Nicotinic Acetylcholine Receptors as Targets for Tobacco Cessation Therapeutics: Cutting-Edge Methodologies to Understand Receptor Assembly and Trafficking

Direct Effects of Nicotine Exposure on Murine Calvaria and Calvarial Cells

Despite the link between adverse birth outcomes due to pre- and peri-natal nicotine exposure, research suggests 11% of US women continue to smoke or use alternative nicotine products throughout pregnancy. Maternal smoking has been linked to incidence of craniofacial anomalies. We hypothesized that pre-natal nicotine exposure may directly alter craniofacial development independent of the other effects of cigarette smoking. To test this hypothesis, we administered pregnant C57BL6 mice drinking water supplemented with 0, 50, 100 or 200 μg/ml nicotine throughout pregnancy. On postnatal day 15 pups were sacrificed and skulls underwent micro-computed tomography (µCT) and histological analyses. Specific nicotinic acetylcholine receptors, α3, α7, β2, β4 were identified within the calvarial growth sites (sutures) and centers (synchondroses). Exposing murine calvarial suture derived cells and isotype cells to relevant circulating nicotine levels alone and in combination with nicotinic receptor agonist and antagonists resulted in cell specific effects. Most notably, nicotine exposure increased proliferation in calvarial cells, an effect that was modified by receptor agonist and antagonist treatment. Currently it is unclear what component(s) of cigarette smoke is causative in birth defects, however these data indicate that nicotine alone is capable of disrupting growth and development of murine calvaria.

Differences in Nicotine Encoding Dopamine Release between the Striatum and Shell Portion of the Nucleus Accumbens

The aim of this work was to determine the effect of nicotine desensitization on dopamine (DA) release in the dorsal striatum and shell of the nucleus accumbens (NAc) from brain slices. In vitro fast-scan cyclic voltammetry analysis was used to evaluate dopamine release in the dorsal striatum and the NAc shell of Sprague–Dawley rats after infusion of nicotine, a nicotinic acetylcholine receptor (nAChR) antagonist mecamylamine (Mec), and an α4β2 cholinergic receptor antagonist (DHβe). DA release related to nicotine desensitization in the striatum and NAc shell was compared. In both structures, tonic release was suppressed by inhibition of the nicotine receptor (via Mec) and the α4β2 receptor (via DHβe). Paired-pulse ratio (PPR) was facilitated in both structures after nicotine and Mec infusion, and this facilitation was suppressed by increasing the stimulation interval. After variable frequency stimulation (simulating phasic burst), nicotine infusion induced significant augmentation of DA release in the striatum that was not seen in the absence of nicotine. In contrast, nicotine reduced phasic DA release in NAc, although frequency augmentation was seen both with and without nicotine. Evaluation of DA release evoked by various trains (high-frequency stimulation (HFS) 100 Hz) of high-frequency stimulation revealed significant enhancement after a train of three or more pulses in the striatum and NAc. The concentration differences between tonic and phasic release related to nicotine desensitization were more pronounced in the NAc shell. Nicotine desensitization is associated with suppression of tonic release of DA in both the striatum and NAc shell that may occur via the α4β2 subtype of nAChR, whereas phasic frequency-dependent augmentation and HFS-related gating release is more pronounced in the striatum than in the NAc shell. Differences between phasic and tonic release associated with nicotine desensitization may underlie processing of reward signals in the NAc shell, and this may have major implications for addictive behavior.

Organelle-specific single-molecule imaging of α4β2 nicotinic receptors reveals the effect of nicotine on receptor assembly and cell-surface trafficking

Nicotinic acetylcholine receptors (nAChRs) assemble in the endoplasmic reticulum (ER) and traffic to the cell surface as pentamers composed of α and β subunits. Many nAChR subtypes can assemble with varying subunit ratios, giving rise to multiple stoichiometries exhibiting different subcellular localization and functional properties. In addition to the endogenous neurotransmitter acetylcholine, nicotine also binds and activates nAChRs and influences their trafficking and expression on the cell surface. Currently, no available technique can specifically elucidate the stoichiometry of nAChRs in the ER versus those in the plasma membrane. Here, we report a method involving single-molecule fluorescence measurements to determine the structural properties of these membrane proteins after isolation in nanoscale vesicles derived from specific organelles. These cell-derived nanovesicles allowed us to separate single membrane receptors while maintaining them in their physiological environment. Sorting the vesicles according to the organelle of origin enabled us to determine localized differences in receptor structural properties, structural influence on transport between organelles, and changes in receptor assembly within intracellular organelles. These organelle-specific nanovesicles revealed that one structural isoform of the α4β2 nAChR was preferentially trafficked to the cell surface. Moreover, nicotine altered nAChR assembly in the ER, resulting in increased production of the receptor isoform that traffics more efficiently to the cell surface. We conclude that the combined effects of the increased assembly of one nAChR stoichiometry and its preferential trafficking likely drive the up-regulation of nAChRs on the cell surface upon nicotine exposure.