Kinetics and pharmacology of the D1- and D2-like dopamine receptors in Japanese quail brain

Abstract

Although the avian brain dopamine system and its functions have been studied much less than the mammalian one, there is an increasing interest in the role of dopamine and its receptors in a wide variety of motor, cognitive and emotional functions in birds with implications for basic research, medicine or agriculture. Pharmacological characterisation of the avian dopamine receptors has had little attention. In this paper we characterise the two classes of dopamine receptors in Japanese quail brain by radioligand binding techniques using [(3)H]SCH 23390 (D(1)) and [(3)H]spiperone (D(2)). Association, dissociation and saturation analyses showed that the binding of both radioligands is time- and concentration-dependent, saturable and reversible. Apparent dissociation constants determined for [(3)H]SCH 23390 and [(3)H]spiperone from concentration isotherms were 1.07 and 0.302 nM and the maximum binding capacities were 89.3 and 389.3 fmol per mg of protein, respectively. Using competitive binding studies with a spectrum of dopamine and other neurotransmitter receptor agonists/antagonists, the [(3)H]SCH 23390 and [(3)H]spiperone binding sites were characterised pharmacologically. Pharmacological profiles of quail dopamine receptors showed a high degree of pharmacological homology with other vertebrate dopamine receptors. The data presented extend the knowledge of kinetics and pharmacology of D(1)- and D(2)-like dopamine receptors in birds, provide data for avian psychopharmacological and comparative studies and represent an important complement to studies using cell expression systems.

Fig. 1

Binding characteristics of [³H]SCH 23390 and [³H]spiperone to slide-mounted brain-mash sections from the Japanese quail basal forebrain. a [³H]SCH 23390 association and dissociation kinetics, b [³H]SCH 23390 saturation, c Scatchard analysis of [³H]SCH 23390 binding, d [³H]spiperone association and dissociation kinetics, e [³H]spiperone saturation, and f Scatchard analysis of [³H]spiperone binding. Each point represents a mean of 3 experiments performed in triplicates

Fig. 2

Pharmacological specificity of [³H]SCH 23390 and [³H]spiperone binding to slide-mounted brain-mash sections from the Japanese quail basal forebrain. a Competitive inhibition of [³H]SCH 23390 binding by selected antagonists, b competitive inhibition of [³H]SCH 23390 binding by selected agonists, c competitive inhibition of [³H]spiperone by selected antagonists, and d competitive inhibition of [³H]spiperone by selected agonists

Fig. 3

Pharmacological homology between quail D₁ dopamine receptor and D₁-like receptors in other species. Correlation plots of estimated inhibitory constants K _i of various dopaminergic agonists and antagonists to inhibit [³H]SCH 23390 binding to quail D₁ receptor and the human dopamine receptors D_1A and D_1B (taken from Sunahara et al. 1991), rat striatum (Faedda et al. 1989), chicken D_1A, D_1B and D_1D receptors (Demchyshyn et al. 1995), xenopus D_1A, D_1B and D_1C receptors (Sugamori et al. 1994) and eel D_1A2, D_1B and D_1C receptors (Cardinaud et al. 1997). The line of identity or equimolarity is indicated

Fig. 4

Pharmacological homology between quail D₂ dopamine receptor and D₂-like receptors in other species. Correlation plots of estimated inhibitory constants K _i of various dopaminergic agonists and antagonists to inhibit [³H]spiperone binding to quail D₂ receptor and the human dopamine receptors D_2L and D₃ (taken from MacKenzie et al. 1994), human D₄ receptors (Van Tol et al. 1991), rat striatum (Andersen et al. 1985), rat D_2L receptor (Castro and Strange 1993) and cynomolgous monkey striatum (Madras et al. 1988). The line of identity or equimolarity is indicated