Inactivation of Serotonergic Neurons in the Rostral Medullary Raphé Attenuates Stress-Induced Tachypnea and Tachycardia in Mice

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.

Involvement of supralemniscal nucleus (B9) 5-HT neuronal system in nociceptive processing: a fiber photometry study

“…1a). We have already confirmed the specificity of TPH2-tTA expression in the previous report [24]. Ten to fourteen-week-old mice were used in this study.…”

“…The tryptophan hydroxylase-2 tetracycline-controlled transactivator (TPH2-tTA) transgenic mice were used [18,23,24] (Fig. 1a).…”

Blockade of 5-HT2A receptors inhibits emotional hyperthermia in mice

“…In supporting this view, a recent study of ours found that intruder stress activates serotonergic neurons in the rostral medullary raphé [36]. It also shows that selective inhibition of these serotonergic neurons causes minor reduction in intruder-elicited tachycardia in mice, indicating that the medullary raphé serotonergic system somewhat contributes to stress-induced tachycardia.…”

Involvement of the Nucleus Accumbens in Chocolate-induced Cataplexy

“…In the present study, we examined a closer relationship between cataplexy and activated brain sites. We used another activation marker, the phosphorylated form of extracellular signal-regulated kinase (pERK), because it is induced more quickly and with a smaller time window than c-Fos 7,8 . We used orexin neuron-ablated (ORX-AB) mice, another animal model of human narcolepsy, rather than prepro-orexin knockout mice 5 because they show more bouts of cataplexy 9 and their loss of orexinergic cells more closely resembles human narcolepsy 10 .…”