Abstract
The medullary raphé nuclei are involved in controlling cardiovascular, respiratory, and thermoregulatory functions, as well as mediating stress-induced tachycardia and hyperthermia. Although the serotonergic system of the medullary raphé has been suggested as the responsible entity, specific evidence has been insufficient. In the present study, we tested this possibility by utilizing an optogenetic approach. We used genetically modified mice [tryptophan hydroxylase 2 (Tph2); archaerhodopsin-T (ArchT) mice] in which ArchT, a green light-driven neuronal silencer, was selectively expressed in serotonergic neurons under the regulation of Tph2 promoters. We first confirmed that an intruder stress selectively activated medullary, but not dorsal or median raphé serotonergic neurons. This activation was suppressed by photo-illumination via a pre-implanted optical fiber, as evidenced by the decrease of a cellular activation marker protein in the neurons. Next, we measured electro cardiogram (ECG), respiration, body temperature (BT), and locomotor activity in freely moving mice during intruder and cage-drop stress tests, with and without photo-illumination. In the intruder test, photo inactivation of the medullary serotonergic neurons significantly attenuated tachycardia (362 ± 58 vs. 564 ± 65 bpm.min, n = 19, p = 0.002) and tachypnea (94 ± 82 vs. 361 ± 138 cpm.min, n = 9, p = 0.026), but not hyperthermia (1.0 ± 0.1 vs. 1.0 ± 0.1°C.min, n = 19, p = 0.926) or hyperlocomotion (17 ± 4 vs. 22 ± 4, arbitrary, n = 19, p = 0.089). Similar results were obtained from cage-drop stress testing. Finally, photo-illumination did not affect the basal parameters of the resting condition. We conclude that a subpopulation of serotonergic neurons in the medullary raphé specifically mediate stress-induced tachypnea and tachycardia, which have little involvement in the basal determination of respiratory frequency (Res) and heart rate (HR), specifically mediate stress-induced tachycardia and tachypnea.
Highlights
Stress causes several physiological responses including tachycardia, tachypnea, hyperactivity, and hyperthermia (Inagaki et al, 2004; Strekalova et al, 2005). These responses are thought to be mediated by medullary raphé neurons, with activation of local neurons leading to subsequent increase in heart rate (HR), blood pressure (Adair et al, 1977; Mccall and Humphrey, 1982), respiration (Lalley, 1986; Besnard et al, 2009), locomotor activity (Dampney, 2015), and body temperature (BT) (Tanaka et al, 2002; Nakamura et al, 2005)
The merged picture shows that ArchT-enhanced green fluorescent protein (EGFP) was exclusively observed in serotonin neurons in tryptophan hydroxylase 2 (Tph2)-transcriptional activator (tTA); tTA-dependent promoter (TetO)-ArchT mice, with few displaying ectopic expression
We focused on midline located nuclei but not laterally located serotonin neurons since we had intended to use single optic fiber to minimize the brain damage
Summary
Stress causes several physiological responses including tachycardia, tachypnea, hyperactivity, and hyperthermia (Inagaki et al, 2004; Strekalova et al, 2005) These responses are thought to be mediated by medullary raphé neurons, with activation of local neurons leading to subsequent increase in heart rate (HR), blood pressure (Adair et al, 1977; Mccall and Humphrey, 1982), respiration (Lalley, 1986; Besnard et al, 2009), locomotor activity (Dampney, 2015), and body temperature (BT) (Tanaka et al, 2002; Nakamura et al, 2005). Airjet stress-induced tachycardia has been shown to be attenuated by inactivation of the medullary raphé region (Zaretsky et al, 2003b). Inactivation of medullary raphé neurons does not change basal HR values (Mccall and Harris, 1987) or respiration (Benarroch, 2007); it does decrease basal BT (Zaretsky et al, 2003a; Ray et al, 2011)
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