No abstract

Nitric oxide (NO) is a messenger molecule that is produced in the brain from the metabolism of L-arginine to L-citruline. Growing evidence suggests a physiological role for NO in long-term potentiation (LTP). Since LTP is a form of synaptic plasticity thought to be involved in learning and memory, we have tested whether inhibition of endogenous NO production affects memory capacities ofrats. We found that the NO synthase [L-arginine, NADPH:oxygen oxidoreductase (nitric oxide-forming), EC 1.14.13.39] inhibitor N01-nitro-Larginine, at doses blocking LTP in hippocampal slices, impairs spatial learning in a radial arm maze and olfactory memory in a social recognition test. In contrast, N"-nitro-L-arginine left shock-avoidance learning unaffected. These results indicate that NO is involved in some but not all forms of memory and further support the existence of a causal link between LTP and spatial learning.Nitric oxide (NO) is a diffusible molecule endowed with intercellular messenger properties in several biological systems including the brain (1, 2). NO mediates the stimulation of soluble guanylate cyclase upon activation of N-methyl-Daspartate (NMDA) receptors (3) and serves as its own negative feedback effector by blocking NMDA-evoked responses (4, 5). This messenger is produced from the enzymatic conversion of L-arginine to L-citrulline by a constitutive NO synthase [NOS; L-arginine, NADPH:oxygen oxidoreductase (nitric oxide-forming), EC 1.14.13.39] which can readily be blocked by arginine analogs, such as Nl-nitro-Larginine [Arg(NO2); also called NG-nitro-L-arginine, where G refers to the guanidino-carbon] (6, 7).Long-term potentiation (LTP) is a persistent increase, which can last for days or weeks, in the synaptic efficacy of pathways produced by brief periods of high-frequency stimulations (HFS) (8). This phenomenon, best characterized in the hippocampus, is thought to be a cellular event involved in the acquisition, storage, or retrieval of information in the brain (9-12). We and others reported recently that NOS inhibitors and NO scavengers block hippocampal LTP in rat brain slices (13)(14)(15)(16) to irreversibly block brain NOS enzymatic activity (7). Separate groups of animals were used for each experiment. Vehicle-treated animals were used as controls.Electrophysiology. Sixteen hours after the last injection, transverse hippocampal slices (0.5-mm thick) were prepared from Sprague-Dawley rats (150-200 g) pretreated with Arg(NO2) (25-100 mg/kg of body weight i.p.) or vehicle. Slices were maintained in a submersion-type recording chamber under superfusion (2.5-3 ml/min) with gassed (95% 02/5% C02) medium containing 124 mM NaCl, 5 mM KCl, 2 mM MgSO4, 2 mM CaCl2, 26 mM NaHCO3, 1.25 mM KH2PO4, and 10 mM glucose at 32°C as described (18). Stimulation and recording electrodes were positioned in the CAl-stratum radiatum, and field excitatory postsynaptic potentials (EPSPs) were evoked every 5 s. The stimulus strength (0.1-ms duration at 2-20 V) was adjusted to evoke EPSPs of at least 0.3-mV amplitude w...

Nitric oxide (NO) functions in several types of synaptic plasticity, including hippocampal long-term potentiation (LTP), in which it may serve as a retrograde messenger after postsynaptic NMDA receptor activation. In accordance with a prediction of this hypothesis, and with previous findings using guinea pig tissue, exogenous NO, when paired with a short tetanus (ST) to afferent fibers, generated a stable NMDA receptor-independent potentiation of rat CA1 hippocampal synaptic transmission that occluded LTP. Contrary to predictions, however, the pairing-induced potentiation was abolished in the presence of NO synthase inhibitors, indicating that endogenous NO is required for exogenous NO to facilitate LTP. Periodic application of NO while endogenous NO synthesis was blocked indicated that a tonic low level is necessary on both sides of the NO-ST pairing for the plasticity to occur. A similar dependence on tonic NO seems to extend to LTP, because application of an NO synthase inhibitor 5 min after tetanic stimulation blocked LTP as effectively as adding it beforehand. The posttetanus time window during which NO operated was restricted to <15 min. Inhibition of the guanylyl cyclase-coupled NO receptor indicated that the potentiation resulting from NO-ST pairing and the NO signal transduction pathway during early LTP are both through cGMP. We conclude that NO does not function simply as an acute signaling molecule in LTP induction but has an equally important role outside this phase. The results resonate with observations concerning the role of the hippocampal NO-cGMP pathway in certain types of learning behavior.

Nitric oxide (NO) is a putative participant in synaptic plasticity and demonstrations that exogenous NO can elicit the same plastic changes have been taken to support such a role. The experiments, carried out on the CA1 region of rat hippocampal slices, were aimed at testing this interpretation. A major component of tetanus-induced long-term potentiation (LTP) was lost in response to L-nitroarginine, which inhibits NO synthase, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), which inhibits NO-sensitive soluble guanylyl cyclase (sGC). At 0.2 Hz afferent fibre stimulation, exogenous NO produced, concentration-dependently, a synaptic depression that reverted on washout to a persistent potentiation that occluded tetanus-induced LTP. The NO concentrations necessary (estimated in the 100-nM range), however, were mostly supramaximal for stimulating hippocampal slice sGC activity. Nevertheless the potentiation, but not the preceding depression, was blocked by ODQ. L-nitroarginine and an NMDA antagonist were similarly effective, indicating mediation by the endogenous NMDA receptor-NO synthase-sGC pathway. At a concentration normally too low to affect synaptic transmission but sufficient to stimulate sGC (estimated to be 50 nM), exogenous NO reversed the effect of L-nitroarginine and caused a potentiation which was blocked by ODQ. At a concentration inducing the depression/potentiation sequence, NO partially inhibited hippocampal slice oxygen consumption. It is concluded that, at physiological levels, exogenous NO can directly elicit a potentiation of synaptic transmission through sGC, provided that the synapses are suitably primed. At higher concentrations, NO inhibits mitochondrial respiration, which can result in an enduring synaptic potentiation due to secondary activation of the endogenous NO-cGMP pathway.

Nitric oxide production in the cerebellum and induction of long-term potentiation (LTP) in the hippocampus have some characteristics in common: both phenomena are induced by activation of N-methyl-d-aspartate receptors and both are highly dependent on calcium-mediated processes. Here we provide evidence that endogenous nitric oxide production is necessary for synaptic plasticity in the CA1 hippocampus of the rat. LTP recorded in slices was blocked in a concentration-dependent manner by the nitric oxide synthase inhibitors l-NG-nitroarginine and l-NG-nitroarginine methyl ester, but l-NG-monomethylarginine was only marginally active. Bathing the slices with haemoglobin, a protein that scavenges nitric oxide, also resulted in a concentration-dependent blockade of LTP. Nitric oxide released locally from hydroxylamine produced a stable potentiation of synaptic transmission that was not additive with LTP induced by high-frequency stimulation. These results are fully consistent with the presumed retrograde messenger role of nitric oxide in LTP.