Anomalous baroreflex functionality inherent in floxed and Cre-Lox mice: an overlooked physiological phenotype

The Solitary Nucleus Connectivity to Key Autonomic Regions in Humans MRI and Literature based Considerations

The nucleus tractus solitarius (NTS), is a key brainstem structure relaying interoceptive peripheral information to the interrelated brain centers for eliciting rapid autonomic responses and for shaping longer-term neuroendocrine and motor patterns. Structural and functional NTS' connectivity has been extensively investigated in laboratory animals. But there is limited information about NTS' connectome in humans. Using MRI, we examined diffusion and resting state data from 20 healthy participants in the Human Connectome Project. The regions within the brainstem (n=8), subcortical (n=6), cerebellar (n=2) and cortical (n=5) parts of the brain were selected via a systematic review of the literature and their white matter NTS connections were evaluated via probabilistic tractography along with functional and directional (i.e., Granger-causality) analyses. The underlying study confirms previous results from animal models and provides novel aspects on NTS integration in humans. Two key findings can be summarized: (i) the NTS predominantly processes afferent input and (ii) a lateralization towards a predominantly left-sided NTS processing. Our results lay the foundations for future investigations into the NTS' tripartite role comprised of interoreceptors' input integration, the resultant neurochemical outflow and cognitive/affective processing. The implications of these data add to the understanding of NTS' role in specific aspects of autonomic functions.

Disproportional cardiovascular depressive effects of isoflurane: Serendipitous findings from a comprehensive re-visit in mice

Employment of anesthetics, including isoflurane, though mandatory in animal experiments, is often regarded as a major limitation because results obtained with anesthetics may be different from those obtained under a conscious state. This study re-visits two issues related to the use of isoflurane. First, does isoflurane exert depression equally on all aspects of cardiovascular functions and their regulations? Second, is the circulatory supply of oxygen to brain tissues sufficient under isoflurane anesthesia? We determined in male C57BL/6J mice the temporal effects of 1.5% (vol/vol) isoflurane on blood pressure (BP), heart rate (HR), cardiac performance, baroreflex-mediated sympathetic vasomotor tone, cardiac vagal baroreflex, functional connectivity within the baroreflex neural circuits, carotid or cerebral blood flow, cortical tissue oxygen level, respiratory rate and blood gas. Over 150 min after exposure to 1.5% isoflurane, BP and HR were sustained at 71% and 79% of their awake levels amid a trend of progressive increase. Cardiac performance was within physiological ranges. Baroreflex-mediated sympathetic vasomotor tone gradually reversed from an 85% reduction toward the conscious level, alongside a parallel decrease in inhibitory connectivity between nucleus tractus solitarii (NTS) and rostral ventrolateral medulla. A decline in excitatory connectivity between NTS and nucleus ambiguus accompanied the decrease in cardiac vagal baroreflex. There were progressive increases in carotid or cerebral blood flow and tissue oxygen tension in cerebral cortex, alongside gradual hypoventilation, mild respiratory acidosis and hypercapnia. We conclude that, by eliciting disproportional depressive actions on cardiovascular functions and their regulations, which sustain circulatory supply of oxygen to brain tissues, 1.5% isoflurane is sufficient to maintain optimal cardiovascular functions in mice.

Physiological and pathophysiological evaluation of baroreflex functionality with concurrent diffusion tensor imaging of its neural circuit in the rat

Background

By measuring the prevalence of neuronal traffic between two brain structures based on the notion that diffusion of water molecules along the axon in parallel bundles will create prominent anisotropy in the direction of the passage of action potentials, diffusion tensor imaging (DTI) may be taken as an effective tool for functional investigations. Demonstration of complementary results obtained from synchronized DTI of the baroreflex neural circuit and physiological or pathophysiological evaluation of baroreflex functionality should validate this notion.

