Rat sympathetic neurons and cardiac myocytes developing in microcultures: correlation of the fine structure of endings with neurotransmitter function in single neurons.

Microcultures containing single sympathetic principal neurons and small numbers of dissociated heart myocytes were prepared from newborn rats. After the transmitter properties of the neuron were studied by electrophysiological experiments, the microculture was examined with the electron microscope. Single neurons of either putative cholinergic or putative adrenergic character made morphological synapses on themselves (autapses), although only cholinargic autapses were detected electrophysiologically. Numerous axonal varicosities were present adjacent to the myocytes but no synaptic specializations were evident. After permanganate fixation to localize endogenous norepinephrine, the endings of neurons which appeared to secrete catecholamines contained many small granular vesicles, while the endings of neurons which appeared to secrete acetylcholine contained none. The endings of neurons which apparently secreted both catecholamines and acetylcholine contained only occasional small granular vesicles.