The human D2long dopamine receptor when expressed heterologously in a human neuronal cell line, SH-SY5Y, produced more robust functional signals than when expressed in a human embryonic kidney cell line, HEK293. Quinpirole (agonist)-induced GTPgamma(35)S binding and high affinity sites were 3 - 4 fold greater in SH-SY5Y than in HEK293 cells. N-type Ca(2+) channel currents present in SH-SY5Y cells, but not HEK293 cells, were inhibited potently by quinpirole with a half-maximal inhibitory concentration of 0.15+/-0.03 nM. Inhibition of adenylyl cyclases by agonists, on the other hand, was of similar potency and efficacy in the two cell lines. GTPgamma(35)S-Bound Galpha subunits from quinpirole-activated and solubilized membranes were monitored upon immobilization with various Galpha-specific antibodies. Galpha(i) and Galpha(o) subunits were highly labelled with GTPgamma(35)S in SH-SY5Y cells, but only Galpha(i) subunits were labelled in HEK293 cells. The additional G(o) coupling in SH-SY5Y cells could arise, at least in part, from the presence of G(o) coupled-effectors, such as the N-type Ca(2+) channel, and may contribute to robust agonist-induced GTPgamma(35)S binding, which is a reliable means for measuring ligand intrinsic efficacy. It appears that expression of neuronal G protein-coupled receptors in neuronal environments could reveal additional functional characteristics that are absent in non-neuronal cell lines. This appears to be due to, at least in part, to the presence of neuron-specific effectors. These findings underscore the importance of the cellular environment in which drug actions are examined, particularly in the face of intensive efforts to develop drugs for G protein-coupled receptors of various origins.