[3H]acetylcholine release from the phrenic nerve is increased or decreased by activation or desensitization of nicotine receptors.

Nicotinic agonists (nicotine, 1,1-dimethyl-4-phenylpiperazinium (DMPP) increased [3H]acetylcholine release when the pre-exposure time was short (20 s). This effect was antagonized by pretreatment with d-tubocurarine (d-TC), which indicates a receptor-mediated effect. After a longer pre-exposure time (3 min) 10 microM nicotine significantly decreased the evoked [3H]acetylcholine release, probably because desensitization of the nicotine autoreceptors had abolished the nicotinic autofacilitation of transmitter release.