Reinforcing stimulus properties of endocoids.

Intracranial self-administration methodologies can be used to directly assess the neurobiological substrates of the reinforcing stimulus properties of drugs and endogenous ligands that may bind to receptors initiating these neuronal events. The reinforcing properties of the intracranial infusion of methionine enkephalin into the nucleus accumbens where it is released from nerve endings were demonstrated. The mechanisms of the reinforcing stimulus resulting from the intracranial self-administration of cocaine into the medial prefrontal cortex were shown to require presynaptic dopaminergic intervations and postsynaptic dopaminergic D2 receptors. The endogenous ligand that competes for cocaine binding may initate a reinforcing stimulus through similar mechanisms which could be directly assessed with intracranial self-administration procedures.