Axotomy-induced change in the properties of ( S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor channels in rat motoneurons

Abstract

Properties of ( S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor channels were studied in fluorescence-labelled control and axotomized motoneurons in spinal cord slices using a patch-clamp technique. Axotomy performed on the third postnatal day resulted in motoneuron death. Application of AMPA or kainate induced large whole-cell currents, but outside-out patches isolated from control motoneurons were either unresponsive or displayed only single-channel activity in response to rapid application of AMPA. Measurement of AMPA receptor channel openings in outside-out patches revealed multiple single-channel conductance levels: 12.2±1.0, 21.9±1.5 and 32.6±3.2 pS. In control motoneurons dialysed with spermine, the current–voltage relationship of responses induced by activation of AMPA receptor channels exhibited various degrees of inward rectification. The rectification index, the ratio of responses at +40 and −60 mV, was used to compare the degree of inward rectification. The mean values of rectification index of responses to focal application of AMPA and AMPA receptor-mediated excitatory postsynaptic currents induced by focal electric stimulation were 0.64±0.17 and 0.50±0.27, respectively. In axotomized motoneurons, the degree of rectification was significantly less for both responses induced by application of AMPA and for excitatory postsynaptic currents (0.91±0.09 and 0.95±0.12, respectively). Deactivation of AMPA receptors assessed from motoneuron excitatory postsynaptic currents at −70 mV was independent of postnatal age, with τ fast=0.88±0.35 ms ( A fast=78.2±11.8%) and τ slow=6.3±3.2 ms. In axotomized motoneurons, the decay time constants of excitatory postsynaptic currents were similar, τ fast=0.91±0.42 ms ( A fast=85.8±12.6%) and τ slow=5.9±3.4 ms. However, the mean amplitude of excitatory postsynaptic currents was only 43% of the amplitude recorded in control motoneurons. The results show that the current induced by activation of AMPA receptors in neonatal motoneurons is mediated by opening of both Ca 2+-permeable and Ca 2+-impermeable channels. As a result of axotomy, an experimental model of neurodegeneration, AMPA receptor channels in injured motoneurons destined to die become predominantly Ca 2+ impermeable. These findings suggest phenotypic control of AMPA receptor channel properties, presumably by affecting their subunit composition.