Cerebral metabolic consequences in the adult brain after neonatal excitotoxic lesions of the amygdala in rats

Modeling heritability of temperamental differences, stress reactivity, and risk for anxiety and depression: Relevance to research domain criteria (RDoC)

Animal models provide important tools to study biological and environmental factors that shape brain function and behavior. These models can be effectively leveraged by drawing on concepts from the National Institute of Mental Health Research Domain Criteria (RDoC) Initiative, which aims to delineate molecular pathways and neural circuits that underpin behavioral anomalies that transcend psychiatric conditions. To study factors that contribute to individual differences in emotionality and stress reactivity, our laboratory utilized Sprague-Dawley rats that were selectively bred for differences in novelty exploration. Selective breeding for low versus high locomotor response to novelty produced rat lines that differ in behavioral domains relevant to anxiety and depression, particularly the RDoC Negative Valence domains, including acute threat, potential threat, and loss. Bred Low Novelty Responder (LR) rats, relative to their High Responder (HR) counterparts, display high levels of behavioral inhibition, conditioned and unconditioned fear, avoidance, passive stress coping, anhedonia, and psychomotor retardation. The HR/LR traits are heritable, emerge in the first weeks of life, and appear to be driven by alterations in the developing amygdala and hippocampus. Epigenomic and transcriptomic profiling in the developing and adult HR/LR brain suggest that DNA methylation and microRNAs, as well as differences in monoaminergic transmission (dopamine and serotonin in particular), contribute to their distinct behavioral phenotypes. This work exemplifies ways that animal models such as the HR/LR rats can be effectively used to study neural and molecular factors driving emotional behavior, which may pave the way toward improved understanding the neurobiological mechanisms involved in emotional disorders.

Long-Lasting Effects of Prenatal Ethanol Exposure on Fear Learning and Development of the Amygdala

Prenatal ethanol exposure (PrEE) produces developmental abnormalities in brain and behavior that often persist into adulthood. We have previously reported abnormal cortical gene expression, disorganized neural circuitry along with deficits in sensorimotor function and anxiety in our CD-1 murine model of fetal alcohol spectrum disorders, or FASD (El Shawa et al., 2013; Abbott et al., 2016). We have proposed that these phenotypes may underlie learning, memory, and behavioral deficits in humans with FASD. Here, we evaluate the impact of PrEE on fear memory learning, recall and amygdala development at two adult timepoints. PrEE alters learning and memory of aversive stimuli; specifically, PrEE mice, fear conditioned at postnatal day (P) 50, showed deficits in fear acquisition and memory retrieval when tested at P52 and later at P70–P72. Interestingly, this deficit in fear acquisition observed during young adulthood was not present when PrEE mice were conditioned later, at P80. These mice displayed similar levels of fear expression as controls when tested on fear memory recall. To test whether PrEE alters development of brain circuitry associated with fear conditioning and fear memory recall, we histologically examined subdivisions of the amygdala in PrEE and control mice and found long-term effects of PrEE on fear memory circuitry. Thus, results from this study will provide insight on the neurobiological and behavioral effects of PrEE and provide new information on developmental trajectories of brain dysfunction in people prenatally exposed to ethanol.

Altered metabolic activity in the developing brain of rats predisposed to high versus low depression-like behavior

Individual differences in human temperament can increase the risk of psychiatric disorders like depression and anxiety. Our laboratory utilized a rat model of temperamental differences to assess neurodevelopmental factors underlying emotional behavior differences. Rats selectively bred for low novelty exploration (Low Responders, LR) display high levels of anxiety- and depression-like behavior compared to High Novelty Responder (HR) rats. Using transcriptome profiling, the present study uncovered vast gene expression differences in the early postnatal HR versus LR limbic brain, including changes in genes involved in cellular metabolism. These data led us to hypothesize that rats prone to high (versus low) anxiety/depression-like behavior exhibit distinct patterns of brain metabolism during the first weeks of life, which may reflect disparate patterns of synaptogenesis and brain circuit development. Thus, in a second experiment we examined activity of cytochrome C oxidase (COX), an enzyme responsible for ATP production and a correlate of metabolic activity, to explore functional energetic differences in the HR/LR early postnatal brain. We found that HR rats display higher COX activity in the amygdala and specific hippocampal subregions compared to LRs during the first 2 weeks of life. Correlational analysis examining COX levels across several brain regions and multiple early postnatal time points suggested desynchronization in the developmental timeline of the limbic HR versus LR brain during the first two postnatal weeks. These early divergent COX activity levels may reflect altered circuitry or synaptic activity in the early postnatal HR/LR brain, which could contribute to the emergence of their distinct behavioral phenotypes.

