Origin of the after-hyperpolarization that follows removal of depolarizing agents from the isolated superior cervical ganglion of the rat

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 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.

The removal of acetylcholine by diffusion at nicotinic synapses in the rat otic ganglion

1. We have examined the clearance of synaptically released acetylcholine in the otic ganglion when acetylcholinesterase was blocked with eserine. 2. Intracellular recordings were made from otic ganglion neurones, in vitro. The decay of the excitatory postsynaptic potential (EPSP), in response to stimulation of afferent fibres, was greatly prolonged in the presence of eserine. Low frequency (0.05-4 Hz) repetitive synaptic stimulation led to a slow depolarization of the postsynaptic cell that persisted throughout the period of stimulation. This slow depolarization was blocked by the nicotinic antagonists mecamylamine (100 microM) or (+)tubocurarine (100 microM), but was unaffected by atropine (1 microM), indicating that the response was due to the activation of nicotinic receptors. 3. Following 2 Hz synaptic stimulation (30 s), the rate of rise of the slow depolarization had a time constant of 3.1 +/- 0.4 s and a peak amplitude of 12 +/- 1 mV. Upon cessation of stimulation, the depolarization decayed to resting levels with a time constant of 18.3 +/- 1.5 s (n = 23). At increasing stimulation frequencies the rate of rise of the depolarization increased. Lowering the probability of release, by adding cadmium to the perfusing solution or by lowering extracellular calcium, slowed the rise time of the response. 4. Both the onset and decay kinetics of the slow depolarization had a low temperature sensitivity, indicating that they reflect diffusional processes. 5. Repetitive stimulation (2 Hz) of the afferent nerve supplying the ganglion, in the presence of eserine, also caused a slow depolarization in cell in which we could not demonstrate a synaptic input. This indicates that synaptically released acetylcholine can spill over onto nearby neurones. 6. We conclude that at parasympathetic synapses, under physiological conditions, transmitter action is terminated by the enzymatic degradation of acetylcholine. When acetylcholinesterase is blocked, acetylcholine accumulates because its removal by diffusion is slow.

Evidence for a sodium-dependent potassium conductance in frog myelinated axon

After blockade of the voltage-dependent potassium conductances by intracellular application of 4-aminopyridine and tetraethylammonium in frog myelinated axons, a set of brief (0.1 ms) intracellular depolarizing pulses or a long (200 ms) depolarizing pulse evoked a train of action potentials. Under both experimental conditions a hyperpolarizing afterpotential appeared (duration 367 ms +/- 34, mean +/- S.E., n = 15). The purpose of this study was to investigate the properties of this hyperpolarizing afterpotential. It was found that the hyperpolarizing afterpotential increases in amplitude with: (1) the number of sodium-dependent action potentials; (2) action potential broadening (following potassium channels blockade); and (3) the level of depolarization during a current step. Application of tetrodotoxin prevented the activation of the hyperpolarizing afterpotential by any of the above stimuli. The hyperpolarizing afterpotential was unaffected by: (1) 8-acetyl-strophanthidin, an agent that poisons the electrogenic pumping in the axon; (2) blocking calcium influx with extracellular 10 mM magnesium or 2 mM manganese; and (3) buffering of the intracellular calcium, using EGTA in the recording microelectrode. Extracellular application of tetraethylammonium, but not 4-aminopyridine, reduced the hyperpolarizing afterpotential. The hyperpolarizing afterpotential reversed at >> -92 mV. Increasing the external potassium concentration from 2 to 10 mM shifted the reversal potential +14.5 mV, indicating that the hyperpolarizing afterpotential is a potassium mediated conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of alpha-adrenoceptors indirectly facilitates sodium pumping in frog motoneurons

The effects of clonidine on Na+ pumping in motoneurons of the isolated frog spinal cord was investigated using sucrose gap recordings from ventral roots. Na+ pump activity, induced in motoneurons either by tetanizing the dorsal root or by rapidly exposing the cord to normal medium following 30 min in K(+)-free Ringer's solution (K(+)-activated hyperpolarizations), was increased by application of clonidine (100 microM). These actions of clonidine were blocked by the preferential alpha 2-adrenergic antagonist yohimbine, but not by alpha 1-adrenergic antagonist prazosin or the beta-blocker propranolol. Clonidine's effects on Na+ pumping appeared to be indirect (presumably via interneurons) because its effects on K(+)-activated hyperpolarizations were reduced by tetrodotoxin (TTX) or high concentrations of Mg2+. This indirect mechanism involved activation of non-NMDA excitatory amino acid receptors. Thus, in the presence of clonidine, CNQX, but not APH, limited the ability of clonidine to enhance K(+)-activated hyperpolarizations. The AMPA receptor may play a role in the process, K(+)-activated hyperpolarizations were augmented by the presence of AMPA; NMDA had no effect. The present results are consistent with the idea that activation of alpha 2-adrenoceptors produces the following: the release of excitatory amino acids from interneurons; the activation of non-NMDA receptors on motoneurons; increased Na+ influx and loading and increased Na+ pump activity.

