Drug-motivated behavior in rats with lesions of the thalamic orosensory area.

Rats suppress intake of a palatable taste cue when paired with a rewarding or an aversive stimulus in appetitive or aversive conditioning, respectively. A similar phenomenon occurs with drugs of abuse, but the nature of this conditioning has been subject for debate. While relatively little is known about the underlying neural circuitry, we recently reported bilateral lesions of the thalamic trigeminal orosensory area isolate drug-induced suppression of intake of a taste cue. The lesion blocks avoidance of the taste cue when paired with experimenter delivered drugs of abuse, yet has no effect on avoidance of the same cue when paired with an aversive agent or when it predicts access to a highly palatable sucrose solution. We hypothesize the lesion may blunt the rewarding properties of the drug. To test this, we used a runway apparatus, as running speed has been shown to increase with increasing reward value. Our hypothesis was supported by failure of the lesioned rats to increase running speed for morphine. Interestingly, lesioned rats did avoid intake of the drug-paired cue when presented in the runway apparatus and displayed naloxone-precipitated withdrawal. Using a partial crossover design, the lesion prevented avoidance of a cocaine-paired cue when presented in the home cage. We conclude that the lesion disrupts avoidance of a taste cue in anticipation of the rewarding properties of a drug but, at least in the presence of contextual cues, allows for avoidance of a taste cue as it elicits the onset of an aversive conditioned state of withdrawal.