Intermittent hypoxia elicits long-term facilitation (LTF), a persistent augmentation (hours) of respiratory motor output. Considerable recent progress has been made toward an understanding of the mechanisms and manifestations of this potentially important model of respiratory plasticity. LTF is elicited by intermittent but not sustained hypoxia, indicating profound pattern sensitivity in its underlying mechanism. During intermittent hypoxia, episodic spinal serotonin receptor activation initiates cell signaling events, increasing spinal protein synthesis. One associated protein is brain-derived neurotrophic factor, a neurotrophin implicated in several forms of synaptic plasticity. Our working hypothesis is that increased brain-derived neurotrophic factor enhances glutamatergic synaptic currents in phrenic motoneurons, increasing their responsiveness to bulbospinal inspiratory inputs. LTF is heterogeneous among respiratory outputs, differs among experimental preparations, and is influenced by age, gender, and genetics. Furthermore, LTF is enhanced following chronic intermittent hypoxia, indicating a degree of metaplasticity. Although the physiological relevance of LTF remains unclear, it may reflect a general mechanism whereby intermittent serotonin receptor activation elicits respiratory plasticity, adapting system performance to the ever-changing requirements of life.

Children with anxious temperament (AT) are particularly sensitive to new social experiences and have increased risk for developing anxiety and depression. The young rhesus monkey is optimal for studying the origin of human AT because it shares with humans the genetic, neural, and phenotypic underpinnings of complex social and emotional functioning. In vivo imaging in young monkeys demonstrated that central nucleus of the amygdala (Ce) metabolism is relatively stable across development and predicts AT. Transcriptome-wide gene expression, which reflects combined genetic and environmental influences, was assessed within the Ce. Results support a maladaptive neurodevelopmental hypothesis linking decreased amygdala neuroplasticity to early-life dispositional anxiety. For example, high AT individuals had decreased mRNA expression of neurotrophic tyrosine kinase, receptor, type 3 (NTRK3). Moreover, variation in Ce NTRK3 expression was inversely correlated with Ce metabolism and other AT-substrates. These data suggest that altered amygdala neuroplasticity may play a role the early dispositional risk to develop anxiety and depression.positron-emission tomography | microarray | brain imaging T he ability to identify brain mechanisms underlying the risk during childhood for developing anxiety and depression is critical for establishing novel early-life interventions aimed at preventing the chronic and debilitating outcomes associated with these common illnesses. To this end, we have optimized a model of anxious temperament (AT), the conserved at-risk phenotype, in young developing rhesus monkeys (1-4). The rhesus monkey is ideal for studying the origin of human AT because these species share the genetic, neural, and phenotypic underpinnings of complex social and emotional functioning (5-10). Importantly, the rhesus developmental model bridges the critical gap between human psychopathology and rodent models, allowing for translation to humans by using in vivo imaging measures and translation to rodents by using ex vivo molecular methods. Thus, the unique hypotheses that can be generated from the rhesus model are invaluable in guiding both imaging studies in children and mechanistic efforts in rodents.Of particular relevance to the AT rhesus model is the relatively recent evolutionary divergence between rhesus monkeys and humans (25 million years) compared with rodents and humans (70 million years) (5). This evolutionary closeness is reflected in the species' similarities in social and emotional behaviors. These homologies, instantiated in their conserved genetic and neural systems, underlie the ability of both humans and rhesus monkeys to form and maintain the relationships necessary for living in complex social environments. In this regard, the experience of anxiety has evolved in primates to motivate the formation of long-lasting attachment bonds that serve to increase security and group cohesion. The comparable rearing practices shared by these species (e.g., close mother-infant bonding) promote early social/emotional le...

