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 alpha 4-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 alpha 4* (Leu9'Ala) receptors, but not wild-type receptors, resulted in significant potentiation of ACh (100 mu M)-elicited responses. Experiments were also performed on midbrain neuronal cultures heterozygous for the alpha 4* (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 10 nM nicotine, there was significantly increased alpha 4* nicotinic-induced Ca2+ influx elicited by low concentration of ACh (3 mu M). Furthermore, chronic exposure to the competitive antagonist dihydro-p-erythroidine, but not to the noncompetitive antagonist mecamylamine, induced up-regulation of ACh elicited nicotinic responses. These results suggest that occupation of alpha 4* 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. (C) 2007 Elsevier Inc. All rights reserved.