Identification of an ATP-sensitive K+ channel in spiny neurons of rat caudate nucleus

Involvement of ATP-sensitive potassium channels and the opioid system in the anticonvulsive effect of zolpidem in mice

Zolpidem is a hypnotic medication that mainly exerts its function through activating γ-aminobutyric acid (GABA)A receptors. There is some evidence that zolpidem may have anticonvulsive effects. However, the mechanisms underlying this effect have not been elucidated yet. In the present study, we used the pentylentetrazole (PTZ)-induced generalized seizure model in mice to investigate whether zolpidem can affect seizure threshold. We also further evaluated the roles of ATP-sensitive potassium (KATP) channels as well as μ-opioid receptors in the effects of zolpidem on seizure threshold. Our data showed that zolpidem in a dose-dependent manner increased the PTZ-induced seizure threshold. The noneffective (i.e., did not significantly alter the PTZ-induced seizure threshold by itself) doses of KATP channel blocker (glibenclamide) and nonselective opioid receptor antagonist (naloxone) were able to inhibit the anticonvulsive effect of zolpidem. Additionally, noneffective doses of either KATP channel opener (cromakalim) or nonselective μ-opioid receptor agonist (morphine) in combination with a noneffective dose of zolpidem exerted a significant anticonvulsive effect on PTZ-induced seizures in mice. A combination of noneffective doses of naloxone and glibenclamide, which separately did not affect zolpidem effect on seizure threshold, inhibited the anticonvulsive effects of zolpidem. These results suggest a role for KATP channels and the opioid system, alone or in combination, in the anticonvulsive effects of zolpidem.

REVIEW ■ : ATP-sensitive Potassium Channels: Diverse Functions in the Central Nervous System

ATP-sensitive potassium channels open when cytoplasmic levels of ATP drop, thus linking membrane potential to the metabolic state of the cell. Cloning studies have suggested that these channels are related structurally to the inward rectifier family of potassium channels, with two putative membrane-spanning regions. Sulfonylurea drugs, which are used in the treatment of diabetes, inhibit these channels by binding to an associated membrane protein. Other drugs, including some vasodilators, activate ATP-sensitive potassium channels. Diverse neurotransmitter and hormone receptors can modulate these channels, in some cases through interactions with guanyl nucleotide binding proteins. There appear to be multiple subtypes of these channels, differing in electrical properties as well as in drug sensitivities. In the brain, these channels appear to play a role in mediating satiety after feeding. They also function in neurons to protect against excitotoxicity, by counteracting the membrane depolarization associated with metabolic stress. Brain dopamine receptors appear to modulate a novel subtype of ATP-sensitive potassium channel. The association of dopamine receptors with a mechanism involved in protection against neurodegeneration may have implications for the causes of diseases in which dopaminergic regions of brain undergo structural changes, possibly including schizophrenia. NEUROSCIENTIST 2:145-152, 1996

Classification of H₂O₂as a neuromodulator that regulates striatal dopamine release on a subsecond time scale

Here we review evidence that the reactive oxygen species, hydrogen peroxide (H(2)O(2)), meets the criteria for classification as a neuromodulator through its effects on striatal dopamine (DA) release. This evidence was obtained using fast-scan cyclic voltammetry to detect evoked DA release in striatal slices, along with whole-cell and fluorescence imaging to monitor cellular activity and H(2)O(2) generation in striatal medium spiny neurons (MSNs). The data show that (1) exogenous H(2)O(2) suppresses DA release in dorsal striatum and nucleus accumbens shell and the same effect is seen with elevation of endogenous H(2)O(2) levels; (2) H(2)O(2) is generated downstream from glutamatergic AMPA receptor activation in MSNs, but not DA axons; (3) generation of modulatory H(2)O(2) is activity dependent; (4) H(2)O(2) generated in MSNs diffuses to DA axons to cause transient DA release suppression by activating ATP-sensitive K(+) (K(ATP)) channels on DA axons; and (5) the amplitude of H(2)O(2)-dependent inhibition of DA release is attenuated by enzymatic degradation of H(2)O(2), but the subsecond time course is determined by H(2)O(2) diffusion rate and/or K(ATP)-channel kinetics. In the dorsal striatum, neuromodulatory H(2)O(2) is an intermediate in the regulation of DA release by the classical neurotransmitters glutamate and GABA, as well as other neuromodulators, including cannabinoids. However, modulatory actions of H(2)O(2) occur in other regions and cell types, as well, consistent with the widespread expression of K(ATP) and other H(2)O(2)-sensitive channels throughout the CNS.

