeared to have a greater spatial distribution before the dominance of the alpha3 subunit transcript around birth. We also revealed expression of alpha3, theta, and epsilon in the developing spinal cord and identified neurons that express epsilon in the postnatal dorsal horn, intermediolateral column and motoneurons. Our findings suggest that various combinations of alpha3-, theta- and epsilon-subunits may be assembled at a regional and developmental level in the brain.

GABAA receptors are expressed and facilitate relaxation in airway smooth muscle

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system and exerts its actions via both ionotropic (GABA(A)) channels and metabotropic (GABA(B)) receptors. GABA(A) channels are ubiquitously expressed in neuronal tissues, and in mature neurons modulate an inward chloride current resulting in neuronal inhibition due to membrane hyperpolarization. In airway smooth muscle (ASM) cells, membrane hyperpolarization favors smooth muscle relaxation. Although GABA(A) channels and GABA(B) receptors have been functionally identified on peripheral nerves in the lung, GABA(A) channels have never been identified on ASM itself. We detected the mRNA encoding of the GABA(A) alpha(4)-, alpha(5)-, beta(3)-, delta-, gamma(1-3)-, pi-, and theta-subunits in total RNA isolated from native human and guinea pig ASM and from cultured human ASM cells. Selected immunoblots identified the GABA(A) alpha(4)-, alpha(5)-, beta(3)-, and gamma(2)-subunit proteins in native human and guinea pig ASM and cultured human ASM cells. The GABA(A) beta(3)-subunit protein was immunohistochemically localized to ASM in guinea pig tracheal rings. While muscimol, a specific GABA(A) channel agonist, did not affect the magnitude or the time to peak contractile effect of substance P, it directly concentration dependently relaxed a tachykinin-induced contraction in guinea pig tracheal rings, which was inhibited by the GABA(A)-selective antagonist gabazine. Muscimol also relaxed a contraction induced by an alternative contractile agonist histamine. These results demonstrate that functional GABA(A) channels are expressed on ASM and suggest a novel therapeutic target for the relaxation of ASM in diseases such as asthma and chronic obstructive lung disease.

GABA(A) receptors in normal development and seizures: friends or foes?

GABA(A) receptors have an age-adapted function in the brain. During early development, they mediate excitatory effects resulting in activation of calcium sensitive signaling processes that are important for the differentiation of the brain. In more mature stages of development and in adults, GABA(A) receptors transmit inhibitory signals. The maturation of GABA(A) signaling follows sex-specific patterns, which appear to also be important for the sexual differentiation of the brain. The inhibitory effects of GABA(A) receptor activation have been widely exploited in the treatment of conditions where neuronal silencing is necessary. For instance, drugs that target GABA(A) receptors are the mainstay of treatment of seizures. Recent evidence suggests however that the physiology and function of GABA(A) receptors changes in the brain of a subject that has epilepsy or status epilepticus.This review will summarize the physiology of and the developmental factors regulating the signaling and function of GABA(A) receptors; how these may change in the brain that has experienced prior seizures; what are the implications for the age and sex specific treatment of seizures and status epilepticus. Finally, the implications of these changes for the treatment of certain forms of medically refractory epilepsies and status epilepticus will be discussed.

Trafficking and potential assembly patterns of epsilon-containing GABAA receptors

Incorporation of the epsilon subunit into the GABAA receptor has been suggested to confer unusual, but variable, biophysical and pharmacological characteristics to both recombinant and native receptors. Due to their structural similarity with the gamma subunits, epsilon subunits have been assumed to substitute at the single position of the gamma subunit in assembled receptors. However, prior work suggests that functional variability in epsilon-containing receptors may reflect alternative sites of incorporation and of not just one, but possibly multiple epsilon subunits in the pentameric receptor complex. Here we present data indicating that increased expression of epsilon, in conjunction with alpha2 and beta3 subunits, results in expression of GABAA receptors with correspondingly altered rectification, deactivation and levels of spontaneous openings, but not increased total current density. We also provide data that the epsilon subunit, like the beta3 subunit, can self-export and data from chimeric receptors suggesting that similarities between the assembly domains of the beta3 and the epsilon subunits may allow the epsilon subunit to replace the beta, as well as the gamma, subunit. The substitution of an epsilon for a beta, as well as the gamma subunit and formation of receptors with alternative patterns of assembly with respect to epsilon incorporation may underlie the observed variability in both biophysical and pharmacological properties noted not only in recombinant, but also in native receptors.

