Jessica Ihne

Strain Differences in Stress Responsivity Are Associated with Divergent Amygdala Gene Expression and Glutamate-Mediated Neuronal Excitability

Stress is a major risk factor for numerous neuropsychiatric diseases. However, susceptibility to stress and the qualitative nature of stress effects on behavior differ markedly among individuals. This is partly because of the moderating influence of genetic factors. Inbred mouse strains provide a relatively stable and restricted range of genetic and environmental variability that is valuable for disentangling gene–stress interactions. Here, we screened a panel of inbred strains for anxiety- and depression-related phenotypes at baseline (trait) and after exposure to repeated restraint. Two strains, DBA/2J and C57BL/6J, differed in trait and restraint-induced anxiety-related behavior (dark/light exploration, elevated plus maze). Gene expression analysis of amygdala, medial prefrontal cortex, and hippocampus revealed divergent expression in DBA/2J and C57BL/6J both at baseline and after repeated restraint. Restraint produced strain-dependent expression alterations in various genes including glutamate receptors (e.g., Grin1, Grik1). To elucidate neuronal correlates of these strain differences, we performed ex vivo analysis of glutamate excitatory neurotransmission in amygdala principal neurons. Repeated restraint augmented amygdala excitatory postsynaptic signaling and altered metaplasticity (temporal summation of NMDA receptor currents) in DBA/2J but not C57BL/6J. Furthermore, we found that the C57BL/6J-like changes in anxiety-related behavior after restraint were absent in null mutants lacking the modulatory NMDA receptor subunit Grin2a, but not the AMPA receptor subunit Gria1. Grin2a null mutants exhibited significant (∼30%) loss of dendritic spines on amygdala principal neurons under nonrestraint conditions. Collectively, our data support a model in which genetic variation in glutamatergic neuroplasticity in corticolimbic circuitry underlies phenotypic variation in responsivity to stress.

Genetic Strain Differences in Learned Fear Inhibition Associated with Variation in Neuroendocrine, Autonomic, and Amygdala Dendritic Phenotypes

Mood and anxiety disorders develop in some but not all individuals following exposure to stress and psychological trauma. However, the factors underlying individual differences in risk and resilience for these disorders, including genetic variation, remain to be determined. Isogenic inbred mouse strains provide a valuable approach to elucidating these factors. Here, we performed a comprehensive examination of the extinction-impaired 129S1/SvImJ (S1) inbred mouse strain for multiple behavioral, autonomic, neuroendocrine, and corticolimbic neuronal morphology phenotypes. We found that S1 exhibited fear overgeneralization to ambiguous contexts and cues, impaired context extinction and impaired safety learning, relative to the (good-extinguishing) C57BL/6J (B6) strain. Fear overgeneralization and impaired extinction was rescued by treatment with the front-line anxiety medication fluoxetine. Telemetric measurement of electrocardiogram signals demonstrated autonomic disturbances in S1 including poor recovery of fear-induced suppression of heart rate variability. S1 with a history of chronic restraint stress displayed an attenuated corticosterone (CORT) response to a novel, swim stressor. Conversely, previously stress-naive S1 showed exaggerated CORT responses to acute restraint stress or extinction training, insensitivity to dexamethasone challenge, and reduced hippocampal CA3 glucocorticoid receptor mRNA, suggesting downregulation of negative feedback control of the hypothalamic–pituitary–adrenal axis. Analysis of neuronal morphology in key neural nodes within the fear and extinction circuit revealed enlarged dendritic arbors in basolateral amygdala neurons in S1, but normal infralimbic cortex and prelimbic cortex dendritic arborization. Collectively, these data provide convergent support for the utility of the S1 strain as a tractable model for elucidating the neural, molecular and genetic basis of persistent, excessive fear.

Effects of subchronic phencyclidine (PCP) treatment on social behaviors, and operant discrimination and reversal learning in C57BL/6J mice

Subchronic treatment with the psychotomimetic phencyclidine (PCP) has been proposed as a rodent model of the negative and cognitive/executive symptoms of schizophrenia. There has, however, been a paucity of studies on this model in mice, despite the growing use of the mouse as a subject in genetic and molecular studies of schizophrenia. In the present study, we evaluated the effects of subchronic PCP treatment (5 mg/kg twice daily × 7 days, followed by 7 days withdrawal) in C57BL/6J mice on (1) social behaviors using a sociability/social novelty-preference paradigm, and (2) pairwise visual discrimination and reversal learning using a touchscreen-based operant system. Results showed that mice subchronically treated with PCP made more visits to (but did not spend more time with) a social stimulus relative to an inanimate one, and made more visits and spent more time investigating a novel social stimulus over a familiar one. Subchronic PCP treatment did not significantly affect behavior in either the discrimination or reversal learning tasks. These data encourage further analysis of the potential utility of mouse subchronic PCP treatment for modeling the social withdrawal component of schizophrenia. They also indicate that the treatment regimen employed was insufficient to impair our measures of discrimination and reversal learning in the C57BL/6J strain. Further work will be needed to identify alternative methods (e.g., repeated cycles of subchronic PCP treatment, use of different mouse strains) that reliably produce discrimination and/or reversal impairment, as well as other cognitive/executive measures that are sensitive to chronic PCP treatment in mice.

A novel role for PSD-95 in mediating ethanol intoxication, drinking and place preference.

