Abstract
The directional selectivity of amacrine and ganglion cells was studied using conventional intracellular recording techniques and drug application in the superfused retina-eyecup preparation of the tiger salamander. Baclofen, a GABAB receptor agonist, enhanced normal directional responses in some directionally selective third-order neurons. In about 30% of the cells that were not normally directional, baclofen induced direction-selective responses. This effect was particularly marked when 2-amino-4-phosphonobutyrate (APB) was used to isolate the OFF pathway. Comparisons of the effects of APB and baclofen on induced directional cells indicate that directional information in the ON and OFF channels is often handled separately and frequently is not aligned. This tends to obscure the observation of directionality as seen from the soma. Application of picrotoxin blocked both normal directional selectivity and baclofen-induced directional selectivity in some cells. Superfusion of picrotoxin and strychnine together blocked directionality in almost all cells. In both normal and induced directionality, the null direction response varied from cell to cell between a small depolarization, no voltage response, or a hyperpolarization. Injection of positive current often revealed “silent” inhibition. Some induced direction-selective cells did not show any evidence of inhibition in the null direction. The similarities in the response to baclofen, the influence of GABA and glycine antagonists, and the characteristics of the null-direction responses suggest that both normal and induced directionality originate from the same sources or mechanisms. Baclofen also induced orientation selectivity, but this was rarely observed.
