Enhancement of NMDA receptor-mediated currents by light in rat neurones in vitro.

1. N-Methyl-D-aspartate (NMDA) receptor function can be modified by the action of several endogenous and exogenous modulatory processes. In the present study, we report that brief pulses of light potentiate NMDA, but not non-NMDA glutamatergic receptor-mediated whole-cell and single channel currents in rat cortical neurones in vitro. In addition, light also potentiated NMDA receptor-mediated whole-cell responses in isolated rat retinal neurones. 2. Potentiation of NMDA whole-cell currents in cortical neurones was readily observed during and following a brief (< 2 s) exposure of neurones to wavelengths of less than 324 nm of relatively bright light (0.09 microW microm-2). In addition, prolonged exposures (> 30 s) to visible wavelengths (> 380 nm) or to attenuated light (1-3 % transmittance of non-attenuated light) were also sufficient to enhance NMDA receptor-mediated responses. 3. The light-induced potentiation of NMDA receptor-mediated currents persisted for several minutes, slowly reversing to control levels with a time constant of approximately 5 min. A subsequent exposure to light could potentiate NMDA receptor-mediated currents for a second time. 4. Light did not alter the apparent affinity of the NMDA receptor for the co-agonists NMDA and glycine. Additionally, potentiation of the NMDA-induced currents was not mediated by a change in the pH sensitivity of the receptor. In excised outside-out membrane patches, the effects of light on NMDA-activated unitary currents were manifested as a twofold increase in channel open frequency without alterations in single channel amplitude or open time. 5. Our results suggest the presence of a light-sensitive moiety within the NMDA receptor, or in a closely associated structure, which affects channel properties. This previously unrecognized form of NMDA receptor modulation may provide a tool for understanding the conformational changes associated with its gating. In addition, it is possible that light may affect NMDA receptor-mediated function or dysfunction in the retina.

NMDA and glutamate evoke excitotoxicity at distinct cellular locations in rat cortical neurons in vitro.

The development of cortical neurons in vivo and in vitro is accompanied by alterations in NMDA receptor subunit expression and concomitant modifications in the pharmacological profile of NMDA-activated ionic currents. For example, we observed that with decreasing NR2B/NR2A subunit expression ratio, the block of NMDA receptor-mediated whole-cell responses by the NR2B-selective antagonist haloperidol was also decreased. In mature cultures (>22 d in vitro), however, NMDA responses obtained from excised nucleated macropatches, which comprised a large portion of the soma, remained strongly antagonized by haloperidol. These results suggest that in more mature neurons NR1/NR2B receptors appear to be preferentially expressed in the cell body. As predicted from the whole-cell recording pharmacological profile, NMDA-induced toxicity was largely unaffected by haloperidol in mature cultures. However, haloperidol effectively blocked glutamate toxicity in the same cultures, suggesting that the neurotoxic actions of this amino acid were mostly due to the activation of somatic NMDA receptors. In experiments in which the potency of glutamate toxicity was increased by the transport inhibitor l-trans-pyrrolidine-2,4-dicarboxylic acid, the neuroprotective effects of haloperidol were significantly diminished. This was likely because of the fact that glutamate, now toxic at much lower concentrations, was able to reach and activate dendritic receptors under these conditions. These results strongly argue that exogenous glutamate and NMDA normally induce excitotoxicity at distinct cellular locations in mature mixed neuronal cultures and that NR1/NR2B receptors remain an important component in the expression of glutamate, but not NMDA-induced excitotoxicity.