Elsevier

Neuroscience

Volume 119, Issue 4, 16 July 2003, Pages 965-977
Neuroscience

Co-expression of α7 and β2 nicotinic acetylcholine receptor subunit mRNAs within rat brain cholinergic neurons

https://doi.org/10.1016/S0306-4522(03)00220-3Get rights and content

Abstract

Nicotine enhances cognitive and attentional processes through stimulation of the basal forebrain cholinergic system. Although muscarinic cholinergic autoreceptors have been well characterized, pharmacological characterization of nicotinic autoreceptors has proven more difficult. The present study used double-labeling in situ hybridization to determine expression of nicotinic acetylcholine receptor (nAChR) subunit mRNAs within basal forebrain cholinergic neurons in order to gain information about possible nAChR autoreceptor properties. Cholinergic cells of the mesopontine tegmentum and striatal interneurons were also examined, as were septohippocampal GABAergic neurons that interact with cholinergic neurons to regulate hippocampal activity. α7 and β2 nAChR mRNAs were found to be co-expressed in almost all cholinergic cells and in the majority of GABAergic neurons examined. α4 nAChR mRNA expression was restricted to cholinergic cells of the nucleus basalis magnocellularis, and to non-cholinergic cells of the medial septum and mesopontine tegmentum.

These data suggest possible regional differences in the pharmacological properties of nicotinic autoreceptors on cholinergic cells. Whereas most cholinergic cells express rapidly desensitizing α7 homomers or α7β2 heteromers, cortical projection neurons may also express a pharmacologically distinct α4β2 nAChR subtype. There may also be differential nAChR regulation of cholinergic and non-cholinergic cells within the mesopontine tegmentum that are implicated in acquisition of nicotine self-administration.

Section snippets

Materials

The following materials were obtained from the indicated sources: bovine serum albumin, polyvinyl pyrrolidone, poly-l-lysine, RNase A (Sigma, St. Louis, MO, USA); pBluescript II SK+ (Stratagene, La Jolla, CA, USA); anti-digoxygenin (Dig)-AP Fab antibody, Dig-rUTP, Genius system nonradioactive nucleic acid detection kit, restriction enzymes, T3, T7 polymerases, proteinase K and yeast tRNA (Boehringer Mannheim Biochemicals, Indianapolis, IN, USA); formamide (Fluka, Ronkonkoma, NY, USA); dextran

Dig probe sensitivity

One caveat of using Dig-labeled riboprobes is their lower tissue penetration compared to the 35S-labeled riboprobes, due to the large size of the Dig molecule. Impaired tissue penetration leads to a lower sensitivity of the non-radioactive in situ hybridization. In order to examine this issue, we performed double-labeling in situ hybridization combining radioactive and non-radioactive riboprobes for both ChAT and GAD. The areas chosen for the analysis were the septohippocampal projection

Discussion

The goal of the present study was to identify which nAChR subunit mRNAs are expressed within cholinergic projection neurons of the basal forebrain and mesopontine tegmentum, and in forebrain cholinergic interneurons, in order to gain insight as to which possible subtype(s) of nAChR may regulate central ACh release. Although the results presented here deal exclusively with mRNA expression, they provide information as to the nAChR subunits that can potentially form functional receptors. The nAChR

Conclusions

In summary, the results of the present study indicate that cholinergic projection neurons and forebrain cholinergic interneurons express a similar pattern of nAChR subunit mRNAs, suggesting a mechanism by which both ACh and nicotine can directly stimulate these neurons. The nAChRs on basal forebrain cholinergic and GABAergic neurons may serve as additional targets that mediate the cognitive-enhancing properties of nicotine and other nicotinic ligands. A number of subtype-selective nicotinic

Acknowledgements

This work was supported by NIH grants DA10612, DA13332 and dissertation fellowship 8DT-0168 (L.A.) from California Tobacco Related Disease Research Program.

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