Characterization of purinoceptors mediating depolarization of rat isolated vagus nerve

1. As part of a broader study to characterize neuronal purinoceptors, the effects of adenosine 5'-triphosphate (ATP) and a range of ATP analogues were investigated on the extracellularly recorded membrane potential of the rat isolated vagus nerve, using a 'grease-gap' technique. 2. ATP evoked depolarization of the rat vagus nerve. The concentration-effect curve to ATP was not monophasic: at the lower concentrations (1 x 10(-5)-1 x 10(-3) M) the curve was shallow (< 50% of the near maximal response to 5-hydroxytryptamine (5-HT)) whilst at higher concentrations the relationship between concentration and amplitude of depolarization was steeper (> 135% of the response to 5-HT at the highest concentration tested, 1 x 10(-2) M). On washout of the high drug concentrations large after-hyperpolarizations were often observed. 3. alpha,beta-methylene ATP (1 x 10(-6)-3 x 10(-4) M), beta,gamma-methylene ATP (1 x 10(-6)-1 x 10(-3) M), and 5'-adenylylimidodiphosphate (beta,gamma-imido ATP; 1 x 10(-6)-1 x 10(-3) M) were all more potent than ATP and produced large depolarizations of the rat vagus nerve at the highest concentrations tested (> 150% of the response to 5-HT). The overall rank order of potency was alpha,beta-methylene ATP > beta,gamma-methylene ATP = beta,gamma-imido ATP > ATP. 4. In contrast, 2-methylthio ATP (1 x 10(-6)-1 x 10(-3) M) produced relatively small depolarizations (< 100% of the response to 5-HT). As was the case with low concentrations of ATP, the concentration-effect curve to 2-methylthio ATP was very shallow. 5. Adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP), adenosine and adenosine 5'-O-(2-thiodiphosphate) (ADP-beta-s; all 1 x 10-6 1 x 10-3M) evoked only small depolarizations of the vagus nerve, amounting to 47 +/- 2.5%, 40.8 +/- 7.8%, 33.7 +/- 3.3% and 62.4 +/- 12.7% of the response to 5-HT, respectively. Uridine 5'-triphosphate (UTP; 1 X 10-6 1 X 10-3M) was inactive.6. The P2 purinoceptor antagonist, suramin (1 x 10-5-M-1 X 10-4 M), antagonized responses to alpha-beta-methylene ATP. The nature of this antagonism was not, however, consistent with simple competitive kinetics between agonist and antagonist. Depolarizations produced by beta,gamma-methylene ATP and beta,gamma-imido ATP were also attenuated by suramin (1 x 10-4 M), but in contrast, suramin had no effect on responses to ADP, 2-methylthio ATP, ADP-beta-S or 5-HT.7. In addition to its antagonist effects, suramin (10-4 M) markedly increased the maximum amplitude of the depolarization produced by ATP.8. It is concluded that a heterogeneous receptor population mediates depolarization of the rat vagus nerve by purine nucleotides. Importantly, the large amplitude depolarizations to alpha,beta-methylene ATP,beta,gamma-methylene ATP and beta,gamma-imido ATP are mediated via receptors that share many characteristics of the classical P2, receptor. In contrast, the relatively small depolarizing effects of ADP, ADP-beta-S and 2-methylthio ATP were suramin-resistant. Although it appears that other purinoceptors are present,these data suggest that the rat vagus nerve may serve as a useful preparation for studying the pharmacology of neuronal P2x receptors.

A component of 5-HT-evoked depolarization of the rat isolated vagus nerve is mediated by a putative 5-HT4 receptor

This study describes a component of 5-HT-evoked depolarization of the rat isolated vagus nerve which was unaffected by the 5-HT3 receptor antagonist ondansetron. A grease-gap extracellular recording technique was used. Ondansetron (10-100 nmol/l) displaced the 5-HT concentration-response curve to the right yielding a pA2 value of 8.6 (8.5-8.8), consistent with 5-HT3 receptor antagonism, and revealing a component of the 5-HT response which was resistant to ondansetron blockade. In the presence of ondansetron (100 nmol/l) the maximum depolarization in the resistant phase was 15.5 (12.6-19.2)% of the initial maximum response to 5-HT and the pEC50 value was 7.0 (6.7-7.3). The mechanism of the ondansetron-resistant component of the 5-HT response resembled a 5-HT4-receptor-effect in being absent in preparations equilibrated with 5-methoxytryptamine (10 mumol/l) and antagonised by ICS 205930 (tropisetron, pA2 6.4). 5-Methoxytryptamine alone was an agonist in the vagus nerve with a maximum response similar to that of the ondansetron resistant phase of the 5-HT response. Similarly renzapride alone evoked small depolarizations of this preparation but antagonized the ondansetron resistant phase of the 5-HT response (pA2 7.3-7.4). These effects of 5-methoxytryptamine and renzapride are also consistent with a 5-HT4 receptor mechanism. Ketanserin (1 mumol/l) and methysergide (1 mumol/l) had little effect on responses to 5-HT. The depolarization evoked by this putative 5-HT4 receptor mechanism was small but prolonged and appears to mask and after-hyperpolarizing phase of the 5-HT response in this tissue.

