Gated currents generate single spike activity in amacrine cells of the tiger salamander retina

Amacrine cells form the neural networks mediating the second level of lateral interactions in the vertebrate retina. Members of a prominent class of amacrine cells, found in most vertebrates, respond at both the onset and termination of steps of illumination with a single, large transient depolarization. We show here how specific relationships between membrane currents control this single spike activity. Using whole-cell patch clamp on living retinal slices, we studied the membrane currents in amacrine cells. The currents elicited by depolarizing voltage steps could be separated into three main ionic components: a transient inward voltage-gated sodium current, a relatively small sustained inward voltage-gated calcium current, and a calcium-dependent outward current. A specific relationship between the sodium and potassium current alone appears to preclude repetitive spike activity. Potassium current is activated at potentials positive to -20 mV, but the sodium inactivation, between -60 and -20 mV, does not intersect potassium activation. Therefore, a steady depolarizing current step elicits an initial spike but then the membrane cannot be sufficiently hyperpolarized by potassium current to remove sodium inactivation and the cell remains refractory.     

The electrophysiological effects of acetylcholine in single human atrial cells

Single cells were isolated from human atria. The ionic currents in these cells were measured using the whole cell clamp technique. Time-dependent currents during depolarizing clamp steps can be described as due to Ca current and transient outward current. Time-dependent inward currents were seen during hyperpolarizing voltage steps. These currents are carried by the inward rectifier iK1, while in some cells also an if pacemaker current is present. The effect of acetylcholine was investigated at potentials negative to -50 mV. Acetylcholine induces an inwardly rectifying K current. The acetylcholine-induced current is different from iK1 in kinetics and Ba-sensitivity. The effect of acetylcholine decreases in time due to desensitization. The electrical response of the human atrial cells to acetylcholine is qualitatively similar to the effect of acetylcholine in guinea-pig atrial cells.     

An inward rectifier and a voltage-dependent K+ current in single, cultured pericytes from bovine heart

OBJECTIVE: The purpose of this study was to describe passive electrical properties and major membrane currents in coronary pericytes. METHODS: 78 single, cultured bovine pericytes were studied with the patch-clamp technique in the whole-cell mode. RESULTS: The membrane potential of the cells was -48.9+/-9.6 mV (mean+/-S.D.) with 5 mM and -23.2+/-2.2 mV with 60 mM extracellular K+. The membrane capacitance was 150.2+/-123.2 pF. The current-voltage relation of the pericytes was dominated by an inward current at hyperpolarized potentials and an outward current at depolarized potentials. Increasing extracellular K+ from 5 to 60 mM led to an increase of the inward current and to a shift of this current to more depolarized potentials. The inward current was very sensitive to extracellular barium (50 microM). The maximum slope conductance of the cells at hyperpolarized potentials was 2.9+/-2.8 nS. Inward rectification of whole-cell currents was steep (slope factor = 6.8 mV). With elevated external K+ the outward current reversed near the potassium equilibrium potential. Onset of the outward current was sigmoid and inactivation of this current was monoexponential, slow (time constant = 12.8 s) and incomplete. Voltage-dependence of outward current steady-state activation was steep (slope factor = 4.6 mV). The outward current was very sensitive to 4-aminopyridine (dissociation constant = 0.1 mM). The maximum slope conductance at depolarized potentials was 16.6+/-15.6 nS. CONCLUSION: We report for the first time, patch-clamp recordings from coronary pericytes. An inward rectifier and a voltage-dependent K+ current were identified and characterized. Regulation of these currents may influence coronary blood flow.     

