Development of spontaneous motility in chick embryos. Effects of drugs acting on monoaminergic synaptic receptors.

The development of the effect of agonists of monoaminergic transmitters--LSD (20 and 200 micrograms/kg egg weight), apomorphine (1 and 3 mg/kg e.w.) and clonidine (2.5 mg/kg e.w.)--was studied on 11- to 19-day chick embryos. All three substances already significantly depressed spontaneous motility in 11 and 13-day-old embryos and their depressant effect increased as embryogenesis progressed. In chronically decapitated embryos (Sedlácek and Doskocil 1978) they had practically the same depressant effect as in normal embryos, showing that their activity is not bound unconditionally to supraspinal compartments of the CNS. These results are further evidence of the participation of central monoaminergic systems in embryonal motility. The finding that the three test drugs take effect during embryogenesis before sensitivity to monoaminergic transmitters appears remains unexplained.