Cell autonomy, receptor autonomy, and thermodynamics in nicotine receptor up-regulation.
Chronic nicotine exposure, in smokers or in experimental rodents administered nicotine, produces elevated levels of nicotinic acetylcholine receptors in several brain regions. However, there are few data on up-regulation of receptors in specific neuronal subtypes. We tested whether functional up-regulation of nicotinic responses occurs in cultured GABAergic neurons of the ventral midbrain. Fura-2 measurements of nicotinic responses were made on ventral midbrain neurons from knock-in mice heterozygous for the alpha4-M2 domain Leu9'Ala mutation, which confers nicotine hypersensitivity. Chronic nicotine exposure at a concentration (10 nM for 3 days) that activates only the hypersensitive alpha4* (Leu9'Ala) receptors, but not wild-type receptors, resulted in significant potentiation of ACh (100 microM)-elicited responses. Experiments were also performed on midbrain neuronal cultures heterozygous for the alpha4* (Leu9'Ala) mutation as well as for a GFP protein fused to a GABA transporter that reliably reveals GABAergic neurons. In cultures chronically treated with 10nM nicotine, there was significantly increased alpha4* nicotinic-induced Ca(2+) influx elicited by low concentration of ACh (3 microM). Furthermore, chronic exposure to the competitive antagonist dihydro-beta-erythroidine, but not to the noncompetitive antagonist mecamylamine, induced up-regulation of ACh elicited nicotinic responses. These results suggest that occupation of alpha4* nicotinic receptor binding site(s), at the interface between two subunits, is sufficient to promote assembly and/or up-regulation of functional receptors in GABAergic neurons. Up-regulation in neurons is both "cell-autonomous", occurring at the cell itself, and "receptor autonomous", occurring at the receptor itself, and may be a thermodynamic necessity of ligand-protein interactions.
Nashmi
Raad
R
Division of Biology, California Institute of Technology, Pasadena, CA 91125, USA.
Lester
Henry
H
D000818:Animals; D002478:Cells, Cultured; D004079:Dihydro-beta-Erythroidine / Q000494:pharmacology; D008464:Mecamylamine / Q000494:pharmacology; D008636:Mesencephalon / Q000187:drug effects* / Q000378:metabolism; D051379:Mice; D009538:Nicotine / Q000494:pharmacology*; D011978:Receptors, Nicotinic / Q000096:biosynthesis*; D013816:Thermodynamics; D015854:Up-Regulation
D011978:Receptors, Nicotinic; D004079:Dihydro-beta-Erythroidine; D008464:Mecamylamine; D009538:Nicotine