Nicotinic acetylcholine receptors containing alpha 7 subunits on rat cortical neurons do not undergo long-lasting inactivation even when up-regulated by chronic nicotine exposure.
Chronic exposure to (-)nicotine has been widely reported to up-regulate nicotinic acetylcholine receptors on neurons and induce long-term inactivation as a possible cause. Nicotinic receptors containing alpha 7 subunits are among the most abundant in brain and influence diverse cellular events. Whole-cell patch clamp recording from embryonic rat cortical neurons in culture was used to identify responses from alpha 7-containing receptors. Immunochemical staining for glutamic acid decarboxylase (GAD) indicated that both GABAergic and non-GABAergic neurons expressed the receptors. Exposure to micromolar concentrations of nicotine for 1-4 days caused up-regulation of the receptors as measured by [alpha-(125)I]-bungarotoxin binding. Carbachol produced the same up-regulation, and cell counts demonstrated that neuronal survival was unchanged. The up-regulation was accompanied by an increased whole-cell response; no evidence was found for long-lasting inactivation. Autonomic alpha 7-containing receptors also avoided long-lasting inactivation, even though the receptors were down-regulated by nicotine. Blocking protein synthesis or protein glycosylation prevented receptor up-regulation on cortical neurons, suggesting that new synthesis was required. No evidence was found for a pre-existing intracellular pool that supplied receptors to the surface. The results indicate that alpha 7-containing receptors differ from other receptor subtypes in their regulation by nicotine and demonstrate further that long-lasting inactivation is not an obligatory requirement for up-regulation in this case.
Kawai
H
H
Neurobiology Section, Division of Biology, University of California, San Diego, La Jolla, California 92093-0357, USA.
Berg
D K
DK
D000818:Animals; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000096:biosynthesis; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009538:Nicotine / Q000494:pharmacology*; D018722:Nicotinic Agonists / Q000494:pharmacology*; D020033:Protein Isoforms / Q000378:metabolism; D051381:Rats; D011978:Receptors, Nicotinic / Q000737:chemistry / Q000187:drug effects* / Q000378:metabolism*; D013997:Time Factors; D015854:Up-Regulation
D009419:Nerve Tissue Proteins; D018722:Nicotinic Agonists; D020033:Protein Isoforms; D011978:Receptors, Nicotinic; D009538:Nicotine