Development of fast synaptic transmission in bullfrog sympathetic ganglia

Extracellular recordings were made of postsynaptic responses originating in sympathetic ganglia 9 and 10 of bullfrog tadpoles and adults. At stage III, when the length and diameter of the developing hindlimb bud are equal, preganglionic stimulation elicits postganglionic action potentials in spinal nerves 9 and 10 near the sciatic plexus. Although they fluctuate in amplitude, these responses follow short trains of repetitive stimuli at 20 Hz. Their mediation by nicotinic synapses was demonstrated by reversible blockade in low Ca2+, high Mg2+ Ringer and in nicotine. Parallel sympathetic B and C systems are clearly defined by stage III. They can be selectively activated by appropriate segmental stimulation of the sympathetic chain and are characterized by distinct conduction velocities which both lie in the C fiber range (less than 1 m/s). Throughout subsequent tadpole stages, the conduction velocities of the developing B and C systems gradually double while the magnitudes of their compound action potentials grow exponentially by 100-fold. Conduction velocities reach adult values after completion of metamorphosis. These results provide physiological evidence that synapse formation in sympathetic ganglia supplying the hindlimbs begins by the earliest stages of limb bud development, is selective, and progresses over a protracted period of months, prior to myelination.