Pyramidal neurons from medial prefrontal cortex (mPFC) project into striatum, thalamus, hypothalamus, and went as far as dorsal raphe nucleus (midbrain) and locus coeruleus (pons). Neural activity in mPFC is reciprocally modulated by monoamine systems arising in these distant brain regions: noradrenergic, dopaminergic and serotonergic. By deregulating these systems genetically, we previously reported that functional projections from mPFC are similarly regulated by these monoaminergic systems using brain-wide tracing based on manganese-enhanced magentic resonance imaging (MEMRI) followed by statistical parametric mapping (SPM). Here we report that a single experience of acute threat alters mPFC projections in wild type mice, and this change correlates with multiple subtle behavioral effects. Mice with or without acute threat (30m exposure to predator odor (2,3,5-Trimethyl-3-thiazoline, TMT) were group housed in environmentally controlled facility for 3 weeks and then underwent MEMRI tract-tracing. Behavior was video recorded before odor, during odor and before tract tracing. MnCl2 (3- 5nL of 0.6M in buffered saline) prepared with 0.5 mg/ml 3k rhodamine dextran-amine was injected into right mPFC and location confirmed by post-injection MRI: x = 0.59 ± 0.13 (R of midline), y = +0.72 ± 0.49mm (bregma), z = -1.04±0.36 mm (deep) (n= 24). Longitudinal images were collected at 6h and 24h after injection. Between imaging sessions animals were returned to their home cages. At conclusion of imaging, animals were sacrificed and perfusion-fixed brains processed for microscopy. Voxel-wise SPM comparisons between time points revealed progression of M(II) accumulation distally. Application of our new in vivo atlas allowed automated digital measurements of Mn(II) signal, revealing dramatic anatomical differences in projections between animals experiencing acute threat and those without which corresponded to subtle behavioral effects. Computational analysis of threat-exposed projections with non-threat exposed mice carrying genetic disruptions of monoaminergic transporters suggested that fear- induced mPFC projections were most similar to altered projections induced by deletion of norepinephrine transporter (NET), which increases the amount of noradrenaline in the synaptic cleft. Acute threat further impacted mPFC projections in mice lacking the serotonergic transporter (SERT). Thus acute threat produces long-term changes in the reward circuitry of the limbic system, which may explain how life-threatening experiences pose a risk for long-term anxiety and substance use disorders. Supported by NIH RO1 MH096093 (ELB) and DA018184 (REJ).
Bearer, E L.; Taylor Uselman; Christopher M. Medina; and Russell E. Jacobs. "Medial prefrontal cortical projections are altered by a single acute fear experience: Evidence for biological mechanisms of anxiety and substance abuse disorders." (2021). https://digitalrepository.unm.edu/hsc-bbhrd/21