Gallamine triethiodide (flaxedil): tetraethylammonium- and pancuronium-like effects in myelinated nerve fibers

Effects of 4-aminopyridine on demyelinated axons, synapses and muscle tension

Several clinical trials have demonstrated that 4-amino-pyridine (4-AP), a potassium channel-blocking agent, improves symptoms in some patients with multiple sclerosis. The beneficial effects have typically been attributed to the restoration of conduction to demyelinated axons, since this effect was previously demonstrated experimentally. However, the clinical dose is approximately 250-1000 times lower than that used experimentally, potentially making extrapolation of the experimental findings unreliable. To examine the action(s) of 4-AP in demyelinating disorders, the drug was administered at clinical doses, both in vivo and in vitro, to rat dorsal column axons which had been experimentally demyelinated by the intraspinal injection of ethidium bromide. 4-AP had no consistent effect in restoring conduction to demyelinated axons, even to axons which were held just on the verge of conducting by adjusting the lesion temperature. However, 4-AP had prominent effects that did not involve demyelinated axons, including the potentiation of synaptic transmission and an increase in skeletal muscle twitch tension. We propose that these latter effects may be largely responsible for the beneficial action of 4-AP in multiple sclerosis patients. If so, the dominant effects of 4-AP in multiple sclerosis patients are independent of demyelination, and it follows that 4-AP may be beneficial in other neurological disorders in which function is diminished.

Conduction properties of central nerve fibers remyelinated by Schwann cells

Demyelination of central axons arises from a number of conditions, including multiple sclerosis and spinal cord compression. The demyelination disrupts conduction and leads directly to the production of symptoms. Repair of the demyelination by peripheral myelinating cells could potentially relieve the symptoms, but the conduction properties of central axons remyelinated by Schwann cells have yet to be studied in detail. This paper examined the conduction properties of such axons. Large focal demyelinating and remyelinating lesions were induced in the dorsal columns of rats by the intraspinal injection of ethidium bromide. Recordings of compound action potentials conducted through these lesions were then made at various recovery times. Thus the changing conduction properties of the affected fibers could be correlated with the different stages of lesion development. During the early stages of demyelination there was widespread conduction block, with no evidence of appreciable conduction occurring with prolonged latency or refractory period of transmission (RPT). However, with the onset of remyelination by Schwann cells, conduction was restored in many axons, and most, if not all, of the affected axons eventually showed successful conduction through the lesion. Initially the conduction was characterized by very prolonged latency, long RPT, and an inability to conduct fast trains of impulses. These deficits became less prominent as remyelination progressed. In chronically remyelinated axons the RPT was restored to within normal limits, although some deficit in both conduction velocity and the ability to conduct trains of impulses persisted. Since these deficits were not severe we conclude that remyelination of central demyelinated axons by Schwann cells should be effective in promoting the restoration of normal function.

Differential effects of the muscarinic M2 antagonists, AF-DX 116 and gallamine, on single neurons of rabbit sympathetic ganglia

Intracellular recording techniques were used to compare the effects of the M2 muscarinic antagonists, AF-DX 116 and gallamine, on membrane potential (Vm), input resistance (Ri), responses induced by methacholine, muscarinic slow postsynaptic potentials and action potentials in the superior cervical ganglion of the rabbit. Gallamine or AF-DX 116 antagonized methacholine-induced or synaptically-evoked muscarinic hyperpolarization, without having significant effect on depolarization induced by methacholine or synaptically. The drug AF-DX 116 reduced evoked muscarinic hyperpolarizing potentials, without significant change in Vm or Ri, recorded in the absence of muscarinic stimulation. In contrast to AF-DX 116, gallamine elicited a concentration-dependent depolarization of the membrane, with a corresponding increase in Ri, when tested in the absence of muscarinic stimulation. These effects of gallamine were accompanied by an increase in duration and decrease in the slope of the descending phase of the action potential. Blockade by gallamine of evoked hyperpolarization was independent of membrane depolarization and readily occurred when gallamine-induced depolarization was prevented by clamping Vm at its pre-gallamine level. The effects of gallamine were maintained during its presence and reversed upon washing with gallamine-free physiological solution. These results indicate that AF-DX 116 and gallamine have a specificity for antagonism of muscarinic responses, mediated by receptors of the M2 type in the superior cervical ganglion. However, gallamine, while an effective antagonist of M2 responses, also has the ability to modify the electrical characteristics of ganglion cells and thus may modify ganglionic transmission by mechanisms other than antagonism of receptors.

