Presynaptic adenosine A_2a receptors on soma and central terminals of rat vagal afferent neurons

The dorsal vagal complex of the medulla oblongata is a key centre involved in the regulation of numerous autonomic functions, including cardiovascular control. Adenosine has been implicated as a potential neuromodulator of the baroreceptor reflex, and therefore the current study has investigated the presence and characteristics of adenosine receptors on rat vagal afferent neurons. In the nodose-vagal grease gap preparation, the adenosine A_2a agonist CGS-21680 evoked a depolarisation only in the presence of the selective adenosine A₁ antagonist PACPX. Autoradiography using [³H]NECA (4 nM) with suppression of A₁ binding enabled the first visualisation of high affinity adenosine A₂ receptors in the nucleus tractus solitarius (NTS). Unilateral nodose ganglionectomy resulted in over 90% reduction in binding in the lesioned (ipsilateral) NTS compared to a sham control. Furthermore, local administration of CGS-21680 increased evoked glutamate release in the NTS, as measured by in vivo microdialysis. These data suggest the presence of presynaptic adenosine A_2a receptors on both the soma and central terminals of rat vagal afferent neurons, and thereby support the hypothesis that adenosine may have a modulatory role in the baroreceptor reflex.