Down-regulation of muscarinic receptors in the striatum of organophosphate-treated swine.
Subacute (daily) administration of diisopropylfluorophosphate (DFP) to male swine (Yorkshire white) resulted in a 97% inhibition of cholinesterase and a decrease of [3H]quinuclidinyl benzilate [( 3H]QNB) binding sites in homogenates of striata by approximately 50% after 14 days. The maximal density of receptors (Bmax) decreased from 2.1 +/- 0.3 to 1.0 +/- 0.2 pmole/mg protein. There was no significant change in the dissociation constant (Kd) for [3H]QNB binding (control: 52.6 +/- 10.7 pM; 7-day: 57 +/- 2.8 pM). Carbachol displacement of [3H]QNB binding yielded data best fit by a two-binding site model. The dissociation constants were KiL = 115 +/- 62 microM (55 +/- 3%) and KiH = 1.8 +/- 0.7 microM (45 +/- 3%), respectively, for the low- and high-affinity states. Seven-Day treatment with DFP reduced the percentage of high-affinity receptors to 22 +/- 8.6%, but affected neither the low- nor the high-affinity Kd (100 +/- 20 and 2 +/- 0.6 microM). With the addition of Mg2+, striatal homogenates had low- and high-affinity receptors in the proportion of approximately 1 to 1. In the presence of Gpp(NH)p + Mg2+ the ratio of high- to low-affinity receptors was 3:1 in homogenates of control tissue (to 26 +/- 5%). This treatment had no effect on this ratio in homogenates of tissue from 7-day DFP-treated swine (3:1) since it was already 3:1. Pirenzepine displacement of [3H]QNB binding was best described by a two-binding site model, with Ki values of 38 +/- 14 and 201 +/- 78 nM, which represent 74 and 26% of the binding sites, respectively. The high affinity Kd value was unchanged following 7 days of DFP treatment (24 +/- 5 nM). There appears to be little change in the displacement curves for pirenzepine inhibition of [3H]QNB binding. This suggests that about 75% of the receptors are of the M1 subtype. Thus, subacute administration of DFP causes not only a decrease in the number of receptors, but also a change in the proportion of agonist affinity states which is related to the interaction of the guanine nucleotide binding protein and the muscarinic receptor.
Yang
C M
CM
Department of Pharmacology/Toxicology, University of Mississippi Medical Center, Jackson 39216-4505.
Dwyer
T M
TM
Mohan
P M
PM
Ho
I K
IK
Farley
J M
JM
D000110:Acetylcholinesterase / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D001667:Binding, Competitive; D002217:Carbachol / Q000494:pharmacology; D003342:Corpus Striatum / Q000378:metabolism*; D015536:Down-Regulation; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D007531:Isoflurophate / Q000494:pharmacology*; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D010890:Pirenzepine / Q000494:pharmacology; D011812:Quinuclidines / Q000378:metabolism*; D011813:Quinuclidinyl Benzilate / Q000378:metabolism*; D011976:Receptors, Muscarinic / Q000187:drug effects / Q000378:metabolism*; D012044:Regression Analysis; D013552:Swine
D011812:Quinuclidines; D011976:Receptors, Muscarinic; D007531:Isoflurophate; D006165:Guanylyl Imidodiphosphate; D010890:Pirenzepine; D011813:Quinuclidinyl Benzilate; D002217:Carbachol; D000110:Acetylcholinesterase; D008274:Magnesium