Endogenous neurotrophin-3 regulates short-term plasticity at lateral perforant path-granule cell synapses

Abstract

In the adult brain, neurotrophin-3 (NT-3) is mainly localized in dentate granule cells, and its expression is decreased by various stimuli, e.g., seizure activity. We have examined the role of endogenous NT-3 for excitatory synaptic transmission at lateral perforant path-dentate granule cell synapses using hippocampal slices from NT-3 knock-out (+/-) and wild-type (+/+) mice. Paired-pulse facilitation (PPF) and also short-term synaptic plasticity induced by a brief, high-frequency train of afferent stimulation were reduced, but the expression of long-term potentiation was not affected in the NT-3+/- mice. Incubation of the slices with recombinant NT-3 reversed the deficit in PPF through a mechanism requiring de novo protein synthesis, implying that the impaired short-term plasticity does not result from a developmental alteration. No changes of overall presynaptic release probability, measured by the progressive block of NMDA receptor-mediated synaptic currents by MK-801, or desensitization of AMPA receptors were detected. Because NT-3 expression is reduced after focal seizures, impaired short-term facilitation may represent a protective response that limits the propagation of epileptiform activity from the entorhinal cortex to the hippocampus.

Fig. 1.

Basal properties of LPP–dentate granule cell synapses are not altered in NT-3+/− mice. A, Relation between the amplitude of the presynaptic fiber volley (PSFV) and the initial slope of field EPSP. Values are taken from individual experiments and are indicated byopen circles (for NT-3+/−) and filled triangles (for NT-3+/+). B, Averaged EPSCs (10 traces) recorded in a dentate granule cell from a NT-3+/+ (top) or NT-3+/− (bottom) mouse at −70 mV before (trace 1) and after (trace 2) application of 5 μm NBQX.C, Averaged EPSCs (10 traces) recorded in the same dentate granule cells at +40 mV with NBQX (5 μm) in the perfusion solution before (trace 3) and after (trace 4) application of 80 μmMK-801.

Fig. 2.

PPF and synaptic responses induced by short high-frequency afferent stimulation are reduced in LPP–dentate granule cell synapses of NT-3+/− mice. A, Averaged EPSCs (10 traces) recorded in a dentate granule cell from an NT-3+/+ and an NT-3+/− mouse, respectively, in response to two stimuli delivered with an ISI of 50 msec. B, Averaged PPF of EPSCs (±SEM) in dentate granule cells from NT-3+/+ (n = 6 cells) and NT-3+/− (n = 11 cells) mice at 50 msec ISI.C, Averaged PPF (±SEM) at different ISIs, as measured using field recordings, in NT-3+/+ (n = 5) and NT-3+/− (n = 8) mice, respectively.D, Averaged EPSCs (10 traces) in dentate granule cells from NT-3+/+ and NT-3+/− mice during short high-frequency stimulation of LPP afferents (10 pulses, 40 Hz). E, Averaged synaptic responses of dentate granule cells from NT-3+/+ (n = 7 cells) and NT-3+/− (n = 10 cells) mice during HFS (10 pulses, 40 Hz) of LPP afferents. The synaptic response was calculated as the integral of the total evoked synaptic currents (0–300 msec) normalized to that of the first evoked synaptic current (0–25 msec).

Fig. 3.

PPF is not altered in Schaffer collateral–CA1 and mossy fiber–CA3 synapses of NT-3+/− mice. A, Averaged PPF (±SEM) of EPSPs, as assessed using field recordings, in Schaffer collateral–CA1 (n = 6 and n = 5 for NT-3+/+ and NT-3+/−, respectively) and (B) mossy fiber–CA3 (n = 6 for both NT-3+/+ and NT-3+/−) synapses of NT-3+/+ and NT-3+/− mice.

Fig. 4.

The impaired PPF in NT-3+/− mice can be rescued by application of recombinant NT-3 with a mechanism requiring protein synthesis. A, Averaged PPF (±SEM) measured using field recordings at different ISIs in slices from NT-3+/− mice incubated with either recombinant NT-3 or cytochrome C450 (Cyt.C) (n = 9 for each group). B, Same experimental paradigm as in A, but CHM was added to the incubation solution 30 min before application of NT-3. In addition, a separate set of slices was exposed to CHM alone (n= 6 for each group).

Fig. 5.

Overall release probability is not altered in NT-3+/− mice. A1, Averaged (5 responses) NMDA receptor-mediated responses recorded in the absence (trace 1) and in the presence of MK-801 (80 μm) (trace 2, average of responses 1–5; trace 3, average of responses 21–25). The dentate granule cell was clamped at +50 mV with 5 μm NBQX added to the perfusion medium. A2, The same traces as in A1normalized to the peak value of trace 1, showing a faster decay of the evoked EPSCs in the presence of MK-801. As can be seen, the accelerated decay of the evoked EPSCs is similar intraces 2 and 3, indicating that blockade of NMDA receptors by MK-801 was equally efficient throughout the whole experiment. A3, Mean amplitude of successive NMDA receptor-mediated EPSCs (same cell as in A₁, average of 5 responses) recorded in the presence of MK-801, normalized to the first response (○). The line represents a first degree exponential curve fit to the amplitude decay. B, Average of the mean amplitude decay of the successive NMDA receptor-mediated EPSCs recorded in the presence of MK-801 (normalized to the first response). The decay rate in cells from NT-3+/+ mice (▴;n = 8 cells) was not different from that of NT-3+/− mice (○; n = 9 cells).

Fig. 6.

NT-3+/− mice can express LTP in LPP–dentate granule cell synapses. Averaged EPSP slopes from NT-3+/+ (▴; n = 12) and NT-3+/− (○;n = 9) mice, respectively, plotted against time. Att = 0, the LPP afferents were tetanized (2 trains, 10 impulses, 100 Hz, 20 sec apart). The averaged (±SEM) changes of EPSP slope are plotted in percentage of the mean EPSP slope value obtained during 10 min before the stimulation. Insetsdemonstrate EPSP traces (average of 20) before and after (at 50 min) the tetanus in representative slices from NT-3+/+ and NT-3+/− mice.