Mice with reduced NMDA receptor expression: more consistent with autism than schizophrenia?

Abstract

Reduced NMDA-receptor (NMDAR) function has been implicated in the pathophysiology of neuropsychiatric disease, most strongly in schizophrenia but also recently in autism spectrum disorders (ASD). To determine the direct contribution of NMDAR dysfunction to disease phenotypes, a mouse model with constitutively reduced expression of the obligatory NR1 subunit has been developed and extensively investigated. Adult NR1(neo-/-) mice show multiple abnormal behaviors, including reduced social interactions, locomotor hyperactivity, self-injury, deficits in prepulse inhibition (PPI) and sensory hypersensitivity, among others. Whereas such phenotypes have largely been interpreted in the context of schizophrenia, these behavioral abnormalities are rather non-specific and are frequently present across models of diseases characterized by negative symptom domains. This study investigated auditory electrophysiological and behavioral paradigms relevant to autism, to determine whether NMDAR hypofunction may be more consistent with adult ASD-like phenotypes. Indeed, transgenic mice showed behavioral deficits relevant to all core ASD symptoms, including decreased social interactions, altered ultrasonic vocalizations and increased repetitive behaviors. NMDAR disruption recapitulated clinical endophenotypes including reduced PPI, auditory-evoked response N1 latency delay and reduced gamma synchrony. Auditory electrophysiological abnormalities more closely resembled those seen in clinical studies of autism than schizophrenia. These results suggest that NMDAR hypofunction may be associated with a continuum of neuropsychiatric diseases, including schizophrenia and autism. Neural synchrony abnormalities suggest an imbalance of glutamatergic and GABAergic coupling and may provide a target, along with behavioral phenotypes, for preclinical screening of novel therapeutics.

Figure 1

Constitutive NMDA-receptor downregulation impairs sociability in mice. (A) Adult male WT or NR1^neo−/− mice were paired with a receptive, WT female for 5 minutes. Mating vocalizations, emitted by the male mouse, were recorded with an ultrasonic-range microphone placed above the testing chamber. (B) Transgenic mice demonstrated significantly reduced interest in the female mouse, as measured by the total time of male initiated interactions. (C) Sociability to a gonadectomized, same-sex wildtype stimulus mouse was measured using a 3-chambered approach/avoidance paradigm. Adult NR1-transgenic mice showed significantly reduced social interest in this task as well. Plots show mean +/− S.E.M. (***P<0.001).

Figure 2

GRIN1 mutant mice demonstrate abnormalities in social communication, as measured by ultrasonic vocalizations (USVs). (A) Male mice emit 70 kHz “pre-mating” vocalizations and 40 kHz “mating” USVs when paired with a receptive female. This sonogram demonstrates four pre-mating calls, recorded from a WT mouse. (B) NR1^neo−/− emit significantly fewer ultrasonic pre-mating calls than do WT littermates. (C) In WT mice, there was a significant, linear association (on a log-log plot) between the number of pre-mating vocalizations emitted and male initiated social interactions, highlighting the relevance of such ultrasonic calls for murine social communication. This relationship was absent for NR1^neo−/− mice. Plots show mean +/− S.E.M. (**P<0.01, ***P<0.001).

Figure 3

NMDA-receptor hypofunction leads to increased repetitive behaviors and self-injury. (A) During a tail-suspension test, 90% of NR1 mutant mice demonstrate a repetitive, hand-wringing stereotypy that was only observed in 10% of WT littermates. See supplemental information for a video. (B) Approximately 25% of NR1^neo−/− mice developed skin lesions due to self-injurious over-grooming behavior. Such lesions were not observed in any WT littermates. Plots show mean +/− S.E.M. (*P<0.05, ***P<0.001).

Figure 4

NR1 mutant mice show increased locomotor activity during light cycle. Group averages of activity are shown over 120-hour testing session.

Figure 5

NMDA-receptor disruption recapitulates several auditory endophenotypes observed in autism. (A) Transgenic mice show elevated acoustic startle responses and (B) reduced prepulse inhibition (PPI). (C) Developmental NMDA-receptor downregulation leads to delayed latency of the N1 component of the auditory-evoked response, as observed in children with autism. (D) Grand-average auditory-evoked response waveforms demonstrating P1 and N1 peaks. (E, F) Transient auditory-evoked phase-locking (PLF; e.g., intertrial coherence) was assessed in the gamma frequency range (30–80 Hz). NR1^neo−/− mice showed significantly reduced gamma synchrony, mirroring results from the clinical population. Plots show mean +/− S.E.M. (*P<0.05).