Nonclassical synaptic functions of transmitters.

Nonclassical synaptic functions are considered in two groups, mainly by reference to the models provided by sympathetic ganglia. Slow postsynaptic potentials (PSPs) are compared with classical fast PSPs. Features include loose delivery of transmitter to receptor, very long synaptic delays and durations of PSPs, slow removal of transmitter or of its effects, integration of repetitive inputs, electrogenesis without large increases in ionic conductances. Neuromodulatory actions affect synaptic efficacy without direct excitatory or inhibitory responses to the transmitter. These include a) control of presynaptic release, and b) contingent postsynaptic actions. In b, a modulatory transmitter alters the efficacy of action by another transmitter. The alteration may persist long after exposure to the modulatory transmitter; in mammalian sympathetic ganglia, exposure to dopamine or to a conditioning train of preganglionic volleys induces a long-term enhancement of the muscarinic slow excitatory PSP. Or the alteration may be restricted mostly to the presence of a modulatory transmitter, with examples cited. Nonclassical synaptic functions may be providing revolutionary possibilities for dealing with slow and broadly distributed cerebral functions, manifested electrophysiologically and behaviorally, that have been difficult to analyze successfully in terms restricted to the fast and discretely localized classical synaptic functions.