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

Note: This article was submitted to Integrative Physiology, a section of the journal Frontiers in Physiology. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.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.This work was supported by JSPS KAKENHI Grants (16H05130, 16K13112 to TK and 18K07353 to IK-Y), CREST JST (JPMJCR1656 to AY), and Research Foundation for Opto-Science and Technology (to IK-Y)

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

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

Graphic plot analysis for estimating binding potential of translocator protein (TSPO) in positron emission tomography studies with [18F]FEDAA1106

Purpose[18F]FEDAA1106 is expected to be used for evaluating the regional density of the peripheral benzodiazepine receptor (also called TSPO) in several neurodegenerative disorders. Regarding the quantification, direct binding potential (BPND) has been reported to be preferable because of the variation of nondisplaceable distribution volume (VND) among individuals. However, the precise calculation of BPND is difficult in small regions or at voxel levels due to noise. Recently, a new graphical analysis (GA) was proposed to estimate VND in a direct way. In this paper, we evaluated two types of GA for reliable quantification of BPND in PET study with [18F]FEDAA1106 using computer simulations and human data.\nMethodsIn the simulations, time-activity curves were generated with various rate constants and noise levels, and the errors of BPND estimated by GA were analyzed by comparing with true values calculated from rate constants given for the simulations. Thereafter, in a human study with [18F]FEDAA1106 for healthy volunteers, BPND was estimated by two types of GA for region-of-interest (ROI) data. Parametric images of BPND were generated by two types of GA with or without wavelet-denoising.\nResultsSimulations showed that BPND by GA was well correlated with true values, despite an underestimation. GA reduced unreasonable estimates compared with a conventional nonlinear least-square fitting (NLS), although larger variation of BPND estimates was observed. In a ROI-based analysis of data obtained in a human study, BPNDs estimated by GA were well correlated with those generated by NLS, though they were underestimated. Parametric BPND images by GA could be improved with wavelet-denoising.\nConclusionGraphical analysis could provide BPND values with high stability and simple calculation in both ROI-based and voxel-based analyses of [18F]FEDAA1106 data

PET kinetic analysis --- Pitfalls and a solution for the Logan plot

The Logan plot is a widely used algorithm for the quantitative analysis of neuroreceptors using PET because it is easy to use and simple to implement. The Logan plot is also suitable for receptor imaging because its algorithm is fast. However, use of the Logan plot, and interpretation of the formed receptor images should be regarded with caution, because noise in PET data causes bias in the Logan plot estimates. In this paper, we describe the basic concept of the Logan plot in detail and introduce three algorithms for the Logan plot. By comparing these algorithms, we demonstrate the pitfalls of the Logan plot and discuss the solution

Effects of point spread function-based image reconstruction on neuroreceptor binding in positron emission tomography study with [11C]FLB 457

The ordered subset expectation maximization with a point spread function (OSEM-PSF) was developed to improve the spatial resolution of reconstructed positron emission tomography (PET) images and has been reported to improve the contrast of hot spots in PET studies for oncology. However, in neuroreceptor imaging, the regional radioactivity concentration changes dynamically during the scan, and the effects of the PSF may differ among various radioligands or quantification methods. In this study, we investigated the effects of the PSF on quantification in PET studies with [11C]FLB 457 of dopamine D2 receptors, using both phantom and human data acquired by the Siemens Biograph 16 imaging platform. In the phantom studies, we evaluated the hot contrast recovery coefficient (HCRC) for variously sized hot spheres and the linearity between the measured and true radioactivities in OSEM-PSF images. Next, in the human studies with [11C]FLB 457, radioactivity concentrations and binding potentials for the cerebral cortex and thalamus were compared between images reconstructed with and without PSF. In the phantom studies, the OSEM-PSF images showed a better HCRC compared to images without PSF, and they showed a good linear correlation with true radioactivity. In the human studies, the radioactivity concentration increased especially in small regions with high accumulation of [11C]FLB 457 when the PSF was included. However, little difference in the binding potentials was observed for the target regions between both types of reconstructed images. In conclusion, PSF-based reconstruction reduced the spill-over phenomena in small hot regions; however, it caused no increase in the binding potentials in the [11C]FLB 457 studies

PET kinetic analysis --- compartmental model

PET enables not only visualization of the distribution of radiotracer, but also has ability to quantify several biomedical functions. Compartmental model is a basic idea to analyze dynamic PET data. This review describes the principle of the compartmental model and categorizes the techniques and approaches for the compartmental model according to various aspects: model design, experimental design, invasiveness, and mathematical solution. We also discussed advanced applications of the compartmental analysis with PET