Yohimbine both increases and decreases nociceptive thresholds in rats: evaluation of the dose-response relationship.

The effect of yohimbine on a response to nociceptive stimulation mediated by supraspinal structures was studied after subcutaneous administration in doses from 0.25 mg/kg up to 25 mg/kg. The results showed that to describe adequately the dose-response relationship of yohimbine in pain modulation, characteristics of opposing effects had to be considered: low doses with an ED50-value of 0.56 mg/kg were found to dose-dependently facilitate nociception whereas high doses instead induced antinociception with an ED50 of 4.74 mg/kg. The possible capacity of alpha 2-adrenoceptors to modulate responses to nociception is discussed.