Modulating BLA-vHPC Synapses to Reduce Anxiety-Like Behavior in Mice

Abstract

Although anxiety disorders are some of the most prevalent mental health conditions, current pharmacotherapies are often ineffective or associated with negative side effects because the neural mechanisms underlying anxiety are not well understood. Previous studies have identified the amygdala and ventral hippocampus (vHPC) as two of the major structures involved in anxiety. The Tye Lab has used optogenetic techniques to show that acute activation of basolateral amygdala (BLA) neurons that project to the vHPC increases anxiety-like behavior in mice, while inhibition of these neurons decreases such behavior. We sought to build on this work to test whether these anxiolytic effects can be sustained long-term. To manipulate neurons, we utilized stabilized step-function opsin (SSFO), a light sensitive cation channel that upon stimulation with a single pulse of light, produces a sustained activation of cells. It is well established that significant increases or decreases in neural activity trigger homeostatic mechanisms that alter the excitability of cells, and we hypothesized that by leveraging this phenomenon we could alter anxiety-related circuits in a long lasting manner. Using a dual viral infection technique, we expressed SSFO tagged with an enhanced yellow fluorescent protein (eYFP) fluorophore only in those BLA cells that projected to the vHPC. Control mice received virus containing eYFP alone. Stereotactic surgery was used to inject the viruses bilaterally into the appropriate brain regions of adult wild-type C57BL/6J mice. Optical fibers were implanted over the BLA, and one minute pulses of blue light (473 nm) were delivered to the mice twice a day for five consecutive days. Two well-validated behavioral assays, the elevated plus maze and the open field test, were used to measure anxiety-like behaviors in these mice. Testing was conducted 7, 30 and 90 days following stimulation and we observed a decrease in anxiety-like behavior in SSFO mice at all three time points, suggesting that reprogramming of the BLA-vHPC circuit had occurred. In order to characterize the synaptic changes that mediated this behavioral phenomenon, we also used dendritic spine analysis and patch clamp techniques to measure mini excitatory post synaptic currents (EPSCs). We found that after SSFO stimulation, BLA-vHPC cells demonstrated lower frequencies of EPSCs and reduced spine densities. Thus, it appears that the changes in synaptic strength mediated through the reduction of functional synapses and excitatory neurotransmitter receptors on BLA-vHPC cells could be some of the mechanisms underlying the anxiolytic effects observed upon SSFO stimulation.