Entry of decamethonium in rat muscle studied by autoradiography

Intraneuronal accumulation and persistence of radiolabel in rat brain following in vivo administration of [3H]-chlorisondamine

1. Chlorisondamine (CHL), a bisquaternary amine, produces a remarkably long-lasting blockade of central responses to nicotine. The mechanism underlying this blockade is not known. The main aim of this study was to test for possible accumulation of [3H]-CHL in rat brain during the period of chronic blockade. 2. Rats received CHL, either systemically (10 mg kg-1) or centrally (10 micrograms i.c.v.). Seven days later, striatal synaptosomes prepared from these animals were tested for nicotine-induced [3H]-dopamine release. This experiment showed that i.c.v. administration of CHL was as effective as systemic administration in producing ex vivo blockade of central nicotinic receptors. 3. Rats received bilateral i.c.v. infusions of [3H]-CHL (10 micrograms) and radioactivity was subsequently quantified in dissected cerebral cortex, striatum, hippocampus, midbrain and cerebellum. Radiolabel was detected at all three survival times (1, 7, and 21 days). Regional heterogeneity was apparent at 7 and 21 days survival. Radiolabel was almost exclusively confined to the insoluble subcellular fraction in all areas sampled. 4. The anatomical distribution of radiolabel was also visualized in brain sections. Rats received bilateral i.c.v. infusions of [3H]-CHL (10 micrograms) and were killed at 1, 7, 21 or 84 days. Immediately before they were killed, all rats were tested behaviourally, and central nicotinic blockade was demonstrated at 1, 7 and 21 days; partial recovery was observed at 84 days. Particularly at longer survival times, tritium was found to be heavily concentrated in the substantia nigra pars compacta, ventral tegmental area, dorsal raphé nucleus, and the granular layer of the cerebellum. 5. The possibility of retrograde axonal transport of radiolabel was then examined. Rats received a unilateral intrastriatal infusion of [3H]-CHL (0.34 or 0.034 micrograms) one week before they were killed. Autoradiographic labelling was largely confined to the site of infusion and to the ipsilateral substantia nigra pars compacta and dorsal raphé nucleus. 6. Thus, after i.c.v. administration, CHL (and/or centrally-formed derivatives) is initially widely distributed within the brain and is then selectively retained within a few brain areas. A persistent accumulation occurs within putative dopaminergic and 5-hydroxytryptaminergic neurones, at least partly through uptake by terminals and/or axons followed by retrograde transport. This persistent and anatomically-selective intraneuronal accumulation possibly underlies the long-term central nicotinic blockade associated with chlorisondamine.

Agmatine acts as an antagonist of neuronal nicotinic receptors

1. Tritiated agmatine has been used by others in ion flux methods to measure nicotinic receptor function in neurones. However, as shown here, agmatine blocks nicotinic receptor function in both the chick retina and the rat superior cervical ganglion at high concentrations. 2. In intact chick retina, agmatine 1 mM decreases dimethylphenylpiperazinium (DMPP)-induced depolarizations measured in the optic nerve by approximately 70%, while having little effect on responses induced by glutamate analogues. DMPP dose-response curves are reduced in a manner consistent with a non-competitive effect of agmatine, and agmatine at 1 mM does not prevent binding of 125I-labelled neuronal bungarotoxin, a snake venom neurotoxin that competitively binds and blocks functional nicotinic receptors in chick retinal homogenates. 3. Agmatine (10 mM) substantially blocks both DMPP-induced depolarizations of rat superior cervical ganglion and synaptic transmission through the ganglion. Others have established that [3H]-agmatine will pass through nicotinic receptor channels in the rat ganglion. These data suggest that agmatine acts both as a cation and as a weak channel blocker at neuronal nicotinic receptors.

Different types of blockade of crustacean acetylcholine-induced currents

The voltage dependence, concentration dependence, and agonist dependence of blocking and unblocking produced by anticholinergic agents on the ionophoretically activated cholinergic currents of the lobster gastric mill 1 (g.m.1) muscle were examined. Although the ionophoretic technique provides only qualitative information as to blocking mechanisms it is useful in revealing slow components of the blocking action of some drugs. At least two qualitatively different types of voltage-dependent block of the crustacean cholinergic currents were observed. For pempidine, mecamylamine and decamethonium (also chlorisondamine: Lingle, 1983), a slowly developing voltage-dependent block was produced that led to the formation of a stable-blocked state. Recovery from this stable-blocked state is largely dependent on subsequent application of agonist. In contrast, recovery from the voltage-dependent block produced by QX-222, atropine, procaine and curare either proceeds independently of agonist application or occurs too rapidly to be observed by the present methods. Blockade by hexamethonium reveals anomalous voltage dependence, being enhanced over some voltages and relieved with additional hyperpolarization. Blockade by trimetaphan is largely independent of membrane potential except at higher concentrations.

