Functional characterization of adenosine receptors in the nucleus tractus solitarius mediating hypotensive responses in the rat

1. The aim of this study was to characterize adenosine receptors located in the nucleus tractus solitarius (NTS) that mediate decreases in blood pressure in the anaesthetized rat. To determine the adenosine receptor subtype involved, a range of selective agonists and antagonists were studied and their relative potencies evaluated. 2. The rank order of agonist potency in inducing decreases in diastolic blood pressure was N⁶-cyclopentyladenosine (CPA) > N⁶-cyclohexyladenosine (CHA) > N-ethyl-carboxamidoadenosine (NECA) ≥ 2-phenylaminoadenosine (CV1808) > 2-p-(carboxyethyl)phenethylamino-5' N-ethylcarboxamidoadenosine (CGS 21680) > N⁶-(2-(4-aminophenyl)ethyl)-adenosine (APNEA). 3. The hypotensive action of CPA following microinjection into the NTS was antagonized by i.v. infusions (50 μg kg^-1 min^-1) of adenosine receptor antagonists, 8-cyclopentyl-1,3 dipropylxanthine (DPCPX), 8-phenyltheophylline (8-PT), 8-(p-sulphophenyl)theophylline (8-SPT), and 1,3-dipropyl-8-N-(2-(diethylamino)ethyl)-N methyl-4-(2,3,6,7-tetrahydro-2,6-dioxo) benzenesulphonamidexanthine (PD 115199). The antagonist potency order was DPCPX > PD115199 ≥ 8-PT. Intravenous infusion of 8-SPT had no effect on blood pressure responses to microinjection of CPA into the NTS. 4. The results suggest that adenosine A₁ receptors in the NTS mediate hypotensive responses in the anaesthetized rat preparation.