Pharmacological characterization of the rat hippocampal muscarinic autoreceptor

The interactions of both agonists and antagonists with the hippocampal presynaptic muscarinic autoreceptor were studied in a release assay with mechanically dissociated tissue. Muscarinic agonists inhibited potassium-ion-evoked release of [3H]acetylcholine maximally by 30 to 40%. Using partial receptor alkylation, the Kd value for oxotremorine-M was determined to be 2.5 microM, whereas the EC50 for this agonist was 0.43 microM; the Kd/EC50 ratio (5.8) indicates that a modest degree of "spare receptors" exist for this response. The relative intrinsic efficacies of partial agonists (pilocarpine, [(4-hydroxy-2-butynyl)trimethyl ammonium chloride] and bethanechol) were compatible with the involvement of a mixture of the M2 and the M4 receptor subtypes. With Schild analysis, pirenzepine was shown to block with low potency (Ki = 370 nM), which indicated that the M1 muscarinic receptor was not involved in this response. The potencies of the cardioselective drugs (11-[[2-(diethylamino)methyl]-1-piperdinyl] acetyl]-5,11-dihydro-6H-pyrido[2,3-b][1,4]benzodiazepine-6-one] and methoctramine (111 and 61 nM, respectively) were somewhat too low to definitely indicate the involvement of an M2 receptor subtype. The Ki values for the smooth muscle/glandular-selective drugs, 4-diphenylacetoxy-N-methylpiperidine methbromide and hexahydrosiladifenidol (2.5 and 87 nM, respectively), were similar to values for these drugs at the M4 subtype, but their potencies did not clearly distinguish this receptor from the M3 subtype. However, the Kd value for himbacine at this receptor (8.6 nM) ruled out involvement of the M3 receptor.(ABSTRACT TRUNCATED AT 250 WORDS)