title: Novel mGluR- and CB1R-independent suppression of GABA release caused by a contaminant of the group I metabotropic glutamate receptor agonist, DHPG.
abstract: BACKGROUND
  Metabotropic glutamate receptors (mGluRs) are ubiquitous throughout the body, especially in brain, where they mediate numerous effects. MGluRs are classified into groups of which group I, comprising mGluRs 1 and 5, is especially important in neuronal communication. Group I actions are often investigated with the selective agonist, S-3,5-dihydroxyphenylglycine (DHPG). Despite the selectivity of DHPG, its use has often led to contradictory findings. We now report that a particular commercial preparation of DHPG can produce mGluR-independent effects. These findings may help reconcile some discrepant reports.
  
  
  METHODS
  We carried out electrophysiological recordings in the rat in vitro hippocampal slice preparation, focusing mainly on pharmacologically isolated GABA(A)-receptor-mediated synaptic currents.
  
  
  RESULTS
  While preparations of DHPG from three companies suppressed GABAergic transmission in an mGluR-dependent way, one batch had an additional, unusual effect. Even in the presence of antagonists of mGluRs, it caused a reversible, profound suppression of inhibitory transmission. This mGluR-independent action was not due to a higher potency of the compound, or its ability to cause endocannabinoid-dependent responses. Field potential recordings revealed that glutamatergic transmission was not affected, and quantal analysis of GABA transmission confirmed the unusual effect was on GABA release, and not GABA(A) receptors. We have not identified the responsible factor in the DHPG preparation, but the samples were 99% pure as determined by HPLC and NMR analyses.
  
  
  CONCLUSIONS
  In certain respects our observations with the anomalous batch strikingly resemble some published reports of unusual DHPG effects. The present findings could therefore contribute to explaining discrepancies in the literature. DHPG is widely employed to study mGluRs in different systems, hence rigorous controls should be performed before conclusions based on its use are drawn.
authors: Department of Physiology, Program in Neuroscience, University of Maryland School of Medicine, Baltimore, MD, USA.
authors: Carlos A
authors: CA
authors: Lafourcade
authors: Longhua
authors: L
authors: Zhang
authors: Bradley E
authors: BE
authors: Alger
mesh_terms: D000818:Animals; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology*; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005998:Glycine / Q000031:analogs & derivatives* / Q000494:pharmacology; D006624:Hippocampus / Q000187:drug effects / Q000502:physiology; D066298:In Vitro Techniques; D051381:Rats; D043884:Receptor, Cannabinoid, CB1 / Q000502:physiology*; D018094:Receptors, Metabotropic Glutamate / Q000819:agonists / Q000037:antagonists & inhibitors / Q000502:physiology*; D012118:Resorcinols / Q000494:pharmacology*; D009435:Synaptic Transmission; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
chemical_list: D018690:Excitatory Amino Acid Agonists; D018691:Excitatory Amino Acid Antagonists; D043884:Receptor, Cannabinoid, CB1; D018094:Receptors, Metabotropic Glutamate; D012118:Resorcinols; D005680:gamma-Aminobutyric Acid; C079215:3,5-dihydroxyphenylglycine; D005998:Glycine