Effects of reversible inactivation of the ventral tegmental area on the firing rate of neurons in the shell sub-region of the nucleus accumbens and on morphine-induced conditioned place preference in the rat

The mesolimbic dopaminergic system that projects from the ventral tegmental area (VTA) to the nucleus accumbens (NAc) is critical for initiation of opioid reinforcement and reward-related effects of drugs of abuse. In the present study, the effects of reversible inactivation of VTA on firing rate of nucleus accumbens neurons and on acquisition and expression of morphine-induced conditioned place preference (CPP) were investigated in rats. Adult male Wistar rats were used in these experiments. In behavioral study, the reversible inactivation of VTA was done through bilateral intra-VTA microinjection of 2% lidocaine during the acquisition and expression of morphine (5 mg/kg; s.c.)-induced CPP and in electrophysiology section, it was done through unilateral intra-VTA microinjection of 2% lidocaine during single unit recording from the NAc neurons. Conditioning score and locomotor activity were recorded by Ethovision software. Firing rate of neurons was recorded by single unit recording technique. The results showed that bilateral intra-VTA administration of lidocaine significantly decreases the acquisition (P<0.01) and expression (P<0.05) of morphine-induced CPP compared to their respective saline-microinjected groups. Moreover, intra-VTA administration of lidocaine had no effect on locomotor activity in these experiments. Also, unilateral intra-VTA administration of lidocaine significantly increased the firing rate of nucleus accumbens neurons. Our results further support the idea that VTA may play an important role in the acquisition and expression of morphine-induced CPP.