Alpha-drenergic inhibition of calcium-dependent potentials in rat sympathetic neurones

Catecholamine inhibition of calcium action potentials in rat locus coeruleus neurones

Intracellular recordings were made from neurones in the nucleus locus coeruleus in a slice of tissue cut from the rat pons. Clonidine (100 nM-10 microM), noradrenaline (10 microM-1 mM) and adrenaline (10 microM-1 mM) all reduced the duration of the spontaneously occurring action potential of the neurones. This effect was also observed on the action potential in the presence of tetrodotoxin, which results from calcium entering the cell. These concentrations of clonidine, noradrenaline and adrenaline always hyperpolarized the membrane. This hyperpolarization was prevented by two procedures which block potassium currents--intracellular caesium and extracellular barium. In conditions of potassium current blockade, noradrenaline (100 microM-1 mM) and adrenaline (20 microM-1 mM) shortened the calcium action potential but clonidine was ineffective even at 10 microM. Adrenaline and noradrenaline also suppressed inward calcium and barium currents measured under voltage clamp. This action of noradrenaline and adrenaline was not prevented by yohimbine (10 microM), propranolol (20 microM) or prazosin (1 microM); it was reduced by a concentration of phentolamine about 100 times higher than its Ke for alpha 2-adrenoceptors on locus coeruleus neurones. It is concluded that noradrenaline and adrenaline can directly inhibit calcium action potentials in locus coeruleus neurones when applied in high concentrations, but that this does not involve an alpha 2-adrenoceptor.

Identification of delayed potassium and calcium currents in the rat sympathetic neurone under voltage clamp

Post-ganglionic neurones of the isolated rat superior cervical ganglion were studied at 37 degrees C under two-electrode voltage-clamp conditions. Membrane depolarization beyond -40 mV from holding levels between -50 and -100 mV produced a delayed outward current which exhibited no inactivation within this voltage range. The current is carried primarily by K+ ions and its instantaneous I-V relation is linear. The total outward current could be separated into two distinct components on the basis of ion-substitution experiments. A voltage-dependent component of the delayed current, termed IK(V), is activated by membrane depolarization beyond -40 mV when Ca2+ fluxes are selectively blocked by Cd2+ or in Ca2+-free solution. IK(V) develops following first-order kinetics and rises to a peak with a voltage-dependent delay (239 ms at -30 mV and 23 ms at +10 mV). GK(V) attains a saturating value of the order of 17 mS/cm2 at about +20 mV and can be described in terms of a simple Boltzmann distribution for a single gating particle with a valency equal to +2.5. A second component of the delayed outward current, termed IK(Ca), depends on Ca2+ entry for its activation and was isolated as difference current before and after block of Ca2+ movements across the membrane. IK(Ca) is larger and faster than IK(V): it is strictly related to Ca2+ influx and also depends on membrane potential depolarization. A distinct Ca2+ current, ICa, was recorded from the neurone exposed to Na+-free or tetrodotoxin solution. ICa was activated by membrane depolarization beyond -30 mV and reached a maximum value near 0 mV. Its activation agrees with fourth-order kinetics and becomes faster with increasing depolarization. The Ca2+ current developed with a voltage-dependent time to peak of 2.9-1.8 ms and thereafter completely inactivated. The relationship between ICa and IK(Ca) is discussed. The Ca2+-k+ repolarizing system is expected to be mainly associated with action potentials arising from a depolarized neurone, whereas the IA current (Belluzzi, Sacchi & Wanke, 1985) dominates the repolarization mechanism at the normal membrane potential. The effect of muscarine was examined. Muscarine (10-50 microM) produced a fall in conductance with a voltage dependence similar to that exhibited by GK(Ca) and was ineffective when removing extracellular Ca2+ or adding Cd2+. A partial suppression of ICa by muscarine is demonstrated. It is suggested that the decrease of the outward current magnitude in the presence of muscarine may be accounted for qualitatively by the reduction in ICa.

Mechanism of alpha 2-adrenergic inhibition of neuroeffector transmission in the mouse vas deferens

The process by which the activation of presynaptic alpha 2-adrenoceptors inhibits the release of noradrenaline from terminals of postganglionic sympathetic nerves was studied in the mouse isolated vas deferens. Clonidine was used as a prototypic agonist. Field stimulation-evoked excitatory junction potentials (e.j.p.s) were recorded from individual muscle cells. The e.j.p. amplitudes were taken as a measure of transmitter release. Changes in the external Ca2+ concentration from 2.5 to 1.25 or 5 mM caused corresponding changes in the size of e.j.p.s. When the normal Ca2+ concentration of the medium (2.5 mM) was substituted by equimolar quantities of Ba2+ or Sr2+, the e.j.p. amplitudes decreased considerably. Clonidine (0.3-30 nM) inhibited the nerve stimulation-evoked e.j.p. amplitudes in a concentration-dependent manner, without altering appreciably the frequency of spontaneous e.j.p.s. Procedures known to enhance Ca2+ entry into nerve terminals, like a high Ca2+ medium (Ca2+ 5 mM) or 4-aminopyridine 30 microM reduced the effect of clonidine. Repetitive nerve stimulation at 3 Hz, which is supposed to lead to an accumulation of free Ca2+ inside nerve terminals, similarly counteracted the effect of clonidine 10 nM. Whereas the alpha 2-adrenergic inhibition of the first e.j.p. in a train was unaffected, the inhibition of all successive e.j.p.s was gradually decreased. At 5 mM Ca2+ only the time-course of facilitation became faster, the decrease in alpha 2-adrenergic inhibition proceeded with the same pulse-dependent rate as at a normal external Ca2+ concentration, although from a lower initial level.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of the repetitive discharge of rat CA 1 pyramidal neurones in vitro

Experiments using intracellular recording techniques were performed on rat hippocampal neurones in vitro, to study the discharge properties of these cells. When CA 1 pyramidal cells were excited by injecting long depolarizing current pulses (approximately 600-800 ms), they responded with an initial rapid action potential discharge which slowed, or accommodated, and then stopped after 200-300 ms. The train of action potentials was followed by a hyperpolarization which was due primarily to calcium-activated potassium conductance (GK(Ca]. The amplitude of this hyperpolarization increased with an increasing number of action potentials in the initial discharge. Blocking the calcium-activated potassium conductance, by injecting EGTA into the cell, by bathing the cell in cadmium, a calcium channel blocker, or by bathing the cell in calcium-free medium, reduced the after-hyperpolarization (a.h.p.) and accommodation such that the frequency of action potential discharge increased and the duration of this discharge was prolonged. Blocking the calcium-activated potassium conductance had a greater effect on discharge frequency later in the action potential train, as late interspike intervals were shortened more than early ones by the application of cadmium or of calcium-free medium. This was presumably because the calcium-activated potassium conductance was more developed later in the train. Accommodation was not completely abolished in the absence of calcium and presence of cadmium, suggesting that other factors, in addition to calcium-activated potassium conductance, contributed to this process. This remaining accommodation was reduced by low doses of carbachol, suggesting that the M-current also plays a role in accommodation. We conclude that accommodation of the action potential discharge of hippocampal pyramidal cells may be regulated by at least two potassium currents: the calcium-activated potassium current and the M-current. Both of these currents are turned on during excitation of the neurone and act in an inhibitory manner on that neurone to limit further action potential discharge.

