The transient hypofunction of NMDARs during early postnatal
development alters the functional expression of CB1R in
the MPP – DG synapse.
Given CB1R’s critical role in the induction of LTD, we
next examined the functional expression of CB1R in the
MPP – DG synapse. We hypothesized that pharmacological activation of
CB1R would fail to induce synaptic depression in the
MK-801-treated slices. To test this prediction, we acquired a stable
baseline of MPP fEPSPs for 20 min prior to perfusing WIN 55,212-2, a
CB1R agonist (WIN, 5 µM; 15 min). Recording continued up
to 90 min, followed by perfusion of DCG-IV. In the control condition, we
found that WIN triggers a stable synaptic depression in the MPP – DG
synapse, sensitive to DCG-IV (fEPSP
90 min post-WIN: 43.01 ± 6.53% of baseline, n = 6 slices / 5
animals; fEPSP in the presence of DCG-IV: 11.23 ± 4.89% of baseline;
traces and black bars in Figure 4a-b, e). By contrast, perfusion of WIN
in the MK-801-treated slices decreased the magnitude of the synaptic
depression of the MPP fEPSP (fEPSP 90 min post-WIN: 74.11 ± 12.16% of
baseline, n = 6 slices / 6 animals, Student’s t-test:
t(10) = 2.25, P < 0.05 vs. control;
fEPSP in the presence of DCG-IV: 28.89 ± 4.61% of baseline; traces and
red bars in Figure 4a-b, e), suggesting functional dysregulation of
presynaptically expressed CB1R.
Because presynaptic activation of CB1R requires
postsynaptic synthesis and release of 2-AG, we next investigated if
neonatal treatment with MK-801 interferes with the postsynaptic
synthesis and release of 2-AG. We stimulated the local production of
2-AG by perfusing the cholinesterase inhibitor physostigmine (10 µM) and
recording the MPP fEPSPs. After a stable baseline was acquired for 20
min, physostigmine was perfused (10 µM for 15 min). Recording continued
up to 90 min and was followed by the perfusion of DCG-IV. In the control
condition, physostigmine perfusion induced synaptic depression,
sensitive to DCG-IV (fEPSP 90 min
post-physostigmine: 52.45 ± 7.06% of baseline, n = 6 slices / 6
animals; fEPSP in the presence of DCG-IV: 9.38 ± 1.2% of baseline;
traces and black bars in Figure 4c-d, f). Strikingly, in the
MK-801-treated slices, physostigmine induced a seemingly diminished
synaptic depression, sensitive to DCG-IV (fEPSP 90 min
post-physostigmine: 77.65 ± 9.65% of baseline, n = 5 slices / 5
animals, Student’s t-test:
t(9) = 2.15; P > 0.05 vs. control;
fEPSP in the presence of DCG-IV:
30.27 ± 1.97% of baseline; traces and red bars in Figure 4c-d, f). The
magnitude of the synaptic depression observed in both WIN and
physostigmine in the MK-801-treated slices was similar (Figure 4g),
suggesting that 2-AG production in granule cells is not affected by
transient hypofunction of NMDARs during early postnatal development.
Likewise, the reduced synaptic depression observed in the MK-801 in the
presence of physostigmine may be ascribed to dysregulated
CB1R functionality. This possibility requires further
exploration.