Embryonic behavior, hatching and neuromuscular development in the chick following a transient reduction of spontaneous motility and sensory input by neuromuscular blocking agents.

Abstract

AbstractEmbryos were immobilized with neuromuscular blocking agents for one to four days between days 10 and 15 of incubation. This treatment reduces spontaneous motility, as well as movement‐initiated proprioceptive and cutaneous stimulation. Although the major aim of these experiments was to determine the effects of such treatment on subsequent behavioral development, several indices of neuromuscular and general morphological development were also examined.A single injection of curare on day 10 continues to depress spontaneous motility for virtually the entire remaining incubation period. This effect is due to the persistence of unmetabolized curare in the closed system of the egg. When a comparable dose of a rapidly metabolized neuromuscular blocking agent (succinylcholine) is given, this long term behavioral depression is not found. Embryos treated with SC can remain totally immobilized for up to 60 hours with no apparent repercussions on subsequent behavior; spontaneous embryonic motility, reflex sensitivity, hatching, and several general posthatching behaviors, all appeared normal following such treatment.Embryos immobilized for as little as 48 hours developed joint malformations and were retarded in general growth by about one day whereas 24 hours of paralysis was not sufficient to induce these effects.Twenty‐four hours of total paralysis from days 10 to 11, plus a 40–50% reduction in motility until days 15 or 16, does not appear to cause any abnormalities in muscle or spinal cord development. Muscle histology, motor endplates, cell number in the spinal cord and choline acetyltransferase and acetylcholinesterase activity in spinal cord and muscle were all comparable to controls. An 80–90% reduction in motility on days 11 through 15, however, induces an apparent alteration of the intensity and distribution of histochemically demonstrable AChE in the anterior and posterior latissimus dorsi muscles.The present findings suggest that the suppression of overt motility for a period comprising ca. 5–12% of the total incubation time (21 days) does not modify subsequent behavioral development or the underlying neurogenetic mechanisms. Though these findings provide some support for the notion that the embryonic nervous system develops in forward reference to and without benefit from function or sensory input, only a more complete reduction of neural function, especially CNS activity, can provide a critical test of this concept.