Generation of an unusual depolarizing response in rabbit primary afferent neurones in the absence of divalent cations

The effects of divalent cations on responses to 5-hydroxytryptamine (5-HT), gamma-aminobutyric acid (GABA) and 1,1-dimethyl-4-phenyl piperazinium (DMPP) were investigated using a sucrose-gap method to record population responses. In Ca-free medium responses to 5-HT were enhanced, those to DMPP depressed and those to GABA unchanged. In Mg-free medium responses to 5-HT were unchanged, while those to DMPP and GABA were depressed. Removal of both Ca and Mg from the superfusion medium caused a small reduction of GABA responses and a large reduction of DMPP responses. Responses to 5-HT were not only greatly potentiated but were changed in character; the depolarizing phase became sigmoid and the dose dependence between quantity of 5-HT and response magnitude was lost as if 5-HT were triggering an all-or-nothing phenomenon. Dose--response relationships for GABA were normal in the large majority of preparations. In about 10% of preparations, supramaximal amounts of GABA or DMPP evoked large responses of a similar character to those evoked by 5-HT. The large responses, generated by an unknown mechanism, were termed X responses. Further reduction in tissue divalent cations by EGTA (1 mM) caused X responses to be generated spontaneously. Ca, Mg, Mn or Co (1 mM) could suppress X responses. DMPP responses, reduced in Ca/Mg-free medium, were largely restored by 1 mM-Ca. Depression of GABA responses in Ca/Mg-free medium could be entirely attributed to the absence of Mg, Mn being able to substitute for Mg. X responses were generated only after equilibration for 1 h with Ca/Mg-free medium. Attempts to manipulate [Ca]i with dinitrophenol or caffeine did not produce the conditions under which X responses were generated. Intracellular records of responses to 5-HT, GABA or DMPP showed that cells with A fibres responded to GABA but not to 5-HT or DMPP. Fifty-four out of sixty-seven cells with C fibre axons (80%) were depolarized by 5-HT, thirty-seven out of forty-nine (76%) by DMPP and forty out of fifty-seven (70%) by GABA. Eighteen out of thirty-eight (47%) C cells were depolarized by all three agents. Some C cells were very sensitive to 5-HT, 10(-6) M evoking a substantial response. In most, responses to 10(-5) M-5-HT had a slower rate of rise than responses to 10(-4) or 10(-3) M-GABA or DMPP, yet lower 5-HT concentrations normally elicited X responses in sucrose-gap experiments whereas GABA or DMPP normally did not.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of a slow cholinergic post-synaptic potential recorded in vitro from rat hippocampal pyramidal cells

Intracellular recording from CA1 pyramidal cells in the hippocampal slice preparation was used to compare the action of exogenously applied acetylcholine (ACh) and cholinomimetics to the effect of electrically stimulating sites in the slice known to contain cholinergic fibres. ACh depolarized pyramidal cells with an associated increase in input resistance, blocked a calcium-activated potassium conductance (GK(Ca], and blocked accommodation of action potential discharge. All of these actions were blocked by the muscarinic antagonist, atropine. Repetitive electrical stimulation of stratum (s.) oriens evoked a series of fast excitatory post-synaptic potentials (e.p.s.p.s) followed by an inhibitory post-synaptic potential. These potentials were followed by a slow e.p.s.p. that lasted 20-30 s. The slow e.p.s.p. was selectively enhanced by eserine and blocked by atropine. Ionophoretic application of ACh closely mimicked the time course of the slow e.p.s.p. The slow e.p.s.p. was blocked by tetrodotoxin and cadmium, indicating that it was dependent on propagated action potentials and on calcium. Considerably higher stimulus strengths were needed to elicit a slow e.p.s.p. than to elicit the earlier synaptic potentials. The size of the slow e.p.s.p. was markedly increased by repetitive stimulation. Stimulation of the alveus, s. oriens, s. pyramidale and fimbria all evoked a slow e.p.s.p., while stimulation of s. radiatum was relatively ineffective. The input resistance of the cell increased during the slow e.p.s.p. Hyperpolarizing the cell decreased the size of the slow e.p.s.p. and at membrane potentials of -70 mV or greater, little response was recorded. Stimulation of s. oriens blocked GK(Ca) and accommodation of action potential discharge. These effects, which could be seen in the absence of any change in membrane potential, were enhanced by eserine and blocked by atropine. The present electrophysiological results establish that CA1 pyramidal cells receive a cholinergic input and demonstrate that this input can dramatically alter the firing properties of these neurones for tens of seconds in the absence of any marked effect on membrane potential. Such an action contrasts with previously characterized synaptic potentials in this region of the brain.

Pharmacological characterization of amine receptors on embryonic chick sensory neurones

The effects of noradrenaline, dopamine and 5-hydroxytryptamine were investigated on the duration of the action potential of embryonic chick sensory neurones in vitro. All three amines, like gamma-aminobutyric acid, decreased the duration of the action potential evoked by current injection. The onset of the noradrenaline-induced decrease in action potential duration was fast (less than 1s) and the recovery phase was dependent upon the dose of noradrenaline applied. Rapid washout of the noradrenaline revealed a minimum 30s recovery time which was independent of the initial noradrenaline concentration. Dopamine and 5-hydroxytryptamine could mimic the effects of noradrenaline on action potential duration. The ED50 for all three amines was approximately 1 microM. At a saturating concentration of 10 microM, noradrenaline was more potent than dopamine and 5-hydroxytryptamine. Saturating doses of noradrenaline and dopamine or 5-hydroxytryptamine were not additive. Responses to all three amines were affected similarly by antagonists: they were antagonized by yohimbine, phentolamine, haloperidol and mianserin but not by propranolol, prazosin, domperidone, spiperone or methysergide. Clonidine and xylazine (alpha 2-adrenoceptor agonists) were also without effect. In contrast to the amines, saturating concentrations of gamma-aminobutyric acid were additive with those of noradrenaline. Responses to GABA were not antagonized by the amine receptor antagonists. The evidence described here suggests that the amines and gamma-aminobutyric acid acid decrease sensory neurone action potential duration via pharmacologically-distinct membrane receptors. In addition, it is likely that the amines are acting via a single class of receptor whose pharmacology is different from classical adrenoceptors, dopamine receptors and 5-hydroxytryptamine receptors.

