Pharmacological and Biochemical Properties of Nicotinic Receptors from Chick Retina

Abstract

Previous work has established that functional nicotinic receptors in the chick retina are blocked by neuronal bungarotoxin (NBT), and that the binding of radio-iodinated NBT to retinal homogenates is displaced by nicotinic ligands. In the present study, we examined the desensitizing effects of agonists on nicotinically-mediated depolarizations recorded from chick retina. The concentrations of five agonists necessary to reduce the amplitude of these depolarizations by 50% were found to correlate closely with the concentrations of these same agonists previously found necessary to displace 50% of NBT binding. In addition, bromoacetylcholine (BAC), a selective affinity alkylating agent for the agonist binding site, irreversibly inactivated the functional responses of intact chick retina with an inhibiting concentration for 50% block (IC50) near 10-6 M, the same concentration of BAC that displaced 50% of labelled NBT binding from alkylated retinal homogenates. These data suggest that NBT acts at the receptor agonist binding site. Furthermore, this binding site has a relatively low affinity for agonists, in the micromolar range, even in the desensitized state. Multiple subtypes of nicotinic receptors are known to exist in neuronal tissue, and receptors that bind agonists in the nanomolar range have been detergent-solubilized and purified using monoclonal antibodies. Under similar conditions, detergent-solubilization of chick retinal homogenates interfere with the interaction between NBT and the low-affinity neuronal nicotinic receptors. These data suggest that the conditions used to purify high-affinity neuronal nicotinic receptors may denature the subtype(s) of neuronal receptors recognized by NBT.

Effects of P‐Aminophenyl Dichloroarsine on Reduced High‐affinity [3H]Nicotine Binding Sites from Chick Brain: A Covalent, Yet Reversible, Agent for Neuronal Nicotinic Receptors

“…Not all subtypes of neuronal receptors bind nicotine with high affinity, and some of the probes for those receptors, such as neuronal bungarotoxin, fail to bind to the receptors after detergent solubilization (e.g. Loring and Zigmond, 1990). Therefore, we have sought to develop probes that should recognize virtually all subtypes of nicotinic receptors, both before and after detergent solubilization.…”

“…Reducing agents have similar effects on high-affinity agonist binding to rat (Schwartz and Kellar, 1983), mouse (Stitzel et al, 1988) and chick nAChRs (this study). Electrophysiological recordings on chick retina show that DTT reduction substantially blocks nicotinic responses (Loring ef al., 1989) and that BAC alkylation of DTT-treated receptors causes an irreversible block in the responses elicited by agonist (Loring and Zigmond, 1990). In addition, reduced subunits immunopurified from rat, chicken, bovine and human brain can be affinity alkylated with BAC and maleimidobenzyltrimethylammonium (Whiting andLindstrom, 1987, 1988).…”