Excitation-secretion coupling: ionic currents in glomerulosa cells: effects of adrenocorticotropin and K+ channel blockers

The ionic conductance of cultured rat glomerulosa cells has been studied using the whole cell variant of the patch-clamp technique. We have identified and partially characterized three currents: a transient outward current, a slow outward current, and a slow inward current. The transient outward current activated rapidly and then inactivated slowly on maintained depolarization. Activation was initiated at -30 mV, and zero current was seen at -60 to -50 mV. The slow outward current did not inactivate with time and was initiated around 0 mV; its zero current voltage was difficult to evaluate. The two outward currents were present in different proportions, which explains the different time course of the total outward current from one cell to another. A slow inward current was also found which activated near -30 mV and reached its reversal potential between 80 and 100 mV. This current was blocked by Co2+, increased with [Ca2+]o, and was insensitive to Na+-free external medium. ACTH, a potent stimulant of steroid output, was found to block the transient outward current, but was ineffective on the slow outward current and the slow inward current. Tetraethylammonium and 4-aminopyridine, K+ channel inhibitors, also blocked the transient outward current.     

Calcium currents in GH3 cultured pituitary cells under whole-cell voltage-clamp: inhibition by voltage-dependent potassium currents.

To isolate inward Ca2+ currents in GH3 rat pituitary cells, an inward Na+ current as well as two outward K+ currents, a transient voltage-dependent current (IKV) and a slowly rising Ca2+-activated current (IKCa), must be suppressed. Blockage of these outward currents, usually achieved by replacement of intracellular K+ with Cs+, reveals sustained inward currents. Selective blockage of either K+ current can be accomplished in the presence of intracellular K+ by use of quaternary ammonium ions. When IKCa and Na+ currents are blocked, the net current elicited by stepping the membrane potential (Vm) from -60 to 0 mV is inward first, becomes outward and peaks in 10-30 msec, and finally becomes inward again. Under this condition, in which both IKV and Ca2+ currents should be present throughout the duration of the voltage step, the Ca2+ current was not detected at the time of peak outward current. That is, plots of peak outward current vs. Vm are monotonic and are not modified by nisoldipine or low external Ca2+ as would be expected if Ca2+ currents were present. However, similar plots at times other than at peak current are not monotonic and are altered by nisoldipine or low Ca2+ (i.e., inward currents decrease and plots become monotonic). When K+ channels are first inactivated by holding Vm at -30 mV, a sustained Ca2+ current is always observed upon stepping Vm to 0 mV. Furthermore, substitution of Ba2+ for Ca2+ causes blockage of IKV and inhibition of this current results in inward Ba2+ currents with square wave kinetics. These data indicate that the Ca2+ current is completely inhibited at peak outward IKV and that Ca2+ conductance is progressively disinhibited as the transient K+ current declines due to channel inactivation. This suggests that in GH3 cells Ca2+ channels are regulated by IKV.     

Development of a delayed outward-rectifying K+ conductance in cultured mouse peritoneal macrophages.

Patch clamp techniques were used to study ionic currents in cultured mouse peritoneal macrophages. Whole-cell voltage clamp studies of cells 1-5 hr after isolation showed only a high-resistance linear membrane. After 1 day in culture, 82 of 85 cells studied had developed a voltage- and time-dependent potassium (K+) conductance similar to the delayed outward rectifier in nerve and muscle cells. The current activated when the membrane was depolarized above -50 mV. The sigmoidally rising current rose to a peak at a rate that increased with depolarization. Inactivation proceeded exponentially with a time constant of approximately equal to 450 ms. Recovery from inactivation was slow (tau = 12 s). The reversal potentials for varying extracellular K+ concentrations followed the Nernst predictions for a K+ -specific channel. The conductance was blocked by extracellular 4-aminopyridine and by intracellular tetraethylammonium chloride, barium, and cesium. Single-channel K+ currents comprising this net current had a conductance of 16 pS, exhibited bursting behavior, and inactivated with time. No inward currents were ever detected in macrophages cultivated for up to 4 days. Short-term exposure to chemoattractant and transmitter agents failed to activate an inward current. Macrophages may change their membrane electrophysiological properties depending on their state of functional activation. We postulate that the K+ conductance develops prior to depolarizing conductances involved in the macrophage's immunological functions.     

