EXCITOTOXIC LESIONS OF THE MEDIODORSAL THALAMIC NUCLEUS ATTENUATE INTRAVENOUS COCAINE SELF-ADMINISTRATION

The present experiments investigated the effects of excitotoxic, axon- sparing lesions of the mediodorsal nucleus of the thalamus (MD) on loc omotor activity and IV cocaine self-administration. Infusion of quinol inic acid into the MD using a glass micropipette produced well-defined neuronal loss restricted to medial and lateral portions of the MD, sp aring adjacent areas such as the lateral habenula and paraventricular thalamic nucleus. MD lesions resulted in delayed habituation to activi ty cages, In addition, lesioned rats self-administered significantly s maller amounts of cocaine than controls during a 14-day acquisition pe riod, and showed attenuated responding for cocaine doses on the descen ding limb of the dose-effect function. Since typical titrating pattern s of responding were maintained in lesioned rats, and responding on th e inactive lever did not differ from sham-operated animals, these pres ent results indicate an enhanced sensitivity to the reinforcing effect s of response-contingent cocaine in rats with excitotoxic lesions of t he MD.