Effects of B-HT 920 on nigrostriatal and mesolimbic dopamine systems in normosensitive and supersensitive rats.

1. B-HT 920, a D2 dopamine receptor agonist, was tested for its ability to exert presynaptic actions in normosensitive rats, and for possible postsynaptic actions in rats made 'supersensitive' to apomorphine. 2. In normosensitive rats, B-HT 920 (0.01-0.3 mg kg-1, i.p.) increased dopamine concentrations and lowered metabolite levels to a similar extent in all four terminal regions examined (medial prefrontal cortex, olfactory tubercle, nucleus accumbens, caudate-putamen). Analogous effects were seen for 5-hydroxytryptamine and its metabolite 5-hydroxyindoleacetic acid. 3. Rats which received bilateral 6-hydroxydopamine (6-OHDA) infusions into the caudate-putamen showed signs of postsynaptic dopamine receptor activation (stereotyped behaviour) in response to B-HT 920 (0.1 and 1.0 mg kg-1, i.p.) and to apomorphine (0.2 mg kg-1, s.c.). Similarly, B-HT 920 (0.1 mg kg-1) induced contralateral circling in rats that had received unilateral 6-OHDA infusions into the medial forebrain bundle; the rate of circling increased gradually over several weeks. 4. In contrast, bilateral 6-OHDA infusions into the nucleus accumbens resulted in a supersensitive (locomotor stimulant) response to a low dose of apomorphine (0.1 mg kg-1, s.c.), but not to B-HT 920 (0.01 and 0.1 mg kg-1). 5. In intact rats, withdrawal of chronic haloperidol treatment induced behavioural supersensitivity to apomorphine but not to B-HT 920.