PHARMACOLOGY AND BIOPHYSICAL PROPERTIES OF ALPHA-7 AND ALPHA-7-ALPHA-8 ALPHA-BUNGAROTOXIN RECEPTOR SUBTYPES IMMUNOPURIFIED FROM THE CHICK OPTIC LOBE

Two chick optic lobe alpha-bungarotoxin receptor subtypes (alpha 7 and alpha 7-alpha 8) were immunopurified using polyclonal antibodies rais ed against synthetic peptides of chick alpha 7 and alpha 8 alpha-bunga rotoxin receptor subunits. The alpha 7 subtype contained the M(r) 57 0 00 alpha 7 subunit, and represented 60-70% of the alpha-bungarotoxin r eceptors; the alpha 7-alpha 8 subtype contained the M(r) 57 000 alpha 7 and alpha 8 subunits, and represented only 20-25% of the receptors. Both subtypes also had an additional M(r) 52 000 subunit. The affinity of these subtypes for alpha-bungarotoxin as well as antagonists was s imilar. However, the alpha 7-alpha 8 subtype displayed consistently hi gher affinities for agonists. When reconstituted in planar lipid bilay ers, the alpha 7-alpha 8 subtype displayed several conductance states of 10-50 pS; the alpha 7 subtype had only one conductance state of 45 pS. The alpha 7-alpha 8 subtype was activated by lower agonist concent rations than the alpha 7 subtype. When expressed in Xenopus oocytes, t he alpha 8 subunit formed functional homomeric receptors that desensit ized rapidly. These channels were blocked by alpha-bungarotoxin and di splayed a higher affinity for agonists than the alpha 7 homomeric rece ptor. Taken together, these data indicate that at least two alpha-bung arotoxin subtypes are present in the chick optic lobe. They operate as ligand-gated channels and display different agonist sensitivities and kinetics/conductance properties.