Effects of pituitary adenylate cyclase activating polypeptide on lumbosacral preganglionic neurons in the neonatal rat spinal cord

The effects of PACAP-38 on phasic and tonic preganglionic neurons (PGN) in L6 and S1 spinal cord slices from neonatal rats (5-11 days old) were studie d using the whole-cell patch clamp technique. PGN were identified by retrog rade axonal transport of a fluorescent dye (Fast Blue, 5 mul of 4% solution ) injected into the intraperitoneal space 3-7 days prior to the study. Bath application of pituitary adenylate cyclase activating polypeptide (PACAP) (20 nM) increased the frequency of spontaneous excitatory postsynaptic pote ntials (EPSPs) and spontaneous firing in both types of PGN. PACAP markedly increased the number (200-800%) and frequency of action potentials elicited by depolarizing current pulses in phasic PGN. but had a smaller effect on tonic PGN. PACAP decreased the threshold for action potential generation by approximately 25% in both types of neurons (e.g. -34.0 +/-1.5 to -38.4 +/- 1.7 mV from a holding potential of -50 mV in phasic PGN, P<0.005). PACAP di d not affect the duration of the action potential. This amplitude of the sp ike after hyperpolarization was not changed but the duration was significan tly reduced by PACAP from 204.4<plus/minus>12.2 to 106.2 +/-8.1 ms in tonic but not in phasic PGN. PACAP suppressed a transient outward current that w as also suppressed by 3-aminopyridine (0.5 mM). These results coupled with the immunohistochemical identification of a dense collection of PACAP fiber s in the region of the PGN, raises the possibility that PACAP may function as an excitatory transmitter in lumbosacral parasympathetic reflex pathways in the neonatal rat. (C) 2001 Elsevier Science B.V. All rights reserved.