Methods

We implemented concurrent changes in neuronal traffic within the neural circuit of the baroreflex-mediated sympathetic vasomotor tone in the brain stem and alterations of its experimental surrogate under physiological and pathophysiological conditions. We further evaluated the functional and clinical implications of results obtained from this experimental paradigm in conjunction with baroreflex induction and a mevinphos intoxication model of brain stem death.

Results

We found that robust connectivity existed between the nucleus tractus solitarii and rostral ventrolateral medulla, the afferent and efferent nuclei of the baroreflex-mediated sympathetic vasomotor. Intriguingly, this connectivity was either reversibly disrupted or irreversibly severed to reflect alterations in baroreflex responses to physiological or pathophysiological challenges.

Conclusions

The capability to observe simultaneous and complementary changes in neuronal traffic within the neural circuit of the baroreflex-mediated sympathetic vasomotor tone and alterations of its experimental surrogate that bears technical, scientific and clinical implications sustains the notion that coupled with relevant physiological phenotypes, DTI can be an effective investigative tool for functional evaluations of brain stem activities.

Baroreflex functionality in the eye of diffusion tensor imaging

By applying diffusion tensor imaging (DTI) as a physiological tool to evaluate changes in functional connectivity between key brainstem nuclei in the baroreflex neural circuits of mice and rats, recent work has revealed several hitherto unidentified phenomena regarding baroreflex functionality. (1) The presence of robust functional connectivity between nucleus tractus solitarii (NTS) and nucleus ambiguus (NA) or rostral ventrolateral medulla (RVLM) offers a holistic view on the moment-to-moment modus operandi of the cardiac vagal baroreflex or baroreflex-mediated sympathetic vasomotor tone. (2) Under pathophysiological conditions (e.g. neurogenic hypertension), the disruption of functional connectivity between key nuclei in the baroreflex circuits is reversible. However, fatality ensues on progression from pathophysiological to pathological conditions (e.g. hepatic encephalopathy) when the functional connectivity between NTS and NA or RVLM is irreversibly severed. (3) The absence of functional connectivity between the NTS and caudal ventrolateral medulla (CVLM) necessitates partial rewiring of the classical neural circuit that includes CVLM as an inhibitory intermediate between the NTS and RVLM. (4) Sustained functional connectivity between the NTS and NA is responsible for the vital period between brain death and the inevitable cardiac death. (5) Reduced functional connectivity between the NTS and RVLM or NA points to inherent anomalous baroreflex functionality in floxed and Cre-Lox mice. (6) Disrupted NTS-NA functional connectivity in Flk-1 (VEGFR2) deficient mice offers an explanation for the hypertensive side-effect of anti-vascular endothelial growth factor therapy (anti-VEGF) therapy. These newly identified baroreflex functionalities revealed by DTI bear clinical and therapeutic implications.

Statistical considerations in reporting cardiovascular research

The problem of inadequate statistical reporting is long standing and widespread in the biomedical literature, including in cardiovascular physiology. Although guidelines for reporting statistics have been available in clinical medicine for some time, there are currently no guidelines specific to cardiovascular physiology. To assess the need for guidelines, we determined the type and frequency of statistical tests and procedures currently used in the American Journal of Physiology-Heart and Circulatory Physiology. A PubMed search for articles published in the American Journal of Physiology-Heart and Circulatory Physiology between January 1, 2017, and October 6, 2017, provided a final sample of 146 articles evaluated for methods used and 38 articles for indepth analysis. The t-test and ANOVA accounted for 71% (212 of 300 articles) of the statistical tests performed. Of six categories of post hoc tests, Bonferroni and Tukey tests were used in 63% (62 of 98 articles). There was an overall lack in details provided by authors publishing in the American Journal of Physiology-Heart and Circulatory Physiology, and we compiled a list of recommended minimum reporting guidelines to aid authors in preparing manuscripts. Following these guidelines could substantially improve the quality of statistical reports and enhance data rigor and reproducibility.