Orthodromic production of non-cholinergic slow depolarizing response in the superior cervical ganglion of the rabbit.

1. A late slow depolarization in the rabbit superior cervical ganglion, recorded extracellularly as ;late late negative' (l.l.n.) response, can be elicited by suitably repetitive stimulation of cervical sympathetic nerve. The l.l.n. response is not blocked by strong nicotinic, muscarinic or adrenergic antagonists; it appears with latencies in seconds, rise times in minutes, durations of up to 20 min or more, and extracellular amplitudes that can exceed 1 mV when recorded in an air-gap chamber.2. The l.l.n. component is a graded post-synaptic response that decreases with a length constant similar to those of the known p.s.p.s (fast e.p.s.p., slow i.p.s.p., and slow e.p.s.p.). This and its other characteristics indicate that the l.l.n. response is neuronally generated and represents a non-cholinergic late slow depolarization. The term ;slow slow e.p.s.p.' is suggested for this response, to replace both ;slow depolarization' and ;late slow e.p.s.p.'.3. The amplitudes, evaluated relative to the compound action potentials, and the durations of l.l.n. responses recorded from intact neurones of rabbit superior cervical ganglion were considerably greater and more consistently producible than the non-cholinergic slow depolarizations recorded by others from impaled neurones of guinea-pig inferior mesenteric ganglion.4. The l.l.n. response does not exhibit the special sensitivity to sodium azide previously found for the muscarinic ;late negative' or slow e.p.s.p. response.5. The total number of orthodromic volleys is the chief determinant of the amplitude and duration of the l.l.n. response. Increases in pulse frequency, with no change in pulse number, exert only a minor influence on amplitude and duration of the l.l.n. response but can markedly decrease latency and rise time.6. Even very low pulse frequencies (e.g. 1/sec) are almost as effective as higher frequencies if a sufficiently large number of stimulus pulses is applied.7. The features of orthodromic production of the l.l.n., slow slow e.p.s.p. response, as well as the amplitudes and durations of this depolarization, indicate that this non-cholinergic post-synaptic response could, like the muscarinic slow e.p.s.p., play a significant role in mediating physiological activities of sympathetic ganglia.