Changes of intracellular sodium and potassium ion concentrations in isolated rat superior cervical ganglia induced by depolarizing agents

Neurons, Receptors, and Channels

Here, I recount some adventures that I and my colleagues have had over some 60 years since 1957 studying the effects of drugs and neurotransmitters on neuronal excitability and ion channel function, largely, but not exclusively, using sympathetic neurons as test objects. Studies include effects of centrally active drugs on sympathetic transmission; neuronal action and neuroglial uptake of GABA in the ganglia and brain; the action of muscarinic agonists on sympathetic neurons; the action of bradykinin on neuroblastoma-derived cells; and the identification of M-current as a target for muscarinic action, including experiments to determine its distribution, molecular composition, neurotransmitter sensitivity, and intracellular regulation by phospholipids and their hydrolysis products. Techniques used include electrophysiological recording (extracellular, intracellular microelectrode, whole-cell, and single-channel patch-clamp), autoradiography, messenger RNA and complementary DNA expression, antibody injection, antisense knockdown, and membrane-targeted lipidated peptides. I finish with some recollections about my scientific career, funding, and changes in laboratory life and pharmacology research over the past 60 years.

Electrophysiological Investigation of the Subcellular Fine Tuning of Sympathetic Neurons by Hydrogen Sulfide

H₂S is well-known as hypotensive agent, whether it is synthetized endogenously or administered systemically. Moreover, the H₂S donor NaHS has been shown to inhibit vasopressor responses triggered by stimulation of preganglionic sympathetic fibers. In contradiction with this latter result, NaHS has been reported to facilitate transmission within sympathetic ganglia. To resolve this inconsistency, H₂S and NaHS were applied to primary cultures of dissociated sympathetic ganglia to reveal how this gasotransmitter might act at different subcellular compartments of such neurons. At the somatodendritic region of ganglionic neurons, NaHS raised the frequency, but not the amplitudes, of cholinergic miniature postsynaptic currents via a presynaptic site of action. In addition, the H₂S donor as well as H₂S itself caused membrane hyperpolarization and decreased action potential firing in response to current injection. Submillimolar NaHS concentrations did not affect currents through K_υ7 channels, but did evoke currents through K_ATP channels. Similarly to NaHS, the K_ATP channel activator diazoxide led to hyperpolarization and decreased membrane excitability; the effects of both, NaHS and diazoxide, were prevented by the K_ATP channel blocker tolbutamide. At postganglionic sympathetic nerve terminals, H₂S and NaHS enhanced noradrenaline release due to a direct action at the level of vesicle exocytosis. Taken together, H₂S may facilitate transmitter release within sympathetic ganglia and at sympatho-effector junctions, but causes hyperpolarization and reduced membrane excitability in ganglionic neurons. As this latter action was due to K_ATP channel gating, this channel family is hereby established as another previously unrecognized determinant in the function of sympathetic ganglia.

Endogenous opioids contribute to insensitivity to pain in humans and mice lacking sodium channel Nav1.7

Loss-of-function mutations in the SCN9A gene encoding voltage-gated sodium channel Nav1.7 cause congenital insensitivity to pain in humans and mice. Surprisingly, many potent selective antagonists of Nav1.7 are weak analgesics. We investigated whether Nav1.7, as well as contributing to electrical signalling, may have additional functions. Here we report that Nav1.7 deletion has profound effects on gene expression, leading to an upregulation of enkephalin precursor Penk mRNA and met-enkephalin protein in sensory neurons. In contrast, Nav1.8-null mutant sensory neurons show no upregulated Penk mRNA expression. Application of the opioid antagonist naloxone potentiates noxious peripheral input into the spinal cord and dramatically reduces analgesia in both female and male Nav1.7-null mutant mice, as well as in a human Nav1.7-null mutant. These data suggest that Nav1.7 channel blockers alone may not replicate the analgesic phenotype of null mutant humans and mice, but may be potentiated with exogenous opioids.

Effects of 1,8-cineole on electrophysiological parameters of neurons of the rat superior cervical ganglion

1. 1,8-Cineole is a non-toxic small terpenoid oxide believed to have medicinal properties in folk medicine. It has been shown to have various pharmacological effects, including blockade of the compound action potential (AP). In the present study, using intracellular recording techniques, we investigated the effects of 1,8-cineole on the electrophysiological parameters of neurons of the superior cervical ganglion (SCG) in rats. 2. 1,8-Cineole (0.1-6 mmol/L) showed reversible and concentration-dependent effects on various electrophysiological parameters. At 3 and 6 mmol/L, but not at 0.1 and 1 mmol/L, 1,8-cineole significantly diminished the input resistance (R(i)) and altered the resting potential (E(m)) to more positive values. At 6 mmol/L, 1,8-cineole completely blocked all APs within 2.7 +/- 0.6 min (n = 12). In neurons exposed to 3 and 1 mmol/L 1,8-cineole, the effects regarding excitability varied from complete AP blockade to minor inhibition of AP parameters. The depolarization of E(m) and the decrease in R(i) induced by 6 mmol/L 1,8-cineole were unaltered by 200 micromol/L niflumic acid, a well known blocker of Ca(2+)-activated Cl(-) currents. 3. Significant correlations (Pearson correlation test) were found between changes in E(m) and decreases in AP amplitude (r = -0.893; P < 0.00282) and maximum ascendant inclination (r = -0.799; P < 0.0173), but not for maximum descendant inclination (r = 0.598; P < 0.117). Application of current to restore the transmembrane potential equal to control E(m) values in the presence of 6 mmol/L 1,8-cineole resulted in the partial recovery of AP. 4. The present study shows that 1,8-cineole effectively blocks the excitability of SCG neurons, probably through various mechanisms, one of which acts indirectly via depolarization of the neuronal cytoplasmatic membrane.

The nicotinic activation of the denervated sympathetic neuron of the rat

Nicotinic responses to endogenous acetylcholine and to exogenously applied agonists have been studied in the intact or denervated rat sympathetic neuron in vitro, by using the two-microelectrode voltage-clamp technique. Preganglionic denervation resulted in progressive decrease of the synaptic current (excitatory postsynaptic current, EPSC) amplitude, which disappeared within 24 h. These effects were accompanied by changes in ion selectivity of the nicotinic channel (nAChR). The extrapolated EPSC null potential (equilibrium potential for acetylcholine action, E(Syn)) shifted from a mean value of -15.9+/-0.7 mV, in control, to -7.4+/-1.6 mV, in denervated neurons, indicating a decrease of the permeability ratio for the main components of the synaptic current (P(K)/P(Na)) from 1.56 to 1.07. The overall properties of AChRs were investigated by applying dimethylphenylpiperazinium or cytisine and by examining the effects of endogenous ACh, diffusing within the ganglion after preganglionic tetanization in the presence of neostigmine. The null potentials of these macrocurrents (equilibrium potential for dimethylphenylpiperazinium action, E(DMPP); and equilibrium potential for diffusing acetylcholine, E(ACh), respectively) were evaluated by applying voltage ramps and from current-voltage plots. In normal neurons, E(Syn) (-15.9+/-0.7 mV) was significantly different from E(DMPP) (-26.1+/-1.0) and E(ACh) (-31.1+/-3.3); following denervation, nerve-evoked currents displayed marked shifts in their null potentials (E(Syn)=-7.4+/-1.6 mV), whereas the amplitude and null potential of the agonist-evoked macrocurrents were unaffected by denervation and its duration (E(DMPP)=-26.6+/-1.2 mV). It is suggested that two populations of nicotinic receptors, synaptic and extrasynaptic, are present on the neuron surface, and that only the synaptic type displays sensitivity to denervation.