Novel gain-of-function mechanism in K(+) channel-related long-QT syndrome: altered gating and selectivity in the HERG1 N629D mutant

The N629D mutation, adjacent to the GFG signature sequence of the HERG1 A K(+) channel, causes long-QT syndrome (LQTS). Expression of N629D in Xenopus oocytes produces a rapidly activating, noninactivating current. N629D is nonselective among monovalent cations; permeation of K(+) was similar to that of Na(+) or Cs(+). During repolarization to potentials between -30 and -70 mV, N629D manifested an inward tail current, which was abolished by replacement of extracellular Na(+) (Na(+)(e)) with extracellular N-methyl-D-glucamine (NMG(e)). Because LQTS occurs in heterozygous patients, we coexpressed N629D and wild type (WT) at equimolar concentrations. Heteromultimer formation was demonstrated by analyzing the response to 0 [K(+)](e). The outward time-dependent current was nearly eliminated for WT at 0 [K(+)](e), whereas no reduction was observed for homomultimeric N629D or for the equimolar coexpressed current. To assess physiological significance, dofetilide-sensitive currents were recorded during application of simulated action potential clamps. During phase 3 repolarization, WT manifested outward currents, whereas homomultimeric N629D manifested inward depolarizing currents. During coexpression studies, variable phenotypes were observed ranging from a reduction in outward repolarizing current to net inward depolarizing current during phase 3. In summary, N629D replaces the WT outward repolarizing tail current with an inward depolarizing sodium current, which is expected to delay later stages of repolarization and contribute to arrhythmogenesis. Thus, the consequences of N629D resemble the pathophysiology seen in LQT3 Na(+) channel mutations and may be considered the first LQTS K(+) channel mutation that exhibits gain of function.     

Gating currents of the cloned delayed-rectifier K+ channel DRK1.

Gating currents of the cloned delayed-rectifier K+ channel DRK1 expressed in Xenopus oocytes were measured with the open-oocyte Vaseline-gap voltage-clamp technique. DRK1 gating charge had the following salient properties: (i) gating-charge amplitude correlated positively with size of the expressed ionic K+ currents; (ii) the time integral of ON and OFF gating currents was similar, indicating charge conservation and lack of charge immobilization; (iii) the gating-charge activation curve was shallower and had a half-activation potential 15 mV more negative than the activation curve for K+ conductance; (iv) effective valence for the gating current was about two electronic charges per gating subunit; (v) for large depolarizations (to > 0 mV) prominent rising phases were observed during the ON and OFF gating charge, which appeared as shoulders in unsubtracted traces; (vi) for small depolarizing pulses (to < 0 mV) ionic-current activation and deactivation had time constants similar to ON and OFF gating-current decay, respectively; (vii) negative prepulses made more prominent the ON rising phase and delayed ionic and gating currents. The results are consistent with a model for K+ channel activation that has an early slow and/or weakly voltage-dependent transition between early closed states followed by more voltage-dependent transitions between later closed states and a final voltage-independent closed-open transition.     

gamma-Aminobutyric acid exerts a local inhibitory action on the axon terminal of bipolar cells: evidence for negative feedback from amacrine cells.

It is well-established morphologically that bipolar cells, the second-order neurons in the vertebrate retina, make reciprocal synapses with amacrine cells in the inner plexiform layer. However, neither the property nor the physiological function of the feedback synapse is understood. Autoradiographic and immunohistochemical studies suggest the presence of gamma-aminobutyric acid (GABA)-ergic amacrine cells, and therefore the bipolar cells are thought to receive GABAergic inputs from amacrine cells. This possibility was investigated in the present study, in which we used solitary bipolar cells dissociated from the goldfish retina enzymatically. Dissociated solitary bipolar cells showed a large variety in morphology. In the present study, we selected the bipolar cells with a huge bulbous axon terminal. Bipolar cells of this subtype were identical in morphology to the on-center cells with rod-dominant inputs as revealed in earlier studies by intracellular staining. Membrane currents were measured under voltage clamp with a patch pipette in the whole cell configuration. In some experiments, GABA-sensitive membrane was excised as an outside-out patch from the axon terminal bulb of solitary bipolar cells. All cells of this type responded to GABA. The highest sensitivity was located at the axon terminal. The minimal effective dose was on the order of 10(-7) M. GABA increased the chloride conductance and evoked a membrane hyperpolarization. Partial desensitization was observed during the application of GABA. The bipolar cells had GABA type A receptors. These results are consistent with the idea that the rod-dominant on-center bipolar cells receive negative feedback inputs from GABAergic amacrine cells.     

