Activation of 5-hydroxytryptamine2C (5-HT2C) receptors by the 5-HT2 receptor agonist m-chlorophenylpiperazine (m-CPP) elicits anxiety in humans and anxiety-like behavior in animals. We compared the effects of m-CPP with the anxiogenic GABAA receptor inverse agonist N-methyl-β-carboline-3-carboxamide (FG-7142) on both anxiety-like behavior and regional brain activation using functional magnetic resonance imaging (fMRI) in the rat. We also determined whether the selective 5-HT2C receptor antagonist SB 242084 [6-chloro-2,3-dihydro-5-methyl-N-[6-[(2-methyl-3-pyridinyl)oxy]-3-pyridinyl]-1H-indole-1-carboxyamide dihydrochloride] would blunt m-CPP or FG-7142-induced neuronal activation. Both m-CPP (3 mg/kg i.p.) and FG-7142 (10 mg/kg i.p.) elicited anxiety-like behavior when measured in the social interaction test, and pretreatment with SB 242084 (1 mg/kg i.p.) completely blocked the behavioral effects of both anxiogenic drugs. Regional brain activation in vivo in response to anxiogenic drug challenge was determined by blood oxygen level-dependent (BOLD) fMRI using a powerful 9.4T magnet. Region of interest analyses revealed that m-CPP and FG-7142 significantly increased BOLD signals in brain regions that have been linked to anxiety, including the amygdala, dorsal hippocampus, and medial hypothalamus. These BOLD signal increases were blocked by pretreatment with SB 242084. In contrast, injection of m-CPP and FG-7142 resulted in BOLD signal decreases in the medial prefrontal cortex that were not blocked by SB 242084. In conclusion, the brain activation signals produced by anxiogenic doses of both m-CPP and FG-7142 are mediated at least partially by the 5-HT2C receptor, indicating that this receptor is a key component in anxiogenic neural circuitry.

Post-transcriptional RNA editing of the G-protein coupled 5-hydroxytryptamine-2C (5-HT2C) receptor predicts an array of 24 receptor isoforms, some of which are characterized by reduced constitutive activity and potency to initiate intracellular signaling. The amygdala is integral to anxiety, fear, and related psychiatric diseases. Activation of 5-HT2C receptors within the amygdala is anxiogenic. Here, we describe the RNA editing profiles from amygdala of two inbred mouse strains (BALB/cJ and DBA/2J) known to be more anxious than a third (C57BL/6J). We confirmed the strain anxiety differences using light ↔ dark exploration, and we discovered that BALB/cJ and DBA/2J are each characterized by a higher functioning RNA editing profile than C57BL/6J. BALB/cJ and DBA/2J exhibit a roughly two-fold reduction in C site editing, and a corresponding two-fold reduction in the edited isoform VSV. C57BL/6J is characterized by a relative decrease in the unedited highly functional isoform INI. We estimated the heritability of editing at the C site to be approximately 40%. By sequencing genomic DNA, we found complete conservation between C57BL/6J, BALB/cJ, DBA/2J and 37 other inbred strains for the RNA edited region of Htr2c, suggesting Htr2c DNA sequence does not influence variation in Htr2c RNA editing between inbred strains of mice. We did, however, discover that serotonin turnover is reduced in BALB/cJ and DBA/2J, consistent with emerging evidence that synaptic serotonin levels regulate RNA editing. These results encourage further study of the causes and consequences of 5-HT2C receptor RNA editing in the amygdala of mice.

The human serotonin 5-HT2C receptor undergoes adenosineto-inosine RNA editing at five positions, generating multiple receptor isoforms with altered G-protein coupling properties. In the current study, we demonstrate that RNA editing regulates the pattern of intracellular signaling. The non-edited human 5-HT2C receptor isoform INI activates phospholipase D via the G13 heterotrimer G-protein. We present evidence that transactivation of the small G-protein RhoA is required for phospholipase D activation. In contrast, neither transactivation of RhoA nor phospholipase D activation was detected in cells expressing the fully edited VGV isoform. The ability to activate phospholipase C is also reduced in VGV-expressing cells, but not to the extent found for the phospholipase D signal. We conclude that RNA editing represents a novel mechanism for regulating 5-HT2C receptor signaling to pathways linked to actin cytoskeletal organization and regulated exocytosis.