Growth hormone promotes myelin repair after chronic hypoxia via triggering pericyte-dependent angiogenesis

White matter injury (WMI) causes oligodendrocyte precursor cell (OPC) differentiation arrest and functional deficits, with no effective therapies to date. Here, we report increased expression of growth hormone (GH) in the hypoxic neonatal mouse brain, a model of WMI. GH treatment during or post hypoxic exposure rescues hypoxia-induced hypomyelination and promotes functional recovery in adolescent mice. Single-cell sequencing reveals that Ghr mRNA expression is highly enriched in vascular cells. Cell-lineage labeling and tracing identify the GHR-expressing vascular cells as a subpopulation of pericytes. These cells display tip-cell-like morphology with kinetic polarized filopodia revealed by two-photon live imaging and seemingly direct blood vessel branching and bridging. Gain-of-function and loss-of-function experiments indicate that GHR signaling in pericytes is sufficient to modulate angiogenesis in neonatal brains, which enhances OPC differentiation and myelination indirectly. These findings demonstrate that targeting GHR and/or downstream effectors may represent a promising therapeutic strategy for WMI.

The Wistar Kyoto Rat: A Model of Depression Traits

There is an ongoing debate about the value of animal research in psychiatry with valid lines of reasoning stating the limits of individual animal models compared to human psychiatric illnesses. Human depression is not a homogenous disorder; therefore, one cannot expect a single animal model to reflect depression heterogeneity. This limited review presents arguments that the Wistar Kyoto (WKY) rats show intrinsic depression traits. The phenotypes of WKY do not completely mirror those of human depression but clearly indicate characteristics that are common with it. WKYs present despair- like behavior, passive coping with stress, comorbid anxiety, and enhanced drug use compared to other routinely used inbred or outbred strains of rats. The commonly used tests identifying these phenotypes reflect exploratory, escape-oriented, and withdrawal-like behaviors. The WKYs consistently choose withdrawal or avoidance in novel environments and freezing behaviors in response to a challenge in these tests. The physiological response to a stressful environment is exaggerated in WKYs. Selective breeding generated two WKY substrains that are nearly isogenic but show clear behavioral differences, including that of depression-like behavior. WKY and its substrains may share characteristics of subgroups of depressed individuals with social withdrawal, low energy, weight loss, sleep disturbances, and specific cognitive dysfunction. The genomes of the WKY and WKY substrains contain variations that impact the function of many genes identified in recent human genetic studies of depression. Thus, these strains of rats share characteristics of human depression at both phenotypic and genetic levels, making them a model of depression traits.

Genetic stress-reactivity, sex, and conditioning intensity affect stress-enhanced fear learning

The Stress-Enhanced Fear Learning (SEFL) model of posttraumatic stress disorder (PTSD) reveals increased fear memory in animals exposed to stress prior to contextual fear conditioning (CFC), similar to the increased likelihood of developing PTSD in humans after prior stress. The present study utilized the SEFL model by exposing animals to restraint stress as the first stressor, followed by CFC using foot-shocks with 0.6 mA or 0.8 mA intensity. Adult males and females from the two nearly isogenic rat strains, the genetically more stress-reactive Wistar Kyoto (WKY) More Immobile (WMI), and the less stress-reactive WKY Less Immobile (WLI) were employed. Percent time spent freezing at acquisition and at recall differed between these strains in both prior stress and no stress conditions. The significant correlations between percent freezing at acquisition and at recall suggest that fear memory differences represent a true phenotype related to the stress-reactivity differences between the strains. This assumption is further substantiated by the lack of effect of either conditioning intensity on percent freezing in WLI males, while WMI males were affected by both intensities albeit with opposite directional changes after prior stress. Differences between the sexes in sensitivity to the two conditioning intensities became apparent by the opposite directional and inverse relationship between fear memory and the intensity of conditioning in WMI males and females. The present data also illustrate that although corticosterone (CORT) responses to prior stress are known to be necessary for SEFL, plasma CORT and percent freezing were positively correlated only in the stress less-reactive WLI strain. These differences in baseline fear acquisition, fear memory, and the percent freezing responses to the SEFL paradigm in the two genetically close inbred WMI and WLI strains provide a unique opportunity to study the genetic contribution to the variation in these phenotypes.