In silico comparative genomic analysis of GABAA receptor transcriptional regulation

Background

Subtypes of the GABA_A receptor subunit exhibit diverse temporal and spatial expression patterns. In silico comparative analysis was used to predict transcriptional regulatory features in individual mammalian GABA_A receptor subunit genes, and to identify potential transcriptional regulatory components involved in the coordinate regulation of the GABA_A receptor gene clusters.

Results

Previously unreported putative promoters were identified for the β2, γ1, γ3, ε, θ and π subunit genes. Putative core elements and proximal transcriptional factors were identified within these predicted promoters, and within the experimentally determined promoters of other subunit genes. Conserved intergenic regions of sequence in the mammalian GABA_A receptor gene cluster comprising the α1, β2, γ2 and α6 subunits were identified as potential long range transcriptional regulatory components involved in the coordinate regulation of these genes. A region of predicted DNase I hypersensitive sites within the cluster may contain transcriptional regulatory features coordinating gene expression. A novel model is proposed for the coordinate control of the gene cluster and parallel expression of the α1 and β2 subunits, based upon the selective action of putative Scaffold/Matrix Attachment Regions (S/MARs).

Conclusion

The putative regulatory features identified by genomic analysis of GABA_A receptor genes were substantiated by cross-species comparative analysis and now require experimental verification. The proposed model for the coordinate regulation of genes in the cluster accounts for the head-to-head orientation and parallel expression of the α1 and β2 subunit genes, and for the disruption of transcription caused by insertion of a neomycin gene in the close vicinity of the α6 gene, which is proximal to a putative critical S/MAR.

Tonically Active GABAA Receptors in Hippocampal Pyramidal Neurons Exhibit Constitutive GABA-Independent Gating

Phasic and tonic inhibitory currents of hippocampal pyramidal neurons exhibit distinct pharmacological properties. Picrotoxin and bicuculline methiodide inhibited both components, consistent with a role for GABAA receptors; however, gabazine, at a concentration that abolished miniature GABAergic inhibitory postsynaptic currents and responses to exogenous GABA, had no effect on tonic currents. Because all GABA-activated GABAA receptors in pyramidal neurons are gabazine-sensitive, it follows that tonic currents are not GABA-activated. Furthermore, picrotoxin-sensitive spontaneous single-channel events recorded from outside-out patches had the same chord conductance as GABA-activated channels and were gabazine-resistant. Therefore, we hypothesize that GABAA receptors, constitutively active in the absence of GABA, mediate tonic current; the failure of gabazine to block tonic current reflects a lack of negative intrinsic efficacy of the antagonist. We compared the negative efficacies of bicuculline and gabazine using the general anesthetic propofol to directly activate GABAA receptors native to pyramidal neurons or alpha1beta3gamma2 receptors recombinantly expressed in human embryonic kidney 293 cells. Propofol activated gabazine-resistant, bicuculline-sensitive currents when applied to either preparation. Although gabazine had negligible efficacy as an inhibitor of propofol-activated currents, it prevented inhibition by bicuculline, which acts as an inverse agonist inhibiting GABA-independent gating. Recombinant alpha1beta1/3gamma2 receptors also mediated agonist-independent tonic currents that were resistant to gabazine and inhibited by bicuculline. Thus, gabazine is a competitive antagonist with negligible negative efficacy and is therefore unable to inhibit GABAA receptors that are active in the absence of GABA because of either anesthetic or spontaneous gating. Moreover, spontaneously active GABAA receptors mediate gabazine-resistant tonic currents in pyramidal neurons.

Impact of epsilon and theta subunits on pharmacological properties of alpha3beta1 GABAA receptors expressed in Xenopus oocytes

Background

γ-Aminobutyric acid type A (GABA_A) receptors provide the main inhibitory control in the brain. Their heterogeneity may make it possible to precisely target drug effects to selected neuronal populations. In situ hybridization using rat brain sections has revealed a unique expression of GABA_A receptor ε and θ subunit transcripts in the locus coeruleus, where they are accompanied at least by α3, α2, β1 and β3 subunits. Here, we studied the pharmacology of the human α3β1, α3β1ε, α3β1θ and α3β1εθ receptor subtypes expressed in Xenopus oocytes and compared them with the γ2 subunit-containing receptors.