Acetylcholine receptors in rat cardiac neurones

Membrane potential changes produced by acetylcholine (ACh), and their underlying mechanisms, were studied in neurones of isolated cardiac ganglia of the rat by means of intracellular microelectrodes. Five components of membrane potential change could be detected in cardiac neurones following 1-5 s micro-application of ACh: (i) fast depolarization resulting from an activation of nonselective cationic conductance; (ii) slow depolarization associated with a decreased membrane conductance, presumably for potassium ions; slow hyperpolarization which consisted of (iii) early and (iv) late parts resulting from an activation of calcium-sensitive potassium current and from inhibition of steady-state inward current, respectively; and (v) delayed slow hyperpolarization associated with an increased conductance, most likely for potassium ions. Components (i), (iii) and (iv) persisted in the presence of atropine and were inhibited by nicotinic antagonists. Thus they were due to activation of nicotinic ACh receptors. However, the sensitivity of component (i) to ganglion-blocking agents appeared to be rather low: IC50s for inhibiting (i) were 226 +/- 34.2 microM, 31.2 +/- 4.31 microM and 15.3 +/- 3.27 microM for hexamethonium, d-tubocurarine, and trimetaphan, respectively. Components (ii) and (v) were abolished by atropine (1 microM) and mimicked by muscarine (component (ii) also persisted in d-tubocurarine), hence they resulted from activation of muscarinic ACh receptors. It is concluded that cardiac neurones are endowed with both nicotinic and muscarinic ACh receptors. Their activation leads to membrane depolarization and discharges followed by hyperpolarization and inhibition of discharges.

Bispyridinium (oxime) compounds antagonize the "ganglion blocking" effect of pyridostigmine in isolated superior cervical ganglia of the rat

The "antidotal effectiveness" of several bispyridinium compounds (HGG 12, HGG 65, HGG 70, HI 6, HLö and HLö 12) against the acetylcholinesterase (AChE) inhibitor pyridostigmine was evaluated in isolated superior cervical ganglia of the rat. Compound action potential amplitudes were inhibited by pyridostigmine in a concentration-dependent manner. HI 6 and atropine proved to be the most effective compounds in antagonizing the "ganglion blocking" action of pyridostigmine. Their relative effectiveness (PE value) was 5.4 and 4.2, respectively. All of the six bispyridinium compounds inhibited carbachol-induced, nicotinic, ganglionic surface depolarizations. The antinicotinic potencies of HI 6 and HLö 7 were about one order of magnitude lower (apparent KI values: 294 and 330 mumol/l) than the antinicotinic potencies of HGG 12, HGG 65, HGG 70 and HLö 12 (apparent KI values ranging from 19 to 41 mumol/l). The antinicotinic potencies of the bispyridinium compounds did not correlate with their in vitro protection of synaptic transmission in sympathetic ganglia. Moreover, the effectiveness of atropine points to the importance of antimuscarinic properties of possible "antidotes" for the maintenance of ganglionic transmission in cases of AChE poisoning.

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.

The effects of anaesthetics and high pressure on the responses of the rat superior cervical ganglion in vitro

The effects of helium pressure and of general anaesthetics were studied on the responses of the isolated superior cervical ganglion of the rat, to determine how far these reflected the pressure reversal of anaesthesia seen in vivo. The method of Brown & Marsh (1974) for extracellular recording of surface potentials was adapted for use in a high-pressure chamber. Helium alone, at 130 atm, did not alter the responses of the ganglion to gamma-aminobutyric acid (GABA) but significantly depressed the depolarizing and hyperpolarizing components of the nicotinic responses, and the muscarinic responses. The potentiation of the responses to GABA caused by pentobarbitone was not altered by the application of helium, at 130 atm. This pressure also decreased further the nicotinic responses which were depressed by pentobarbitone. Nitrogen, at 34 atm (the anaesthetic ED50 in vivo) and at 68 atm, significantly decreased the nicotinic responses of the ganglia, and the addition of helium to a total of 130 atm further increased this depression. At pressures of 3.3-68 atm, nitrogen caused small decreases in the responses to GABA. Nitrous oxide at 1.5 atm (the ED50 for loss of righting reflex in mice) and at 3 atm, significantly depressed the responses to GABA and to the nicotinic agonist, but did not alter the responses to methylfurmethide. The effects of nitrous oxide were unaltered when helium was added to a total of 130 atm, although this pressure of helium added alone significantly depressed the cholinergic responses. A mixture of 50% nitrous oxide and 50% oxygen, when added to the pressure chamber, at normal atmospheric pressure, caused transient increases in the responses to GABA. The effects of temperature on GABA responses and on nicotinic responses were very different from those of pressure. Preliminary evidence suggested that raising the temperature may decrease the extent of potentiation of GABA responses by pentobarbitone. The results are discussed in relation to the pressure reversal of anaesthesia in vivo. It was concluded that there was no evidence that the basis of this interaction lay in the potentiation of GABA responses by general anaesthetics, or the depression of cholinergic responses, although the changes seen were not in all cases simply additive. It was considered that effects of general anaesthetics such as the potentiation of GABA may contribute to the effects used to measure general anaesthesia in vivo, such as loss of righting reflex, but may not be related to the non-specific actions which cause anaesthesia.

