Interaction between mGluR8 and calcium channels in photoreceptors is sensitive to pertussis toxin and occurs via G protein betagamma subunit signaling.
OBJECTIVE
The most recently identified metabotropic glutamate receptor (mGluR), type 8 mGluR (mGluR8), has been identified functionally as a presynaptic autoreceptor in rod photoreceptors. This study analyzed the mechanism of action underlying mGluR8 activity and modulation of the cytosolic Ca2+ concentration in mouse photoreceptors.


METHODS
The cytosolic Ca2+ concentration of acutely isolated rod photoreceptors was monitored optically with microspectrofluorimetry and in the presence of modulators of G protein activity.


RESULTS
mGluR8 activation by the group III mGluR agonists l-2-amino-4-phosphonobutyrate and l-serine-O-phosphate or the physiological ligand l-glutamate produced a decrease in influx of extracellular Ca2+ into the cytosol. Pretreatment of isolated rod photoreceptors with the G protein uncoupler suramin or pertussis toxin, which inactivates Gi/o/z proteins and Gt protein/transducin, or a G protein betagamma subunit-inhibiting peptide abolished this activity. Preincubation of cells with cholera toxin (CTX), an activator of Gs protein, had no effect.


CONCLUSIONS
These results suggest that the function of mGluR8 of modulating the cytosolic Ca2+ concentration and thereby potentially the release of neurotransmitter from rod spherules, the axon terminal systems of rod photoreceptors, is mediated by a pertussis toxin-sensitive G protein potentially via the betagamma subunit. The absence of Go and Gz proteins, as reported previously, implies a novel potential interaction between Gi2 and/or Gt protein/transducin and mGluR8 in photoreceptors. These results have potential implications for the regulatory function and pharmacologic targeting of mGluR8 in photoreceptors.
Koulen
Peter
P
Department of Pharmacology and Neuroscience, University of North Texas Health Science Center at Fort Worth, Fort Worth, Texas 76107-2699, USA. pkoulen@hsc.unt.edu
Liu
Jiyuan
J
Nixon
Everett
E
Madry
Christian
C
D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D005456:Fluorescent Dyes / Q000378:metabolism; D044387:GTP-Binding Protein beta Subunits / Q000502:physiology*; D044388:GTP-Binding Protein gamma Subunits / Q000502:physiology*; D051379:Mice; D015591:Microspectrophotometry; D037342:Pertussis Toxin / Q000494:pharmacology*; D018094:Receptors, Metabotropic Glutamate / Q000378:metabolism*; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism*; D015398:Signal Transduction / Q000502:physiology*
D015220:Calcium Channels; D018690:Excitatory Amino Acid Agonists; D005456:Fluorescent Dyes; D044387:GTP-Binding Protein beta Subunits; D044388:GTP-Binding Protein gamma Subunits; D018094:Receptors, Metabotropic Glutamate; C111435:metabotropic glutamate receptor 8; D037342:Pertussis Toxin; D002118:Calcium