In the cerebellar cortex, brief, 8 Hz activation of parallel fibres (PFs) induces a cyclic adenosine 3′5’‐monophosphate (cAMP) and protein kinase A (PKA)‐dependent form of long‐term potentiation between PFs and Purkinje cells. With 10 mm BAPTA in the recording pipette, potentiation evoked by raised frequency stimulation (RFS) to one of two, synaptically independent PF inputs to the same Purkinje cell did not remain input specific but consistently spread to synapses that did not receive RFS, up to the maximum distance tested of 168 μm. LTP at activated and non‐activated sites was accompanied by a decrease in paired pulse facilitation (PPF). The PKA inhibitor H‐89 blocked both of these effects. Inhibition of nitric oxide synthase (NOS), either by 7‐nitro‐indazole (7‐NI) or NG‐nitro‐l‐arginine methyl ester (l‐NAME), completely prevented heterosynaptic potentiation and associated reduction in PPF. LTP at distant synapses was selectively prevented by the nitric oxide scavenger 2‐(4‐carboxyphenyl)‐4,4,5,5‐tetramethylimidazoline‐1‐oxyl‐3‐oxide (cPTIO). Inhibition of soluble guanylate cyclase or protein kinase G had no effect on either pathway. Synaptic potentiation at PF‐PC synapses, induced by the adenylate cyclase activator forskolin, was also prevented by inhibition of NOS. Forskolin‐induced increases in mEPSC frequency were similarly prevented by NOS inhibition and mimicked by the NO donor spermine NONOate. These results are consistent with the notion that heterosynaptic potentiation is of pre‐synaptic origin and dependent upon activation of cAMP/PKA and NO. Moreover, they suggest that cAMP/PKA activation stimulates NO production and this diffusible messenger facilitates pre‐synaptic transmitter release at synapses within a radius of upwards of 150 μm, through a mechanism that does not involve cGMP.

cGMP is thought to play a role in cerebellar signalling yet its production within Purkinje cells has never been detected. In the present study, the hydrolysis of a fluorescent substrate analogue, 2'-O-anthranyloyl cyclic GMP, by type 5 phosphodiesterase was monitored within Purkinje cells in slices and in culture. Nitric oxide, either endogenously released from adjacent neurons or pharmacologically applied, accelerated the rate of hydrolysis in a manner that was dependent on soluble guanylyl cyclase, demonstrating that nitric oxide triggers cyclic GMP production in Purkinje cells, which in turn activates type 5 phosphodiesterase. We conclude that NO acts as an intercellular messenger in the cerebellar cortex and that parallel fibre terminals are a probable source of nitric oxide.