Delta opioid receptors contribute to the cardiorespiratory effects of biphalin in anesthetized rats.

Biphalin expresses almost equal affinity for μ- and δ-opioid receptors. The aim of this study was to delineate a possible role of δ-opioid receptors in the cardio-respiratory effects of systemic injection of biphalin in anesthetized, spontaneously breathing rats. In control animals, an intravenous bolus of biphalin (0.3 μmol/kg) evoked apnea, followed by a decreased breathing rate and increased tidal volume, hypotension and bradycardia. Blockade of δ-opioid receptors with naltrindole (4.2 μmol/kg) significantly reduced the duration of apnea, slowdown of respiration, immediate post-challenge hypotension and bradycardia induced by biphalin administration. These results indicate that the activation of δ-opioid receptors adds to the depressive response produced by biphalin.