Stimulation-produced descending inhibition from the periaqueductal gray and nucleus raphe magnus in the rat: mediation by spinal monoamines but not opioids.

Focal electrical stimulation in the midbrain periaqueductal gray (PAG) or medullary nucleus raphe magnus (NRM) inhibits spinal nociceptive transmission and nociceptive reflexes. The purpose of this study was to evaluate, in lightly pentobarbital-anesthetized rats, the spinal neurotransmitter(s) mediating descending inhibition of the nociceptive tail-flick (TF) reflex produced by focal electrical stimulation in the PAG or NRM. To characterize the neurotransmitter(s) mediating inhibition of the TF reflex, selective pharmacologic antagonists were administered into the lumbar intrathecal space. Stimulation thresholds in the PAG or NRM for inhibition of the TF reflex were established and the effects of intrathecally administered phentolamine, yohimbine, prazosin, methysergide (15 micrograms initially, 30 micrograms cumulative) or naloxone (10 micrograms initially, 20 micrograms cumulative) on TF inhibitory thresholds determined. Phentolamine, yohimbine and methysergide increased the intensity of stimulation in the PAG and the NRM for inhibition of the TF reflex; prazosin and naloxone had no effect. Descending inhibition produced by focal electrical stimulation in the PAG or NRM is mediated in part by spinal serotonergic and/or alpha 2-adrenergic receptors. Naloxone was administered both intrathecally and intravenously; however, a role for opioid receptors in descending inhibition from the midbrain or medulla was not found.