An intracellular analysis of gamma-aminobutyric-acid-associated ion movements in rat sympathetic neurones

Double-barrelled ion-sensitive micro-electrodes were used to measure the changes of the intracellular activities of Cl-, K+, and Na+ (aiCl, aiK, aiNa) in neurones of isolated rat sympathetic ganglia during the action of gamma-aminobutyric acid (GABA). The membrane potential of some of the neurones was manually 'voltage clamped' by passing current through the reference barrel of the ion-sensitive micro-electrode. This enabled us to convert the normal depolarizing action of GABA into a hyperpolarization. A GABA-induced membrane depolarization was accompanied by a decrease of aiCl, aiK and no change in aiNa, whereas a GABA-induced membrane hyperpolarization resulted in an increase of aiCl, aiK and also no change in aiNa. GABA did not change the free intracellular Ca2+ concentration, as measured with a Ca2+-sensitive micro-electrode, whereas such an effect was seen during the action of carbachol. pH-sensitive electrodes, on the other hand, revealed a small GABA-induced extracellular acidification. The inward pumping of Cl- following the normal, depolarizing action of GABA required the presence of extracellular K+ as well as Na+, whereas CO2/HCO3--free solutions did not influence the uptake process. Furosemide, but not DIDS, blocked the inward pumping of Cl-. In conclusion, our data show that only changes in intracellular activities of K+ and Cl- are associated with the action of GABA. Furthermore, they indicate that a K+/Cl- co-transport, and not a Cl-/HCO3- counter-transport, may be involved in the homoeostatic mechanism which operates to restore the normal transmembrane Cl- distribution after the action of GABA.

Neurophysiology and pharmacology of long-term potentiation in the rat sympathetic ganglion

Brief tetanic stimulation of the preganglionic nerve induced a persistent potentiation of nicotinic synaptic transmission in the rat superior cervical sympathetic ganglion. Quantitative measurements of the post-tetanic increase in synaptic efficacy revealed two distinct time courses. The early, rapidly decaying component, termed post-tetanic potentiation (p.t.p.), had a decay time constant of 2-3 min, as reported elsewhere. The duration of the more persistent component, called long-term potentiation (l.t.p.), was extremely temperature dependent, lasting much longer at 32 degrees C than at 22 degrees C. In half of the experiments performed at 32 degrees C, l.t.p. showed no detectable decay over the course of 1 h or more after a brief tetanic stimulation. Other experiments were conducted at 22 degrees C. The induction of l.t.p. was dependent on the extracellular [Ca2+]. Transient elevation of the extracellular [K+] also produced a long-term enhancement of synaptic efficacy, and this effect was also Ca2+ dependent. The tetani that were effective in inducing l.t.p. (5-20 Hz for 5-20 s) were well within the physiological range of preganglionic activity. The magnitude and time course were related to frequency and duration of stimulation. The occurrence of l.t.p. was restricted to those preganglionic fibres that were tetanically stimulated. This lack of heterosynaptic or generalized effects was demonstrated by splitting the preganglionic nerve into two branches that could be independently tested and conditioned. Physiological activation of muscarinic or nicotinic receptors apparently does not play an essential role in causing ganglionic l.t.p., which is expressed as an enhancement of nicotinic transmission. A muscarinic antagonist (2 microM-atropine) did not block l.t.p. Preganglionic stimulation induced l.t.p. even when a high concentration of a nicotinic antagonist (3 mM-hexamethonium) was present during the tetanic stimulation. Furthermore, bath application of a cholinergic agonist (100-1000 microM-carbachol) could not substitute for tetanic stimulation in provoking l.t.p. Activation of adrenergic receptors also appeared not to play an essential role. Neither a beta-adrenergic antagonist (10 microM-sotolol or 1 microM-propranolol) nor an alpha-adrenergic antagonist (1 microM-phentolamine) had any significant effect on the magnitude or duration of l.t.p. The results indicate that ganglionic l.t.p. is a Ca2+- and temperature-dependent process that can be created independently of the activation of nicotinic, muscarinic or adrenergic receptors.

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.

Examination of the role of the electrogenic sodium pump in the adrenaline-induced hyperpolarization of amphibian neurones

The effect of adrenaline and acetylcholine (ACh) on the membrane potential of Rana pipiens sympathetic ganglia was examined by means of the sucrose gap recording technique. Adrenaline (1-50 microM) consistently produced a hyperpolarization (Adrh) which was not reduced by Ringer solution containing 10 mM-Mn2+, nor by Ringer solution where the Na+ concentration was reduced from 100 to 30 mM. High doses of ACh (10 mM) produced a biphasic response, a depolarization (AChd) followed by an after-hyperpolarization (ACha.h.p.). Ringer solution containing 100 mM-Li+ (rather than 100 mM-Na+) or 10 microM-ouabain blocked the ACha.h.p. and reduced the Adrh. The AChd was essentially unchanged. Ringer solution containing 0.2 mM-K+ (rather than 2 mM-K+) blocked part of the ACha.h.p. whereas the Adrh was enhanced. Ringer solution containing 6 mM-K+ reduced the amplitude of the Adrh. The Adrh and the antidromically evoked action potential after-hyperpolarization (antidromic a.h.p.) reversed polarity at approximately the same membrane potential. These data do not support the hypothesis that the Adrh results from activation of the electrogenic sodium pump. It is tempting to speculate that the response may be generated by an increase in potassium conductance (gK) which is especially sensitive to manipulations which result in sodium pump inhibition.

