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Chronotropic and Dromotropic Responses to Localized Glutamate Microinjections in the Rat $ Nucleus Ambiguus

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Chronotropic and Dromotropic Responses to Localized Glutamate Microinjections in the Rat $ Nucleus Ambiguus brain research 1542 (2014) 93–103 Available online at www.sciencedirect.com www.elsevier.com/locate/brainres Research Report Chronotropic and dromotropic responses to localized glutamate microinjections in the rat $ nucleus ambiguus Karla N. Sampaio1,He´lder Mauad2, K. Michael Spyer, Timothy W. Fordn Division of Biosciences, Faculty of Life Sciences, University College London, Gower Street, London WC1E 6BT, UK article info abstract Article history: The cardioinhibitory effects of cardiac vagal motoneurons (CVMs) are mediated by Accepted 18 October 2013 activation of postganglionic neurons in the epicardial ganglia which have been shown to Available online 24 October 2013 exert functionally selective effects on heart rate and atrioventricular conduction in the rat. Here we investigate whether CVMs producing these responses may occupy different Keywords: rostrocaudal positions within the nucleus ambiguus. Excitation of CVMs was attempted Autonomic nervous system by microinjections of glutamate into the nucleus ambiguus of an arterially perfused Vagus nerve preparation in a grid extending over 2 mm in the rostrocaudal plane using the obex as a Nucleus ambiguus reference point. Microinjections were paired, one made during pacing to measure changes Heart rate in atrioventricular conduction (P-R interval) independent of changes in heart rate and the Glutamate other looking for changes in heart period (P-P interval) un-paced. Although evidence of a differential distribution was found in 7 cases, in the majority (13/20), sites producing maximal effects on both variables coincided. Maximal changes in atrioventricular conduc- tion resulted from more rostral sites in 6 cases and from a more caudal site in only one. Overall, the ratio of the change in atrioventricular conduction to the change in heart rate for a given site was significantly greater 1 mm rostral to the obex than at either end of the test grid. We conclude that while CVMs controlling atrioventricular conduction are distributed with a peak somewhat rostral to those controlling heart rate in a number of animals, there is a significant overlap and much greater variability in this distribution in the rat than in cats and dogs. & 2013 The Authors. Published by Elsevier B.V. All rights reserved. Abbreviation: CVM, cardiac vagal motoneuron ☆ This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. n Corresponding author. Present address: University of Nottingham School of Health Sciences, South Block Link, Queens Medical Centre, Nottinghamshire NG7 2HA, UK. Fax: þ44 115 823 1211. E-mail address: [email protected] (T.W. Ford). 1Present address: Department of Pharmaceutical Sciences, Federal University of Espírito Santo, Av. Marechal Campos, 1468, Maruípe, Vitória, ES, Brazil. 2Present address: Department of Physiological Sciences, Federal University of Espírito Santo, Av. Marechal Campos, 1468, Maruípe, Vitória, ES, Brazil. 0006-8993/$ - see front matter & 2013 The Authors. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.brainres.2013.10.035 94 brain research 1542 (2014) 93–103 dromotropic responses with no effects on heart rate were 1. Introduction reported. The present experiments investigate whether differ- ential cardiac effects can be elicited by focal application of The vagus carries the parasympathetic motor supply to the heart glutamate along the rostrocaudal extent of the rat nucleus where it exerts a predominantly inhibitory action. Increased ambiguus while monitoring the effects on heart rate and cardiac vagal activity thus results in a fall not only in the atrioventricular conduction. heart rate (negative chronotropic effect) but also in the rate of A preliminary report has been published in abstract form propagation of the cardiac action potential (negative dromo- (Sampaio et al., 2000). tropic effect) and in the force of myocardial contraction (negative inotropic effect) (see Jones (2001)). The vagal motor path- way comprises a preganglionic component, originating in the medulla oblongata, which in turn synapses with a postganglionic 2. Results component within the cardiac ganglia which are located pre- dominantly in the atrial epicardium. Although axonal tracing 2.1. Effects of microinjections of glutamate studies have shown that mammalian preganglionic cardiac vagal motoneurons (CVMs) arise from the nucleus ambiguus, dorsal Glutamate microinjections commonly elicited changes in motor vagal nucleus and the intermediate reticular formation, heart rate and atrioventricular conduction when the elec- the nucleus ambiguus represents the principal site of origin of trode tip was between 1.5 and 1.8 mm lateral to the midline myelinated cardioinhibitory B-fibers (McAllen and Spyer, 1976, and at a depth of 1.6–1.9 mm depending on the rostrocaudal 1978; Nosaka et al., 1979) while the dorsal