Spinal serotonergic and kappa opioid receptors mediate facilitation of the tail flick reflex produced by vagal afferent stimulation.

Facilitation of the nociceptive tail flick (TF) reflex produced by low intensity (2.0 msec, 20 Hz) electrical stimulation of cervical vagal afferents (VAS) was studied in rats lightly anesthetized with pentobarbital. Serotonin (methysergide, LY 53857 or ICS 205-930), adrenergic (prazosin or yohimbine) or opioid (naloxone, nor-binaltorphimine or naltrindole) receptor antagonists were administered into the subarachnoid space of the lumbar enlargement to characterize the spinal receptors mediating VAS-produced facilitation. Spinal 5-HT1 and kappa-opioid receptors were determined to be involved in facilitation of the TF reflex produced by VAS, suggesting that facilitation of spinal nociceptive transmission by activation of cervical vagal afferents is mediated by subtypes of spinal serotonergic and opioid receptors.