Effects of cold stress on morphine-induced nitric oxide production and mu-opiate receptor gene expression in Mytilus edulis pedal ganglia.

OBJECTIVES: Subjecting the marine bivalve Mytilus edulis to an immediate temperature change has been shown to rapidly alter the animals' ganglionic monoamine levels, as well as its ciliary activity. Recently, we extended this observation to include the organism's ganglionic mu opiate receptor and morphine levels. In the past, we demonstrated that M. edulis ganglionic mu receptors exposed to morphine was coupled to the immediate release nitric oxide (NO). In this study, we measured morphine-induced NO release in M. edulis subjected to acute cold stress.

METHODS: NO release was monitored with an NO-selective microprobe. Temporal changes in mu opiate receptor expression were also examined over 24 hours.

RESULTS: In this study, we demonstrate that after 12h cold exposure (4 degrees C from 24 degrees C), the estimated relative mu opiate receptor (MOR) gene expression in M. edulis pedal ganglia, measured by real-time PCR, did not differ significantly from the control group (1.23+/-0.25, p>0.05). However, the measured M. edulis pedal ganglia MOR expression demonstrated that ganglia significantly (0.77+/-0.05, p<0.001) down regulated their mu opiate receptor mRNA expression after 24h exposure to the cold water. The mean value for control animal (24 degrees C, n=14) morphine-stimulated NO release was 36.7 +/- 9.8 nM. Morphine additions to cold-treated tissues (4 degrees C, n=7) produced an average of 6.7 +/- 4.9 nM NO, which was a statistically significant difference between 25 degrees C and 4 degrees C animals (p=0.025).

CONCLUSION: The study further demonstrates that mu opiate receptor expression is coupled to NO release.

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