Investigation of Morphine-Induced Dopamine Release in the Nucleus Accumbens by Fiber Photometry

Objective: Morphine is an opiate frequently used in the management of acute and chronic pain. Sequential use of morphine increases dopamine levels in the nucleus accumbens (NAc) through activation of dopaminergic neurons in the ventral tegmental area. The aim of the study was to investigate dopaminergic signals in the NAc during morphine dependence and withdrawal using fiber photometry system with dopaminergic sensor (GRABDA).
Materials and Methods: Male Wistar rats were divided into two groups as morphine (M) and morphine+naloxone (M+N). GRABDA was injected into the right NAc region. GRABDA contains a recognition domain based on the dopamine receptor 2 (DRD2) in its structure. When dopamine binds to this domain, the green fluorescent protein in the sensor is activated. The resulting fluorescence change reflects dopaminergic activity and signal transmission in the region. Afterwards, a fiber optic cable was fixed to NAc. After 15 days of rest, 10mg/kg morphine was injected intraperitoneally for 5 days. 3mg/kg naloxone was injected into the M+N group after the last injection. Average ΔF/F (%) values expressing dopaminergic signalling were calculated with Python after collecting fiber photometric records. The ΔF/F (%) value expresses the normalised level of dopamine binding to DRD2 receptors as a percentage. Statistical comparisons were made using a mixed-ANOVA test and post-hoc comparisons were made using estimated marginal means.
Results: The ΔF/F (%) values of the M and M+N groups increased with morphine injection compared to baseline period (p<0.05). As a result of naloxone injections, the ΔF/F (%) values of the M+N group significantly decreased compared to the M group (p<0.05). Morphine addiction increases dopaminergic signalling in the NAc, while naloxone suppresses this effect.
Conclusions: Fiber photometry with a novel biosensors enables more efficient detection of dopaminergic activity compared to older methods such as liquid chromatography. Further studies are needed for a better understanding of specific neuronal and regional processes.