Suppression of adenosine responses by nitric oxide in hippocampal slices A. Shahraki and T.W. Stone. Institute of Biomedical & Life Sciences, West Medical Building, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K.	Print Abstract Search PubMed for: Shahraki A Stone TW

Nitric oxide (NO) is thought to play a role in long-term potentiation (LTP), and can be generated in central neurons in response to activation of glutamate receptors. It has been reported that activation of NMDA receptors can suppress neuronal responses to adenosine (Nikbakht and Stone, 2001). We have examined whether NO is able to modify neuronal responses to adenosine and mediate the actions of NMDA.

Hippocampal slices from male Wistar rats (100-150g) were maintained in artificial cerebrospinal fluid (ACSF) gassed with 95% O2 and 5% CO2 and containing (in mM): KH₂PO₄ 2.2, KCl 2, NaHCO₃ 25, NaCl 115, CaCl₂ 2.5, MgSO₄ 1.2, glucose 10. Stimuli were delivered to the Schaffer collaterals in the stratum radiatum, and evoked field excitatory postsynaptic potentials (fEPSPs) were recorded extracellularly from the CA1 region. Paired stimuli were delivered through the same electrode at 10, 20, and 50 ms intervals. Each experiment was performed on 5-6 slices. Data are presented as means ± S.E.M and were analysed for statistical significance with repeated measures ANOVA followed by the Student-Newman-Keuls test. P values <0.05 were considered significant.

Adenosine 10µM depressed single fEPSPs by 54.46% ± 5.6 (n = 4) of the control size. The superfusion of S-nitroso-N-acetylpenicillamine (SNAP) at 100µM induced a long-lasting potentiation (LTP) of fEPSP slope (141% * 10.98, n = 4) compared with baseline. During applications of SNAP two adenosine responses 10 mins apart were significantly smaller than before SNAP, (30.5% ± 3.29 and 20.6% ± 2.02, n = 4, respectively).

Superfusion of xanthine (100µM) / xanthine oxidase (0.02U/ml) (X/XO) induced LTP (126.2% ± 6.17, n = 5) and significantly suppressed the two responses to adenosine (32.6% ± 3.02, P<0.05; 28.5% ± 4.15, P<0.001, Student-Newman-Keuls), compared with controls (48.7% ± 4.12). SNAP and X/XO were able to reduce significantly the effects of adenosine on paired-pulse phenomena.

The guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) 10µM prevented the inhibitory effects of SNAP on adenosine responses and its induction of LTP, suggesting that the action of NO is mediated through cyclic GMP. Application of superoxide dismutase (SOD) 120u.ml^-1could prevent the suppression of inhibitory effects of adenosine by NMDA 4µM while ODQ could not.

In conclusion, the action of NO is mediated through cyclic GMP and might be involved in presynaptic depression in the CA1 region of hippocampus. Blockade of SNAP and X/XO effects by superoxide dismutase implies that oxygen free radicals are involved, probably due to formation of peroxynitrite. The effects of ODQ and SOD on the interaction between adenosine and NMDA indicate that cGMP does not mediate this interaction but oxygen free radicals might contribute.

Nikbakht, M.R. and Stone, T.W. (2001). Europ. J. Pharmacol. 427, 13-25.