α₂‐Adrenergic drugs modulate the binding of [¹⁸F]fallypride to dopamine D2/3 receptors in striatum of living mouse

AIM: To test for alpha(2) adrenergic modulation of dopamine D(2/3) receptor availability in striatum of living mice using the high-affinity ligand [(18)F]fallypride and microPET. METHODS: Groups of anesthetized mice were pretreated with saline, the alpha(2)-agonist clonidine (1 mg/kg), and the alpha(2)-antagonists RX821002 (1 mg/kg) and yohimbine (1 mg/kg). Dynamic microPET recordings lasting 120 min were then initiated upon i.v. tracer injection of [(18)F]fallypride. Parametric maps of [(18)F]fallypride binding potential (BP(ND)) were calculated using the Logan method, with cerebellum serving as the reference region. RESULTS: Mean striatal [(18)F]fallypride BP(ND) was 10.6 +/- 1.7 in the saline control animals, 8.9 +/- 1.7 (-16%; P < 0.05) in the RX821002 group, 8.3 +/- 2.6 (-22%; P < 0.05) in the yohimbine group and 10.3 +/- 2.2 (n.s.) in the clonidine group. CONCLUSIONS: These findings are consistent with a tonic inhibition of dopamine release by alpha(2) adrenergic receptors, such that alpha(2) blockade increased the competition from endogenous dopamine at D(2/3) receptors, thus reducing the [(18)F]fallypride BP(ND) by about 20%. Absent effects of clonidine suggest a ceiling effect in the tonic inhibition of dopamine release. This in vivo PET evidence for alpha(2)/dopaminergic interaction may be relevant to putative actions of atypical antipsychotic medications via adrenergic receptors.