The serotonin neurotransmitter system controls diverse biological and psychological functions namely, the regulation of mood, anxiety states, cognition, reward-related behavior, motor function and appetitive behavior. Treatment of neuropsychiatric disorders by drugs that directly affect serotonin transmission also provide evidence for the involvement of the serotonin system in these disorders including generalized anxiety disorder, panic disorder, obsessive-compulsive disorder and major depressive disorder.
Serotonin and Depression
There are at least 14 different serotonin receptors, each of which is differentially regulated, but only one serotonin transporter. Many widely used antidepressant/anti-anxiety medications are thought to work by decreasing serotonin reuptake. Acute inhibition of serotonin reuptake by these drugs is not sufficient, however, to induce a therapeutic response and the full benefits are only evident after a number of weeks of continuous administration. These observations have led to the theory that the efficacy of serotonin reuptake inhibiting antidepressants is not simply due to the blockade of reuptake but results from long term increases in extracellular serotonin that cause cellular adaptation in presynaptic and postsynaptic neurons. The prevailing hypothesis for this process states that initially, inhibition of the serotonin transporter increases extracellular serotonin and this leads to activation of presynaptic serotonin autoreceptors, which decreases serotonergic firing rates and results in only small net increases in extracellular serotonin levels. Over the course of weeks, desensitization of serotonin autoreceptors occurs, leading to a return in neuronal firing rates to normal levels. In the long run, greatly increased levels of extracellular serotonin become available in the face of continued serotonin reuptake inhibition.
However, the study of alterations in basal extracellular serotonin levels after chronic administration of SRIs using microdialysis in rats and mice has produced contrasting results with some studies demonstrating increases in extracellular serotonin while others show no changes. Microdialysis has some inherent weaknesses including low and variable recovery of neurotransmitters from the extracellular space, large probe size, which causes damage to the surrounding tissue and low temporal resolution. Microdialysis, with its low temporal resolution, is limited in its ability to differentiate serotonin release and reuptake rates. By contrast, FCV with its high temporal resolution can give information about alterations in serotonin release versus reuptake rates. Together with microdialysis and its advantages for measuring homeostatic alterations in timeaveraged basal extracellular serotonin levels, FCV determinations of serotonin releaseand reuptake rates in mice administered serotonin transporter inhibiting antidepressants will better elucidate the mechanisms of how these important medications work.
In humans, the SERT gene-linked polymorphic region (5-HTTLPR) is comprised of a 43 bp indel in the gene promoter that influences the expression of SERT such that the short or S allele drives lower SERT expression compared to the long L allele. Individuals expressing S/L or S/S genotypes show increased anxiety-related personality traits, particularly neuroticism compared to individuals expressing the L/L genotype. We hypothesize that subtle but measurable variations in 5-HT transporter function exist between the S/S and S/L groups that have not been explored until now. To test this hypothesis, we measure the 5-HT uptake rates in cultured human lymphoblasts, which express the same SERT gene as that in brain. For this purpose we utilize high-speed chronoamperometry, a highly sensitive microelectrode voltammetry method that has single second temporal resolution. We have previously demonstrated that using this technique, we can detect modest decreases in Vmax (and measure Km) in brain synaptosomes prepared from mice lacking one intact copy of the SERT gene versus wildtype mice that are not detectable by radiochemical methods. With high-speed chronoamperometrywe have been able to differentiate uptake rates on the order of 50% between the S/S and L/L genotypes in lymphoblasts. We are currently investigating uptake rates in S/L expressing lymphoblasts, as well as examining the effects of other SERT polymorphisms including the rs25531 single nucleotide polymorphism and the VNTR (STin repeats). Additionally, we are investigating rarer point mutations including a gain-of-function isoleucine to valine mutation (I425V) in the SERT coding region that has been investigated only in transfected non-neuronal cells and is associated with a severe cluster of psychiatric disorders that includes obsessive compulsive disorder.
- Chronoamperometry detects differential changes in synaptosomal uptake in serotonin transporter knockout mice. X. A. Perez and A. M. Andrews, Analytical Chemistry 77:818-826 (2005) (ABSTRACT or PDF).
- Filtration compromises synaptosomal membranes during radiochemical assay of serotonin uptake: Comparison with chronoamperometry is SERT knockout mice. X. A. Perez, L. E. Bianco and A. M. Andrews, Journal of Neuroscience Methods, 154:245-255 (2006) (ABSTRACT or PDF).