Identification of a Stress‐Inhibitory Pathway from the Nucleus of the Solitary Tract (NTS) to the Bed Nucleus of the Stria Terminalis (BNST)

We and others have previously shown that the NTS influences physiological responses to both acute and chronic stress. The NTS sends a robust neuronal projection to the BNST consisting of both catecholaminergic and 11βhydroxysteroid dehydrogenase‐expressing neurons. The BNST contributes importantly to regulation of the stress response, however, the functional significance of the NTS to BNST pathway is not known. We conducted the present experiments to test the hypothesis that activating this pathway would influence stress‐associated physiological endpoints. The Cre‐inducible channelrhodopsin construct, rAAV2/EF1a‐DIO‐hChR2(H134R), was microinjected into the NTS (500 nl bilaterally and at the midline) and the retrograde Cre recombinase expressing virus, CAV2‐Cre, was injected into the BNST (1000 nl bilaterally) in six adult male Wistar‐Kyoto rats. After recovery, rats were implanted with chronic indwelling fiber optics targeting the BNST. At least 12 weeks later, tests were performed in conscious rats to measure the effect of stimulation of NTS neuron terminals in the BNST. Optogenetic activation of the NTS to BNST pathway reduced baseline plasma corticosterone concentration from 5.4±0.9 to 2.1±0.8 μg/dl (P<0.05, within subjects design) and increased the percent of time the rats spent in the open arms of the elevated plus maze from 20±7 to 87±6 % (P<0.05, between subjects design). Rats also underwent tests for sucrose and saline preference. Activation of the NTS to BNST pathway reduced 1.8% saline intake over a 30 min period from 1.40±0.32 to 0.55±0.16 ml (P<0.05, within subjects design). Sucrose (1% solution) intake and water intake were not affected by activation of the pathway. Sucrose intake was 2.01±0.64 ml without stimulation and 1.59±0.24 ml with stimulation. Average water intake was 0.94±0.37 ml without stimulation and 0.89±0.29 ml with stimulation. We conclude that in unstressed normal male rats activation of the neuronal pathway from the NTS to the BNST inhibits the hypothalamic‐pituitary‐adrenal axis, anxiety‐like behavior and saline intake. The pathway may act to restrain the stress response under these conditions. Future studies will determine the function of this pathway under conditions of chronic stress, its role in the control of cardiovascular function, and the role of mineralocorticoid receptors in both catecholaminergic and 11βhydroxysteroid dehydrogenase‐expressing NTS neurons. Support or Funding Information R01 HL076807, R01 HL‐122494