Opiate-like substances mediate norepinephrine-induced but not serotonin-induced antinociception at spinal level: reevaluation by an electrophysiological model of formalin test in rats.
After subcutaneous injection of formalin (5%, 50 microl) into a hindpaw of rats, biphasic excitatory nociceptive discharges were recorded extracellularly in thalamic parafascicular neurons. Intrathecal (i.t.) administration of either norepinephrine (NE. 6 nmol, 10 microl) or serotonin (5-HT, 120 nmol, 10 microl) prior to the second phase significantly inhibited the second phase of the formalin-induced parafascicular nociceptive discharges. Intrathecal naloxone (Nal, 50 nmol, 10 microl) did not show any effect on the parafascicular nociceptive discharges. However, when i.t. Nal was given 5 min before NE, Nal prevented the NE antinociceptive effect. Pre-administration of Nal before 5-HT did not affect the antinociceptive effects of 5-HT on the second phase of nociceptive discharges. These results indicate that opiate-like substances are involved in the mediation of NE-induced antinociception. It is suggested that endogenous NE and 5-HT released from brainstem descending terminals at the spinal level carry out their antinociceptive actions differently.
Ma
J
J
Department of Neurobiology, Shanxi Medical University, Taiyuan, P.R. China.
Qiao
J T
JT
Dafny
N
N
D000818:Animals; D004347:Drug Interactions; D004594:Electrophysiology; D005260:Female; D005557:Formaldehyde; D008297:Male; D009270:Naloxone / Q000494:pharmacology*; D009619:Nociceptors / Q000187:drug effects*; D009638:Norepinephrine / Q000494:pharmacology*; D010147:Pain Measurement / Q000187:drug effects; D051381:Rats; D017208:Rats, Wistar; D012701:Serotonin / Q000494:pharmacology*
D005557:Formaldehyde; D012701:Serotonin; D009270:Naloxone; D009638:Norepinephrine