Origin of the background sodium current and effects of sodium removal in cultured embryonic cardiac cells

Cardiac automaticity is partly due to a diastolic sodium current. Possible mediators of this include tetrodotoxin-sensitive "fast" channels, cesium-sensitive time-dependent pacemaker current channels, calcium-gated nonspecific channels, and electrogenic sodium-calcium exchange. We have studied the effects of abrupt sodium removal on membrane current and conductance in voltage-clamped chick embryonic myocardial cell aggregates, in the presence of various sodium flux inhibitors. Total replacement of sodium by lithium, Tris, or tetraethylammonium ions in aggregates clamped in the pacemaker range caused a brief outward current followed by a sustained net inward current. The outward current reached a peak value of 1.1 +/- 0.5 microA/cm2 at a mean latency of 5.4 +/- 1.2 sec. (n = 6; V = -70.5 +/- 8.9 mV; Tris). Conductance often decreased during the outward current. The inward current developed exponentially (t = 19 +/- 5 sec) and reached a steady state value of -1.6 +/- 0.4 microA/cm2. This current was reversed by depolarization (mean reversal potential = -13 +/- 13 mV), and was accompanied by increased conductance and spontaneous mechanical activity. Neither of the sodium-removal currents was affected by 20 microM tetrodotoxin. Cesium (up to 20 mM) had no effect on the late inward current or the mechanical activity, but decreased the early outward current by 80 +/- 12%. Manganese (25 mM), which blocks sodium-calcium exchange, abolished the late inward current and the mechanical activity. Manganese also reduced the early outward current by 27 +/- 10%. Manganese and cesium together blocked all the effects of sodium removal. We conclude that removal of extracellular sodium interrupts a cesium-sensitive "background" current, that may be related to the time-dependent pacemaker current, If. Sodium removal also causes gradual activation of a nonspecific conductance, which can ultimately depolarize the cells, and which may be gated by cytoplasmic calcium.     

Presynaptic inhibition in Aplysia involves a decrease in the Ca2+ current of the presynaptic neuron.

By voltage clamping presynaptic cell L10 and using pharmacologic separation techniques, we have analyzed the specific ionic currents in the presynaptic cell that correlate with presynaptic inhibition while assaying transmitter release with intracellular recordings from postsynaptic cells. We have found that presynaptic inhibition can be elicited in conditions in which the Na+ and the various K+ channels are pharmacologically blocked and depolarizing current pulses produce only an inward Ca2+ current. Both inward currents and tail currents at and above the K+ reversal potential were always less inward during presynaptic inhibition. The changes in conductance associated with presynaptic inhibition were voltage sensitive and paralleled the voltage sensitivity of the Ca2+ channel. We therefore conclude that presynaptic inhibition is caused by a direct transmitter-mediated decreased of presynaptic Ca2+-channel conductance.     

Voltage-gated and synaptic currents in rat Purkinje cells in dissociated cell cultures.

