A Novel Closed-Head Model of Mild Traumatic Brain Injury Caused by Primary Overpressure Blast to the Cranium Produces Sustained Emotional Deficits in Mice

Emotional disorders are a common outcome from mild traumatic brain injury (TBI) in humans, but their pathophysiological basis is poorly understood. We have developed a mouse model of closed-head blast injury using an air pressure wave delivered to a small area on one side of the cranium, to create mild TBI. We found that 20-psi blasts in 3-month-old C57BL/6 male mice yielded no obvious behavioral or histological evidence of brain injury, while 25–40 psi blasts produced transient anxiety in an open field arena but little histological evidence of brain damage. By contrast, 50–60 psi blasts resulted in anxiety-like behavior in an open field arena that became more evident with time after blast. In additional behavioral tests conducted 2–8 weeks after blast, 50–60 psi mice also demonstrated increased acoustic startle, perseverance of learned fear, and enhanced contextual fear, as well as depression-like behavior and diminished prepulse inhibition. We found no evident cerebral pathology, but did observe scattered axonal degeneration in brain sections from 50 to 60 psi mice 3–8 weeks after blast. Thus, the TBI caused by single 50–60 psi blasts in mice exhibits the minimal neuronal loss coupled to “diffuse” axonal injury characteristic of human mild TBI. A reduction in the abundance of a subpopulation of excitatory projection neurons in basolateral amygdala enriched in Thy1 was, however, observed. The reported link of this neuronal population to fear suppression suggests their damage by mild TBI may contribute to the heightened anxiety and fearfulness observed after blast in our mice. Our overpressure air blast model of concussion in mice will enable further studies of the mechanisms underlying the diverse emotional deficits seen after mild TBI

The effects of lesions of the auditory pathway on the processing of an auditory safety signal for conditioned fear

Includes bibliographical references (pages [118]-136)Over the past few decades, Pavlovian conditioning procedures have been utilized to advance our understanding of the neural systems involved in the production of fear. While much is known about the neural systems involved in the production of fear, little is known about the neural systems involved in the inhibition of fear. In rats, the inhibition of fear can be examined using a feature-negative discrimination procedure in which a noise stimulus acquires the ability to inhibit fear to a light which signals danger. Because the “safety” properties of this noise must be transmitted through auditory pathways in the brain, identification of these auditory pathways will help to identify a component of the neural circuit involved in the inhibition of fear. The purpose of the present study was to examine the effects of lesions of structures within the auditory system in an effort to disrupt the detection of the noise inhibitor. To accomplish this, four experiments were conducted in which rats were first given feature-negative discrimination training followed by lesions of the inferior colliculus (IC), mediate geniculate body (MGB), auditory thalamus (ADT), or auditory cortex (CTX). Next, rats were tested for the ability to inhibit fear in the presence of the noise safety signal. The results of these experiments indicated that bilateral lesions of either the IC or ADT disrupted the ability of the noise inhibitor to inhibit fear to a light CS. In contrast, lesions largely restricted to the MGB or CTX did not disrupt the inhibition of fear. These results suggest that an auditory pathway(s) which includes the IC and ADT is used to detect the safety properties previously conditioned to an auditory stimulus.Ph.D. (Doctor of Philosophy

The effects of auditory thalamic lesions on the processing of an auditory safety signal for conditioned fear

