Local NMDA receptor hypofunction evokes generalized effects on gamma and high-frequency oscillations and behavior

Highlights

• MK801 elevates gamma (30–80 Hz) and high high-frequency oscillations (HFO).

• Local MK801 infusion increases oscillations in local circuits and in distant regions.

• Locomotion and stereotypy increase after infusion of MK801 into the PFC HIPP and NAcc.

Abstract

The NMDA receptor (NMDAr) hypofunction theory of schizophrenia suggests that aberrant signaling through NMDAr underlies the pathophysiology of this disease. This is commonly modeled in rodents via treatment with NMDAr antagonists, which causes a range of behavioral effects that represent endophenotypes related to schizophrenia. These drugs also disrupt high-frequency neural oscillations within the brain, also potentially relevant to disease. We studied the effect of localized NMDAr hypofunction on the generation of neural oscillations occurring both locally and in distant brain regions, and on behaviors routinely used as endophenotypes to model psychosis in rodents. Wistar rats were implanted with local field potential recording electrodes in the prefrontal cortex, dorsal hippocampus and nucleus accumbens, as well as cannulae in these regions to facilitate drug infusion. Rats received bilateral infusions of MK801 (0, 5 μg, 20 μg, 50 μg) into one of the three target regions and their behavior measured in an open field. We also assessed the effects of systemic MK801 injection (0.16 mg/kg sc). Electrophysiological signals were recorded continuously, allowing assessment of gamma oscillations (30–80 Hz) and high-frequency oscillations (HFO: 130–180 Hz) occurring as a result of infusions. Regardless of MK801 infusion location, gamma oscillations and HFOs significantly and consistently increased in all three regions studied, similar to that observed following systemic injection. Locomotor activity, stereotypies and ataxia were also observed following infusion into all regions. We conclude that localized regions exhibiting NMDAr hypofunction are sufficient to disrupt local as well as diffuse neural circuits and global brain function, and concomitantly cause psychosis-related behavioral effects.

Introduction

The NMDA receptor (NMDAr) hypofunction theory of schizophrenia posits that a deficit in signaling through NMDAr is a key pathophysiological component of the disease. This is primarily driven by observations that exposure to NMDAr antagonists such as PCP and ketamine evoke in healthy volunteers a constellation of behavioral effects with a high degree of similarity to those observed in schizophrenia, and selectively exacerbate pre-existing symptoms in schizophrenia patients (Lahti et al., 1995, Adler et al., 1998, Newcomer et al., 1999). Molecular evidence also supports a role for NMDAr hypofunction in the pathophysiology of the disorder: post-mortem comparisons of dorsolateral prefrontal cortex between schizophrenia and healthy controls identified significantly reduced NMDAr subunit expression in patient samples (Beneyto and Meador-Woodruff, 2008, Weickert et al., 2013), and several genetic associations have been identified linking polymorphisms in NMDAr subunits and schizophrenia (e.g.: Ohtsuki et al., 2001, Zhao et al., 2006).

A popular preclinical model which attempts to replicate this hypofunction involves treatment of NMDAr antagonists to rodents (Adell et al., 2012). This causes a variety of behavioral effects, including hyperlocomotion, stereotypic behavior, cognitive deficits, and sensorimotor gating impairments, features which are considered to be endophenotypes of schizophrenia (Adell et al., 2012). In addition to producing behavioral disturbances, NMDAr antagonists also cause disturbances in electrophysiological rhythms within the brain, particularly oscillations, including in the 30- to 80-Hz (i.e. gamma) and 130- to 180-Hz bands (high-frequency oscillations – HFOs) (Hunt et al., 2006, Hunt et al., 2010, Pinault, 2008, Hakami et al., 2009, Olszewski et al., 2013a, Olszewski et al., 2013b, Anderson et al., 2014), which are similar to those reported in schizophrenia (Uhlhaas and Singer, 2010). Such acute pharmacology studies are supported by genetic models of schizophrenia, where deletion of NMDAr from specific populations of interneurons similarly disturbs neural oscillations and behavior (eg: Carlen et al., 2012, Billingslea et al., 2014). These observations have promoted the idea that NMDAr hypofunction alters the regulation of gamma oscillations and HFOs, which in turn lead to schizophrenia symptoms, and this concept represents a major focus of ongoing research.

Recent models of schizophrenia have proposed that the underlying pathophysiology can be attributed to a state of functional disconnectivity, whereby communication between distinct regions of the brain is disturbed (Friston, 2002). This model broadly suggests that, in a healthy brain, appropriate cognitive functioning is dependent on the coherent interaction of regionally distinct and functionally specialized brain regions. In the brain of a person with schizophrenia, the interaction between regions becomes aberrant, leading to disturbances in executive processing and thus a state of cognitive dysfunction. One extension of the NMDAr hypofunction model suggests that elevated gamma oscillations represent diffuse electrophysiological noise (Pinault, 2008), impairing the ability of brain regions to appropriately communicate, hence leading to functional disconnectivity and cognitive disruption. Systemic administration of NMDAr antagonists leads to increased gamma oscillations in several different brain regions (Hakami et al., 2009), and also leads to aberrant coherence between regions (Palenicek et al., 2011). However whether localized NMDAr hypofunction in discrete brain regions can influence functional connections in distant sites has not, to our knowledge been studied.

This study set out to assess the influence of localized NMDAr hypofunction – elicited by microinfusion of the NMDAr antagonist MK801 – on high-frequency electrophysiological oscillations in local and distant brain regions, focussing on key regions associated with schizophrenia pathophysiology: prefrontal cortex (PFC), dorsal hippocampus (HIPP) and nucleus accumbens (NAcc). While the PFC can exert top-down regulatory control of these structures (Vertes, 2006, Floresco et al., 2009), whether this control includes the regulation of high-frequency neural oscillations has not been studied. We were also intrigued to ascertain whether local NMDAr hypofunction was sufficient to trigger behavioral abnormalities commonly observed following systemic injection of NMDAr antagonists, and whether these behaviors were associated with any changes to neural oscillations. The systemic effects of NMDAr antagonists are well-studied, and at least some of these effects may be primarily caused specifically by inhibition of NMDAr within the PFC (Schwabe and Koch, 2004). One of our secondary goals therefore was to determine whether behavioral disturbances caused by local NMDAr hypofunction were related to, or could be dissociated from, local electrophysiological effects.