European Journal of Pharmacology, 166 (1989) 303-305 303 Elsevier EJP 20411 Short communication BHT-920 and LY-171555 (quinpirole) have similar affinities for striatal D-2 dopamine receptors, and similar affinities for striatal D-1 dopamine receptors Sergi Ferr6 *, Teresa Guix, Joan Sall6s 1, Laura Cuffi 1, Albert Badia 1 and Miquel Casas Laboratori de Neuropsicofarmacologia, Programa Sant Pau-CITRAN, Fundaci6 d'Investigaci6 Santa Creu i Sant Pau, Hospital de la Santa Creu i de Sant Pau and 1 Departament de Farmacologia i Psiquiatria, Universitat Autbnoma de Barcelona, Bellaterra, Barcelona, Spain Received 25 April 1989, accepted 16 May 1989 The affinity of LY-171555 (quinpirole) and BHT-920 for both states of rat striatal dopamine D-1 and D-2 receptors was determined. Although these drugs have different pharmacological effects in experimental animals, we found that they had similar affinities for both D-1 and D-2 receptors. BHT-920; Quinpirole; Dopamine D-1 receptors; Dopamine D-2 receptors; Striatum; (Rat) 1. Introduction Although both LY-171555 (quinpirole) and BHT-920 seem to be selective D-2 dopaminergic agonists (And6n and Grabowska-And6n, 1988; Seeman and Niznik, 1988), they cause different effects in experimental animals. In rodents BHT- 920 produces either a decrease or no change in locomotor activity whereas LY-171555 causes an increase in locomotor activity (Hinzen et al., 1986; Waiters et al., 1987; And6n and Grabowska- And6n, 1988). However, the decrease in locomotor activity which follows dopaminergic depletion (produced by the administration of reserpine or a-methyl-parathirosine) is not reversed by any of these substances, unless a selective D-1 dopamine receptor agonist is administered simultaneously (Barone et al., 1986; Waiters et al., 1987; And6n and Grabowska-And6n, 1988). Furthermore, without dopamine depletion, the coadministration of a selective D-1 dopamine receptor agonist pro- duces an increase in locomotor activity in animals treated with BHT-920 and a potentiation of the locomoter activation induced by LY-171555 (Barone et al., 1986; Waiters et al., 1987; And6n and Grabowska-And6n, 1988; Pifl and Horny- kiewicz, 1988). Because of these pharmacological differences between LY-171555 and BHT-920 we wanted to study the affinities of these substances for D-1 and D-2 dopamine receptors. In fact, this is the first report describing that BHT-920 has affinity for D-1 receptors. 2. Materials and methods * To whom all correspondence should be addressed: Labora- tori de Neuropsicofarmacologia, Programa Sant Pau- CITRAN, Fundaci6 d'Investigaci6 Santa Creu i Sant Pan, Hospital de la Santa Creu i Sant Pau, Av. Sam Antoni Ma. Claret, 167, Barcelona 08025, Spain. Male Sprague-Dawley rats (280-320 g) were decapitated and the striata were immediately re- moved and pooled. Membranes were prepared by homogenizing the striata in 40 volumes (w/v) of ice-cold Tris-HC1 buffer (50 mM, pH 7.4) and 0014-2999/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)