The electrical properties of rat Purkinje cells and synapses from granule cells were studied in dissociated cell cultures. To identify the cells we used an immunohistochemical method and recorded voltage-gated and synaptic currents with the patch-clamp technique (the whole-cell mode). Cultured Purkinje cells generated action potentials similar to those recorded from in vitro slices or in vivo preparations. Early Na, late Ca inward current, and K outward currents were distinguished by ion substitution under voltage clamp. We also recorded spontaneous synaptic currents in Purkinje cells cultured with granule cells. These synaptic currents reversed direction at the membrane potential of 2.5 mV, which was similar to currents induced by L-glutamate. Therefore, these are most likely excitatory synaptic currents from granule cells. Since these properties of Purkinje cells examined here are similar to those in situ, the cells in dissociated cell cultures offer a great opportunity to study biophysical properties of identified neurons in the central nervous system.     

Transient outward current carried by potassium and sodium in quiescent atrioventricular node cells of rabbits

Single atrioventricular node cells were dispersed by treating the rabbit heart with collagenase. In Tyrode's solution, the cells became rounded, and about 20% of them showed spontaneous activity, whereas the rest remained quiescent. When those quiescent cells were whole-cell clamped, depolarizing clamp pulses from the holding potential of -83 mV induced an outward current which decayed quickly, with a time course similar to that of the transient outward current in the Purkinje fiber. The amplitude of the current became larger when progressively more positive clamp pulses were given from a very negative holding potential. The inactivation time course of this current consisted of two exponential components. Single-channel current recordings from those cells revealed a class of channels that activated more frequently during the initial part of depolarizing pulses. Summation of those unitary currents reproduced activation and inactivation time courses of the macroscopic current well, suggesting that this channel corresponds to the transient outward current. The current-voltage relationship of the channel was linear with the slope conductance of 19.9 +/- 1.8 pS (n = 7), and the reversal potential was near the resting potential of the atrioventricular node cell with 5.4 mM potassium chloride and 134.6 mM sodium chloride in the pipette. The channel was passing mainly potassium ions, but sodium ions also seemed to carry a fraction of the current. The possible role of the transient outward current in the quiescent node cell is discussed.     

Presynaptic membrane potential affects transmitter release in an identified neuron in Aplysia by modulating the Ca2+ and K+ currents

We have examined the relationships between the modulation of transmitter release and of specific ionic currents by membrane potential in the cholinergic interneuron L10 of the abdominal ganglion of Aplysia californica. The presynaptic cell body was voltage-clamped under various pharmacological conditions and transmitter release from the terminals was assayed simultaneously by recording the synaptic potentials in the postsynaptic cell. When cell L10 was voltage-clamped from a holding potential of -60 mV in the presence of tetrodotoxin, graded transmitter release was evoked by depolarizing command pulses in the membrane voltage range (-35 mV to + 10 mV) in which the Ca(2+) current was also increasing. Depolarizing the holding potential of L10 results in increased transmitter output. Two ionic mechanisms contribute to this form of plasticity. First, depolarization inactivates some K(+) channels so that depolarizing command pulses recruit a smaller K(+) current. In unclamped cells the decreased K(+) conductance causes spike-broadening and increased influx of Ca(2+) during each spike. Second, small depolarizations around resting potential (-55 mV to -35 mV) activate a steady-state Ca(2+) current that also contributes to the modulation of transmitter release, because, even with most presynaptic K(+) currents blocked pharmacologically, depolarizing the holding potential still increases transmitter release. In contrast to the steady-state Ca(2+) current, the transient inward Ca(2+) current evoked by depolarizing clamp steps is relatively unchanged from various holding potentials.     

