Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, C6
Does the dorsal motor nucleus of the vagus control cardiac chronotropism?
L. M. Salo1, R. M. McAllen2, J. Hewinson1, M. T. Ambler1, J. F. Paton1, A. E. Pickering1
1. Physiology and Pharmacology, Bristol Heart Institute, University of Bristol, Bristol, United Kingdom. 2. Howard Florey Institute, Melbourne, Victoria, Australia.
Both the nucleus ambiguus (NA) and dorsal motor nucleus of the vagus (DMV) contain vagal preganglionic neurons with extensive projections to the cardiac vagal ganglia. While NA cardiac vagal preganglionics exert a strong cardioinhibitory influence on heart rate, it is unclear whether a similar function exists for DMV preganglionics. To test this, we used a viral vector-mediated optogenetic strategy to obtain repeatable neuronal specific activation and investigated the relative influence of NA and DMV neurons on cardiac chronotropy. Vagal preganglionic neurons were transduced with a lentiviral vector with a PRS8 promotor driving channelrhodopsin2 (ChR2) expression, and one week later were light-activated in the working heart-brainstem preparation (WHBP) (1), and heart rate responses recorded. Male Wistar rats (P19, n=16) were injected with of Lenti-PRS8-ChR2-EYFP (1μl) into either the NA or DMV (left and/or right) under anaesthesia (ketamine 60mg.kg-1/metadomidine 25μg.kg-1 i.p.). Subsequently, they were anaesthetised with Halothane (5%) (discontinued after decerebration) for surgical setup of the WHBP, and heart rate, perfusion pressure and phrenic nerve activity was monitored. Preganglionic neurons were light activated using a laser optrode (445nm) inserted over the DMV or NA, and heart rate recorded. In some cases a vagus nerve was recorded or atropine (10μg i.a.) was given. The brains were fixed (4% formalin), cut into 40μm coronal sections and injections sites identified using fluorescence microscopy. Values are mean ± S.E.M., compared by paired t-tests. Optoactivation of NA or DMV neurons produced bradycardias that were titratable and temporally locked to the stimulus. Brisk responses were evoked by NA optoactivation (-37±4 bpm, n=9, p<0.05), compared to modest DMV responses (-9±1 bpm, n=8, p<0.05) (NA vs DMV, p<0.05). Where both NA and DMV neurons were transduced in the same rat, a similar pattern was seen: strong NA with small DMV responses (-20 vs -4 bpm, respectively, n=1). Co-activation of DMV preganglionics with vagal cardiorespiratory afferent evoked reflexes (eg peripheral chemoreflex) showed no evidence of potentiation of the bradycardia. Thoracic vagal recordings showed DMV and NA optoactivation was effective in evoking discharge and histology showed many more transduced cholinergic neurons in the DMV (>10x) than NA. All bradycardias were without change in phrenic nerve activity (p>0.05) or perfusion pressure (p>0.05), and were parasympathetically mediated as atropine abolished them (reduced by 95%). This study shows the feasibility of using optogenetics to manipulate cardiac parasympathetic drives and produce changes in end organ function. Our results have revealed a novel, albeit modest, cardio-inhibitory chronotropic role for the DMV. Further, activation of these neurons appears not to facilitate the peripheral chemoreceptor reflex.
Where applicable, experiments conform with Society ethical requirements