Slow excitatory post-synaptic currents in bull-frog sympathetic neurones

Spontaneous Slow Fluctuation of EEG Alpha Rhythm Reflects Activity in Deep-Brain Structures: A Simultaneous EEG-fMRI Study

The emergence of the occipital alpha rhythm on brain electroencephalogram (EEG) is associated with brain activity in the cerebral neocortex and deep brain structures. To further understand the mechanisms of alpha rhythm power fluctuation, we performed simultaneous EEGs and functional magnetic resonance imaging recordings in human subjects during a resting state and explored the dynamic relationship between alpha power fluctuation and blood oxygenation level-dependent (BOLD) signals of the brain. Based on the frequency characteristics of the alpha power time series (APTS) during 20-minute EEG recordings, we divided the APTS into two components: fast fluctuation (0.04–0.167 Hz) and slow fluctuation (0–0.04 Hz). Analysis of the correlation between the MRI signal and each component revealed that the slow fluctuation component of alpha power was positively correlated with BOLD signal changes in the brain stem and the medial part of the thalamus and anterior cingulate cortex, while the fast fluctuation component was correlated with the lateral part of the thalamus and the anterior cingulate cortex, but not the brain stem. In summary, these data suggest that different subcortical structures contribute to slow and fast modulations of alpha spectra on brain EEG.

A model for pleiotropic muscarinic potentiation of fast synaptic transmission

The predominant form of muscarinic excitation in the forebrain and in sympathetic ganglia arises from m1 receptors coupled to the G(q/11) signal transduction pathway. Functional components of this system have been most completely mapped in frog sympathetic B neurons. Presynaptic stimulation of the B neuron produces a dual-component muscarinic excitatory postsynaptic potential (EPSP) mediated by suppression of voltage-dependent M-type K(+) channels and activation of a voltage-insensitive cation current. Evidence from mammalian systems suggests that the cation current is mediated by cyclic GMP-gated channels. This paper describes the use of a computational model to analyze the consequences of pleiotropic muscarinic signaling for synaptic integration. The results show that the resting potential of B neurons is a logarithmic function of the leak conductance over a broad range of experimentally observable conditions. Small increases (<4 nS) in the muscarinically regulated cation conductance produce potent excitatory effects. Damage introduced by intracellular recording can mask the excitatory effect of the muscarinic leak current. Synaptic activation of the leak conductance combines synergistically with suppression of the M-conductance (40 --> 20 nS) to strengthen fast nicotinic transmission. Overall, this effect can more than double synaptic strength, as measured by the ability of a fast nicotinic EPSP to trigger an action potential. Pleiotropic muscarinic excitation can also double the temporal window of summation between subthreshold nicotinic EPSPs and thereby promote firing. Activation of a chloride leak or suppression of a K(+) leak can substitute for the cation conductance in producing excitatory muscarinic actions. The results are discussed in terms of their implications for synaptic integration in sympathetic ganglia and other circuits.

Synaptically activated low-threshold muscarinic inward current sustains tonic firing in rabbit prevertebral sympathetic neurons

Whole-cell patch-clamp experiments were performed on non-dissociated rabbit coeliac sympathetic neurons in the presence of nicotinic blockers. Coeliac neurons were classified as either silent or spontaneously active (pacemaker) cells. Under voltage-clamp conditions, pacemaker cells exhibited a steady-state N-shaped current-voltage relationship due to the presence of a persistent voltage-dependent inward current in the potential range of -100 to approximately -20 mV. This inward current sustained the regular firing activity of pacemaker cells and was absent from quiescent neurons. It disappeared in the presence of tetrodotoxin and in low Ca(2+)-high Mg2+ external solutions and was enhanced by eserine. Splanchnic nerve stimulation induced slow regenerative depolarizations and firing discharges in silent neurons by activating a low-threshold voltage-sensitive inward current. The synaptic current had a U-shaped voltage-dependence from -96 to approximately -20 mV and exhibited the dynamic properties of the muscarinic voltage-dependent inward current INa,M. It gave the current-voltage relationship an N shape similar to that observed in spontaneously active cells. The muscarinic antagonists atropine and pirenzepine abolished the inward current present in pacemaker cells and that induced by nerve stimulation in silent neurons. These data provide evidence that both spontaneous firing activity and nerve-evoked depolarizing responses in coeliac neurons are sustained by the activation of the muscarinic Na,M current. The tonic activation of INa,M in spontaneously firing cells results from a sustained Ca(2+)-dependent tetrodotoxin-sensitive release of acetylcholine. This study provides evidence that the role of the muscarinic receptors is not purely a neuromodulatory one, but that these receptors are directly involved in ganglionic neurotransmission.

Muscarinic activation of a novel voltage-sensitive inward current in rabbit prevertebral sympathetic neurons

The muscarinic activation of rabbit prevertebral sympathetic neurons was studied in non-dissociated coeliac and superior mesenteric ganglia using whole-cell patch-clamp techniques. In the presence of nicotinic blockers, carbachol, muscarine and oxotremorine-M (1-50 microM) induced tonic firing by activating a persistent inward current. These effects were abolished by atropine. They persisted when the M-current was blocked with Ba2+ (1 mM) and intracellular Cs+. The muscarinic inward current was found to be time- and voltage-dependent. It peaked at -60 mV, decreased at large hyperpolarizations and was tonically activated between -110 and -20 mV, which gave steady-state I-V curves an N-shape between -96 and -54 mV. The negative slope accounted for the large hyperpolarizing responses generated by current pulses in carbachol-treated cells. The muscarinic current was abolished when Na+ was replaced by choline, Tris+, sucrose, N-methyl-D-glucamine and Cs+ but not Li+. It was resistant to tetrodotoxin (3 microM), amiloride (3 microM), benzamil (10 microM) and tetraethylammonium (5-20 mM). No involvement of K+ and Cl- could be detected. We therefore styled it INa,M, in reference to its ionic selectivity and its coupling to muscarinic receptors. Low Ca(2+)-Mg2+ salines enhanced the Na,M-current. The current was blocked by Cd2+, Co2+, La3+ (1 mM) and Ba2+ (5 mM) but insensitive to methoxyverapamil hydrochloride, nicardipine, nifedipine and omega-conotoxin MVII A (2-20 microM). These effects were ascribed to the binding of di- and trivalent ions to the Na,M-channels. Spike bursts transiently blocked INa,M. With high intracellular ethylene glycol bis(b-aminoethyl ether)-N,N'-tetraacetic acid or 1,2-bis (2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (20-50 mM), this effect was reduced, whereas INa,M persisted in long-term recordings and its amplitude increased twofold, indicating that intracellular calcium negatively regulated the Na,M-channels. We conclude that we have described a novel muscarinic receptor-coupled channel which appears to play a major part in regulating the firing behaviour of sympathetic neurons.