G protein activation inhibits gating charge movement in rat sympathetic neurons

G protein-coupled receptors (GPCRs) control neuronal functions via ion channel modulation. For voltage-gated ion channels, gating charge movement precedes and underlies channel opening. Therefore, we sought to investigate the effects of G protein activation on gating charge movement. Nonlinear capacitive currents were recorded using the whole cell patch-clamp technique in cultured rat sympathetic neurons. Our results show that gating charge movement depends on voltage with average Boltzmann parameters: maximum charge per unit of linear capacitance (Q(max)) = 6.1 +/- 0.6 nC/microF, midpoint (V(h)) = -29.2 +/- 0.5 mV, and measure of steepness (k) = 8.4 +/- 0.4 mV. Intracellular dialysis with GTPgammaS produces a nonreversible approximately 34% decrease in Q(max), a approximately 10 mV shift in V(h), and a approximately 63% increase in k with respect to the control. Norepinephrine induces a approximately 7 mV shift in V(h) and approximately 40% increase in k. Overexpression of G protein beta(1)gamma(4) subunits produces a approximately 13% decrease in Q(max), a approximately 9 mV shift in V(h), and a approximately 28% increase in k. We correlate charge movement modulation with the modulated behavior of voltage-gated channels. Concurrently, G protein activation by transmitters and GTPgammaS also inhibit both Na(+) and N-type Ca(2+) channels. These results reveal an inhibition of gating charge movement by G protein activation that parallels the inhibition of both Na(+) and N-type Ca(2+) currents. We propose that gating charge movement decrement may precede or accompany some forms of GPCR-mediated channel current inhibition or downregulation. This may be a common step in the GPCR-mediated inhibition of distinct populations of voltage-gated ion channels.

The identification of the sympathetic neurons innervating the hamster submandibular gland and their electrophysiological membrane properties

The neuron innervating the hamster submandibular (SM) gland was identified in the superior cervical ganglion (SCG) in vitro by recording the antidromic response using the intracellular recording technique. After the cellular response was recorded, methylene blue was injected iontophoretically into the neuron from the recording electrode, and the location of the cell soma was determined. The salivatory neurons of the SM gland were in the small- to medium-sized group of the entire cell population of the SCG. The cell size was 36.3 x 24.4 microm (mean, n=45). The postganglionic fibers were entirely unmyelinated (mean: 0.34 m/sec at 28-30 degrees C, n=141). Eighty-seven percent of the cells were distributed in the central one-third of area between the external carotid nerve origin and the caudal pole in the SCG. The resting membrane potential, membrane input resistance, membrane time constant and membrane input capacitance of the salivatory neuron were as follows: -49.2+/-7.6 mV (n=102), 52.9+/-23.6 Mohms (n=71), 8.0+/-3.4 msec (n=71) and 147+/-50 pF (n=71). Fast- and slow-excitatory postsynaptic potentials (EPSPs) were evoked, but not slow-inhibitory postsynaptic potentials (IPSPs). The fast EPSP was 13.1+/-5.7 mV in amplitude and 46.2+/-17.1 msec in duration (n=35). The slow EPSP (20 Hz, 5 sec) was 6.9+/-11 .9 mV in amplitude and 101+/-43 sec in duration (n=16). The directly-evoked spike was 63.0+/-11.9 mV in amplitude and 5.9+/-1.3 msec in duration (n=54). The spike after-hyperpolarization (AHP) was 12.5+/-3.5 mV in amplitude and 353+/-161 msec in duration. Na+ and Ca+ channels were involved in the spike generation. The voltage-dependent K+ channels (delayed rectifier), A channels and rapidly Ca2+-activated K+ channels (BK channels) regulated the spike-falling phase. The delayed rectifiers, A channels, and BK and SK (slowly Ca2+-activated) channels were involved in generation of spike-AHP. Muscarine suppressed the Ca2+ component of spike via muscarinic receptors.

Electron Probe X-Ray Microanalysis: a Tool for Elucidating the Role of Ions in Neuronal Physiology and Pathophysiology

Electron probe x-ray microanalysis (EPMA) is a quantitative electron microscope technique that measures both water content (percentage water) and total (free plus bound) concentrations of biological elements in selected morphological compartments. Unlike other methods for determination of ion/element concentrations, EPMA permits simultaneous quantitation of several elements (Na, P, S, Cl, K, Ca, and Mg) and allows optical differentiation of nervous tissue cell types (i.e, neurons, glia) with subsequent analysis of respective submembrane regions or organelles (e.g, axoplasm, mitochondria, nuclei). EPMA, therefore, represents a powerful tool for extending our current understanding of elements/ions in neurophysiological processes. In addition, it is presumed that neuropathic injury disrupts normal intraneuronal Na+, K+, and Ca2+ distribution and that the structural and functional consequences are mediated by ion translocation. However, little specific information is available regarding how translocated ions distribute among subcellular anatomical compartments after injury. EPMA quantification of ion/element changes associated with various nervous tissue injury models has helped to elucidate corresponding pathophysiological mechanisms. In this review, we will discuss EPMA and the realized, as well as potential, contributions of this technique to deciphering the role of ions in neuronal physiology and pathophysiology. Our recent studies of axon degeneration during acrylamide intoxication will be described to illustrate the utility of EPMA.

A new method for recording surface compound potentials in sympathetic ganglia from mouse, rat, and guinea pig--application to muscarinic and nicotinic depolarizations

We present a new method for electrophysiologic investigations in isolated autonomous ganglia of a variety of laboratory animals. This method enables determination of surface compound potentials in ganglia and changes induced by pharmacologic compounds. Advantages of our methods are as following: (1) the method is relatively simple and does not require sophisticated experimental setups, with minor modifications it is adaptable to investigate ganglia of varying sizes; (2) the signal amplitude is comparable or even higher when compared with signals obtained by other methods: (3) the apparatus allows fast addition and removal of the investigational compounds and thus the determination of acute and subacute desensitizing effects; and (4) fast preparation and minor tissue injuries during preparation of the ganglia allow determination of surface potential changes over a time period of up to 2 days without qualitative changes of the parameters. In this report we demonstrate the validity of this method using superior cervical ganglia from rat, mouse, and guinea pig. Agonists used to trigger potential changes are the cholinergic agonists acetylcholine, muscarine, nicotine, and carbachol. The possibility of receptor desensitization by these compounds is investigated by repeated application over 5 h.