Adrenergic transmission in the dog mesenteric vein and its modulation by alpha-adrenoceptor antagonists

Adrenergic transmission was investigated in the dog mesenteric vein by recording electrical responses of single smooth muscle cells to perivascular nerve stimulation. Perivascular nerve stimulation generated an excitatory junction potential (e.j.p.) and a slow depolarization of the membrane. The amplitude of the e.j.p. was increased by increasing the stimulus intensity, and at high intensity, a spike potential was generated. Repetitive stimulation of the nerves showed facilitation of e.j.ps and enhanced the amplitude of slow depolarization. A linear relationship was observed between the amplitude of the e.j.p. and of slow depolarization. The slow depolarization was inhibited by application of yohimbine or phentolamine, but not by prazosin. The amplitude of e.j.p. was increased by prazosin and was decreased by yohimbine. Both e.j.p. and slow depolarization were inhibited by guanethidine or tetrodotoxin. Exogenously applied noradrenaline depolarized the muscle membrane and, in high concentrations (greater than 10(-7)M), generated slow waves. These effects of noradrenaline were blocked by yohimbine. High concentrations of prazosin (greater than 10(-6)M) showed weak inhibitory effects on the noradrenaline actions. The amplitude of e.j.p. was decreased by exogenously applied noradrenaline in a dose-dependent manner. The inhibitory effect of noradrenaline on the e.j.p. was suppressed by yohimbine, but not by prazosin or phentolamine. Phentolamine, but not prazosin, enhanced the facilitation process of e.j.ps. This effect was not suppressed by exogenously applied noradrenaline. Application of neostigmine but not of atropine, reduced the e.j.p. amplitude without affecting the slow depolarization. It was concluded that, in the dog mesenteric vein, perivascular nerve stimulation produced three types of electrical responses of the smooth muscle membrane, i.e., e.j.p., slow depolarization and spike potential. The slow depolarization was generated by activation of alpha 2-adrenoceptors. Exogenously applied noradrenaline reduced the e.j.p. amplitude through activation of prejunctional alpha 2-adrenoceptors, but the reduction may not involve alpha-autoinhibitory mechanisms.

Modulation by dopamine of population responses and cell membrane properties of hippocampal CA1 neurons in vitro

Dopamine (DA) was applied to rat hippocampal slices maintained in vitro. Extracellular and intracellular recording techniques were used to study the effect of DA on population responses, membrane potentials, and membrane responses to hyperpolarizing current pulses in CA1 pyramidal cells. Temporary exposure of hippocampal slices to DA has a dual effect. The initial action of DA is to produce a suppression of the extra-cellularly recorded population responses. In individual neurons, this initial effect is seen as a membrane hyperpolarization accompanied by a decrease in the amplitude of responses to hyperpolarizing current pulses. The frequency of occurrence of spontaneous depolarizations and spikes is reduced. The early action of DA is followed by a profound potentiation of the population responses that can last for hours. This long-lasting potentiation of the population response, induced by DA, is depressed by spiroperidol, a DA antagonist. In individual neurons, the late effect of DA is a long-lasting membrane depolarization associated with an increase in the amplitude of responses to hyperpolarizing current pulses. During this late phase, spontaneous activity is increased, as are single cell responses to stimulation of afferents. The evidence presented here indicates that DA is able to induce a long-lasting modification of the excitability of CA1 hippocampal neurons. This modulation of excitability by DA may be similar in nature to previously described DA-modulatory actions in the peripheral nervous system.

Frequency dependent intermittency and ionic basis of impulse conduction in postganglionic sympathetic fibres of guinea-pig vas deferens

Some characteristic features of the functional innervation of guinea-pig vas deferens have been determined. Both ganglionic transmission from the hypogastric nerves and impulse propagation in proximal regions (main branch bundles within about 15 mm from the prostatic end of the organ) of the majority of single postganglionic sympathetic fibres of vas deferens nerve, had a high safety factor. Failure at these levels cannot account for the intermittent pattern of electrically-evoked secretion of transmitter from the individual varicosity of the terminals of vas deferens nerves, observed under identical experimental conditions. The shape of the extracellular single fibre action potential recorded by small calibre suction electrodes remained constant in proximal regions of vas deferens nerve, when the frequency of stimulation was varied between 0.5 and 8 Hz. Therefore, frequency-dependent facilitation of transmitter secretion in this tissue cannot be explained by frequency-dependent growth in the amplitude of nerve action potentials, as earlier assumed. However, when recordings were made in distal regions of vas deferens nerve (in small axon bundles, close to their points of insertion into the substance of the epididymal end of the organ), on two occasions fibres were found in which the safety factor for impulse conduction was low and frequency-dependent. The possibility is discussed that this feature, which was an exception in these non-terminal regions of vas deferens nerve, may be shared by the majority of fibres as they proceed distally towards the terminals. Clearly, if this is the case, intermittent failure of transmitter secretion from the individual varicosity may be due, at least in part, to intermittent failure of conduction of the nerve impulse to the terminals. Some useful qualitative information on the ionic basis of the extracellular nerve action potential, that might underlie a proximo-distal decline in the safety factor for impulse conduction in these nerves, was obtained by determining the effects on the shape of the signal, caused by varying the ionic composition of the medium (sodium, calcium), and by local addition of agents with known actions on sodium (tetrodotoxin), potassium (tetra-ethyl ammonium, 4-aminopyridine, rubidium, barium) and calcium channels (cobalt, manganese, lanthanum, nickel, D-600). By these criteria, the action potential that was shown to be a "normal" sodium-potassium spike, in proximal regions of vas deferens nerve, was found to have a different "pharmacological profile", in distal regions of the nerve, in a manner suggesting that here nerve impulse conduction had become somehow "calcium-dependent".(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological and pharmacological characterization of serotonergic dorsal raphe neurons recorded extracellularly and intracellularly in rat brain slices

Extracellular and intracellular recordings were made from dorsal raphe (DR) neurons in frontal rat brain slices maintained in vitro. A population of neurons was found which displayed electrophysiological and pharmacological characteristics of serotonin-containing DR neurons recorded in vivo. Recorded extracellularly, these neurons displayed biphasic or triphasic action potentials of 1.5-3.0 ms duration, and discharged with a slow and steady rhythm. Recorded intracellularly these neurons displayed action potentials of about 1.8 ms duration, which were followed by large (10-20 mV) after hyperpolarizations which normally lasted 200-800 ms. These presumed serotonergic DR neurons were inhibited by LSD and serotonin. They were excited by norepinephrine, or the alpha-agonist phenylephrine, and these activations could be reduced or blocked by alpha-adrenoreceptor antagonists including the selective alpha 1-antagonist, prazosin. The major difference between the in vitro recordings and previous in vivo recordings from anesthetized animals was a reduction in the number of spontaneously firing DR neurons. This was probably due, at least in part, to a disfacilitation of serotonergic DR neurons in the slice caused by the functional removal of a tonic noradrenergic input.

