Effect of Fear Extinction and Myocardial Infarction on Corticotropin Releasing Hormone Receptor 1 mRNA in Rostral Locus Coeruleus of Rats

Abstract

We found that 8 weeks after myocardial infarction (MI) female, but not male rats showed deficits in extinction of conditioned fear (P<0.05). Small hairpin RNA-mediated knockdown of corticotropin releasing hormone receptor 1 (CRF1) around the rostral locus coeruleus (rLC) reversed deficits in fear extinction. Female and male rats subjected to MI and sham knockdown showed reduced CRF1 mRNA levels 24 hours after extinction training compared to sham MI/sham knockdown controls (P<0.01). CRF1 mRNA levels in the rLC region were directly related to the extent of freezing observed during extinction trials in rats subjected to sham knockdown (P<0.05). These findings suggest either that exposure to feared cues during extinction contributed to acute reductions in CRF1 mRNA in the rLC region, or that chronic reductions in CRF1 mRNA levels resulted from MI. Here, we determined if CRF1 mRNA levels in the rLC region were reduced after MI in male rats. Fresh frozen brains of rats exposed to coronary artery ligation (n=4) or sham surgery (n=3) 8 weeks prior to euthanasia were processed for fluorescent in situ hybridization. Brain sections containing the rLC were processed for detection of CRF1-, vesicular glutamate transporter 2- (VGLUT2, a glutamatergic cell marker) and glutamate decarboxylase 2 mRNA (GAD2, a GABAergic cell marker). Assessments from the medial parabrachial nucleus to the lateral border of the posterodorsal tegmental nucleus and ventral border of Barrington's nucleus were performed. Cells were counted bilaterally and averaged within rat. No differences in CRF1 mRNA were observed between groups. A small number of CRF1 mRNA positive cells (39±6) close to the rLC did not show VGLUT2 or GAD2 mRNA label. Numbers of GAD2- (90±15) and VGLUT2 mRNA-positive cells (365±30) did not differ between groups. CRF1- and VGLUT2 mRNA-positive cells were found in high density in the medial parabrachial nucleus and in the lateral dorsal tegmental nucleus. CRF1- and GAD2 mRNA-positive cells were found medially, near the posterodorsal tegmental nucleus. The percent of VGLUT2 mRNA-positive cells (72±4) that co-expressed CRF1 mRNA was greater than the percent of GAD2 mRNA-positive cells that co-expressed CRF1 mRNA (60±5, p<0.001). The integrated density of CRF1 mRNA fluorescence was greater in VGLUT2 mRNA-positive cells than in GAD2 mRNA-positive cells (P<0.05). The data indicate that CRF1 mRNA is expressed primarily by glutamatergic cells in the rLC region of the male rat and that reductions in CRF1 mRNA levels observed in the rLC region 24 hours after extinction in MI rats are likely due to stress responses during fear extinction rather than to chronic effects of MI.

Support or Funding Information

Cardiovascular Research Institute, Loyola University Chicago

This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.