Parvalbumin cell ablation of NMDA-R1 causes increased resting network excitability with associated social and self-care deficits

Abstract

NMDA-receptor (NMDAR) hypofunction is strongly implicated in the pathophysiology of schizophrenia. Several convergent lines of evidence suggest that net excitation propagated by impaired NMDAR signaling on GABAergic interneurons may be of particular interest in mediating several aspects of schizophrenia. However, it is unclear which behavioral domains are governed by a net increase of excitation and whether modulating downstream GABAergic signaling can reverse neural and thus behavioral deficits. The current study determines the selective contributions of NMDAR dysfunction on PV-containing interneurons to electrophysiological, cognitive, and negative-symptom-related behavioral phenotypes of schizophrenia using mice with a PVcre-NR1flox-driven ablation of NR1 on PV-containing interneurons. In addition, we assessed the efficacy of one agent that directly modulates GABAergic signaling (baclofen) and one agent that indirectly modifies NMDAR-mediated signaling through antagonism of mGluR5 receptors (2-methyl-6-(phenylethynyl) pyridine (MPEP)). The data indicate that loss of NMDAR function on PV interneurons impairs self-care and sociability while increasing N1 latency and baseline gamma power, and reducing induction and maintenance of long-term potentiation. Baclofen normalized baseline gamma power without corresponding effects on behavior. MPEP further increased N1 latency and reduced social behavior in PVcre/NR1+/+ mice. These two indices were negatively correlated before and following MPEP such that as N1 latency increases, sociability decreases. This finding suggests a predictive role for N1 latency with respect to social function. Although previous data suggest that MPEP may be beneficial for core features of autism spectrum disorders, current data suggest that such effects require intact function of NMDAR on PV interneurons.

Figure 1

Ablation of NMDARs on PV interneurons impairs self-care behavior and sociability yet enhances performance on cognitive tasks without affecting anxiety or locomotion. (a) PVcre/NR1fl/fl mice had a significantly lower score on the nest-building test than PVcre/NR1+/+ mice (t=2.117, df=22, p<0.04). (b) PVcre/NR1fl/fl mice had significantly lower social index scores than PVcre/NR1+/+ mice (t=3.44, df=19, p=0.002). (c) PVcre/NR1fl/fl mice performed significantly better on the continuous T-maze task relative to controls (t=−2.26, df=19, p=0.03). (d) Time in center is not significantly different between PVcre/NR1fl/fl and PVcre/NRI+/+ mice. (e) Total distance is not significantly different between PVcre/NR1fl/fl and PVcre/NR1+/+ mice. *p<0.05, **p<0.01.

Figure 2

Loss of NMDARs on PV-positive interneurons results in increased baseline gamma power and N1 latency. (a) N1 latency is significantly increased in PVcre/NR1fl/fl mice relative to PVcre/NR1+/+ mice (t=−7.179, df=14, p<0.000005) and N1 amplitude trends toward increasing in PVcre/NR1fl/fl mice relative to controls (t=2.129, df=14, p=0.051). (c, d) N1 latency is not correlated with (c) cognition; however, there is a significant negative correlation between N1 latency and (d) sociability (r²=0.3069, p=0.02). (e) Baseline gamma power is significantly higher in PVcre/NR1fl/fl relative to PVcre/NR1+/+ mice (t=−2.37, df=26, p=0.02). The data show evoked gamma power was not significantly different between PVcre/NR1+/+ and PVcre/NR1fl/fl mice. (g) Data indicate a significant negative correlation between baseline and evoked gamma power (r²=0.2672, p=0.004). (h, i) Sociability was not correlated with either (h) baseline gamma power or (i) evoked gamma power. (j, k) Cognition was significantly correlated with (j) baseline gamma power (r²=0.3743, p=0.003), but there was no correlation between (k) cognition and evoked gamma power. *p<0.05, ***p<0.001.

Figure 3

Ablation of NMDA receptors on PV interneurons does not affect membrane potential but reduces initiation and maintenance of long-term potentiation (LTP) after tetanus. (a) There is no difference between PVcre/NR1+/+ and PVcre/NR1fl/fl mice for the plot of evoked amplitude over time. (b, c) There is no difference in amplitude or frequency of spontaneous EPSCs in PVcre/NR1fl/fl mice compared with controls. (d) Time course of LTP before and after tetanic stimulation. (e) Strength of LTP at induction is significantly reduced in PVcre/NR1fl/fl mice compared with controls (t=4.800, df=138, p<0.0001). (f) Strength of LTP at 1 h after tetanic stimulation remains significantly lower in PVcre/NR1fl/fl mice compared with controls (t=7.300, df=138, p<0.0001). ***p<0.001.

Figure 4

MPEP further impairs N1 latency and sociability in animals with ablated NMDARs on PV-positive cells. (a, b) Nest building is not improved by (a) baclofen or (b) MPEP. (c) MPEP further significantly increases N1 latency across both genotypes (F=6.72, df=1, p=0.02). (d) MPEP further significantly decreases sociability across genotypes (F=5.33, df=1, p=0.03). (e) Sociability and N1 latency are significantly correlated in the presence of MPEP (r²=0.3134, p=0.03). ***PV_cre/NR1 fl/fl>PV_cre/NR1^+/+, *MPEP>saline.

Figure 5

Baseline and evoked gamma power are not dissociable in the presence of either baclofen or MPEP following ablation of NMDARs on PV interneurons. (a) Baclofen normalizes baseline gamma power in PVcre/NR1fl/fl mice relative to PVcre/NR1+/+. (b) Following MPEP, there is a trend toward increased baseline gamma power in PVcre/NR1fl/fl mice relative to control mice (F=4.19, df=1, p=0.052). (c, d) Baseline and evoked gamma power are significantly correlated in the presence of (c) baclofen (r²=0.3458, p=0.002) and (d) MPEP (r²=0.3960, p=0.0008).