Modulation of action potential during the late slow excitatory postsynaptic potential in bullfrog sympathetic ganglia

The spike peak and after-hyperpolarization of the action potential of bullfrog sympathetic ganglion cells were depressed during the late slow excitatory postsynaptic potential (EPSP). These changes in the action potential were mimicked by luteinizing hormone-releasing hormone (LH-RH), a neurotransmitter candidate for the late slow EPSP. LH-RH (5 microM) suppressed the voltage-dependent K+ currents, both the delayed rectifier K+ current (IK1) and the M current (IK2). It is suggested that the depression of the after-hyperpolarization of the action potential during the late slow EPSP may be due to suppression of IK1 and IK2.

Ca-activated potassium current in vertebrate sympathetic neurons

Ca-activated K-currents (IC) in sympathetic neurones have been triggered by intracellular Ca-injection or by activating ICa. IC is strongly voltage-dependent, with a peak slope of 11 mV/e-fold depolarization above -50 mV. Relaxation, fluctuation and single channel analysis suggests this to result from voltage-dependent opening and closing rates. Time-constants for channel opening and closing are about 15 msec near zero mV. Single channel conductance is about 100 pS. Currents can be blocked by TEA. IC is activated very rapidly (less than or equal to 5 msec) and sometimes transiently by a depolarizing voltage-step. It is suggested that IC contributes to both spike repolarization and spike after-hyperpolarization. Spontaneous miniature ICs have also been recorded, probably activated by the release of packets of intracellular Ca.

Substance P inhibits the action potentials in bullfrog sympathetic ganglion cells

Substance P (0.5-5 microM) depressed the spike peak and after-hyperpolarization of action potentials of bullfrog sympathetic ganglion cells. It also depressed the after-hyperpolarization and prolonged the falling phase in Ca2+ spikes. The voltage-dependent K+ currents, both the delayed rectifier K+ current (Ik1) and the M current (Ik2), were suppressed by substance P, suggesting that the depression of the after-hyperpolarization may be due to suppression of these K+ currents.

Acetylcholine mediates a slow synaptic potential in hippocampal pyramidal cells

The hippocampal slice preparation was used to study the role of acetylcholine as a synaptic transmitter. Bath-applied acetylcholine had three actions on pyramidal cells: (i) depolarization associated with increased input resistance, (ii) blockade of calcium-activated potassium responses, and (iii) blockade of accommodation of cell discharge. All these actions were reversed by the muscarinic antagonist atropine. Stimulation of sites in the slice known to contain cholinergic fibers mimicked all the actions. Furthermore, these evoked synaptic responses were enhanced by the cholinesterase inhibitor eserine and were blocked by atropine. These findings provide electrophysiological support for the role of acetylcholine as a synaptic transmitter in the brain and demonstrate that nonclassical synaptic responses involving the blockade of membrane conductances exist in the brain.

An intracellular characterization of neurones and neural connexions within the left coeliac ganglion of cats

Intracellular recordings were made in vitro from neurones located within the left coeliac ganglion of the cat solar plexus. Thirty percent of the neurones within left coeliac ganglia were identified as efferent neurones. Within this neuronal population, splenic-efferent and renal-efferent neurones were identified specifically. Neurones within left coeliac ganglia were characterized as either phasic (fast adapting) neurones or tonic (slowly adapting) neurones depending upon their prolonged firing behaviour. Electrophysiological properties of neurones varied considerably. The wide range of values obtained for both input resistance and input capacitance suggest that sizeable differences in either specific membrane resistance or cell geometry exist within the over-all neurone population. Frequency distributions of input resistance, time constant, input capacitance and current threshold for tonic and phasic neurones were found to be significantly different. Compound excitatory post-synaptic potentials were produced by stimulation of the ipsilateral splanchnic nerves in 69% of the neurones tested and in 3% of the neurones tested upon stimulation of the contralateral splanchnic nerves. Electrical stimulation of nerve fibres located in the coeliac plexus, the superior mesenteric plexus or the left renal nerves generated excitatory synaptic potentials in neurones located within left coeliac ganglia. It is concluded that neurones within the left coeliac ganglion are innervated by splanchnic nerve fibres primarily contained within the left splanchnic nerves, receive excitatory synaptic input from splenic, renal and other peripheral preganglionic fibres and have extremely varied electrophysiological properties.

Mechanism of negative feed-back inhibition of norepinephrine release by alpha-adrenergic agonists

The mechanism of alpha-adrenergic receptor-mediated feedback regulation of [3H]norepinephrine release was studied in the guinea-pig heart. The overflow of [3H]norepinephrine evoked by stimulation (1 Hz for 60 s) was inhibited up to 90% in a dose-dependent manner by norepinephrine (58.5 and 177 nM) and epinephrine (54 and 164 nM). These inhibitory effects were antagonized by 20 mM tetraethylammonium. The overflow of [3H]norepinephrine was near normal when the calcium concentration of the perfusion medium was reduced from a control value of 2.5 to 0.25 mM in the presence of 20 mM tetraethylammonium. Norepinephrine exerted its typical inhibitory effect on the overflow induced in low [Ca2+] and high [tetraethylammonium] solution. Similarly, the inhibitory effects of epinephrine became evident in the presence of 20 mM tetraethylammonium and 0.25 mM calcium ions. Another K-channel blocker, 4-aminopyridine, was also effective in antagonizing the presynaptic actions of norepinephrine; the effect was more pronounced at lower than higher concentrations of norepinephrine. However, after reduction of the calcium ion concentration to 0.2 mM in the presence of 1 mM 4-aminopyridine, all concentrations of norepinephrine significantly reduced the overflow of [3H]norepinephrine. We conclude that the alpha-adrenergic agonists directly influence the availability of calcium ions needed for the transmitter release, and that this effect is brought about without altering the electrical properties (i.e. nerve conduction, action potential size, resting membrane potential of the nerves, etc.). This is in contrast to the actions of acetylcholine and adenosine which, in an earlier study, were shown to inhibit [3H]norepinephrine release primarily by modifying the electrical properties of the nerves.

