How stress and fluoxetine modulate serotonin 2C receptor pre-mRNA editing

Abstract

In two inbred strains of mice, C57BL/6 and 129Sv, the majority of forebrain neocortical pre-mRNA encoding the serotonin 2C (5-HT2C) receptor is altered by adenosine-to-inosine editing. As a result, >60% of all mRNAs encode receptors with reduced constitutive and agonist-stimulated activity. However, in the BALB/c strain, a genetically distinct inbred strain with lower forebrain serotonin levels, spontaneously elevated anxiety, and increased stress reactivity, the majority of 5-HT2C mRNA is nonedited and encodes receptors with the highest constitutive activity and the highest agonist affinity and potency. Neither acute stress (the forced swim test) nor chronic treatment with the serotonin-selective reuptake inhibitor fluoxetine elicit significant changes in 5-HT2C pre-mRNA editing in C57BL/6 mice. In contrast, exposure of BALB/c mice to acute stress and chronic treatment of nonstressed BALB/c mice with fluoxetine elicit significant, site-specific increases in 5-HT2C pre-mRNA editing that increase the pool of mRNA encoding receptors with reduced function. These changes in 5-HT2C pre-mRNA editing resemble those detected previously in the prefrontal cortex of subjects with major depression. However, when chronic fluoxetine treatment is combined with stress exposure of BALB/c mice, these changes in 5-HT2C pre-mRNA editing are no longer detected. These findings illustrate that 5-HT2C pre-mRNA editing responses to stress and chronic fluoxetine are modulated by the genetic background, as well as the behavioral state of the animal. They suggest further that the changes in 5-HT2C pre-mRNA editing found in major depression reflect a previously unrecognized molecular response to stress that can be prevented by chronic antidepressant treatment.

Figure 1.

Percentages of major edited and nonedited (NE) 5-HT_2C mRNA isoforms (top) and percentages of site-specific editing (bottom) in the forebrain neocortex of 129Sv, C57BL/6, and BALB/c mice. Adult male mice (P60-P70) were used in this study. Data represent means ± SEM of determinations made in four (129Sv and C57BL/6) or five (BALB/c) animals. For each animal, a minimum of 50 sequences was obtained (totaling 213 sequences for 129Sv, 204 for C57BL/6, and 266 for BALB/c). The data obtained for 129Sv mice were reported previously (Gurevich et al., 2002a). The percentages of ABD- and ABCD-edited isoforms of 129Sv and C57BL/6 mice did not significantly differ from each other, but they differed significantly from corresponding percentages determined for BALB/c mice (ANOVA; post hoc Tukey-Kramer multiple-comparison test; p < 0.001). The percentages of nonedited and AB edited 5-HT_2C mRNA are significantly higher in BALB/c mice compared with 129Sv and C57BL/6 mice (p < 0.001 and p < 0.05, respectively).

Figure 2.

Percentages of major edited and nonedited (NE) 5-HT_2C mRNA isoforms (top) and percentages of site-specific editing (bottom) in the forebrain neocortex of nonstressed and stressed C57BL/6 (A, B) and BALB/c (C, D) mice. fluox, Fluoxetine. Data represent means ± SEM of determinations made in 36 animals (5 BALB/c mice and 4 C57Bl/6 mice per treatment group), and a minimum of 48 sequences was obtained for each of the animals. Data were compared by one-way ANOVA, and statistical differences were resolved post hoc using the Tukey-Kramer multiple-comparison test.