Differential involvement of dopamine receptors in conditioned suppression induced by cocaine

Inactivation of ERK1/2 Signaling in Dopaminergic Neurons by Map Kinase Phosphatase MKP3 Regulates Dopamine Signaling and Motivation for Cocaine

The mesolimbic dopamine system is a crucial component of reward and reinforcement processing, including the psychotropic effects of drugs of abuse such as cocaine. Drugs of abuse can activate intracellular signaling cascades that engender long-term molecular changes to brain reward circuitry, which can promote further drug use. However, gaps remain about how the activity of these signaling pathways, such as ERK1/2 signaling, can affect cocaine-induced neurochemical plasticity and cocaine-associated behaviors specifically within dopaminergic cells. To enable specific modulation of ERK1/2 signaling in dopaminergic neurons of the ventral tegmental area, we utilize a viral construct that Cre dependently expresses Map kinase phosphatase 3 (MKP3) to reduce the activity of ERK1/2, in combination with transgenic rats that express Cre in tyrosine hydroxylase (TH)-positive cells. Following viral transfection, we found an increase in the surface expression of the dopamine transporter (DAT), a protein associated with the regulation of dopamine signaling, dopamine transmission, and cocaine-associated behavior. We found that inactivation of ERK1/2 reduced post-translational phosphorylation of the DAT, attenuated the ability of cocaine to inhibit the DAT, and decreased motivation for cocaine without affecting associative learning as tested by conditioned place preference. Together, these results indicate that ERK1/2 signaling plays a critical role in shaping the dopamine response to cocaine and may provide additional insights into the function of dopaminergic neurons. Further, these findings lay important groundwork toward the assessment of how signaling pathways and their downstream effectors influence dopamine transmission and could ultimately provide therapeutic targets for treating cocaine use disorders.

Cocaine-induced reinstatement in rats: evidence for a critical role of cocaine stimulus properties

RATIONALE

The exact behavioral nature of drug-induced reinstatement of drug seeking is still debated. As an incentive, the drug can have general facilitatory influences on appetitive behaviors. As an interoceptive stimulus, the drug can acquire discriminative properties and control behavior.

OBJECTIVE

This study assessed the relative contribution of the incentive versus discriminative properties of cocaine in food-seeking reinstatement.

METHODS

In Experiment 1, eight groups of rats were trained to press a lever for food pellets and experienced cocaine (0, 5, 10, or 15 mg/kg; i.p.), either during the operant conditioning sessions or 4 h after, in another environment without food access. In Experiment 2, to dissociate the role of the operant response per se from the consummatory response, two groups of rats experienced food consumption under cocaine (10 mg/kg; i.p.) either during operant conditioning sessions or during alternate sessions of free access to the food. Then, for both experiments, food pellets were withheld and cocaine injections ceased (extinction). The reinstating effects of noncontingent cocaine (10 mg/kg; i.p.) and food pellet delivery were assessed. Locomotor activity was recorded to probe expression of behavioral sensitization.

RESULTS

Cocaine reinstated lever pressing only in rats having previously performed the operant responses under cocaine. In contrast, food pellet delivery reinstated lever pressing independently of rats' history with cocaine. Locomotor sensitization was evidenced for all cocaine-pre-exposed rats, dissociating sensitization from reinstatement.

CONCLUSIONS

When present during operant conditioning, the stimulus "cocaine" acquires conditioned properties which can then promote reinstatement of the extinguished behavior.