A proposed model is shown for the role of serotonin in the adaptation of β-cells to pregnancy. Panel (A) outlines the effects of increased expression of the serotonin synthetic enzyme TPH and the serotonin receptor Htr2b at mid gestation. Panel (B) shows the effects of decreased expression of Htr2b and increased expression of Htr1d at the end of pregnancy and during the postpartum period. Trp, tryptophan; 5HT, 5 hydroxytrytomine, serotonin; TPH, tryptophan hydroxylase; PRLR, prolactin receptor.