Pain responses to perineuromal injection of normal saline, gallamine, and lidocaine in humans

Rat neurons have shown an increase of spontaneously active fibers to systemically administered potassium channel blocking agents such as tetraethylammonium chloride (TEA) and gallamine. Neuroma formation and spontaneous activity have been associated with autotomy in rats and pain in humans. To evaluate the chemosensitivity of human neurons to potassium channel blocking agents, 9 subjects with neuroma pain underwent perineuromal injection in a single-blinded fashion of normal saline, gallamine, and lidocaine. Sodium had no effect on control pain levels, while gallamine significantly increased and lidocaine significantly decreased pain from control levels. Three of 4 patients with accompanying phantom limb pain noted an increase in pain after the injection of gallamine. The data suggest that peripheral input plays a modulating but not solitary role in both neuroma and phantom limb pain. Agents which increase potassium channel permeability or decrease sodium influx would be predicted to decreased perceived pain.

A quantitative study of the effects of some muscarinic antagonists on the guinea-pig olfactory cortex slice

1. Muscarinic depression of the electrically-evoked surface-negative field potential (N-wave) was measured in guinea-pig olfactory cortex slices maintained in vitro. 2. The effects of three muscarinic receptor antagonists, pirenzepine, atropine and gallamine on this muscarinic response were analysed in detail. 3. Pirenzepine was a potent competitive antagonist of carbachol (CCh)-evoked responses. Schild plot analysis yielded a pA2 value of 7.9 (Schild slope constrained to unity). A similar analysis for atropine versus CCh responses gave a pA2 of 8.9. 4. Combination experiments using pirenzepine and atropine produced dose-ratio shifts close to those expected for two antagonists competing for a similar receptor site. 5. Gallamine was only a weak antagonist of responses to CCh. 6. Oxotremorine behaved as a competitive antagonist at this muscarinic receptor (pA2 = 6.1). 7. It is concluded that the presynaptic muscarinic receptor mediating depression of the N-wave in the olfactory cortex slice is of the M1-subtype.

Identification of the teleost Edinger-Westphal nucleus by retrograde horseradish peroxidase labeling and by electrophysiological criteria

A homolog of the Edinger-Westphal nucleus of other vertebrates is described in two species of serranid basses of the genus Paralabrax, a group possessing a wide range of ocular accommodation but lacking a pupillary reflex to light. The nucleus was found by retrograde labeling from the ciliary ganglion and lies dorsolateral to the ipsilateral oculomotor nucleus. The nucleus consists of 60 to 100 neurons with an average soma diameter of about 20 microns in animals weighing 70 to 150 g. Electrophysiological experiments support the identification. Microstimulation of the nucleus evokes contraction of the ipsilateral lens retractor muscle and slight constriction of the caudal ipsilateral iris. Multi- and single-unit recordings in the nucleus reveal spontaneous firing (about 30 spikes/s in single units), the rate of which decreases during visually-evoked lens retractor relaxations (accommodation to near stimuli). Recordings of muscle fiber activity in the lens retractor show essentially the same behavior, which suggests that the ciliary ganglion and neuromuscular junctions simply relay impulses with little if any synaptic integration. The existence of a discrete Edinger-Westphal nucleus devoted largely to accommodation makes Paralabrax a good model system for the further tracing of central accommodation control pathways.

Cholinergic regulation of impulse frequency in peripheral nerve

Previous studies have demonstrated the existence of receptors for gamma-aminobutyric acid (GABA), beta-adrenergic catecholamines and acetylcholine in vertebrate peripheral nerve, and provided functional correlates for activation of both GABA and beta-adrenergic receptors. The present studies show that a cholinergic receptor present on the nerve can also modify impulse pattern. In frog sciatic nerve, both carbamylcholine and dibutyryl cyclic GMP increased the amplitude of the response to the second stimulus of a train at very short interstimulus intervals. The effect of carbamylcholine was blocked by 4-aminopyridine. The results are consistent with the hypothesis that cholinergic agonists mediate an increase in endogenous cyclic guanosine monophosphate (cGMP), which increases the ability of the nerve to follow closely spaced stimuli by inhibiting potassium channels.

Actions of convulsants, 4-aminopyridine and pentylenetetrazole, on the transient outward current of single isolated nodose ganglion neurons

The actions of 4-aminopyridine (4-AP) and pentylenetetrazole (PTZ) were examined on the transient outward current (IA) of single isolated neurons of the rabbit nodose ganglion. Both agents reduced the amplitude of the current and shortened its duration in a dose-dependent manner. The steady-state inactivation of the IA channel was shifted in a hyperpolarizing direction in the presence of these agents. It is suggested that convulsants such as 4-AP and PTZ modify the IA channels resulting in increased cell excitability possibly contributing to their convulsant action.