Depolarization of rat isolated superior cervical ganglia mediated by beta 2-adrenoceptors

Depolarizations of freshly-dissected isolated superior cervical ganglia of the rat were recorded extracellularly. The following sympathomimetic amines (in order of decreasing potency) produced depolarizations of up to 0.4 mV: isoprenaline, salbutamol, adrenaline, noradrenaline. Depolarizations were lost after overnight storage, leaving only hyperpolarizing responses. Depolarizations by isoprenaline were antagonized by (-)-propranolol (pA2 8.94 +/- 0.15), (+/-)-butoxamine (pA2 7.36 +/- 0.12) and (+/-)-practolol (pA2 5.14 +/- 0.13). They were not blocked by phentolamine (1 microM) or phenoxybenzamine (1 microM). Isoprenaline and salbutamol were antagonized with equal facility by practolol or butoxamine. In concentrations producing ganglionic depolarization, these compounds also produced a smaller depolarization of presynaptic elements in the ganglion, but not preganglionic trunk fibres. Presynaptic depolarization was blocked by 100 nM propranolol but not by 1 microM phentolamine. Isoprenaline and salbutamol increased the amplitude of the compound ganglionic action potential recorded following single preganglionic nerve stimuli when transmission had been rendered submaximal by adjusting the Ca/Mg ratio, but not in normal solution. Isoprenaline (0.1 microM) also increased the amount of [3H]-acetylcholine released by preganglionic stimulation in low Ca/high Mg solution. It is concluded that facilitatory adrenoceptors are present on pre- and postsynaptic elements in rat superior cervical ganglia, which resembles the 'beta 2' subclass of beta-receptors.

The ionic basis of adenosine receptor actions on post-ganglionic neurones in the rat

Adenosine inhibited three Ca2+-dependent potentials recorded intracellularly from post-ganglionic neurones of the rat superior cervical ganglion. A shoulder on the falling phase of the action potential elicited in normal Locke solution, a hyperpolarizing after-potential (h.a.p.) that follows the spike, and a regenerative Ca2+ spike elicited in Locke solution containing TTX and TEA were all reversibly inhibited by adenosine analogues in a dose-dependent fashion. The maximum rate of rise of the Ca2+ spike (dV/dt) was markedly reduced suggesting that the underlying mechanism of adenosine action is inhibition of the Ca2+ conductance mechanism and thus, the voltage-sensitive Ca2+ current. I/V curves in low Ca2+, high Mg2+, TTX, TEA, and Co2+ to block the Ca2+ current show no change in resistance in the presence of 2-chloroadenosine. The actions of adenosine were nearly eliminated in the presence of 1 mM-theophylline, an adenosine receptor antagonist. The order of agonist potency on the inhibition of the h.a.p. was: N-6-[L-phenylisopropyl] adenosine (L-PIA) greater than 2-chloroadenosine greater than adenosine greater than cyclic AMP = 5' AMP. The concentration of L-PIA which produced a half-maximal effect (EC50) was 0.5 microM and that for cyclic AMP was 100 microM. Dipyridamole, an adenosine uptake blocker, potentiated the effects of low concentrations of adenosine and shifted the dose-response curve for adenosine towards that of 2-chloroadenosine (EC50 = 1 microM). These results are consistent with the concept of an external adenosine receptor, but we are unable to assign a receptor subtype. Cyclic AMP mimicked the effects of adenosine, but these effects were eliminated by adenosine deaminase. Our results suggest that the electrogenic effects of bath-applied cyclic AMP may result from the metabolism of cyclic AMP to adenosine by ganglionic tissue. We conclude that adenosine activates a receptor on the neuronal cell surface to inhibit the voltage-dependent Ca2+ current.

Presynaptic alpha 2-adrenoceptor antagonism by verapamil but not by diltiazem in rabbit hypothalamic slices

1 Rabbit hypothalamic slices prelabelled with [3H]-noradrenaline and superfused with Krebs solution were stimulated electrically at a frequency of 5 Hz. Exposure to verapamil (0.1 to 10 microM) significantly increased, in a concentration-dependent manner, the electrically-evoked overflow of tritium, without affecting the spontaneous outflow of radioactivity. 2 Exposure to diltiazem in concentrations up to 100 microM had no effect on the electrically evoked release of [3H]-noradrenaline, but increased the basal outflow of radioactivity at 10 and 100 microM. 3 The preferential alpha 2-adrenoceptor antagonist, yohimbine (0.1 microM) significantly antagonized the inhibitory effect of clonidine or adrenaline on [3H]-noradrenaline overflow elicited by electrical stimulation. Verapamil (3 microM) also antagonized this inhibitory effect of the alpha 2-adrenoceptor agonists on [3H]-noradrenaline release. In contrast to these results, exposure to diltiazem (10 microM) was ineffective in blocking the action of the alpha 2-adrenoceptor agonist. 4 These results suggest that the two Ca2+-antagonists verapamil and diltiazem differ in their ability to affect central noradrenergic neurotransmission. While verapamil is a relatively potent alpha 2-adrenoceptor antagonist, diltiazem is devoid of presynaptic alpha 2-adrenoceptor antagonist properties.

Calcium channel modulation by neurotransmitters, enzymes and drugs

Calcium channels in excitable membranes are of great importance for many cellular functions. Modulation of these channels by neurotransmitters and drugs regulates calcium influx into the cell and thereby alters the functional state of the cell. Recently it has become possible to measure properties of single calcium channels directly and to obtain evidence on mechanisms of their modulation.

