Potentiation of acute opioid-induced respiratory depression and reversal of tolerance by the calcium antagonist nimodipine in awake rats.

The interaction between sufentanil, a mu-opioid agonist, and the Ca2+ antagonist nimodipine on respiration and on the development of opioid tolerance in awake rats has been analyzed. Our previous work demonstrated that chronic treatment with nimodipine together with sufentanil increases the analgesic potency of the opioid 50 fold. Therefore, we have investigated whether the opioid-induced respiratory depression is potentiated in parallel with the analgesia. Ventilation was measured by the whole body plethysmographic method. In naive rats, sufentanil (10-80 micrograms/kg) consistently induced a dose-dependent respiratory depression. Pretreatment with nimodipine (200 micrograms/kg) potentiated this effect but to a lesser extent than it potentiated analgesia. After chronic administration of the opioid (2 micrograms/h, 7 days) tolerance was manifested as a reduction in both the area under the time course curve and in the maximum effect. Nimodipine (1 microgram/h) administered concurrently with sufentanil for 7 days counteracted the tolerance to respiratory depression but no additional potentiation was observed. These results demonstrate that the interaction between nimodipine and sufentanil is not limited to antinociception but also extends to respiratory depression. However, compared with analgesia, the clinical relevance of a potential increase in opioid-induced respiratory depression by nimodipine may be negligible.