Effects of magnesium on fictive locomotion induced by activation of N-methyl-D-aspartate (NMDA) receptors in the lamprey spinal cord in vitro

It has previously been demonstrated that an activation of N-methyl-D-aspartate (NMDA) receptors can induce fictive locomotion as well as tetrodotoxin (TTX)-resistant membrane potential oscillations in certain types of neurones in the in vitro preparation of the lamprey spinal cord. These oscillations in individual neurones depend on voltage-sensitive properties of NMDA-activated channels which are only manifested in the presence of Mg2+. To evaluate the role of these pacemaker-like oscillations in the generation of locomotion, the motor patterns induced by N-methyl-D,L-aspartate (NMA) before and after removal of Mg2+ were compared. It was found that the ventral root burst pattern of fictive locomotion was more irregular after removal of Mg2+, particularly at low burst rates. This suggests that the membrane properties underlying the NMDA-induced TTX-resistant membrane potential oscillations are of importance for the generation of a stable and regular locomotor activity in particular at low rates of fictive locomotion. When fictive locomotion was induced instead by an activation of kainate receptors a removal of Mg2+ had no effect on the motor pattern. The effects of the two K+-channel blockers, tetraethylammonium (TEA) and gallamine were also tested on NMA-induced fictive locomotion. Both compounds caused an increase in the burst frequency. The Mg2+-dependent NMDA-induced bistable membrane properties thus appear to be of importance for the operation of the network which generates the locomotor pattern.

Simultaneous secretion of catecholamines from the adrenal medulla and of [3H]norepinephrine from sympathetic nerves from a single test preparation: different effects of agents on the secretion

The uptake and release of catecholamines was investigated in the isolated perfused adrenal gland of the rat after preloading the preparation with [3H]norepinephrine, and the effects of various agents were examined on the stimulation-evoked secretion of catecholamines and total tritium. Large quantities of tritium were found in the adrenal medulla after either intravenous injection of [3H]norepinephrine to the rat, or perfusion of the isolated adrenal gland with Krebs-bicarbonate solution containing [3H]norepinephrine. The retention of the tritium was inhibited 90% by desipramine. Acute treatment with guanethidine and chronic treatment with 6-hydroxydopamine abolished the secretion of tritium without affecting the secretion of catecholamines evoked at 1 Hz. Nicotine, muscarine and acetylcholine enhanced the secretion of catecholamines but not tritium, whereas tyramine and ephedrine enhanced the secretion of tritium but not catecholamines. It is concluded that chromaffin cells do not possess the norepinephrine uptake mechanism and that the uptake of [3H]norepinephrine occurs mainly in sympathetic nerve terminals present in the adrenal gland and the surrounding blood vessels (adrenal and renal veins). The differential localization of [3H]norepinephrine and catecholamines allowed us to test the effects of a variety of pharmacological agents that alter neurotransmitter release by acting on receptors on the neuronal membrane, acting on sodium and potassium channels, or acting to alter the intracellular concentrations of adenosine 3',5'-cyclic monophosphate and protein kinase C. Transmural stimulation (1 Hz for a total of 300 pulses) markedly enhanced the release of catecholamines and tritium which was blocked by tetrodotoxin (sodium channel-blocker) and potentiated by tetraethylammonium and gallamine (potassium channel-blockers). Phentolamine, an alpha adrenergic blocking agent which acts on both alpha-1 and alpha-2 receptors, caused a 3- to 4-fold facilitation of the tritium secretion while inhibiting catecholamine secretion by 45%. [Met]enkephalin almost completely inhibited the evoked-secretion of tritium but had very little effect on the secretion of catecholamines. Forskolin inhibited the tritium secretion by 80% but produced more than a 2-fold facilitation of catecholamine secretion. Phorbol 12,13-dibutyrate caused facilitation of evoked secretion of both catecholamines and tritium. A combination of phorbol ester and forskolin had a synergistic effect on stimulation-evoked secretion of catecholamines, whereas phorbol ester partially reversed the inhibitory effects of forskolin on the tritium secretion.(ABSTRACT TRUNCATED AT 400 WORDS)

