BRAIN RESEARCH ELSEVIER Brain Research 667 (1994) 291-294 Short communication Immunohistochemical evidence for flupirtine acting as an antagonist on the N-methyl-D-aspartate and homocysteic acid-induced release of GABA in the rabbit retina N.N. Osborne a,* G. Pergande c, F. Block b, M. Schwarz b a Nuffield Laboratory of Ophthalmology, Unwersttyof Oxford, Walton Street, Oxford OX2 6AW,, UK b Department of Neurology, RWTHAachen, Pauwelsstr 30, 52057Aachen, Germany c German AS TA Medtca A.G., Medtcal Department, Wetsmullerstr 45, 60314 Frankfurt, Germany Accepted 11 October 1994 Abstract When rabbit retinas are exposed in vitro to specific excitatory amino acid receptor agonists certain GABAergic amacrine cells are activated to cause a release of GABA. The GABA that is not released can be detected by immunohistochemlstry. Exposure of tissues to kainate or NMDA each caused a characteristic change m the GABA immunoreactivity. CNQX antagonlsed the kamate effect specifically while MK-801 counteracted the influence of NMDA The effect produced by kainate was mimicked by domoic acid while the influence of homocysteic acid was identical with NMDA. Flupirtine alone did not influence the nature of the GABA immunoreactivity and so did not act as a kainate or NMDA agonist. However, flupirtine counteracted the influence produced by NMDA and homocysteic acid but had no effect on the kainate and domoic acid responses. Thus in this system flupirtine acts as an NMDA antagonist. Keywords: N-Methyl-o-aspartate; Kainate; Domoic acid; Homocysteic acid; Flupirtlne; GABA-immunoreactivity; Retina Recent studies by Osborne and Herrera [7] have shown that when the rabbit retina is exposed in vitro to glutamatergic agonists some of these substances induce a change in the GABA immunoreactivity. Of the exci- tatory amino acid agonists tested (ACPD, APB, NMDA, AMPA and kainate) only NMDA, kainate and AMPA caused changes in the GABA immunoreactiv- ity. The NMDA effect was antagonised by MK-801 but not by CNQX and was characterised by the GABA 'staining' in the inner plexiform layer appearing as four clear bands. In contrast, AMPA/kainate caused three bands of GABA immunoreactivity to appear in the inner plexiform layer and this effect was antagonised by CNQX and not MK-801. The results were inter- preted as suggesting that NMDA and kainate/AMPA receptors are not associated in identical ways with the various GABAergic amacrine cells and that when these receptors are specifically activated a release of the * Corresponding author. Fax: (44) (865) 794508. 0006-8993/94/$07.00 © 1994 Elsevier Science B V All rights reserved SSDI 0006-8993(94)01208-3 neurotransmitter is elicited from those GABAergic cells containing the activated receptors [7]. The use of the in vitro retinal tissue preparation as described by Osborne and Herrera [7] to screen for drugs possibly interacting with kainate/AMPA or NMDA receptors is demonstrated in the present study. Several studies have suggested that L-homocysteic and domoic acids act as agonists at NMDA [6,10,12] and kainate [1,12] receptors, respectively. It was therefore decided to study their effects specifically. In addition, experiments were undertaken to see whether flupirtine influences the NMDA and kainate-induced effects on retinal GABA immunoreactivity. Flupirtine is a central acting non-opioid analgesic and appears to interact with neither serotonin, dopamine, nicotinic nor adren- ergic receptors [4]. However, recent elctrophysiological studies by Schwarz et al. [9] suggest that flupirtine may have an antagonistic action on NMDA receptors. Anaesthetised rabbits were killed with an overdose of pentobarbitol, the retinas rapidly dissected, cut into four wedge-shaped triangular quarters and placed in Locke's solution (magnesium free) containing (in