Oxygen/glucose deprivation in hippocampal slices: altered intraneuronal elemental composition predicts structural and functional damage

Effects of oxygen/glucose deprivation (OGD) on subcellular elemental composition and water content were determined in nerve cell bodies from CA1 areas of rat hippocampal slices. Electron probe x-ray microanalysis was used to measure percentage water and concentrations of Na, P, K, Cl, Mg, and Ca in cytoplasm, nucleus, and mitochondria of cells exposed to normal and oxygen/glucose deficient medium. As an early (2 min) consequence of OGD, evoked synaptic potentials were lost, and K, Cl, P, and Mg concentrations decreased significantly in all morphological compartments. As exposure to in vitro OGD continued, a negative DC shift in interstitial voltage occurred ( approximately 5 min), whereas general elemental disruption worsened in cytoplasm and nucleus (5-42 min). Similar elemental changes were noted in mitochondria, except that Ca levels increased during the first 5 min of OGD and then decreased over the remaining experimental period (12-42 min). Compartmental water content decreased early (2 min), returned to control after 12 min of OGD, and then exceeded control levels at 42 min. After OGD (12 min), perfusion of hippocampal slices with control oxygenated solutions (reoxygenation) for 30 min did not restore synaptic function or improve disrupted elemental composition. Notably, reoxygenated CA1 cell compartments exhibited significantly elevated Ca levels relative to those associated with 42 min of OGD. When slices were incubated at 31 degreesC (hypothermia) during OGD/reoxygenation, neuronal dysfunction and elemental deregulation were minimal. Results show that in vitro OGD causes loss of transmembrane Na, K, and Ca gradients in CA1 neurons of hippocampal slices and that hypothermia can obtund this damaging process and preserve neuronal function.

Oxygen/glucose deprivation in hippocampal slices: altered intraneuronal elemental composition predicts structural and functional damage

Effects of oxygen/glucose deprivation (OGD) on subcellular elemental composition and water content were determined in nerve cell bodies from CA1 areas of rat hippocampal slices. Electron probe x-ray microanalysis was used to measure percentage water and concentrations of Na, P, K, Cl, Mg, and Ca in cytoplasm, nucleus, and mitochondria of cells exposed to normal and oxygen/glucose deficient medium. As an early (2 min) consequence of OGD, evoked synaptic potentials were lost, and K, Cl, P, and Mg concentrations decreased significantly in all morphological compartments. As exposure to in vitro OGD continued, a negative DC shift in interstitial voltage occurred ( approximately 5 min), whereas general elemental disruption worsened in cytoplasm and nucleus (5-42 min). Similar elemental changes were noted in mitochondria, except that Ca levels increased during the first 5 min of OGD and then decreased over the remaining experimental period (12-42 min). Compartmental water content decreased early (2 min), returned to control after 12 min of OGD, and then exceeded control levels at 42 min. After OGD (12 min), perfusion of hippocampal slices with control oxygenated solutions (reoxygenation) for 30 min did not restore synaptic function or improve disrupted elemental composition. Notably, reoxygenated CA1 cell compartments exhibited significantly elevated Ca levels relative to those associated with 42 min of OGD. When slices were incubated at 31 degreesC (hypothermia) during OGD/reoxygenation, neuronal dysfunction and elemental deregulation were minimal. Results show that in vitro OGD causes loss of transmembrane Na, K, and Ca gradients in CA1 neurons of hippocampal slices and that hypothermia can obtund this damaging process and preserve neuronal function.

Regulation of gene expression by trans-synaptic activity: a role for the transcription factor NF-kappa B

Earlier studies in the sympathetic ganglion have led to the proposal that adaptation of transcription to trans-synaptic activity is controlled by a signal transduction pathway featuring a transcription factor which translocates to the nucleus upon its release from the post-synaptic membrane by after-hyperpolarization. In light of recent progress, it is proposed here that NF-kappa B constitutes the postulated transcription factor.

Effects of mono- and divalent ions on the binding of the adenosine analogue CGS 21680 to adenosine A2 receptors in rat striatum

The effect of monovalent and divalent cations on equilibrium binding of the adenosine A2-selective agonist ligand CGS 21680 (2-[p-(2-carbonylethyl)phenylethylamino]-5'-N-ethylcarboxami doadenosine) to membranes prepared from rat striatum was examined. Competition experiments with cyclohexyladenosine, 2-chloroadenosine, N-ethylcarboxamidoadenosine and CGS 21680 suggest that at 2 nM [3H]CGS 21680 binds to a single site with the pharmacology of an A2a receptor. Magnesium and calcium ions caused a concentration-dependent increase in binding that reached about 10-fold at 100 mM. Manganese ions had a biphasic effect on binding with a maximal increase at 5 mM. Lithium, sodium and potassium ions all caused a concentration-dependent decrease of binding. Sodium was most potent, potassium least. At 200 mM ion concentration, the inhibition of binding was 88% by sodium, 47% by lithium and 29% by potassium ions. The effect of sodium chloride was the same as that of sodium acetate. The effect of sodium ions was essentially similar to that of Gpp(NH)p. However, sodium ions produced a larger effect than even maximally effective concentrations of Gpp(NH)p. The maximal inhibition by Gpp(NH)p was about 55% at 2 nM radioligand concentration irrespective of the magnesium concentration. The maximal effect of sodium ions was reduced by increasing concentrations of magnesium ions. Increasing magnesium ion concentration from 1 to 100 mM increased the half-maximally effective concentration of Gpp(NH)p almost 10-fold and that of sodium ions less than 2-fold. Furthermore, sodium ions and Gpp(NH)p had additive effects. The binding of an agonist to striatal A2a receptors shows an unusually large dependence on both divalent and monovalent cations that can only partly be explained by a change in the coupling to Gs proteins.

A five-conductance model of the action potential in the rat sympathetic neurone

The origin of the action potential in neurones has yet to be answered satisfactorily for most cells. We present here a five-conductance model of the somatic membrane of the mature and intact sympathetic neurone studied in situ in the isolated rat superior cervical ganglion under two-electrode voltage-clamp conditions. The neural membrane hosts five separate types of voltage-dependent ionic conductances, which have been isolated at 37 degrees C by using simple manipulations such as conditioning-test protocols and external ionic pharmacological treatments. The total current could be separated into two distinct inward components: (1) the sodium current, INa, and (2) the calcium current, ICa; and three outward components: (1) the delayed rectifier, IKV, (2) the transient IA, and (3) the calcium-dependent IKCa. Each current has been kinetically characterized in the framework of the Hodgkin-Huxley scheme used for the squid giant axon. Continuous mathematical functions are now available for the activation and inactivation (where present) gating mechanisms of each current which, together with the maximum conductance values measured in the experiments, allow for a satisfactory reconstruction of the individual current tracings over a wide range of membrane voltage. The results obtained are integrated in a full mathematical model which, by describing the electrical behaviour of the neurone under current-clamp conditions, leads to a quantitative understanding of the physiological firing pattern. While, as expected, the fast inward current carried by Na+ contributes to the depolarizing phase of the action potential, the spike falling phase is more complex than previous explanations. IKCa, with a minor contribution from IKV, repolarizes the neurone only under conditions of low cell internal negativity. Their role becomes less pronounced in the voltage range negative to -60 mV, where membrane repolarization allows IA to deinactivate. In the spike arising from these voltage levels the membrane repolarization is mainly sustained by IA, which proves to be the only current sufficiently fast and large enough to recharge the membrane capacitor at the speed observed during activity. Different modes of firing coexist in the same neurone and the switching from one to another is fast and governed by the membrane potential level, which makes the selection between the different voltage-dependent channel systems. The neurone thus seems to be prepared to operate within a wide voltage range; the results presented indicate the basic factors underlying the different discrete behaviours.