The role of the electrogenic sodium pump in the glutamate afterhyperpolarization of frog spinal cord

Drug responses of isolated hemisected frog spinal cords were examined by means of the sucrose-gap technique. The glutamate-induced depolarizations (glu-d) of motoneurones (recorded from ventral roots), and primary afferents (recorded from dorsal roots), were followed by an afterhyperpolarization (glu-a.h.). The depolarization induced by DL-homocysteic acid (DLH) was only occasionally followed by an afterhyperpolarization (DLH-a.h.). The glu-a.h. on both roots persisted in the presence of tetrodotoxin (TTX, 0.1-1 microM), or Ringer solution containing 10 mM-Mg2+; 0.1 mM-Ca2+ or 2 mM-Mn2+; 0.2 mM-Ca2+. This indicated that the response was neither due to the release of endogenous neurally active substances nor to the activation of a Ca2+-sensitive K+ conductance. The glu-a.h. was reduced or blocked by K+-free Ringer solution, 3-acetylstrophanthin (3-Ac-Str; 1 microM) or Li+ ions, and was therefore attributed to the activity of the electrogenic Na+ pump. The duration of depolarization induced by glu or DLH was increased in the presence of K+-free Ringer solution, 1 microM 3-Ac-Str or Li ions. It is therefore suggested that the electrogenic Na+ pump may play a role in limiting the duration of depolarization induced by the action of excitatory amino acids. The re-admission of K+ ions to preparations which had been incubated in K+-free Ringer solution produced a transient hyperpolarization (K-a.h.) of the membrane potential of ventral roots which is also attributable to the activation of the electrogenic Na+ pump. Both the K-a.h. and the glu-a.h. were enhanced in Ca2+-free Ringer solution. It is therefore suggested that the Ca2+ ions may modulate the activity of the electrogenic pump in central nervous tissue.

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.

Effects of divalent cations on responses of a sympathetic ganglion to 5-hydroxytryptamine and 1,1-dimethyl-4-phenyl piperazinium

1 The effects of raising or lowering [Ca(2+)](o) or [Mg(2+)](o) on potential changes evoked by 5-hydroxytryptamine (5-HT) and by the nicotinic agonist, 1,1-dimethyl-4-phenyl piperazinium (DMPP) have been investigated.2 Changes in membrane potential were measured at the ganglion or in postganglionic axons by the sucrose-gap technique. The ganglionic response to both 5-HT and DMPP was a depolarization followed by an after-hyperpolarization (AH). AH decayed exponentially over most of its time course; the time constant of decay for 5-HT responses was 4.4 +/- 0.3 min (mean +/- s.e.mean, rate constant 0.23 min(-1)) and that for DMPP responses was not significantly different, being 3.9 +/- 0.3 min (rate constant 0.26 min(-1)).3 Increasing [Ca(2+)](o) to 5.1 or 7.6 mM caused some hyperpolarization of the ganglion, reduced the amplitude of depolarizations evoked by 5-HT by 29% and usually potentiated responses to DMPP (average 12%). Ca-free solutions caused a depolarization of the ganglion, increased the amplitude of depolarizations evoked by 5-HT by 23% and reduced that of depolarizations to DMPP by 32%. [Mg(2+)](o) 12.7 and 25.4 mM caused depolarizations of the ganglion and reduced the amplitude of depolarizations evoked by 5-HT by 34 and 84%, respectively, and those to DMPP by 10 and 75%, respectively. Mg-free solutions or low [Mg(2+)](o) caused a slow depolarization of the ganglion and reduced the amplitude of depolarizations to both 5-HT and DMPP by approx. 20%. Ca/Mg-free solutions produced a slow depolarization of the ganglion, increased the amplitude of depolarizations evoked by 5-HT by 78% and reduced those to DMPP by 58%.4 Increasing [Ca(2+)](o) reduced the amplitude of AH evoked by 5-HT by 50% and increased that to DMPP by 73%, while prolonging AH duration and increasing the time constant of decay. Ca-free solutions had complex effects on AH evoked by 5-HT, which were increased on average by 116%, and depressed AH evoked by DMPP; in both cases there was a decrease in the time constant of decay. [Mg(2+)](o) 12.7 mM reduced the amplitude of AH evoked by 5-HT more than that evoked by DMPP, and increased the rate of decline of the exponential phase. Low Mg solutions reduced in amplitude the AH evoked by 5-HT by 56% and the AH evoked by DMPP by 38%. The time constant of decay was increased. Ca/Mg-free solutions reduced AH amplitude in both 5-HT and DMPP responses. The effects on time constant are consistent with the generation of AH by an electrogenic sodium pump, the ATP-ase of which is Mg(2+)-dependent and inhibited by Ca(2+).5 Responses to 5-HT could be recorded from postganglionic axons and consisted of a rapid depolarization, sometimes followed by an AH whose time constant of decay was smaller than that of ganglionic responses. Full dose-response curves in control and test media could be obtained. In Ca/Mg-free solutions, 5-HT depolarizations were potentiated but no significant shift in the curve was observed.6 It is suggested that divalent cations modulate the coupling between 5-HT receptor and ion channel, an increase in [Ca(2+)](o) reducing the coupling or stabilizing the ion channel in the closed conformation. Ca(2+) and Mg(2+) may compete for the same binding site. This mechanism does not appear to be involved at nicotinic receptors and their related ion channels.