Pirenzepine-sensitive component of forelimb vascular resistance and heart rate in cats

In vagotomized, midcollicular decerebrate, non-anesthetized cats, in which the preganglionic input to the stellate ganglion was intact and the right forelimb was autoperfused at constant flow, the muscarinic receptor antagonist pirenzepine (PZP, 50 micrograms/kg i.v.), which does not cross the blood-brain barrier, produced a decrease in forelimb perfusion pressure (FLPP), heart rate (HR) and systemic arterial pressure (SAP). Administration of the nicotinic receptor antagonist hexamethonium (30 mg/kg i.v.) after PZP produced a further, larger drop in FLPP, HR and SAP. The dose of PZP used blocked the increase in FLPP and HR evoked by the muscarinic receptor agonist McN-A-343, but not the increase evoked by the nicotinic receptor agonist DMPP, when these drugs were injected into the arterial supply of the ganglion. Following administration of phentolamine and propranolol (2 mg/kg i.v. of each) which caused bradycardia and forelimb vasodilation, PZP had no effect on FLPP and HR. This finding suggests that PZP decreases sympathetic tone in resistance vessels and in the heart. Pirenzepine did not depress the forelimb vasoconstriction and the cardioacceleration evoked by electrical stimulation of the postganglionic vertebral and inferior cardiac nerves, respectively, suggesting that PZP does not act at the neuro-effector junctions. On the other hand, PZP blocked the forelimb vasoconstriction and the cardioacceleration produced by stimulating the preganglionic input of the stellate ganglion in presence of hexamethonium (30 mg/kg i.v.), indicating the stellate ganglion as the likely site of action of the drug. These findings suggest that the ganglion cell firing that underlies the sympathetic tone of cardiovascular effector cells is generated, in part, by a muscarinic, PZP-sensitive, synaptic mechanism.

Delayed onset and slow time course of the non-M-type muscarinic current in bullfrog sympathetic neurons

The onset and time course of the muscarinic currents induced by brief applications of acetylcholine (ACh) were examined in voltage-clamped neurons of bullfrog sympathetic ganglia bathed in a solution containing d-tubocurarine. At a potential of -40 mV, the ACh-induced current (IACh) appeared within 1.2 s and rapidly increased to its peak with a half-activation time of 2.2 s. This initial current was termed the fast IACh and was blocked by 4 mM Ba2+. At a potential more negative than -60 mV, the fast IACh disappeared and the remaining IACh activated with a delay of 3.9 s and slowly increased to its peak with a half-activation time of 8.2 s. This delayed current was termed the slow IACh and is thought to be associated with inhibition of a K+ current, or IM, as well as activation of an inward current through non-M-type muscarinic cation channels. The slow IACh was not inhibited by Ba2+, but its amplitude was reduced with depolarization (the extrapolated reversal potential was +3 mV). In Na(+)-free solution, the amplitude of the slow IACh reduced, but its polarity did not reverse in the voltage region examined (-30 to -100 mV). The slow excitatory postsynaptic current was also recorded, and was shown to have a similar delay in onset and slow time course. The results demonstrate that ACh activates the non-M-type muscarinic current three times more slowly than it inhibits IM.

Muscarinic inhibition of two potassium currents in guinea-pig prevertebral neurons: differentiation by extracellular cesium

Muscarinic responses were studied in dissociated guinea-pig celiac ganglion neurons using the whole-cell voltage-clamp technique. Muscarine (0.025-1 mM; EC50 = 95 microM) administered to cells for 1.5 s evoked inward shifts in holding current in 53 of 74 cells. The amplitude of the inward current transients decreased with hyperpolarization and the null potential averaged -71 +/- 3.4 mV (n = 11). The currents that underlie the responses to muscarine were examined with hyperpolarizing voltage stepping protocols to -100 mV from a holding potential of -30 mV. Eighty-one per cent of cells displayed voltage-dependent current relaxations characteristic of the M-potassium current. Twenty per cent of responding cells displayed no M-current but only a voltage-independent current consistent with a leak current. In the latter type of cells, the muscarine-evoked inward currents reversed near EK and became outward at more hyperpolarized potentials. Analysis of steady state I-V relationships before and after bath application of muscarine showed that the two muscarine-sensitive potassium currents were distributed differently among three types of cells: (i) with M-current (18%); (ii) with leak current (18%); and (iii) with M-current and with leak current (64%). Cesium and barium were used to differentiate the M-current and the muscarine-sensitive leak current. Barium (2 mM) reduced the M-current and the leak potassium current, whereas cesium (2 mM) reduced the M-current but did not affect leak current. Thus, barium reduced the amplitude of muscarinic responses by 79% but cesium reduced them by only 14%. We conclude that muscarinic responses in guinea-pig celiac neurons are produced by suppression of two K+ currents: the M-current and a muscarine-sensitive leak current. These two currents are differentially susceptible to the potassium channel blockers barium and cesium.

Two signal transduction mechanisms of substance P-induced depolarization in locus coeruleus neurons

Effects of substance P on cultured neurons of the locus coeruleus of the rat were studied using the whole-cell patch clamp technique. In some cells substance P produced a decrease in a K conductance which showed an inwardly rectifying property. In other cells substance P produced an initial inward current which was accompanied by a conductance increase. The rest of the cells showed responses which were mixtures of the above two responses. The measurement of the reversal potential of the initial inward current after suppressing the voltage-gated Ca and K conductances suggests that it is caused by an increase in a non-selective ionic conductance. In cells loaded with 260 microM GTP gamma S, application of substance P produced an irreversible reduction of the K conductance, while the initial inward current could still be recorded, suggesting that the former is mediated by a G protein, whereas the latter may be activated by a different signal transduction mechanism. The initial inward current was not eliminated by external application of high concentrations of tetrodotoxin, d-tubocurarine or amiloride. Nor was it affected by the intracellular application of cyclic GMP or cyclic AMP.

