3H-Nicotine in cat superior cervical and nodose ganglia after close-arterial injection in vivo

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.

Neuropharmacological effects of nicotine in relation to cholinergic mechanisms

Studies with nicotine have, from the outset, been of fundamental importance for the elucidation of physiological mechanisms. However, many nicotinic cholinoreceptors do not appear to subserve a physiological role, at least within the present framework of knowledge. The main challenge in the immediate future is to determine the functional role of nicotinic cholinoceptors in the CNS. This knowledge will not only add to the physiological mechanisms that have been unravelled as a result of studies with nicotine, but will also provide a major contribution to the understanding of tobacco use.

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.

The mechanism of action of nicotine on vascular adrenergic neuroeffector transmission

The aim of this study was to determine the site and mechanism of action of nicotine on sympathetic neuroeffector transmission in the isolated pulmonary artery of the rabbit. Nicotine and cocaine potentiated the constrictor response elicited by electrical-field stimulation of postganglionic adrenergic neurones. The potentiation was reversible and in the case of nicotine, no tachyphylaxis developed. The nicotine-induced potentiation was characterized by a rapid onset and an initial, transitory peak, while the enhancement caused by cocaine progressed more slowly and was monophasic. Hexamethonium and (+)-tubocurarine prevented the potentiation caused by nicotine. Nicotine did not prevent the adrenergic neurone blocking effect of bretylium on the response to field stimulation. Nicotine increased the stimulation-induced outflow of tritium from pulmonary artery preloaded with 3H-(--)-noradrenaline. Contractions of the artery elicited by tyramine were enhanced by pargyline, unaltered by nicotine and blocked by cocaine. Nicotine did not alter the concentration--response curve of exogenous (--)-noradrenaline while cocaine moved it to the left. The accumulation of 3H-(--)-noradrenaline by rabbit isolated aorta was not altered by nicotine, hexamethonium and (+)-tubocurarine. The accumulation of 3H-nicotine by the aorta was much lower than that seen with 3H-(--)-noradrenaline. The disposition of the 3H-nicotine accumulation into adventitia and media was concentration-independent. These results suggest (1) that nicotine potentiates the neurogenic vasoconstriction response in part by increasing the stimulation-induced release of transmitter from adrenergic neurone terminals; (2) that the site of the nicotinic receptors mediating this action is located on the outer surface of the neurones; and (3) that the potentiation is not due to blockade of noradrenaline re-uptake.

Prevention by drugs of tachyphylaxis at nicotinic receptors in the cat superior cervical ganglion in situ

A new compound, AF3 (4-ethyl-6-oxa-1-azatricyclo)4.2.2.02,7)dodecan-5-one), and its 4-phenyl analogue, AF6, embodying the structural elements of acetylcholine in a highly rigid framework, were shown to evoke nicotine-like responses in the nictitating membrane (NM) and blood pressure when applied to the superior cervical ganglion in anaesthetized cats. In this respect, their equiactive molar ratio was (nicotine : 1),20-30. However, at doses that were too low to evoke any response, AF3 appeared to potentiate the responses to nicotine or tetra-methylammonium (TMA) by preventing or abolishing tachyphylaxis to the two latter drugs, the effect being dose-dependent with AF3. DMPP which produces much less tachyphylaxis, or preganglionic nerve stimulation, was little or not potentiated in presence of AF3. It is proposed that potentiation to nicotine or TMA occurs following occupancy by AF3 of a regulatory subsite, thereby preventing further access to it by nicotine or TMA. In this respect, AF3 plays the role of a "neutral" molecule.