Muscarinic inhibition of sympathetic C neurones in the bullfrog

1. The muscarinic inhibitory post-synaptic potential (i.p.s.p.) in sympathetic C neurones has been characterized in an isolated preparation of bullfrog paravertebral chain ganglia. Interactions between the i.p.s.p. and two other synaptic potentials have also been examined. 2. A single presynaptic stimulus to a C cell produces a nicotinic excitatory post-synaptic potential (e.p.s.p.) followed by a muscarine i.p.s.p. The latency of the i.p.s.p. is 50 msec or longer and the response lasts for seconds. C cells receive multiple cholinergic innervation but the thresholds for activation of the e.p.s.p. and i.p.s.p. are inseparable. Trains of 50 or more presynaptic stimuli produce a non-cholinergic e.p.s.p. which follows the nicotinic e.p.s.p. and i.p.s.p. and which lasts for tens of seconds. 3. The i.p.s.p. produced by a single presynaptic stimulus can be 30 mV in amplitude. However, in most cells, a short train of stimuli applied at an optimal frequency of 10 Hz is required to produce a large i.p.s.p. 4. The i.p.s.p. is blocked by atropine but is not affected by catecholamine antagonists. 5. Ionophoretically applied acetylcholine (ACh) mimics the i.p.s.p. in its latency, time course and amplitude. In addition, the i.p.s.p. and the muscarinic response to ACh reverse polarity at the same membrane potential: -102 mV in normal Ringer solution. The i.p.s.p. reversal potential shifts by 55 mV/decade change in extracellular K+ concentration and is insensitive to the Cl- gradient. 300 microM-Ba2+ totally blocks the muscarinically activated conductance in a reversible manner. 6. Action potentials, when initiated by a supramaximal nicotinic e.p.s.p. or by an antidromic impulse, are not blocked by the i.p.s.p. 7. Near resting potential (-50 to -60 mV), C cells can fire repetitively. The non-cholinergic slow e.p.s.p. is often accompanied by oscillations in membrane potential and firing of action potentials. This repetitive firing of C cells, which appears to be enhanced by the non-cholinergic e.p.s.p., is strongly inhibited by the i.p.s.p. The inhibition can be mimicked by injection of very small hyperpolarizing currents (e.g. 25 pA). Interactions between the i.p.s.p. and the non-cholinergic e.p.s.p. can generate phasic bursting patterns in C cells. 8. The mechanism underlying the i.p.s.p. and the consequences of these findings for ganglionic integration are discussed.

Morphine enhances and depresses Ca2+-dependent responses in visceral primary afferent neurons

Morphine at 1 to 100 nM enhances and depresses Ca2+-dependent potentials, the shoulder of the action potential, the hyperpolarizing afterpotential, and the calcium spike in visceral primary afferent neurons of rabbits. These effects could be reversed by naloxone. The enhancement and depression of the Ca2+-dependent potentials are not only concentration-dependent but they are also time-dependent and can be observed with a given concentration of morphine.

Electrotonic synapses are formed by fetal rat sympathetic neurons maintained in a chemically-defined culture medium

Principal neurons from the superior cervical ganglia of rat fetuses were maintained for up to 101 days in dissociated cell cultures in a serum-free, chemically-defined medium; non-neuronal cells were killed by the periodic addition of fluorodeoxyuridine to the medium. Intracellular recordings, obtained at various times between 16th and 98th day in vitro, showed that these neurons could generate substantial (up to 90 mV) action potentials in response to depolarizing current injections; these responses were dependent on tetrodotoxin-sensitive Na+ channels, cobalt-sensitive Ca++ channels, and tetraethylammonium-sensitive K+ channels. Action potentials were often followed by prominent, long hyperpolarizing after-potentials (10-15 mV, greater than 150 ms); the duration of these after-potentials was reduced by the addition of Co++ (2-5 mM) to the perfusate. Acetylcholine depolarized these neurons by a hexamethonium-sensitive mechanism. To determine whether sympathetic neurons formed synapses in a defined medium, intracellular recordings were obtained from pairs of neighboring neurons. Synaptic interactions were frequently observed at all times in vitro (up to 60% of all pairs tested). At many synapses, both hyperpolarizing and depolarizing DC potential changes spread from one neuron to another. At other synapses, the spread of DC potential changes could not be directly demonstrated; however, interactions at such synapses were not inhibited by antagonists of several neurotransmitters, by elevation of the Mg++/Ca++ ratio, or by the addition of Co++. Thus most, if not all, of the synaptic interactions among sympathetic neuron were electronic; such electrical synapses were not observed among dorsal root ganglion neurons maintained in the same medium. These data indicate that, when maintained in a chemically-defined culture medium, sympathetic neurons of rat fetuses express many of the basic membrane properties observed in neurons of superior cervical ganglia recently removed from adult rats. However, fetal sympathetic neurons maintained in this defined medium also differ from their counterparts in vivo; they adopt a mode of synaptic transmission (electrical) that has not been observed in the sympathetic ganglia of the adult rat. Thus, as late as the 21st embryonic day, not only the choice of neurotransmitter, but also the mode of transmission has not been irrevocably determined in sympathetic neurons.

The appearance and development of chemosensitivity in Rohon-Beard neurones of the Xenopus spinal cord

1. We have examined the onset and subsequent development of chemosensitivity in Rohon-Beard neurones from the Xenopus spinal cord. These cells become sensitive to bath-applied gamma-aminobutyric acid (GABA) around stage 25 (early tailbud, about 1 d old), and remain so at least until stage 49 (9 d old). In contrast, a number of other neurotransmitter candidates tested caused no potential or conductance change during the same period.2. We examined ionophoretic dose-response relations of the cells at stage 26, a couple of hours after the first acquisition of GABA sensitivity. Sensitivities as high as 450 mV/nC were recorded. Comparable sensitivities were recorded between stages 46-49 (5-9 d old).3. Measurements of ionophoretic sensitivities and input resistances during several periods from stage 26 to maturity show that the underlying conductance change for a given GABA dose is likely to increase steadily during this time. A ;sensitivity index' (ionophoretic sensitivity/input resistance) was calculated, which is low at stage 26, higher at intermediate stages (stages 31-42), and highest for mature cells (stages 46-49; 5-9 d of development).4. The reversal potential of the ionophoretic GABA response is the same at stage 26 (-30 mV) as it is in mature cells. Ion substitution experiments show that Na(+) and K(+), but not Cl(-) or Ca(2+), are involved in the response.5. GABA responses at stage 26 are pharmacologically similar to those of mature cells. The responses are blocked by 10 muM-picrotoxin or curare, and muscimol is an agonist in concentrations as low as 1 muM.6. GABA responses at stage 26 desensitize in a manner similar to that seen for mature cells, either with prolonged bath application of GABA or with repetitive ionophoretic application.7. Nearly half of the cells tested at stage 26 respond to glycine, in concentrations as low as 5 muM. This sensitivity is absent by 3(1/2) d of development.8. The responses of Rohon-Beard neurones to GABA are similar to those of other cells in that they involve a conductance increase, are mimicked by muscimol, and are blocked by picrotoxin. These responses are different in that they do not involve Cl(-) and are blocked by low concentrations of curare.9. Many of the characteristics of GABA receptors, i.e. the reversal potential, desensitization, and pharmacology, are constant during development. However, the sensitivity of the cells to GABA and the spectrum of transmitters to which they are sensitive appear to change.

