text: First, basal AGB entry is dependent upon the overall ion channel activity in the ‘resting’ state and may be further modified if L-2amino-4-phosphono butyric acid (L-APB or L-AP4), a specific metabotropic glutamate receptor agonist, is used to modify AGB entry into ON bipolar cells.(93,128,197,202) The second reason relates to the diverse disposition of glutamate receptors within the Figure 12.