Seizure responses and induction of Fos by the NMDA agonist (tetrazol-5-yl)glycine in a genetic model of NMDA receptor hypofunction

Booting up the organism during development: Pre-behavioral functions of the vertebrate brain in guiding body morphogenesis

A recent study in Xenopus laevis embryos showed that the very early brain has important functions long before behavior. While the nascent brain is being constructed, it is required for normal patterning of the muscle and peripheral nerve networks, including those far away from the head. In addition to providing important developmental signals to remote tissues in normal embryogenesis, its presence is also able to render harmless exposure to specific chemicals that normally act as teratogens. These activities of the early brain can be partially compensated for in a brainless embryo by experimental modulation of neurotransmitter and ion channel signaling. Here, we discuss the major findings of this paper in the broader context of developmental physiology, neuroscience, and biomedicine. This novel function of the embryonic brain has significant implications, especially for understanding developmental toxicology and teratogenesis in the context of pharmaceutical and environmental reagents.

The brain is required for normal muscle and nerve patterning during early Xenopus development

Possible roles of brain-derived signals in the regulation of embryogenesis are unknown. Here we use an amputation assay in Xenopus laevis to show that absence of brain alters subsequent muscle and peripheral nerve patterning during early development. The muscle phenotype can be rescued by an antagonist of muscarinic acetylcholine receptors. The observed defects occur at considerable distances from the head, suggesting that the brain provides long-range cues for other tissue systems during development. The presence of brain also protects embryos from otherwise-teratogenic agents. Overexpression of a hyperpolarization-activated cyclic nucleotide-gated ion channel rescues the muscle phenotype and the neural mispatterning that occur in brainless embryos, even when expressed far from the muscle or neural cells that mispattern. We identify a previously undescribed developmental role for the brain and reveal a non-local input into the control of early morphogenesis that is mediated by neurotransmitters and ion channel activity.Functions of the embryonic brain prior to regulating behavior are unclear. Here, the authors use an amputation assay in Xenopus laevis to demonstrate that removal of the brain early in development alters muscle and peripheral nerve patterning, which can be rescued by modulating bioelectric signals.

Increased sensitivity to kainic acid in a genetic model of reduced NMDA receptor function

The pathophysiology of schizophrenia may involve reduced NMDA receptor function and experimental models of NMDA receptor hypofunction have proven useful for characterizing neurobiological abnormalities potentially relevant to schizophrenia. The present study assessed behavioral responses and induction of Fos after administration of kainic acid to wild type mice (NR1(+/+)) and mice with genetically reduced NMDA receptor expression (NR1(neo/neo)). At a dose of 20 mg/kg, kainic acid induced lethal seizures in 100% of the NR1(neo/neo) mice tested but produced no lethal seizures in the wild type mice. The NR1(neo/neo) mice also exhibited enhanced behavioral responses to kainic acid at a dose of 15 mg/kg but no lethal seizures were produced by this dose. A greater induction of Fos was observed in neocortical and limbic cortical regions of the NR1(neo/neo) compared to NR1(+/+) mice after administration of 15 mg/kg kainic acid. In contrast, there were no differences between the genotypes in kainic acid induced Fos in the amygdala, hippocampus, lateral septum, and nucleus accumbens. In order to determine if altered behavioral phenotypes of the NR1(neo/neo) mice could be related to increased sensitivity of kainate receptors to endogenous glutamate, effects of the highly selective kainate antagonist LY382884 were examined. The kainate antagonist reduced the exaggerated acoustic startle responses, deficits in prepulse inhibition of acoustic startle, and motor hyperactivity in the NR1(neo/neo) mice. These findings suggest that selective kainate receptor antagonists could be novel therapeutic candidates for schizophrenia.

Assessment of NMDA receptor NR1 subunit hypofunction in mice as a model for schizophrenia

N-methyl-D-aspartate receptors (NMDARs) play a pivotal role in excitatory neurotransmission, synaptic plasticity and brain development. Clinical and experimental evidence suggests a dysregulation of NMDAR function and glutamatergic pathways in the pathophysiology of schizophrenia. We evaluated electrophysiological and behavioral properties of NMDAR deficiency utilizing mice that express only 5-10% of the normal level of NMDAR NR1 subunit. Auditory and visual event related potentials yielded significantly increased amplitudes for the P20 and N40 components in NMDAR deficient (NR1(neo)-/-) mice suggesting decreased inhibitory tone. Compared to wild types, NR1(neo)-/- mice spent less time in social interactions and showed reduced nest building. NR1(neo)-/- mice displayed a preference for open arms of a zero maze and central zone of an open field, possibly reflecting decreased anxiety-related behavioral inhibition. However, locomotor activity did not differ between groups in either home cage environment or during behavioral testing. NR1(neo)-/- mice displayed hyperactivity only when placed in a large unfamiliar environment, suggesting that neither increased anxiety nor non-specific motor activation accounts for differential behavioral patterns. Data suggest that NMDAR NR1 deficiency causes disinhibition in sensory processing as well as reduced behavioral inhibition and impaired social interactions. The behavioral signature in NR1(neo)-/- mice supports the impact of impaired NMDAR function in a mouse model with possible relevance to negative symptoms in schizophrenia.

Neural activation deficits in a mouse genetic model of NMDA receptor hypofunction in tests of social aggression and swim stress

Mice with reduced expression of the NR1 subunit of the NMDA receptor (NR1 hypomorphic mice) display altered behavioral phenotypes that may relate to behavioral characteristics of schizophrenia. Altered phenotypes in the NR1 hypomorphs include marked deficits in species-typical behavioral interactions in tests of social aggression and social affiliation. To gain insight into neuroanatomical circuits disrupted by reduced NMDA receptor function, the present work compared regional brain activation in NR1 hypomorphic mice and their wild type controls after a resident-intruder test. Induction of Fos protein was used as an index of neuronal activation. Wild type mice exhibited robust induction of Fos in select brain regions, including specific nuclei of the hypothalamus and amygdala, lateral septum, and widespread regions of the cerebral cortex. Although the behavioral patterns were different for male and female mice, neuroanatomical patterns of Fos induction were remarkably similar for the two sexes. To determine socially specific components of Fos induction by the resident-intruder test, responses were compared for mice assessed in a test of general arousal and stress involving forced swim. Some common brain regions were activated by both tests but regionally specific differences were also found. The NR1 hypomorphic mice tested in the resident-intruder procedure displayed distinctly different behavioral interactions compared to the wild type mice and exhibited a significantly blunted Fos response in almost all brain regions. The mutant mice also exhibited reduced Fos in response to swim stress in specific brain regions. These data suggest that the NR1 hypomorphic mice have functional activation deficits in response to social challenge and swim stress.