Short and long-term heterologous sensitization of adenylate cyclase by D-4dopamine receptors

The D-4 dopamine receptor, a member of the D-2-like dopamine receptor famil y, may be important in the etiology and treatment of schizophrenia. The pre sent study was designed to examine the effects of dopamine agonist exposure on adenylate cyclase activity in HEK293 cells stably expressing recombinan t-D-4 receptors. Two hour pretreatment with dopamine receptor agonists resu lted in heterologous sensitization of forskolin-stimulated cyclic AMP accum ulation in intact cells expressing the D-4.2, D-4.4, or D-4.7 dopamine rece ptor variant. The potency and efficacy of dopamine for sensitization of cyc lic AMP accumulation was comparable at all D-4 receptor variants. D-4 dopam ine receptor-mediated sensitization was blocked by the D-4 antagonist, cloz apine, and prevented by overnight pretreatment with pertussis toxin, implyi ng a role for G(i)/G(o), proteins in heterologous sensitization. Further, l ong-term (18 h) agonist exposure resulted in a greater degree of sensitizat ion of forskolin-stimulated cyclic AMP accumulation in both intact cells an d membrane preparations of cells expressing the D-4 receptor, compared to 2 h agonist exposure, without altering the density of the receptors. In addi tion, long-term agonist exposure decreased the abundance of G(i alpha) with out altering the abundance of G(s alpha), whereas short-term agonist treatm ent had no effect on the immunoreactivity of either G protein. In summary, long-term agonist-induced sensitization of adenylate cyclase by the D-4 rec eptor may involve mechanisms that do not contribute to short-term sensitiza tion.