Previous human postmortem brain tissue research has implicated abnormalities of 5-HT receptor availability in depression and suicide. Although altered abundance of 5-HT 1A, 5-HT 2A, and 5-HT 2C receptors (5-HT1A, 5-HT2A, and 5-HT2C) has been reported, the causes remain obscure. This study evaluated the availability of these three receptor subtypes in postmortem brain tissue specimens from persons with a history of major depression (MDD) and normal controls and tested the relationships to protein kinases A and C (PKA, PKC). Samples were obtained from postmortem brain tissue (Brodmann area 10) from 20 persons with a history of MDD and 20 matched controls as determined by a retrospective diagnostic evaluation obtained from family members. Levels of 5-HT1A, 5-HT2A, and 5-HT2C receptor were quantitated via Western blot analyses. Basal and stimulated PKA and PKC activity were also determined. The depressed samples showed significantly increased 5-HT2A receptor abundance relative to controls, but no differences in 5-HT1A or 5-HT2C receptors. Basal and cyclic AMP-stimulated PKA activity was also reduced in the depressed sample; PKC activity was not different between groups. 5-HT2A receptor availability was significantly inversely correlated with PKC activity in controls, but with PKA activity in the depressed sample. Increased 5-HT2A receptor abundance and decreased PKA activity in the depressed sample are consistent with prior reports. The correlation of 5-HT2A receptor levels with PKA activity in the depressed group suggests that abnormalities of 5-HT2A receptor abundance may depend on receptor uncoupling and heterologous regulation by PKA. © 2009 IBRO.