Relative activities of substances related to 5-hydroxytryptamine as depolarizing agents of superior cervical ganglion cells

Membrane potential changes evoked by 5-Ht and related substances were recorded by the sucrose-gap method from rabbit ganglia superfused with Krebs solution at 20 degrees C. A solution of the substance under test was injected into the superfusion stream. The activity of 23 substances was compared to that of 5-HT in respect of depolarizing capacity. 0.01 mumol 5-HT produced a near-threshold depolarization, while 0.6-0.8 mumol induced a maximal one. Some 5-HT analogues evoked prolonged responses distinctly different from the rapid depolarization and repolarization characteristic of 6-HT, while others were inactive. Compounds di- or trimethylated at the side-chain nitrogen atom were capable in addition of activating nicotinic receptors. The results suggest that: (1) the optimal requirements for activating ganglionic 5-HT receptors are a hydroxyl group at position 5 on the indole nucleus and a side-chain bearing an ethylamine amino group; (2) methyl substituents around the terminal nitrogen atom are well tolerated and a quaternary nitrogen may increase activity at the 5-HT receptor; and (3) substitution of a methyl group at carbon atom 2 of the indole nucleus reduces activity. A limitation of the technique is the difficulty of obtaining more than one dose-response curve from a particular preparation; a reduction in potency due to lower affinity cannot be readily distinguished from one due to lower intrinsic activity.

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.

Hyperpolarization of frog primary afferent fibres caused by activation of a sodium pump

1. In the isolated frog spinal cord repetitive stimulation of a lumbar dorsal root produced a sustained negative potential recorded from an adjacent inactive dorsal root by sucrose gap techniques. This negative potential was followed by a positive potential, an indication that the dorsal root terminals were hyperpolarized. Increasing the duration of the tetanus applied to the active root increased the amplitude and duration of the after-hyperpolarization which could be up to 6 mV and 3 min respectively. 2. The hyperpolarization presumably reflected an increased rate of active sodium pumping. Since it was reversibly reduced by metabolic inhibitors (dinitrophenol, NaCN) and cooling (Q10, 2 . 6) it was clearly dependent upon intact metabolic activity. In addition, a variety of procedures used to inhibit sodium pumps (including application of ouabain, elimination of potassium from the superfusate, and partial substitution of lithium for sodium ions) significantly and reversibly decreased the potential. 3. The hyperpolarization was not dependent upon intact chemical synaptic transmission since it could survive prolonged immersion of the cord in Ringer solution containing manganese or magnesium ions. 4. It is suggested that the hyperpolarization of inactive fibres resulted from a decreased extracellular potassium concentration in the dorsal horn produced as a result of a pumping mechanism which extruded sodium and transported potassium inwards by dorsal root fibres directly activated by the tetanus.

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.

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.

Hyperpolarizing 'alpha 2'-adrenoceptors in rat sympathetic ganglia

1 Receptors mediating catecholamine-induced hyperpolarization of isolated superior cervical sympathetic ganglia of the rat have been characterized by means of an extracellular recording method.2 (-)-Noradrenaline (EC(50), 1.7 +/- 0.6 muM) produced an immediate low-amplitude (< 400 muV) hyperpolarization. The hyperpolarization was increased on removal of external Ca(2+) or on reduction of external K(+) from 6 to 2 mM. Hyperpolarization was unaffected by changing the temperature from 25 degrees to 37 degrees C.3 Hyperpolarization was also produced by the following agonists (potencies relative to (-)-noradrenaline): (-)-noradrenaline 1; (+/-)-isoprenaline 0.41; (-)-phenylephrine 0.40; (+)-noradrenaline 0.13; 2-amino-6,7-dihydroxy tetrahydronaphthalene (ADTN) 0.25; dopamine 0.1; methoxamine 0.012; amidephrine 0.0015.4 Responses were antagonized by phentolamine (1 muM) but not by (+/-)-propranolol (1 muM), haloperidol (10 muM) or alpha-flupenthixol (1 muM). This suggested that hyperpolarization was mediated solely through alpha-receptor stimulation not through stimulation of beta-receptors or dopamine-receptors.5 Dose-ratio shifts produced by phentolamine varied with different agonists. The shift increased in inverse proportion to the ability of the agonists to inhibit [(3)H]-(-)-noradrenaline uptake, suggesting that uptake of agonists limited the dose-ratio shift. Cocaine and nortriptyline reduced catecholamine-induced hyperpolarization in concentrations (10 muM and 1 muM respectively) necessary to inhibit [(3)H]-(-)-noradrenaline uptake.6 Clonidine (0.01 to 1 muM), oxymetazoline (0.01 to 1 muM) and ergometrine (0.1 to 10 muM) produced a persistent, low-amplitude hyperpolarization, as though they were partial agonists. Responses to the agonists were blocked by yohimbine (1 muM) but not be prazosin (1 muM).7 It is concluded that the adrenergic cell bodies in the ganglion were hyperpolarized through activation of the same type of alpha-receptor (;alpha(2)-receptors') as those present at adrenergic nerve terminals.

