Crucial Role of α4 and α6 Nicotinic Acetylcholine Receptor Subunits from Ventral Tegmental Area in Systemic Nicotine Self-Administration

Abstract

The identification of the molecular mechanisms involved in nicotine addiction and its cognitive consequences is a worldwide priority for public health. Novel in vivo paradigms were developed to match this aim. Although the ␤2 subunit of the neuronal nicotinic acetylcholine receptor (nAChR) has been shown to play a crucial role in mediating the reinforcement properties of nicotine, little is known about the contribution of the different ␣ subunit partners of ␤2 (i.e., ␣4 and ␣6), the homo-pentameric ␣7, and the brain areas other than the ventral tegmental area (VTA) involved in nicotine reinforcement. In this study, nicotine (8.7-52.6 g free base/kg/inf) selfadministration was investigated with drug-naive mice deleted (KO) for the ␤2, ␣4, ␣6 and ␣7 subunit genes, their wild-type (WT) controls, and KO mice in which the corresponding nAChR subunit was selectively re-expressed using a lentiviral vector (VEC mice). We show that WT mice, ␤2-VEC mice with the ␤2 subunit re-expressed exclusively in the VTA, ␣4-VEC mice with selective ␣4 re-expression in the VTA, ␣6-VEC mice with selective ␣6 re-expression in the VTA, and ␣7-KO mice promptly self-administer nicotine intravenously, whereas ␤2-KO, ␤2-VEC in the substantia nigra, ␣4-KO and ␣6-KO mice do not respond to nicotine. We thus define the necessary and sufficient role of ␣4␤2-and ␣6␤2-subunit containing nicotinic receptors (␣4␤2*-and ␣6␤2*-nAChRs), but not ␣7*-nAChRs, present in cell bodies of the VTA, and their axons, for systemic nicotine reinforcement in drug-naive mice.

Differential contribution of genetic variation in multiple brain nicotinic cholinergic receptors to nicotine dependence: recent progress and emerging open questions

“…When the chrna6 and chrna4 genes were re-expressed (using lentiviral vector) in the a6À/À VTA and a4À/À VTA (respectively), the reinforcement property of nicotine was restored. 145 This important study confirms that CHRNA6 and CHRNA4 (as well as CHRNB2) expression in the VTA is both necessary and sufficient for self-administration of nicotine. 145 Drenan et al 132 used genetically manipulated a6-nicotine-hypersensitive mice (substitution of Leu with Ser on the 9 0 residue in the M2 domain) to show the role of this subunit in modulating DA transmission.…”

“…145 This important study confirms that CHRNA6 and CHRNA4 (as well as CHRNB2) expression in the VTA is both necessary and sufficient for self-administration of nicotine. 145 Drenan et al 132 used genetically manipulated a6-nicotine-hypersensitive mice (substitution of Leu with Ser on the 9 0 residue in the M2 domain) to show the role of this subunit in modulating DA transmission. The hypersensitive a6-mice showed increased DA neuron firing in the VTA compared with WT.…”

“…In addition to the role of the CHRNA4 subunit in the nicotine self-administration model of acute reinforcement described above, 145 involvement of the a4-nAChR subunit in nicotine-induced reward, tolerance, and sensitization was demonstrated in engineered mutant. 146 Single-point mutation in the poreforming M2 domain of the a4-subunit-encoding gene (Leu9 0 to Ala9 0 ) was introduced by the researchers.…”

“…77,131 a7-KO studies have failed to support a role for this subunit in sensitivity to the behavioral effects of nicotine. a7À/À nicotinenaive mice do not display different IV self-administration of nicotine (model of acute reinforcement) than WT, 145 and a7-KO mice did not show weakened responses to nicotine in a drug discrimination experiment nor difference from WT in sensitivity to the locomotor depressant effects of nicotine. 152,153 The development of nicotine tolerance did not differ between a7-KO and WT mice.…”

A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors

“…Transgenic approaches have also been used to study mutations associated with congenital myasthenic syndromes (in the α1, δ and ε subunit) [135,136] and mutations associated with nocturnal frontal lobe epilepsy (in the α4 subunit) [137][138][139]. A particularly powerful approach is the targeted re-expression of nAChR subunits in knockout mice by viral-based gene delivery [140], a technique that has been used successfully to examine neuronal nAChR α4 [141], α6 [141], α7 [142], and β2 [143] subunits.…”