SummaryBehavioral inhibition (BI) is an adaptive defensive response to threat; however, extreme BI is associated with anxiety-related psychopathology. When rats are exposed to a natural predator they display stress-and anxiety-related behavioral alterations and physiological activation. To develop a preclinical rodent model to study mechanisms underlying human BI and anxiety, we examined the extent to which ferret exposure elicits anxiety-related BI and HPA and amygdala activation of the CRF system. In the first experiment, BI and other behaviors were assessed in the presence or absence of a ferret. In the second experiment, ferret-induced corticosterone release and changes in brain cfos expression were assessed. In the final experiment, gene chip and quantitative real time-PCR analyses were performed on amygdala tissue from control and ferret-exposed rats. Ferret exposure increased BI and submissive posturing, as well as plasma corticosterone and the number of Fospositive cells in several brain regions including the amygdala. Gene expression analysis revealed increased amygdalar mRNA for CRF-binding protein, but not the CRF 1 receptor, CRF 2 receptor or CRF. In rodents, ferret exposure can be used to elicit anxiety-related BI, which is associated with HPA and amygdala activation. Since the amygdala and the CRF system have been implicated in adaptive and maladaptive anxiety responses in humans, these data support use of our rodent model to further investigate mechanisms underlying anxiety-related psychopathology in humans.

Psychological stressors precipitate and maintain stress-induced psychopathology, and it is likely that altered amygdala function underlies some of the deleterious effects of psychological stress. To understand the mechanisms underlying the linkage between the response to psychological stressors and maladaptive or psychopathological responses, we have focused on amygdala responsivity in animal models employing species-specific psychological stressors. In the present study, we characterized the effects of a 15-min exposure to a natural predator, the ferret, on rat behavior and the expression of the somatostatin family of genes in the amygdala. We examined the somatostatin family of genes because substantial evidence shows that central somatostatin systems are altered in various neuropsychiatric illnesses. We report that rats respond to acute ferret exposure with a significant increase in fearful and anxious behaviors that is accompanied by robust amygdala activation and an increase in somatostatin receptor 2 (sst2) messenger RNA expression within the amygdala and anterior cingulate cortex. These studies are the first to show stress-induced changes in amygdala sst2 expression and may represent one mechanism by which psychological stress is linked to adaptive and maladaptive behavioral responses.

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Background Anxious temperament (AT) is identifiable early in life and predicts the later development of anxiety disorders and depression. Neuropeptide Y (NPY) is a putative endogenous anxiolytic neurotransmitter that adaptively regulates responses to stress and may confer resilience to stress-related psychopathology. Using a well-validated non-human primate model of AT, we examined expression of the NPY system in the central nucleus (Ce) of the amygdala, a critical neural substrate for extreme anxiety. Methods In twenty-four young rhesus monkeys, we measured Ce mRNA levels of all members of the NPY system that are detectable in the Ce using quantitative real time polymerase chain reaction (qRT-PCR). We then examined the relationship between these mRNA levels and both AT expression and brain metabolism. Results Lower mRNA levels of NPY receptor 1 (NPY1R) and NPY receptor 5 (NPY5R), but not NPY or NPY receptor 2 (NPY2R) in the Ce predicted elevated AT; mRNA levels for NPY1R and NPY5R in the motor cortex were not related to AT. In situ hybridization analysis provided for the first time a detailed description of NPY1R and NPY5R mRNA distribution in the rhesus amygdala and associated regions. Lastly, mRNA levels for these two receptors in the Ce predicted metabolic activity in several regions that have the capacity to regulate the Ce. Conclusion Decreased NPY signaling in the Ce may contribute to the altered metabolic activity that is a component of the neural substrate underlying AT. This suggests that enhancement of NPY signaling may reduce the risk to develop psychopathology.

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Behavioral inhibition (BI) is an adaptive defensive response to threat; however, children who display extreme BI as a stable trait are at risk for development of anxiety disorders and depression. The present study validates a rodent model of BI based on an ethologically relevant predator exposure paradigm. We show that individual differences in rat BI are stable and trait-like from adolescence into adulthood. Using in situ hybridization to quantify expression of the immediate early genes homer1a and fos as measures of neuronal activation, we show that individual differences in BI are correlated with the activation of various stress-responsive brain regions that include the paraventricular nucleus of the hypothalamus and CA3 region of the hippocampus. Further supporting the concept that threat-induced BI in rodents reflects levels of anxiety, we also show that BI is decreased by administration of the anxiolytic, diazepam. Finally, we developed criteria for identifying extreme BI animals that are stable in their expression of high levels of BI and also show that high BI (HBI) individuals exhibit maladaptive appetitive responses following stress exposure. These findings support the use of predator threat as a stimulus and HBI rats as a model to study mechanisms underlying extreme and stable BI in humans.