Three experiments were performed in order to add further support to the hypothesis that the exaggerated aggressiveness elicited by apomorphine in REM sleep deprived rats was due to a state of supersensitivity of post-synaptic dopaminergic receptors in brain.
In the first experiment, REM deprived rats displayed much aggressiveness when challenged with 10 and 20 mg/kg of bromocriptine and piribedil. Thus, the intensification of responses by REM sleep deprivation is not restricted to apomorphine, as it was also obtained with two other dopaminergic agonists. In the second experiment, the association of REM deprivation with an injection of haloperidol 24 h before apomorphine administration induced still more aggressive behavior when compared to the rats that were only sleep deprived. It has been claimed that 24 h after haloperidol a state of supersensitivity to dopamine agonists occurs in the brain; therefore, it is probable that REM deprivation could also act similarly. The third experiment showed that haloperidol administered 2 h before apomorphine administration blocked the aggressive behavior in rats either submitted to REM deprivation alone or to REM deprivation plus a previous injection of haloperidol 24 h before. This also favors the proposed hypothesis.
Alternative possibilities for explaining the observed hyperresponsiveness of REM deprived rats to apomorphine and other dopaminergic agonists are also analysed.