title: Evidence for presynaptic adenosine A2a receptors associated with norepinephrine release and their desensitization in the rat nucleus tractus solitarius.
abstract: Rat medullary brain segments containing primarily nucleus tractus solitarius (NTS) were used for superfusion studies of evoked transmitter release and for isotherm receptor binding assays. Isotherm binding assays with [3H]CGS-21680 on membranes prepared from NTS tissue blocks indicated a single high-affinity binding site with a KD of 5.1 +/- 1.4 nM and a Bmax of 20.6 +/- 2.4 fmol/mg of protein. The binding density for [3H]CGS-21680 on NTS membranes was 23 times less than comparable binding on membranes from striatal tissue. Electrically stimulated (1 min at 25 mA, 2 ms, 3 Hz) release of [3H]norepinephrine ([3H]NE) from 400-microns-thick NTS tissue slices resulted in an S2/S1 ratio of 0.96 +/- 0.02. Superfusion of single tissue slices with 0.1-100 nM CGS-21680, a selective adenosine A2a receptor agonist, for 5 min before the S2 stimulus produced a significant concentration-dependent increase in the S2/S1 fractional release ratio that was maximal (31.3% increase) at 1.0 nM. However, superfusion of tissue slices with CGS-21680 over the same concentration range for 20 min before the S2 stimulus did not alter the S2/S1 ratio significantly from control release ratios. The augmented release of [3H]NE mediated by 1.0 nM CGS-21680 with a 5-min tissue exposure was abolished by 1.0 and 10 nM CGS-15943 as well as by 100 nM 8-(3-chlorostyryl)caffeine, both A2a receptor antagonists, but not by 1.0 nM 8-cyclopentyl-1,3-dipropylxanthine, the A1 receptor antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Physiology, Wayne State University School of Medicine, Detroit, MI 48201, USA.
authors.forename: R A
authors.initials: RA
authors.lastname: Barraco
authors.forename: C
authors.initials: C
authors.lastname: Clough-Helfman
authors.forename: B P
authors.initials: BP
authors.lastname: Goodwin
authors.forename: G F
authors.initials: GF
authors.lastname: Anderson
mesh_terms: D000241:Adenosine / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D066298:In Vitro Techniques; D008297:Male; D008526:Medulla Oblongata / Q000378:metabolism*; D009638:Norepinephrine / Q000378:metabolism*; D010627:Phenethylamines / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D018047:Receptors, Purinergic P1 / Q000378:metabolism*; D013997:Time Factors
chemical_list: D010627:Phenethylamines; D018047:Receptors, Purinergic P1; C061282:2-(4-(2-carboxyethyl)phenethylamino)-5'-N-ethylcarboxamidoadenosine; D000241:Adenosine; D009638:Norepinephrine