Excitatory action of pituitary adenylate cyclase activating polypeptide on rat sympathetic preganglionic neurons in vivo and in vitro

Abstract

In vivo and in vitro experiments were undertaken to evaluate the effects of pituitary adenylate cyclase activating polypeptide-38 (PACAP-38) on rat sympathetic preganglionic neurons (SPNs). Intrathecal injection of PACAP-38 (0.1–1 nmol) via an implanted cannula to the T2-T3 segments of urethane-anesthetized adult rats caused a dose-dependent increase of mean arterial blood pressure from minutes to over 1 h. The pressor response was not antagonized by prior injection of the PACAP type II receptor antagonist PACAP 6-38 (0.5 nmol), but was significantly attenuated by prior intravenous administration of phentolamine (1 mg/kg). As a positive control, intrathecal injection of glutamate (1 μmol) and substance P (SP, 5 nmol) caused a short- and long-lasting pressor response. Vasoactive intestinal polypeptide (VIP, 1 nmol) had no significant pressor effect. In the second series of experiments, whole-cell patch recordings were made from antidromically identified SPNs of immature (12–16-day-old) rat thoracolumbar spinal cord slices. Applied to the spinal cord slices by superfusion, PACAP-38 (10–30 nM) caused intense neuronal discharges with or without a long-lasting membrane depolarization. The depolarization was not prevented by superfusing the slices with tetrodotoxin (0.3 μM) or low Ca 2+ (0.25 mM) solution, indicating that PACAP-38 directly depolarized the SPNs. The depolarization was insensitive to the type II PACAP receptor antagonist PACAP 6-38. Collectively, these results provide evidence that PACAP-38 exerts a potent and long-lasting excitatory effect on SPNs, leading to an increase of spinal sympathetic outflow and one of the consequences of which is an elevation of blood pressure.