Evidence for initiation of calcium spikes in C-cells of the rabbit nodose ganglion

In Na+-free solution or in tetrodotoxin (TTX, 10(-6) M) solution, direct electrical stimulation of nodose ganglion C-cells of the rabbit elicited regenerative action potentials. The amplitude of the action potentials generated in these neurons is dependent on the external Ca2+ concentration ([Ca2+]0). These action potentials were characterized by reduced amplitude, prolonged duration, and graded responses to changes of the stimulation intensity. Either removal of [Ca2+]0 or application of organic Ca2+-blocking agents, diltiazem (10(-5) M) or verapamil (10(-5) M), abolished these action potentials. The conduction of action potentials along the axon was blocked in Na+-free solution or by application of TTX. The present results provide evidence for the initiation of Ca2+-dependent action potentials in the soma membrane of mammalian nodose ganglia, and suggest that the Ca2+ ion plays an important role in the development of action potentials in the C-cell of rabbit nodose ganglia.

Effects of cyclic AMP analogues and phosphodiesterase inhibitors on K+-induced [3H]noradrenaline release from rat brain slices and on its presynaptic alpha-adrenergic modulation

The possible role of cyclic AMP in the presynaptic alpha-adrenoceptor-mediated modulation of [3H]noradrenaline (NA) release induced by 13 mM K+ from superfused rat cerebral cortex slices was investigated. Both dibutyryl-cyclic AMP (db-cAMP) and 8-bromo-cyclic AMP (8-Br-cAMP) dose-dependently (10(-4) - 10(-2) M) enhanced K+-induced (3H]NA release, maximally to about 160% of control. In contrast, db-cAMP had no effect on calcium-induced [3H]NA release in the presence of the calcium ionophore A 23187. Surprisingly, the phosphodiesterase (PDE) inhibitors 3-isobutyl-1-methylxanthine (IBMX). 7-benzyl-IBMX, 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrrolidone (ZK 62771), and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro 20-1724) appeared to inhibit K+-induced [3H]NA release in a dose-dependent (10(-5) - 10(-3) M) manner. At a concentration of 10(-4) M, AK 62771 caused an inhibition of [3H]NA release by 30%, and this inhibitory effect was not affected by 10(-6) M phentolamine nor by 10(-3) M db-cAMP or 10(-4) M theophylline. Theophylline by itself enhanced [3H]NA release to about 135% of control. The inhibitor effect of the alpha-adrenoceptor agonist oxymetazoline (1 micro M) and the enhancing effect of the antagonist phentolamine (1 micro M) on [3H]NA release were significantly decreased in the presence of 10(-3) M db-cAMP or 8-Br-cAMP, whereas 10(-4) M ZK 62771 had no effect. In the presence of 10(-2) M NaF, a potent activator of adenylate cyclase, the inhibitory effect of oxymetazoline (1 micro M) on [3H]NA release was significantly decreased. The data obtained with the cyclic AMP analogues support the hypothesis that activation of presynaptic alpha-receptors modulating NA release results in an inhibition of a presynaptic adenylate cyclase. Possible causes for the anomalous effects of th PDE inhibitors are discussed.

Control of calcium current in rat sympathetic neurons by norepinephrine

Inward voltage-dependent calcium currents were recorded from clamped rat sympathetic ganglion cells using either one or two microelectrodes. Suppression of potassium current was achieved by applying tetraethylammonium (TEA) externally and TEA plus cesium internally. Peak ICa was observed at 0 mV. ICa was abolished by perfusing cadmium or low calcium medium. ICa was reduced by adding norepinephrine (1-50 micrometers). This effect was not accompanied by any major change in the voltage sensitivity or time course of the residual calcium current. It is suggested that norepinephrine acts by reducing the number of available calcium channels.

Modulation of nicotinic transmission by biogenic amines in bullfrog sympathetic ganglia

Studies of transmission in isolated paravertebral sympathetic ganglia of the bullfrog and at the sciatic-sartarius muscle synapse in the frog yielded evidence that biogenic amines such as catecholamines or 5-hydroxytryptamine can modulate transmission in sympathetic ganglia and at the neuromyal junction. These two transmitter substances are able to modulate transmission by affecting the amount of ACh release from presynaptic terminals and also by affecting the sensitivity of nicotinic Ach receptors of the subsynaptic membrane. Information is presented as to how these compounds exert their modulatory effects on these synapses.

Voltage-sensitive K-currents in sympathetic neurons and their modulation by neurotransmitters

A description is given of methods for voltage clamping and for studies of the several varieties of K-currents occurring in sympathetic neurons. Conclusions were derived chiefly from experiments conducted on bullfrog lumbar paravertebral sympathetic ganglia but observations made were compared with results obtained from studies of neurons of the rat superior cervical ganglion. Discussion is given of means used for revealing and identifying these currents and for studying actions of drugs, calcium, etc. and circumstances such as membrane state on the four quite distinct time- and voltage-dependent K-currents: IK, IC, IA and IM. The latter (IM) is under direct presynaptic control and the major focus of the paper is on this current and its role. However, considerable information is given concerning the characteristic of the other three (IK, IC and IA).

Alteration of the electrophysiological activity in sympathetic ganglia infected with a neurotropic virus. I. Presynaptic origin of the spontaneous bioelectric activity

The bioelectric activity of the rat superior cervical ganglion (SCG) infected with pseudorabies virus (PRV) was examined in vitro 30-38 h after inoculation. Simultaneous intra- and extracellular recordings on the internal (ICN) and external carotid nerves (ECN) revealed a synchronized spontaneous activity. This synchronization can be ascribed either to the functional organization of the ganglion or to the mechanism of initiation itself. In the infected ganglia two categories of cells were observed: cells displaying abnormal spontaneous discharges, and silent cells whose electrophysiological behavior was similar to control cells. Spontaneously active cells showed intermittent spiking and bursting activity. The discharge pattern was associated with the firing rate of the emitting cell: sporadically active cells emitted single spikes whereas highly active cells fired bursts of action potentials (APs). Long lasting intracellular recordings demonstrated that the cells undergo gradual changes evolving from sporadic on to high activity. Spontaneous APs usually rode on prepotentials similar to the excitatory postsynaptic potentials (EPSPs). A comparative study of spontaneous prepotentials and orthodromically evoked EPSPs in the same cell demonstrated that the spontaneous prepotentials are real synaptic potentials. No pace-maker potentials were observed. The passive and active electrical membrane properties of spontaneously active neurons were not different from those of silent cells or control cells impaled in uninfected ganglia. D-Tubocurarine abolished the spontaneous activity in the whole ganglion. Ortho- and antidromic electrical stimulations of suprathreshold intensity elicited an evoked response in neurons displaying spontaneous activity, followed by a delayed burst whose shape was similar to the spontaneous burst of the cell. Stimuli of subthreshold intensities induced this delayed burst independently from the evoked response. We conclude that the spontaneous bioelectrical activity is of presynaptic, but not necessarily of preganglionic origin. The possible existence of a cholinergic intraganglionic pathway revealed by the viral infection is discussed.

alpha 2-adrenoceptor-mediated hyperpolarization of locus coeruleus neurons: intracellular studies in vivo

Intracellular recordings in vivo from noradrenergic neurons in the rat locus coeruleus showed that membrane potential was hyperpolarized by the administration of clonidine (an alpha 2-adrenoceptor agonist) or after a burst of spikes evoked by intracellular pulses; both types of hyperpolarization were associated with a decrease in membrane input resistance, and both could be blocked by the alpha 2-adrenoceptor antagonist piperoxane. These results suggest that a hyperpolarization of membrane potential mediated by an alpha 2-adrenoceptor underlies both clonidine- and activation-induced inhibition of locus coeruleus cell firing.

