DEPENDENCE OF RETINOGENICULATE TRANSMISSION IN CAT ON NMDA RECEPTORS

We have examined the possibility that N-methyl-D-aspartate (NMDA) receptors may be involved in the visual response of relay cells in the cat dorsal lateral geniculate nucleus (dLGN). The selective NMDA receptor antagonists D-2-amino-5-phosphonovalerate (APV) and 3-[(±)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) have been iontophoretically applied to X and Y cells in the dLGN and effects of the visual response to a light spot flashed within the receptive field center determined. The antagonist effects were assessed at ejection current levels producing a selective blockade of the responses to iontophoretically applied NMDA with respect to those elicited by the non-NMDA receptor agonists quisqualate and kainate. These selective effects were determined in an experimental paradigm where the visual response and responses to NMDA and the non-NMDA receptor agonists were compared in the same test run. The data refer to a total population of 52 cells (28 X, 24Y). Application of APV abolished or greatly reduced the visual responses of both X and Y cells. The mean percentage reduction in the visual response for the X cells studied was 59 ± 10% (SE; n = 7) and for the Y cells 66 ± 8% (SE; n = 11). Both the early onset transient and the sustained component of the visual response to the flashed stimulus were equally affected. The antagonist CPP produced a similar pattern of effect to APV, substantially reducing or abolishing the visual response in both X and Y cells. In general, iontophoretically applied CPP appeared to be more potent and selective than APV. The mean percentage reduction in the visual response for X cells was 80 ± 4% (SE; n = 21) and for Y cells 66 ± 8% (SE; n = 14). There was no systematic distinction in the vulnerability of the early onset transient and sustained components of the visual response to NMDA receptor blockade with CPP, although in certain cases visual responses were abolished save for a single spike in the onset transient. Subdividing the X and Y cells into their on- and off-center subgroups did not reveal any systematic distinction in the vulnerability to NMDA receptor blockade, with the proviso that on-center Y cells were slightly less affected by CPP application than the other groups. The data provide strong support for the view that NMDA receptors play a pivotal role in the visual response of both X and Y cells in the dLGN. This refers to all components of the visual response and provides a further example of the contribution of NMDA receptors to fast synaptic processing.