Quantitative effects of some muscarinic agonists on evoked surface-negative field potentials recorded from the guinea-pig olfactory cortex slice

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Abstract
1 The effects of muscarinic receptor agonists on the electrically-evoked surface-negative field potential (N-wave) were measured in the guinea-pig olfactory cortex slice maintained in vitro. 2 Bath-superfusion of (±)-muscarine, acetylcholine (ACh), carbachol (CCh), or methacholine (MCh) (10–200 μm) produced reversible, dose-dependent depressions of the N-wave (ACh and MCh effects were observed in the presence of 10 μm neostigmine). The order of potencies (based on agonist dose causing 50% field depression: IC50) was: ACh ≥ muscarine > CCh > MCh. All four agonists depressed the field potential by 100% at doses greater than 500 μm. 3 Pilocarpine and bethanechol were weak agonists and only produced measurable effects at high doses (1–2 mm). Neither agonist evoked a maximum response at doses up to 10 mm. 4 The muscarinic ganglion stimulant, McN-A-343 yielded inconsistent results, depressing the field potential in some slices, but having no effect in others. Pre-application of a conditioning dose (100 μm) of McN-A-343 reduced subsequent responses to CCh, suggesting possible partial agonist properties. 5 Oxotremorine (up to 100 μm) did not depress the field potential, but it reversibly antagonized the effects of CCh. 6 It is concluded that reproducible, quantifiable responses to muscarinic agonists can be evoked in the olfactory cortex slice. We suggest this preparation may be useful for conducting pharmacological studies of ‘intact’ central muscarinic receptors.