Direct intracerebellar injections of N-methyl-D-aspartate (NMDA) or D-serine elicited dose-dependent increases in cerebellar cyclic GMP levels, in vivo in the mouse. The actions of D-serine were antagonized by the competitive NMDA receptor antagonist 3-(2-carboxypiperazin-4-yl) propyl-1-phosphonic acid and by the phencyclidine receptor agonist MK-801, observations supporting actions at the NMDA-coupled glycine receptor. In addition, the actions of D-serine were antagonized by a partial agonist (D-cycloserine) and an antagonist (HA-966) of the NMDA-coupled glycine receptor. These data are all consistent with D-serine acting at the NMDA-coupled glycine receptor and represent the first demonstration of glycine receptor potentiation of ongoing NMDA-mediated neuronal activity in the CNS, rather than potentiation of exogenous NMDA.

The synthesis of nitric oxide by brain slices has been demonstrated in several laboratories. In addition, in vitro studies have demonstrated stimulation of nitric oxide synthesis by excitatory amino acid receptor agonists. These data have led to the hypothesis that this readily diffusible "intercellular messenger molecule" acts to generate a cascade effect by activating guanylate cyclase in several cell types and thereby augment levels of the second messenger cyclic GMP (cGMP). Therefore, we evaluated this hypothesis in vivo, by testing the actions of the nitric oxide synthase inhibitor N-mono-methyl-L-arginine (NMMA) on elevations in level of mouse cerebellar cGMP generated by excitatory amino acid receptor agonists. The stimulatory effects of D-serine, quisqualate, and kainate were all found to be antagonized by this enzyme inhibitor. In addition, NMMA antagonized the increases in cerebellar cGMP level elicited by harmaline and pentylenetetrazole, pharmacological agents that augment endogenous excitatory amino acid transmission. Our data are, therefore, the first in vivo demonstration that nitric oxide is an important "messenger molecule" in the cerebellum, mediating the actions of kainate, quisqualate, and N-methyl-D-aspartate receptor agonists on guanylate cyclase. These data are consistent with previous in vitro findings with kainate and N-methyl-D-aspartate.

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