Differential modification of striatal D1 dopamine receptors and effector moieties by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline in vivo and in vitro

Abstract

Both in vivo and in vitro treatments with the irreversible protein-modifying reagent, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), were used to investigate rat striatal D1 dopamine receptor/effector interactions. Peripherally administered EEDQ markedly reduced D1 dopamine receptor binding and D1 dopamine receptor-stimulated adenylate cyclase in a dose-dependent manner. However, EEDQ administered in vivo did not result in functional modification of either the guanine nucleotide-regulatory protein (Ns) or the catalytic subunit of striatal adenylate cyclase as assessed via guanine nucleotide- or forskolin-stimulated cAMP production. Interestingly, the loss in D1 dopamine receptor binding did not correlate directly with observed reductions in dopamine-stimulated adenylate cyclase activity; 40% of D1 dopamine receptor binding was lost with no significant reduction in the Vmax of dopamine-stimulated adenylate cyclase activity. Conversely, the reduction by EEDQ of the adenylate cyclase activity stimulated by the partial agonist SKF38393 was reduced in parallel with EEDQ-induced reductions in the D1 dopamine receptor Bmax. However, when SKF38393-stimulated adenylate cyclase activity was potentiated by forskolin, approximately 30% of receptors could be lost with no significant reduction in cAMP production, resembling the pattern observed utilizing the full agonist dopamine. In vivo pretreatment with the specific D1 antagonist, SCH23390, prevented reductions in dopamine-stimulated adenylate cyclase activity and D1 dopamine receptor binding, suggesting that EEDQ acts at the ligand recognition site of the receptor. Unlike in vivo treatment, in vitro EEDQ treatment resulted in dose-dependent decreases in catalytic subunit activity as assessed by forskolin-stimulated cAMP production, indicating that, in vitro, the adenylate cyclase catalytic subunit is vulnerable to EEDQ-induced modification. These data indicate that EEDQ is an effective tool for elucidating the mechanisms and biochemistry of D1 dopamine receptor/effector coupling.