Effects of neonatal amygdala lesions on fear learning, conditioned inhibition, and extinction in adult macaques

Fear conditioning studies have demonstrated the critical role played by the amygdala in emotion processing. Although all lesion studies until now investigated the effect of adult-onset damage on fear conditioning, the current study assessed fear-learning abilities, as measured by fear-potentiated startle, in adult monkeys that had received neonatal neurotoxic amygdala damage or sham-operations. After fear acquisition, their abilities to learn and use a safety cue to modulate their fear to the conditioned cue, and, finally, to extinguish their response to the fear conditioned cue were measured with the AX+/BX- Paradigm. Neonatal amygdala damage retarded, but did not completely abolish, the acquisition of a learned fear. After acquisition of the fear signal, four of the six animals with neonatal amygdala lesions discriminated between the fear and safety cues and were also able to use the safety signal to reduce the potentiated-startle response and to extinguish the fear response when the air-blast was absent. In conclusion, the present results support the critical contribution of the amygdala during the early phases of fear conditioning that leads to quick, robust responses to potentially threatening stimuli, a highly adaptive process across all species and likely to be present in early infancy. The neonatal amygdala lesions also indicated the presence of amygdala-independent alternate pathways that are capable to support fear learning in the absence of a functional amygdala. This parallel processing of fear responses within these alternate pathways was also sufficient to support the ability to flexibly modulate the magnitude of the fear responses.

Dendritic morphology of neurons in prefrontal cortex and ventral hippocampus of rats with neonatal amygdala lesion

Neonatal basolateral amygdala (nBLA) lesions in rats have been widely used as a neurodevelopmental model that mimics schizophrenia-like behaviors. Recently, we reported that nBLA lesions result in significant decreases in the dendritic spine number of layer 3 prefrontal cortex (PFC) pyramidal cells and medium spiny neurons of the nucleus accumbens (NAcc), which all changes after puberty. At present, we aimed to evaluate the effect of this lesion in pyramidal neurons of CA1 of the ventral hippocampus (VH) and layer 5 of the PFC. In order to assess the effects of nBLA lesions on the dendritic morphology of the PFC and VH neurons, we carried out nBLA lesions in rats on postnatal day (PD) 7, and then we studied the dendritic morphology of these two limbic subregions at prepubertal (PD35) and postpubertal (PD60) ages. Dendritic characteristics were measured by Golgi-Cox procedure followed by Sholl analysis. We also evaluated the effects of nBLA lesions on the prepulse inhibition (PPI) and acoustic startle responses. The nBLA lesion induced a significant increase in dendritic length of layer 5 pyramidal neurons of the PFC at both ages, with a decrease in the dendritic spines density after puberty. The spine density of CA1 VH pyramidal neurons showed significant decreases at both ages. PPI was decreased in adulthood in the animals with an nBLA lesion. These results show that an nBLA lesion alters the dendritic morphology at the level of the PFC and VH in distinct ways before puberty, suggesting a disconnection between these limbic structures at an early age, and increasing our understanding of the implications of the VH in early amygdala dysfunction in schizophrenia.

Changes in cortical morphology resulting from long-term amygdala damage

The amygdala's contribution to emotion, cognition and behavior depends on its interactions with subcortical and cortical regions. Amygdala lesions result in altered functional activity in connected regions, but it is not known whether there might be long-term structural sequelae as well. We hypothesized that developmental bilateral amygdala lesions would be associated with specific gray matter morphometric abnormalities in the ventromedial prefrontal cortex (vmPFC), anterior cingulate cortex (ACC) and the ventral visual stream. We conducted regions of interest and vertex-based analyses of structural MRI data acquired in two patients with long-standing focal bilateral amygdala lesions (S.M. and A.P.), compared to gender- and age-matched healthy comparison subjects. Both patients showed significant proportional increases in gray matter volume of the vmPFC. Cortical thickness was increased in the vmPFC and ACC and decreased in the ventral visual stream. There were no morphometric changes in dorsolateral prefrontal cortex or dorsal visual stream cortices. These findings support the hypothesis that cortical regions strongly connected with the amygdala undergo morphometric changes with long-standing amygdala damage. This is the first evidence in humans of the remote alteration of brain morphology in association with amygdala lesions, and will help in interpreting the structural and functional consequences of amygdala pathology in neuropsychiatric disorders.

