Movements of labelled sodium ions in isolated rat superior cervical ganglia

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.

Regulation of the subthreshold chloride conductance in the rat sympathetic neuron

The mechanisms that control chloride conductance (gCl) in the rat sympathetic neuron have been studied by the two-electrode voltage-clamp technique in mature, intact superior cervical ganglia in vitro. In addition to voltage dependence in the membrane potential range -120/-50 mV, gCl displays time- and activity-dependent regulation (sensitization). The resting membrane potential is governed by voltage-dependent gK and gCl, which determine values of cell input conductance ranging from 7 to 18 nS (full deactivation) to an upper value of about 130 nS (full activation and maximal gCl sensitization). The quiescent neuron, held at constant membrane potential, spontaneously and gradually moved from a low- to a high-conductance status. An increase (about 40 nS) in gCl accounted for this phenomenon, which could be prevented by imposing intermittent hyperpolarizing episodes. Following spike firing, gCl increased by 20-33 nS, independent of the cell conductance value preceding tetanization, and thereafter decayed to the pre-stimulus level within 5 min. Intracellular sodium depletion and its successive ionophoretic restoration moved the neuron from a stable low-conductance state to maximum gCl sensitization, pointing to a link between gCl sensitization and [Na+]i. The dependence of gCl build-up on [Na+]i and the time-course of such Na+-related modulation have been examined: gCl sensitization was absent at 0 [Na+]i, was well developed (20 nS) at 15 mM and tended towards a saturating value of 60 nS for higher [Na+]i. Sensitization was transient in response to neuron activity. In the silent neuron, sensitization of gCl shifted membrane potential over a range of about 15 mV.

Effects of Corticotrophin Releasing Factor, Muscarine and Somatostatin on Rubidium and Potassium Efflux from Mouse AtT-20 Pituitary Cells

Effects of secretagogues and anti-secretagogues of ACTH secretion on K+ permeability in the clonal pituitary cell line AtT-20 were measured by recording 86Rb or 42K efflux. Efflux was accelerated by the secretagogues K+, corticotrophin, forskolin, isoprenaline, and the Ca-ionophore A23187. Efflux was reduced by the inhibitors somatostatin, muscarine, and oxotremorine, or by removing external Ca. Efflux was also reduced by the K+-channel blocking compound d-tubocurarine but not by tetraethylammonium. Muscarine, oxotremorine, somatostatin, and 0 Ca2+ also reduced intracellular Ca2+ measured by quin-2 fluorescence. It is suggested that most of the resting 86Rb or 42K efflux measured under these conditions occurs via tubocurarine-sensitive Ca2+-dependent K+-channels, and that changes in efflux rate produced by secretagogues or anti-secretagogues are secondary to changes in intracellular Ca2+.

Origin of 5-hydroxytryptamine-induced hyperpolarization of the rat superior cervical ganglion and vagus nerve