Subsecond regulation of striatal dopamine release by pre-synaptic KATP channels

ATP-sensitive K(+) (K(ATP)) channels are composed of pore-forming subunits, typically Kir6.2 in neurons, and regulatory sulfonylurea receptor subunits. In dorsal striatum, activity-dependent H(2)O(2) produced from glutamate receptor activation inhibits dopamine release via K(ATP) channels. Sources of modulatory H(2)O(2) include striatal medium spiny neurons, but not dopaminergic axons. Using fast-scan cyclic voltammetry in guinea-pig striatal slices and immunohistochemistry, we determined the time window for H(2)O(2)/K(ATP)-channel-mediated inhibition and assessed whether modulatory K(ATP) channels are on dopaminergic axons. Comparison of paired-pulse suppression of dopamine release in the absence and presence of glibenclamide, a K(ATP)-channel blocker, or mercaptosuccinate, a glutathione peroxidase inhibitor that enhances endogenous H(2)O(2) levels, revealed a time window for inhibition of 500-1000 ms after stimulation. Immunohistochemistry demonstrated localization of Kir6.2 K(ATP)-channel subunits on dopaminergic axons. Consistent with the presence of functional K(ATP) channels on dopaminergic axons, K(ATP)-channel openers, diazoxide and cromakalim, suppressed single-pulse evoked dopamine release. Although cholinergic interneurons that tonically regulate dopamine release also express K(ATP) channels, diazoxide did not induce the enhanced frequency responsiveness of dopamine release seen with nicotinic-receptor blockade. Together, these studies reveal subsecond regulation of striatal dopamine release by endogenous H(2)O(2) acting at K(ATP) channels on dopaminergic axons, including a role in paired-pulse suppression.

Partial mitochondrial inhibition causes striatal dopamine release suppression and medium spiny neuron depolarization via H2O2 elevation, not ATP depletion

Mitochondrial dysfunction is a potential causal factor in Parkinson's disease. We show here that acute exposure to the mitochondrial complex I inhibitor rotenone (30-100 nM; 30 min) causes concentration-dependent suppression of single-pulse evoked dopamine (DA) release monitored in real time with carbon-fiber microelectrodes in guinea pig striatal slices, with no effect on DA content. Suppression of DA release was prevented by the sulfonylurea glibenclamide, implicating ATP-sensitive K+ (KATP) channels; however, tissue ATP was unaltered. Because KATP channels can be activated by hydrogen peroxide (H2O2), as well as by low ATP, we examined the involvement of rotenone-enhanced H2O2 generation. Confirming an essential role for H2O2, the inhibition of DA release by rotenone was prevented by catalase, a peroxide-scavenging enzyme. Striatal H2O2 generation during rotenone exposure was examined in individual medium spiny neurons using fluorescence imaging with dichlorofluorescein (DCF). An increase in intracellular H2O2 levels followed a similar time course to that of DA release suppression and was accompanied by cell membrane depolarization, decreased input resistance, and increased excitability. Extracellular catalase markedly attenuated the increase in DCF fluorescence and prevented rotenone-induced effects on membrane properties; membrane changes were also largely prevented by flufenamic acid, a blocker of transient receptor potential (TRP) channels. Thus, partial mitochondrial inhibition can cause functional DA denervation via H2O2 and KATP channels, without DA or ATP depletion. Furthermore, amplified H2O2 levels and TRP channel activation in striatal spiny neurons indicate potential sources of damage in these cells. Overall, these novel factors could contribute to parkinsonian motor deficits and neuronal degeneration caused by mitochondrial dysfunction.

