FUNCTIONAL CONTRIBUTION OF THE ALPHA-7 SUBUNIT TO MULTIPLE SUBTYPES OF NICOTINIC RECEPTORS IN EMBRYONIC CHICK SYMPATHETIC NEURONS

1. Madly studies of the alpha 7 subunit of the neuronal nicotinic acet ylcholine receptor (nAChR) family have demonstrated that this cc-bunga rotoxin (alpha-BgTx)-binding neuronal receptor can participate in ACh- gated channels. Heterologous expression studies reveal that alpha 7 su bunits form homomeric channels of unusually high Ca2+ permeability. Ho wever, the physiological role of the alpha 7 subunit in native neurona l nAChR channels is less clear. 2. We present evidence that the alpha 7 subunit contributes to the function of at least three subtypes, of n ative nAChR expressed by embryonic chick sympathetic neurones. These s ubtypes functionally distinct from heterologously expressed homomeric alpha 7 nAChRs as well as homomeric-like currents described in studies of hippocampal and parasympathetic neurones. 3. The proposed nAChRs d iffer from one another and from homomeric alpha 7 nAChRs in their sens itivity to block by alpha 7 subunit-specific antagonists: alpha-BgTx a nd methyllycaconitine (MLA). While MLA blocks 60% of the macroscopic A Ch response, alpha-BgTx inhibits a small component of the macroscopic current described by slow-on and slow-off kinetics. 4. Functional dele tion of the alpha 7 subunit by antisense oligonucleotide treatment eli minates the susceptibility of the nAChRs to block by both MLA and alph a-BgTx. 5. Single channel recordings combined with pharmacological and antisense-mediated 'deletion' techniques reveal that alpha-BgTx-sensi tive alpha 7-containing nAChRs have a small unitary conductance (18 pS ), brief open time kinetics and relatively low open probability (P-o). MLA-sensitive alpha 7 nAChRs are characterized by a conductance of si milar to 35 pS, intermediate burst duration, and a relatively high P-o . 6. The third nAChR subtype deleted by alpha 7 antisense treatment is characterized by a unitary conductance of 50 pS and prolonged opening duration. 7. We propose that these three populations of native alpha 7-containing nAChRs are distinct heteromeric complexes that include ot her alpha and/or beta nAChR subunits.