| Neuronal nicotinic acetylcholine receptors (nAChRs) are present throughout the central and peripheral nervous systems. Native nAChRs are not only heterogeneous in subtypes but also complex in subunit composition. In the present study, we used receptor binding and immunoprecipitation methods to examine the nAChRs in a number of peripheral ganglia and brain regions from adult rat. In the periphery, nAChRs were found in superior cervical ganglia (SCG), nodose, stellate, and pelvic ganglia. However, density of the nAChRs in SCG was ∼10 to 50 times higher than those in the other ganglia. A majority of the nAChRs in SCG and nodose ganglia contained alpha3 and beta4 subunits, but a significant fraction of the nAChRs in these ganglia also contained alpha5 and beta2 subunits. Sequential immunoprecipitations in SCG revealed that all alpha5 subunits were associated with alpha3 and beta4 subunits, forming the mixed heteromeric alpha3beta4alpha5 subtype, representing 25--30% of the total heteromeric nAChRs in SCG; also, 55--60% and 10--15% of the nAChRs were simple alpha3beta4 and mixed alpha3beta4beta2 subtype, respectively. In brain, in addition to the predominant alpha4 and beta2 subunits, we found significant fractions of nAChRs contain alpha5 subunit, ranging from ∼one-tenth of the total nAChRs in superior colliculus to more than one-third in hippocampus. Moreover, alpha5 subunit was exclusively incorporated in the alpha4beta2 nAChRs, forming the mixed heteromeric alpha4beta2alpha5 subtype in hippocampus, striatum, cerebral cortex and thalamus. Most importantly, during chronic administration of nicotine in rats, density of the alpha4beta2alpha5 subtype did not change, despite significant up-regulation of the alpha4beta2* and alpha3beta2* nAChRs in brain. This differential regulatory effect of nicotine also extended to other nAChR subtypes. Similar to the alpha4beta2alpha5 subtype, nAChRs containing beta3 subunit in striatum and superior colliculus, most likely in the composition of alpha6beta2beta3*, appeared resistant to regulation by nicotine, whereas the overall density of the alpha6-containing nAChRs decreased in striatum, but remained the same in superior colliculus. The differential regulation by chronic nicotine may not only provide new clues to the mechanisms of nicotine-induced up-regulation of nAChRs in brain, but also have important implications in various aspects of nicotine addiction. |
