Diverse effects of intrathecal pituitary adenylate cyclase-activating polypeptide on nociceptive transmission in mice spinal cord.

Pituitary adenylate cyclase-activating polypeptide (PACAP) immunoreactive neural elements have been detected in the mouse spinal cord. The discrepancy of PACAP actions in the role of sensory transmission has been proposed to have potentiation and inhibition on nociceptive responses after intrathecal application of PACAP. The aim of the present study was to assess nociceptive transmission of PACAP in the mouse spinal cord by comparison with that of substance P (SP). The intrathecal injection of PACAP induced licking or scratching behavior similar to that of SP. These PACAP-induced aversive behaviors showed different manner from SP-induced responses in point of time course. SP-induced aversive responses quickly increased and suddenly disappeared almost within 1 min. Meanwhile, following a long latency after the injection, PACAP-induced aversive responses gradually appeared, and then persisted more than 60 min. In the early phase, PACAP produced an increase of tail flick latency. Pretreatment with 6-hydroxydopamine (6-OHDA) which destroys noradrenaline neuron of descending pain inhibitory systems in the spinal cord markedly abridged the latency and augmented the duration of PACAP-induced aversive responses. In this way, PACAP exhibits diverse effects on nociception, such as an analgesic role in early phase of the injection and subsequently lasting algesia. These results suggest that PACAP as a neurotransmitter or neuromodulator might have crucial role in nociceptive transmission system.