1 5-Hydroxytryptamine (5-HT)-induced membrane potential changes were recorded extracellularly from rat superior cervical ganglia (SCG) and cervical vagus nerves in vitro. 2 On the SCG, low concentrations of 5-HT (1 X 10(-8)-3 X 10(-7) M) induced concentration-related hyperpolarization responses. Higher concentrations of 5-HT (1 X 10(-6) 1 X 10(-4) M) induced complex responses which typically consisted of an initial hyperpolarization, followed by a depolarization and subsequent after-hyperpolarization. The depolarization, but not the initial hyperpolarization, was blocked by metoclopramide (3 X 10(-5) M), quipazine (1 X 10(-6) M) or MDL 72222 (1 X 10(-5) M). 3 5-HT-induced hyperpolarization of the SCG was potentiated when the amount of calcium chloride added to the superfusion medium was reduced from 2.5 to 0.15 mmol l-1. Hyperpolarization responses recorded from SCG preparations superfused with this low-calcium medium were unaffected by the substitution of lithium chloride for sodium chloride and were potentiated by the omission of potassium ions. Ouabain (1 X 10(-3) M) abolished both the hyperpolarization and the depolarization induced by 5-HT. 4 On the vagus nerve, 5-HT (1 X 10(-7) - 3 X 10(-5)M) did not induce initial hyperpolarization in either normal or low-calcium Krebs-Henseleit medium. However, in the latter solution only, depolarization responses induced by 5-HT at concentrations of 1 X 10(-6)M or greater were followed by hyperpolarization. Both the depolarization and the post-5-HT hyperpolarization were blocked by metoclopramide (3 X 10(-5)M) but were unaffected by spiperone (1 X 10(-7)M). 5 On the vagus nerve, post-5-HT hyperpolarization responses were selectively and reversibly inhibited by ouabain, and by superfusion with Krebs-Henseleit medium that was either potassium-free or contained lithium chloride in place of sodium chloride. 7 These results demonstrate the generation in the rat SCG of a 5-HT-induced hyperpolarization response that is not mediated through 5-HT3 receptors and is unlikely to be a consequence of depolarization. In contrast, on the rat vagus nerve, the post-5-HT hyperpolarization observed in the present study had the characteristics expected of depolarization-dependent activation of a sodium ion pump.

Influence of 5-hydroxytryptamine uptake on the apparent 5-hydroxytryptamine antagonist potency of metoclopramide in the rat isolated superior cervical ganglion

Metoclopramide, 1 X 10(-6) -1 X 10(-4) M, was found to behave as a reversible, competitive antagonist of 5-hydroxytryptamine (5-HT)-induced depolarization of the rat isolated vagus nerve (VN) and superior cervical ganglion (SCG). The pKB values were 6.60 (+/- 0.04) and 5.74 (+/- 0.07), respectively. The possibility that this apparent difference in potency was due to saturable 5-HT uptake was investigated. The SCG, but not the VN, accumulated tritium-labelled 5-HT via a saturable, sodium- and temperature-dependent mechanism. Ganglionic 5-HT uptake was blocked by desmethylimipramine (IC50 1.4 X 10(-6)M), chlorimipramine (8.7 X 10(-9) M), zimelidine (1.5 X 10(-7) M), paroxetine (4.3 X 10(-8) M) and citalopram (6.2 X 10(-8) M). The 5-HT uptake inhibitor paroxetine, 1 X 10(-6) M, did not modify the apparent 5-HT antagonist potency of metoclopramide on the VN, but raised the pKB obtained against 5-HT on the SCG from 5.74 (+/- 0.07) to 6.25 (+/- 0.03). It is suggested that the observed difference in the potency of metoclopramide as a 5-HT antagonist on the rat VN and SCG was due to saturable 5-HT uptake in the latter preparation. The results do not support a difference in the 5-HT receptors mediating depolarization on the VN and SCG.

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.

