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Expression of mu opioid receptor mRNA in rat brain: an in situ hybridization study at the single cell level
1Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104.
Abstract:
The mu (mu) opioid receptors, which mediate the effects of morphine, are widely distributed in brain. We have examined the distribution of mRNA encoding a mu opioid receptor in rat brain with in situ hybridization histochemistry at the single-cell level to obtain information about the cell types synthesizing this receptor. Only neurons, not glia, were labeled in discrete brain regions. High levels of labeling were detected in the thalamus, striosomes of the caudate-putamen, globus pallidus, and brain regions involved in nociception, arousal, respiratory control, and, possibly, addiction. The general distribution of the receptor mRNA paralleled that of mu opioid binding sites with some notable exceptions. These include the cerebral cortex, which contains binding sites, but very few labeled neurons. No labeling was observed in the cerebellum, a region devoid of mu binding sites. Three main findings emerged from these experiments: 1) the mRNA was present in regions mediating both the therapeutic (analgesia) and the unwanted (respiratory depression, addiction) effects of morphine, 2) the mRNA was very densely expressed by neurons known to receive dense enkephalin-containing inputs, and 3) the dissociation between the presence of binding sites and absence of mRNA in some brain regions supports a presynaptic localization of mu opioid receptors in these areas. Alternatively, other subtypes of mu opioid receptors may be encoded by a different mRNA. These results provide new insights into the receptor types and neuronal circuits involved in the effects of endogenous opioids and morphine.
Insights
This study maps mu opioid receptor mRNA in rat brains, revealing neurons, not glia, synthesize these receptors in key areas. This offers insights into morphine
Area of Science:
- Neuroscience
- Molecular Biology
- Pharmacology
Background:
- Mu (μ) opioid receptors are crucial for mediating morphine's effects and are widely distributed in the brain.
- Understanding the cellular localization of these receptors is essential for elucidating their roles in various physiological and pathological processes.
Purpose of the Study:
- To investigate the distribution of messenger RNA (mRNA) encoding the mu opioid receptor in the rat brain at a single-cell level.
- To identify the specific cell types responsible for synthesizing mu opioid receptors.
- To correlate receptor mRNA distribution with brain regions involved in nociception, arousal, respiratory control, and addiction.
Main Methods:
- In situ hybridization histochemistry was employed to detect mu opioid receptor mRNA in rat brain tissue.
- Analysis was performed at the single-cell level to precisely map the location of labeled neurons.
- Comparison of mRNA distribution with known distribution of mu opioid binding sites.
Main Results:
- Mu opioid receptor mRNA was exclusively detected in neurons, not glial cells, within specific brain regions.
- High levels of mRNA were found in the thalamus, striosomes, globus pallidus, and areas associated with pain, arousal, respiration, and addiction.
- Dissociation between mRNA presence and binding sites in certain areas (e.g., cerebral cortex) suggests presynaptic localization or alternative receptor subtypes.
Conclusions:
- Mu opioid receptor mRNA is expressed in neuronal populations involved in both therapeutic (analgesia) and adverse (respiratory depression, addiction) effects of morphine.
- High mRNA expression in neurons receiving enkephalin inputs highlights potential endogenous opioid signaling pathways.
- Findings provide crucial insights into the cellular basis of opioid action and the neuronal circuits underlying morphine's diverse effects.