Effects of optogenetic photoexcitation of infralimbic cortex inputs to the basolateral amygdala on conditioned fear and extinction

Abstract

Deficiencies in the ability to extinguish fear is a hallmark of Trauma- and stressor-related disorders, Anxiety disorders, and certain other neuropsychiatric conditions. Hence, a greater understanding of the brain mechanisms involved in the inhibition of fear is of significant translational relevance. Previous studies in rodents have shown that glutamatergic projections from the infralimbic prefrontal cortex (IL) to basolateral amygdala (BLA) play a crucial instructional role in the formation of extinction memories, and also indicate that variation in the strength of this input correlates with extinction efficacy. To further examine the relationship between the IL→BLA pathway and extinction we expressed three different titers of the excitatory opsin, channelrhodopsin (ChR2), in IL neurons and photostimulated their projections in the BLA during partial extinction training. The behavioral effects of photoexcitation differed across the titer groups: the low titer had no effect, the medium titer selectively facilitated extinction memory formation, and the high titer produced both an acute suppression of fear and a decrease in fear during (light-free) extinction retrieval. We discuss various possible explanations for these titer-specific effects, including the possibility of IL-mediated inhibition of BLA fear-encoding neurons under conditions of sufficiently strong photoexcitation. These findings further support the role of IL→BLA pathway in regulating fear and highlight the importance of methodological factors in optogenetic studies of neural circuits underling behavior.

Keywords: Basolateral amygdala; Channelrhodopsin-2; Extinction; Fear; Infralimbic cortex; Optogenetics; Prefrontal cortex.

Figure 1:. BLA expression of ChR2-YFP fluorescence from IL projections.

Representative images of YFP expression in the BLA following injection of low (A), medium (B) or high (C) titers of a YFP-fused ChR2-containing AAV construct into the IL. All 3 titer groups had optical fibers chronically implanted in BLA to shine blue light onto the transfected IL projections in BA. Scale bars = 50 μm.

Figure 2:. Effects of photoexcitation of the IL→BLA pathway on fear and extinction.

(A) Schematic of experimental design showing photoexcitation of the IL→BLA pathway during CS presentation during partial (10-trial) extinction training. (B) Relative to YFP controls photoexcitation of IL→BLA pathway decreased freezing on the first trial-block of extinction training and on subsequent (light-free) retrieval in mice expressing a high-titer viral construct, decreased freezing on retrieval only in medium titer expressing mice, and was without effect at any stage of testing in low titer expressing mice. (C) Freezing during first trial-block of extinction training inversely correlated with fluorescence intensity in the BA. Data are means ± SEM. eYFP n=6, high titer n=5, medium n=5, low n=8. *P<0.05 versus YFP.