AN INVESTIGATION OF THE MOLECULAR AND CELLULAR SUBSTRATES OF ANXIETY AND DEPRESSION-LIKE DISORDERS

Open Access
Author:
Earnheart, John Clinton
Graduate Program:
Biochemistry, Microbiology, and Molecular Biology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
February 28, 2007
Committee Members:
  • Bernhard Luscher, Committee Chair
  • Graham Hugh Thomas, Committee Member
  • Wendy Hanna Rose, Committee Member
  • Randen Patterson, Committee Member
  • Byron C Jones, Committee Member
Keywords:
  • hippocampal neurogenesis
  • brain development
  • anxiety disorder
  • conditional knockout mouse
  • depression
Abstract:
In humans, heightened trait anxiety is a vulnerability factor for diverse psychiatric disorders, especially generalized anxiety disorder (GAD) and major depression. Besides multiple genetic factors, environmental factors such as early life stress are believed to increase vulnerability for anxiety and mood disorders. Early life stress has been shown to modulate the stress response in adulthood through changes in gene expression and changes in neurotransmitter systems. Stress, including especially early life stress, is also a potent inhibitor of adult hippocampal neurogenesis, a mechanism that has been implicated in the etiology of depression. Conversely, deficits in neurotransmission, including neurotransmission by ?-aminobutyric acid (GABA), in the cortex and hippocampus are implicated in the etiology of anxiety and depressive disorders. GABA type A (GABAA) receptors are key control elements of anxiety states based on the anxiolytic properties of benzodiazepines (BZs), which act as allosteric GABAA receptor agonists. Heterozygous deletion of the ?2 subunit of GABAA receptors results in modest but significant deficits in BZ binding sites as well as altered channel conductance, deficits in trafficking and clustering of GABAA receptors at postsynaptic sites. However, there is no change in the number of GABA binding sites indicating that the number of GABAA receptors was unaltered in these mice. This subtle deficit in GABAA receptors is associated with behavioral, pharmacological and cognitive alterations indicative of elevated trait anxiety. The ?2+/- mice exhibit neophobia and marked behavioral avoidance to natural or learned stressors. This phenotype is reversed to wildtype (wt) levels following BZ treatment reflecting sensitivity of pathologically anxious patients to anxiolytics. Altered behavior of ?2+/? mice is associated with cognitive deficits including enhanced 1-s trace conditioning and impaired ambiguous cue conditioning but unimpaired spatial memory. In the present thesis, cell-type-specific and developmentally controlled inactivation of the ?2 subunit gene was used to further analyze the mechanism underlying anxiety-like behavior of ?2+/? mice. Recombination of a ‘floxed’ ?2 subunit (f?2) allele in either of two different Cre-expressing mouse lines results in a reduction of GABAA receptors specifically in glutamatergic neurons of the forebrain. In adulthood, Emx1Cre x f?2/+ and CaMKIICre2834 x f?2/+ mice exhibited comparable deficits in BZ binding in the forebrain. Specifically, both mouse lines showed substantial reductions in BZ binding in the forebrain, including the cortex, CA1 region of the hippocampus and striatum, with less severe deficits in the amygdala and the dentate gyrus. However, in Emx1Cre mice Cre-mediated recombination occurs in immature glutamatergic neurons of the embryonic and adult forebrain while in CaMKIICre2834 mice recombination is developmentally delayed to mature neurons of the adult brain. The GABAA receptor deficits in immature glutamatergic neurons of the embryonic forebrain resulted in heightened anxiety-like behavior similar of that seen previously in ?2+/? mice. This included increased risk assessment behavior and neophobia in paradigms without an implicit threat and increased avoidance to naturally aversive stimuli. In addition, Emx1Cre x f?2/+ mice and ?2+/? mice showed increased behavioral inhibition in paradigms with predictive validity for antidepressant drug effects in humans. By contrast, the developmentally delayed GABAA receptor deficit of CaMKIICre x f?2/+ mice was without behavioral effect. Thus, the first three weeks of life represent a GABAA receptor dependent critical period in the establishment of normal emotionality of mice. In addition to behavioral deficits, ?2 subunit inactivation in immature forebrain glutamatergic neurons was associated with significant deficits in adult hippocampal neurogenesis. Decreases in adult born neurons in the adult hippocampus were associated with normal cell proliferation, but a reduction in the number of cells expressing mature neuronal markers. Reductions in neurogenesis were characterized by normal proliferation and initial neuronal differientiation, indicating a selective vulnerability of maturing neurons to modest functional deficits in GABAA receptors. By contrast, a developmentally delayed GABAA receptor deficit in mature glutamatergic neurons of the forebrain did not affect adult hippocampal neurogenesis. Therefore, normal emotionality and adult hippocampal neurogenesis have in common that they are dependent on developmental GABAA receptor in immature glutamatergic neurons. Thus, GABAA receptor deficits in immature neurons of the developing and/or adult brain are implicated in the etiology of heightened negative emotionality, which in humans is associated with generalized anxiety disorder and major depression.