Diversity and distribution of nicotinic acetylcholine receptors in the locus ceruleus neurons

Abstract

The neurons of the locus ceruleus are responsible for most of the noradrenergic innervation in the brain and nicotine potentiates noradrenaline release from their terminals. Here we investigated the diversity and subcellular distribution of nicotinic acetylcholine receptors (nAChRs) in the locus ceruleus both somatically, by combining single-cell reverse transcription-PCR with electrophysiological characterization, and at the level of nerve terminals, by conducting noradrenaline efflux experiments. The proportion of neurons in the locus ceruleus expressing the nicotinic subunit mRNAs varied from 100% (␤2) to 3% (␣2). Yet, two populations of neurons could be distinguished on the basis of the pattern of expression of nAChR mRNAs and electrophysiological properties. One population (type A) of small cells systematically expressed ␣3 and ␤4 mRNAs (and often ␣6, ␤3, ␣5, ␣4), and nicotinic agonists elicited large currents with a potency order of cytisine > nicotine. Another population (type B) of cells with large soma did not contain ␣3 and ␤4 mRNAs but, systematically, ␣6 and ␤3 (and often ␣4) and responded to nicotinic agonists in the order of nicotine > cytisine. The nicotinic modulation of noradrenaline release in the hippocampus displayed an order of potency nicotine > cytisine, suggesting that noradrenergic terminals in the hippocampus originate largely from type B cells of the locus ceruleus. Accordingly, immunocytochemical labeling showed that ␤3 is present in hippocampal terminals. The ␣6␤3␤2(␣4) heterooligomer thus behaves as the main nicotinic regulator of the ceruleo-hippocampal pathway. Brain nicotinic acetylcholine receptors (nAChRs) are heteropentameric ligand-gated ion channels that result from the combinatorial assembly of up to nine different subunits (␣2-7, ␤2-4) (1). These subunits are differentially expressed throughout the central nervous system (2-6) and contribute to a wide diversity of brain functions and pathologies (7,8). The large number of possible subunit combinations allows the formation of many different subtypes of nAChRs. However, the small number of subunits expressed by a single neuron restrains the number of possible combinations of subunit. Furthermore, preferential association of subunits has been demonstrated in neurons containing a large number of nAChR subunits (9).Immunoprecipitation studies have demonstrated the existence of two main subtypes of nAChRs in the mammalian brain: ␣4␤2 heteromers (10) and ␣7 homomers (11). Several other associations have been demonstrated in chicken autonomic ganglia, such as ␣3␤4(␤2)␣5 (12), and in chicken retina, such as ␣4␤2␣5 (13) and ␣6␤4␤3 (14). Furthermore, only a limited number of pharmacological tools is available to discriminate between various forms of nAChR in the nervous system. However, at least three pharmacological classes of nAChRs may be distinguished on the basis of a subunit composition including ␣7, ␤2, or ␤4 (15, 16). Cytisine is a partial agonist in ␤2-containing nAChRs but it is more efficient than nicotine in ␤4-containing...

The diversity of subunit composition in nAChRs: Evolutionary origins, physiologic and pharmacologic consequences

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Nicotinic Receptors in the Brain Links between Molecular Biology and Behavior

“…In a familiar environment nicotine administration can result Léna et al 1999;Marubio et al 1999;andXu et al 1999a, 1999b. Abbreviations: NIC, nicotine; CYT, nicotinic agonist cytisine; EPI, nicotinic agonist epibatidine; MCC, nicotinic agonist methyl carbamylcholine; ACH, acetylcholine; MCA, nicotinic antagonist mecamylamine; DH␤E, nicotinic antagonist dihydro-␤-erythroidine; MLA, ␣7 antagonist methyllyaconitine; ␣BTX, ␣7 antagonist ␣-bungarotoxin; IPN, interpeduncular nucleus; SN, substantia nigra; SC, superior colliculus; MhB, medial habenula; LC, locus coeruleus.…”

Cellular and Behavioral Outcomes of Dorsal Striatonigral Neuron Ablation: New Insights into Striatal Functions

“…After cell-attached and whole-cell recordings, the cell content was aspirated, expelled into a test tube where the RT reaction was performed overnight at 40 1C. The single-cell RT-PCR protocol used here was adapted from Lena et al (1999) to simultaneously detect the expression of DTR-GFP and ChAT mRNAs. Genomic DNA amplification was systematically assessed using a NMDA receptor gene intron (NR1 subunit, GRIN1) as genomic control (Supplementary Table S4).…”