Nitric oxide (NO) is a candidate retrograde messenger in long-term potentiation (LTP). The NO metabolic pathway is expressed in the cerebellar granule cell layer but its physiological role remained unknown. In this paper we have investigated the role of NO in cerebellar mossy fiber-granule cell LTP, which has postsynaptic N-methyl-D-aspartate (NMDA) receptor-dependent induction. Pre- and postsynaptic current changes were simultaneously measured by using extracellular focal recordings, and NO release was monitored with an electrochemical probe in P21 rat cerebellar slices. High-frequency mossy fiber stimulation induced LTP and caused a significant NO release (6.2 ± 2.8 nM; n = 5) in the granular layer that was dependent on NMDA receptor as well as on nitric oxide synthase (NOS) activation. Preventing NO production by perfusing the NOS inhibitor 100 μM NG-nitro-L-arginine (L-NNA), blocking extracellular NO diffusion by 10 μM MbO2, or inhibiting the NO target guanylyl cyclase (sGC) with 10 μM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-dione (ODQ) prevented LTP. Moreover, the NO donor 10 μM 2-(N,N-diethylamino)-diazenolate-2-oxide·Na (DEA-NO) induced LTP, which was mutually occlusive with LTP generated by high-frequency stimulation, prevented by ODQ, and insensitive to NMDA channel blockade (50 μM APV + 25 μM 7-Cl-kyn) or interruption of mossy fiber stimulation. Thus NO is critical for LTP induction at the cerebellar mossy fiber-granule cell relay. Interestingly, LTP manipulations were accompanied by consensual changes in the presynaptic current, suggesting that NO acts as a retrograde signal-enhancing presynaptic terminal excitability.
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