Intracellular pH in rat isolated superior cervical ganglia in relation to nicotine-depolarization and nicotine-uptake

1. The intracellular pH (pH(i)) of rat isolated superior cervical ganglia incubated in normal Krebs solution (pH(o)=7.37) was estimated to be 7.33 from the uptake of a weak acid, (14)C-5,5-dimethyloxazolidine-2,4-dione (DMO). Addition of 30 muM nicotine for 30 min reduced the DMO-estimated pH(i) by 0.15 units to 7.18. This effect was prevented by hexamethonium (2.5 mM) or by depolarizing the ganglion with K(+) (124 mM).2. (3)H-Nicotine (30 muM) was concentrated within the ganglia to an intracellular/extracellular concentration ratio (C(i)/C(o)) of 5.54 in normal Krebs solution and 4.61 in 2.5 mM hexamethonium. This would suggest an intracellular pH of 6.54 and 6.63 respectively. In ganglia previously depolarized by K(+) the corresponding values for C(i)/C(o) were 4.02 (minus hexamethonium, estimated pH(i) 6.95) and 4.17 (plus hexamethonium, estimated pH(i) 6.94).3. A multicompartment cell interior comprising an acid cytoplasm (pH approximately 6.6) and more alkaline nucleus and mitochondria is proposed to explain the difference between the values of pH(i) estimated from the uptake of DMO and nicotine. It is suggested that the fall in pH(i) during nicotine-depolarization results from metabolic stimulation following Na(+) entry.

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.

Uptake of nicotine and extracellular space markers by isolated rat ganglia in relation to receptor activation

1. Uptake of (3)H-nicotine by isolated rat superior cervical sympathetic (SCG) and nodose (NG) ganglia was measured in vitro. Depolarization of the ganglia by nicotine was measured electrically.2. Nicotine depolarized the SCG but not the NG. The mean ED50 for depolarization was 5.3 x 10(-6)M.3. Both ganglia accumulated nicotine when incubated in 3.1 x 10(-5)M(3)H-nicotine: after 30 min incubation the ratios of tissue to medium concentrations were (mean +/- S.E. of mean): SCG, 3.49 +/- 0.13; NG, 2.50 +/- 0.09.4. Total water contents, estimated by drying to constant weight, were: SCG, 83.8 +/- 0.12%; NG, 80.1 +/- 0.21%. Extracellular spaces, measured as (3)H-mannitol space, were: SCG, 38.8 +/- 1.3; NG, 40.3 +/- 0.8% wet weight. These values were not significantly altered by nicotine.5. Correction for tissue fluid spaces indicated that the ratio of the mean intracellular fluid concentration to the extracellular fluid concentration for (3)H-nicotine at 3.1 x 10(-5)M were: SCG, 7.4; NG, 5.6. The ratios were not altered in any consistent manner on varying the nicotine concentration between 3.1 x 10(-7) and 1.6 x 10(-4)M.6. When the nicotine concentration was sufficiently great (6.2 x 10(-6)M or more) to evoke large SCG depolarizations, hexamethonium (2.5 x 10(-3)M) reduced (3)H-nicotine uptake by the SCG by up to 19% without affecting uptake by the NG, and thereby reduced the uptake difference between the two ganglia. With nicotine concentrations <6.2 x 10(-6)M, hexamethonium did not modify uptake by either ganglion.7. It was concluded that nicotine may be concentrated within neurones, and that such intracellular accumulation may be augmented during depolarization induced by nicotine.

Entry of decamethonium in rat muscle studied by autoradiography

1. When a steady plasma level of decamethonium was maintained by infusion in rats, the labelled compound became concentrated in the region of the end-plate of skeletal muscle, as shown by scintillation counting.2. The distribution of tritium-labelled decamethonium in single muscle fibres was studied by autoradiography of frozen sections, with resolution less than 1 mu.3. After intravenous injection of a dose of decamethonium which produced partial paralysis it was shown that the labelled compound had entered muscle fibres in the region of the end-plate, and for several hundred microns on either side of the end-plate.4. Entry of decamethonium could be demonstrated as early as 30 sec after intra-arterial injection. There was no evidence of any redistribution of labelled drug for a period of 2 hr after the initial entry.5. Previous administration of tubocurarine markedly reduced the entry of labelled decamethonium.