In vivo morphine decreases [3H]nimodipine receptor binding in rat brain regions

Thein vivo effect of the mu agonist morphine and antagonist naloxone on [³H]nimodipine receptor binding in rat brain regions has been investigated. Morphine administration (15 mg/s.c.) for thirty minutes produced a 19% decrease in [³H]nimodipine receptor binding (B_max 158.2 fmol to 128.9 fmol) in cortex and 29% decrease in cerebellum (65.3 fmol to 46.0 fmol). Lesser changes were observed in hippocampal and striatal regions with no changes in hypothalamus and brain stem. All effects were completely antagonized by naloxone pretreatment (1 mg/kg). The studies suggest that opiates in vivo can alter [³H]nimodipine binding to the Ca²⁺ channel receptor protein. These findings agree with the previously observed decreases in Ca²⁺ influx in nerve ending preparations and inhibition of I_Ca²⁺ following opiate treatment and suggest opiates reduce Ca²⁺-dependent neurotransmitter release by altering the Ca²⁺ channel receptor protein in an allosteric fashion.