Results

The GABA sensitivites and effects of several positive modulators of GABA_A receptors were studied in the absence and the presence of EC₂₅ GABA using the two-electrode voltage-clamp method. We found 100-fold differences in GABA sensitivity between the receptors, α3β1ε subtype being the most sensitive and α3β1γ2 the least sensitive. Also gaboxadol dose-response curves followed the same sensitivity rank order, with EC₅₀ values being 72 and 411 μM for α3β1ε and α3β1γ2 subtypes, respectively. In the presence of EC₂₅ GABA, introduction of the ε subunit to the receptor complex resulted in diminished modulatory effects by etomidate, propofol, pregnanolone and flurazepam, but not by pentobarbital. Furthermore, the α3β1ε subtype displayed picrotoxin-sensitive spontaneous activity. The θ subunit-containing receptors were efficiently potentiated by the anesthetic etomidate, suggesting that θ subunit could bring the properties of β2 or β3 subunits to the receptor complex.

Conclusion

The ε and θ subunits bring additional features to α3β1 GABA_A receptors. These receptor subtypes may constitute as novel drug targets in selected brain regions, e.g., in the brainstem locus coeruleus nuclei.

Kinetics and spontaneous open probability conferred by the epsilon subunit of the GABAA receptor

GABAA receptors mediate synaptic and extrasynaptic inhibition. Native receptors consist of alpha and beta subunits, which are required for function, and another "modulatory" subunit, for example, gamma, delta, or epsilon. Of these, the epsilon subunit has the most restricted distribution, confers resistance to neurosteroid and anesthetic modulation, and causes spontaneous channel opening. Little is known, however, about how epsilon affects receptor kinetics, which in turn shape responses to both ambient and synaptic GABA exposure. Here, we expressed human alpha2beta1, alpha2beta1gamma2, or alpha2beta1epsilon subunit combinations in human embryonic kidney 293 cells and used rapid solution exchange to study receptor kinetics in outside-out patches. The epsilon subunit greatly slowed deactivation and recovery after brief GABA pulses. During long, saturating GABA pulses, the rate of desensitization was slower for alpha2beta1epsilon and alpha2beta1gamma2 than for alpha2beta1. However, in alpha2beta1epsilon, the final extent of desensitization was large compared with that of alpha2beta1gamma2. Responses in alpha2beta1epsilon, but not the others, were often followed by an "overshoot" above the baseline, suggesting that a fraction of channels are spontaneously open and are transiently silenced by receptor activation and subsequent desensitization. The baseline current and associated noise were reduced by picrotoxin, revealing that epsilon-containing channels are open approximately 4% of the time in the absence of GABA. These results suggest that, if epsilon-containing receptors are expressed at synapses, the synaptic currents would be long-lasting but may rundown quickly under high-frequency activation. In addition, silencing of spontaneous openings by desensitization raises the possibility that tonic inhibition mediated by epsilon-containing receptors may be regulated by phasic inhibition.

Pharmacological properties of GABAA receptors in rat hypothalamic neurons expressing the epsilon-subunit

The pharmacological properties and functional role of native GABA(A) receptors (GABA(A)Rs) were investigated in rat hypothalamic neurons expressing the epsilon-subunit with the help of whole-cell patch-clamp recording and single-cell reverse transcription-PCR. Two cell groups were identified: histaminergic tuberomamillary and orexinergic/hypocretinergic neurons. Approximately 25% of histaminergic and 70% of orexinergic neurons contained mRNA encoding for the epsilon-subunit. Double-immunofluorescence staining revealed a somatic localization of this protein in these two neuronal groups. Constitutive activity, diazepam modulation, fast desensitization of maximal currents, and activation by propofol (6-98 microm) of GABA(A)Rs did not correlate with epsilon-subunit expression. Propofol at 3-12 microm potentiated GABA-mediated currents similarly in all neurons. However, noise variance analysis of GABA-mediated currents enhanced by propofol revealed a significant difference between epsilon-positive and epsilon-negative neurons. The former displayed no difference between control and potentiated responses, and, in the latter, noise was decreased in the presence of propofol. Spontaneous IPSCs recorded in cultured hypothalamic neurons were prolonged in the presence of propofol in all epsilon-negative neurons, whereas propofol-resistant IPSCs were recorded in epsilon-positive cells. The infrequent expression of the epsilon-subunit may be a key factor in the recently discovered central role of the tuberomamillary nucleus in anesthesia.