High impact AMPAkines induce a Gq-protein coupled endoplasmic calcium release in cortical neurons: a possible mechanism for explaining the toxicity of high impact AMPAkines

Positive allosteric modulators of the AMPA receptor (AMPAkines) have a multitude of promising therapeutic properties. The pharmaceutical development of high impact AMPAkines has however been limited by the appearance of calcium dependent neuronal toxicity and convulsions in vivo. Such toxicity is not observed at exceptionally high concentrations of low impact ampakines. Since most AMPAR are somewhat impermeable to calcium, the current study sought to examine the extent to which different mechanisms contribute to the rise in intracellular calcium in the presence of high impact ampakines. In the presence of AMPA alone, cytosolic calcium elevation is shown to be sodium-dependent. In the presence of high impact AMPAkines such as cyclothiazide (CTZ) or CX614, however, AMPAR potentiation also activates an additional mechanism that induces calcium release from endoplasmic reticular (ER) stores. The pathway that connects AMPAR to the ER system involves a Gq protein, phospholipase C mediated inositol triphosphate (InsP3) formation and ultimately stimulation of InsP3 receptors located on the ER. The same linkage was not observed using high concentrations of the low impact AMPAkines, CX516 (Ampalex) and CX717. We also demonstrate that CX614 produces neuronal hyperexcitability at therapeutic doses while the newer generation low impact AMPAkine CX1739 is safe at exceedingly high doses. While earlier studies have demonstrated a functional linkage between AMPAR and G proteins, this report demonstrates that in the presence of high impact AMPAkines, AMPAR also couple to a Gq protein, which triggers a secondary calcium release from the ER and provides insight into the disparate actions of high and low impact AMPAkines. ### Competing Interest Statement RespireRx Pharmaceuticals is developing low impact ampakines to treat ADHD, spinal cord injury, and opiate-induced respiratory depression.