Alcohol-induced neuronal death in central extended amygdala and pyriform cortex during the postnatal period of the rat

Mothers who consume alcohol during pregnancy may cause a neurotoxic syndrome defined as fetal alcohol spectrum disorder (FASD) in their offspring. This disorder is characterized by reduction in brain size, cognitive deficits and emotional/social disturbances. These alterations are thought to be caused by an alcohol-induced increase in apoptosis during neurodevelopment. Little is known about neuroapoptosis in the central extended amygdala and the pyriform cortex, which are key structures in emotional/social behaviors. The goal of this study was to determine the vulnerability of neuroapoptotic alcohol effects in those areas. Rats were administered alcohol (2.5g/kg s.c. at 0 and 2h) or saline on postnatal day (PND) 7, 15 and 20. The Amino-cupric-silver technique was used to evaluate neurodegeneration and immunohistochemistry to detect activated caspases 3-8 and 9 at 2h, 4, 6, 8, 12 and 24h after drug administration. We measured blood alcohol levels each hour, from 2 to 8h post second administration of alcohol in each of the ages studied. Results showed alcohol induced apoptotic neurodegeneration in the central extended amygdala on PND 7 and 15, and pyriform cortex on PND 7, 15 and 20. These structures showed activation of caspase 3 and 9 but not of caspase 8 suggesting that alcohol-induced apoptosis could occur by the intrinsic pathway. The pharmacokinetic differences between ages did not associate with the neurodegeneration age dependence. In conclusion, these limbic areas are damaged by alcohol, and each one has their own window of vulnerability during the postnatal period. The possible implications in emotional/social features in FASD are discussed.

Neonatal amygdala lesions: co-occurring impact on social/fear-related behavior and cocaine sensitization in adult rats

Neurodevelopmental abnormalities of temporal-limbic structures may underlie both adult psychiatric syndromes and increased addiction vulnerability, leading to high frequencies of "dual diagnosis" disorders. Although the amygdala is implicated in various mental disorders and drug addiction, no studies have explored the impact of early developmental damage to the amygdala on phenotypes relating to mental illness and addictions as co-occurring processes. We tested rats with neonatal amygdala lesions (NAML) vs. SHAM-operated controls in a battery of tests--novel field activity, elevated plus maze (EPM), and social interaction (SI) at baseline and after odor and restraint stress--followed by measures of cocaine sensitization (15 mg/kg vs. saline x 5 days + challenge session 2 weeks later) and remeasurement of SI. NAMLs showed increased novelty-related locomotion, less fear responding in the EPM, and resistance to predator-odor- but not to restraint-induced suppression of SI. NAMLs also had elevated cocaine sensitization profiles, and cocaine history differentially affected subsequent SI in NAMLs compared with SHAMs. NAMLs may provide models for understanding a shared neurobiological basis for and complex interactions among psychiatric symptoms, drug exposure history, and addiction vulnerability.

Effects of neonatal amygdala or hippocampus lesions on resting brain metabolism in the macaque monkey: a microPET imaging study

Longitudinal analysis of animals with neonatal brain lesions enables the evaluation of behavioral changes during multiple stages of development. Interpretation of such changes, however, carries the caveat that permanent neural injury also yields morphological and neurochemical reorganization elsewhere in the brain that may lead either to functional compensation or to exacerbation of behavioral alterations. We have measured the long-term effects of selective neonatal brain damage on resting cerebral glucose metabolism in nonhuman primates. Sixteen rhesus monkeys (Macaca mulatta) received neurotoxic lesions of either the amygdala (n=8) or hippocampus (n=8) when they were two weeks old. Four years later, these animals, along with age- and experience-matched sham-operated control animals (n=8), were studied with high-resolution positron emission tomography (microPET) and 2-deoxy-2[(18)F]fluoro-d-glucose ([(18)F]FDG) to detect areas of altered metabolism. The groups were compared using an anatomically-based region of interest analysis. Relative to controls, amygdala-lesioned animals displayed hypometabolism in three frontal lobe regions, as well as in the neostriatum and hippocampus. Hypermetabolism was also evident in the cerebellum of amygdala-lesioned animals. Hippocampal-lesioned animals only showed hypometabolism in the retrosplenial cortex. These results indicate that neonatal amygdala and hippocampus lesions induce very different patterns of long-lasting metabolic changes in distant brain regions. These observations raise the possibility that behavioral alterations in animals with neonatal lesions may be due to the intended damage, to consequent brain reorganization or to a combination of both factors.