Effects of quinidine on plateau currents of guinea-pig ventricular myocytes

Effects of quinidine on membrane currents forming the plateau of action potentials were studied using an isolated single ventricular cell from guinea-pig hearts. Quinidine (5 mg/l) produced a fall and shortening of the early part of the plateau, and delayed its later part and final repolarization, without changes in resting membrane potential. Application of quinidine caused a reversible depression of the peak Ca2+ current by about 30% of the control. Delayed outward K+ current, iK, also decreased to less than 20% of the control. Thus, an outward tail current upon repolarization to -40 mV from depolarizing voltage steps of the plateau ranges became inward. Current values at the end of 200 ms pulses in response to voltage steps to -60-0 mV were always positive and were not changed by the drug. The inward current elicited at potentials negative to resting potential level, also, decreased by 13% to 23% of the control in the presence of the drug, but the effect was not reversible upon wash-out of the drug. These results suggest that quinidine causes a non-specific depression of inward rectifier K+ current, iK1, with minor degree but has little effect on the window sodium current. Therefore, changes in the action potential repolarization produced by quinidine can be explained by its effects on both calcium current and delayed outward K+ current.     

Effects of trimetazidine on action potentials and membrane currents of guinea-pig ventricular myocytes

The effects of trimetazidine on membrane potentials and membrane currents of enzymatically isolated guinea-pig ventricular cells were studied with the use of giga-seal suction pipettes for patch clamp. Trimetazidine (3 X 10(-5) M) decreased the action potential duration from 433 +/- 179 ms (mean and S.D., n = 9) to 319 +/- 156 ms within 8 mins. In voltage clamp experiment, trimetazidine at a concentration of 1.5 X 10(-4) M decreased the peak amplitude of calcium current by 40% (0.92 +/- 0.46 nA to 0.55 +/- 0.19 nA, mean +/- S.D., n = 5). The effect on calcium current was rate-dependent, e.g., at 1 Hz, trimetazidine blocked a larger fraction of the calcium current than at 0.2 Hz. The drug decreased the conductance of potassium current which flows via inward rectifier potassium channel from 28 +/- 11 nS to 19 +/- 10 nS, n = 5, P less than 0.05). Trimetazidine shifted the steady state current-voltage relationship outward at potentials positive to -20 mV. This shift was not due to the enhanced time- and voltage-dependent outward current (Ik). From these findings, it was concluded that trimetazidine shortens action potential duration by blocking the calcium channels with increases in steady state outward current or a possible blockade of non-inactivated component of the calcium current, at the plateau potentials. The reduction of calcium current and of inward rectifier potassium current may protect the cardiac cells from accumulation of calcium ions and from loss of potassium ions, in the presence of ischemia.     

TRPM1 is required for the depolarizing light response in retinal ON-bipolar cells

The ON pathway of the visual system, which detects increases in light intensity, is established at the first retinal synapse between photoreceptors and ON-bipolar cells. Photoreceptors hyperpolarize in response to light and reduce the rate of glutamate release, which in turn causes the depolarization of ON-bipolar cells. This ON-bipolar cell response is mediated by the metabotropic glutamate receptor, mGluR6, which controls the activity of a depolarizing current. Despite intensive research over the past two decades, the molecular identity of the channel that generates this depolarizing current has remained elusive. Here, we present evidence indicating that TRPM1 is necessary for the depolarizing light response of ON-bipolar cells, and further that TRPM1 is a component of the channel that generates this light response. Gene expression profiling revealed that TRPM1 is highly enriched in ON-bipolar cells. In situ hybridization experiments confirmed that TRPM1 mRNA is found in cells of the retinal inner nuclear layer, and immunofluorescent confocal microscopy showed that TRPM1 is localized in the dendrites of ON-bipolar cells in both mouse and macaque retina. The electroretinogram (ERG) of TRPM1-deficient (TRPM1^−/−) mice had a normal a-wave, but no b-wave, indicating a loss of bipolar cell response. Finally, whole-cell patch-clamp recording from ON-bipolar cells in mouse retinal slices demonstrated that genetic deletion of TRPM1 abolished chemically simulated light responses from rod bipolar cells and dramatically altered the responses of cone ON-bipolar cells. Identification of TRPM1 as a mGluR6-coupled cation channel reveals a key step in vision, expands the role of the TRP channel family in sensory perception, and presents insights into the evolution of vertebrate vision.     

Endothelin induces a nonselective cation current in vascular smooth muscle cells.

