A metabotropic glutamate receptor in amphibian retina

In the first part of this project, current clamp and voltage clamp experiments were performed in the amphibian retina in both eyecup and slice preparations to examine the effects of endogenous photoreceptor transmitter, APB (2-amino-4-phosphonobutyrate), a selective ON bipolar cell receptor agonist, and 1S,3R ACPD (1-amino-1,3-cyclopentanedicarboxylic acid), a selective metabotropic glutamate receptor agonist. The major finding was that both APB and 1S,3R ACPD mimicked the effect of the photoreceptor transmitter at the ON bipolar cell synapse. They reduced ON bipolar cell membrane conductance during the light response. In the dark, when the photoreceptor transmitter release was high, the effects of APB and 1S,3R ACPD were occluded. In the retinal network, they both selectively blocked the ON bipolar cell light response. It is concluded that the retinal APB receptor is a member of the mGluR family and it is expressed predominantly at ON bipolar cell synapses in the retina.; Secondly, the ERG b-wave was quantitatively correlated with ON bipolar cell membrane potential and synaptic current. Simultaneous ERG and intracellular ON bipolar cell recordings showed that APB produced a similar suppression of both waveforms. In addition, the ON bipolar cell light response revealed a strong correlation between membrane potential and synaptic current. The rank order potency of glutamate analogs tested using the ERG b-wave as an indicator of ON bipolar cell activity was: APB {dollar}>{dollar} 1S,3R ACPD {dollar}>{dollar} ibotenate {dollar}gg{dollar} quisqualate. This matches the pharmacology of the cloned mGluR4, mGluR6 and mGluR7 receptors.; In the final part, conformational searching among nine APB receptor agonists suggested that an extended conformation of glutamate preferentially binds to the APB receptor. The CoMFA model, which correlates the structural properties of analogs with activity, identified one conformation that closely correlated with activity. Steric potentials play an important role in determining activity, particularly at a position near the plane of the three binding sites. The CoMFA model was able to predict the activity of several putative glutamate analogs.