As mentioned in the Introduction the presence
As mentioned in the Introduction the presence of the GluA2 subunit prevents Ca transport, while activation of AMPA receptors lacking GluA2 increases intracellular Ca , . This suggests that the reduced membrane expression of GluA2 and the increase in GluA1 in hyperammonemic rats would result in a larger increase in Ca in post-synaptic neurons following activation of AMPA receptors. This would lead to activation of different signal transduction pathways, thus contributing to altered neurotransmission in hyperammonemia. This has been already shown in Imipenem of hyperammonemic rats in vivo. We have previously shown by in vivo microdialysis in freely moving rats that chronic hyperammonemia reduces the activation of the glutamate-NO-cGMP pathway by low-affinity (GluA2-containing) AMPA receptors and increases the activation of the pathway by high-affinity (GluA2-lacking) AMPA receptors . The results reported here provide a mechanistic explanation for this alteration in activation of signal transduction pathways by activation of AMPA receptors in cerebellum of hyperammonemic rats. Extracellular glutamate can only activate AMPA receptors present in the membrane. The reduced activation of the glutamate-NO-cGMP pathway through GluA2-containing receptors would be a consequence of the reduced membrane expression of the GluA2 subunit in hyperammonemia. The increased activation of the glutamate-NO-cGMP pathway through GluA2-lacking (GluA1-containing) receptors would be a consequence of the increased membrane expression of the GluA1 subunit in hyperammonemia. The altered membrane expression of GluA1 and GluA2 would contribute therefore to altered modulation of the glutamate-NO-cGMP pathway in cerebellum of hyperammonemic rats. We have shown that the function of this pathway is reduced in cerebellum of hyperammonemic rats and that impairment of this pathway is responsible for their reduced ability to learn a Y maze task. Treatments that restore the glutamate-NO-cGMP pathway function in cerebellum also restore the ability to learn the Y maze task , . The altered membrane expression of GluA1 and GluA2 would also contribute therefore to reduce learning ability in hyperammonemic rats. AMPA receptors in cerebellum also modulate fine motor coordination , , , , which is also reduced in rats with chronic hyperammonemia . It is likely that altered membrane expression of GluA1 and GluA2 could also contribute to motor in-coordination in hyperammonemic rats. AMPA receptors in cerebellum also modulate long-term depression , a form of synaptic plasticity considered the bases of some forms of learning such as motor learning . The effects of chronic hyperammonemia on motor learning have not been reported. It is possible that altered membrane expression of GluA1 and GluA2 could also affect to motor learning in hyperammonemic rats. We propose in Fig. 6 some possible mechanisms by which hyperammonemia increases membrane expression of GluA1 and reduces that of GluA2 in cerebellum. Chronic hyperammonemia enhances basal activation of NMDA receptors in cerebellum both in vivo and in freshly isolated slices . This leads to an adaptive mechanism to prevent excitotoxicity resulting in reduced activity of nitric oxide synthase and formation of NO which reduces activation of soluble guanylate cyclase and the levels of cGMP in cerebellum of hperammonemic rats . We show now that, in hyperammonemic rats, the reduced levels of cGMP result in reduced activity of phosphodiesterase 2 (PDE2), which is activated by cGMP and degrades cAMP . The reduced activity of PDE2 thus results in increased cAMP. This is supported by the following facts: a) cAMP levels are increased in hyperammonemic rats compared to controls; b) the inhibitor of PDE2, EHNA, strongly enhances cAMP levels in control rats but not in hyperammonemic rats. This supports that EHNA may not inhibit PDE2 in hyperammonemic rats because it was already inhibited; c) addition of 8-Br-cGMP, a membrane-permeable analog of cGMP which activates PDE2, reduces cAMP levels in control and hyperammonemic rats to similar levels. This supports that the increased levels of cAMP in hyperammonemia would be due to reduced cGMP as increasing cGMP reduces cAMP to levels similar to controls; d) the role of enhanced NMDA receptor activation in the increased levels of cAMP in hyperammonemic rats is supported by the fact that blocking NMDA receptors with MK-801 reduces cAMP levels in hyperammonemic rats (similarly to increasing cGMP levels) but not in control rats. This is in agreement with previous reports showing that blocking NMDA receptors with MK-801 strongly increases cGMP in cerebellum of hyperammonemic rats, but not in control rats .