Effects of two different reversible monoamine oxidase-A inhibitors on nociceptive thresholds in the rat

The acute effect of two different reversible inhibitors of monoamine oxidase-A on nociceptive thresholds was evaluated in the rat by the tail-flick and hot-plate tests. CGP 11305-A, a monoamine oxidase-A inhibitor that also blocks serotonin reuptake, elicited an increase of latency in the tail-flick and the hot-plate test. Ineffective doses of CGP 11305-A increased nociceptive thresholds when administered in combination with other serotoninergic agents, i.e. chlorimipramine or 5-hydroxytryptophan, at doses that were ineffective alone. CGP 22364-A, a pure inhibitor of monoamine oxidase-A, increased latency only in the hot-plate test. Both compounds decreased spontaneous locomotor activity at the doses effective in the hot-plate test, suggesting that the responses observed in this test are not related to a pure effect on nociceptive thresholds. The data suggest that the increase in serotonin availability induced by monoamine oxidase-A inhibition alone is not sufficient to affect nociceptive thresholds.