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Effect of morphine on proenkephalin gene expression in the rat brain
1Department of Cell Biology, Neurobiology and Anatomy, Loyola University Medical Center, Maywood, IL 60153, USA.
Abstract:
Previous studies indicate that an acute injection of morphine does not effect the level of opioid peptides and their mRNA in the brain. However, due to the presence of a large pool of mRNA and possible opposing changes in turnover rate it is often difficult to visualize the transitory and relatively small alterations in gene transcription by examining mRNA level. Therefore, in situ hybridization with probes directed against intronic sequences to measure the primary transcript of proenkephalin (PPE) mRNA (heteronucleic RNA, hnRNA) in the rat brain following morphine administration was used in this study. The distribution of the hybridization signal of probes against both the A and B intron of the PPE gene were identical and coincide with the distribution PPE mRNA. Thus, to increase the sensitivity of this assay both probes were concurrently hybridized. Female and male Sprague-Dawley rats were gonadectomized and injected with morphine (10 mg/kg, SC). We detected no changes in PPE mRNA levels in the striatum, olfactory tubercle (OT) and n. accumbens core (NAC) at any time following morphine administration. However, from 0.5 h until 24 h following morphine injection, the levels of PPE hnRNA in NAC and OT but not in the dorsal striatum were significantly decreased. The level of c-fos mRNA was increased only the dorsal striatum following morphine injections. These data show that morphine administration can acutely change opioid peptide gene transcription. The observed decrease of PPE hnRNA levels for 24 h following a single morphine injection may indicate its importance for the development of acute and chronic dependence. However, the significance of these alterations in PPE gene transcription in term of the acute effect of morphine is not clear, because the steady-state level of mRNA was not changed.
Insights
Morphine acutely decreases proenkephalin gene transcription in specific brain regions, as measured by primary transcripts (hnRNA), but not mature mRNA levels. This suggests changes in opioid peptide gene transcription occur before steady-state mRNA alterations.
Area of Science:
- Neuroscience
- Molecular Biology
- Pharmacology
Background:
- Acute morphine administration typically shows no effect on opioid peptide and mRNA levels.
- Measuring gene transcription directly is challenging due to mRNA pool size and turnover rates.
Purpose of the Study:
- To investigate the acute effects of morphine on proenkephalin (PPE) gene transcription.
- To differentiate between changes in gene transcription and mature mRNA levels following morphine exposure.
Main Methods:
- Utilized in situ hybridization with probes targeting intronic sequences of the proenkephalin gene (PPE hnRNA) in rat brains.
- Administered morphine (10 mg/kg, SC) to gonadectomized female and male Sprague-Dawley rats.
- Measured PPE hnRNA and mRNA levels in the striatum, olfactory tubercle (OT), and nucleus accumbens core (NAC), alongside c-fos mRNA.
Main Results:
- No significant changes in PPE mRNA levels were observed in the striatum, OT, or NAC post-morphine.
- A significant decrease in PPE hnRNA levels was detected in the NAC and OT from 0.5 h to 24 h after morphine injection.
- Increased c-fos mRNA levels were observed exclusively in the dorsal striatum.
Conclusions:
- Morphine administration acutely alters opioid peptide gene transcription, evidenced by decreased PPE hnRNA.
- The observed decrease in PPE hnRNA suggests a potential role in the development of opioid dependence.
- The lack of change in steady-state PPE mRNA levels indicates that transcriptional changes precede alterations in mature mRNA during acute morphine exposure.