This chapter summarizes the recent investigations of the transmitter status of sympathetic principal neurons derived from neonatal or adult rats and grown singly in microcultures with cardiac cells. The work began out of an interest in the status of individual neonatal sympathetic neurons during a transition from an initial (at least) adrenergic state to a predominantly cholinergic state, under the influence of non-neuronal cells. Under the influence of nerve growth factor, neurites grow progressively over the microculture but not beyond its borders. Many microcultures survive for 1–3 months; after such periods, the density of neurites over the myocytes is often greater than that of the normal innervation of sympathetic target tissues in vivo. The classical view of transmitter status in adult mammalian sympathetic principal neurons is that two transmitters—norepinephrine and acetylcholine—are expressed. Each neuron secretes only one transmitter (monofunction), and that transmitter is expressed approximately full-on; once the appropriate transmitter is adopted, the neuron does not change status.
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