Muscarinic receptors in rat sympathetic ganglia

1 Potential changes in isolated superior cervical ganglia of the rat produced by muscarinic-receptor agonists were recorded by an extracellular ;air-gap' method.2 Muscarinic agonists produced a delayed low-amplitude ganglion depolarization, frequently preceded by a hyperpolarization. Potentials were enhanced by reducing [K(+)](o) or [Ca(2+)](o).3 Mean ED(50) values (muM) for depolarization at 25 degrees C were: oxotremorine 0.004, methylfurmethide 0.11, (+/-)-muscarine 0.24, furmethide 1.56, pilocarpine 4.81 and AHR-602 (N-benzylpyrrolidylacetate methobromide) 10.8. Responses produced by oxotremorine, pilocarpine and AHR-602 showed some characteristics of ;partial agonism'. ED(50) values (muM) for choline esters (measured in the presence of 2.5 mM hexamethonium) were: acetylcholine 3.2, methacholine 59 and bethanechol 78.4 Responses to muscarine were antagonized by hyoscine (K(I) 0.49 nM) atropine (K(I) 0.24 nM) methylscopolamine (K(I) 0.09 nM) lachesine (K(I) 0.15 nM) and (weakly) by hexamethonium (K(I) 0.2 mM). Propylbenzilylcholine mustard produced irreversible antagonism with an apparent onset rate constant of 2 x 10(5) M(-1)S(-1).5 Depolarization was accompanied by facilitation of submaximal ganglionic transmission.6 Muscarine (1 to 100 muM) initially reduced, then increased, the rate of (86)Rb(+)-efflux from isolated ganglia at both 6 and 120 mM [K(+)](o). These effects were reduced by 1 muM hyoscine.7 No consistent change in the amounts of cyclic 3',5'-guanosine monophosphate in isolated ganglia accompanying muscarinic depolarization could be detected.8 Mean against ED(50) values (muM) for contracting the rat isolated ileum were: oxotremorine 0.012, methylfurmethide 0.29, (+/-)-muscarine 0.48, pilocarpine 7.8 and AHR-602 9.9. Mean antagonist K(I) values (nM) were: hyoscine 0.17, atropine 0.34 and lachesine 0.27.9 It is concluded that ganglionic muscarinic receptors are quite similar to ileal receptors in terms of agonist ED(50) and antagonist K(I) values, and that the major difference between them lies in the greater ;efficacy' of certain agonists (pilocarpine, AHR-602 and McN-A-343) on the ganglion.

The role of sodium influx mediated by nicotinic receptors as an initial event in trans-synaptic induction of tyrosine hydroxylase in adrenergic neurons

1. Isolated superior cervical ganglia of the rat were incubated for 2--30 min (37 degrees C) in Krebs' solution or tissue culture medium (BGJb) containing 22Na and then washed for 30 min in ice-cold 22Na-free Krebs' solution (to clear extracellular space). The radioactivity remaining in the ganglia was taken as a measure of 22Na influx into the intracellular compartment of the ganglion. 2. Addition of cholinomimetics (100 microM nicotine or 100 microM carbachol) to the incubation led to an increase in 22Na influx. This increase reached maximal values after 10 min of incubation; it was more pronounced after incubation in Krebs' solution than in BGJb medium. 3. While chlorisondamine (3 microM) or dopamine (100 microM) greatly reduced the carbachol-induced 22Na influx, tetrodotoxin (2 microM) did not have any effect. 4. In ganglia obtained from animals treated with 6-hydroxydopamine in the early postnatal phase (resulting in an extensive destruction of peripheral sympathetic neurons) neither carbachol (100 microM) nor nicotine (100 microM) produced an increase in 22Na influx demonstrating that the intraneuronal compartment is responsible for this enhanced influx. 5. The effects of dopamine, chlorisondamine and tetrodotoxin on the carbachol-induced 22Na uptake into superior cervical ganglia are similar to their effects on carbachol-mediated induction of tyrosine hydroxylase in superior cervical ganglia kept in tissue culture (Thoenen and Otten 1977b). It is concluded that the induction of tyrosine hydroxylase via nicotinic receptors is closely linked to the enhanced sodium influx into the adrenergic neurons mediated by the same receptors.

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.

[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.

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.

The effect of hypoxia on evoked potentials in the in vitro hippocampus

1. We have studied the effect of hypoxia on transmission of electrical activity between the perforant path and the dentate granule cells in the in vitro guinea-pig hippocampus. 2. Hypoxia abolishes the evoked field potential within about 3 min, a time similar to that occurring in vivo (Andersen, 1960). 3. The evoked potential is very rapidly abolished by extracellular K+ concentrations greater than 13.4 mM; it is abolished by ouabain concentrations greater than 10(-5) M. The rate at which it is abolished increases with increasing ouabain concentrations: concentrations of about 8 x 10(-5) M abolish the evoked potential at the same rate as does hypoxia. 4. The time required to abolish the evoked potential during hypoxia decreases markedly as the extracellular K+ concentration is elevated from 4.4 to 13.4 mM. The time to abolish the potential during hypoxia is also decreased by partial replacement of the Cl- in the bathing medium by less permeant anions and by the presence of a low (10(-7) M) concentration of ouabain. All these are conditions which are expected to depolarize neuronal cell membranes. None of these alterations in the perfusing medium affect the concentrations of ATP or creatine phosphate in the hippocampal slice. Increasing extracellular Mg2+/Ca2+ to levels which reduce the evoked response by about 50% has no effect upon the time required to abolish the evoked potential during hypoxia at any concentration of extracellular [K+]. 5. These results provide evidence that the basis for the hypoxic block of the evoked potential is a depolarization of neuronal processes. They are consistent with the hypothesis that this depolarization is a result of inhibition of the Na+/K+ pump.

