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In vivo morphine decreases [3H]nimodipine receptor binding in rat brain regions
1Department of Pharmacology, University of Texas Health Science Center, San Antonio 78284-7764.
Abstract:
The in 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 (Bmax 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 ICa2+ following opiate treatment and suggest opiates reduce Ca2+-dependent neurotransmitter release by altering the Ca2+ channel receptor protein in an allosteric fashion.
Insights
Morphine, a common opiate, alters calcium channel binding in rat brains, affecting neurotransmitter release. Naloxone, an antagonist, reverses these effects, suggesting a role for calcium channels in opiate action.
Area of Science:
- Neuroscience
- Pharmacology
- Molecular Biology
Background:
- Opiates are known to modulate neuronal activity.
- Calcium channels play a critical role in neurotransmitter release.
Purpose of the Study:
- To investigate the in vivo effect of morphine and naloxone on [3H]nimodipine binding to calcium channels in rat brain regions.
- To explore the interaction between opiates and calcium channel receptor proteins.
Main Methods:
- Administration of morphine (15 mg/s.c.) and naloxone (1 mg/kg) to rats.
- Measurement of [3H]nimodipine receptor binding in various rat brain regions (cortex, cerebellum, hippocampus, striatum, hypothalamus, brain stem).
Main Results:
- Morphine significantly decreased [3H]nimodipine binding in the cortex (19%) and cerebellum (29%).
- Lesser decreases were observed in hippocampal and striatal regions; no changes were noted in the hypothalamus and brain stem.
- Naloxone pretreatment completely antagonized the effects of morphine.
Conclusions:
- In vivo opiates, such as morphine, can alter [3H]nimodipine binding to the Ca2+ channel receptor protein.
- These findings support the hypothesis that opiates reduce Ca2+-dependent neurotransmitter release via allosteric modulation of calcium channel receptor proteins.