Concentrations of carbachol stimulating phosphoinositide hydrolysis cause a sustained decrease in membrane potential and firing rate: role of inositol and inositol polyphosphate second messengers

We have investigated the relationship between muscarinic agonist-stimulated phosphoinositide (PI) hydrolysis and electrophysiological responses in rat hippocampal slice preparations. In a previous extracellular study, we found that muscarinic agonists at concentrations that stimulate PI hydrolysis result in a biphasic firing response; an initial increase in firing followed by loss of firing at higher concentrations. To test the hypothesis that variability in obtaining consistent loss of firing is related to depletion of intracellular inositol, we investigated the effects of adding exogenous inositol to the buffer. We now report that concentrations of inositol similar to those in cerebral spinal fluid (30-100 microM) augment carbamylcholine (carbachol, CCh) mediated loss of firing and [3H]inositol-1,3,4,5-tetrakisphosphate ([3H]Ins(1,3,4,5)P4) formation. Inhibition of firing produced by 30 microM CCh in the presence of inositol was associated with a sustained depolarization of 20-25 mV, an increased slope resistance in the depolarized range (-60 to -40 mV), and a parallel shift in the hyperpolarized (-100 to -70 mV) range of the voltage-current curve and increased frequency of spontaneous IPSPs. Under voltage-clamp, measurements of the M-current (IM) showed sustained inactivation by CCh with reversal after washout of CCh. Manual depolarization of cells by current injection to the same level of depolarization as attained with CCh did not usually lead to the same loss of firing. These findings suggest that IM, and possibly other voltage-independent currents or ion pumps, may cause loss of firing only in part through a depolarization blockade of firing and not through desensitization. Furthermore, CCh treatment without inositol did not depolarize neurons as much as CCh with inositol, and usually did not cause a delayed loss of firing. Brain slice preparations may thus require physiological concentrations of inositol to show consistent or maximum phosphoinositide-mediated electrophysiological responses.

Bethanechol-induced responses in mudpuppy parasympathetic neurons

The effect of bethanechol on membrane potential and excitability was determined in mudpuppy parasympathetic postganglionic neurons. Bethanechol induced a large amplitude hyperpolarization, which was followed by a smaller amplitude depolarization, in 115 out of 135 cells tested. In approximately 20% of these cells, a brief depolarization preceded the hyperpolarization. During the bethanechol-induced hyperpolarization, the membrane input resistance decreased markedly, whereas the input resistance was increased during the subsequent depolarization. The hyperpolarization and depolarization were blocked by atropine and were unaffected by d-tubocurarine, thus, both appeared to be mediated by muscarinic receptors. The bethanechol-induced hyperpolarization was inhibited by the M2 muscarinic receptor antagonist AF-DX 116, whereas the bethanechol-induced depolarization was unaffected. Both a nonselective increase in membrane conductance and a decrease in membrane potassium conductance appeared to be involved in the generation of the bethanechol-induced depolarization. Evidence for the first mechanism was obtained in barium-treated cells in which bethanechol initiated a rapid onset depolarization, which was reversed at membrane potentials near 0 mV. Evidence for the second mechanism was obtained when the hyperpolarization was inhibited by AF-DX 116. In AF-DX 116-treated cells, the membrane input resistance was increased during most of the bethanechol-induced depolarization. Mudpuppy neurons initiate repetitive action potential activity in response to long depolarizing current pulses. Following application of bethanechol, with the hyperpolarization negated electrotonically, the number of action potentials produced by a depolarizing current pulse was greater than that produced prior to application of bethanechol. It is suggested that activation of muscarinic receptors on mudpuppy cardiac neurons influences multiple conductance systems and determines the excitability of these neurons.

A patch-clamp study on the muscarine-sensitive potassium channel in bullfrog sympathetic ganglion cells

1. A voltage-independent K+ channel was characterized and effects of muscarine were studied in cultured bullfrog sympathetic ganglion cells using the cell-attached patch-clamp configuration. 2. Three types of single-channel current were recorded from 2- to 10-day-old cultured cells in the presence of tetraethylammonium (2-20 mM), tetrodotoxin (1-2 microM), Cd2+ (0.1 mM) and apamin (20 nM). 3. The most frequently observed channel was a voltage-independent K+ channel which was open at the resting membrane potential and had a conductance of 52.6, 78.9 and 114.9 pS at a [K+]o of 2, 40 and 100 mM, respectively. This channel was designated background K+ channel. 4. Two other channel types were observed less frequently. One had a conductance of 26 pS (external K+, 118 mM) and a long open time of several seconds at the resting membrane potential. The second channel had a smaller conductance (20 pS) and displayed a voltage-dependent activation. 5. The open probability of the background K+ channel varied between patches, ranging from 0.0005 to 0.486. The open time distribution was fitted by a single exponential with a time constant of 0.51 ms. Both of these parameters were independent of the membrane potential. The closed time distribution consisted of at least four exponentials having time constants of 0.17, 3.7, 120 ms and several seconds. 6. Muscarine (10-20 microM) applied to the membrane outside the patch pipette reversibly enhanced the activity of the background K+ channel. This effect was associated with an increase in the open probability, which resulted from an increase in the mean open time concomitant with a decrease in the mean closed time. Muscarine did not change the single-channel conductance of this channel. 7. The effects of muscarine were blocked by atropine (1 microM). 8. It is concluded that there exists a muscarine-sensitive, voltage-independent K+ channel in cultured bullfrog ganglion cells. This K+ channel appears to contribute to the generation of the resting membrane potential and underlie the slow inhibitory postsynaptic potential of these neurones in situ.

Comparison of atropine pre- and post-treatment in ganglion neurons exposed to soman

The prevention and reversal of the effects of soman (pinacoloxymethylphosporyl fluoride) have been examined on the electrical properties of sympathetic ganglion neurons in vitro from the adult bullfrog Rana catesbeiana. Atropine (10 microM) pre-treatment (before soman) blocked the soman-induced decrease in the membrane potential, membrane resistance, and afterhyperpolarization (AHP) duration. Atropine post-treatment (after soman exposure) restored the soman-induced decrease in the membrane potential but was ineffective in reversing either the membrane resistance or the duration of the AHP. These results demonstrate: 1) that the effects of soman on the electrical properties of these neurons are mediated by the activation of muscarinic receptors, 2) that following receptor activation different cellular mechanisms may be involved in the regulation of the electrical properties of the neuron, and 3) pre-treatment rather than post-treatment with atropine is more effective in blocking these direct effects of soman. These results are discussed in reference to the increased effectiveness of atropine pretreatment in the protocols for soman-induced neurotoxicity.

