Presynaptic cholinergic mechanisms in the rat cerebellum: evidence for nicotinic, but not muscarinic autoreceptors.
The present study shows that N-[3H]methylcarbamylcholine ([3H]MCC) binds to a single population of high-affinity/low-density (KD = 5.0 nM; Bmax = 8.2 fmol/mg of protein) nicotinic binding sites in the rat cerebellum. Also, there exists a single class of high-affinity binding sites (KD = 4.8 nM; Bmax = 24.2 fmol/mg of protein) in the cerebellum for the M1 specific muscarinic ligand [3H]pirenzepine. In contrast, the M2 ligand, [3H]AF-DX 116, appears to bind to two classes of binding sites, i.e., a high-affinity (KD = 3 nM)/low-capacity (Bmax = 11.7 fmol/mg of protein) class, and a second class of lower affinity (KD = 28.4 nM) and higher capacity (Bmax = 36.3 fmol/mg of protein) sites. The putative M3 selective ligand [3H]4-diphenylacetoxy-N-methylpiperidine also binds to two distinct classes of binding sites in cerebellar homogenates, one of high affinity (KD = 0.5 nM)/low capacity (Bmax = 19.5 fmol/mg of protein) and one of low affinity (KD = 57.5 nM)/high capacity (Bmax = 140.6 fmol/mg of protein). In experiments which tested the effects of cholinergic drugs on acetylcholine release from cerebellar brain slices, the nicotinic agonist MCC enhanced spontaneous acetylcholine release in a concentration-dependent manner, and the maximal increase in acetylcholine release (59.0-68.0%) occurred at 10(-7) M. The effect of MCC to increase acetylcholine release was Ca2+-dependent and tetrodotoxin-insensitive, suggesting an action on cholinergic terminals. Also, the MCC-induced increase in acetylcholine release was effectively antagonized by dihydro-beta-erythroidine, d-tubocurarine, and kappa-bungarotoxin, but was insensitive to either atropine or alpha-bungarotoxin.(ABSTRACT TRUNCATED AT 250 WORDS)
Lapchak
P A
PA
Department of Pharmacology, McGill University, Montreal, Quebec, Canada.
Araujo
D M
DM
Quirion
R
R
Collier
B
B
D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D001921:Brain / Q000378:metabolism; D002217:Carbachol / Q000031:analogs & derivatives / Q000378:metabolism; D002531:Cerebellum / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008024:Ligands; D008297:Male; D010276:Parasympatholytics / Q000378:metabolism; D010890:Pirenzepine / Q000031:analogs & derivatives / Q000378:metabolism; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000032:analysis*; D011978:Receptors, Nicotinic / Q000032:analysis / Q000378:metabolism*
D008024:Ligands; D010276:Parasympatholytics; D011976:Receptors, Muscarinic; D011978:Receptors, Nicotinic; C053765:N-methylcarbamylcholine; D010890:Pirenzepine; D002217:Carbachol; D000109:Acetylcholine; C048593:otenzepad