Role of Ca2+ channels in the ability of membrane depolarization to prevent neuronal death induced by trophic-factor deprivation: evidence that levels of internal Ca2+ determine nerve growth factor dependence of sympathetic ganglion cells

Sympathetic neurons depend on nerve growth factor (NGF) for their survival both in vivo and in vitro; these cells die upon acute deprivation of NGF. We studied the effects of agents that cause membrane depolarization on neuronal survival after NGF deprivation. High-K+ medium (greater than or equal to 33 mM) prevented cell death; the effect of K+ was dose-dependent (EC50 = 21 mM). The protection by high K+ was abolished either by withdrawal of extracellular Ca2+ or by preloading the cells with a Ca2+ chelator. The involvement of Ca2+ flux across membranes in high-K+ saving of NGF-deprived neurons was also supported by experiments using Ca2+-channel antagonists and agonists. The Ca2+ antagonists nimodipine and nifedipine effectively blocked the survival-promoting effect of high K+. The Ca2+ agonists Bay K 8644 and (S)-202-791 did not by themselves save neurons from NGF deprivation but did strongly augment the effect of high K+; EC50 was shifted from 21 mM to 13 mM. These data suggest that dihydropyridine-sensitive L-type Ca2+ channels play a major role in the high-K+ saving. The depolarizing agents choline (EC50 = 1 mM) and carbamoylcholine (EC50 = 1 microM), acting through nicotinic cholinergic receptors, also rescued NGF-deprived neurons. The saving effect of nicotinic agonists was not blocked by withdrawal of extracellular Ca2+ but was counteracted by a chelator of intracellular Ca2+, suggesting the possible involvement of Ca2+ release from internal stores. Based on these findings we propose a "Ca2+ set-point hypothesis" for the degree of trophic-factor dependence of sympathetic neurons in vitro.

Distribution of elements in rat peripheral axons and nerve cell bodies determined by x-ray microprobe analysis

X-ray microprobe analysis was used to determine concentrations (millimoles of element per kilogram dry weight) of Na, P, Cl, K, and Ca in cellular compartments of frozen, unfixed sections of rat sciatic and tibial nerves and dorsal root ganglion (DRG). Five compartments were examined in peripheral nerve (axoplasm, mitochondria, myelin, extraaxonal space, and Schwann cell cytoplasm), and four were analyzed in DRG nerve cell bodies (cytoplasm, mitochondria, nucleus, and nucleolus). Each morphological compartment exhibited characteristic concentrations of elements. The extraaxonal space contained high concentrations of Na, Cl, and Ca, whereas intraaxonal compartments exhibited lower concentrations of these elements but relatively high K contents. Nerve axoplasm and axonal mitochondria had similar elemental profiles, and both compartments displayed proximodistal gradients of decreasing levels of K, Cl, and, to some extent, Na. Myelin had a selectively high P concentration with low levels of other elements. The elemental concentrations of Schwann cell cytoplasm and DRG were similar, but both were different from that of axoplasm, in that K and Cl were markedly lower whereas P was higher. DRG cell nuclei contained substantially higher K levels than cytoplasm. The subcellular distribution of elements was clearly shown by color-coded images generated by computer-directed digital x-ray imaging. The results of this study demonstrate characteristic elemental distributions for each anatomical compartment, which doubtless reflect nerve cell structure and function.

A quantitative description of the sodium current in the rat sympathetic neurone

The somata of rat sympathetic neurones were voltage-clamped in vitro at 27 degrees C using separate intracellular voltage and current micro-electrodes. Na currents were isolated from other current contributions by using: Cd to block the Ca current (ICa) and the related Ca-dependent K current (IK(Ca)), and external tetraethylammonium to suppress the delayed rectifier current (IK(V) ). The holding potential was maintained at -50 mV to inactivate the fast transient K current (IA) when the IA contamination was unacceptable. Step depolarizations beyond -30 mV activated a fast, transient inward current carried by Na ions; it was suppressed by tetrodotoxin and was absent in Na-free solution. Once activated, INa declined exponentially to zero with a voltage-dependent time constant. The underlying conductance, gNa, showed a sigmoidal activation between -30 and +10 mV, with half-activation at -21.1 mV and a maximal value (mean gNa) of 4.44 microS per neurone. The steady-state inactivation level, h infinity, varied with membrane potential, ranging from complete inactivation at -30 mV to minimal inactivation at about -90 mV with a midpoint at -56.2 mV. Double-pulse experiments showed that development and removal of inactivation followed a single-exponential time course; tau h was markedly voltage-dependent and ranged from 46 ms at -50 mV to 2.5 ms at -100 mV. Besides the fast inactivation, the Na conductance showed a slow component of inactivation. The steady-state value, s infinity, was maximal at -80 mV and minimal at -40 mV. The removal of slow inactivation is a two-time-constant process, the first with a time constant in the order of hundreds of milliseconds and the second with a time constant of seconds. Slow inactivation onset appeared to be a faster process than its removal. When slow inactivation was fully removed the peak INa increased by a factor of 1.8. INa was well described by assuming it to be proportional to m3h. The temperature dependence of peak INa, tau m and tau h was studied in the temperature range 17-27 degrees C and found similar to that reported for other preparations. The Q10 of these parameters allowed the reconstruction of the INa kinetic properties at 37 degrees C.