Electrical properties of neurones in the olfactory cortex slice in vitro

1. Slices of guinea-pig olfactory cortex were maintained in vitro. Electrical properties of neurones in the prepyriform region were studied using single high resistance glass micro-electrodes filled with potassium acetate, connected to a resistance-compensating circuit to allow passage of current through the electrode. 2. Neurones showed a high, stable resting membrane potential (75.4 +/- 2.7 mV, mean +/- S.D.; n = 47). Input resistance measured with small depolarizing currents varied over a range of 9-280 Momega. The time constant for decay of depolarizing potentials was 19.4 +/- 7.5 msec (mean +/- S.D.).. 4. Depolarization produced repetitive action potentials (maximum frequency of 85 Hz) having peak amplitudes of +16 to +47 mV. The action potential was followed by a depolarizing after potential of about 20 mV positive to the membrane potential. 5. In these and other respects, the prepyriform neurones appear to behave like most other neurones in the mammalian brain, after allowing for the more stable recording conditions in this preparation.

Determination of transmitter function by neuronal activity

The role of neuronal activity in the determination of transmitter function was studied in cultures of dissociated sympathetic neurons from newborn rat superior cervical ganglia. Cholinergic and adrenergic differentiation were assayed by incubating the cultures with radioactive choline and tyrosine and determining the rate of synthesis and accumulation of labelled acetylcholine and catecholamines. As in previous studies, pure neuronal cultures grown in control medium displayed much lower ratios of acetylcholine synthesis to catecholamine synthesis than did sister cultures grown in medium previously conditioned by incubation on appropriate nonneuronal cells (conditioned medium). However, here we report that neurons treated with the depolarizing agents elevated K(+) or veratridine, or stimulated directly with electrical current, either before or during application of conditioned medium, displayed up to 300-fold lower acetylcholine/catecholamine ratios than they would have without depolarization, and thus remained primarily adrenergic. Elevated K(+) and veratridine produced this effect on cholinergic differentiation without significantly altering neuronal survival. Because depolarization causes Ca(2+) entry in a number of cell types, the effects of several Ca(2+) agonists and antagonists were investigated. In the presence of the Ca(2+) antagonists D600 or Mg(2+), K(+) did not prevent the induction of cholinergic properties by conditioned medium. Thus depolarization, either steady or accompanying activity, is one of the factors determining whether cultured sympathetic neurons become adrenergic or cholinergic, and this effect may be mediated by Ca(2+).

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.

Indirect effects of amino-acids on sympathetic ganglion cells mediated through the release of gamma-aminobutyric acid from glial cells

