Serotonin synthesis, release and reuptake in terminals: a mathematical model

<p>Abstract</p> <p>Background</p> <p>Serotonin is a neurotransmitter that has been linked to a wide variety of behaviors including feeding and body-weight regulation, social hierarchies, aggression and suicidality, obsessive compulsive disorder, alcoholism, anxiety, and affective disorders. Full understanding of serotonergic systems in the central nervous system involves genomics, neurochemistry, electrophysiology, and behavior. Though associations have been found between functions at these different levels, in most cases the causal mechanisms are unknown. The scientific issues are daunting but important for human health because of the use of selective serotonin reuptake inhibitors and other pharmacological agents to treat disorders in the serotonergic signaling system.</p> <p>Methods</p> <p>We construct a mathematical model of serotonin synthesis, release, and reuptake in a single serotonergic neuron terminal. The model includes the effects of autoreceptors, the transport of tryptophan into the terminal, and the metabolism of serotonin, as well as the dependence of release on the firing rate. The model is based on real physiology determined experimentally and is compared to experimental data.</p> <p>Results</p> <p>We compare the variations in serotonin and dopamine synthesis due to meals and find that dopamine synthesis is insensitive to the availability of tyrosine but serotonin synthesis is sensitive to the availability of tryptophan. We conduct <it>in silico </it>experiments on the clearance of extracellular serotonin, normally and in the presence of fluoxetine, and compare to experimental data. We study the effects of various polymorphisms in the genes for the serotonin transporter and for tryptophan hydroxylase on synthesis, release, and reuptake. We find that, because of the homeostatic feedback mechanisms of the autoreceptors, the polymorphisms have smaller effects than one expects. We compute the expected steady concentrations of serotonin transporter knockout mice and compare to experimental data. Finally, we study how the properties of the the serotonin transporter and the autoreceptors give rise to the time courses of extracellular serotonin in various projection regions after a dose of fluoxetine.</p> <p>Conclusions</p> <p>Serotonergic systems must respond robustly to important biological signals, while at the same time maintaining homeostasis in the face of normal biological fluctuations in inputs, expression levels, and firing rates. This is accomplished through the cooperative effect of many different homeostatic mechanisms including special properties of the serotonin transporters and the serotonin autoreceptors. Many difficult questions remain in order to fully understand how serotonin biochemistry affects serotonin electrophysiology and vice versa, and how both are changed in the presence of selective serotonin reuptake inhibitors. Mathematical models are useful tools for investigating some of these questions.</p

Epistasis among Presynaptic Serotonergic System Components

Epistatic interactions among regulatory components of the serotonin (5-HT) neurotransmitter system may be an important aspect of 5-HT function. Because 5-HT dysregulation is associated with several common psychiatric disorders, the potential for epistasis among genetic variants in the 5-HT transporter (SERT), 5-HT 1B terminal autoreceptor and the 5-HT 1A somatodendritic autoreceptor should be examined. In this study, output from a dynamic minimal model of 5-HT function was compared to empirical results in the literature. Parameters representing extracellular 5-HT clearance rates (SERT), 5-HT release levels (5-HT 1B ) and inhibitory thresholds (the amount of extracellular 5-HT above which cell firing is inhibited, an indication of 5-HT 1A autoreceptor sensitivity) were varied to simulate genetic deletion (i.e. knockout) of each component singly, and in combination. Simulated knockout effects on extracellular 5-HT level and presynaptic neural firing rates were in the same direction and of similar relative magnitude as studies in the literature. Epistasis among presynaptic components appears to be important in the 5-HT system’s regulation of extracellular 5-HT levels, but not of firing rates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44113/1/10519_2004_Article_1019.pd

Effects of acute treatment with paroxetine, citalopram and venlafaxine in vivo on noradrenaline and serotonin outflow: a microdialysis study in Swiss mice

1. This study investigated whether a single administration of a range of doses (1, 4 and 8 mg kg(−1), i.p.) of paroxetine, citalopram or venlafaxine may simultaneously increase extracellular levels of 5-HT ([5-HT]ext) and noradrenaline ([NA]ext) by using in vivo microdialysis in the frontal cortex (FCx) of awake, freely moving Swiss mice. 2. In vivo, paroxetine induced similar increases in cortical [5-HT]ext at the three doses tested, and induced a statistically significant increase in cortical [NA]ext at 4 and 8 mg kg(−1). Citalopram increased neither [5-HT]ext nor [NA]ext at the lowest dose, but increased both neurotransmitter levels at 4 and 8 mg kg(−1). At these doses, citalopram induced greater increases in cortical [5-HT]ext than in [NA]ext. Venlafaxine increased [5-HT]ext and [NA]ext to about 400 and 140% of the respective basal values at 8 mg kg(−1). 3. Citalopram and paroxetine have the highest potency to increase cortical [5-HT]ext and [NA]ext, respectively. In addition, the rank of order of efficacy of these antidepressant drugs to increase [5-HT]ext in vivo in the FCx of mice was as follows: venlafaxine>citalopram>paroxetine, while the efficacy to increase cortical [NA]ext in mice of paroxetine and citalopram is similar, and greater than that of venlafaxine. 4. In conclusion, extracellular levels of cortical [NA]ext increase with the highest doses of the very selective SSRI citalopram, as well as with the very potent SSRI paroxetine. Surprisingly, the SNRI venlafaxine increased cortical [5-HT]ext to a greater extent rather than [NA]ext in the range of doses studied in mice