M-currents in frog sympathetic ganglion cells: manipulation of membrane phosphorylation

1. The inward current and the M-current (IM) suppression produced when muscarine is applied to frog sympathetic ganglion cells was recorded by means of the whole-cell patch-clamp technique. The holding potential was -30 mV and [K+]o was 6 mM. 2. The steady-state IM was maintained for at least 20 min when the patch pipette contained neither adenosine 5'-triphosphate (ATP) nor adenosine 3':5'-cyclic monophosphate (cyclic AMP). Inclusion of these substances or the ATP antagonist, beta,gamma-methyleneadenosine 5'-triphosphate (beta,gamma-MethATP; 1 or 2 nM) (failed to alter the rate of IM 'run down'. By contrast, inclusion of adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma-S, 1 or 2 mM) resulted in a 60% reduction of the current within 18 min. 3. Despite the inability of ATP-gamma-S to maintain steady-state IM, it had no effect on the ability of muscarine (2-100 microM) to suppress a constant fraction of the available current. ATP-gamma-S and beta,gamma-MethATP increased the rise time and duration of the response to muscarine. 4. Inclusion of a phosphatase inhibitor, diphosphoglyceric acid (DPG, 1-2.5 mM) or alkaline phosphatase (100 micrograms ml-1) failed to affect the amplitude of muscarinic responses. 5. These results question the role of the phosphorylation and/or dephosphorylation reactions in the transduction mechanism for muscarine-induced IM suppression but are consistent with the possibility that M-channels are 'directly coupled' via G-protein to the muscarinic receptor.

Kinetics of G protein-mediated modulation of the potassium M-current in bullfrog sympathetic neurons

The inhibition of the voltage-dependent, K+ M-current (IM) following receptor-independent G protein activation with controlled intracellular perfusion of nonhydrolyzable GTP analogs had an exponential time course, with rates hyperbolically dependent on GTP analog concentration, and a limiting value of 0.53 min-1. The inhibitory agonist muscarine caused a concentration-dependent acceleration of the rate of nucleotide-induced inhibition, with a plateau of about 20 min-1 and an exponential time course. In neurons not treated with nucleotide analogs the IM recovery rate following agonist removal was 3-7 min-1. It is proposed that the overall kinetics of the transduction pathway for IM modulation is governed by the agonist-dependent kinetics of nucleotide interaction with G proteins. A simple model of IM modulation based on G proteins' kinetics has been developed. These data suggest a possible cellular process responsible for the time course of slow synaptic potentials caused by IM inhibition in sympathetic neurons.

A muscarine-activated voltage-independent K+ channel in cultured bullfrog sympathetic neurones

In cultured bullfrog sympathetic neurones, cell-attached patch clamp revealed a voltage-independent K+ channel having a conductance of 46-113 pS at an external K+ of 2-120 mM. Muscarine (10-20 microM), applied to the cell membrane outside a recording pipette, increased its open probability and mean open time in an atropine-sensitive manner. This muscarine-activated K+ channel could underlie some of the muscarinic inhibitory postsynaptic potentials in both central and peripheral nervous systems.

Effects of muscarine and adrenaline on neurones from Rana pipiens sympathetic ganglia

1. Neurones dissociated from Rana pipiens paravertebral sympathetic ganglia were studied by means of the whole-cell patch-clamp technique. Responses to agonists were best recorded when cyclic AMP was included in the patch pipette. 2. Two populations of cells were identified on the basis of size (input capacitance, Cin) and the presence or absence of a fast, transient outward current (A-current, IA). This current was usually present in the 'large' cells (Cin = 40.5 +/- 1.5 pF, n = 66) but absent from 'small' cells (Cin = 21.0 +/- 0.8 pF, n = 70). 3. Both cell types exhibited a slowly activating, non-inactivating K+ current (M-current, IM) which was suppressed by luteinizing hormone-releasing hormone (LHRH, 10-100 microM). Threshold for activation of IM was about -75 mV, half-maximal activation was at -50 mV and the M-conductance GM increased e-fold for at 7 mV change in membrane potential. The maximum value for IM studied in large cells by patch-clamp procedures was less than 0.2 nA. More M-channels were available per unit membrane area in the small cells (GM = 1495 microS cm-2) than in the large cells (GM = 1034 microS cm-2). Time constants for IM deactivation at -70 mV were faster in the large cells (37.2 +/- 4.6 ms, n = 16) than in the small cells (66.1 +/- 5.9 ms, n = 9). 4. Muscarine (10 microM) produced inward current in the large cells as a result of IM suppression. In 40% of the large cells, some of the M-channels were also sensitive to adrenaline (10-100 microM). In a few large cells (less than 10%) adrenaline produced outward current by increasing IM. 5. Muscarine failed to effect IM in the small cells and instead produced an inwardly rectifying K+ current which activated within 5 ms at -110 mV. The outward current produced in twenty out of thirty-seven small cells by adrenaline was occluded by that produced by muscarine, suggesting that both agonists affect the same K+ channels. 6. Inclusion of the protein kinase inhibitors, 1-(5-isoquinolinyl-sulphonyl)-2-methyl piperazine (H-7, 50 microM) or gold sodium thiomalate (GST, 50 microM) in the pipette solution failed to antagonize either muscarine-induced current. Both currents were prolonged when the 'internal solution' contained GTP-gamma-S (50 microM). 7. Phorbol-12-myristate-13-acetate (PMA, 2-5 microM) produced an inward current as a result of IM suppression in both small and large cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of Ca2+ and K+ channels in sympathetic neurons by neuropeptides and other ganglionic transmitters

Neuropeptides are known to modulate the excitability of frog sympathetic neurons by inhibiting the M-current and increasing the leak current, but their effects on Ca2+ channels are poorly understood. We compared effects of LHRH, substance P, epinephrine, and muscarine on Ca2+, K+, and leak currents in dissociated frog sympathetic neurons. At concentrations that inhibit M-current, LHRH and substance P strongly reduced N-type Ca2+ current and induced a leak conductance that may contribute to slow EPSPs. In contrast, muscarine produced little reduction of Ca2+ current, even in cells in which it strongly suppressed the M-current. We find that peptidergic inhibition of Ca2+ channels involves G proteins, but does not require protein kinases. In addition, it leads to reductions in Ca2(+)-activated K+ current and catecholamine release.

