Dopamine delta receptor effect

Enkephalins could be implicated in dopamine mesoaccumbens-depen-dent appetitive motivation and behavioral positive reinforcement by separated mechanisms involving both mu and delta receptors. Activation of the former in heroin abusers produces sedation and overstimulation, lack of vigilance, and possibly fear. These effects cannot be reached with dual inhibitors, and one can speculate that this is related to an increased level of endogenous enkephalins leading to a preferential interaction with delta receptors for both binding and anatomical reasons. 

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