Renal sympathetic nerve activity during morphine abstinence in sino-aortic baroreceptor-denervated rats

This study was undertaken to investigate the influence of the arterial baroreceptors on the response of renal sympathetic nerve activity (rSNA) during naloxone-precipitated morphine abstinence in rats. In chronically baroreceptor-denervated, morphine-dependent rats, rSNA, mean arterial pressure (MAP) and heart rate (HR) were studied before and after repeated i.v. bolus doses of naloxone (0.005-5 mg kg-1), during chloralose anaesthesia or in the conscious state. In the anaesthetized animals, naloxone doses of 0.05-5 mg kg-1 caused a pronounced inhibition of rSNA, reaching a level 61% below pre-naloxone activity. This was accompanied by increases in MAP and HR. In the conscious rats, the lower doses of naloxone elicited an initial state of increased somatomotor activity. This was paralleled by slight increases in rSNA and MAP. After 4-5 min, the behavioural excitation faded and was replaced by lethargy. The rats exhibited still signs of withdrawal in the form of piloerection, chromodacryorrhoea and defaecations. Concomitantly, rSNA returned towards the pre-naloxone level, while MAP showed a sustained increase. The higher naloxone doses exacerbated the hypertension without any further changes in rSNA or in behaviour. We conclude that the influence of the baroreceptors is of minor significance for the inhibition of rSNA during naloxone-precipitated abstinence in anaesthetized rats. In conscious, intact rats, however, the baroreceptors seem to contribute to rSNA inhibition since no significance decrease of rSNA occurred in baroreceptor-denervated rats in the present study. This is in contrast to our previous finding of a marked inhibition of rSNA in rats with intact baroreceptors.