Developmental downregulation of ATP-sensitive potassium conductance in astrocytes in situ

In many neural and non-neural cells, ATP-sensitive potassium (K(ATP)) channels couple the membrane potential to energy metabolism. We investigated the activation of K(ATP) currents in astrocytes of different brain regions (hippocampus, cerebellum, dorsal vagal nucleus) by recording whole-cell currents with the patch-clamp technique in acute rat brain slices. Pharmacological tools, hypoglycemia and specific compounds in the pipette solution (cAMP, UDP), were used to modulate putative K(ATP) currents. The highest rate of K(ATP) specific currents was observed with a pipette solution containing cAMP and external stimulation with diazoxide (0.3 mM). The diazoxide-activated current had a reversal potential negative to -80 mV and was inhibited by tolbutamide (0.2 mM). We found that not all cells activated a K(ATP) current, and that the portion of cells with functional K(ATP) channel expression was developmentally downregulated. Whereas diazoxide activated K(ATP) currents in 57% of the astrocytes in rats aged 8-11 days (n = 21), the rate decreased to 38% at 12-15 days (n = 29) and to 8% at 16-19 days (n = 12). No significant difference was observed for the three brain regions. In recordings without cAMP in the internal solution, only 21% (12-15 days; n = 19) or none (16-19 days; n = 7), respectively, showed a potassium current upon diazoxide application. This metabolically regulated potassium conductance may be of importance, particularly in immature astrocytes with a complex current pattern, which have a relatively high input resistance: K(ATP) currents activated by energy depletion may hyperpolarize the cells, or stabilize a negative resting potential during depolarizing stimuli mediated, e.g., by glutamate receptors and/or uptake carriers.

Molecular biology of adenosine triphosphate-sensitive potassium channels

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

Cell-type specific expression of ATP-sensitive potassium channels in the rat hippocampus

1. The distribution of ATP-sensitive K+ channels (KATP channels) was investigated in four cell types in hippocampal slices prepared from 10- to 13-day-old rats: CA1 pyramidal cells, interneurones of stratum radiatum in CA1, complex glial cells of the same area and granule cells of the dentate gyrus. The neuronal cell types were identified visually and characterized by the shapes and patterns of their action potentials and by neurobiotin labelling. 2. The patch-clamp technique was used to study the sensitivity of whole-cell currents to diazoxide (0.3 mM), a KATP channel opener, and to tolbutamide (0.5 mM) or glibenclamide (20 microM), two KATP channel inhibitors. The fraction of cells in which whole-cell currents were activated by diazoxide and inhibited by tolbutamide was 26% of pyramidal cells, 89 % of interneurones, 100% of glial cells and 89% of granule cells. The reversal potential of the diazoxide-induced current was at the K+ equilibrium potential and a similar current activated spontaneously when cells were dialysed with an ATP-free pipette solution. 3. Using the single-cell RT-PCR method, the presence of mRNA encoding KATP channel subunits (Kir6.1, Kir6.2, SUR1 and SUR2) was examined in CA1 pyramidal cells and interneurones. Subunit mRNA combinations that can result in functional KATP channels (Kir6.1 together with SUR1, Kir6.2 together with SUR1 or SUR2) were detected in only 17% of the pyramidal cells. On the other hand, KATP channels may be formed in 75% of the interneurones, mainly by the combination of Kir6.2 with SUR1 (58% of all interneurones). 4. The results of these combined analyses indicate that functional KATP channels are present in principal neurones, interneurones and glial cells of the rat hippocampus, but at highly different densities in the four cell types studied.

Identification of an ATP-sensitive potassium channel current in rat striatal cholinergic interneurones