Facilitation of noradrenaline release by gallamine in the rat salivary gland

The effect of gallamine on spontaneous and stimulation-evoked overflow of tritium was studied in the submandibular gland of the rat. The gland was perfused retrogradely and labeled with 3H-noradrenaline. The stimulation-evoked (1 Hz for 60 s) overflow of tritium was facilitated by increasing concentrations of gallamine (0.3-20 mM). None of the concentrations of gallamine increased the spontaneous overflow of the tritium. The facilitatory effect of gallamine was observed in 0.3 to 5 mM calcium medium; the maximum facilitation was observed at the normal concentration of calcium (2.5 mM). The facilitatory effect of gallamine was inversely related to the frequency of stimulation (10-fold facilitation at 1 Hz and 3-fold at 10 Hz). Stimulation of the salivary gland by a single pulse (1 ms duration) in the normal medium did not evoke an overflow of tritium; however, the same stimulus produced a marked increase in the overflow in the presence of gallamine. The facilitatory action of gallamine on the release of sympathetic transmitter is ascribed to the enhanced availability of calcium ions to the secretory process resulting from blockade of potassium conductance during nerve activity.

Gallamine and vagotomy enhance respiratory modulation of reticular units

Respiration related units ( RRU ) were recorded in the brainstem of cats with spinal transection at the C7-Th 1 level and breathing N2O. The proportion of RRU in several structures was compared in control and in 3 experimental groups: (1) paralyzed with gallamine triethiodide; (2) vagotomized; and (3) both vagotomized and paralyzed. After gallamine, RRU percentage was multiplied by 3 in the bulbo-pontine reticular formation (RF) and as much as 20 in the mesencephalic RF. Vagotomy multiplied RRU proportions by 2 in the bulbo-pontine RF, by 12 in the mesencephalic RF and by 3 in the pneumotaxic complex (nucleus parabrachialis medialis ( NPBM ) and K olliker -Fuse (KF) nucleus). The effects of gallamine and vagotomy were not additive. Gallamine still increased proportion of RRU in the RF after vagotomy showing that the gallamine effect is not vagally mediated; this was in contrast to the previously reported suppression by vagotomy of phrenic discharge facilitation induced by gallamine. The total number of firing units in the RF was not modified by gallamine or vagotomy. It is concluded that the respiratory modulation of reticular neurons is selectively enhanced by gallamine and vagotomy through two independent mechanisms.

Actions of potassium channel blockers on guinea-pig lateral olfactory tract axons

Population action potentials were recorded from the guinea-pig isolated lateral olfactory tract. At 30 degrees C, the conduction velocity of the fibres was about 4 m/s and the absolute refractory period was less than or equal to 1.5 ms. The population spike was unaffected by removal of calcium ions from the superfusate but was abolished in tetrodotoxin. Tetraethylammonium ions (10 mmol/l) had no effect on the population spike, however the following potassium channel blocking drugs increased the duration in a concentration-dependent manner (in order of decreasing potency): 3,4-diaminopyridine, 4-aminopyridine, 3-aminopyridine, sparteine, cesium ions and barium ions. In addition to a prolongation, these substances also reduced the amplitude of the conducted spike. It is concluded that the rising phase of the spike is generated by a voltage-dependent increase in sodium conductance and that an increase in potassium conductance contributes to the falling phase. The potassium channels are potently blocked by aminopyridine like drugs.

Accelerating effects of pentobarbitone on the inactivation process of the calcium current in Helix neurones

The effect of pentobarbitone on Ca2+ current (ICa), separated from other ionic currents was studied under voltage clamp using a suction pipette technique in Helix neurones. Pentobarbitone depressed the maximal peak amplitude (MPA) of ICa in a concentration-dependent manner without shifting the current-voltage (I-V) relationships along the voltage axis. Increases in external Ca2+-concentration [( Ca2+]o) overcame the inhibitory action of the agent on MPA. Pentobarbitone markedly accelerated the decay phase of ICa which took a distinctly different time course from that of the control. The accelerating action of the agent on the decay phase of ICa was not overcome by increases in [Ca2+]o. In the presence of internal EGTA (20 mM), pentobarbitone also accelerated the decay of ICa. Changes in pH of the external perfusing solution altered the potency of pentobarbitone in depressing MPA; in the presence of pentobarbitone (3 X 10(-4) M) at pH of 7.0, 8.0 and 9.0, fractional inhibition was approx. 46%, 21% and 4%, respectively. Internal application of pentobarbitone (10(-4)-10(-3) M) inhibited MPA, but exerted no effect on the decay phase of ICa. Pentobarbitone (10(-4) M) markedly accelerated the decrease of MPA of ICa induced by repetitive stimuli applied at an interval of 150 ms, indicating a use-dependent depression of MPA. Results provide evidence that pentobarbitone has a dual action on ICa, inhibiting MPA and accelerating the decay phase of ICa.