Effects of 2‐amino‐4‐phosphonobutyric acid on responsivity and spatial summation of X cells in the cat retina.

Abstract

SUMMARY1. We studied the effect of locally applied 2-amino-4-phosphonobutyric acid (APB) on the response of X-type cat retinal ganglion cells to stimulation by sinusoidal gratings of photopic mean luminance.2. Application of APB produced a decrease in the mean firing rate of on-centre X cells, but an increase in the mean firing rate of off-centre X cells.3. The response and responsivity of both on-and off-centre X cells to drifting sinusoidal gratings were suppressed by APB. Under control conditions, the response amplitude was linear with grating contrast for contrasts less than 10 %. During APB application, the slope of the contrast response curve was reduced, and linearity was maintained for a larger range of contrast.4. Assuming that APB acts selectively on the cone to depolarizing bipolar cell synapse, the suppression of the response of on-and off-centre X cells suggests a functional link between depolarizing bipolar cells and both types of X cell.5. Because APB reduces the centre response of both on-and off-centre X cells, caution should be observed in interpreting the effect of APB on higher visual centres in terms of the blockage of 'on' signals leaving the retina.6. By analysing the spatial frequency tuning curve before and during APB application in terms of the difference-of-Gaussians model of receptive field structure, it was found that the balance between the integrated strengths of the centre and surround was not modified by APB. APB, however, had a differential effect on the summating area of the centre and surround. While the centre radius was unaffected by APB, the surround radius was reduced. This suggests that the peak sensitivity of the surround mechanism was reduced less severely than the peak sensitivity of the centre mechanism.7. Strychnine also suppressed the response of the centre mechanism of both onand off-centre X cells to drifting gratings.

Centre components of cone‐driven retinal ganglion cells: differential sensitivity to 2‐amino‐4‐phosphonobutyric acid.

“…The differential effects described above also cannot be attributed to an increase in surround contribution to the response. This is because (1) the surround contribution to the response is relatively small under the stimulus conditions used (see Methods) and (2) APB suppresses both centre and surround responses and does not change the centre-surround balance of at least the X cells (Chen & Linsenmeier, 1989). Figure 6 shows the change in time course of the 'off' response of off-centre cells in a different way.…”

“…3) the right). Fortunately, a small dose of APB increased the mean rate sufficiently (Bolz et al 1984;Chen & Linsenmeier, 1989) to show that the cell's firing rate did drop (transient 'on' response) during the bar's bright phase (second histogram). This elevated mean rate also allowed the observation of the sharp drop in firing rate at the end of the dim phase.…”

“…The methods for animal preparation, recording and visual stimulation, and pressure ejection have been reported previously (Chen & Linsenmeier, 1989).…”

“…Significantly, the transient 'off' response to 20% contrast was also reduced by 24 % under the same dose of APB, which shows that the suppressive power of APB is independent of contrast. Furthermore, in experiments with sinusoidal gratings (Chen & Linsenmeier, 1989), APB suppressed responses of on-and off-centre cells at all contrasts, including those high enough to saturate the response. The differential effects described above also cannot be attributed to an increase in surround contribution to the response.…”

“…In the cat retina, numerous studies (Enroth-Cugell & Pinto, 1972;Bolz, Wiissle & Thier, 1984; McGuire, Stevens & Sterling, 1986;Wassle, Schiifer-Trenkler & Voigt, 1986;Freed & Nelson, 1987;Muller, Wiissle & Voigt, 1988;Chen & Linsenmeier, 1989) have shown that the centre response of the retinal X cells, and maybe Y cells, may also be mediated by multiple synaptic pathways. These studies, however, did not deal with the possible differences between the transfer properties of these pathways.…”