1. Whole-cell patch-clamp recordings were made from rat striatal cholinergic interneurones in slices of brain tissue in vitro. In the absence of ATP in the electrode solution, these neurones were found to gradually hyperpolarize through the induction of an outward current at -60 mV. This outward current and the resultant hyperpolarization were blocked by the sulphonylureas tolbutamide and glibenclamide and by the photorelease of caged ATP within neurones. 2. This ATP-sensitive outward current was not observed when 2 mM ATP was present in the electrode solution. Under these conditions, 500 microM diazoxide was found to induce an outward current that was blocked by tolbutamide. 3. Using permeabilized patch recordings, neurones were shown to hyperpolarize in response to glucose deprivation or metabolic poisoning with sodium azide (NaN3). The resultant hyperpolarization was blocked by tolbutamide. 4. In cell-attached recordings, metabolic inhibition with 1 mM NaN3 revealed the presence of a tolbutamide-sensitive channel exhibiting a unitary conductance of 44.1 pS. 5. Reverse transcription followed by the polymerase chain reaction using cytoplasm from single cholinergic interneurones demonstrated the expression of the ATP-sensitive potassium (KATP) channel subunits Kir6.1 and SUR1 but not Kir6.2 or SUR2. 6. It is concluded that cholinergic interneurones within the rat striatum exhibit a KATP channel current and that this channel is formed from Kir6.1 and SUR1 subunits.

KATP channel formation by the sulphonylurea receptors SUR1 with Kir6.2 subunits in rat dorsal vagal neurons in situ

1. Functional and molecular properties of ATP-sensitive K+ (KATP) channels were studied in dorsal vagal neurons (DVNs) of rat brainstem slices using patch-clamp and single-cell antisense RNA amplification-polymerase chain reaction (PCR) techniques. 2. In the cell-attached configuration, 1 mM cyanide resulted in block of spontaneous firing and concomitant opening of single channels with a mean single open time of 2-3 ms and a burst duration of up to several hundred milliseconds. Inhibition of such single-channel activity with 200 microM tolbutamide led to the reappearance of spontaneous discharge. 3. Whole-cell recordings during anoxia revealed a hyperpolarization of the DVNs. Harvesting of cytoplasm, antisense RNA amplification and subsequent PCR showed coexpression for single DVNs of mRNA for the sulphonylurea receptor SUR1 isoform and for the inwardly rectifying K+ channel subunit Kir6.2, but not for the SUR2 or Kir6.1 isoforms of these channel/receptor subclasses. 4. Upon anoxia, a stable depolarization by less than 10 mV was observed in non-excitable cells in the dorsal vagal nucleus. These cells, which expressed glial fibrillary acidic protein (GFAP), showed a high level of mRNA for Kir6.2, a weak signal for SUR1, whereas SUR2 or Kir6.1 were not detected. 5. The results suggest that functional KATP channels in DVNs are constituted by the formation of Kir6.2 subunits with SUR1 receptors.

A view of sur/KIR6.X, KATP channels

ATP-sensitive potassium channels, termed KATP channels, link the electrical activity of cell membranes to cellular metabolism. These channels are heteromultimers of sulfonylurea receptor (SUR) and KIR6.X subunits associated with a 1:1 stoichiometry as a tetramer (SUR/KIR6.X forms the pores, whereas SUR regulates their activity. Changes in [ATP]i and [ADP]i gate the channel. The diversity of KATP channels results from the assembly of SUR and KIR6.X subtypes KIR6.1-based channels differ from KIR6.2 channels mainly by their smaller unitary conductance. SUR1- and SUR2-based channels are distinguished by their differential sensitivity to sulfonylureas, whereas SUR2A-based channels are distinguished from SUR2B channels by their differential sensitivity to diazoxide. Mutations that result in the loss of KATP channels in pancreatic beta-cells have been identified in SUR1 and KIR6.2. These mutations lead to familial hyperinsulinism. Understanding the mutations in SUR and KIR6.X is allowing insight into how these channels respond to nucleotides, sulfonylureas, and potassium channel openers, KCOs.

KATP-channel on the somata of spiny neurones in rat caudate nucleus: regulation by drugs and nucleotides

