Neuropharmacology of earthworm giant fibers

Abstract

The effects of a variety of neurotropic drugs on excitability and action potential form of the giant nerve fibers of the earthworm ventral cord were studied with external electrodes.

Earthworm giant fibers were like frog sciatic myelinated fibers in the types of changes induced by most drugs tested, the worm fibers being generally more sensitive. Many centrally active drugs, such as chlordiazepoxide, LSD and methamphetamine, resembled local anesthetics in causing conduction block by increased threshold; unlike procaine, the threshold elevation caused by the above drugs could not be restored transiently toward normal by repetitive maximal stimulation. With most imipraminics, phenothiazines and related agents the mechanism of conduction block did not seem to depend on threshold increase.

Earthworm fibers differed from frog sciatic in several respects: they were not made hyper-excitable by calcium depletion; they showed repetitive firing with high concentration of calcium (the repetitive firing could be antagonized by barbital and trimethadione, not by bromide or diphenylhydantoin); they showed transient lowering of threshold by cholinesterase inhibitors; catelectrotonic repetitive firing could be blocked byd-tubocurarine. Some of the above findings suggest that N-cholinergic receptors may function in earthworm but not in frog myelinated fibers.

The following relationships held true over a wide range of variation of nerve properties by drugs and physiological maneuvers. The ratio of spike voltage when at maximum rate of activation to the maximum spike voltage itself remained relatively constant (0·7 approximately): spike amplitude was modifiable independently of threshold or maximum rate of activation; usually there was an inverse relation between threshold and maximum rate of activation, the only notable exception being initial threshold-lowering action of typical depolarizing monovalent cations (K, Rb, Cs).