- zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Neuroohamcoloav, Vol. 36, No. 10, m. 1405-1415, 1997 Pergamon PII: SOO28-3908(97)00113-S 0 199yElsevier Science Lti: All rights reserved Printed in Great Britain 0028-3908/97 $17.00 + 0.00 In vivo Microdialysis Study of GABAA and GABAB Receptors Modulating the Glutamate Receptor/NO/ Cyclic GMP Pathway in the Rat Hippocampus ERNEST0 FEDELE, GIORGIA VARNIER and MAURIZIO RAITERI” Institute of Pharmacology and Pharmacognosy, University of Genova, Viale Cembrano 4, 16148 Genova, Italy (Accepted 2 June 1997) Summary-Intrahippocampal perfusion of bicuculline (50 PM) in Mg2+-free medium caused elevation of extracellular cGMP and epileptic-like behaviour. Both effects were partially prevented by blocking NMDA receptors with MK-801 or Mg*+ ions. Similarly, the GABAu receptor antagonists CGP52432 (0.1-30 PM) and CGP35348 (0.3-l n&I) evoked increases of extracellular cGMP. CGP52432 also elicited behavioural responses ranging from wet dog shakes to convulsions. MK-801 or Mg2+ ions reduced the effects of CGP52432. Local application of muscimol (100-300 PM) or (-)baclofen (300 PM) caused inhibition of extracellular cGMP. Administration of the AMPA/kainate receptor antagonist NBQX (100 PM) caused cGMP elevation which was almost abolished by co-perfusion of muscimol and (-)baclofen. In the presence of physiological Mg”+, perfusion of AMPA (30 PM) failed to affect cGMP levels, although rats displayed wet dog shakes episodes. When AMPA was co-perfused with low concentrations of bicuculline or CGP52432, cGMP elevations were observed in 60% of the rats. Addition of both antagonists to AMPA resulted in 85% of rats displaying a cGMP response. To conclude: (a) extracellular hippocampal cGMP is controlled by inhibitory GABA* and GABAa receptors tonically activated through GABAergic interneurons receiving AMPA/kainate- mediated glutamatergic inputs; (b) the GABAergic receptors are not endogenously saturated and can be further stimulated by exogenous agonists; (c) blockade of the GABA-mediated inhibition causes increase of cGMP and epileptic-like behaviour, due largely to endogenous activation of NMDA receptors; (d) reproducible cGMP responses to AMF’A can be observed when the inhibitory GABAergic inputs to the NO/guanylyl cyclase system are blocked, coni’nming the previously proposed existence of AMPA/kainate receptors able to increase the nucleotide synthesis. 0 1997 Elsevier Science Ltd. KeywordsGABAA receptor, GABAu receptor, GABA-NMDA receptor interaction, nitric oxide (NO), cGMP, microdialysis, hippocampus, epilepsy. It is well known that stimulation of glutamate receptors results in an increase of 3’:5’-cyclic monophosphate (cGMP) levels which is mediated by the gaseous intra and intercellular messenger nitric oxide (NO), and a large body of evidence indicates that the activation of this biochemical pathway is, instrnmental in triggering key Abbreviations: AMPA, a-amino-3-hydroxy-5-methyl-4-isoxa- zolepropionic acid; cGMP, 3’:5’-cyclic monophosphate; CGP 35348, 3-amino-propyl (diethoxymethyl)phosphinic acid; CGP 52432, [3-[[(3,4-dichlorophenyl) methyl]atnino]- propyl] (diethoxymethyl) phosphenic acid; DMSO, di- methyl sulphoxide; DNQX, 6,7-dinitroquinoxaline-2,3- dione; MK-801, dizocilpine; NBQX, 6-nitro-7-sulphamoyl- benzo(f)quinoxaline-2-3-dione; NMDA, N-methyl-D-aspa- rate; ODQ, IH-[ 1,2,4]#oxadiazolo[4,3-alquinoxaline-I-one; WDS, wet dog shakes; WR, wild running. *To whom correspondence should be addressed. Fax: 39 10 3993360; e-mail: M.Raiteri@pharmatox.unige.it. events in a variety of physiological and pathophysiolo- gical processes (for reviews see Schuman and Madison, 1994; Zhang and Snyder, 1995). In the past few years, several studies have shown that microdialysis sampling of extracellular cGMP in the cerebellum (Vallebuona and Raiteri, 1993, 1994; Fedele and Raiteri, 1996; Fedele et al., 1997a), hippocampus (Vallebuona and Raiteri, 1994, 1995; Fedele et al., 1996) and frontal cortex (Laitinen et al., 1994) of freely moving rats represents a suitable experimental model to study in vivo the glutamate receptor/NO-synthase/guanylyl cyclase pathway. How- ever, while in the cerebellum stimulation of both N- methyl-D-aspartate (NMDA) and a-amino-3-hydroxy-5 methyl-4-isoxazolepropionic acid (AMPA) receptors resulted in the NO-mediated increase of cGMP (Valle- buona and Raiteri, 1993,1995; Fedele and Raiteri, 1996), in the hippocampus local infusion of AMPA was not able to affect the nucleotide levels in almost all the 1405