Successfully differentiating safety from danger is an essential skill for survival. While decreased activity in the medial prefrontal cortex (mPFC) is associated with fear generalization in animals and humans, the circuit level mechanisms used by the mPFC to discern safety are not clear. To answer this question, we recorded activity in the mPFC, basolateral amygdala (BLA), and dorsal (dHPC) and ventral hippocampus (vHPC) in mice during exposure to learned (differential fear conditioning) and innate (open field) anxiety. We found increased synchrony between the mPFC and BLA in the theta frequency range (4–12 Hz) only in animals that differentiate between averseness and safety. Moreover, during recognized safety across learned and innate paradigms, BLA firing becomes entrained to theta input from the mPFC. These data suggest that selective tuning of BLA firing to mPFC input provides a safety-signaling mechanism whereby the mPFC taps into the microcircuitry of the amygdala to diminish fear.

The mediodorsal thalamus (MD) shares reciprocal connectivity with the prefrontal cortex (PFC) and decreased MD-PFC connectivity is observed in schizophrenia patients. Patients also display cognitive deficits including impairments in working memory, but a mechanistic link between thalamo-prefrontal circuit function and working memory is missing. Here, using pathway-specific inhibition we found directional interactions between MD and medial PFC (mPFC), with MD-to-mPFC supporting working memory maintenance and mPFC-to-MD supporting subsequent choice. We further identify mPFC neurons that display elevated spiking during the delay, a feature that was absent on error trials and required MD inputs for sustained maintenance. Strikingly, delay-tuned neurons had minimal overlap with spatially-tuned neurons and each mPFC population exhibited mutually exclusive dependence on MD and hippocampal inputs. These findings indicate a role for the MD in sustaining prefrontal activity during working memory maintenance. Consistent with this idea we found that enhancing MD excitability was sufficient to enhance task performance.

Summary Despite the increasing use of optogenetics in vivo, the effects of direct light exposure to brain tissue are understudied. Of particular concern is the potential for heat induced by prolonged optical stimulation. We demonstrate that high intensity light, delivered through an optical fiber, is capable of elevating firing rate locally, even in the absence of opsin expression. Predicting the severity and spatial extent of any temperature increase during optogenetic stimulation is therefore of considerable importance. Here we describe a realistic model that simulates light and heat propagation during optogenetic experiments. We validated the model by comparing predicted and measured temperature changes in vivo. We further demonstrate the utility of this model by comparing predictions for various wavelengths of light and fiber sizes, as well as testing methods for reducing heat effects on neural targets in vivo.

Summary Theta oscillations synchronize the basolateral amygdala (BLA) with the hippocampus (HPC) and medial prefrontal cortex (mPFC) during fear expression. The role of gamma-frequency oscillations in the BLA is less well characterized. We examined gamma- and theta-frequency activity in recordings of neural activity from the BLA-HPC-mPFC circuit during fear conditioning, extinction, and exposure to an open field. In the BLA, slow (40-70 Hz) and fast (70-120 Hz) gamma oscillations were coupled to distinct phases of the theta cycle and reflected synchronous high frequency unit activity. During periods of fear, BLA theta-fast gamma coupling was enhanced, while fast gamma power was suppressed. Periods of relative safety were associated with enhanced BLA fast gamma power, mPFC-to-BLA directionality, and strong coupling of BLA gamma to mPFC theta. These findings suggest that switches between states of fear and safety are mediated by changes in BLA gamma coupling to competitive theta frequency inputs.

Summary Stimuli that possess inherently rewarding or aversive qualities elicit emotional responses and also induce learning by imparting valence upon neutral sensory cues. Evidence has accumulated implicating the amygdala as a critical structure in mediating these processes. We have developed a genetic strategy to identify the representations of rewarding and aversive unconditioned stimuli (USs) in the basolateral amygdala (BLA) and have examined their role in innate and learned responses. Activation of an ensemble of US-responsive cells in the BLA elicits innate physiological and behavioral responses of different valence. Activation of this US ensemble can also reinforce appetitive and aversive learning when paired with differing neutral stimuli. Moreover, we establish that the activation of US-responsive cells in the BLA is necessary for the expression of a conditioned response. Neural representations of conditioned and unconditioned stimuli must therefore ultimately connect to US-responsive cells in the BLA to elicit both innate and learned responses.