Anatomical and physiological observations on synapses formed on isolated autonomic neurons in tissue culture

Neurons from the superior cervical ganglion of perinatal rats were freed of their supporting cells and established in culture by the method of Bray. To these were added meninges-free explants of embryonic rat thoracic spinal cord to allow interaction between the outgrowing cord neurites and the isolated autonomic neurons. Electron microscopic observations on the autonomic neurons in these preparations revealed the presence of axon terminals of two types. One type (presumably adrenergic) contained pleomorphic vesicles after aldehyde fixation; these vesicles contained dense cores after KMnO₄ fixation. The second type of terminal (at least some of which were presumably cholinergic) contained uniform round vesicles without demonstrable dense cores. Extirpation of the cord explant demonstrated that the latter (but not the former) were of spinal cord origin. Intracellular recordings indicated that the electrical properties of dissociated superior cervical ganglion neurons were comparable to observations made in vivo. In ganglion neurons, grown with cord, spontaneous, small local depolarizations were frequently observed and these were sometimes associated with the generation of action potentials. The synaptic nature of the local potentials was suggested by increases in their amplitude during hyperpolarization of the postsynaptic membrane (discussed in ref. 18), and their reduction in amplitude and frequency in the presence of 15 mM MgCl₂.