Upregulation of [3H]methyllycaconitine binding sites following continuous infusion of nicotine, without changes of α7 or α6 subunit mRNA: an autoradiography andin situhybridization study in rat brain

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Abstract
It is well established that exposure of experimental animals to nicotine results in upregulation of the α4β2-subtype of neuronal nicotinic acetylcholine receptors (nAChRs). The aim of this study was to determine the effect of nicotine on the levels of α7-nAChRs in rat brain, for which only partial information is available. Rats were infused with nicotine (3 mg/kg/day) or saline for 2 weeks and their brains processed for receptor autoradiography with [3H]methyllycaconitine (MLA), a radioligand with nanomolar affinity for α7-nAChRs. In control rats binding was high in hippocampus, intermediate in cerebral cortex and hypothalamus, and low in striatum, thalamus and cerebellum. There was high correlation between the distribution of [3H]MLA binding sites and α7 subunit mRNA (r = 0.816). With respect to saline-treated controls, nicotine-treated rats presented higher [3H]nicotine binding in 11 out of 15 brain regions analysed (average increase 46 ± 6%). In contrast, only four regions showed greater [3H]MLA binding, among which the ventral tegmental area (VTA) and cingulate cortex (mean increase 32 ± 3%). No changes in α7 mRNA levels were observed after nicotine treatment. Similarly, there was no variation of α6 subunit transcript in the VTA, a region which may contain MLA-sensitive (non-α7)-α6*-nAChRs ( Klink et al., 2001 ). In conclusion, nicotine increased [3H]MLA binding, although to a smaller extent and in a more restricted regional pattern than [3H]nicotine. The enhancement of binding was not paralleled by a significant change of α7 and α6 subunit transcription. Finally, the present results provide the first anatomical description of the distribution of [3H]MLA binding sites in rat brain.