We evaluated the opioid antinociceptive mechanism of the calcium channel blocker flunarizine. Flunarizine produced a dose-dependent analgesia in the hot plate test which was antagonized by general (naloxone) and μ [beta-fualtrexamine (β-FNA)]-opioid antagonists, indicating a role for μ receptors in flunarizine analgesia. Naltrindole (δ1 antagonist) did not affect flunarizine analgesia. A fixed subthreshold dose of flunarizine augmented μ (morphine) analgesia and blocked δ1, [enkephalin, [D-Pen2,5] (DPDPE)] analgesia. Apparently, flunarizine has μ agonistic activity and δ1, antagonist or mixed agonistic-antagonistic activity.
- Opioid receptor subtypes