Differential Expression and Function of Nicotinic Acetylcholine Receptors in Subdivisions of Medial Habenula

Abstract

Neuronal nAChRs in the medial habenula (MHb) to the interpeduncular nucleus (IPN) pathway are key mediators of nicotine's aversive properties. In this paper, we report new details regarding nAChR anatomical localization and function in MHb and IPN. A new group of knock-in mice were created that each expresses a single nAChR subunit fused to GFP, allowing high-resolution mapping. We find that ␣3 and ␤4 nAChR subunit levels are strong throughout the ventral MHb (MHbV). In contrast, ␣6, ␤2, ␤3, and ␣4 subunits are selectively found in some, but not all, areas of MHbV. All subunits were found in both ChAT-positive and ChAT-negative cells in MHbV. Next, we examined functional properties of neurons in the lateral and central part of MHbV (MHbVL and MHbVC) using brain slice patch-clamp recordings. MHbVL neurons were more excitable than MHbVC neurons, and they also responded more strongly to puffs of nicotine. In addition, we studied firing responses of MHbVL and MHbVC neurons in response to bath-applied nicotine. Cells in MHbVL, but not those in MHbVC, increased their firing substantially in response to 1 M nicotine. Additionally, MHbVL neurons from mice that underwent withdrawal from chronic nicotine were less responsive to nicotine application compared with mice withdrawn from chronic saline. Last, we characterized rostral and dorsomedial IPN neurons that receive input from MHbVL axons. Together, our data provide new details regarding neurophysiology and nAChR localization and function in cells within the MHbV.

Nicotine Dependence Reveals Distinct Responses from Neurons and Their Resident Nicotinic Receptors in Medial Habenula

“…We noted that the a4, a6, b2, and b3 subunits exhibit a localization gradient from inferior to lateral across the ventral portion of MHb. In particular, a4 subunits are concentrated in MHbVL and a6 subunits are preferentially confined to MHbVI (Shih et al, 2014). Given that the patterns seen for various nAChR subunits suggest that specific subdivisions exist in MHb, we first sought to determine whether nAChRs in MHbVL and MHbVI exhibit functional differences.…”

“…Mice received a mean dose of 1 mg/kg per hour of nicotine for 14 days. The nicotine dose used in this study to develop physical dependence was chosen based on the results obtained in previous studies (Matta et al, 2007;Hilario et al, 2012;Shih et al, 2014). This dose yields plasma levels of ∼0.3 mM, a concentration similar to that observed in human smokers consuming an average of 17 cigarettes a day (Matta et al, 2007).…”

“…Nicotine dependence was induced with osmotic minipumps (model 2004; Alzet, Cupertino, CA) as previously reported (Shih et al, 2014). These minipumps, implanted subcutaneously under brief isoflurane anesthesia, were filled with either sterile saline or (-)-nicotine hydrogen-tartrate salt (Glentham Life Sciences, Corsham, UK) dissolved in sterile saline.…”

“…Cells in the dorsal 1/3 of the MHb preferentially express substance P, whereas cells in the ventral 2/3 of the MHb preferentially express choline acetyltransferase (ChAT), a cholinergic marker (Contestabile et al, 1987;Yamaguchi et al, 2013). The dorsal and ventral MHb project to anatomically divergent components of the IPN (Contestabile and Flumerfelt, 1981;Yamaguchi et al, 2013;Shih et al, 2014), and different regions of IPN have recently been appreciated to mediate diverse aspects of nicotine withdrawal. For example, the dorsal IPN, which receives projections from the ventrolateral MHb (MHbVL), is a key mediator of the somatic aspect of nicotine withdrawal (ZhaoShea et al, 2013).…”

Mechanisms of Action and Persistent Neuroplasticity by Drugs of Abuse

“…The MHb-IPN tract highly expresses a-bungarotoxininsensitive nAChRs (Mulle et al, 1991), likely containing a4, and a5, a3, and b4 nAChR subunits encoded by the CHRNA5-A3-B4 gene cluster (Aizawa et al, 2012;Shih et al, 2014). This gene cluster has been linked with increased nicotine consumption and dependence in human genetics studies (Berrettini et al, 2008;Bierut et al, 2008;Lips et al, 2010;Ware et al, 2011).…”