Monoamine oxidase inhibitors and serotonin uptake inhibitors: Differential effects on [³H]serotonin binding sites in rat brain

Rats were administered either monoamine oxidase inhibitors or serotonin uptake inhibitors for either 1, 4 or 16 days. The binding of [³H]serotonin to brain homogenates and the concentration of serotonin in brain was measured at these times. Treatment with inhibitors of serotonin uptake did not change the specific binding of [³H]serotonin in either cerebral cortex or hippocampus, nor did it produce any consistent alterations in the concentration of serotonin in the cerebral cortex. In contrast, monoamine oxidase inhibitors capable of inhibiting A-type monoamine oxidase significantly decreased [³H]serotonin binding after both 4 and 16 days of treatment; serotonin concentrations were significantly elevated at all time intervals. Inhibitors of B-type monoamine oxidase had no effect on either [³H]serotonin binding or serotonin concentrations in cerebral cortex. The reduction in labeled serotonin binding caused by monoamine oxidase inhibitors is due to a decrease in the maximum number of specific binding sites with no change in the affinity of the binding sites for labeled serotonin.