5-HT7 Receptor Neuroprotection against Excitotoxicity in the Hippocampus

Abstract

Introduction and Objectives: The PDGFβ receptor and its ligand, PDGF-BB, are expressed throughout the central nervous system (CNS), including the hippocampas. Several reports confirm that PDGFβ receptors are neuroprotective against N-methyl-D-asparate (NMDA)-induced cell death in hippocampal neurons. NMDA receptor dysfunction is important for the expression of many symptoms of mental health disorders such as schizophrenia. The serotonin (5-HT) type 7 receptor was the most recent of the 5-HT receptor family to be identified and cloned. 5-HT receptors interact with several signaling systems in the CNS including receptors activated by the excitatory neurotransmitter glutamate such as the NMDA receptor. Although there is extensive interest in targeting the 5-HT7 receptor with novel therapeutic compounds, the function and signaling properties of 5-HT7 receptors in neurons remains poorly characterized. Methods: The SH-SY5Y neuroblastoma cell line, primary hippocampal cultures, and hippocampal slices were treated with 5-HT7 receptor agonists and antagonists. Western blotting was used to measure PDGFß receptor expression and phosphorylation as well as NMDA receptor subunit expression and phosphorylation levels. Real-time RT-PCR was used to measure mRNA level of PDGFß receptor in neuronal cultures. Cell death assays (MAP2, MTT) were used to measure the neuroprotective effects of 5-HT7 and PDGFß receptor activation. Results: My research involved elucidating the molecular mechanisms of neuroprotection after 5-HT7-induced PDGFß receptor upregulation. I demonstrated that 24 h treatment with the selective 5-HT7 receptor agonist, LP 12, increased not only the expression but also the activation of PDGFß receptors as measured by the phosphorylation of tyrosine 1021, the phospholipase Cγ binding site. Activation of the 5-HT7 receptor also selectively changed the expression and phosphorylation state of the NR2B subunit of the NMDA receptor. Activation of 5-HT7 receptors was neuroprotective against NMDA-induced toxicity in primary hippocampal neurons and this effect required PDGFß receptor kinase activity. Thus, long-term (24 h) activation of 5-HT7 receptors was neuroprotective via increasing the expression of a negative regulator of NMDA activity, the PDGFß receptor. In contrast, acute activation (5-30 min) of 5-HT7 receptor increased NMDA evoked current and altered NMDA receptor subunit phosphorylation in hippocampal neurons in a manner that was different from what we observed in our 24 h experiments. Conclusions: I identified two 5-HT7 receptor to NMDA receptor pathways: acute activation of the receptor increased NMDA-evoked currents whereas long-term 5-HT7 agonist treatment prevented NMDA-induced excitotoxicity in a PDGFß receptor-dependent manner. This research is significant in the ongoing advances for the treatment of mental heath disorders, such as schizophrenia and depression, that involve the 5-HT, glutamate, and neuronal growth factor systems.