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Opioid-neuroleptic interaction in brainstem self-stimulation
1Department of Psychology, Concordia University, Montreal, Que., Canada.
Abstract:
Injection of morphine into the ventral tegmental area (but not dorsal to it) induced a dose-dependent decrease in the frequency threshold for midline metencephalic brain stimulation reward. Facilitating doses of ventral tegmental morphine also reversed, in 4 of 6 animals, the threshold-increasing effects of pimozide (0.35 mg/kg, i.p.). This reversal was itself reversed by naloxone (2 mg/kg, i.p.), suggesting a direct action of morphine at ventral tegmental opiate receptors. These data fit with electrophysiological evidence that ventral tegmental morphine stimulates or disinhibits dopamine impulse flow, which would result in increased synaptic dopamine concentrations and decreased synaptic pimozide effectiveness. In the remaining two animals, the combined neuroleptic-opiate treatment resulted in a complete cessation of responding that was not reversed by a 5-fold increase in stimulation frequency. This finding suggested a complete inactivation of the reward mechanism, which might be expected from the interaction of high doses of two drugs that are each known to be capable of producing depolarization inactivation of dopaminergic neurons. These data confirm that brainstem self-stimulation, like medial forebrain bundle self-stimulation, depends critically on the function of the mesocorticolimbic dopamine system.
Insights
Morphine in the ventral tegmental area lowers brain stimulation reward thresholds by acting on opiate receptors. This suggests the mesocorticolimbic dopamine system is crucial for reward mechanisms.
Area of Science:
- Neuroscience
- Neuropharmacology
- Reward System Research
Background:
- The mesocorticolimbic dopamine system is implicated in reward and motivation.
- Understanding the precise mechanisms of dopamine's role in reward is essential for treating addiction and other disorders.
Purpose of the Study:
- To investigate the role of ventral tegmental area (VTA) morphine in modulating brain stimulation reward.
- To explore the interaction between morphine, pimozide, and naloxone within the VTA's reward circuitry.
Main Methods:
- Rats were administered morphine injections directly into the VTA.
- Midline metencephalic brain stimulation was used to assess reward thresholds.
- The effects of VTA morphine were examined in conjunction with pimozide and naloxone administration.
Main Results:
- Ventral tegmental area morphine dose-dependently decreased the frequency threshold for brain stimulation reward.
- Morphine's effects were reversed by naloxone, indicating action at opiate receptors.
- In some animals, combined neuroleptic-opiate treatment led to a complete cessation of responding, suggesting reward mechanism inactivation.
Conclusions:
- These findings confirm the critical role of the mesocorticolimbic dopamine system in brainstem self-stimulation.
- Morphine in the VTA directly influences reward pathways, likely through dopaminergic mechanisms.
- The study highlights the complex interactions between dopaminergic and opiate systems in reward processing.