Nucleus accumbens local field potential power spectrums, phase-amplitude couplings and coherences following morphine treatment

In the past decade, neural processing has been extensively studied in cognitive neuroscience. However, neural signaling in the nucleus accumbens (NAc) that might clarify reward process remained to be investigated. Male Swiss albino ICR mice implanted with intracranial electrodes into the NAc and the ventral tegmental area (VTA) were used for morphine administration and local field potential (LFP) recording. One-way ANOVA revealed significant increases in low (30.3-44.9 Hz) and high (60.5-95.7 Hz) gamma powers in the NAc following morphine administration (5 and 15 mg/kg, i.p.). These gamma activities oscillated independently with different time-course responses. Locomotor activity was also significantly increased by morphine administration. Regression analyses revealed that high gamma activity induced by morphine was positively correlated with distance travelled by animals. Low and high gamma powers were completely abolished by injection of naloxone, a non-specific opiate antagonist. Analysis of phase-amplitude coupling confirmed that slow oscillations at 1-4 Hz (delta) and 4-8 Hz (theta) for phase were found to significantly increase modulation index of broad (30.27-80.77 Hz) and narrow (59.48-70.34 Hz) frequency ranges for amplitude, respectively. Moreover, significant increases in coherence values between the NAc and the VTA during 30-40 min following morphine administration were seen for 22.46-44.90 Hz frequency range. Altogether, this study demonstrated changes of LFP oscillations in the NAc with low and high gamma activities, delta- and theta-gamma couplings and interplay with VTA in response to morphine administration. These findings represent neural signaling in the mesolimbic dopamine pathway that might process reward function.