Interest in the role of nitric oxide (NO) in the nervous system began with the demonstration that glutamate receptor activation in cerebellar slices causes the formation of a diffusible messenger with properties similar to those of the endothelium-derived relaxing factor. It is now clear that this is due to the Ca2+/calmodulin-dependent activation of the enzyme NO synthase, which forms NO and citrulline from the amino acid L-arginine. The cerebellum has very high levels of NO synthase, and although it has low levels of guanylyl cyclase, cerebellar cyclic guanosine monophosphate (cGMP) levels are an order of magnitude higher than in other brain regions. A transcellular metabolic pathway is also present in the cerebellar cortex to recycle citrulline back to arginine. The NO formed binds to and activates soluble guanylyl cyclase to elevate cGMP levels in target cells. Studies employing NADPH-diaphorase, a selective histochemical marker for NO synthase, together with immunohistochemistry, in situ hybridization and biochemical studies have indicated that NO production occurs in granule and basket cells in the cerebellar cortex, whereas cGMP formation appears to occur largely in other cells, including Purkinje cells. Given that a long-term depression of AMPA currents can be seen in isolated Purkinje cells, this anatomical localization suggests that NO cannot play an essential role in the induction of this form of synaptic plasticity.
In vitro studies using cerebellar slices or cultures, as well as in vivo studies, have demonstrated that NO formation and release occur in the cerebellar cortex in response to NMDA receptor activation. Using intracerebellar microdialysis in awake-behaving animals, we have found that this is associated with a large increase in extracellular cGMP levels in the cerebellar cortex. A similar increase in cGMP efflux is seen in response to activation of AMPA or metabotropic glutamate receptors, or to activation of the climbing fiber input. The increase in extracellular cGMP required the Ca2+-dependent activation of NO synthase, and was potentiated by inhibition of phosphodiesterases or organic anion transport. These results suggest a possible role for cGMP as an intercellular messenger in the cerebellar cortex.
In summary, stimuli that elevate Ca2+ levels in granule or in basket cells will activate NO synthase, which by binding to its receptor-soluble guanylyl cyclase – causes an increase in cGMP. cGMP may then act through protein kinases, phosphodiesterases, and ion channels or receptors to affect cerebellar function.