Isoguvacine, isonipecotic acid, muscimol and N-methyl isoguvacine on the GABA receptor in rat sympathetic ganglia

The GABA-mimetic activities of 4 analogues muscimol, isonipecotic acid, isoguvacine and N-methyl isoguvacine have been examined at the GABA receptor in the rat isolated superior cervical ganglion. The depolarizing action of all 4 analogues could be selectively antagonized by bicuculline methochloride and isopropyl bicyclophosphate. Muscimol was the only analogue more potent than GABA (molar potency ratio = 5.08 +/- 0.707). The potency of isoguvacine was 0.23 +/- 0.026 and isonipecotic acid 0.011 +/- 0.0028. N-methyl isoguvacine was less than 0.001 GABA.

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.

Action of surugatoxin on nicotinic receptors in the superior cervical ganglion of the rat

Surugatoxin (SGTX, 0.1-2 muM) reversibly depressed orthodromic transmission and antagonized the depolarizing action of carbachol on the isolated superior cervical ganglion of the rat. The apparent dissociation equilibrium constant against carbachol-induced depolarization (measured in the presence of hyoscine) was 58 and 76 nM determined at 0.2 and 2 muM respectively. SGTX (2muM) did not reduce the depolarizing effects of (+/-)-muscarine, gamma-aminobutyric acid or angiotensin, but did reduce that to 5-hydroxytryptamine. Release of [3H]-acetylcholine following repetitive (10 Hz) preganglionic sympathetic stimulation was maintained in the presence of 2 muM SGTX. It is concluded that SGTX has a high and selective affinity for ganglionic nicotinic receptors.

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.

Membrane potential changes induced by 5-hydroxytryptamine in the rabbit superior cervical ganglion

1. Changes in resting membrane potential induced by 5-hydroxytryptamine (5-HT) have been measured in the excised ganglion by the sucrose-gap technique. 2. 5-HT produced a rapid depolarization, the threshold concentration for depolarization being around 10 muM. With concentrations of 100 muM or greater, repolarization began during the course of the superfusion; this was followed by prolonged tachyphylaxis. 3. Tachyphylaxis was largely avoided by making injections into the superfusion stream. Standard injections of 0.2 mumol 5-HT dissolved in 0.2 ml of Krebs solution were used routinely and could be given at 20-30 min intervals to evoke relatively constant responses. 4. The response to an injection consisted of a rapid depolarization, followed by a rapid repolarization and subsequent after-hyperpolarization. The threshold quantity for depolarization was around 0.01 mumol, while the ED50 estimated from 6 dose-response curves was 0.12 +/- 0.02 mumol (mean +/- s.e. mean). 5. Injections of 5-HT (0.2 mumol), choline (10 mumol) and acetylcholine (9.9 mumol) produced depolarizations of similar magnitude. 6. Monoamine oxidase inhibitors failed to alter substantially the amplitude of depolarizations to 5-HT. 7. 5-HT depolarizations were unaltered in amplitude when the impermeant anion benzenesulphonate was substituted for the chloride ion in Krebs solution, but were initially markedly reduced in amplitude in a sodium-deficient medium; some recovery of the response subsequently occurred. The depolarization which persisted in sodium-deficient solutions was much reduced or abolished when calcium ions were then removed from the superfusion medium. Removal of either calcium ions alone or potassium ions from the superfusion fluid did not reduce depolarization amplitude. 8. The after-hyperpolarization was abolished in sodium-deficient solutions, usually increased in potassium-free solutions, reduced or abolished by ouabain or nicotine, but unaffected by calcium free solutions. 9. A depolarizing action of 5-HT on presynaptic terminals in the ganglion appears probable.

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).

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.

Influence of SKF-525A congeners, strophanthidin and tissue-culture media on desensitization in frog skeletal muscle

1 Microelectrodes have been used to follow changes in membrane potential at end-plate regions of frog skeletal muscle fibres exposed to carbachol; the depolarizing drug was applied to narrow strips of muscle in a rapidly flowing solution containing relatively impermeant anions rather than chloride.2 During prolonged applications of carbachol (10 to 20 muM), the depolarization caused by the drug showed a gradual decline which was attributed to desensitization.3 Desensitization was little if at all affected by supplementing the external solution with factors present in tissue-culture media, or by treating the muscle with strophanthidin (25 muM).4 The rate of repolarization in the presence of carbachol (10 to 20 muM) was greatly increased by the SKF-525A congeners pipenzolate bromide (10 muM) and adiphenine hydrochloride (1 muM). The desensitization-enhancing action of these compounds is discussed.

