Two distinct classes of functional alpha 7-containing nicotinic receptor on rat superior cervical ganglion neurons

1. Nicotinic acetylcholine receptors (nAChRs) that bind alpha-bungarotoxin (alpha Bgt) were studied on isolated rat superior cervical ganglion (SCG) n eurons using whole-cell patch clamp recording techniques. 2. Rapid application of ACh onto the soma of voltage clamped neurons evoked a slowly desensitizing current that was reversibly blocked by alpha Bgt (5 0 nM). The toxin-sensitive current constituted on average about half of the peak whole-cell response evoked by ACh. 3. Nanomolar concentrations of methyllycaconitine blocked the alpha Bgt-sen sitive component of the ACh-evoked current as did intracellular dialysis wi th an anti-alpha 7 monoclonal antibody. The results indicate that the slowl y reversible toxin-sensitive response elicited by ACh arises from activatio n of an unusual class of alpha 7-containing receptor (alpha 7-nAChR) simila r to that reported previously fur rat intracardiac ganglion neurons. 4. A second class of functional alpha 7-nAChR was identified on some SCG ne urons by using rapid application of choline to elicit responses. In these c ases a biphasic response was obtained, which included a rapidly desensitizi ng component that was blocked by alpha Bgt in a pseudo-irreversible manner. The pharmacology and kinetics of the responses resembled those previously attributed to alpha 7-nAChRs in a number of other neuronal cell types. 5. Experiments measuring the dissociation rate of I-125-labelled alpha Bgt from SCG neurons revealed two classes of toxin-binding site. The times for toxin dissociation were consistent with those required to reverse blockade of the two kinds of alpha Bgt-sensitive response. 6. These results indicate that rat SCG neurons express two types of functio nal alpha 7-nAChR, differing in pharmacology, desensitization and reversibi lity of alpha Bgt blockade.