AUTORADIOGRAPHIC DISTRIBUTION OF GLUTAMATERGIC LIGAND-BINDING SITES IN XENOPUS BRAIN - EVIDENCE FOR INTRACELLULAR [H-3] AMPA BINDING-SITES

The binding of a series of [H-3]glutamatergic ligands was determined by receptor autoradiography of membrane homogenate pellets and horizontal sections of Xenopus brain. Consistent with previous reports that a 'unitary' glutamate receptor is present in Xenopus CNS, the radioligands showed similar densities of binding sites in the membrane homogenate pellets. Furthermore, [H-3]kainate binding was completely displaced by AMPA or CNQX, [H-3]AMPA binding was completely displaced by kainate or CNQX and [H-3]CNQX binding was completely displaced by AMPA or kainate. However, in whole brain sections there were apparently 2- to 5-fold more [H-3]AMPA and [H-3]CNQX than [H-3]kainate sites. The absence of these extra sites in broken-washed membrane preparations suggests that the additional [H-3]AMPA and [H-3]CNQX binding may be due to cytosolic sites. The observation that all [H-3]AMPA and [H-3]CNQX binding in the brain sections is displaced by kainate indicates that the putative cytosolic sites are sensitive to relatively high concentrations of kainate but that they differ from the previously characterised Xenopus CNS unitary receptors.