On the role of nitric oxide in hippocampal long-term potentiation

Abstract

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.

Fig. 1.

Potentiation of CA1 hippocampal synaptic transmission by pairing exogenous NO with an ST.A, Lack of effect of DEA/NO (5 min perfusion) on baseline synaptic transmission (0.033 Hz stimulation). In this and subsequent figures, the administration of DEA/NO is indicated by ahorizontal bar, the progressive thinning of which reflects the exponential decline in DEA/NO concentration (half-life, 6 min). Thirty minutes later, the slices were given HFS (atarrow; n = 5). B, Facilitation of LTP by combined ST and DEA/NO. In each slice, an ST was delivered to the presynaptic fibers, and, 30 min later, a second ST was given in the absence or presence of DEA/NO, as indicated (n = 5–6). HFS was subsequently applied (atarrow) to all slices. C, Lack of potentiation in response to DEA/NO delivered 5 min after the ST (n = 4). The 40–50 sec delay in the perfusion system has not been corrected for in this and subsequent figures. Theinsets show representative fEPSPs (average of 4 consecutive traces) recorded in the presence or absence of DEA/NO at the times indicated by the letters.

Fig. 2.

Facilitation of LTP by exogenous NO is independent of NMDA receptors. A, Positive control showing that the NMDA antagonist d-AP-5 inhibits the short-term potentiation induced by the ST (n = 3). B, Similar administration of d-AP-5 did not affect the potentiation of synaptic transmission brought about by giving an ST in the presence of DEA/NO (n = 4). Controls (−d-AP-5) were taken from Figure 1B. The insets show representative fEPSPs in the presence ofd-AP-5 (average of 4 consecutive traces) recorded in the presence of d-AP-5 at the times indicated by theletters.

Fig. 3.

Effect of NO synthase inhibitors on LTP facilitation by exogenous NO. A, Block of facilitation by l-NOArg (n = 5). B, Block by l-NIO (n = 6). Theinsets show representative fEPSPs (average of 4 consecutive traces) recorded in the presence of l-NOArg orl-NIO at the times indicated by theletters.

Fig. 4.

Excess l-arginine (L-Arg) reversesl-NOArg-induced inhibition of LTP facilitation by NO. Slices were subjected to ST–DEA/NO pairing in the presence ofl-NOArg, with (●; n = 4) or without (○; n = 6) l-arginine. Theinsets show representative fEPSPs (average of 4 consecutive traces) recorded in the absence (left) or presence (right) of l-arginine at the times indicated by the letters.

Fig. 5.

Timing of the participation of endogenous NO in the facilitation of LTP by exogenous NO. In all cases, the NO synthase inhibitor l-NIO was present (as indicated) during the pairing of DEA/NO (applied at 3 μm) and an ST, to inhibit the potentiation that would normally result; a lower DEA/NO concentration (initially 0.3 μm) was added (as indicated) in an attempt to overcome the effect of NO synthase inhibition.A, The low DEA/NO concentration was added together with the l-NIO 15 min before pairing (n = 4). B, Two fresh solutions of DEA/NO at the low concentration were superfused successively after washout of the higher concentration used in the pairing protocol (n = 4).C, Combined administration of the low DEA/NO concentration before and after the pairing protocol (n = 4). The insets show representative fEPSPs (average of 4 consecutive traces) recorded at the time indicated by the letters.

Fig. 6.

Role of NO synthase in LTP. A,l-NIO applied from 15 min before HFS until the end of the experiment resulted in a gradual loss of LTP compared with interleaved controls (n = 6). B,l-NIO superfused for 30 min starting 5 min after HFS also inhibited late-phase LTP (n = 5; 4 interleaved with controls). C, When the l-NIO application was delayed until 15 min after HFS, LTP was sustained (n = 4–5). The insets show representative fEPSPs (average of 4 consecutive traces) froml-NIO-treated slices recorded at the times indicated by theletters.

Fig. 7.

Involvement of the guanylyl cyclase-coupled NO receptor in NO-induced LTP facilitation and LTP. A, Pairing of DEA/NO with ST fails to elicit a potentiation in the presence of ODQ (n = 4). B, Late delivery of ODQ (5 min after HFS) inhibits late-phase LTP (interleaved experiment; n = 5). The insets show representative fEPSPs (average of 4 successive traces) from ODQ-treated slices.