NMDA Receptor Blockade Reverses Forebrain Deficits in Neuronal Activity and Behavioral Dysfunction in Mouse Models of Rett Syndrome

Background: Rett syndrome (RTT) is a complex autism spectrum disorder (ASD) caused by loss-of-function mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2), a transcriptional regulatory protein. RTT patients suffer from severe impairments in cognition, motor control, respiration and autonomic regulation, and many exhibit autistic behaviors. Altered functional connectivity has been identified in some brain regions in mouse models of RTT and other ASDs and in imaging studies of individuals with autism. However, a map of circuit dysfunction in the RTT brain as a whole has been lacking, hampering our ability to understand the complexity of neurologic dysfunction in this disorder and identify therapeutic targets. Mapping global network dysfunction is especially important in understanding RTT because prominent features of the disease, such as abnormal cardiorespiratory control, are highly dependent on behavioral state, suggesting that interactions between distant structures, such as the forebrain and brainstem, are abnormal.

Objectives:  1) To map regional differences in brain activity between Mecp2 mutant and wildtype (Wt) mice with high anatomic resolution, and 2) Determine whether or not circuit dysfunction in Mecp2 mutants is reversible by treatment with small molecule therapeutic agents in vivo.

Methods:  Neural circuit function in Mecp2 mutant (Mecp2tm1.1Jae/y and Mecp2tm1.1Jae/+, 129/Bl6/BalbC) and Wt mice was mapped in situ using immunochemical localization of the activity-dependent, immediate early gene product Fos, combined with whole-cell patch clamp recording from isolated brain slices in vitro. Prepulse inhibition of acoustic startle (PPI), an index of sensorimotor gating, was used as a behavioral outcome measure. Animals were randomly assigned to treatment groups and observers were blinded to genotype and treatment.

Results:  Fos mapping revealed significant differences in brain activity across the neuraxis between Mecp2 mutants and Wt mice. Specifically, we identified a large domain, rostral to the pons, in which RTT mice exhibit significantly lower activity than Wt, including key nodes in the default mode network (DMN; medial prefrontal, cingulate and retrosplenial cortices) as well as sensory and motor cortices. Forebrain hypoactivity in mutants was associated with markedly exaggerated PPI responses compared to Wt. In contrast, medullary regions important for cardiorespiratory homeostasis exhibited higher activity than Wt, including increased frequency of spontaneous excitatory postsynaptic currents (EPSC) and increased evoked EPSC amplitudes. Acute treatment of mutant mice with the NMDA receptor antagonist ketamine restored wildtype levels of Fos expression in the forebrain and completely reversed deficits in PPI, even at sub-psychotomimetic doses (8 mg/kg, i.p.).

Conclusions: Neurological abnormalities in Mecp2 mutant mice are associated with excitatory/inhibitory imbalance across the forebrain-midbrain-hindbrain axis, including hypoactivity in the forebrain default mode network and hyperactivity in the caudal brainstem. In light of recent findings that the default mode network is also hypofunctional in autism, our data raise the possibility that reduced activity within this meta-circuit is a shared feature of RTT and other ASDs and is reversible by treatments targeting ketamine-sensitive signaling pathways. On the other hand, hyperactivity within the caudal brainstem may underlie the disturbances in respiratory and autonomic control that generally distinguish RTT from other disorders within the autism spectrum.