Chimeric D-1/D-2 receptors were constructed to identify structural det
erminants of drug affinity and efficacy. We previously reported that c
himeras that had D-1 receptor transmembrane domain VII together with a
mino-terminal sequence from the D-2 receptor were nonfunctional. D-2/D
-1 chimeras were constructed that contained D-2 receptor sequence at t
he amino-and carboxyl-terminal ends and D-1 receptor sequence in the i
ntervening region. Chimeric receptors with D-2 sequence from transmemb
rane domain 7 to the carboxyl terminus together with D-2 receptor sequ
ence from the amino terminus through transmembrane helix 4 (D-2[1-4,D-
7]) and 5 (D-2[1-5,D-7]) bound [H-3]spiperone with high affinity, cons
istent with the hypothesis that D-2 receptor transmembrane domain I or
II is incompatible with D-1 receptor transmembrane domain VII. D-2[1-
4,D-7] and D-2[1-5,D-7] had affinities similar to D-1 and D-2 receptor
s for most nonselective dopamine antagonists and had affinities for mo
st of the selective antagonists that were intermediate between those o
f the parent receptors. D-2[1-4,D-7] and D-2[1-5,D-7] mediated dopamin
e receptor agonist-induced stimulation and inhibition, respectively, o
f cAMP accumulation, The more efficient coupling of D-2[1-5,D-7] to in
hibition of cAMP accumulation, compared with the coupling of D2[5-7] a
nd D2[3-7], supports the view that multiple D-2 receptor cytoplasmic d
omains acting in concert are necessary for receptor activation of G(i)
. In contrast, D-2[1-4,D-7], which contains only one cytoplasmic loop
(the third) from the D-1 receptor, is capable of activating G(s). D-2[
1-4,D-7] exhibited several characteristics of a constitutively active
receptor, including enhanced basal (unliganded) stimulation of cAMP ac
cumulation, high affinity for agonists even in the presence of GTP, an
d blunted agonist-stimulated cAMP accumulation. A number of dopamine r
eceptor antagonists were inverse agonists at D-2[1-4,D-7], inhibiting
basal cAMP accumulation, Some of these drugs were also inverse agonist
s at the D-1 receptor. Interestingly, several antagonists also potenti
ated forskolin-stimulated cAMP accumulation via D-2[1-5,D-7] and via t
he D-2 receptor, which could reflect inverse agonist inhibition of nat
ive constitutive activity of this receptor.