Nicotinic and muscarinic cholinergic receptors in honey bee ( Apis mellifera) brain

Abstract

1. Honey bee head homogenates contained particulate components capable of binding the nicotinic receptor antagonist α-bungarotoxin (BGT) and the muscarinic receptor antagonist quinuclidinyl benzilate (QNB). Specific binding of [ 125I]BGT (denned by nicotine) and [ 3H]QNB (defined by atropine) was heat sensitive, linear with tissue concentration, and saturable. B max values were 204 fmol mg protein −1 for [ 125I]BGT and 57 fmol mg protein −1 for [ 3H]QNB yielding a binding site ratio of 3.6:1. Hill coefficients were 1.0 for each radioligand. 2. Binding by both radioligands was rapid and reversible. Association ( k +1) and dissociation ( k −1) rates were 1.38 × 10 6 s −1 M −1 and 6.2 × 10 −4 s −1 for [ 125I]BGT and 3.27 × 10 6 s −1 M −1 and 9.4 × 10 −5 s −1 for [ 3H]QNB. The dissociation rate constants ( K D) were 450 pM ( k −1 k +1 ) and 743 pM (saturation) for [ 125I]BGT and 30pM ( k −1 k +1 ) and 96 pM (saturation) for [ 3H]QNB. 3. Pharmacological profiles were nicotinic for [ 125I]BGT with nicotine ( K i 2.6 × 10 −7 M), d-tubocurarine ( K i 1.0 × 10 −6 M), and ACh + dichlorvos ( K i 4.5 × 10 −6 M ) being the most potent inhibitors and muscarinic for [ 3H]QNB with (±)-QNB ( K i 2.2 × 10 −12M), S( + )-dexetimide ( K i 2.9 × 10 −11 M ), atropine ( K i 6.9 × 10 −10M ), and scopolamine ( K i 4.1 × 10 −10M ) being most potent. 4. It appeared that [ 125I]BGT and [ 3H]QNB were binding with high affinity to honey bee brain populations of nicotinic and muscarinic cholinergic receptors, respectively.