Noradrenergic mediation of spinal antinociception by 5-hydroxytryptamine: characterization of receptor subtypes

The present study examined the involvement of spinal noradrenergic mechanisms in spinal antinociception by the 5-hydroxy-tryptamine (5-HT) receptor-selective agonists CGS 12066B (5-HT_1B; 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxalinc), TFMPP (5-HT_IC; M-trifluoromethylphenyl-piperazine) and DOI (5-HT₂; 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) using the rat hot plate test. Effects of α-adrenoreceptor antagonists (phentolamine, yohimbine), the adrenergic neurotoxin 6-hydroxydopamine, and the selective noradrenergic uptake blocker desipramine were determined. CGS 12066B, TFMPP and DOI produced dose-related antinociception. The antinockeptive effect of each agent was reduced by pretreatment with both phentolamine and yohimbine (15-60 μ g). Pretreatment with 6-hydroxydopamine (100 μg, intrathecal) for 7-10 days. which reduced spinal cord levels of noradrenaline by 87%, inhibited the action of TFMPP (and 5-HT), but not CGS 12066B or DDI Pretreatment with desipramine (25 mg/kg, systemic) potentiated the action of TFMPP but not CGS 12066B or DOI (or 2-methyl-5-HT). These results suggest that antinociception by TFMPP is dependent on release of endogenous noradrenaline from the spinal cord, while that produced by CGS 12066B and DOI is not. As TFMPP exhibits a close similarity to 5-HT in these experiments, the 5-HT receptor subtype being activated to induce noradrenaline release may either be a 5-HT_IC or a 5-HT_IS subtype. Other mechanisms account for the observed blockade of the action of CGS 12066B and DOI by α-adrenoreceptor antagonists.