Effect of cytochalasin B on neuromuscular transmission in tissue culture

Stereospecific glucose transport across motor nerve terminal membrane: an electrophysiological study

Spontaneous transmitter release at the neuromuscular junction of the frog and rat was monitored during exposures to hyperosmotic solutions containing different sugars. Raising the osmolarity of the medium with D-glucose causes a marked, but transient, increase in the frequency of miniature end-plate potentials (MEPPs): after the initial elevation in frequency there is a subsequent decline towards the control levels, in spite of a continuous perfusion with the hyperosmotic solution. This decline occurs more rapidly in the frog. Two nonmetabolized analogues of glucose, 2-deoxy-D-glucose and 3-O-methylglucose, cause a transient hyperosmotic increase in MEPP frequency, which is very similar to the effect of D-glucose. The elevation of MEPP frequency with hyperosmotic glucose is stereospecific. Hyperosmotic solutions of L-glucose cause a sustained increase in transmitter release in the rat and frog. Insulin dramatically reduces the response of the frog nerve terminal to hyperosmotic D-glucose. Phenolphthalein, a glucose transport blocker, reduces or eliminates the secondary decline in MEPP frequency. It is suggested that the transient nature of the response to hyperosmotic solutions reflects the penetration of the hyperosmotic agent into the nerve terminal. The rate of decline of the MEPP frequency presumably indicates the rate of transport, which determines the rate of osmotic equilibration. This rate can then serve as an index of the relative permeability of the functioning presynaptic membrane to different sugars.

Muscle fibrillation caused by cytochalasin-B applied to the motor nerve

Cytochalasin-B, a drug known to interfere with axoplasmic transport, evoked fibrillary potentials in the geniohyoid muscle when applied to its motor nerve. Despite this denervation-like effect, neuromuscular transmission remained normal. Some contractile characteristics of the muscle were studied. It was found that contraction time, isometric twitch tension, and half-relaxation time were not altered by the drug treatment. The present findings show that neurogenic molecular factors conveyed by axoplasmic transport to the nerve terminal are involved in the regulation of some muscle membrane characteristics but do not modify the muscle contractile features.

Effect of concanavalin A on black widow spider venom activity at the neuromuscular junction: implications for mechanisms of venom action

Concanavalin A (Con A) inhibits black widow spider venom-induced transmitter release at both tissue-cultured and adult neuromuscular junctions and also inhibits the venom-induced destruction of cultured neurites. This inhibitory action is partially or completely prevented by prior treatment with colchicine. Neither colchicine nor Con A interacts significantly with depolarization-induced transmitter release. These results are analogous to those obtained from experiments on lymphocyte surface receptor capping. They suggest that redistribution of neuronal membrane components may be a crucial step in spider venom action. This membrane redistribution appears to be modulated in neurons, as in other cell types, by microtubule-microfilament array. How such a redistribution causes increased transmitter release cannot as yet be specified. Changes in the ionic permeability of sodium and potassium were examined as likely mechanisms. Increased sodium influx (and seondary release of calcium from intracellular stores) cannot be the basis for spider venom action. Increased potassium efflux remains a possibility, but is not consistent with all of the data. Other possible mechanisms are also suggested.