Proceedings of The Physiological Society

University of York (2002) J Physiol 539P, S004

Communications

Adenosine release in the nucleus of the solitary tract during hypothalamic stimulation and hypoxia in anaesthetised rats

N. Dale*, A.V. Gourine†, T. Thomas†, E. Llaudet* and K.M. Spyer†

*Department of Biological Sciences, University of Warwick, Coventry CV4 7AL and †Department of Physiology, Royal Free and University College London Medical School, Royal Free Campus, London NW3 2PF, UK


Adenosine at the level of the nucleus of the solitary tract (NTS) modulates the baroreceptor and chemoreceptor reflexes (Spyer & Thomas, 2000; Thomas et al. 2000). There is evidence that adenosine acting through A1 receptors, which are dense in the caudal NTS, partially mediate the cardiovascular responses evoked by stimulation of the hypothalamic defence area (HDA; Spyer & Thomas, 2000). We have used an enzyme-based adenosine microsensor (see Dale, 1998; Dale et al. 2000) to test the hypothesis that hypothalamic stimulation evokes adenosine release in the NTS.

Experiments were performed in 29 anaesthetised (pentobarbitone sodium 60 mg kg-1 I.P., then 10 mg kg-1 I.V. as required) rats, which were injected with gallamine triethiodide (10 mg kg-1, I.V., then 2-4 mg kg-1 h-1 I.V.) and artificially ventilated. All studies were carried out in accordance with the UK Animals (Scientific Procedures) Act, 1986. Adequate anaesthesia was ensured by maintaining stable levels of blood pressure, heart and central respiratory rate. The rat was humanely killed by overdose of anaesthetic at the end of the experiment. The adenosine sensor (1 mm length, 50-100 µm diameter) was inserted into the NTS at discrete positions along its rostro-caudal axis. The HDA was stimulated electrically (1 ms pulses, 100-200 µA, 100 Hz for 5 s). The responses to systemic hypoxia were determined by ventilating the animal with a mixture of 0 % O2 and 100 % N2. Electrical stimulation of HDA induced a reproducible cardiovascular response, consisting of an initial increase in blood pressure followed by a secondary increase following termination of the stimulus. It was found that (i) the cardiovascular response induced by stimulation of the HDA is accompanied by profound and rapid adenosine release, reaching levels as high as 8 µM, which occurred within a restricted area of the NTS -0.3 to +0.2 mm from obex); (ii) markedly smaller or no increases in adenosine levels at all sites following stimulation of the HDA were observed in recordings made either caudal or rostral to this area; (iii) adenosine release in the NTS evoked by stimulation of the HDA is abolished by ecto-5fi-nucleotidase inhibitor α,β-methylene ADP, suggesting that adenosine is derived from the extracellular breakdown of ATP; (iv) smaller but significant increases in adenosine levels were observed along the rostro-caudal extent of the NTS during systemic hypoxia.

This direct demonstration of rapid and localised adenosine release in the NTS supports the hypothesis that adenosine, as a major neuromodulator, plays an important physiological role in the areas of the brainstem involved in cardiovascular control.

We thank the Cunningham Trust, The Wellcome Trust and the BBSRC for generous support.


Where applicable, experiments conform with Society ethical requirements