Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, C109
Activity-dependent long-term potentiation produced by exogenous nitric oxide during NMDA receptor blockade in hippocampal slices.
B. M. Pigott1, J. Garthwaite1
1. The Wolfson Institute for Biomedical Research, University College London, London, United Kingdom.
Nitric oxide (NO) is a freely diffusible intercellular signalling molecule in most brain areas and has been widely implicated in synaptic plasticity and other phenomena. NO generation is linked to NMDA receptor activation and physiological NO signal transduction is achieved through guanylyl cyclase activation and cGMP accumulation (Garthwaite, 2008). In the hippocampus, NO has long been implicated as a retrograde messenger in NMDA receptor-dependent long-term potentiation (LTP), but its precise role remains unsettled. If NO were acting purely presynaptically and if presynaptic changes contribute to LTP, it is predicted that exogenous NO should partially overcome the inhibitory effect of NMDA receptor blockade on LTP. To test this hypothesis, NMDA receptor-dependent LTP was studied at Schaffer collateral-CA1 synapses using field potential recording in hippocampal slices of male mice aged 6-8 weeks. LTP was induced using a 1-s, 100-Hz tetanus. Values of potentiation given below refer to mean field EPSP slopes 55-60 minutes after the tetanus, normalised to baseline responses (100 %) ± SEM. As expected, the NMDA antagonist, D-AP5 (50 µM), reversibly blocked LTP (110 ± 6 % following tetanus during NMDA blockade; 169 ± 14 % following a second tetanus after D-AP5 washout; n = 5). When present during tetanic stimulation in the presence of D-AP5, the NO donor PAPA/NONOate produced a long-lasting potentiation (152 ± 9 %; n = 5), as predicted by the retrograde messenger hypothesis. This NO-induced potentiation was, however, slow to develop (20 min to plateau) and, unexpectedly, it did not occlude subsequent LTP, which was, instead, substantially enhanced (266 ± 9 %; n = 5). Both the NO-induced potentiation and the subsequent enhancement of LTP were dependent on exogenous NO concentration (maximal at 3 µM PAPA/NONOate) and on there being a coincident tetanic stimulation (115 ± 4 % during NMDA blockade with exogenous NO but no tetanus; 190 ± 11 % following a subsequent tetanus after washout of D-AP5 and PAPA/NONOate; n = 5). Both effects were also blocked by the guanylyl cyclase antagonist, ODQ (10 µM; 114 ± 7 % following tetanus during NMDA blockade and with exogenous NO; 159 ± 10 % following subsequent tetanus; n = 5), implying that they both involve the generation of cGMP and so could be mechanistically linked. It is concluded that the apparent rescue of LTP by exogenous NO paired with a tetanus when NMDA receptors are blocked cannot simply be explained by the NO compensating for a missing component of normal LTP. Instead, the results show that exogenous NO, in an activity-dependent manner, evokes a long-term enhancement of synaptic transmission of similar amplitude to that normally produced during LTP, but that is additive to normal LTP.
Where applicable, experiments conform with Society ethical requirements