Two components of muscarine-sensitive membrane current in rat sympathetic neurones

Membrane currents induced by muscarine (Imus) were recorded in voltage-clamped neurones in isolated rat superior cervical ganglia. Two components of Imus were regularly recorded: an inward current resulting from inhibition of the outward K+ current, IM; and an outward current attributable to the reduction of a steady inward current. The presence of these two components caused a 'cross-over' in the current-voltage curves at -50 +/- 3 mV in neurones impaled with KCl-filled micro-electrodes or at -63 +/- 4 mV in neurones impaled with K-acetate-filled electrodes. Both components of Imus were prevented by atropine. Both persisted in Krebs solution containing tetrodotoxin (1 microM), Cd2+ (200 microM) or 0 Ca2+. When IM was inhibited by external Ba2+ or internal Cs+ only the outward component of Imus could be detected. This component reversed at +3 +/- 2 mV in cells impaled with CsCl-filled electrodes or at -20 +/- 3 mV in cells impaled with Cs-acetate-filled electrodes. The reversal potentials agreed with those for the currents induced by gamma-aminobutyric acid (+4 +/- 2 mV and -16 +/- 3 mV with CsCl and Cs acetate electrodes respectively). Replacement of external NaCl with Na acetate (so reducing external Cl- concentration ( [Cl-]o) from 155 to 22 mM) shifted the reversal potential for Imus by +25 and +14.5 mV in two cells impaled with CsCl-filled electrodes. A tenfold reduction of external [Na+] (by glucosamine replacement) did not significantly alter the reversal potential for Imus in KCl or CsCl-impaled cells. Under conditions where IM is already inhibited, the residual outward component of Imus can lead to hyperpolarization and inhibition of neuronal activity in unclamped cells. We conclude that both inward and outward components of Imus result from direct activation of muscarinic receptors on the ganglion cells. The inward component results from IM inhibition. We suggest that the outward component results from inhibition of another, voltage-independent current IX which largely comprises a Cl- current. The inward component induces membrane depolarization and an increased excitability; the outward component can lead to hyperpolarization and reduced excitability.

A fast transient outward current in the rat sympathetic neurone studied under voltage-clamp conditions

Post-ganglionic neurones of the isolated rat superior cervical ganglion were voltage clamped at 37 degrees C using separate intracellular voltage and current micro-electrodes. Control experiments in current clamp suggested that the neurone is electrotonically compact, the soma and the proximal dendritic membranes being under good spatial voltage uniformity. Depolarizing voltage steps from membrane potentials near -50 mV evoked: (i) a voltage-dependent inward Na+ current, (ii) an inward Ca2+ current, (iii) a voltage-dependent outward K+ current, (iv) a Ca2+-activated K+ outward current. Depolarizations from holding potentials more negative than -60 mV elicited, besides the currents mentioned above, a fast transient outward current IA which peaked in 1-2.5 ms and then decayed to zero following an exponential time course. The IA current was shown to be primarily, if not exclusively, carried by K+. It was unaffected by removal of external Ca2+ or addition of Cd2+ and was weakly blocked by tetraethylammonium ions and partially by 4-aminopyridine. The IA current showed a linear instantaneous current-voltage relationship. Its activation ranged from -60 to 0 mV with a mid-point at -30 mV. The A conductance could be described in terms of a simple Boltzmann distribution for a single gating particle with a valency of +3. Both the development and removal of inactivation followed a single exponential time course with a voltage-dependent time constant which was large near the resting potential (42 ms at -70 mV) and small (11 ms) near -100 and -40 mV. Steady-state inactivation h infinity ranged from -100 to -50 mV, with a mid-point at -78 mV, suggesting that approximately 50% of the IA channels are available at the physiological resting potential. Action potentials elicited from various holding potentials showed maximal repolarization rates dependent on the holding potential itself. This voltage dependence was found to be in reasonably good agreement with that of h infinity curve. These data are consistent with the view that in the rat sympathetic neurone, under physiological conditions, it is the IA current rather than the delayed outward current that is responsible for the fast action potential repolarization.

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.

Different types of potassium transport linked to carbachol and gamma-aminobutyric acid actions in rat sympathetic neurons

Carbachol and gamma-aminobutyric acid depolarize mammalian sympathetic neurons and increase the free extracellular K+-concentration. We have used double-barrelled ion-sensitive microelectrodes to determine changes of the membrane potential and of the free intracellular Na+-, K+- and Cl- -concentrations ( [Na+]i, [K+]i and [Cl-]i) during neurotransmitter application. Experiments were performed on isolated, desheathed superior cervical ganglia of the rat, maintained in Krebs solution at 30 degrees C. Application of carbachol resulted in a membrane depolarization accompanied by an increase of [Na+]i, a decrease of [K+]i and no change in [Cl-]i. Application of gamma-aminobutyric acid also induced a membrane depolarization which, however, was accompanied by a decrease of [K+]i and [Cl-]i, whereas [Na+]i remained constant. Blockade of the Na+/K+-pump by ouabain completely inhibited both the reuptake of K+ and the extrusion of Na+ after the action of carbachol, and also the post-carbachol undershoot of the free extracellular K+-concentration. On the other hand, in the presence of ouabain, no changes in the kinetics of the reuptake of K+ released during the action of gamma-aminobutyric acid could be observed. Furosemide, a blocker of K+/Cl- -cotransport, inhibited the reuptake of Cl- and K+ after the action of gamma-aminobutyric acid. In summary, the data reveal that rat sympathetic neurons possess, in addition to the Na+/K+-pump, another transport system to regulate free intracellular K+-concentration. This system is possibly a K+/Cl- -cotransport.

Intracellular electrolyte concentrations in rat sympathetic neurones measured with an electron microprobe

Intracellular element concentrations were measured in rat sympathetic neurones using energy dispersive electron microprobe analysis. The resting intracellular concentrations of sodium potassium and chloride measured in ganglia maintained for about 90 min in vitro at 25 degrees C were 3, 155 and 25 mmol/kg total tissue wet weight respectively. Recalculated in mmol/l cell water, these values are 5, 196 and 32 respectively. There were no significant differences between the nuclear and cytoplasmic values of these ions. Incubation in either carbachol (180 mumol/l, 4 min) or ouabain (1 mmol/1, 60 min) significantly increased the intracellular sodium and decreased the intracellular potassium concentrations. Neither substance materially altered the intracellular chloride concentration. The data obtained are compared and contrasted to those obtained in mammalian sympathetic neurones using chemical analysis and ion-sensitive microelectrodes.

Intracellular free sodium and potassium, post-carbachol hyperpolarization, and extracellular potassium-undershoot in rat sympathetic neurones

Double-barrelled ion-sensitive microelectrodes were used to record the free intracellular Na+- and K+-concentrations [( Na+]i, [K+]i) and to determine their relation to changes in membrane potential and extracellular K+ [( K+]e) in rat sympathetic ganglia. The application of 50 mumol/l carbachol resulted in an elevation of [K+]e followed by a post-carbachol [K+]e-undershoot. The membrane depolarization of the sympathetic neurones was associated with an increase in [Na+]i and a decrease in [K+]i. A membrane hyperpolarization and a recovery of [K+]i and [Na+]i to their baseline levels were observed during the [K+]e-undershoot. The time course of the [K+]e-undershoot correlated exactly with the duration of the rise in [Na+]i and decrease of [K+]i. No K+-reuptake occurred in the presence of ouabain. These data confirm, by direct measurements of intracellular ion concentration changes, the contribution of the Na+,K+-pump to the post-carbachol membrane hyperpolarization and [K+]e-undershoot.

