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APB is a glutamate analogue specific to the mGluR6 receptor of ON retinal bipolar cells. It is used in research into receptive field processing by blocking the ON channel. Why is it not called an antagonist?

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    $\begingroup$ I have changed the title of this question to reflect its more restricted scope, corrected the spelling of glutamate — the poster should take the trouble to ensure the spelling of key molecules is correct — and changed "function" to "use in research". It is now in a suitable form for SE Biology, so that it is appropriate to vote the answer of @BryanKrause as correct. $\endgroup$ – David Dec 31 '17 at 17:01
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APB (2-Amino-4-phosphonobutyric acid) is a metabotropic glutamate receptor agonist, not an antagonist, and in many publications it is indeed referred to as an agonist, so I think your question is somewhat ill-posed. For example:

Yamashita, M., & Wassle, H. (1991). Responses of rod bipolar cells isolated from the rat retina to the glutamate agonist 2-amino-4-phosphonobutyric acid (APB). Journal of Neuroscience, 11(8), 2372-2382.

You can find many many more by searching "2-Amino-4-phosphonobutyric acid glutamate" on Google Scholar.

The reason that APB suppresses ON bipolar cells is because metabotropic glutamate receptors are often inhibitory rather than excitatory. Glutamate is typically thought of as excitatory because ionotropic glutamate receptors such as AMPA and NMDA receptors are non-specific cation channels that depolarize and therefore excite cells. Metabotropic glutamate receptors operate through g-protein second messenger cascades. In ON bipolar cells in particular, there are cation channels that are closed by mGluR6 activation, which therefore inhibits them.

Here's a review article that describes this as well as other basic information about bipolar cell function:

Euler, T., Haverkamp, S., Schubert, T., & Baden, T. (2014). Retinal bipolar cells: elementary building blocks of vision. Nature Reviews Neuroscience, 15(8), 507-519.

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  • $\begingroup$ Thank you Bryan. As you can tell from my question I am an interested amateur! My source describes the molecule as an analog for glutamate but its function is to block the G protein coupled receptor and prevent any neurotransmitter action that would otherwise result in the closing of the sodium channels. So, in a sense, the analog is performing the role of an antagonist. This was my confused thinking anyway! $\endgroup$ – adlibber Nov 30 '17 at 18:57
  • $\begingroup$ @adlibber No worries, interested amateurs are good targets for StackExchange sites. :) "My source describes the molecule as an analog for glutamate but its function is to block the G protein coupled receptor" - Then your source is either wrong or they aren't actually using 2-Amino-4-phosphonobutyric acid but a different molecule. APB might prevent glutamate from binding the receptor if it is higher affinity than glutamate (not sure if it is but could be), but it's still activating the receptor like glutamate and is therefore an agonist, and it would cause the sodium channels to close. $\endgroup$ – Bryan Krause Nov 30 '17 at 20:51
  • $\begingroup$ OK so its an agonist. Thanks for taking me through it :-) $\endgroup$ – adlibber Dec 1 '17 at 12:03

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