The actions of tubocurarine at the frog neuromuscular junction

1. The action of tubocurarine on voltage-clamped frog muscle end-plates has been re-examined by means (a) equilibrium dose-ratio measurements, (b) current fluctuation measurements and (c) voltage-jump relaxation measurements. 2. The equilibrium measurements can be interpreted as implying that tubocurarine has (a) a competitive blocking action, with a dissociation constant of 0.34 microM, which is not dependent on membrane potential, and (b) an additional voltage-dependent blocking action. 3. In the presence of tubocurarine two kinetic components can be seen. The faster one is similar to, but rather faster than, the normal ion channel closing rate. The other is much slower (1--3 sec), and, in relaxation experiments it is in the opposite direction to the fast relaxation. 4. A number of alternative explanations for the results are discussed. The mechanism that fits them best appears to be a combination of competitive block (or block of shut channels), with a strongly voltage-dependent block of open ion channels by tubocurarine. Estimates of the rate constants for channel blocking (and their voltage dependence) are derived. From these estimates the dissociation constant for the binding of tubocurarine to open channels appears to be roughly 0.12 microM at --70 mV and 0.02 microM at --12 mV. 5. Several potential sources of error in the experiments, and in their interpretation, are discussed. The most serious of these are problems associated with diffusion in the small volume of the synaptic cleft, viz. (a) changes in cleft concentration consequent on changes in binding, and (b) ionophoretic flux of antagonist and agonist into the synaptic cleft.

Sodium entry in rat diaphragm induced by depolarizing drugs

1. An increased uptake of labelled sodium was found in the end-plate region of rat diaphragm following brief exposure to solution containing (24)Na plus carbachol (100 muM), with a wash in inactive saline. Tetrodotoxin (0.1 muM) was also present. Comparable results were obtained with decamethonium and suberyldicholine.2. With carbachol (100 muM) the influx of labelled sodium at the end-plate region was increased by a factor of at least three as compared with that at the end of the fibres.3. After entry the labelled sodium spread along the fibres with an apparent diffusion coefficient which was half that expected in the external solution.4. The dose-response curve for the effect of carbachol gave a half-maximal value of 72 muM.5. In muscles depolarized by potassium methyl sulphate the effect of carbachol on the entry of sodium was reduced although demonstrable.6. The entry of labelled sodium at the end-plate was maintained during prolonged exposure to carbachol (100 muM) or decamethonium (100 muM).7. The rate of entry of (24)Na obtained with carbachol, after corrections for the wash, was estimated as 1.5 x 10(3) ions channel(-1) sec(-1), measured over a period of 15 sec.8. Labelled decamethonium and labelled carbachol also accumulated at the end-plate region. After extrapolation to allow for the effects of the wash the entry of decamethonium when expressed as a clearance (pl. mg(-1) sec(-1)) was comparable to that of sodium, as expected if decamethonium and sodium enter through the same channels.

Movement of labelled decamethonium in muscle fibres of the rat

1. Tritium-labelled decamethonium accumulated in diaphragm muscles of the rat in vitro with a peak at the end-plate region and the distributions were fitted by Gaussian curves. 2. Prolonged wash in physiological saline (10 hr) produced some loss in radioactivity but no detectable spread of the labelled compound along the fibres, which indicated that the decamethonium was not in a mobile form. 3. Rats injected with labelled decamethonium showed radioactivity in the diaphragm muscles after 21 days. 4. A slow spread of the labelled compound along the fibres was detected, and from the widening of the Gaussian curves the apparent diffusion coefficient was 1.2 X 10(-8) cm2sec-1, which is less than 1/500 of that in free solution.

