Figure 4.

Increasing Hippocampal PV Neuron Excitability Creates a Vulnerable Network State that Is Sensitive to Ab

(A) PV-Cre mice were injected with hM3Dq virus at 8–10 weeks of age. After 4 weeks, CNO (1 mg/kg) or saline were i.p. injected daily for a period of 3 weeks. Bilateral infusion of 0.15 fmole of Ab or PBS control in the CA1 region of the hippocampus of PV-Cre mice was completed 6.5 weeks after discontinuation of CNO injections. An MWM test was performed 1.5 week after Ab infusions. Electrophysiological recordings were performed following the MWM test.

(B) Spatial learning was assessed measuring the latency to find the hidden platform on four consecutive training days (T1–4). Both Ab-infused and PBS-infused PV-Cre mice that expressed hM3Dq and had received CNO injections showed significant learning during training (training two-way repeated measures ANOVA: n = 17/13 mice per group, F_3,72 = 47.65, p < 0.000).

(C) During the one-minute probe trial, Ab-infused PV-Cre mice spent significantly less time in the target quadrant (TQ) compared with PBS-infused PV-Cre mice (two-way ANOVA: n = 17/13 mice per group, F_3,112 = 48.98, p = 0.000, post-hoc LSD test: ∗∗∗p = 0.000). Compared with chance level (dashed line), PBS-infused PV-Cre mice spent significantly more time in the target quadrant, whereas Ab-infused PV-Cre did not (Student's t test: p < 0.001 for CNO and p = 0.059 for CNO + Ab).

(D) PV interneuron resting membrane potential was significantly increased in Ab-infused PV-Cre mice compared with PBS-infused PV-Cre mice (Student's t test: n = 16/15 cells from 4 mice per group, ∗∗p = 0.004).

(E) There was no difference in PV interneuron input resistance between Ab-infused and PBS-infused PV-Cre mice that expressed hM3Dq and had received CNO injections (Student's t test: n = 16/15 cells from 4 mice per group, p = 0.740).

(F) Voltage responses to 1 s hyperpolarizing or depolarizing current steps from a PV interneuron in PBS-infused (blue) and Ab-infused (red) PV-Cre mice that expressed hM3Dq and had received CNO injections.

(G) Average action potential (AP) frequency in response to 0–250 pA depolarizing current steps illustrating a significant decrease in PV interneuron firing frequency in Ab-infused compared with PBS-infused PV-Cre mice expressing hM3Dq and had received CNO injections (group x current two-way repeated measures ANOVA: n = 16/15 cells from 4 mice per group, F_1,30 = 5.76, ∗p = 0.023).

(H) Pyramidal neuron resting membrane potential was unaltered in Ab-infused compared with PBS-infused PV-Cre mice expressing hM3Dq and had received CNO injections (Student's t test: n = 14/15 cells from 4 mice per group, p = 0.300).

(I) Pyramidal neuron input resistance was not changed in Ab-infused compared with PBS-infused PV-Cre mice that expressed hM3Dq and had received CNO injections (Student's t test: n = 14/15 cells from 4 mice per group, p = 0.070).

(J) Voltage responses to 1 s hyperpolarizing or depolarizing current steps from a pyramidal neuron in PBS-infused (blue) and Ab-infused (red) PV-Cre mice that expressed hM3Dq and had received CNO injections.

(K) AP frequency in response to 0–250 pA depolarizing current steps illustrating a significant increase in pyramidal neuron excitability in Ab-infused compared with PBS-infused PV-Cre mice that expressed hM3Dq and had received CNO injections starting at 150 pA (group x current two-way repeated measures ANOVA: n = 14/15 cells from 4 mice per group, F_1,30 = 1.25, ∗p = 0.02).

(L) Example traces of spontaneous inhibitory postsynaptic currents (sIPSC) recorded from hippocampal pyramidal neurons in PBS-infused (blue) or Ab-infused (red) PV-Cre mice that expressed hM3Dq and had received CNO injections.

(M and N) No significant alterations were observed in the frequency (M) or in the amplitude (N) of sIPSCs in Ab-infused compared with PBS-infused PV-Cre mice that expressed hM3Dq and had received CNO injections (Mann-Whitney test: n = 14/16 cells from 5 mice per group, p = 0.983 and p = 0.822).

(O) Example traces of spontaneous excitatory postsynaptic currents (sEPSC) recorded from hippocampal pyramidal neurons in PBS-infused (blue) or Ab-infused (red) PV-Cre mice that expressed hM3Dq and had received CNO injections.

(P) Decreased sEPSC frequency in Ab-infused compared with PBS-infused PV-Cre mice that expressed hM3Dq and had received CNO injections (Mann-Whitney test: n = 18/13 cells from 5 mice per group, ∗∗∗p = 0.000).

(Q) No alterations were observed in the amplitudes of sEPSCs (Mann-Whitney test: n = 18/13 cells from 5 mice per group, p = 0.370). Values are mean ± SEM.