Serum PRL responses to a dopamine D2R agonist and antagonist in 15-month-old male mice measured one hour after treatment. (a) Bromocriptine decreased PRL levels in Prlr–/– mice, but not the other mutant genotypes. (b) Haloperidol increased PRL levels in wild-type mice but in none of the other genotypes. Because of their low basal values, the wild-type responses are shown magnified in the insets. Statistical analyses were by one-tailed paired Student t test for each drug within genotype. *P < 0.05 compared with pretreatment; n = 5–8.