Extracellular K+ concentration during electrical stimulation of rat isolated sympathetic ganglia, vagus and optic nerves

Recordings of extracellular potassium concentration ( [K+]e) were made in rat isolated sympathetic ganglia, vagus and optic nerves using ion sensitive microelectrodes. Repetitive orthodromic stimulation of ganglia resulted in [K+]e increases of up to 7 mmol/l above resting level (6 mmol/l), which were followed by post-stimulus undershoots. Activation of vagal A and B fibres did not significantly alter [K+]e but C-fibre activity induced rises of up to 5 mmol/l. Repetitive stimulation of the predominantly myelinated optic nerve resulted in [K+]e rises of up to 2.5 mmol/l. In the ganglion and vagus nerve, application of ouabain (30-1000 mumol/l) led to a raised baseline [K+]e concentration, an increase in the peak achieved during stimulation and a reduced undershoot amplitude. The amplitude of the undershoot in normal solution was shown to be dependent on the duration of the preceding stimulation period as well as the amplitude of the preceding [K+]e rise. In ganglia and vagus nerves, bath application of gamma-aminobutyric acid (10-100 mumol/l) and carbachol (10-100 mumol/l) also elevated [K+]e. It is concluded that repetitive activity in rat peripheral and central nerve fibres leads to significant changes in extracellular K+ ion-concentration and that the restoration of these levels is strongly dependent on the intact activity of the membrane Na+/K+ pump.

Slow inhibitory postsynaptic potentials and hyperpolarization evoked by noradrenaline in the neurones of mammalian sympathetic ganglion

Slow IPSPs evoked in the neurones of rabbit isolated superior cervical ganglion by repetitive orthodromic stimulation, and a response evoked in the neurones of this ganglion by perfusion of noradrenaline, were studied using intracellular microelectrodes. Slow IPSPs were observed in 36% of neurones studied, and when investigated after treatment with D-tubocurarine and neostigmine, had a mean amplitude of 4.4 +/- 0.2 mV (mean +/- S.E.) and duration of 5 sec to 1.5 min. Two types of slow IPSPs occurring in different neurones were found. The slow IPSP of the first type was followed by a decrease in cell input resistance, was increased by depolarization and decreased by hyperpolarization of the membrane, with the reversal potential, if estimated by extrapolation method, equal to -77.8 +/- 3.3 mV. The slow IPSP of the second type was not followed by any change in cell input resistance, was increased by hyperpolarization and decreased by depolarization. The slow IPSP of the second type was reversibly blocked by phentolamine (1.4 X 10(-4) M). Noradrenaline (1 X 10(-4) M) evoked hyperpolarization or hyperpolarization followed by depolarization in 55% of the neurones studied. Hyperpolarization evoked by noradrenaline had a mean amplitude to 5.0 +/- 0.2 mV, was not followed by any change in cell input resistance, was reversibly blocked by phentolamine (1.4 X 10(-4) M), and was decreased by both depolarization and hyperpolarization of the cell membrane. It has been concluded that there are two groups of neurones in superior cervical ganglia, different with respect to the ionic mechanisms underlying the slow IPSP. In the first group of neurones the slow IPSP is probably due to an increase in potassium permeability of the membrane. The ionic mechanisms underlying the slow IPSP in the second group of neurones of noradrenaline-induced hyperpolarization remain unclear.

Intracellular pH and the distribution of weak acids and bases in isolated rat superior cervical ganglia

1. The steady-state intracellular/extracellular concentration ratios (Ci/Co) of a number of radiolabelled weak bases in isolated rat superior cervical ganglia were measured. 2. Observed values for Ci/Co (mean +/- S.E. of mean) were [3H]nicotine, 6.17 +/- 0.12; [14C]morphine, 6.08 +/- 0.14 [3H]atropine, 7.10 +/- 0.16; [14C]trimethylamine, 6.73 +/- 0.13; [14C]procaine, 10.13 +/- 0.26. If Ci/Co were determined by the transmembrane pH gradient, the intracellular pH (pHi) appropriate to these concentration gradients lay between 6.4 and 6.6 at an extracellular pH (pHo) of 7.4. 3. the steady-state value of Ci/Co for the weak acid 5,5-dimethyl-2,4-oxazolidinedione (DMO) was 0.87 +/- 0.007. The appropriate pHi was 7.31 +/- 0.003. 4. The difference between the values of pHi calculated from the distribution of the weak bases and of DMO could not be attributed to (i) experimental error, (ii) partial permeation of protonated base, (iii) intracellular binding or carrier-mediated transport of base, (iv) lipid uptake of base or (v) different pK'a inside and outside cells. 5. The difference between the measurements of pHi made with DMO and nicotine (pHDMO-pHnic) was reduced or abolished by uncoupling agents, which act as transmembrane proton carriers. This effect was not reproduced by respiratory inhibitors or by exposure to lactate. 6. pHDMO-pHnic was small (less than 0.1 units) in human erythrocytes, which contain no intracellular organelles, and was exaggerated (1.0 unit) in slices of lipid-depleted brown adipose tissue which contained an abundance of mitochondria. 7. It is concluded that the different values of pHi determined using weak acids and bases arise from the presence of membrane-bound intracellular compartments of differing pH, and that where the use of pH-sensitive micro-electrodes is impracticable, it is desirable to measure pHi with both a weak acid and a weak base unless these can be shown equal over a wide range of pHi values.

gamma-Aminobutyric acid efflux from sympathetic glial cells: effect of 'depolarizing' agents

1. Isolated desheathed rat superior cervical ganglia were incubated in [3H]2,3,-gamma-aminobutyric acid ([3H]GABA) solution (1--10 microM for 2--3 hr) in the presence of 10 microM-amino-oxyacetic acid (AOAA). The subsequent efflux of tritium into a stream of superfused non-radioactive GABA-free Krebs solution at 25 degrees C was measured. 2. In the presence of 10 micrometer-AOAA the mean basal efflux rate coefficient (k0) for exit of tritium into the superfusion fluid was 0.7 x 10(-3) min-1. More than 98% of effluent tritium comprised unchanged [3H]GABA. The rate coefficient showed no correlation with the amount of [3H]GABA previously accumulated by the ganglion. 3. Elevation of [K+]o to greater than 50 mM increased the rate coefficient for [3H]GABA release by up to four times. Changes in efflux rate were not correlated with osmotic changes, and persisted after re-accumulation of effluent [3H]GABA by the inward carrier was inhibited. The effect of alkali metal cations diminished in the order Rb+ greater than K+ greater than Cs+Li+. Effects of K+ solutions were not reduced by omitting Ca2+ ions, with or without the addition of Mg2+. 4. Application of electrical pulses (0.1--1 msec duration, 1--10 Hz, 4 min trains) to the ganglion soma or to the preganglionic nerve trunk also raised k0. This effect declined with repeated stimulus trains, without an accompanying diminution in the response to K+. Responses to electrical stimulation were not reduced by amethocaine (300 microM), tetrodotoxin (3 microM) or raised [Mg2+i1 (0 mM-[Ca2+]/30 mM-[Mg3+]). Separate local superfusion of the pre- and post-ganglionic nerve trunks and of the ganglion soma showed that the response to electrical stimulation was localized to the vicinity of the stimulus and was not propagated along the nerve trunks or across the synapses. 5. Electrical recording from impaled 'inexcitable' cells (presumed to be neuroglial cells (Appendix)) indicated that the quantities of K+ ion accumulating during repetitive nerve stimulation are insufficient to stimulate the release of GABA from the glial cells. No physiological role for the release process in modulating neuronal excitability could be adduced.

