The role of adenosine receptors in the rostral ventrolateral medulla in the cardiovascular response to defence area stimulation in the rat

The effects of microinjections of adenosine and an adenosine receptor antagonist into the rostral ventrolateral medulla (RVLM) on the cardiovascular changes associated with the defence reaction were investigated in anaesthetized rats. Responses to electrical and chemical stimulation in RVLM were determined in alpha-chloralose-anaesthetized, paralysed and artificially ventilated rats. Microinjections of adenosine (10 nM) and the adenosine receptor antagonist 8-sulphophenyl-theophylline (8-SPT; 0.12 microM) were made into the RVLM and their effects on arterial pressure (ABP), heart rate (HR) and skeletal muscle blood flow determined. Microinjections of adenosine into the RVLM evoked either an increase or a decrease in ABP, with variable effects on HR. These actions of adenosine were blocked by prior injection of 8-SPT at the same site. Histological analysis showed that adenosine evoked an increase in arterial pressure when injected into rostral areas of the RVLM, and a depressor response when injected into caudal regions. Furthermore, microinjections of adenosine into the RVLM augmented the increase in ABP evoked by electrical stimulation of the hypothalamic defence area (HDA). Whilst microinjection of 8-SPT into RVLM had no effect on the baseline levels of the variables measured, it reduced the HDA-evoked increase in ABP. From these results we propose that adenosine modulates the cardiovascular changes evoked upon stimulation of the HDA via an action on sympatho-excitatory neurones within the rostral ventrolateral medulla.