The potent vasoconstrictor endothelin leads to smooth muscle cell depolarization and increases in intracellular Ca2+. Although effects of endothelin on calcium channels have been described, it also has been speculated that endothelim may activate additional ion channels. The purpose of the present study was to identify an alternative ion current that could play a role in depolarizing cells in response to vasoconstrictors like endothelin and vasopressin. The effects of endothelin, vasopressin, sarafotoxin S6b, and phenylephrine were assessed using whole-cell patch-clamp recordings from primary dissociated rat aortic or mesenteric arterial smooth muscle cells cultured for 24-72 hours. From the usual resting potentials of these cells of -50 to -60 mV, endothelin (1-100 nM) induced a depolarization via an increase in membrane conductance. This depolarization was phasic, oscillating repeatedly from the resting potential to a relatively depolarized level and back to the resting potential. From a holding potential of -60 mV, endothelin-1, endothelin-3, vasopressin, or sarafotoxin S6b (but not phenylephrine) induced transient inward currents that also could be phasic. In external sodium, lithium, or cesium (but not Tris) and in internal potassium or cesium, these currents reversed near 0 mV. Although nifedipine-insensitive, the inward currents were absent in zero calcium, barium, or strontium, or in the presence of cobalt or nickel. These results represent the first report of a nonselective cation current in primary vascular smooth muscle cells that is calcium dependent and that could be responsible for the depolarizations induced from the resting potential by vasoconstrictors such as endothelin.     

Somatostatin depresses excitability in neurons of the solitary tract complex through hyperpolarization and augmentation of IM, a non-inactivating voltage-dependent outward current blocked by muscarinic agonists.

The synaptic function of somatostatin-containing fibers in the nervous system is controversial. Therefore, we used a slice preparation of the rat brain stem to test the electrophysiological effects of prosomatostatin-derived peptides on neurons of the solitary tract complex, which contains an abundance of somatostatin-containing fibers and cell bodies. Superfusion of both somatostatin-14 and somatostatin-28 (the precursor for somatostatin-14), but not somatostatin-28-(1-12) or -(1-10), predominantly inhibited spontaneous spike and subthreshold (probably synaptic) activity. In intracellular recordings, somatostatin-14 and -28 hyperpolarized most neurons in association with a slight (10-35%) but reproducible decrease in input resistance. These hyperpolarizing responses were augmented in depolarized cells and persisted in cells in which spontaneous inhibitory postsynaptic potentials became depolarizing after Cl- injection. These data suggest that somatostatin receptors regulate a K+ conductance. In voltage-clamp studies, somatostatin-28 and -14 induced a steady outward current and augmented the voltage-dependent, nonactivating outward K+ conductance (IM) shown to be blocked by activation of muscarinic cholinergic receptors. These results suggest (i) that somatostatin-containing elements in the solitary tract complex play an inhibitory role through the activation of postsynaptic permeability to potassium ions and (ii) that the same ion channel type may be coregulated by two neurotransmitter candidates, somatostatin and acetylcholine, through a reciprocal control mechanism.     

Gated currents in isolated olfactory receptor neurons of the larval tiger salamander.

The electrical properties of enzymatically isolated olfactory receptor cells were studied with whole-cell patch clamp. Voltage-dependent currents could be separated into three ionic components: a transient inward sodium current, a sustained inward calcium current, and an outward potassium current. Three components of the outward current could be identified by their gating and kinetics: a calcium-dependent potassium current [IK(Ca)], a voltage-dependent potassium current [IK(V)], and a transient potassium current (Ia). Typical resting potentials were near -54 mV, and typical input resistance was 3-6 G omega. Thus, only 3 pA of injected current was required to depolarize the cell to spike threshold near -45 mV. The response to a current step consisted of either a single spike regardless of stimulus strength, or a train of less than 8 spikes, decrementing in amplitude and frequency over approximately equal to 250 msec. Thus, the receptor response cannot be finely graded with stimulus intensity.