Pituitary adenylate cyclase activating peptide produces a marked and long-lasting depression of a C-fibre-evoked flexion reflex.

Pituitary adenylate cyclase activating peptide (PACAP) is a novel vasoactive intestinal peptide-like peptide. It has a neuronal localization and occurs in two forms, PACAP-38 and the C-terminally truncated PACAP-27. In a recent report we described a dense accumulation of PACAP-27-immunoreactive nerve fibres in the superficial layer of the dorsal horn in the spinal cord and a few PACAP-27-immunoreactive nerve cell bodies in the spinal ganglia in the rat. Double-immunostaining showed that immunoreactive PACAP-27 occurred in a subpopulation of the calcitonin gene related peptide/substance P-containing cell bodies and fibres. In this study, PACAP-27 (0.47-0.63 pmol) given intrathecally produced a significant and long-lasting suppression of C-fibre-evoked flexion reflex produced by the electrical stimulation of the plantar nerve and recorded as a reflex discharge in the peroneal nerve. With a higher dose, 1.25 pmol, the magnitude of the response was not increased further but the effect became longer-lasting. PACAP-38 and vasoactive intestinal peptide were tested in doses up to 5 pmol and produced a significant and, in the case of PACAP-38, long-lasting suppression of the spinal flexion reflex. PACAP-27 seemed to be much more potent than PACAP-38 and vasoactive intestinal peptide. Intrathecal PACAP-27 was without effect on the monosynaptic stretch reflex (evoked by electrical stimulation of the gastrocnemius nerve and recorded as a reflex discharge in the sciatic nerve), suggesting that its effect on the flexion reflex is specific.(ABSTRACT TRUNCATED AT 250 WORDS)