Nicotinic acetylcholine currents of cultured Kenyon cells from the mushroom bodies of the honey bee Apis mellifera

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Abstract
1 Acetylcholine‐induced currents of mushroom body Kenyon cells from the honey bee Apis mellifera were studied using the whole‐cell configuration of the patch clamp technique. Pressure application of 1 mM acetylcholine (ACh) induced inward currents with amplitudes between ‐5 and ‐500 pA. 2 The cholinergic agonists ACh and carbamylcholine had almost equal potencies of current activation at concentrations between 0·01 and 1 mM; nicotine was less potent. The muscarinic agonist oxotremorine did not elicit any currents. 3 Approximately 80 % of the ACh‐induced current was irreversibly blocked by 1 μM α‐bungarotoxin. Atropine (1 mM) did not block the ACh‐induced current. 4 Upon prolonged ACh application the current desensitized with a time course that could be approximated by the sum of two exponentials (τ1= 276 ± 45 ms (mean ± s.e.m.) for the fast component and τ2= 2·4 ± 0·7 s for the slow component). 5 Noise analyses of whole‐cell currents yielded elementary conductances of 19·5 ± 2·4 pS for ACh and 23·7 ± 5·0 pS for nicotine. The channel lifetimes, calculated from the frequency spectra, were τo= 1·8 ms for ACh and τo= 2·5 ms for nicotine. 6 Raising the external calcium concentration from 5 to 50 mM shifted the reversal potential of the ACh‐induced current from +4·6 ± 0·9 to +37·3 ± 1·3 mV. The calcium‐to‐sodium permeability ratio (PCa : PNa) was 6·4. 7 In high external calcium solution (50 mM) the ACh‐induced current rectified in an outward direction at positive membrane potentials. 8 We conclude that Kenyon cells express nicotinic ACh receptors with functional profiles reminiscent of the vertebrate neuronal nicotinic ACh receptor subtype.