The transient hypofunction of NMDARs during early postnatal development differentially affects the synaptic strength of the PP – DG synapses.
It is already established that neonatal treatment with MK-801 or related antagonists of NMDA receptors such as phencyclidine alters the glutamatergic transmission of the hippocampus (Kjaerby et al., 2017; Griego et al., 2022; Márquez et al., 2023). Therefore, we explored the effects of MK-801 on the synaptic strength of the LPP and the MPP inputs to DG granule cells. In the control condition, the averaged I-O curve of the LPP fEPSP (0-800 µA; 100 µA steps, at 0.067 Hz) reached a maximal amplitude of 4.16 ± 0.46 mV (black symbols in Figure 1c; n = 9 slices / 6 animals); remarkably, neonatal treatment with MK-801 did not alter the maximal amplitude of the LPP fEPSP (maximal amplitude: 3.54 ± 0.34 mV; green symbols in Fig 1c, n = 10 slices / 6 animals). Contrary to this observation, the MPP fEPSP reached a maximal amplitude of 7.5 ± 0.68 mV (black symbols in Figure 1d; n = 8 slices / 6 animals), and neonatal treatment with MK-801 reduced the maximal MPP fEPSP amplitude to 3.87 ± 0.52 mV (P < 0.001 vs. control, two-way RM ANOVA treatment effect: F(1, 16) = 8.31; red symbols in Figure 1d; n = 10 slices / 6 animals). Our findings indicate that neonatal treatment with MK-801 selectively dysregulates the MPP inputs to DG granule cells. While the MPP exhibits a ≈50% reduction in its maximal amplitude, the LPP transmission is not affected by MK-801. Likewise, the pharmacological selectivity of MK-801 suggests a differential composition in the presynaptic and postsynaptic components mediating glutamatergic transmission of the LPP and the MPP onto DG granule cells. In addition, we performed an analysis of the fEPSP kinetics from MPP and LPP synapses; these parameters corroborate the differences in the strength of both synapses (Table 1).
Because the fEPSPs acquired for the I-O curves were preceded by presynaptic fiber volleys (FV), we also analyzed the relationships between stimulus intensity, FV amplitude, and fEPSP amplitude. The left panel in Figure 1e shows the relationship between stimulus intensity (I) and LPP FV amplitude. The right panel depicts the relationship between FV amplitude and LPP fEPSP slope in both experimental conditions (black and green symbols). We found that neonatal treatment with MK-801 did not alter either relationship at the LPP – DG synapse. However, the same analysis at the MPP synapse uncovered a different scenario. The left panel in Figure 1f contrasts the FV response in control vs. MK-801-treated animals (black and red symbols). Compared to the control slices, the MK-801-treated slices exhibited a significant decrease in FV amplitude and faster response saturation [two-way RM ANOVA, interaction effect (treatment x current intensity): F(8, 88) = 2.57, P < 0.01; red symbols; left panel in Figure 1f]. Likewise, the coupling analysis of the FV vs. MPP fEPSP slope shows a dysregulation between FV and MPP fEPSP (slope in control: 0.87 ± 0.24 mV/ms; in MK-801: 0.16 ± 0.04 mV/ms; Mann-Whitney test,P < 0.01; right panel in Figure 1f). These results suggest that neonatal treatment with MK-801 alters the propagation of presynaptic action potentials and the concomitant synaptic response in the MPP but not the LPP synapse.