Presynaptic nitric oxide/cGMP facilitates glutamate release via hyperpolarization-activated cyclic nucleotide-gated channels in the hippocampus

In hippocampal neurons, synaptic transmission is affected by a variety of modulators, including nitric oxide (NO), which was proposed as a retrograde messenger as long as two decades ago. NO signals via two NO‐sensitive guanylyl cyclases (NO‐GCs) (NO‐GC1 and NO‐GC2) and the subsequent increase in cGMP. Lack of long‐term potentiation in mice deficient in either one of the two NO‐GCs demonstrates the involvement of both NO‐GCs in synaptic transmission. However, the physiological consequences of NO/cGMP and the cellular mechanisms involved are unknown. Here, we analyzed glutamatergic synaptic transmission, most likely reflecting glutamate release, in the hippocampal CA1 region of NO‐GC knockout mice by single‐cell recording, and found glutamate release to be reduced under basal and stimulated conditions in the NO‐GC1 knockout mice, but restorable to wild‐type‐like levels with a cGMP analog. Conversely, an inhibitor of NO/cGMP signaling, ODQ, reduced glutamate release in wild‐type mice to knockout‐like levels; thus, we conclude that presynaptic cGMP formed by NO‐GC1 facilitates glutamate release. In this pathway, NO is supplied by endothelial NO synthase. In search of a cGMP target, we found that two mechanistically distinct blockers of hyperpolarization‐activated cyclic nucleotide‐gated (HCN) channels (ZD7288 and DK‐AH269) abolished the cGMP‐induced increase in glutamate release, suggesting that cGMP either directly or indirectly signals via HCN channels. In summary, we unravel a presynaptic role of NO/cGMP most likely in glutamate release and propose that HCN channels act as effectors for cGMP.