M1 and M2 muscarinic receptors mediate excitation and inhibition of guinea-pig intracardiac neurones in culture

1. The effects of muscarine upon intracardiac neurones cultured from ganglia within the atria and interatrial septum of the newborn guinea-pig heart were studied using intracellular recording techniques. 2. Muscarine applied to the neuronal soma typically produced a biphasic change in membrane potential which consisted of a small hyperpolarization followed by a depolarization. In addition, muscarine (0.01-10 microM) inhibited the calcium-dependent, after-hyperpolarization (AHP) and greatly increased the number of action potentials that could be evoked by a given depolarizing current. 3. The hyperpolarization was associated with a decrease in input resistance and it reversed to become a depolarization at a potential of -86.5 mV. This response was antagonized by 4-diphenylacetoxy-N-methyl-piperidine (4-DAMP; 100 nM) and AF-DX 116 (500 nM), but was unaffected by pirenzepine (0.1-5 microM). 4. Two types of slow depolarization were observed in the presence of muscarine. The most common was associated with an increase in input resistance in the potential range -70 to -40 mV. Pirenzepine (100 nM) selectively antagonized this response, 4-DAMP (100 nM) similarly antagonized the response, but was non-selective. AF-DX 116 (0.5-5 microM) showed no antagonist effect. The less common depolarization (5% of cells) had a long latency and was associated with a decrease in input resistance. 5. Muscarine reduced the duration of the action potential and inhibited the AHP. Cadmium chloride (100 microM) mimicked these actions of muscarine. Application of muscarine immediately following a train of action potentials did not inhibit the AHP, suggesting that muscarine did not directly inhibit the calcium-activated potassium current (IK(Ca)). Muscarine-induced depression of the slow AHP was antagonized by 4-DAMP (100 nM) but was not antagonized by either pirenzepine (0.1-0.5 microM) or AF-DX 116 (0.5-5 microM). 6. It is concluded that the muscarine-induced depolarization of guinea-pig intracardiac neurones results from reduction of a potassium conductance similar to the M-conductance, through activation of M1 muscarinic receptors. The hyperpolarization results from an increase in potassium conductance, through activation of M2 muscarinic receptors.

A G Protein Mediates the Inhibition of the Voltage-Dependent Potassium M Current by Muscarine, LHRH, Substance P and UTP in Bullfrog Sympathetic Neurons

The involvement of G proteins in the transduction mechanism of M current (Im) inhibition by extracellular ligands in bullfrog sympathetic neurons was examined using the hydrolysis resistant nucleotide analogues GTPgammaS and GDPbetaS. Im was recorded in large (40 - 60 microm) isolated neurons using the patch-clamp technique in the whole-cell configuration, as well as in neurons from the intact ganglion impaled with conventional microelectrodes. In whole-cell recordings Im could be recorded without significant loss for 1 h or more provided ATP was present in the patch pipette. Muscarine, D-Ala6-LHRH, substance P and UTP reversibly inhibited Im in isolated control neurons, with full and rapid recovery of the current following agonist washout. Dialysis of isolated neurons with various concentrations of GTPgammaS (1 - 100 microM) affected, in a dose-dependent manner, the recovery of Im after its inhibition by brief agonist application. With 50 microM GTPgammaS, Im inhibition became completely irreversible. Similarly, the reversibility of Im inhibition by muscarine was reduced or abolished by the iontophoretic injection of GTPgammaS through a second microelectrode into neurons of the intact ganglion. GTPgammaS by itself caused a slow, agonist-independent suppression of Im in dialysed neurons, thus mimicking agonist action. Dialysis of isolated neurons with GDPbetaS (100 - 500 microM) attenuated by half or more the magnitude of Im inhibition by agonist as compared to control neurons. In addition, GDPbetaS attenuated the response of a given neuron to muscarine and D-Ala6-LHRH, and caused slow increase of Im, as a function of dialysis time. Incubation (2 - 72 h, 4 - 36 degrees C) of isolated neurons or intact ganglions with activated pertussis toxin had no effect on the response to muscarine. Toxin injections to experimental animals were equally ineffective. In contrast to Im, the additional inward current with increase in conductance induced by muscarine and D-Ala6-LHRH reversed with agonist washout in GTPgammaS-dialysed neurons, although more slowly than in control neurons. The results in this study indicate that a G protein, possibly pertussis toxin-insensitive, provides a common coupling step linking muscarinic, substance P, D-Ala6-LHRH and UTP receptors to the inhibition of M current.

Neurotoxicological effects of trimethyltin on the stellate ganglion

Hamsters treated with trimethyltin (TMT), 3 or 4 mg/kg IP, developed neurological symptoms, including tremor, within 24 hours. Postganglionic action potentials were recorded from isolated stellate ganglia of untreated hamsters (control ganglia) and TMT-treated hamsters (TMT ganglia). Compound action potentials (nicotinic transmission) of control and TMT ganglia were not significantly different. The afterdischarges induced by preganglionic stimulation at 30 Hz for 2 sec in the presence of 10(-3) M hexamethonium (muscarinic transmission) were significantly smaller in TMT ganglia than in control ganglia. The discharges induced by the muscarinic cholinoceptor agonist. McN-A-343, were also smaller in the TMT ganglia. Two other muscarinic processes, posttetanic potentiation and potentiation of the compound action potential by McN-A-343, were not significantly reduced in the TMT ganglia. Morphological studies of the ganglia revealed marked changes in the TMT ganglia with severe neuronal degeneration including vacuole formations and accumulations of lysosomes in the cytoplasm. These results demonstrate that TMT has marked anatomical effects on the stellate ganglion that may lead to the reduction in muscarinic cholinergic transmission.

Adrenaline inhibits muscarinic transmission in bullfrog sympathetic ganglia

The effect of adrenaline (Ad) on muscarinic transmission was examined in B neurones of bullfrog sympathetic ganglia by using intracellular and voltage-clamp recording methods. Bath-application of Ad (5-500 microM) caused a depression of the slow excitatory postsynaptic potential (EPSP) elicited by repetitive stimulations of preganglionic nerve fibres in the presence of curare (30 microM). Ad also depressed the 'muscarinic' ACh potential induced by ionophoretic application of ACh directly to curarized sympathetic neurones in a concentration-dependent manner. Isoprenaline mimicked the effect of Ad in producing the inhibition of the 'muscarinic' ACh potential. Propranolol antagonized the inhibitory action of Ad. Dibutyryl adenosine 3',5'-monophosphate had no significant effect on the 'muscarinic' ACh potential. Under voltage-clamp conditions, Ad caused an inward current associated with inhibition of the M-current (Brown and Adams 1980). Ad depressed the amplitude of slow postsynaptic currents produced by applications of ACh and muscarinic. At a concentration of 100 microM, Ad produced a 68 +/- 8% (n = 12) depression of the amplitude of the muscarinic ACh current. The inhibition of muscarinic transmission induced by Ad is due to a direct suppression of the muscarinic current at the postsynaptic membrane in bullfrog sympathetic ganglia.

