Increased amphetamine-induced locomotor activity, sensitization and accumbal dopamine release in M5 muscarinic receptor knockout mice

Muscarinic M5 receptors are the only muscarinic receptor subtype expressed by dopamine-containing neurons of the ventral tegmental area. These cells play an important role for the reinforcing properties of psychostimulants and M5 receptors modulate their activity. Previous studies showed that M5 receptor knockout (M 5 −/− ) mice are less sensitive to the reinforcing properties of addictive drugs. Here, we investigate the role of M5 receptors in the effects of amphetamine and cocaine on locomotor activity, locomotor sensitization, and dopamine release using M 5 −/− mice backcrossed to the C57BL/6NTac strain. Sensitization of the locomotor response is considered a model for chronic adaptations to repeated substance exposure, which might be related to drug craving and relapse. The effects of amphetamine on locomotor activity and locomotor sensitization were enhanced in M 5 −/− mice, while the effects of cocaine were similar in M 5 −/− and wild-type mice. Consistent with the behavioral results, amphetamine-, but not cocaine, -elicited dopamine release in nucleus accumbens was enhanced in M 5 −/− mice. The different effects of amphetamine and cocaine in M 5 −/− mice may be due to the divergent pharmacological profile of the two drugs, where amphetamine, but not cocaine, is able to release intracellular stores of dopamine. In conclusion, we show here for the first time that amphetamine-induced hyperactivity and dopamine release as well as amphetamine sensitization are enhanced in mice lacking the M5 receptor. These results support the concept that the M5 receptor modulates effects of addictive drugs.