Cannabinoid Regulation of Serotonin 2A (5-HT_2A) Receptor Signaling

Abstract

Accumulating evidence indicates that sustained cannabinoid agonist exposure may precipitate the onset of some neuropsychiatric disorders that are associated with dysfunction of serotonin 2A (5-HT2A ) receptor neurotransmission in the brain. Recent behavioral evidence suggests that non-selective cannabinoid agonists can regulate 5-HT2A receptor signaling in the brain. However, the molecular mechanisms of this cannabinoid-induced change in 5-HT2A receptor signaling are unknown. Here we present experimental evidence that repeated treatment with a non-selective cannabinoid agonist, CP55940, can enhance 5-HT2A receptor activity and expression in the rodent prefrontal cortex (PFCx) and two neuronal cell culture models, CLU13 and A1A1v cells. Cannabinoids mediate many of their physiological effects through two cannabinoid receptors, cannabinoid type 1 (CB1) and cannabinoid type 2 (CB2 ), which are expressed in the brain. Our evidence indicates that CB2 receptors would mediate this phenomenon because cannabinoid-induced upregulation and enhanced activity of 5-HT2A receptors was: (1) induced by selective CB2 receptor but not selective CB1 receptor agonists and (2) inhibited by CB2 but not CB1 shRNA lentiviral particles in neuronal cells. CB2 receptors are positively coupled to the Extracellular Regulated Kinase 1/2 (ERK1/2) signaling cascade which has been shown to regulate transcription factors, such as cyclic AMP response binding protein (CREB) and activator protein 1 (AP-1), which have consensus sequences in the rat 5-HT2A receptor promoter. Interestingly, we found that cannabinoid-induced upregulation of 5-HT2A receptors was inhibited by ERK1/2 inhibitors and AP-1, but not CREB, inhibitor. Studies show that G-protein receptor kinases (GRKs), such as GRK5, can phosphorylate G-protein coupled receptors (GPCRs) to regulate the formation of the Beta ( β )-Arrestin 2/ERK scaffolding complex that can modulate the long term activation of ERK1/2 signaling. Indeed, we found that â-Arrestin2 shRNA lentiviral treatment prevents cannabinoid-induced upregulation of 5-HT2A receptor and significantly reduces cannabinoid activation of ERK1/2 signaling. Moreover, GRK5 shRNA lentiviral particle treatment inhibits cannabinoid-induced: (1) enhanced CB2 receptor phosphorylation (2) enhanced co-immunopreciptiation of β -Arrestin 2 and ERK1/2 and (3) upregulation and enhanced activity of 5-HT2A receptors. The enhanced function of 5-HT2A receptors could also involve cannabinoid-induced enhanced interaction between 5-HT2A and dopamine D2 (D2) receptors. We found that Sprague-Dawley rats treated with a non-selective cannabinoid receptor agonist (CP55940) showed enhanced co-immunoprecipitation of 5-HT2A and D2 receptors. Formation of the functional 5-HT2A and D2 receptor heteromer in the PFCx is suggested to contribute to the pathophysiology of neuropsychiatric disorders such as schizophrenia. Furthermore, enhanced activity of cortical 5-HT2A receptors is associated with several physiological functions and neuropsychiatric disorders such as stress response, anxiety, depression, and schizophrenia. Therefore, these cannabinoid-regulated molecular mechanisms may be relevant to some neuropsychiatric disorders in humans.