Muscarinic excitation and inhibition of neurons in the submucous plexus of the guinea-pig caecum

Intracellular recordings were made from neurons in the submucous plexus of the guinea-pig caecum. Muscarinic agonists (acetylcholine, bethanechol and muscarine) depolarized about 70%, and hyperpolarized about 30% of the submucous plexus neurons. Low concentrations of pirenzepine reversibly antagonized both responses. The measured dissociation constants (KD) of 10-30 nM for the depolarizations and 1-3 nM for the hyperpolarizations suggest that each response was mediated by muscarinic M1 cholinoceptors. The muscarinic depolarization and hyperpolarization were associated with a decreased and an increased conductance, respectively, and the reversal potential for the muscarinic responses varied as the potassium concentration varied, always being around the potassium equilibrium potential. In cells depolarized by muscarinic agonists these agents appeared to decrease a potassium conductance that could also be inactivated by substance P. In approximately 30% of the submucous neurons, the slow inhibitory postsynaptic potential, elicited in response to single or repetitive focal stimuli (1-10 pulses at 20-40 Hz), appeared to consist of a large component which was sensitive to the blocking action of idazoxan (100-300 nM) and a small component which was idazoxan-insensitive. The latter (muscarinic slow inhibitory postsynaptic potential) was completely abolished by pirenzepine. The concentrations of pirenzepine which caused a 50% depression ranged from 5 to 20 nM. The muscarinic slow inhibitory postsynaptic potential was increased in amplitude and duration by physostigmine (100-300 nM). The muscarinic slow inhibitory postsynaptic potential was accompanied by a decrease in membrane input resistance, and was reversed in polarity near the potassium equilibrium potential. When muscarine induced a hyperpolarization and/or focal stimulation elicited a muscarinic slow inhibitory postsynaptic potential in the presence of idazoxan (100-300 nM), the intracellular injection of guanosine 5'-O-(3-thiotriphosphate) produced a progressive membrane hyperpolarization during which the muscarinic hyperpolarizing responses were attenuated. It is concluded that the muscarine-induced reduction in potassium conductance is mediated through a muscarinic M1 receptor which has a relatively low affinity for pirenzepine. The muscarine-induced increase in potassium conductance is probably produced by the association of a guanine nucleotide-binding regulatory protein with another muscarinic M1 receptor that has a relatively high affinity for pirenzepine.

Clindamycin-induced alteration of ganglionic function. I. Direct effects on ganglion cell properties

The influence of the lincosamide antibiotic, clindamycin, on the properties of bullfrog sympathetic ganglion B cells has been determined in vitro using conventional voltage recording methods or single microelectrode voltage-clamp recording techniques. Individual neurons were depolarized with both bath application or local perfusion of clindamycin. The amplitude of the depolarization was not altered by pretreatment with 50 microM (+)-tubocurarine, 10-microM atropine, or 1.5 microM tetrodotoxin (TTX), indicating that the clindamycin-induced depolarization does not result from either the activation of (1) nicotinic receptors, (2) muscarinic receptors, or (3) voltage-gated sodium channels. Clindamycin partially inhibited IM, an action which accounts for part of the clindamycin-induced depolarization. The duration of the hyperpolarizing afterpotential (HAP) following the action potential was decreased in the presence of clindamycin. Clindamycin decreased the amplitude and maximum rate of rise (MRR) of TTX-insensitive action potentials. As calcium influx is thought to contribute to the depolarizing phase of the TTX-insensitive spikes, we suggest that the decrease in HAP duration by clindamycin results from a decrease in the somal calcium current. Further, it is suggested that a decrease in IM and HAP duration may be responsible for the increased excitability exhibited during exposure to clindamycin.

Muscarinic regulation of two ionic currents in the bullfrog sympathetic neurone

The mechanism of slow muscarinic excitation in bullfrog sympathetic ganglia was studied using a single-electrode voltage clamp technique. The membrane current induced by muscarine (0.01-30 microM: Imus) consisted of a voltage-dependent component (dIM) induced by the inhibition of K+-current (known as IM), a voltage-independent cation-selective current (ID), or both. In the last case, the magnitude of either component varied in different cells. Generation of both dIM and ID were equally suppressed by pirenzepine with a dissociation constant (Ki) of 30 nM, while they were inhibited less by AF-DX116 with a Ki of 600 nM. Kd values for muscarinic induction of dIM and ID were 0.35 and 5 microM respectively. This difference was also seen even after reducing the receptor population by pretreatment with propylbenzilylcholine mustard. ID was enhanced after blockade of dIM by Ba2+ (4 mM), with a little change in Kd. This effect was larger when the control ID was smaller. These results suggest that a single subtype (M1) of muscarinic receptor generates both the dIM and ID with different efficacies and that Ba2+, known as a blocker of IM (accordingly of a high-efficacy current, dIM), potentiates a low-efficacy current (ID).

Adrenaline depolarization in paravertebral sympathetic neurones of bullfrogs

Responses to adrenaline (Ad) and their ionic mechanisms were analysed using intracellular recording and voltage-clamp methods in neurones of bullfrog sympathetic ganglia. Ad (5 microM-1 mM) applied directly to sympathetic neurones by pressure ejection through a micropipette produced three types of depolarizing responses (2-20 mV). Under voltage-clamp conditions, Ad (100 microM) produced fast, slow and mixed types of inward currents (AdIs) with amplitude of 2.9 +/- 1.3 nA. beta-Adrenoceptors may be responsible for the generation of these AdDs. The slow AdI which lasted for 1-5 min was associated with a decreased membrane conductance. The slow AdI decreased at hyperpolarized potential level and eventually nullified at -70 mV. No reversal of the slow AdI polarity was observed in the Ringer solution. Injection of Cs2+ into the ganglion cells produced a marked depression of the amplitude of the slow AdI. The slow AdI was blocked by bath-applied Ba2+ but not by TEA. Ad reduced the slow current relaxation, the M current, associated with voltage jumps in the membrane potential range -35 to -55 mV. The fast Ad response was associated with an increase in membrane conductance. When the membrane was depolarized, the fast AdI decreased and reversed its polarity at -36 +/- 8.3 mV. Removal of Cl ion from superfusing solution depressed the fast AdI, suggesting that activation of Cl- conductances may be involved in the generation of the fast AdI. The mixed type of Ad response exhibited characteristics of both the fast and slow Ad responses. The results suggest that Ad increases the excitability of neurones in bullfrog sympathetic ganglia.