1 All experiments were performed on rat isolated desheathed superior cervical ganglia maintained in Krebs solution containing amino-oxyacetic acid (10 muM) at 25 degrees C. 2 Influx rates of gamma-amino-n-butyric acid (GABA) were measured by incubating ganglia in 0.5 muM [3H]-GABA for 30 minutes. Influx was inhibited by 50% on adding 14.3 muM unlabelled GABA, 59.2 muM beta-alanine (BALA) or 424 muM beta-amino-n-butyric acid (BABA). 3 Efflux of [3H]-GABA into non-radioactive solution superfused over ganglia previously incubated for 60 min in 1 muM [3H]-GABA was measured. The mean resting efflux rate coefficient (k) was 0.64 +/- 0.05 X 10(-3) min-1. Addition of high concentrations of unlabelled GABA, BABA or BALA to the superfusing solution increased k by (maximally) 3.6-4.3 times; half-maximal increases occurred at the following concentrations: GABA, 16 muM; BALA, 85 muM; BABA, 606 muM. Replacement of external Na+ with Li+ or TRIS increased the resting value of k and inhibited acceleration by external amino acids. Prior incubation in 1 muM [3H]-GABA with 1 mM unlabelled GABA increased resting k 1.5 times, but did not alter the peak rate coefficient produced by external amino acids. 4 Neuronal depolarization produced by the amino acids was measured with surface electrodes. Pre-incubation in 1 mM GABA for 60 min potentiated low-amplitude responses to BALA or BABA but not those to GABA or 3-aminopropanesulphonic acid (a potent agonist with low affinity for the GABA carrier). Omission of external Na+ reduced responses to BABA but increased those to GABA. 5 Incubation in 1 mM GABA for 60 min (as required to potentiate BABA or BALA actions) increased the amount of GABA in the tissue from 0.21 to 0.73 mmol/kg wet weight. Autoradiographs in which labelled GABA was used indicated that uptake into neuroglial cells was responsible for this accumulation. 6 It is suggested that: (i) BALA and BABA are substrates for the inward GABA carrier responsible for GABA entry into ganglionic glial cells; (ii) they accelerate efflux by inhibiting carrier-mediated reaccumulation of effluent GABA by the glial cells; (iii) interstitial GABA concentrations are thereby increased to a level capable of depolarizing adjacent neurones; and (iv) this, rather than direct GABA-receptor activation, accounts for the depolarization produced by low concentrations of BALA and BABA. Potentiation of their depolarizing action after pre-incubation in 1 mM GABA is suggested to result from the increased amount of intracellular GABA available for release, and is quantitatively compatible with this increase; inhibition in Na+-free solution is due to their inability to inhibit reaccumulation of GABA under these conditions. 7 A model for the action of carrier substrates is described in an Appendix. Calculations based thereon yield increments in interstitial GABA concentration in the presence of carrier substrates compatible with those determined experimentally (up to 1 muM at rest or 3.4 muM after pre-incubation in GABA).

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...

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

1. Na and K contents of isolated rat superior cervical ganglia were measured by flame photometry, and intracellular Na and K concentrations ([Na](i) and [K](i)) calculated using Li and (35)SO(4) to determine extracellular space (e.c.s.).2. Resting concentrations after 1-2 hr incubation at 25 degrees C in normal Krebs solution were: [Na](i), 19.8 +/- 0.9 m-mole (kg cell water)(-1); [K](i), 192.7 +/- 2.8 m-mole (kg cell water)(-1) (mean +/- S.E. of mean of thirty-five ganglia). Correction for losses during e.c.s. measurement gave 22 mM [Na](i) and 207 mM [K](i) as probable fresh concentrations.3. Carbachol (180 muM for 4 min) increased [Na](i) by 47.8 +/- 2.9 m-mole (kg cell water)(-1) and decreased [K](i) by 54.6 +/- 4.3 m-mole (kg cell water)(-1). Maximal exchange with carbachol or nicotine (at approximately 1 mM for 4 min) amounted to 80-100 m-mole (kg cell water)(-1). On washing with Krebs solution containing 2.5 mM hexamethonium recovery of ionic concentrations occurred with a rate constant of 0.3-0.4 min(-1).4. Restitution of ganglionic Na and K after carbachol was inhibited by washing with K-free solution, and slowed by ouabain (0.14 mM), cyanide (2 mM) or cooling (Q(10) 2.7 between 17 and 27 degrees C).5. Equilibrium potentials for Na and K (E(Na), E(K)) at rest were calculated to be +49 and -88 mV. At a membrane potential (E(m)) of -70 mV, the permeability ratio P(Na):P(K) was calculated at 0.04:1 (assuming P(Cl):P(K) < 0.1).