THE BEHAVIORAL-EFFECTS OF HABITUATION AND CHALLENGE WITH APOMORPHINE AFTER 6-OHDA LESIONING OF THE NUCLEUS-ACCUMBENS IN RATS

In rats the function of the dopamine system in the nucleus accumbens w as tested after 6-OHDA lesioning of this brain area and after ORG 2766 induced facilitation of recovery in 6-OHDA lesioned animals. A low do se of systemically administered apomorphine (50 mu g/kg) decreased mot ility when sham operated rats were placed in a novel environment. A si milar decrease was found in saline treated rats tested for the second time 1 day later. In thus habituated animals, the low dose of apomorph ine did not induce hypomotility. Thus habituation and hypomotility aft er a low dose of apomorphine may be due to a similar mechanism, viz. d iminished dopamine release. A higher dose of apomorphine (125 mu g/kg) increased motility, but only when the rats were habituated to the tes t environment. Animals with a bilateral 6-OHDA lesion of the nucleus a ccumbens showed hypomotility when tested for the first time 1 week aft er the lesion. The low and the higher dose of apomorphine elicited hyp ermotility in both nonhabituated and habituated lesioned rats. Their a ctivity was higher than in sham operated animals, suggesting supersens itivity of postsynaptically located dopamine receptor systems in lesio ned rats. Treatment with the ACTH(4-9) analog ORG 2766 during the firs t week after induction of the lesion counteracted the hypomotility of the lesioned rats. Furthermore ORG 2766 enhanced the supersensitivity as revealed by challenge with the low dose of apomorphine.