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Evidence for Presynaptic Adenosine A 2a Receptors Associated with Norepinephrine Release and Their Desensitization in the Rat Nucleus Tractus Solitarius
Author(s) -
Barraco Robin A.,
CloughHelfman Carolyn,
Goodwin Bradley P.,
Anderson Gordon F.
Publication year - 1995
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1046/j.1471-4159.1995.65041604.x
Subject(s) - cgs 21680 , medicine , endocrinology , agonist , chemistry , adenosine , solitary nucleus , receptor , biophysics , adenosine a2a receptor , adenosine receptor , biology
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 [ 3 H]CGS‐21680 on membranes prepared from NTS tissue blocks indicated a single high‐affinity binding site with a K D of 5.1 ± 1.4 n M and a B max of 20.6 ± 2.4 fmol/mg of protein. The binding density for [ 3 H]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 [ 3 H]norepinephrine ([ 3 H]NE) from 400‐µm‐thick NTS tissue slices resulted in an S 2 /S 1 ratio of 0.96 ± 0.02. Superfusion of single tissue slices with 0.1–100 n M CGS‐21680, a selective adenosine A 2a receptor agonist, for 5 min before the S 2 stimulus produced a significant concentration‐dependent increase in the S 2 /S 1 fractional release ratio that was maximal (31.3% increase) at 1.0 n M . However, superfusion of tissue slices with CGS‐21680 over the same concentration range for 20 min before the S 2 stimulus did not alter the S 2 /S 1 ratio significantly from control release ratios. The augmented release of [ 3 H]NE mediated by 1.0 n M CGS‐21680 with a 5‐min tissue exposure was abolished by 1.0 and 10 n M CGS‐15943 as well as by 100 n M 8‐(3‐chlorostyryl)caffeine, both A 2a receptor antagonists, but not by 1.0 n M 8‐cyclopentyl‐1,3‐dipropylxanthine, the A 1 receptor antagonist. Taken together, these results suggest that CGS‐21680 augmented the evoked release of [ 3 H]NE in the NTS via activation of presynaptic A 2a receptors within the same concentration range as the binding affinity observed for [ 3 H]CGS‐21680. It was also apparent that this population of presynaptic adenosine A 2a receptors in the NTS desensitized within 20 min because the augmenting action of CGS‐21680 on evoked transmitter release was not evident at the longer interval.