The synaptic signaling mechanisms mediating the behavioral effects of ethanol (EtOH) remain poorly understood. Post‐synaptic density 95 (PSD‐95, SAP‐90, Dlg4) is a key orchestrator of N‐methyl‐D‐aspartate receptors (NMDAR) and glutamatergic synapses, which are known to be major sites of EtOHs behavioral actions. However, the potential contribution of PSD‐95 to EtOH‐related behaviors has not been established. Here, we evaluated knockout (KO) mice lacking PSD‐95 for multiple measures of sensitivity to the acute intoxicating effects of EtOH (ataxia, hypothermia, sedation/hypnosis), EtOH drinking under conditions of free access and following deprivation, acquisition and long‐term retention of EtOH conditioned place preference (CPP) (and lithium chloride‐induced conditioned taste aversion), and intoxication‐potentiating responses to NMDAR antagonism. PSD‐95 KO exhibited increased sensitivity to the sedative/hypnotic, but not ataxic or hypothermic, effects of acute EtOH relative to wild‐type controls (WT). PSD‐95 KO consumed less EtOH than WT, particularly at higher EtOH concentrations, although increases in KO drinking could be induced by concentration‐fading and deprivation. PSD‐95 KO showed normal EtOH CPP 1 day after conditioning, but showed significant aversion 2 weeks later. Lithium chloride‐induced taste aversion was impaired in PSD‐95 KO at both time points. Finally, the EtOH‐potentiating effects of the NMDAR antagonist MK‐801 were intact in PSD‐95 KO at the dose tested. These data reveal a major, novel role for PSD‐95 in mediating EtOH behaviors, and add to growing evidence that PSD‐95 is a key mediator of the effects of multiple abused drugs.

Convergent effects of mouse Pet-1 deletion and human PET-1 variation on amygdala fear and threat processing

Serotonin is critical for shaping the development of neural circuits regulating emotion. Pet-1 (FEV-1) is an ETS-domain transcription factor essential for differentiation and forebrain targeting of serotonin neurons. Constitutive Pet-1 knockout (KO) causes major loss of serotonin neurons and forebrain serotonin availability, and behavioral abnormalities. We phenotyped Pet-1 KO mice for fear conditioning and extinction, and on a battery of assays for anxiety- and depression-related behaviors. Morphology of Golgi-stained neurons in basolateral amygdala (BLA) and prelimbic cortex was examined. Using human imaging genetics, a common variant (rs860573) in the PET-1 (FEV) gene was tested for effects on threat-related amygdala reactivity and psychopathology in 88 Asian-ancestry subjects. Pet-1 KO mice exhibited increased acquisition and expression of fear, and elevated fear recovery following extinction, relative to wild-type (WT). BLA dendrites of Pet-1 KO mice were significantly longer than in WT. Human PET-1 variation associated with differences in amygdala threat processing and psychopathology. This novel evidence for the role of Pet-1 in fear processing and dendritic organization of amygdala neurons and in human amygdala threat processing extends a growing literature demonstrating the influence of genetic variation in the serotonin system on emotional regulation via effects on structure and function of underlying corticolimbic circuitry.

Tolerance to ethanol intoxication after chronic ethanol: role of GluN2A and PSD-95

The neural and genetic factors underlying chronic tolerance to alcohol are currently unclear. The GluN2A N‐methyl‐D‐aspartate receptors (NMDAR) subunit and the NMDAR‐anchoring protein PSD‐95 mediate acute alcohol intoxication and represent putative mechanisms mediating tolerance. We found that chronic intermittent ethanol exposure (CIE) did not produce tolerance [loss of righting reflex (LORR)] or withdrawal‐anxiety in C57BL/6J, GluN2A or PSD‐95 knockout mice assayed 2–3 days later. However, significant tolerance to LORR was evident 1 day after CIE in C57BL/6J and PSD‐95 knockouts, but absent in GluN2A knockouts. These data suggest a role for GluN2A in tolerance, extending evidence that human GluN2A gene variation is involved in alcohol dependence.

Tolerance to ethanol intoxication after chronic ethanol

The neural and genetic factors underlying chronic tolerance to alcohol are currently unclear. The GluN2A N‐methyl‐D‐aspartate receptors (NMDAR) subunit and the NMDAR‐anchoring protein PSD‐95 mediate acute alcohol intoxication and represent putative mechanisms mediating tolerance. We found that chronic intermittent ethanol exposure (CIE) did not produce tolerance [loss of righting reflex (LORR)] or withdrawal‐anxiety in C57BL/6J, GluN2A or PSD‐95 knockout mice assayed 2–3 days later. However, significant tolerance to LORR was evident 1 day after CIE in C57BL/6J and PSD‐95 knockouts, but absent in GluN2A knockouts. These data suggest a role for GluN2A in tolerance, extending evidence that human GluN2A gene variation is involved in alcohol dependence.

Tolerance to ethanol intoxication after chronic ethanol: role of GluN2A and PSD-95: Role of GluN2A and PSD-95

The neural and genetic factors underlying chronic tolerance to alcohol are currently unclear. The GluN2A N‐methyl‐D‐aspartate receptors (NMDAR) subunit and the NMDAR‐anchoring protein PSD‐95 mediate acute alcohol intoxication and represent putative mechanisms mediating tolerance. We found that chronic intermittent ethanol exposure (CIE) did not produce tolerance [loss of righting reflex (LORR)] or withdrawal‐anxiety in C57BL/6J, GluN2A or PSD‐95 knockout mice assayed 2–3 days later. However, significant tolerance to LORR was evident 1 day after CIE in C57BL/6J and PSD‐95 knockouts, but absent in GluN2A knockouts. These data suggest a role for GluN2A in tolerance, extending evidence that human GluN2A gene variation is involved in alcohol dependence.