An inhibitor of nitric oxide synthesis prevents memory formation in the chick.

Memory formation is presumed to require retrograde communication across synaptic junctions. Nitric oxide (NO) is a putative retrograde messenger at N-methyl-D-aspartate (NMDA)-mediated synapses [8, 9]. Inhibitors of nitric oxide synthesis block initiation of long-term potentiation [2, 3, 19]. Memory for a one-trial passive avoidance task in the young chick involves an NMDA-linked intracellular cascade culminating in lasting modulation of synaptic morphology and [6, 18]. Here we show that injection of the nitric oxide synthase inhibitor N-nitro-L-arginine prior to training results in amnesia for the passive avoidance task; the amnesia can be overcome by injecting L-arginine along with the inhibitor. Thus we have verified for the first time experimentally that NO plays a role in memory formation.