Neonatal hypofunction of NMDARs during early postnatal development
does not interfere with the expression of LTP in the MPP – DG synapse.
We also explored the synaptic potentiation of the MPP from control and
MK-801-treated slices with the same TBS protocol we delivered at the LPP
synapse. A stable baseline response of MPP fEPSPs was recorded for 20
min. This was followed by TBS and 90 min of continuous recording and
then, finally, perfusion of DCG-IV (5 µM). In the control slices, MPP
TBS triggered a PTP and a robust, long-lasting enhancement of the MPP
fEPSP slope, sensitive to DCG-IV (fEPSP at PTP: 199.8 ± 11.65% of
baseline; fEPSP 90 min post-TBS: 171.4 ± 16% of baseline; fEPSP in
presence of DCG-IV: 18.27 ± 2.24% of baseline; n = 8 slices / 7
animals; traces and black symbols in Figure 7a-b). Interestingly, MPP
TBS delivered on the MK-801-treated slices also triggered PTP, followed
by a sustained increase in the slope of the fEPSP not statistically
different from the synaptic potentiation observed in the control slices
(fEPSP at PTP: 198.9 ± 29.47% of baseline; fEPSP 90 min post-TBS: 135 ±
9.43% of baseline, Mann-Whitney test, P > 0.05 vs.
control; fEPSP in presence of DCG-IV: 32.46 ± 5.16% of baseline;n = 7 slices / 7 animals; traces and red symbols in Figure 7a-b).
Likewise, we did not find differences in 𝜏 values between the control
and MK-801-treated slices (𝜏 in control: 65 ± 3 s; 𝜏 in MK-801: 83.94 ±
16.34 s, Mann-Whitney test, P > 0.05: inset in
Figure 7b).
The average post-TBS responses at 90 min for the slices analyzed in the
control condition and MK-801-treated slices are depicted in the bar
graphs in Figure 7c. The heatmaps in Figure 7d contrast the magnitude of
the synaptic potentiation obtained in each slice, and the cumulative
probability graph in Figure 7e shows the potentiation for control slices
(black symbols) vs. MK-801-treated slices (red symbols). These results
suggest that transient hypofunction of NMDA receptors during early
postnatal development does not suppress the MPP – DG synapse’s ability
to undergo LTP in young rats.