Regulation of two ion channels by a common muscarinic receptor-transduction system in a vertebrate neuron

In bullfrog sympathetic ganglion cells, muscarine produced an inward current (Imus) through the activation of a subtype (M1) of muscarinic acetylcholine receptor (mAChR) by suppressing an outward M-current (IM), and/or activating cation-selective current (ID; see below). The former was induced with a potency (Kd = 0.5 microM) higher than the latter (Kd = 5 microM) before and after blocking a fraction of the receptor with an irreversible blocker. Activators of protein kinase C mimicked muscarine's actions. Blocking IM by Ba2+ increased ID. These results suggest that activation of M1-mAChR both closes M-channel and opens cation-selective D-channel through phosphoinositide breakdown and the subsequent activation of protein kinase C and that a difference in potency at the last step of the cascade determines the order in which channels are regulated.

Slow postsynaptic potentials in neurones of submucous plexus of guinea-pig caecum and their mimicry by noradrenaline and various peptides

Intracellular recordings of membrane potential and membrane currents were made from neurones in the submucous plexus of the guinea-pig caecum in vitro. Fast and slow excitatory postsynaptic potentials and slow inhibitory postsynaptic potentials were recorded from the majority of neurones following focal stimulation of presynaptic fibres in the plexus. The slow inhibitory postsynaptic potential was associated with an increase in membrane conductance and reversed its polarity at -90 mV; it was reversibly blocked by yohimbine. The slow excitatory postsynaptic potential and its underlying current was associated with a decrease in membrane conductance. Two kinds of voltage-dependence both of the slow excitatory postsynaptic potential and current were observed; in 80% of cells, the excitatory postsynaptic potential and current became smaller with membrane hyperpolarization and reversed polarity at -90 mV (reversing type) but in 20% of cells both the excitatory postsynaptic potential and current simply disappeared when the membrane potential reached -70 mV (non-reversing type). The effects of acetylcholine, adenosine 5'-triphosphate, bombesin, 5-hydroxytryptamine, neurotensin, noradrenaline, substance P and vasoactive intestinal polypeptide were examined. The only substance which mimicked the slow inhibitory postsynaptic potential was noradrenaline; brief applications of noradrenaline caused hyperpolarizations which had the same time-course, reversal potential and sensitivity to yohimbine as the slow inhibitory postsynaptic potential. The non-reversing type of slow excitatory postsynaptic potential was mimicked only by adenosine 5'-triphosphate. The reversing type of slow excitatory postsynaptic potential was mimicked by bombesin, neurotensin, substance P and vasoactive intestinal polypeptide. 5-Hydroxytryptamine and vasoactive intestinal polypeptide (in some neurones) caused a depolarization with an increase in membrane conductance. All three synaptic potentials were reversibly depressed by superfusion of noradrenaline but noradrenaline did not affect the potential changes evoked by brief application of exogenous acetylcholine or substance P. It is concluded that, in guinea-pig submucous plexus neurones, the slow inhibitory postsynaptic potential is mediated by noradrenaline and results from a potassium conductance increase.(ABSTRACT TRUNCATED AT 400 WORDS)

Noradrenaline mediates slow excitatory synaptic potentials in rat dorsal raphe neurons in vitro

Repetitive focal stimulation to the slice surface within the region of the dorsal raphe (DR) nucleus of rat brain elicited a slow excitatory synaptic potential (slow EPSP), which followed a slow inhibitory synaptic potential (slow IPSP) in a majority of the DR neurons. The slow EPSPs were associated with either an increase of a decrease in membrane resistance. Noradrenaline (NA) application caused a membrane depolarization in most of the DR neurons. The NA-induced depolarization was also accompanied by either an increase or a decrease in membrane resistance. Both the slow EPSP and NA-induced depolarization were inhibited by phentolamine and prazosin but not by yohimbine and propranolol. The result suggests that slow EPSPs in rat DR neurons are mediated by NA interacting with an alpha 1-adrenoceptor.

Cholinergic agonists suppress a potassium current in freshly dissociated smooth muscle cells of the toad

Single micro-electrode voltage-clamp and current-clamp techniques were used to study cholinergic responses in single freshly isolated gastric smooth muscle cells from the toad Bufo marinus. Acetylcholine (ACh) or muscarine caused membrane depolarization, which sometimes gave rise to action potentials and contractions. The agonist-induced depolarization is due to the suppression of a voltage-dependent K+ conductance, a conclusion based on the following observations. Depolarization was accompanied by an apparent membrane conductance decrease, seen as the increased size of voltage deflexions in response to constant current pulses. The conductance decrease was confirmed under voltage clamp, where current deflexions in response to constant voltage jumps were smaller in the presence of cholinergic agonists. Muscarine induced net inward currents at potentials positive to the K+ equilibrium potential (EK), and net outward currents at potentials negative to EK. In experiments where external K+ concentration ([K+]o) ranged from 20 to 90 mM the reversal potentials shifted 58 mV positive per tenfold elevation of [K+]o, as expected for a K+ current. The steady-state current-voltage relationship revealed that the K+ current inhibited by muscarine was larger at more positive potentials than expected from driving force considerations alone. Therefore, the underlying conductance suppressed by cholinergic agonists was voltage dependent, with almost complete deactivation at potentials more negative than approximately -70 mV and exhibiting a sigmoidal activation curve upon depolarization. The deactivation of this voltage-dependent K+ conductance caused slow current relaxations to occur in response to hyperpolarizing voltage commands from depolarized holding potentials. In experiments where [K+]o ranged from 3 to 30 mM, these current relaxations reversed direction at potentials near EK and the reversal potential shifted 52 mV positive per tenfold elevation of [K+]o, indicating that K ions carry most of the charge. The current relaxations that occurred in response to hyperpolarizing voltage commands were suppressed by ACh, muscarine and oxotremorine. The effects of muscarine persisted in nominally Ca2+-free solutions containing Mn2+. Ba2+ mimicked the effects of muscarinic agonists. Thus, isolated smooth muscle cells exhibit a K+ current resembling the M-current of sympathetic and other neurones, which is reversibly suppressed by cholinergic agonists. The existence of a cholinergic K+ conductance decrease is of interest because it has not previously been demonstrated in smooth muscle.

