Genes within the serotonergic system are differentially expressed in human brain

Abstract

Background: Serotonin is an important neurotransmitter with wide-ranging functions throughout the central nervous system. There is strong evidence to suggest that regulation of serotonergic gene expression might be related to genetic variability, and several studies have focused on understanding the functional effects of specific polymorphisms within these genes on expression levels. However, the combination of genotype together with gender and brain region could have an overall effect on gene expression. In this study, we report expression patterns of five serotonergic genes (TPH1, TPH2, 5-HT2A, 5-HT2C, 5-HTT) in seven different human post-mortem brain regions (superior frontal gyrus, superior temporal gyrus, striatum, cerebellum, hippocampus, midbrain and thalamus) using TaqMan real-time quantitative PCR. In addition, the effect of genotype and gender on their expression levels was determined.

Results: The data revealed that mRNA from the five genes investigated was detected in all brain regions and showed an overall significant difference in expression levels. Furthermore, the expression of 5-HT2C, 5-HT2A and TPH2 was found to be significantly different between the various brain regions. However, neither gender nor genotype showed significant effects on the expression levels of any of the genes assayed. Interestingly, TPH1 and TPH2 were expressed in all brain regions similarly except for within the striatum and cerebellum, where TPH1 was expressed at a significantly higher level than TPH2.

Conclusion: The effect of brain region has a greater influence on serotonergic gene expression than either genotype or gender. These data add to the growing body of evidence that effects of functional polymorphisms on gene expression in vitro are not observed ex vivo, and provide information that will aid in the design of expression studies of the serotonergic gene system within human post-mortem brain.

Figure 1

Bar graph representing the expression (2^-ΔCt) of five serotonergic genes before controlling for brain region, genotype or gender. All genes are expressed in the brain, 5-HTT showing the lowest expression and 5-HT2A the highest. Bars represent mean and error bars represent standard error of the mean (s.e.m).

Figure 2

Bar graph representing the effect of gender on expression (2^-ΔCt) of the five genes in all brain regions. Males (grey bars) and females (white bars) represent the mean and error bars representing s.e.m. No significant differences in expression are observed.

Figure 3

Bar graph describing the expression (2^-ΔCt) of 5-HT2C in various brain regions. There are significant differences in expression (denoted by ** [p < 0.001]) across the brain regions, mainly due to lower expression of genes in the cerebellum. The symbol * represents a significant difference P = 0.003 between 5-HT2C expression in striatum and STG brain regions (see Table 1). Bars represent the mean and error bars represent s.e.m. The groups representing expression in the thalamus have N = 2.

Figure 4

Bar graph describing the expression (2^-ΔCt) of 5-HT2A in various brain regions. There are significant differences in expression (denoted by ** [p < 0.001] and * [p ≤ 0.04]) across the brain regions, mainly due to lower expression of genes in the cerebellum (see Table 1). Bars represent the mean and error bars represent s.e.m.

Figure 5

Bar graph describing the expression (2^-ΔCt) of TPH2 in various brain regions. There are significant differences in expression (denoted by ** [p = 0.001] and * [p = 0.011]) across the brain regions, mainly due to lower expression of genes in the cerebellum (see Table 1). Bars represent the mean and error bars represent s.e.m. The groups representing expression in the thalamus have N = 2.

Figure 6

Bar graph of mean expression (2^-ΔCt) of TPH1 (grey bars) versus TPH2 (white bars) and error bars representing s.e.m. Both genes are expressed in all brain regions, and TPH1 expression is significantly higher than TPH2 in both the striatum (t = 2.487, p = 0.022, denoted by *) and cerebellum (t = 4.297, p < 0.001, denoted by **). The groups representing TPH2 expression in the hippocampus and thalamus have a much larger variance than that of TPH1, probably due to the smaller number of individuals in these groups (hippocampus N = 5, thalamus N = 2).