Abnormal open field behavior after anterolateral hypothalamic injection of 6-hydroxydopamine

To investigate the importance of forebrain catecholamine terminals in open field behavior, rats were microinjected bilaterally in the medial forebrain bundle at the level of the anteralteral hypothalamus with 6-hydroxydopamine (6-OHDA), a specific catecholaminergic neurotoxin. These 6-OHDA microinjections produced extensive loss of forebrain catecholamine terminals; identical vehicle microinjections did not. When 6-OHDA rats were given 8 open field (OF) tests in the first or the fifth postinjection week, they had a longer latency to enter the OF, crossed fewer squares and reared less than normal rats or rats microinjected with vehicle. The abnormal OF behavior of 6-OHDA rats was not a generalized loss of locomotor activity because 6-OHDA rats were normally active in the home cage. The abnormal OF behavior of 6-OHDA rats was also not a result of a generalized lack of reactivity because the OF test elicited an increase of plasma corticosterone in 6-OHDA rats. The possibility that 6-OHDA rats were abnormal in the OF because they were hyperractive to it was not consistent with the observations that the OF activity of 6-OHDA rats did not change with repetitive testing, 6-OHDA rats did not defecate more than vehicle rats, and 6-OHDA rats did not display freezing behavior. These results suggest, but do not prove, that the abnormal OF behavior of 6-OHDA rats reflects a deficit of exploratory behavior that is correlated with extensive loss of forebrain catecholamine terminals.