A novel selective allosteric modulator potentiates the activity of native metabotropic glutamate receptor subtype 5 in rat forebrain.
We found that N-[4-chloro-2-[(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)methyl]phenyl]-2-hydroxybenzamide (CPPHA), is a potent and selective positive allosteric modulator of the metabotropic glutamate receptor subtype 5 (mGluR5). CPPHA alone had no agonist activity and acted as a selective positive allosteric modulator of human and rat mGluR5. CPPHA potentiated threshold responses to glutamate in fluorometric Ca(2+) assays 7- to 8-fold with EC(50) values in the 400 to 800 nM range, and at 10 microM shifted mGluR5 agonist concentration-response curves to glutamate, quisqualate, and (R,S)-3,5-dihydroxyphenylglycine (DHPG) 4- to 7-fold to the left. The only effect of CPPHA on other mGluRs was weak inhibition of mGluR4 and 8. Neither CPPHA nor the previously described 3,3'-difluorobenzaldazine (DFB) affected [(3)H]quisqualate binding to mGluR5, but although DFB partially competed for [(3)H]3-methoxy-5-(2-pyridinylethynyl)pyridine binding, CPPHA had no effect on the binding of this 2-methyl-6-(phenylethynyl)-pyridine analog to mGluR5. Although the binding sites for the two classes of allosteric modulators seem to be different, these different allosteric sites can modulate functionally and mechanistically similar allosteric effects. In electrophysiological studies of brain slice preparations, it had been previously shown that activation of mGluR5 receptors by agonists increased N-methyl-D-aspartate (NMDA) receptor currents in the CA1 region of hippocampal slices. We found that CPPHA (10 microM) potentiated NMDA receptor currents in hippocampal slices induced by threshold levels of DHPG, whereas having no effect on these currents by itself. Similarly, 10 microM CPPHA also potentiated mGluR5-mediated DHPG-induced depolarization of rat subthalamic nucleus neurons. These results demonstrate that allosteric potentiation of mGluR5 increases the effect of threshold agonist concentrations in native systems.
O'Brien
Julie A
JA
Neuroscience-WP, Merck Research Laboratories, West Point, Pennsylvania 19486, USA.
Lemaire
Wei
W
Wittmann
Marion
M
Jacobson
Marlene A
MA
Ha
Sookhee N
SN
Wisnoski
David D
DD
Lindsley
Craig W
CW
Schaffhauser
Hervé J
HJ
Rowe
Blake
B
Sur
Cyrille
C
Duggan
Mark E
ME
Pettibone
Douglas J
DJ
Conn
P Jeffrey
PJ
Williams
David L
DL
D000494:Allosteric Regulation; D000818:Animals; D001549:Benzamides / Q000494:pharmacology*; D016466:CHO Cells; D006224:Cricetinae; D004594:Electrophysiology; D006801:Humans; D008954:Models, Biological; D010797:Phthalimides / Q000494:pharmacology*; D016548:Prosencephalon / Q000187:drug effects* / Q000378:metabolism; D011869:Radioligand Assay; D051381:Rats; D017207:Rats, Sprague-Dawley; D064529:Receptor, Metabotropic Glutamate 5; D018094:Receptors, Metabotropic Glutamate / Q000187:drug effects / Q000378:metabolism*
D001549:Benzamides; C578935:GRM5 protein, human; C578937:Grm5 protein, rat; C481448:N-(4-chloro-2-((1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)methyl)phenyl)-2-hydroxybenzamide; D010797:Phthalimides; D064529:Receptor, Metabotropic Glutamate 5; D018094:Receptors, Metabotropic Glutamate