Altered serotonin synthesis, turnover and dynamic regulation in multiple brain regions of mice lacking the serotonin transporter

To evaluate the consequences of inactivation of the serotonin transporter (SERT) gene on 5-HT homeostasis and function, 5-HT synthesis and turnover rates were measured using the decarboxylase inhibition method in multiple brain regions (frontal cortex, striatum, brainstem, hippocampus and hypothalamus) from mice with a genetic disruption of SERT. 5-HT synthesis rates were increased 30-60% in the different brain regions of SERT −/− mice compared to littermate +/+ control mice despite 55-70% reductions in tissue 5-HT concentrations. Brain regions that possessed a greater capacity to increase synthesis and turnover (frontal cortex, striatum) demonstrated lesser reductions in tissue 5-HT. Female SERT −/− mice had greater increases (79%) in brain 5-HT synthesis than male −/− mice did (25%), a finding associated with higher brain tryptophan concentrations in females. Despite increased 5-HT synthesis, there was no change in either TPH2 or TPH1 mRNA levels or in maximal in vitro TPH activity in the brainstem of SERT −/− mice. Catecholamine homeostasis as reflected in brain tissue concentrations and in synthesis and turnover of dopamine and norepinephrine was unchanged in SERT −/− mice. Taken together, the results demonstrate a markedly altered homeostatic situation in SERT −/− mice that lack 5-HT reuptake, resulting in markedly depleted tissue stores that are inadequately compensated for by increased 5-HT synthesis, with brain region and gender specificity observed.