Zaprinast stimulates extracellular adenosine accumulation in rat pontine slices

Regulation of long-term depression by increases in [guanosine 3',5'-cyclic monophosphate] in the hippocampal CA1 region of freely behaving rats

A role for guanosine 3',5'-cyclic monophosphate (cGMP) and the protein kinase G (PKG) pathway in synaptic long-term depression (LTD) in the hippocampal CA1 region has been proposed, based on observations in vitro, where, for example, increases of [cGMP] result in short-term depression (STD) coupled with a reduction in presynaptic glutamate release. To date, no evidence exists to support that LTD in the intact, freely behaving animal involves these mechanisms. We examined the effect of increases of [cGMP] on basal transmission and electrically-induced STD at hippocampal CA1 synapses in vivo. We found that elevating [cGMP] dose-dependently caused a chemically-induced STD which occluded electrically-induced STD. Repeated administration of Zaprinast, an inhibitor of cGMP-degrading phosphodiesterase, resulted in persistent LTD (>24 h). Paired-pulse analysis supported a presynaptic mechanism of action. Application of an inhibitor of soluble guanylate cyclase prevented LTD induced by low-frequency stimulation (LFS), and impaired LFS-STD elicited in the presence of Zaprinast. These data suggest the involvement of cGMP in LTD in the CA1 region of freely behaving adult rats.

Nitric oxide-induced adenosine inhibition of hippocampal synaptic transmission depends on adenosine kinase inhibition and is cyclic GMP independent

Adenosine is an important inhibitory neuromodulator that regulates neuronal excitability. Several studies have shown that nitric oxide induces release of adenosine. Here we investigated the mechanism of this release. We studied the effects of nitric oxide on evoked field excitatory postsynaptic potentials (fEPSPs) recorded in the CA1 area of rat hippocampal slices. The nitric oxide donor 1,1-diethyl-2-hydroxy-2-nitroso-hydrazine sodium (DEA/NO; 100 microm) depressed the fEPSP by 77.6 +/- 4.1%. This effect was abolished by the adenosine A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 400 nm), indicating that the nitric oxide effect was mediated by adenosine accumulation. The DEA/NO effect was unaltered by the 5'-ectonucleotidase inhibitor alpha,beta-methylene-adenosine 5'-diphosphate (AMP-CP; 100 microm), indicating that extracellular adenosine did not derive from ATP or cAMP release. The guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazole[4,3-a]quinoxaline-1-one (ODQ; 5 microm) did not affect nitric oxide depression of the fEPSPs, indicating that nitric oxide-mediated adenosine release was not mediated through a cGMP signaling cascade. This conclusion was confirmed by the observation that 8-(4-chlorophenylthio)-guanosine-3',5'-cyclic monophosphate (8-pCPT-cGMP; 1 mm) reversibly depressed the fEPSP by 24.9 +/- 4.5%, but this effect was not blocked by adenosine antagonists. Adenosine kinase inhibitor 5-iodotubercidin (ITU; 7 microm) occluded the nitric oxide effects by 74%, suggesting that inhibition of adenosine kinase activity contributes to adenosine release. In conclusion, exogenous nitric oxide evokes adenosine release by a cGMP-independent pathway. Intracellular cGMP elevation partially inhibits the fEPSP but not through adenosine release. Although a direct block of adenosine kinase by nitric oxide can not be excluded, the depression of adenosine kinase activity may be due to inhibition by its own substrate adenosine.