Agonist Stimulation of the Serotonin 1A Receptor Causes Suppression of Anoxia‐Induced Apoptosis via Mitogen‐Activated Protein Kinase in Neuronal HN2‐5 Cells

Previous studies have indicated that stimulation of neuronal inhibitory receptors, such as the serotonin 1A receptor (5‐HT 1A ‐R), could cause attenuation of the activity of both N‐type Ca 2+ channels and N ‐methyl‐D‐aspartic acid receptors, thus resulting in protection of neurons against excitotoxicity. The purpose of this study was to investigate if the 5‐HT 1A ‐R is also coupled to an alternative pathway that culminates in suppression of apoptosis even in cells that are deficient in Ca 2+ channels. Using a hippocampal neuron‐derived cell line (HN2‐5) that is Ca 2+ channel‐deficient, we demonstrate here that an alternative pathway is responsible for 5‐HT 1A ‐R‐mediated protection of these cells from anoxia‐triggered apoptosis, assessed by deoxynucleotidyl‐transferase‐mediated dUTP nick end‐labeling (TUNEL). The 5‐HT 1A ‐R agonist‐evoked protection was eliminated in the presence of pertussis toxin and also required phosphorylation‐mediated activation of mitogen‐activated protein kinase (MAPK), as evidenced by the elimination of the agonist‐elicited rescue of neuronal cells by the MAPK kinase inhibitor PD98059 but not by the phosphatidylinositol 3‐kinase (PI‐3K) inhibitor wortmannin. Furthermore, agonist stimulation of the 5‐HT 1A ‐R caused a 60% inhibition of anoxia‐stimulated caspase 3‐like activity in the HN2‐5 cells, and this inhibition was abrogated by PD98059 but not by wortmannin. Although agonist stimulation of the 5‐HT 1A ‐R caused an activation of PI‐3Kγ in HN2‐5 cells, our results showed that this PI‐3Kγ activity was not linked to the 5‐HT 1A ‐R‐promoted regulation of caspase activity and suppression of apoptosis. Thus, in the neuronal HN2‐5 cells, agonist binding to the 5‐HT 1A ‐R results in MAPK‐mediated inhibition of a caspase 3‐like enzyme and a 60‐70% suppression of anoxia‐induced apoptosis through a Ca 2+ channel‐independent pathway.