The results of this study suggest that patients carrying the HTR3A c.-42T allele are at increased risk of having severe dyspeptic symptoms. This risk seems to be even higher for women and patients homozygous for the 5-HTTLPR L allele.
The association could be explained as follows; Noxious and non-noxious visceral sensations are carried by extrinsic primary afferents to the dorsal horn of the spinal cord. Sensory transmission in the spinal dorsal horn is attenuated by endogenous inhibitory systems that originate at the brainstem. One of the main descending systems to the spinal dorsal horn is serotonergic . 5-HT3 receptors present on spinal inhibitory interneurons receive input from the descending serotonergic fibers. Activation of these 5-HT3 receptors evokes release of GABA, which in turn reduces the excitability of dorsal horn neurons . Consequently, the output of visceral sensory information to the brainstem and thereby symptom perception is reduced. It has been demonstrated in a model of visceral pain that 5-HT3 receptors in the spinal cord mediate antinociception . The HTR3A c.-42T allele promotes translation of the HTR3A transcript resulting in enhanced production of the 5-HT3A subunit [27, 28]. In vitro experiments indicate that homomeric 5-HT3A receptors have lower affinity for 5-HT and desensitize more rapidly as compared to heteromeric 5-HT3AB receptors . Consistently, it has been found in 5-HTT knockout mice, in which 5-HT availability at the receptor is enhanced, that the expression of 5-HT3 subunits is altered, apparently leading to a relatively increased proportion of homomeric 5-HT3A receptors . Thus, the increased expression of 5-HT3A subunits in HTR3A c.-42T allele carriers may result in a higher proportion of homomeric 5-HT3A receptors and as a consequence decreased response to 5-HT of the 5-HT3 receptor involved in the descending antinociceptive pathway reflected in higher symptom severity. The additive effect of the LL genotype of the 5-HTTLPR polymorphism is conceivable as homozygosity for the long allele results in more rapid re-uptake of 5-HT and earlier termination of 5-HT induced signalling. As a consequence activation of the 5-HT3 receptor on inhibitory interneurons in the spinal cord is even more diminished; this reduces antinociception.
The association between severe dyspeptic symptoms and the HTR3A c.-42C > T and 5-HTTLPR genotypes appears to be stronger in females than in males. A possible explanation for this finding would be different availability of serotonin in males and females. It has been shown that ovarian hormones regulate the expression of tryptophan hydroxylase (rate-limiting enzyme of serotonin synthesis), 5-HTT and inhibitory 5-HT1A autoreceptors . Lack of control for the variations in female sex hormones during the menstrual cycle, use of hormone replacement therapy and hormonal contraception has likely confounded many of the reported findings of gender differences in indices of serotonin availability. Interestingly, an in vivo investigation with positron emission tomography and α-[11C]methyl-L-tryptophan trapping as a proxy for brain serotonin synthesis, showed a lower rate in healthy women compared to men . Although the pre-menopausal women were studied in the same phase of the menstrual cycle, post-menopausal women were also included. The latter may have affected the extent of the reported difference in men and women. Moreover, this finding in healthy subjects does not necessarily reflect the situation in individuals with dyspepsia. Anyhow, a lower rate of serotonin synthesis in women coincides with a lower level of 5-HT available for receptor activation in females, which is consistent with reduced 5-HT3 receptor mediated antinociceptive effects.
In short, the presence of a 5-HT3 receptor with lower response to 5-HT due to polymorphism, rapid 5-HT re-uptake by 5-HTT and less serotonin available for receptor activation due to gender differences in 5-HT synthesis predisposes to increased perception of visceral stimuli. The influence of the HTR3A c.-42C > T genotype has also been evaluated in (female) patients with irritable bowel syndrome (IBS) . In contrast to what one might expect, since both dyspeptic and IBS symptoms have a visceral sensitivity component, the CC genotype appeared to be associated with greater IBS severity. Enhanced activity of amygdala-related emotional arousal circuits has been implicated in the pathophysiology of IBS . Consistent with a study in healthy subjects, in IBS CC genotype subjects showed increased amygdala responsiveness to emotional facial stimuli compared with T carriers [30, 41]. Activation of 5-HT3 receptors on GABAergic interneurons innervating the amygdala exerts an inhibitory influence on amygdala reactivity, whereas those on excitatory interneurons have the opposite effect . The finding of lower amygdala reactivity in T carriers was interpreted as T-allele-related increased expression of 5-HT3 receptors on GABAergic interneurons resulting in greater inhibition of the amygdala [30, 41] These associations and interpretation of increased 5-HT3A subunit expression appear in conflict with our findings, but 5-HT3 receptor subunit composition may vary in the different regions of the central nervous system as well as the influence of 5-HT3 receptors on excitatory interneurons. Moreover, in dyspepsia increased amygdala responsiveness may be secondary to decreased spinal antinociception. It is noteworthy that another study in patients with IBS suggests that the HTR3A c.-42T allele is associated with the diarrhea-predominant phenotype of the disease . Patients with this phenotype seem to be more often affected by visceral hypersensitivity . Furthermore, the HTR3A c.-42T allele was found to be associated with visceral hypersensitivity in patients with gastroesophageal reflux disease .
The HTR3A c.-42C > T polymorphism likely represents a predisposing genetic variant in common to psychiatric disorders and dyspepsia. Recently, effects of the HTR3A c.-42C > T genotype on emotional brain correlates of susceptibility to depression have been reported . Furthermore, in healthy subjects and patients with IBS the CC genotype was found to be associated with greater anxiety ratings  and in Caucasians the C-allele was associated with elevated scores for the anxiety-related trait harm avoidance . Although anxiety is comorbid with dyspepsia, our findings indicate that in dyspepsia the opposite allele is associated with greater overall symptom severity. Additional research is needed to directly examine the relationship between HTR3A c.-42C > T polymorphism and anxiety in patients with dyspepsia.
To appreciate results of this study several limitations should be mentioned. To date, most studies on 5-HT3 receptor composition and function have been performed in vitro or in rodents, which lack expression of the HTR3C, D, and E subunits . It is not known yet how native receptors in humans are composed, including the 5-HT3 receptors on spinal inhibitory interneurons. Also the consequences of increased expression of 5-HT3A subunits for receptor composition and antinociceptive function have not been assessed in vivo. Secondly, there is a possibility that association between HTR3A c.-42T allele and severe dyspeptic symptoms is due to the effect of some other polymorphism, even in another gene, which is in linkage disequilibrium with HTR3A c.-42C > T. Indeed, the serotonin receptor subunit gene HTR3B maps in close proximity to HTR3A and the possibility has been raised that the HTR3A c.-42C > T polymorphism is not the susceptibility variant but a common variant in HTR3B, found to be associated with depression in Japanese . Furthermore, we could not discriminate between patients with organic and functional dyspepsia. This could influence the results if the association would be specific only for one of them. In this respect it has to be mentioned that previously no significant association of HTR3A c.-42C > T and 5-HTTLPR polymorphisms with dyspeptic symptoms was detected in tertiary referral patients . In the latter study organic disease was excluded, but we believe that the discrepant results can be explained by taking into account not merely the presence but also the severity of dyspeptic symptoms in the current study.