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

The electrophysiological properties of parasympathetic preganglionic neurons (PGN) in L6 and S1 spinal cord slices from neonatal rats were studied using the patch clamp techniques. PGN were identified by retrograde axonal transport of a fluorescent dye (Fast Blue) injected intraperitoneally before the experiment. PGN in the intermediolateral region of the spinal cord were divided into two classes (tonic PGN and phasic PGN) on the basis of firing properties during prolonged (300 ms) depolarizing current pulses. Tonic neurons exhibited a prolonged discharge (average maximum: 5.6); whereas phasic PGN fired on average only 1.4 spikes during depolarizing pulses. PGN were usually oval in shape. The mean long axis of tonic PGN (20.7+/-0.5 microm) was significantly (P<0.05) larger than that of phasic PGN (16.7+/-0.3 microm). Tonic and phasic PGN had similar resting membrane potentials, thresholds for spike activation, input resistances and action potential durations. The duration 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 mM) 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 during 300 ms depolarizing current pulses) and increased firing frequency by 19% in tonic PGN. These data indicate that the different discharge patterns of parasympathetic PGN are dependent in part on differences in the expression of 4-AP-sensitive K(+) channels. The two types of PGN may provide an innervation to different targets in the pelvic viscera.