Depolarizing actions of gamma-aminobutyric acid and related compounds on rat superior cervical ganglia in vitro

1 Potential changes in rat superior cervical ganglia were recorded in vitro with surface electrodes.2 gamma-aminobutyric acid (GABA) produced a transient, low-amplitude ganglion depolarization at rest, and a transient hyperpolarization in ganglia depolarized by carbachol. Depolarization was not prevented by preganglionic denervation. The log dose-response curve for depolarization was sigmoid with a mean ED(50) of 12.5 muM.3 The ganglion was depolarized in similar manner by the following compounds (mean molar potencies relative to GABA (=1) in brackets): 3-aminopropane sulphonic acid (3.4), gamma-amino-beta-hydroxybutyric acid (0.27), beta-guanidino-propionic acid (0.12), guanidinoacetic acid (0.057), delta-aminovaleric acid (0.048), beta-alanine (0.01), 2,4-diaminobutyric acid, gamma-guanidinobutyric acid, taurine and N-methyl-GABA (all <0.01). The following compounds did not depolarize the ganglion at 10 mM concentrations: alpha- and beta-amino-n-butyric acids, alpha-amino-iso-butyric acid, glycine and glutamic acid.4 Depolarization declined in the continued presence of GABA. Ganglia thus ;desensitized' to GABA showed a diminished response to other amino acids but not to carbachol.5 The effect of GABA was not antagonized by hyoscine and hexamethonium in combination, in concentrations sufficient to block responses to carbachol.6 Responses to GABA were blocked more readily than those to carbachol by bicuculline (IC(50), 14 muM) and picrotoxin (IC(50), 37 muM). Strychnine (IC(50), 73 muM) was a relatively weak and less selective GABA-antagonist.7 It is concluded that sympathetic ganglion cells possess receptors for GABA and related amino acids which are (a) different from the acetylcholine receptors and (b) similar to GABA receptors in the central nervous system.

Hyperpolarization of rabbit superior cervical ganglion cells due to activity of an electrogenic sodium pump

1 The mechanisms underlying the hyperpolarization which follows depolarization of rabbit superior cervical ganglion cells by acetylcholine, have been investigated and compared with the mechanisms responsible for the hyperpolarizations induced by orthodromic stimulation of the ganglion.2 The amplitude of the drug-induced hyperpolarization (after-hyperpolarization) was diminished when [Na(+)](0) and the duration of the preceding depolarization were reduced.3 In K(+)-free solutions, the amplitude of the after-hyperpolarization was often diminished and its rate of development was reduced. In 12.5 mM K(+)-Krebs solutions, the amplitude and rate of development of the after-hyperpolarization were increased; the potential was still present when the resting potential was at or close to E(K).4 Ouabain (10 muM) prevented or greatly diminished the after-hyperpolarization. The rates of onset and decay of the after-hyperpolarization were reduced in glucose-free solutions.5 It is, therefore, concluded that the after-hypolarization is due to the activity of an electrogenic sodium pump.6 The positive after-potential associated with the ganglionic action potential was increased in K(+)-free solutions and diminished when the resting potential approached E(K), indicating that it is due to a period of increased K(+) conductance. In the presence of high concentrations of hexamethonium (276 muM), the P wave was not selectively depressed by ouabain and has been shown by other workers to be due to a mechanism not involving an increased potassium conductance. It is concluded, therefore, that the positive after-potential, the P wave and the after-hyperpolarization are due to different mechanisms.

Nicotine washout rates from isolated rat ganglia in relation to recovery from nicotine depolarization

1. Isolated rat superior cervical ganglia recovered more slowly from the depolarizing action of nicotine than from that of carbachol or acetylcholine. This was due to sustained high nicotine concentrations in the vicinity of the receptors, since recovery was hastened by adding hexamethonium to the washout fluid.2. Ganglia incubated for 4 min in 80 muM (3)H-nicotine accumulated nicotine to a level exceeding the extracellular space, as judged from the uptake of (3)H-mannitol.3. The subsequent efflux of (3)H-nicotine into non-radioactive solution could be largely resolved into two exponential components, with rate constants of 0.55+/-0.04 and 0.094+/-0.007 min(-1). The former was similar to that for total mannitol efflux, and so might be largely ascribed to clearance of extracellular nicotine. The slower efflux might be due to clearance from intracellular compartments. Nicotine efflux rates were not affected by hexamethonium indicating that receptor-activation did not modify the slow efflux.4. Efflux of choline compounds ((3)H-acetylcholine, (3)H-choline and (3)H-carbachol) showed an additional, very slow component (rate constant 0.001 to 0.002 min(-1)).5. It was suggested that slow efflux of intracellular nicotine might sustain depolarization on washing by maintaining high perineuronal concentrations of nicotine. With choline compounds the efflux rate from such sources may be too slow to affect perineuronal concentrations.