1. The aim of the present study was to characterize the pharmacological properties of the adenosine 5'-triphosphate(ATP)-sensitive K+ channel (KATP-channel) on the somata of spiny neurones in rat caudate nucleus and to compare them with those of beta-cells. For that purpose we tested the effects of several KATP-channel-inhibiting and -activating drugs on the opening activity of the KATP-channel in caudate nucleus by use of the patch-clamp technique. In addition, the modulation of drug responses by cytosolic nucleotides was examined. 2. When KATP-channels in caudate nucleus were activated in cell-attached patches by inhibition of mitochondrial energy production, meglitinide (a benzoic acid derivative), Hoe36320 (a sulphonylurea of low lipophilicity) and glipizide reduced KATP-channel activity half-maximally at 0.4 microM, 0.4 microM and about 0.5 nM, respectively. 3. In inside-out patches (presence of 0.7 mM free Mg2+ at the cytoplasmic membrane side), tolbutamide (0.1 mM) caused only partial inhibition of KATP-channels in the absence of cytosolic nucleotides but complete inhibition in the simultaneous presence of the channel-activating nucleotide guanosine 5'-diphosphate (GDP; 1 mM) and the channel-inhibiting nucleotide adenylyl-imidodiphosphate (AMP-PNP; 0.2 mM). 4. Diazoxide (0.3 mM) strongly increased channel activity in the presence of ATP (0.1 mM) or GDP (0.03 mM), but was ineffective in the presence of AMP-PNP (0.1 mM). In the absence of cytosolic nucleotides diazoxide even decreased channel activity. 5. In the presence of 0.1 mM ATP, diazoxide activated KATP-channels half-maximally at 38 microM. 6. When KATP-channel activity was inhibited by 0.1 mM ATP, (-)-pinacidil (0.5 mM) elicited a slight activation of KATP-channels in caudate nucleus, whereas (+)-pinacidil (0.5 mM) and lemakalim (0.3 mM) were ineffective. 7. Since our data indicate similar control by drugs and nucleotides of KATP-channels in the somata of spiny neurones and pancreatic beta-cells, we conclude that the high affinity sulphonylurea receptors of these tissues are probably closely related.

ATP-sensitive K+ channels are functional in expiratory neurones of normoxic cats

1. We analysed spontaneously active expiratory neurones (n = 48) of anaesthetized cats for the presence of ATP-sensitive K+ (KATP) channels. 2. Intracellular injection of ATP reversibly depolarized neurones during all phases of the respiratory cycle. During expiration, membrane potential depolarized by an average of 1.5 +/- 0.1 mV leading to a 25% increase of discharge frequency. During inspiration, ATP induced a 1.8 +/- 0.2 mV depolarization, which was accompanied by a maximum of 20% increase of input resistance (Rn). 3. Extracellular application of diazoxide, an agonist of KATP channels, resulted in reversible membrane hyperpolarization in 68% of neurones (n = 19). This hyperpolarization (2.5 mV during expiration and 3.1 mV during inspiration) was accompanied by a 22% decrease in Rn. 4. Extracellular application of tolbutamide and glibenclamide, two antagonists of KATP channels, evoked reversible depolarizations in 76% of neurones (n = 21). The depolarization was relatively constant throughout the respiratory cycle (1.4 mV during expiration and 2.3 mV during inspiration). Rn increased by 22%. 5. The same sulphonylureas also changed the steepness of membrane depolarization when neurones escaped spontaneous synaptic inhibition during postinspiration. Extracellularly applied tolbutamide and glibenclamide increased the steepness of depolarization by 21%, while diazoxide reduced it by 20%. 6. Antagonism of drugs was verified by simultaneous extra- and intracellular application of diazoxide and glibenclamide, respectively. 7. During voltage clamp at holding potential at -60 to -67 mV, intracellular or extracellular application of tolbutamide and glibenclamide blocked a persistent outward current. 8. We conclude that KATP channels are functional in expiratory neurones of adult cats and contribute to the control of excitability even during normoxia.

Sulfonylurea-sensitive potassium current evoked by sodium-loading in rat midbrain dopamine neurons