Discrepancies in the extracellular space of sympathetic ganglia measured using different isotopes of mannitol and sucrose

The extracellular space of rat superior cervical ganglia in vitro was measured using mannitol and sucrose labelled with tritium and carbon-14. The volumes of distribution of the 3H-labelled derivatives, especially [3H]mannitol, exceeded those of the 14C-derivatives. The divergence increased with increasing lengths of incubation. Thus, after 30 min incubation, 'spaces' (ml . g-1) were: [14Cu]mannitol, 0.407; [3H]mannitol 0.447; [14C]mannitol, 0.458; [3H]mannitol, 0.645; [14C]sucrose, 0.430; [3H]sucrose, 0.497. Using thin layer chromatography, it was shown that an average of 22% of the label in ganglia incubated for 120 min with [3H]mannitol, but only 4% with [14C]mannitol, was not associated with the parent compound. Both [3H]- and [14C]sucrose appeared to be metabolized by 11%. It is concluded that mannitol and sucrose can be metabolized in isolated ganglia and that this may lead to substantial errors in estimating the extracellular space, particularly when [3H]markers are used.

[3H]gamma-Aminobutyric acid uptake into neuroglial cells of rat superior cervical sympathetic ganglia

1. The influx of [3H]gamma-aminobutyric acid ([3H]GABA) into isolated rat superior cervical ganglia has been measured by radioassay, supplemented by autoradiography. Ganglia were incubated in oxygenated Krebs solution at 25 degrees C, containing 10 microM-amino-oxyacetic acid. Under these conditions more than 95% of accumulated tritium was unmetabolized [3H]GABA. 2. Ganglionic radioactivity increased linearly with incubation time, to yield an intracellular fluid/extracellular fluid concentration ratio (Ci/Co) of about 200 after 6 hr in 0.5 microM-external [3H]GABA. 3. Uptake showed saturation with an apparent transport constant (KT) of 6.8 microM and maximum influx velocity (Jmaxi) of 7 mumole 1. cell fluid-1- min-1. 4. The influx rate at Co = 0.5 microM was unaltered by raising intracellular GABA from 0.2 to 1 mM. 5. Influx velocity increased with temperature (5--35 degrees C) in a monotonic manner with an apparent activation energy of 14 kcal mole-1. 6. Concentrative uptake was depressed by reducing external [Na+] with ouabain, by raising [K+]o above 20 mM, or by removing external Cl-. Uptake was not particularly sensitive to Ca2+ or Mg2+ ions. 7. Utake of [3H]GABA (0.5 microM) was inhibited by beta-guanidinopropionic acid (apparent KI, 28 microM), beta-alanine (KI, 55 microM), gamma-amino-beta-hydroxybutyric acid (KI, 220 microM), beta-amino-n-butyric acid (KI, 708 microM), 3-aminopropanesulphonic acid (KI, 832 microM) and taurine (KI greater than 1 mM). Uptake was not depressed by 1 mM-glycine, alpha-alanine, leucine, serine, methionine or alpha-amino-iso-butyric acid. 8. Radioactively labelled methionine, leucine, glycine, serine, beta-alanine and taurine (concentrations less than or equal to 5 microM) were also taken up by ganglia. Of these, only uptake of beta-alanine and taurine were significantly depressed by 1 mM-GABA. 9. Autoradiographs confirmed that [3H]GABA and [3H] beta-alanine were taken up predominantly into extraneuronal sites (presumed to be neuroglial cells). Methionine, leucine, glycine and serine showed preferential accumulation in neurones. Neuronal uptake of leucine was not prevented by inhibiting protein synthesis. 10. Calculations of net fluxes from unidirectional tracer fluxes suggest that the sympathetic glial cells are capable of promoting net uptake of GABA at external concentrations above 1 microM.

Calcium-dependent potentials in the mammalian sympathetic neurone

1. Intracellular recordings from post-ganglionic neurones of the rat superior cervical ganglion revealed two non-synaptic potentials dependent upon Ca2+, a hyperpolarizing afterpotential (h.a.p.) and a tetrodotoxin (TTX)-insensitive spike. 2. The h.a.p. followed regeneration discharge of the membrane potential in normal and TTX-containing Locke solution. 3. The h.a.p. appeared to arise from an increased K+ conductance because it was associated with a decrease in input resistance, reversed at -90 mV, and was proportional in magnitude to the extracellular K+ concentration. 4. Tetraethylammonium (TEA) and 4-aminopyridine (4-AP) apparently antagonized a voltage-sensitive K+ conductance because they broadened the action potential. However, these substances reduced only slightly the peak amplitude and earliest phases of the h.a.p. 5. The TTX-insensitive spike was most apparent when TEA was present and was invariably followed by an h.a.p. with a magnitude proportional to that of the spike. 6. The magnitude of the h.a.p. and the TTX-insensitive spike was directly proportional to the external Ca2+ concentration and was antagonized by Co2+ and Mn2+ in a dose-dependent fashion. 7. In normal Locke solution, Ba2+ antagonized the h.a.p. and allowed the neurone to sustain discharge during prolonged depolarization. In Locke solution containing TTX and TEA, Ba2+ reduced the magnitude of the h.a.p. but greatly increased the duration of the TTX-insensitive spike. 8. The h.a.p. was not significantly affected by altering external Cl- concentration and the TTX-insensitive spike was not reduced by altering external Na+ concentration. 9. It is concluded that the post-ganglionic neurone supports a regenerative Ca2+ conductance mechanism which in turn triggers an increased K+ conductance. The h.a.p. appears to result from outward K+ current in both a Ca2+ and voltage-dependent fashion.

Effects of nicotine and hexamethonium on discharges in the stellate ganglion

The effects of nicotine and hexamethonium on postganglionic discharges elicited by tetanic preganglionic stimulation or muscarinic agonists were observed on the isolated hamster stellate ganglion. The amplitude and duration of the afterdischarges from tetanic preganglionic stimulation in hexamethonium (10-3 M) were smaller than the amplitude and duration of the afterdischarges in nicotine (10-3 M). Also, hexamethonium decreased the amplitude and duration of the afterdischarges from repetitive stimulation in the presence of nicotine. The mechanism of these effects was explored. After application of nicotine for 30 min, the discharges from McN-A-343, a muscarinic agonist, were the same as before the nicotine. Hexamethonium did not reverse the block of the single evoked potential by nicotine. The potentials during a train in the presence of hexamethonium plus atropine were the same as the potentials during a train in the presence of nicotine plus atropine. Hexamethonium did depress the McN-A-343 discharges in the presence of nicotine and also in the control solution. These results indicate that hexamethonium has a direct depressant effect on the muscarinic synaptic membrane.

