The action of acetylcholine on the locomotor central pattern generator for swimming in Xenopus embryos.

Rhythmic locomotion in Xenopus laevis embryos is controlled by a central pattern generator in the spinal cord. This was demonstrated in experiments in which movements were blocked by tubocurarine or o--bungarotoxin (Kahn and Roberts, 1982a; Boothby and Roberts, 1988) and activity was recorded either extracellularly with suction electrodes on the ventral roots or intracellularly with microelectrodes from spinal neurones. Fictive swimming activity was evoked by skin stimulation, dimming of the illumination or could occur spontaneously, and its parameters were similar to those of actual swimming. This preparation, immobilised with neuromuscular blocking agents, has been very useful for the analysis of the cellular mechanisms underlying locomotor rhythm production (Roberts, 1990). The possibility remained that these antagonists also had central effects. To check this we have used an isolated nervous system preparation which enabled us to record fictive swimming activity in the absence of neuromuscular antagonists. This also allowed us to look at the effects of acetylcholine (ACh) and its antagonists on the central pattern generator for swimming locomotion. Experiments were carried out using stage 37/38 Xenopus laevis embryos (Nieuwkoop and Faber, 1956). Animals were anaesthetised in 0.1% MS222 for 5min and transferred to fresh saline (Soffe, 1990), where the central nervous system (CNS) was removed using etched tungsten pins. The isolated CNS consisted of about half to two-thirds of the spinal cord and the whole brain with pineal organ but without lateral eyes or olfactory bulbs. The CNS was then transferred to 5 ml of fresh saline in a Petri dish, where electrical activity (spinalogram) was recorded with a tapering polyethylene suction electrode applied to the caudal end of the spinal cord (Fig. 1). If a new Petri dish is used the CNS will stick to the bottom, facilitating intracellular recordings which were made with glass microelectrodes filled with 2moll" potassium acetate, resistance 100-200 MQ.