Labelled decamethonium in cat muscle

1 Tritium-labelled decamethonium was infused intravenously in 12 cats at final rates of 1.3-4.2 nmol kg-1 min-1 to produce a steady plasma concentration which ranged between 0.21-1.3 mumol/l in different experiments. Muscle contractions were elicited by nerve stimulation and the potential at the end-plate regions of superficial fibres was recorded by extracellular electrodes. 2 Under these conditions, it was not possible to obtain a steady degree of neuromuscular block. The initial decrease in muscle contractions was followed by recovery towards the original value although the concentration of decamethonium in the plasma remained constant, or in some cases rose. The initial depolarization of the end-plate region also waned during the constant infusion of the drug. 3 Once the twitch tension had returned to control values during infusion of the drug, prolongation of the infusion for a total of four hours did not produce a secondary neuromuscular block. 4 Scintillation counting showed that during infusion of labelled decamethonium the radioactivity of the muscles increased progressively with time. The uptake was less in the soleus muscle than in the fast-contracting flexor longus digitorum and extensor longus digitorum muscles. Muscles which had been denervated 12-13 days previously showed a greater uptake of labelled drug than control muscles from the contralateral limb. 5 The labelled drug was localized by autoradiography of frozen sections of leg muscles following intra-arterial injection of decamethonium. Grain counts in individual fibres showed that small amounts of decamethonium had entered the muscle fibres along their entire length, and there was increased uptake of the drug into the cell in the region of the end-plate. 6 The mechanisms underlying the waning of the pharmacological response during constant application of depolarizing drugs are discussed.

Decamethonium in the perfused and immersed rat diaphragm

1 The water content and mannitol space of rat diaphragms which were perfused through the inferior vena cava was increased compared with immersed diaphragms. The potassium content of both preparations, when expressed in terms of dry weight, was maintained at similar values to that found in vivo. 2 Despite the application of a constant concentration of decamethonium, a steady level of neuromuscular block was not obtained in either the perfused or immersed rat diaphragm. The immersed preparation differed from the perfused preparation in that recovery from paralysis occurred despite the continued presence of the drug. 3 The rate of uptake of labelled decamethonium (100 muM) at the end-plate region was similar in the perfused and immersed diaphragm. The slopes of the regressions were 0.059 and 0.054 mul mg-1 min-1 (based on dry weight) respectively which were not significantly different. This implies that the rate of uptake of the drug at the end-plate is slow and limited by the rate of entry into the fibre rather than by diffusion to the site of entry.

Acetylcholine receptors

The idea that certain drugs and neurotransmitters produce their effects by combining with specific receptors was first clearly expressed by Langley (1905) on the basis of the selective and localized effect of nicotine on striated muscle fibres. In 1914, Langley published a paper in which the antagonism between ‘curari’ and nicotine was analysed and measured as the ratio by which the nicotine concentration had to be increased in order to produce a standard response in the presence of tubocurarine. It was clear that Langley had in mind the idea of competition between nicotine and curare for the receptor sites and it was surprising that he did not formulatethe theory quantitatively, particularly since Hill, working in Langley's laboratory in 1909, published a mathematical analysis of the action of nicotine on frog muscle giving kinetic and equilibrium equations based on the law of mass action, which could easily have been extended to give an account of competitive antagonism. Barger & Dale (1910) did not favour the idea of specific receptors for sympathomimetic amines.

Structure and physiology of denervated and re-innervated cockroach muscle

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Decamethonium in depolarized muscle and the effects of tubocurarine

1. In rat diaphragms immersed in solution containing 5 mM potassium the maximum uptake of labelled decamethonium was found at the end-plate region. In muscles depolarized in solution containing potassium methyl sulphate the uptake was reduced but a peak concentration at the end-plate region was still demonstrated.2. The uptake of labelled decamethonium increased steadily with time and was interpreted in terms of the entry of decamethonium into the fibres. The permeability at 10-100 muM was similar to that of sodium.3. The uptake of decamethonium at the end-plate region was dependent on the concentration. At low values the uptake in depolarized muscle was uniform along the fibres. Increase in concentration produced a peak at the end-plate region. This was interpreted as a change in permeability such that half the maximum effect was present at a concentration of approximately 5 muM.4. At high concentrations the influx showed saturation and carrier-like kinetics with a half-saturation concentration of 400 muM.5. Tubocurarine inhibited the peak uptake in depolarized diaphragm. The results were consistent with competitive inhibition with an inhibitory constant of 0.07 muM.6. Acetylcholine in high concentration also inhibited the uptake of decamethonium in the end-plate region of depolarized muscle.