Electrophysiological properties of lumbosacral preganglionic neurons in the neonatal rat spinal cord

The electrophysiological properties of parasympathetic preganglionic neuron s (PGN) in L6 and S1 spinal cord slices from neonatal rats were studied usi ng the patch clamp techniques. PGN were identified by retrograde axonal tra nsport of a fluorescent dye (Fast Blue) injected intraperitoneally before t he experiment. PGN in the intermediolateral region of the spinal cord wen d ivided into two classes (tonic PGN and phasic PGN) on the basis of firing p roperties during prolonged (300 ms) depolarizing current pulses. Tonic neur ons exhibited a prolonged discharge (average maximum: 5.6); whereas phasic PGN fired on average only 1.4 spikes during depolarizing pulses. PGN were u sually oval in shape. The mean long axis of tonic PGN (20.7+/-0.5 mu m) was significantly (P<0.05) larger than that of phasic PGN (16.7+/-0.3 mu m). T onic and phasic PGN had similar resting membrane potentials, thresholds for spike activation, input resistances and action potential durations. The du ration of the after-hyperpolarization (AHP) in tonic PGN (200.5+/-11.9 ms) was longer than in phasic PGN (137.6+/-9.8 ms). 4-aminopyridine (4-AP, 0.5 mu M) reduced the threshold for spike activation in tonic and phasic PGN. 4 -AP also unmasked tonic firing in phasic PGN (average maximum: 5.5 spikes d uring 300 ms depolarizing current pulses) and increased firing frequency by 19% in tonic PGN. These data indicate that the different discharge pattern s of parasympathetic PGN are dependent in part on differences in the expres sion of 4-AP-sensitive K+ channels. The two types of PGN may provide an inn ervation to different targets in the pelvic viscera. (C) 2000 Elsevier Scie nce B.V. All rights reserved.