Includes bibliographical references (pages [53]-64)Although much is known about the neural systems responsible for the acquisition and expression of conditioned fear, little is known about the neural systems responsible for the inhibition of fear. One procedure well suited for the investigation of inhibition of fear is the conditioned inhibition of fear-potentiated startle developed by Falls and Davis in 1995. The purpose of this study was to examine the effects of auditory thalamic lesions on the acquisition of conditioned inhibition to an auditory conditioned inhibitor. Rats were given either complete electrolytic auditory thalamic lesions or sham lesions prior to conditioned inhibition training. The training consisted of 15 light+shock pairings on each of 2 days followed by 5 light+shock trials intermixed with 15 noise—»light no shock trials on each of 5 days. This training procedure was designed to establish the noise as a conditioned inhibitor. Lesion, sham, and unoperated groups showed equivalent levels of conditioned inhibition of fear, defined as a reduction in fear-potentiated startle to the light when accompanied by the noise. These data suggest that the auditory thalamus is not critical for the acquisition of conditioned inhibition of fear-potentiated startle to an auditory conditioned inhibitor. Because lesions of the auditory thalamus have been shown to prevent the acquisition of fear-potentiated startle to an auditory CS, we evaluated whether our lesions would prevent acquisition of fear-potentiated startle to an auditory CS. The lesioned and a subset of sham animals were given 10 noise+shock pairings on each of 2 days. Lesioned animals showed significantly less fear-potentiated startle than sham animals, suggesting that the lesions were sufficient to prevent acquisition to an auditory CS. These results indicate that although the auditory thalamus may be important for the acquisition of fear to a noise CS, it is not critical for the acquisition of conditioned inhibition to a noise inhibitor.M.A. (Master of Arts

Identification of candidate genes that underlie the QTL on chromosome 1 that mediates genetic differences in stress-ethanol interactions

Alcoholism, stress, and anxiety are strongly interacting heritable, polygenetic traits. In a previous study, we identified a quantitative trait locus (QTL) on murine chromosome (Chr) 1 between 23.0 and 31.5 Mb that modulates genetic differences in the effects of ethanol on anxietyrelated phenotypes. The goal of the present study was to extend the analysis of this locus with a focus on identifying candidate genes using newly available data and tools. Anxiety-like behavior was evaluated with an elevated zero maze following saline or ethanol injections (1.8 g/kg) in C57BL/6J, DBA2J, and 72 BXD strains. We detected significant effects of strain and treatment and their interaction on anxiety-related behaviors, although surprisingly, sex was not a significant factor. The Chr1 QTL is specific to the ethanol-treated cohort. Candidate genes in this locus were evaluated using now standard bioinformatic criteria. Collagen 19a1 (Col19a1) and family sequence 135a (Fam135a) met most criteria but have lower expression levels and lacked biological verification and, therefore, were considered less likely candidates. In contrast, two other genes, the prenylated protein tyrosine phosphate family member Ptp4a1 (protein tyrosine phosphate 4a1) and the zinc finger protein Phf3 (plant homeoDomain finger protein 3) met each of our bioinformatic criteria and are thus strong candidates. These findings are also of translational relevance because both Ptp4a1 and Phf3 have been nominated as candidates genes for alcohol dependence in a human genome-wide association study. Our findings support the hypothesis that variants in one or both of these genes modulate heritable differences in the effects of ethanol on anxiety-related behaviors

Motor, Visual and Emotional Deficits in Mice after Closed-Head Mild Traumatic Brain Injury Are Alleviated by the Novel CB2 Inverse Agonist SMM-189

We have developed a focal blast model of closed-head mild traumatic brain injury (TBI) in mice. As true for individuals that have experienced mild TBI, mice subjected to 50–60 psi blast show motor, visual and emotional deficits, diffuse axonal injury and microglial activation, but no overt neuron loss. Because microglial activation can worsen brain damage after a concussive event and because microglia can be modulated by their cannabinoid type 2 receptors (CB2), we evaluated the effectiveness of the novel CB2 receptor inverse agonist SMM-189 in altering microglial activation and mitigating deficits after mild TBI. In vitro analysis indicated that SMM-189 converted human microglia from the pro-inflammatory M1 phenotype to the pro-healing M2 phenotype. Studies in mice showed that daily administration of SMM-189 for two weeks beginning shortly after blast greatly reduced the motor, visual, and emotional deficits otherwise evident after 50–60 psi blasts, and prevented brain injury that may contribute to these deficits. Our results suggest that treatment with the CB2 inverse agonist SMM-189 after a mild TBI event can reduce its adverse consequences by beneficially modulating microglial activation. These findings recommend further evaluation of CB2 inverse agonists as a novel therapeutic approach for treating mild TBI