NICOTINIC AND MUSCARINIC CHOLINERGIC RECEPTORS IN HONEY-BEE (APIS-MELLIFERA) BRAIN

1. Honey bee head homogenates contained particulate components capable of binding the nicotinic receptor antagonist alpha-bungarotoxin (BGT) and the muscarinic receptor antagonist quinuclidinyl benzilate (QNB). Specific binding of [I-125]BGT (defined by nicotine) and [H-3]QNB (defined by atropine) was heat sensitive, linear with tissue concentration, and saturable. B(max) values were 204 fmol mg protein-1 for [I-125]BGT and 57 fmol mg protein-1 for [H-3]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 x 10(6) S-1 M-1 and 6.2 x 10(-4) S-1 for [I-125]BGT and 3.27 x 10(6) S-1 M-1 and 9.4 x 10(-5) S-1 for [H-3]QNB. The dissociation rate constants (K(D)) were 450 pM (k-1/k+1) and 743 pM (saturation) for [I-125]BGT and 30 pM (k-1/k+1) and 96 pM (saturation) for [H-3]QNB. 3. Pharmacological profiles were nicotinic for [I-125]BGT with nicotine (K(i) 2.6 x 10(-7) M), D-tubocurarine (K(i) 1.0 x 10(-6) M), and ACh + dichlorvos (K(i) 4.5 x 10(-6) M) being the most potent inhibitors and muscarinic for [H-3]QNB with (+/-)-QNB (K(i) 2.2 x 10(-12) M), S(+)-dexetimide (K(i) 2.9 x 10(-11) M), atropine (K(i) 6.9 x 10(-10) M), and scopolamine (K(i) 4.1 x 10(-10) M) being most potent. 4. It appeared that [I-125]BGT and [H-3]QNB were binding with high affinity to honey bee brain populations of nicotinic and muscarinic cholinergic receptors, respectively.