RETROTRAPEZOID NUCLEUS MUSCARINIC RECEPTOR SUBTYPES LOCALIZED BY AUTORADIOGRAPHY

Microinjection of muscarinic receptor subtype antagonists into the reg ion of the cat retrotrapezoid nucleus (RTN) decreases blood pressure ( greatest efficacy; M2 subtype) and both baseline phrenic activity and CO2 sensitivity (greatest efficacy; M3/M1 subtype). Here we examine, i n cat medullary sections at the level of the RTN, the effects of the s ame antagonists on binding of the high affinity muscarinic agonist qui nuclidinyl benzilate (QNB). H-3-QNB binding was saturated and highly s pecific at 1 nM concentration and stable over 30 to 120 min (K-d, 0.49 nM; B-max, 136 fm/mg protein). Studied biochemically, we found IC50 v alues for whole sections of 4.9 x 10(-6) M (M1 antagonist pirenzepine) ; 1.0 x 10(-6) M (M2 antagonist AFDX); and 0.64 x 10(-7) M (M3 antagon ist DAMP; P<0.03 vs PZ). Densitometric analysis of whole medullary cro ss section autoradiograms resulted in similar IC50 values as in the bi ochemical approach. Specific analysis of the RTN region demonstrated t he presence of H-3-QNB binding and similar competition by the antagoni sts. Average IC50 values determined by densitometry were 14 x 10(-6) M (pirenzepine); 1.3 x 10(-6) M (AFDX; P<0.01 vs PZ); and 0.53 x 10(-7) M (DAMP; P<0.01 vs PZ). All three subtypes of muscarinic receptors id entifiable via pharmacological antagonists appear to be present in the RTN region but we could not distinguish a subtype-specific pattern of receptor distribution.