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 was tested after 6-OHDA lesioning of this brain area and after ORG 2766 induced facilitation of recovery in 6-OHDA lesioned animals. A low dose of systemically administered apomorphine (50 micrograms/kg) decreased motility when sham operated rats were placed in a novel environment. A similar decrease was found in saline treated rats tested for the second time 1 day later. In thus habituated animals, the low dose of apomorphine did not induce hypomotility. Thus habituation and hypomotility after a low dose of apomorphine may be due to a similar mechanism, viz. diminished dopamine release. A higher dose of apomorphine (125 micrograms/kg) increased motility, but only when the rats were habituated to the test environment. Animals with a bilateral 6-OHDA lesion of the nucleus accumbens showed hypomotility when tested for the first time 1 week after the lesion. The low and the higher dose of apomorphine elicited hypermotility in both nonhabituated and habituated lesioned rats. Their activity was higher than in sham operated animals, suggesting supersensitivity of postsynaptically located dopamine receptor systems in lesioned rats. Treatment with the ACTH(4-9) analog ORG 2766 during the first 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.