Spinal 5-HT pathways and the antinociception induced by intramedullary clonidine in rats.
The possible involvement of spinal 5-hydroxytryptamine (5-HT) pathways in antinociception induced by microinjection of clonidine into the ventrolateral surface of the medulla oblongata was investigated in rats. Microinjection of clonidine (10-20 micrograms), but not yohimbine (1 microgram) or 0.9% saline, into the lateral medulla prolonged the hot plate latency in rats. This clonidine-induced antinociception was abolished by intramedullary injection of the alpha 2-adrenoceptor antagonist, yohimbine. Selective destruction of spinal 5-HT neurons produced by intraspinal injection of 5,7-dihydroxytryptamine (5,7-DHT; 10 micrograms) or postsynaptic blockade of spinal 5-HT receptors produced by intrathecal injection of cyproheptadine (1 microgram; a mixed 5-HT1/5-HT2 antagonist) also abolished clonidine-induced antinociception. Rats given 5,7-DHT intraspinally or cyproheptadine intrathecally showed a decrease in hot plate latency as compared with the controls. In anesthetized rats, the 5-HT release from the thoracic spinal cord was enhanced by microinjection of clonidine into the lateral medulla. This enhanced spinal 5-HT release evoked by intramedullary injection of clonidine was abolished by pretreatment of rats with intraspinal injection of 5,7-DHT. These results indicate that 5-HT pathways to the spinal cord mediate the antinociceptive effect induced by microinjection of clonidine into the ventrolateral surface of the medulla oblongata in rats.
Lin
M T
MT
Department of Physiology, National Cheng Kung University Medical College, Tainan City, Taiwan, Republic of China.
Su
C F
CF
D015116:5,7-Dihydroxytryptamine / Q000494:pharmacology; D000700:Analgesics / Q000494:pharmacology*; D000818:Animals; D003000:Clonidine / Q000008:administration & dosage / Q000494:pharmacology*; D003533:Cyproheptadine / Q000494:pharmacology; D007278:Injections, Spinal; D008297:Male; D008526:Medulla Oblongata; D008845:Microinjections; D009434:Neural Pathways / Q000187:drug effects / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000502:physiology*; D013116:Spinal Cord / Q000502:physiology*; D015016:Yohimbine / Q000494:pharmacology
D000700:Analgesics; D015016:Yohimbine; D003533:Cyproheptadine; D015116:5,7-Dihydroxytryptamine; D012701:Serotonin; D003000:Clonidine