motor vagal nucleus level relative to the obex. A representative example of the gives rise to unmyelinated C-fibers which have a lesser effect on time course and extent of changes in P-P and P-R intervals at theheartandlittledemonstrableinputfromthebaroreceptors different rostrocaudal levels is illustrated in Fig. 1. (Ford and McWilliam, 1986; Ford et al., 1990; Jones et al., 1995, Responses typically occurred with an onset latency of less 1998; Nosaka et al., 1982). than 5 s, peaked some 5–30 s later and returned to base- Earlier studies from this laboratory have shown that it is line levels within 1–2 min. The responses were reproducible possible to produce excitation of discrete epicardial ganglia within any series of injections and could not be elicited by resulting in selective effects on the rat heart (Sampaio et al., injections of equivalent volumes of vehicle (20 nL Ringer's 2003). These data are consistent with the existence of func- solution). The sizes of the responses varied quite consider- tionally selective ganglia that have been demonstrated in ably however from one preparation to another. Sites produ- other species (Ardell and Randall, 1986; Gatti et al., 1995, 1997; cing maximal responses were well-localized; microinjections Randall et al., 1986a, 1986b). 0.5–1.0 mm medial or lateral to these sites were typically In the cat, the vagal preganglionic axons running from the ineffective or produced comparatively small responses at nucleus ambiguus to the heart appear to target specific longer onset latencies. Significant increases in both P-P and ganglionic clusters and therefore may also be functionally P-R interval were observed following microinjection of gluta- defined at the preganglionic level (Gatti et al., 1996; Massari mate at all levels of the nucleus ambiguus studied when et al., 1995). That preganglionic CVMs in the rat nucleus compared to pre-injection levels (Table 1). ambiguus may also be functionally specific is suggested by the results of preliminary tracing studies performed in this laboratory. Single discrete microinjections of anterograde 2.2. Rostrocaudal distribution tracers that label small numbers of vagal preganglionic motoneurons in the nucleus ambiguus result in a restricted For each injection level the overall change in P-P interval distribution of labeled axon terminals in the cardiac ganglia (unpaced) is illustrated in Fig. 2A. Comparing the effective- (Jones et al., 1999). While precise mapping of the injection ness of different sites in eliciting changes in P-P interval it sites to anatomically identified ganglia was not performed in can be seen that microinjection of glutamate at 0, þ0.5 and this study, labeled axons were traced running through gang- þ1 induced significantly greater changes in P-P interval lionated plexuses without making any apparent synaptic compared to the other two sites. Changes in P-R interval contact until reaching an apparent target zone where dense however (Fig. 2B) were observed to be significantly greater innervation of neighboring ganglion cells occurred. This only at the levels 0 and þ1.0 compared to the changes elicited suggests that single preganglionic CVMs may preferentially at the most rostral and most caudal sites. To compare the innervate ganglion cells projecting to one particular target relative effectiveness of microinjections on chronotropic and rather than multiple sites within the heart. That such a dromotropic responses, the ratio of the change in P-P interval targeting exists is supported by anterograde tracing experi- to the change in P-R interval (ΔP-R/ΔP-P) was calculated for ments which have shown that CVMs from the left and right all sites where both measurements were made (i.e. micro- sides of the brain stem appear to innervate different subdivi- injections during both unpaced and paced runs). The inten- sions within the cardiac ganglionic plexuses (Cheng et al., tion was to avoid loss of valuable paired data which would 2004). otherwise occur when data were pooled. If the CVMs con- In the study by Gatti et al. (1996), sites in the nucleus trolling P-P interval and those controlling P-R interval were ambiguus producing negative dromotropic effects to gluta- mixed homogeneously across the rostrocaudal extent of the mate tended to be located rostral to those producing negative nucleus then one would not expect to find peaks in this ratio chronotropic effects. In some rostral locations, sites producing at any particular rostrocaudal location. brain research 1542 (2014) 93–103 95 P-P interval P-R interval 300 95 P-P P-R (ms) (ms) 200 85 100 100 60 60 (mmHg) (mmHg) Pressure - 0.5 Pressure -0.5 L-Glu L-Glu 300 95 P-P P-R (ms) (ms) 200 85 100 100 60 60 (mmHg) (mmHg) Pressure Pressure Obex Obex L-Glu L-Glu 300 100 P-R P-P (ms) 200 (ms) 90 100 100 60 60 (mmHg) (mmHg) Pressure Pressure + 0.5 + 0.5 L-Glu L-Glu 300 90 P-R P-P (ms) (ms) 200 80 100 100 60 60 (mmHg) (mmHg) Pressure Pressure + 1.0 + 1.0 L-Glu L-Glu 300 90 P-P P-R (ms) 200 (ms) 80 100 100 60 60 (mmHg) (mmHg) Pressure + 1.5 Pressure + 1.5 L-Glu L-Glu 70 sec 70 sec Fig.
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