Changes in plasma prolactin during SSRI treatment: evidence for a delayed increase in 5-HT neurotransmission

We studied the effect of the selective serotonin reuptake inhibitor (SSRI), paroxetine, on basal plasma prolactin concentrations in 11 healthy subjects. Subjects were tested before paroxetine, and after 1 and 3 weeks of treatment (20mg daily). On each test occasion prolactin levels were sampled before and following administration of a placebo capsule, for a total of 4h. After 3 weeks paroxetine treatment plasma prolactin levels were significantly higher than those seen either pre-treatment or after 1 week of treatment. In contrast, 1 week of paroxetine treatment did not significantly increase prolactin concentrations over pre-treatment values. Plasma concentrations of paroxetine did not differ between 1 and 3 weeks of treatment. The secretion of plasma prolactin is, in part, under the tonic regulation of serotonergic pathways and the present results therefore support animal experimental data suggesting that SSRIs produce a delayed increase in some aspects of brain serotonin neurotransmission.