Assessment of NMDA receptor activation in vivo by Fos induction after challenge with the direct NMDA agonist (tetrazol-5-yl)glycine: effects of clozapine and haloperidol

N-methyl-D-aspartate receptor agonism and antagonism within the amygdaloid central nucleus suppresses pain affect: differential contribution of the ventrolateral periaqueductal gray

The amygdala contributes to the generation of pain affect, and the amygdaloid central nucleus (CeA) receives nociceptive input that is mediated by glutamatergic neurotransmission. The present study compared the contribution of N-methyl-d-aspartate (NMDA) receptor agonism and antagonism in the CeA to generation of the affective response of rats to an acute noxious stimulus. Vocalizations that occur following a brief tail shock (vocalization afterdischarges) are a validated rodent model of pain affect and were preferentially suppressed, in a dose-dependent manner, by bilateral injection into the CeA of NMDA (.1, .25, .5, or 1 μg/side) or the NMDA receptor antagonist d-(-)-2-amino-5-phosphopentanoic acid (AP5; 1, 2, or 4 μg/side). Vocalizations that occur during tail shock were suppressed to a lesser degree, whereas spinal motor reflexes (tail flick and hind limb movements) were unaffected by injection of NMDA or AP5 into the CeA. Injection of NMDA, but not AP5, into the CeA increased c-Fos immunoreactivity in the ventrolateral periaqueductal gray, and unilateral injection of the μ-opiate receptor antagonist H-d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP; .25 μg) into ventrolateral periaqueductal gray prevented the antinociception generated by injection of NMDA into the CeA. These findings demonstrate that although NMDA receptor agonism and antagonism in the CeA produce similar suppression of pain behaviors, they do so via different neurobiologic mechanisms.

PERSPECTIVE

The amygdala contributes to production of the emotional dimension of pain. NMDA receptor agonism and antagonism within the CeA suppressed rats' emotional response to acute painful stimulation. Understanding the neurobiology underlying emotional responses to pain will provide insights into new treatments for pain and its associated affective disorders.

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.

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.

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

Effects of the direct NMDA agonist (tetrazol-5-yl)glycine (TZG) were examined in a genetic mouse model of reduced NMDA receptor function. In this model, expression of the NR1 subunit is reduced but not eliminated and the mice are therefore designated as NR1 hypomorphic. Previous work suggested that the reduced NR1 subunit expression produced a functional subsensitivity as judged by a blunted Fos induction response to a sub-seizure dose of TZG. In the present study seizure threshold doses of TZG were tested in the wild type and mutant mice. Surprisingly, there was no difference in the seizure sensitivity between the wild type mice and mice presumed to express very low levels of the NR1 subunit. An extensive neuroanatomical analysis of Fos induction was conducted after the threshold seizure doses of TZG. The results demonstrate that some brain regions of the NR1 -/- mice exhibit much lower Fos induction in comparison to the NR1 +/+ mice. These regions include hippocampus, amygdala, and cerebral cortical regions. However, in other regions, similar induction of Fos was observed in both genotypes in response to the NMDA agonist. Regions showing similar Fos induction in the NR1 +/+ and NR1 -/- mice include the lateral septum, nucleus of the solitary tract, and medial hypothalamic regions. The results suggest that the NMDA receptor hypofunction in the NR1 -/- mice is not global but regionally specific and that subcortical structures are responsible for the seizure-inducing effects of TZG.