ROLE OF SOMATOSTATIN-POSITIVE GABAERGIC NEURONS IN ANXIETY AND DEPRESSION-RELATED PHENOTYPES

Open Access
- Author:
- Jefferson, Sarah J
- Graduate Program:
- Neuroscience
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 11, 2018
- Committee Members:
- Bernhard Luscher, Dissertation Advisor/Co-Advisor
Bernhard Luscher, Committee Chair/Co-Chair
Timothy J Jegla, Committee Member
Melissa Rolls, Committee Member
Sonia Angele Cavigelli, Outside Member - Keywords:
- Depression
GABA
Somatostatin
Interneuron
Anxiety
Stress - Abstract:
- Major Depressive Disorder (MDD) is a highly prevalent and debilitating psychiatric syndrome marked by depressed mood and lack of interest. Current antidepressant drugs require several weeks of treatment to achieve therapeutic efficacy and most patients do not achieve remission with the first antidepressant prescribed. These limitations of current antidepressant drugs highlight the need for a better understanding of the pathophysiology of MDD and identification of novel therapeutic targets. Clinical evidence suggests that MDD is associated with reduced concentrations of the inhibitory neurotransmitter γ-aminobutyric acid (GABA) and altered expression of GABA type A receptors (GABAARs) in brain regions that are implicated in the pathophysiology of MDD. Somatostatin (SST), a neuropeptide marker of a major subset of primarily dendrite-targeting GABAergic interneurons, is also reduced in many of the same brain regions of depressed patients. Reducing GABAergic synaptic transmission in mice by hemizygously deleting the γ2 subunit of GABAARs results in an anxious-depressive-like phenotype that includes hypothalamic-pituitary-adrenal (HPA) axis hyperactivity and is normalized by treatment with currently used antidepressants. We show here that this phenotype is also normalized with a subanesthetic dose of the rapid-acting antidepressant ketamine and that ketamine potentiates inhibitory synapses in these mice in a region-specific manner. This finding is specific to γ2+/- mice and suggests that stimulation of specific inhibitory synapses contributes to anxiolytic and antidepressant-like phenotypes. Based on these findings and the fact that SST expression is reduced in MDD patients, we predicted that disinhibition of SST-positive (SST+) GABAergic interneurons through deletion of the γ2 subunit of GABAARs selectively in these neurons (SSTCre:γ2f/f mice) would result in an antidepressant-like phenotype. Indeed, these mice show an anxiolytic and antidepressant-like phenotype, along with enhanced GABAergic input to principal cells. Biochemical analyses of extracts from the hippocampus and medial prefrontal cortex (mPFC) of these mice further show reduced phosphorylation of eukaryotic elongation factor 2 (eEF2), consistent with corresponding changes in eEF2 phosphorylation upon treatment of rodents with the antidepressants fluoxetine, ketamine and 5-HT2C antagonists and suggestive of altered dendritic translation. Based on these findings we hypothesized that altered protein translation, particularly in dendrites of principal cells in the hippocampus and mPFC, may underlie the phenotype of SSTCre:γ2f/f mice that mimics antidepressant drug treatment. To identify targets of differential translation in principal cells of SSTCre:γ2f/f mice, we used cell type-specific isolation of ribosome-associated RNAs coupled with RNAseq to measure actively translated messenger RNAs from CA1 pyramidal neurons. We show that in SSTCre:γ2f/f mice there is altered translation of mRNAs thought to be involved in neuroprotection, synaptogenesis, neurogenesis and neurotransmitter regulation, including GPR37l1, Psd2, Cbln1, Snap23, Grip2ip, and Erbb4. Furthermore GPR37l1 protein, which is normally expressed mostly in glia in the forebrain, is highly upregulated in CA1 pyramidal neurons from SSTCre:γ2f/f mice. This receptor, which is thought to be involved in cell survival, may be involved in antidepressant phenotypes by initiating neuroprotective mechanisms. Since stress is an important risk factor for MDD and is known to impair the function of SST+ neurons, we wanted to determine whether SSTCre:γ2f/f mice were resilient to anxious and depressive-like phenotypes induced by chronic stress. Using the unpredictable chronic mild stress (UCMS) paradigm, we show that male SSTCre:γ2f/f mice are resilient to stress at the level of synaptic function in that they show no reduction in frequency of inhibitory currents on pyramidal neurons. They are also resilient to chronic stress-induced increases in eEF2 phosphorylation in the hippocampus and some of the stress-induced anxious and depressive-like behaviors observed in normal mice. Female SSTCre:γ2f/f mice, however, remain sensitive to stress-induced behavioral changes and resilience is only evident in SST neuron density. Others have shown that, in mice, chronic stress leads to decreased transcription of genes in the eukaryotic initiation factor 2α (eIF2α) pathway in SST+ neurons, a pathway that is regulated by inhibitory phosphorylation of eIF2α by PKR-like endoplasmic reticulum kinase (PERK). They also showed that systemic pharmacological PERK inhibition protected mice from chronic stress-induced anxious behaviors. We have begun to address the effects of cell-type specific PERK knockout on behavior to determine whether PERK inhibition in SST+ neurons protects against stress-induced behavioral deficits. We find that knockout of PERK in SST+ neurons (SSTCre:PERKf/f mice) or glutamatergic neurons (CAMKIICre:PERKf/f mice) increases despair-like behavior in the learned helplessness test (LHT), which assesses failure to escape a stressful stimulus after exposure to subchronic inescapable stress. Taken together, the data presented in this dissertation point to a role for potentiation of specific inhibitory synapses, such as distal dendritic synapses targeted by SST+ neurons, in antidepressant biochemical and behavioral phenotypes and resilience to chronic stress.