In Parkinson's disease, there is evidence of impaired mitochondrial function which reduces the capacity to synthesize ATP in dopamine neurons. This would be expected to reduce the activity of the sodium pump (Na+/K+ ATPase), causing increased intracellular levels of Na+. Patch pipettes were used to introduce Na+ (40 mM in pipette solutions) into dopamine neurons in the rat midbrain slice in order to study the electrophysiological effects of increased intracellular Na+. We found that intracellular Na+ loading evoked 100-300 pA of outward current (at -60 mV) and increased whole-cell conductance; these effects developed gradually during the first 10 min after rupture of the membrane patch. Extracellular Ba2+ reduced most of the outward current evoked by Na+ loading; this Ba(2+)-sensitive current reversed direction at the expected reversal potential for K+ (EK), and was also blocked by extracellular tetraethylammonium (30 mM) and intracellular Cs+ (which replaced K+ in pipette solutions). The sulfonylurea drugs glipizide (IC50 = 4.9 nM), tolbutamide (IC50 = 23 microM) and glibenclamide (1 microM) were as effective as 300 microM Ba2+ in reducing the K+ current evoked by Na+ loading. When recording with "control" pipettes containing 15 mM Na+, diazoxide (300 microM) increased chord conductance and evoked outward current at -60 mV, which also reversed direction near EK. Effects of diazoxide were blocked by glibenclamide (1 microM) or glipizide (300 nM). Diazoxide (300 microM) and baclofen (3 microM), which also evoked K(+)-mediated outward currents recorded with control pipettes, caused little additional increases in outward currents during Na+ loading. Raising ATP concentrations to 10 mM in pipette solutions failed to significantly reduce currents evoked by diazoxide or Na+ loading, suggesting that these currents may not be mediated by ATP-sensitive K+ channels. Finally, Na+ loading using pipettes containing Cs+ in place of K+ evoked a relatively small outward current (50-150 pA at -60 mV), which developed gradually over the first 10 min after rupturing the membrane patch. This current was reduced by dihydro-ouabain (3 microM) and a low extracellular concentration of K+ (0.5 mM instead of 2.5 mM), but was not affected by Ba2+. We conclude that intracellular Na+ loading evokes a current generated by Na+/K+ ATPase in addition to sulfonylurea-sensitive K+ current. This Na(+)-dependent K+ current is unusual in its sensitivity to sulfonylureas, and could protect dopamine neurons against toxic effects of intracellular Na+ accumulation.

KATP channel mediation of anoxia-induced outward current in rat dorsal vagal neurons in vitro

1. Thin brainstem slices (150 microns thickness) were taken from mature rats, and membrane potentials (Em) and currents (Im) in the dorsal vagal neurons (DVN) were analysed with whole-cell patch clamp techniques during anoxia. 2. At a holding potential (Vh) of -50 mV, a sustained anoxia-induced outward current (AOC) of 92 +/- 44 pA (reversal potential (Erev), -78 +/- 12 mV) and a concomitant increase of membrane conductance (gm) from 2.2 +/- 0.45 to 5.9 +/- 2.4 nS were revealed in 40% of 142 DVN analysed. The AOC led to a hyperpolarization of the cells by 14.4 +/- 6.1 mV from a mean resting Em of -51 +/- 6 mV, and to blockade of spontaneous action potential discharges. In the remaining DVN, anoxia had almost no effect on Em, Im or gm and did not block spontaneous action potential discharges. 3. The AOC was not affected by 0.5 microM tetrodotoxin (TTX), 2 mM Mn2+, 50 microM cyanonitroquinoxaline dione (CNQX) or 100 microM bicuculline. 4. Elevation of the extracellular [K+] from 3 to 10 mM resulted in a positive shift of Erev of the AOC by 23 mV, whereas an increase in the [Cl-] of the patch pipette solution from 5 to 144 mM had no effect on Erev. 5. In DVN responding with an AOC, addition of 200 microM diazoxide, an activator of ATP-sensitive K+ (KATP) channels, to oxygenated solutions elicited a similar outward current (Erev = -79 +/- 5.5 mV, n = 12) and increase in gm. Diazoxide did not affect Em, Im or gm in cells which did not show an AOC. 6. In a subpopulation of DVN (n = 26), spontaneous activation of a KATP current with an Erev of -80 +/- 6 mV was observed. As analysed in four of these cells, an AOC was revealed during the initial phase of development of the spontaneous outward current but not under steady-state conditions. 7. The AOC, the diazoxide-induced current, and the spontaneous outward current were completely blocked upon bath application of the KATP channel blocker tolbutamide (100-200 microM). 8. The results indicate that the sustained anoxia-induced outward current of dorsal vagal neurons is due to activation of KATP channels. A possible physiological role of functional inactivation of these cells during metabolic disturbances is discussed.