Serotonin, But Not N -Methyltryptamines, Activates the Serotonin 2A Receptor Via a β-Arrestin2/Src/Akt Signaling Complex In Vivo

Hallucinogens like psilocybin activate serotonin 2A receptors (5-HT2AR) to produce psychoactive effects. Serotonin itself, the natural neurotransmitter, also activates these receptors but does not normally cause hallucinations. This study shows that serotonin triggers a specific signaling pathway involving β-arrestin2, phosphoinositide 3-kinase, Src, and Akt in the frontal cortex of mice, whereas N-methyltryptamines (hallucinogens) do not. In mice lacking β-arrestin2, serotonin-induced head-twitch responses (a behavioral proxy for receptor activation) were greatly reduced unless doses were elevated, and N-methyltryptamines produced stronger responses. Blocking N-methyltransferase prevented serotonin precursor-induced head twitches in knockout mice, suggesting N-methyltryptamines, not serotonin, mediate that response. This agonist-directed signaling bifurcation may inform drug development for conditions like schizophrenia or depression where hallucinations occur.

Study at a glance

Abstract

Hallucinogens mediate many of their psychoactive effects by activating serotonin 2A receptors (5-HT 2A R). Although serotonin is the cognate endogenous neurotransmitter and is not considered hallucinogenic, metabolites of serotonin also have high affinity at 5-HT 2A R and can induce hallucinations in humans. Here we report that serotonin differs from the psychoactive N -methyltryptamines by its ability to engage a β-arrestin2-mediated signaling cascade in the frontal cortex. Serotonin and 5-hydroxy- l -tryptophan (5-HTP) induce a head-twitch response in wild-type (WT) mice that is a behavioral proxy for 5-HT 2A R activation. The response in β-arrestin2 knock-out (βarr2-KO) mice is greatly attenuated until the doses are elevated, at which point, βarr2-KO mice display a head-twitch response that can exceed that of WT mice. Direct administration of N -methyltryptamines also produces a greater response in βarr2-KO mice. Moreover, the inhibition of N -methyltransferase blocks 5-HTP-induced head twitches in βarr2-KO mice, indicating that N -methyltryptamines, rather than serotonin, primarily mediate this response. Biochemical studies demonstrate that serotonin stimulates Akt phosphorylation in the frontal cortex and in primary cortical neurons through the activation of a β-arrestin2/phosphoinositide 3-kinase/Src/Akt cascade, whereas N -methyltryptamines do not. Furthermore, disruption of any of the components of this cascade prevents 5-HTP-induced, but not N -methyltryptamine-induced, head twitches. We propose that there is a bifurcation of 5-HT 2A R signaling that is neurotransmitter and β-arrestin2 dependent. This demonstration of agonist-directed 5-HT 2A R signaling in vivo may significantly impact drug discovery efforts for the treatment of disorders wherein hallucinations are part of the etiology, such as schizophrenia, or manifest as side effects of treatment, such as depression.