Movements of radioactive potassium in isolated rat ganglia

1. Isolated rat superior cervical ganglia were continuously superfused with (42)K (or (86)Rb) solution and the amount of radioactivity taken up was monitored using scintillation counting.2. Entry of (42)K into the ganglia could be resolved into two components, one amounting to 83% of the total (42)K uptake, with a rate constant of 0.015 min(-1), and the other of 17% of the total, with a rate constant of 0.15 min(-1).3. With 6 mM-K in the bathing solution, the equilibrium uptake of (42)K after 4 hr corresponded to an intracellular concentration of 147 mM-K. Changes in the K concentration of the bathing solution (0.5-20 mM) had little effect on this value.4. Carbachol or nicotine caused a rapid net loss of (42)K. (42)K was recaptured on washing out the depolarizing agents, with a rate constant of about 0.3 min(-1). This recapture rate was slowed by ouabain, dinitrophenol, cyanide, mersalyl and by reducing the K concentration in the bathing solution.5. Efflux of (42)K from preloaded ganglia occurred with a rate constant of 0.017 min(-1). This rate was increased about sixfold by 180 muM carbachol in 6 mM-K but not in 150 mM-K suggesting that the increase in efflux was mainly a consequence of the depolarization caused by carbachol.6. (86)Rb fluxes and the effects of carbachol thereon were similar.

Post-tetanic spontaneous spike activity in rat sympathetic neurons exposed to low potassium ion concentration

Preganglionic tetanic stimulation (30 sec at 50/sec) of rat superior cervical ganglia, performed in the presence of reduced external potassium concentration (0-1 mM), is followed by a long-lasting postganglionic afterdischarge which fails to appear if stimulation is repeated in normal (5.6 mM) postassium solution. Intracellular recordings revealed that tetanus is followed by 15-30 mV membrane hyperpolarization when the neuron is exposed to normal concentrations of potassium. Conversely, after the ganglion is soaked in low potassium, stimulation results in long-lasting depolarization of the nerve cell with the consequent appearance of spontaneous spikes. This effect is reversed on returing to normal external potassium. Spontaneous activity also occurs after antidromic activation of the cell. It is suggested that tetanus causes sodium loading of the neuron, which leads to stimulation of an electrogenic sodium pump. If potassium is available, the membrane will hyperpolarize, whereas depolarization and pacemaker activity ensues if external potassium is removed. The electrogenic sodium pump thus endows. the rat sympathetic neuron with a mechanism which enables it excitability to be controlled.

A note on the effect of dithiothreitol (DTT) on the depolarization of isolated sympathetic ganglia by carbachol and bromo-acetylcholine

The S-S reducing agent, dithiothreitol (DTT) altered the properties of nicotinic receptors in rat superior cervical ganglia such that (i) carbachol became less active as a depolarizing agent and (ii) bromo-acetylcholine produced an irreversible depolarization. The latter was temporarily annulled by hexamethonium (which retained antagonist properties), but returned when hexamethonium was removed. It is concluded that ganglionic nicotinic receptors might be quite similar to those for monoquaternary agonists in leech dorsal muscle.

Actions of gamma-aminobutyric acid on sympathetic ganglion cells

1. Responses of single ganglion cells in the isolated rat superior cervical ganglion to gamma-aminobutyric acid (GABA) applied via the bathing medium were recorded using intracellular micro-electrodes. 2. GABA produced a large fall in cell input resistance, frequently to immeasurable levels. In thirteen cells showing a modest response to 100 muM GABA, input resistance fell from 50-5 +/-9-5 to 15.9 +/- 3-2 Momega (means +/- S.E. of mean). After correction for resistance leaks introduced by the impaling electrode, the resting membrane resistance Rm and the resistance of the GABA-shunt Rg in these cells were calculated to be 79-3 +/- 16-6 and 35-0 +/- 9-5 Momega respectively. 3. Cells with recorded resting membrane potentials greater than -42 mV were depolarized by GABA; at resting potential less than -42 mV they were hyperpolarized...

Movements of labelled sodium ions in isolated rat superior cervical ganglia

1. Isolated rat superior cervical ganglia were incubated in Krebs solution containing (24)Na and carbachol for 4 min at 25 degrees C. They were then washed at 3 degrees C for 15 min to remove extracellular (24)Na and the efflux of residual intracellular (24)Na stimulated by warming to 25 degrees C.2. During the 15 min wash at 3 degrees C desaturation curves became exponential with a rate constant of 0.012 +/- 0.001 min(-1) (n = 24). This was assumed to represent loss of intracellular (24)Na, and initial uptake of (24)Na was calculated therefrom by back-extrapolation to zero wash-time. After 4 min in (24)Na + 180 muM carbachol intracellular [(24)Na] so calculated was 61.6 +/- 3.1 mM (n = 18), representing 83% labelling of intracellular Na. In the absence of carbachol intracellular [(24)Na] was 10.0 +/- 0.5 mM, representing 49% labelling. Extracellular Na was labelled by > 90% after 4 min in (24)Na. The apparent rate constant for washout of extracellular (24)Na was 0.6 min(-1) at 3 degrees C and 0.95 min(-1) at 25 degrees C.3. The loss of the residual intracellular (24)Na during temperature stimulation was interpreted quantitatively in terms of an exponential decline of the bulk of intracellular (24)Na with an extrusion rate constant of 0.39 +/- 0.1 min(-1) (n = 18), efflux being delayed by passage through the extracellular space with an effective rate constant of 0.8-1.2 min(-1).4. The peak rate constant (k(C)) for the desaturation curve at 25 degrees C was 0.35 +/- 0.01 min(-1). An Arrhenius plot of log k(C)/T degrees K(-1) yielded a two-stage linear regression with a transition at 20 degrees C. Activation energies of 8 and 31 kcal. mole(-1) were calculated above and below this transition respectively.5. Omission of K from the 25 degrees C temperature-stimulating solution reduced k(C) by 62%. The K-sensitive component of extrusion rate constant was a hyperbolic function of [K](e) with half-saturation at 5.6 mM-[K](e) and maximum k(C) of 0.58 min(-1).6. Cyanide (2 mM), 2,4-dinitrophenol (1 mM) and ouabain (1.4 mM) reduced k(C) by 50-90%. The half-maximally inhibiting concentration of ouabain was about 60 muM.7. Substitution of sucrose, Li or choline for external Na did not reduce the extrusion rate of (24)Na in either 6 mM-[K](e) or 0 mM-[K](e). Li stimulated (24)Na extrusion in Na-free, K-free solution.8. The properties of the ganglionic Na pump deduced from rates of temperature-stimulated (24)Na extrusion accord with the view that the ganglion hyperpolarization observed after Na loading by exposure to nicotinic depolarizing agents results from electrogenic Na extrusion. A comparable hyperpolarization is observed after temperature stimulation following Na loading.