Neuropharmacological Analysis of Mechanisms Controlling Larval Behaviour in the Sea Urchin; 2. Effects of physostigmine and the role of cholinergic mechanisms

Abstract

The purpose of this paper is to shed some light on the systems controlling the contractile activities in the sea urchin pluteus, their pharmacological properties, the regional organization of the structures involved, their interactions, and their physiological roles. Physostigmine may bring about tetanic celomic activity of long duration. In spite of this it is possible to recognize the two activity patterns elicited by carbachol. The effects of the two agents, together with some histochemical data, suggests the occurrence of two control systems. The main muscular control system may be linked to adjoining cholinergic and mono-aminergic structures close to the mouth and is presumably responsible for the “reciprocal antagonism” between swallowing and celomic movements when the larva is stimulated (activity pattern I). It may involve intricate interactions between the cholinergic and monoaminergic components in the generation of muscular impulses. Another control system is presumably linked to neurons extending from the main ciliated band and responsible for the intestinal cycling and the “non-antagonistic” muscular activity (activity pattern II). After strong stimulation both control systems may become “exhausted” in some respects, but may eventually recover. In comparison with the effects of carbachol, the physostigmine effects are not quickly reversible which indicates that, to a great extent, they are due to inhibition of cholinesterase in deeper parts of the two control systems.