Method for in vivo calibration of renal sympathetic nerve activity in rabbits Sandra L. Burke, Geoffrey A. Head * Neuropharmacology Laboratory, Baker Heart Research Institute, Commercial Road Prahran, PO Box 6492, St Kilda Road Central, Melbourne, Vic. 8008, Australia Received 16 January 2003; received in revised form 8 April 2003; accepted 18 April 2003 Abstract A major difficulty of recording from peripheral sympathetic nerves is that microvolt values reflect characteristics of the recording conditions and limit comparisons between different experimental groups. In this study we assessed methods of calibrating renal sympathetic nerve activity (RSNA) in conscious rabbits. Calibration values were obtained from maximum RSNA responses to nasopharyngeal stimulation, airjet stress or unloading baroreceptors. Curves relating RSNA to blood pressure were produced by raising and lowering blood pressure with vasoactive drugs. To assess whether normalization would eliminate differences between RSNA curves which were most likely due to recording conditions, rabbits were first divided into two groups with high or low basal microvolt levels of RSNA, then again into two groups with high or low heart rate. In both cases, curves were similar if values were normalized by nasopharyngeal stimulation or by the upper plateau value. In hypertensive rabbits, where the baroreflex is suppressed, only the nasopharyngeal method showed this attenuated pattern. This method also eliminated the 50% decay in basal RSNA measured over 5 weeks. We conclude that expressing RSNA in terms of the maximum response to nasopharyngeal stimulation provides a calibration method suitable for comparing nerve activity over the long term as well as showing valid differences in baroreflex curves between different experimental groups. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Renal nerve activity; Nasopharyngeal stimulation; Airjet stress; Baroreflex curves; Normalization method; Rabbits 1. Introduction Direct recordings of sympathetic nerve activity have been an extremely important tool in assessing the role of the sympathetic nervous system in cardiovascular con- trol. Conscious recordings from multifiber sympathetic nerve preparations have been developed for a number of animal species (Dorward et al., 1985; Gross and Kirchheim, 1980; Ricksten and Thore ´n, 1980; Riedel et al., 1982; Schad and Seller, 1975) and in humans (Wallin and Elam, 1994). In human studies, sympathetic nerve activity is usually measured by counting the number of bursts (Wallin and Elam, 1994). A disadvan- tage of this approach is that information about the size of the bursts is lost. In animal studies, the most common method of analysis is to integrate nerve activity over time (Dorward et al., 1985; Riedel et al., 1982). However, there is considerable variation in the levels of nerve activity occurring between animals and within animals studied on different days. This may be due to the different number of nerve bundles isolated for recording as well as changes in recording conditions with time. The latter results from increasing resistance at the recording electrode due to growth of inflammatory tissue or infiltration of fluid and loss of nerve fibers. Development of a chronic electrode for recording renal sympathetic nerve activity (RSNA) in conscious rabbits has meant that recordings can be made for several weeks after implantation of the electrode (Dor- ward et al., 1985; Riedel et al., 1982). Several techniques have been employed by investigators to deal with the problem of the gradual attenuation of the recorded signal over time. A frequently used method is to scale all RSNA as a percent of the basal level of RSNA (Burke et * Corresponding author. Tel.:  /61-3-8532-1111; fax:  /61-3-8532- 1100. E-mail address: geoff.head@baker.edu.au (G.A. Head). Journal of Neuroscience Methods 127 (2003) 63 /74 www.elsevier.com/locate/jneumeth 0165-0270/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0165-0270(03)00121-3