Effects of bifeprunox and aripiprazole on rat serotonin and dopamine neuronal activity and anxiolytic behaviour.

The atypical antipsychotic bifeprunox is a partial dopamine D(2) and 5-HT(1A) receptor agonist. Using in-vivo electrophysiological and behavioural paradigms in the rat, the effects of bifeprunox and aripiprazole were assessed on ventral tegmental area (VTA) dopamine and dorsal raphe serotonin (5-HT) cell activity and on foot shock-induced ultrasonic vocalisation (USV). In VTA, bifeprunox and aripiprazole decreased (by 20-50%) firing of dopamine neurons. Interestingly, bursting activity was markedly reduced (by 70-100%), bursting being associated with a larger synaptic dopamine release than single spike firing. Both ligands reduced inhibition of firing rate induced by the full dopamine receptor agonist apomorphine, whereas the D(2) receptor antagonist haloperidol prevented these inhibitory effects, confirming partial D(2)-like agonistic properties. On 5-HT neurons, bifeprunox was more potent than aripiprazole to suppress firing activity. The 5-HT(1A) receptor antagonist WAY-100,635 prevented their effects. In the USV test of anxiolytic-like activity, bifeprunox had higher potency than aripiprazole to reduce vocalisations. Both WAY-100,635 and haloperidol reversed the effects of both agonists. The present in-vivo study shows that bifeprunox is a potent partial D(2)-like and 5-HT(1A) receptor agonist reducing preferentially the phasic activity of dopamine neurons. Thus, bifeprunox would be expected to be an effective compound against positive and negative symptoms of schizophrenia.