Serotonin uptake in blood platelets and the dexamethasone suppression test in depressed patients

Two putative biological markers of some forms of depressive illness, the dexamethasone suppression test (DST) and the V_max of serotonin (5- hydroxytryptamine, 5-HT) uptake in blood platelets, were studied in 40 unipolar, bipolar, and schizoaffective depressed patients. The V_max levels in those whose cortisol levels suppressed normally after dexamethasone (n=25) were not significantly different from those of the nonsuppressors(n=15). When a criterion V_max>-8.5 pmoles/10⁷ platelets/minute of ¹⁴C-5-HT uptake was used as the lower limit of normal, 18 patients had V_max values lower than normal, only four of whom were nonsuppressors. There was a tendency for the incidence of lower than normal V_max levels in nonsuppressors (4/15, 26.7%) to be less than that of the suppressors (14/25, 56.0%). These results suggest that the two abnormalities are independent of each other but tend to support the hypothesis that decreased V_max may be an adaptive response which restores serotonergic function to normal. Twenty-nine of the 40 patients (72.5%) had one or both abnormalities, a finding which suggests that determination of both parameters would significantly increase the proportion of depressed patients who could be diagnosed by these biological tests.