The role of the AMPA receptor GluR2 subunit in anesthetic neurodepression and excitotoxic cell death

Abstract

The AMPA subtype of glutamate receptor mediates fast excitatory neurotransmission. This ligand-gated ionotropic receptor is composed of multiple subunits (GluR1, GluR2, GluR3, and GluR4). In particular, the GluR2 subunit is present in the majority of AMPA receptors in the forebrain and imparts reduced calcium permeability, insensitivity to polyamine inhibition, sensitivity to barbiturate inhibition and influences the localization of these receptors to the synapse. The objective of this thesis was to determine the importance of the GluR2 subunit in physiological, pharmacological, and pathophysiological processes. Specifically, the roles of the AMPA receptor GluR2 subunit in anesthetic-induced neurodepression, excitatory neurotransmission, and neuronal excitotoxicity were investigated. 'In vitro' electrophysiological studies revealed a decreased potency of barbiturates for the inhibition of GluR2-deficient AMPA receptors. In contrast, behavioral studies demonstrated that GluR2 null mutant (-/-) mice were more sensitive to pentobarbital than wildtype (+/+) mice. These results suggest that the inhibition of AMPA receptors does not contribute to the neurodepressive effects of barbiturates in this mouse model. These findings were corroborated by behavioral studies demonstrating that the (-/-) mice were also more sensitive to volatile anesthetics, which do not appreciably inhibit AMPA receptors. Therefore, the inhibition of AMPA receptors does not underlie anesthetic-mediated neurodepression. Instead, we postulated that a reduced excitatory neurotransmission in (-/-) mice renders then more sensitive to general anesthetics. A reduced excitatory neurotransmission 'in vitro' was demonstrated by the decrease in the fast component of evoked excitatory postsynaptic current (EPSC) recorded from (-/-) hippocampal slice. In contrast, AMPA receptor-mediated miniature EPSCs in cultured (-/-) hippocampal neurons were unaltered, suggesting that a normal AMPA receptor complement exists at synapses activated by the spontaneous release of glutamate. The increased kainate-evoked current density measures in (-/-) neurons were suggestive of an enhanced expression of extrasynaptic AMPA receptors. These results are consistent with a reduced localization of GluR2-deficient AMPA receptors to some synapses. In addition, kainate excitotoxicity was studied in the and (-/-) mice. 'In vitro' and 'in vivo' experiments demonstrated unchanged neuronal death, despite increased calcium influx into (-/-) hippocampal cultured neurons. In summary, absence of the GluR2 subunit contributes to a reduced synaptic neurotransmission and increased anesthetic-induced neurodepression, without enhancing excitotoxic neuronal injury.