Muscarinic and peptidergic excitation of bull-frog sympathetic neurones

The large B cells of bull-frog sympathetic ganglia are well known to be depolarized by slow synaptic transmission, muscarinic agonists, analogues of luteinizing hormone-releasing hormone (LHRH), and substance P. Voltage-clamp analysis shows that these actions result from two underlying mechanisms: inhibition of the M-current, a voltage-dependent potassium current; and in some cells, an inward current associated with an increase in conductance. The additional inward current appears as a voltage-insensitive change in the instantaneous conductance (i.e. apparent leak conductance). The additional inward current is typically slower in onset and offset than is M-current inhibition. It is typically seen for higher concentrations and longer durations of agonist application. In many cells, only a decrease in M-current can be demonstrated. Muscarine inhibits the M-current with 50% inhibition (I50) at 0.7 microM. At least 86% of the M-current is muscarine sensitive. At comparable concentrations, oxotremorine produces less M-current inhibition than does muscarine. Some analogues of teleost LHRH (T-LHRH) are more potent as M-current inhibitors than T-LHRH itself. Those peptides tend to act more slowly than T-LHRH. Substance P shows variable potency for M-current inhibition, with I50 s ranging from 2 nM to greater than 2 microM on different cells. The response to long applications of substance P desensitizes in some cells, which has not been observed for comparable applications of muscarinic or LHRH agonists. Other tachykinins (including substance K) inhibit the M-current. C-terminal fragments of substance P are ineffective, and M-current inhibition by substance P is not blocked by [D-Pro2,D-Trp7,9]- or [D-Arg1,D-Pro2, D-Trp7,9,Leu11] substance P. The slow muscarinic excitatory post-synaptic potential (e.p.s.p.) produces a graded inhibition of up to 90% of the M-current. Occasional cells show an additional inward current with an associated increase in conductance during the slow e.p.s.p. This effect is less marked than with exogenous muscarinic agonists. The late, slow e.p.s.p., which is produced by stimulation of high threshold C-fibre inputs and is resistant to cholinergic antagonists, also involves M-current inhibition. An additional inward current can be observed in some cells. M-current inhibition (by agonists or slow synaptic potentials) increases the number of spikes produced by a given depolarizing current, often allowing maintained firing. This action is not mimicked by equivalent depolarization, and is still seen when the cell is manually clamped to the original resting potential.(ABSTRACT TRUNCATED AT 400 WORDS)

An intracellular study of synaptic transmission and dendritic morphology in sympathetic neurons of the chick embryo

The characteristics of synaptic transmission in whole embryonic avian sympathetic ganglia have been examined by intracellular recording. Neurons in lumbar paravertebral ganglia of chick embryos exhibit both fast nicotinic excitatory postsynaptic potentials (EPSPs) and non-cholinergic slow EPSPs. Fast nicotinic transmission is mediated by at least 3-5 convergent preganglionic inputs and can be detected at the earliest embryonic stage examined (Stage 38; 12 days of incubation). Two types of non-cholinergic slow EPSPs have been observed and distinguished by their time course and the resulting changes in input resistance. One of these slow synaptic potentials is mimicked by exogenously applied substance P, but not by exogenous luteinizing hormone-releasing hormone (LH-RH). Muscarinic agonists also evoke slow depolarizations in the ganglia, mediated at least in part by inhibition of the M-current. Intracellular labeling with horseradish peroxidase reveals cells with 5-10 primary dendrites at Stage 42 (16 days of incubation), the earliest stage to exhibit slow EPSPs. The active and passive membrane properties of avian sympathetic neurons, including the presence of the M-current, generally resemble those of adult mammalian and amphibian sympathetic neurons. Functional activity in chick sympathetic neurons is present at a developmental stage where both biochemical and morphological indices of synapse maturation are at low levels. Since this progression has also been observed in the avian ciliary ganglion, it may be of general relevance to neuronal development.

Membrane currents underlying the cholinergic slow excitatory post-synaptic potential in the rat sympathetic ganglion

Non-nicotinic slow synaptic currents were recorded from voltage-clamped neurones in isolated rat superior cervical ganglia bathed in a solution containing d-tubocurarine and (usually) 1 microM-neostigmine. Three components of slow synaptic current could be detected following repetitive preganglionic stimulation: a net inward current resulting from inhibition of the voltage-dependent outward K+ current IM; a net outward current associated with a fall in membrane conductance when IM was deactivated by membrane hyperpolarization or inhibited with external Ba2+ or internal Cs+; and an occasional late inward current associated with an increased membrane conductance. As a result, synaptic current amplitudes showed complex changes with changes in membrane potential. Both the inward current associated with IM inhibition and the outward current were enhanced by neostigmine and blocked by atropine or pirenzepine, and therefore resulted from activation of muscarinic receptors. In unclamped neurones, equivalent stimulation produced a membrane depolarization and induced or facilitated repetitive spike discharges. It is concluded that the principal synaptic response to muscarinic receptor activation is IM inhibition, leading to a net inward current and increased excitability, but that this response may be modified under certain circumstances by other synaptic currents.

Noradrenaline hyperpolarization and depolarization in cat vesical parasympathetic neurones

Responses to noradrenaline (NA) applied by superfusion, ionophoresis or pressure pulse were analysed using conventional intracellular recording and voltage-clamp methods in cat vesical parasympathetic ganglia. NA (1 microM) hyperpolarized 60% of the neurones, depolarized 25%, and produced a biphasic potential, which comprised a membrane hyperpolarization followed by a membrane depolarization, in 10%. About 5% of the neurones did not respond to NA. The NA hyperpolarization was blocked by yohimbine (1 microM), an alpha 2-adrenoceptor antagonist, whereas the NA depolarization was blocked by prazosin (0.1-1 microM), an alpha 1-adrenoceptor antagonist. These data indicated that the NA hyperpolarization was mediated through alpha 2-adrenoceptors and the NA depolarization through alpha 1-adrenoceptors. The NA hyperpolarization was accompanied by an increase in conductance, while the NA depolarization was associated with a decrease in conductance measured under manual-clamp conditions. Similar conductance changes were observed under voltage clamp. NA hyperpolarizations became smaller as the membrane was hyperpolarized and reversed polarity beyond -100 mV. NA depolarizations also became smaller at hyperpolarized membrane potentials and reversed polarity around -90 mV. The NA responses were enhanced in low-K media and depressed in high-K Krebs solution. The NA hyperpolarization was blocked by the Ca antagonists, Cd, Mn and Co. Intracellular injection of EGTA caused a slowly developing, progressive block of the NA hyperpolarization. The NA depolarization was not affected by low Ca concentrations, Ca antagonists or intracellular injection of EGTA. In some neurones the NA depolarization was unmasked in solutions containing Ca antagonists and after intracellular EGTA injection. The NA hyperpolarization was depressed by intracellular injection and extracellular superfusion of Cs but not by TEA. Ba (10-100 microM) depressed the NA hyperpolarization by 30%. The NA depolarization persisted in the presence of muscarine (10 microM) and was not blocked by Cs or TEA but was depressed 70% by Ba (10 microM). These data are consistent with the hypotheses that alpha 2-adrenoceptor activation produces a membrane hyperpolarization that is mediated through a Ca-dependent K conductance, and that alpha 1-adrenoceptor activation produces a membrane depolarization through closure of a voltage-insensitive K channel.