Central monoaminergic mechanisms in mice and analgesic activity of spiradoline mesylate, a selective kappa-opioid receptor agonist.

We assessed the roles of brain monoaminergic systems in the analgesic action of spiradoline, a novel kappa-opioid agonist, behaviorally and biochemically by using noradrenaline (NE) and serotonin (5-HT) uptake inhibitors. Analgesic activity was evaluated by measuring latency time in the mouse tail-pinch test. Spiradoline at intramuscular doses of 0.3 mg/kg or more elicited a significant analgesic effect and the metabolism of both NE and 5-HT was significantly increased in brainstem and cortex. Pretreatment of the mice with imipramine, desipramine or clomipramine caused marked potentiation of spiradoline analgesia, whereas reserpine and phenoxybenzamine inhibited it. Morphine analgesia was enhanced by clomipramine but not by imipramine, desipramine or phenoxybenzamine. These results suggest that excitation of noradrenergic and serotonergic pathways in the brain appears to be involved in spiradoline analgesia, and that, as regards tail-pinch nociception, the kappa-opioid agonist acts on the noradrenergic pathway more potently than morphine.