Antinociceptive action of dopamine agonists in the nucleus raphe magnus of rats is mediated by D2 receptors.
We have shown previously that the dopamine agonist apomorphine is antinociceptive when injected into the nucleus raphe magnus of rats, presumably via stimulation of descending antinociceptive pathways. We have now extended the investigation to receptor-specific agonists. We measured nociception in conscious Sprague-Dawley rats, using the latency of the first flexor response following tail immersion in water at 49 degrees C, before and after injection of agonists into the nucleus raphe magnus through chronically implanted cannulae. The D2 agonist PPHT HCl produced an increase in latency, i.e. hypoalgesia, for 30 min after injection of doses of 10 and 20 nmol. The lowest dose of the D1 agonist SKF 38393 produced no change in latency, but a bimodal response was found at the higher two doses (20 and 200 nmol), with hypoalgesia at 15 min, followed by a sustained hyperalgesia. The dopamine autoreceptor agonist R (+)-3PPP produced a significant hypoalgesia at 10 and 20 nmol 15 min after injection, with no subsequent hyperalgesia. Our results are consistent with the hypothesis that the antinociceptive action of dopamine in the rat nucleus raphe magnus is mediated by the D2 receptor.
Phillips
S
S
Department of Physiology, University of the Witwatersrand Medical School, Parktown, Republic of South Africa.
Gelgor
L
L
Mitchell
D
D
D000700:Analgesics / Q000494:pharmacology*; D000818:Animals; D015259:Dopamine Agents / Q000494:pharmacology*; D008297:Male; D011903:Raphe Nuclei / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011930:Reaction Time / Q000187:drug effects; D017448:Receptors, Dopamine D2 / Q000187:drug effects / Q000502:physiology*
D000700:Analgesics; D015259:Dopamine Agents; D017448:Receptors, Dopamine D2