Synaptic inputs and action potentials of magnocellular neuropeptidergic cells: intracellular recording and staining in slices of rat hypothalamus

Excitatory postsynaptic potentials (EPSPs) and action potentials of magnocellular neuropeptidergic cells (MNCs) in the paraventricular (PVN) and supraoptic nuclei (SON) were studied with intracellular recording in coronal slices of rat hypothalamus. The fluorescent dye Lucifer Yellow (LY) was injected intracellularly and the cells were subsequently identified as magnocellular (somata greater than 15 x 15 micrometer). These cells generally had a large cytoplasm-to-nucleus ratio. In PVN it was frequently possible to trace filled dendrites to the ependyma of the third ventricle, and occasionally dendritic spines could be seen. Electrical stimuli in areas dorsolateral and ventrolateral to the fornix column evoked EPSPs in some anatomically identified MNCs of PVN, which indicates that presynaptic fibers innervating MNCs approach PVN from this region. Short-latency (less than 1 msec) spikes could be evoked in many MNCs of PVN by stimulation near SON, which is consistent with the known projection to the neurohypophysis of many MNCs. Action potentials in MNCs of PVN and SON had significantly longer durations at one-third spike height (mean +/- S.D. = 2.06 +/- 0.6 msec) than hippocampal CA1 pyramidal cells (1.17 +/- 0.29 msec). This suggests that neuroendocrine cells in mammals and some lower vertebrates and invertebrates are similar in this regard.

Slowing effects of dopamine and calcium-channel blockers on frequency of sodium spikes in rat pars intermedia cells

1. Spontaneous discharge of action potentials (Na spikes) in cells isolated from rat pars intermedia was slowed or arrested by Co(2+), Ni(2+) or Mn(2+), which block voltage-dependent Ca channels in these cells. The amplitude of persisting spikes was undiminished. The effects resembled those of dopamine.2. Action potential frequency decreased when the Ca(2+) concentration was lowered to 0.1 mM and increased when the Ca(2+) concentration was raised from this level to 1 mM or 2 mM or when Ba(2+) (2 mM) was introduced. These effects, together with those of Co(2+), Ni(2+) and Mn(2+), are consistent with the possibility that Ca(2+) participates in the regulation of spike discharge.3. Verapamil, methoxyverapamil (D600), and nifedipine reduced the amplitude of the individual Na spikes in concentrations that had little effect on voltage-dependent Ca channels. Action potential frequency was comparatively little affected by these drugs.4. K(+) (15 mM) stimulated action potential frequency and this effect too was suppressed by dopamine or Co(2+).5. The effect which dopamine had of slowing spontaneous discharge, like the inhibitory effect on secretion, was blocked by metoclopramide. But otherwise the mechanism is unclear: dopamine blocked voltage-dependent Ca channels in some cells but not in most others.6. The effects of K(+) and Ba(2+) of eliciting spikes, the suppression of Na-spike discharge by Co(2+) and related Ca-channel blocking cations, and the unspecific effects of the organic ;Ca channel blockers', all have implications for the use of these substances as tools to analyse stimulus-secretion coupling.

A study of renal-efferent neurones and their neural connexions within cat renal ganglia using intracellular electrodes

1. Intracellular recordings were made, in vitro, fron neurones located within left renal ganglia and left coeliac ganglia of cat solar plexus. 2. Forty-three percent of the neurones of the renal ganglia tested were identified by antidromic activation as renal-efferent neurones. 3. Electrical stimulation of all nerve trunks emanating from renal ganglia, other than the renal nerves, did not antidromically activate renal ganglia neurones. Neurones not antidromically activated were designated as non-efferent neurones. 4. Neurones within the renal ganglia were also characterized as phasic or tonic neurones depending on their pattern of discharge. 5. Electrical stimulation of renal nerves produced excitatory synaptic potentials (e.p.s.p.s) in 18% of the renal-efferent neurones. 6. Compounded e.p.s.p.s were produced in 50% of the renal ganglia neurones tested by stimulation of the ipsilateral splanchnic nerves and in 84% of the neurones upon stimulation of the vertebral nerve. 7. Synaptic responses demonstrating characteristics typical of those induced by activation of multisynaptic neural pathways were produced upon stimulation of renal and coeliac nerves. 8. The results of this study indicate that the electrophysiological properties of renal-efferent neurones vary considerably from neurone to neurone and that these neurones receive synaptic inputs from a variety of preganglionic fibres and possibly from other neurones having their soma located within the solar plexus.

Clonidine activates membrane potassium conductance in myenteric neurones

1 Intracellular recordings were made from neurones in the myenteric plexus of the ileum removed from guinea-pigs. The effects of clonidine and adrenaline on membrane potential and resistance were observed.2 Clonidine (100 pM-30 nM) caused a concentration-dependent membrane hyperpolarization associated with a fall in neurone input resistance.3 The amplitude of the clonidine hyperpolarization, but not the conductance increase, was greater in cells with lower resting potentials and smaller in more polarized neurones. In a given cell, membrane hyperpolarization decreased and membrane depolarization increased the clonidine effect.4 Low potassium solutions enhanced and high potassium solutions reduced the hyperpolarizing action of clonidine but did not significantly change the conductance increase caused by clonidine.5 The concentration-effect curve for clonidine was displaced to the left when the extracellular calcium concentration was reduced. Conversely, clonidine was almost ineffective in elevated calcium concentrations. This was true for both the hyperpolarization and the conductance increase.6 It is suggested that clonidine activates a potassium conductance by causing an elevation in the free intracellular calcium concentration.7 Clonidine reversibly depressed the amplitude of the nicotinic fast excitatory postsynaptic potential and the noncholinergic slow excitatory postsynaptic potential.8 All the effects of clonidine were shared by adrenaline and the actions of both were reversed or prevented by phentolamine (100 nM-1 muM).