AMPA receptor regulation mechanisms: future target for safer neuroprotective drugs.
The post-synaptic AMPA receptors play an important role in mediating fast excitatory transmission in the mammalian brain. Over-activated AMPA receptors induce excitotoxicity, implicated in a number of Chronic neurodegenerative disorders such as Parkinson's disease, Huntington's disease, and AIDS encephalitis. AMPA receptor antagonists offer protection against neurodegeneration in the experimental models even if they are given 24 h after the injury. Because AMPA receptors seem to be involved in the neurodegenerative diseases, modulating the activity of the AMPA receptors could be an attractive approach to reduce or prevent excitotoxicity. Studies conducted recently have exhibited a number of new mechanisms for AMPA receptor regulation. Modulations of these were found to have protective implications. AMPA receptor depolarization and desensitization are protective to the neurons. Receptor desensitization depends on the receptor subunit composition. The R/G editing site and the flip/flop cassettes in AMPA receptor subunits contribute to a great extent in receptor desensitization and recovery rates. Molecules that could quicken receptor desensitization or delay recovery could be of use. AMPA receptors limit neuronal entry of Ca2+ ions by regulating Ca2+-permeability. Ca2+-permeable receptor channels are made up of GluR1, GluR3, or GluR4 subunits, whereas presence of the GluR2 subunit restricts Ca2+ entry and renders the receptor Ca2+-impermeable. GluR2 levels, however, experience a fall after neuronal insult rendering the AMPA receptors Ca2+-permeable, thus factors that could interfere with this event might prove to be very beneficial against excitotoxicity. AMPA receptor clusters are stabilized by PSD-95, which requires palmitoylation at two sites. Targeting palmitoylation of the PSD-95 can also be a useful approach to disperse AMPA clusters at the synapse. In the perisynaptic region, mGluRs are present a little away from the synapse and are among the glutamate transporters, which require high-frequency firing for activation. On activation they might enhance the activity of NMDA receptors at the synapse to increase the levels of AMPA receptors. AMPA receptors surfaced at this juncture can contribute to heavy Ca2+ influx. Thus, blocking this pathway could be of considerable importance in preventing the excitotoxicity. A number of proteins such as the GRIP, PICK, and NSF also modulate the functions of AMPA receptors. Polyamines also block Ca2+ permeable AMPA receptors and thus are protective. NO and cGMP also play an important role in negatively regulating AMPA receptors and thus could offer protection. Modulation of AMPA receptor by different mechanisms has been discussed in the present review to implicate importance of these targets/pathways for safer and future neuroprotective drugs.