Neuropharmacology 38 (1999) 1883 – 1891 Role of cyclic GMP in glutamate neurotoxicity in primary cultures of cerebellar neurons Carmina Montoliu, Marta Llansola, Elena Kosenko, Regina Corbala ´n, Vicente Felipo * Instituto de Inestigaciones Citologicas, Fundacio ´n Valenciana de Inestigaciones Biome ´dicas, Amadeo de Saboya 4, 46010 Valencia, Spain Accepted 16 March 1999 Abstract The role of cGMP in the mediation of glutamate neurotoxicity remains controversial. Some reports indicate that cGMP mediates glutamate neurotoxicity while others indicate that cGMP is neuroprotective. We have studied the role of cGMP in the mediation of glutamate and nitric oxide neurotoxicity in primary cultures of cerebellar neurons. Inhibition of soluble guanylate cyclase prevents glutamate and nitric oxide neurotoxicity. There is a good correlation between inhibition of cGMP formation and neuroprotection. Moreover 8-Br-cGMP, a cell permeable analog of cGMP, induced neuronal death. These results indicate that increased intracellular cGMP is involved in the mechanism of neurotoxicity. Inhibitors of phosphodiesterase did not increase intracellular cGMP but increased the content of cGMP in the extracellular medium and prevented glutamate neurotoxicity. Moreover, addition of cGMP to the extracellular medium also prevented glutamate neurotoxicity in cerebellar neurons in culture. These results are compatible with a neurotoxic effect of increased intracellular cGMP and a neuroprotective effect of increased extracellular cGMP. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: Cyclic GMP; Glutamate; Neurotoxicity; Guanylate cyclase; Phosphodiesterase; Neuroprotection www.elsevier.com/locate/neuropharm 1. Introduction Glutamate is the main excitatory neurotransmitter in mammals. However, excessive activation of glutamate receptors leads to neuronal degeneration and death. Excitatory amino acid neurotoxicity has been proposed to contribute to the pathogenesis of different neurode- generative diseases, including amyotrophic lateral scle- rosis and Huntington disease. Glutamate neurotoxicity is also involved in the neuronal damage found in cerebral ischemia and it has been proposed to be in- volved in the origin of Alzheimer’s and Parkinson’s diseases. The understanding of the molecular mecha- nism of glutamate neurotoxicity and of the possible mechanisms to prevent it would be therefore of great interest for the treatment of the above situations. In many systems, including primary cultures of cere- bellar neurons, glutamate neurotoxicity is mainly medi- ated by excessive activation of the NMDA type of glutamate receptor (Choi, 1987; Novelli et al., 1988; Min ˜ ana et al., 1996). Increased intracellular Ca 2 + is an essential step leading to neuronal death (Choi, 1987; Manev et al., 1989). In several types of neurons in culture one step in the neurotoxic process is the gluta- mate-induced formation of nitric oxide. This is sup- ported by experiments showing that glutamate neurotoxicity is prevented by inhibitors of nitric oxide synthase such as nitroarginine (Dawson et al., 1991, 1993; Cazevieille et al., 1993; Lafon-Cazal et al., 1993). Moreover, it has been proposed that prolonged applica- tion of nitroarginine enhances the neuroprotective effi- cacy (Vige ´ et al., 1993). In the cerebellar neurons used in the present work, glutamate neurotoxicity is com- pletely prevented by nitroarginine (Marcaida et al., 1995). It seems therefore that formation of nitric oxide is an essential step in the mediation of glutamate neuro- toxicity. However, the subsequent events in the neuro- toxic process are not well understood. * Corresponding author. Tel.: +34-96-3391250; fax: +34-96- 3601453. E-mail address: vfelipo@ochoa.fib.es (V. Felipo) 0028-3908/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII:S0028-3908(99)00071-4