ß2* nAChR sensitivity modulates acquisition of cocaine self-administration in male rats.

Signaling through nicotinic acetylcholine receptors (nAChRs) plays a role in cocaine reward and reinforcement, suggesting that the cholinergic system could be manipulated with therapeutics to modulate aspects of cocaine use disorder (CUD). We examined the interaction between nAChRs and cocaine reinforcement by expressing a hypersensitive ß2 nAChR subunit (ß2Leu9'Ser) in the ventral tegmental area of male Sprague Dawley rats. Compared to control rats, ß2Leu9'Ser rats acquired (fixed ratio) intravenous cocaine self-administration faster and with greater likelihood. By contrast, ß2Leu9'Ser rats were approximately equivalent to controls in their intake of cocaine on a progressive ratio schedule of reinforcement, suggesting differential effects of cholinergic signaling depending on experimental parameters. Like progressive ratio cocaine SA, ß2Leu9'Ser rats and controls did not differ significantly in food SA assays, including acquisition on a fixed ratio schedule or in progressive ratio sessions. These results highlight the specific role of high-affinity, heteropentameric ß2* (ß2-containing) nAChRs in acquisition of cocaine SA, suggesting that mesolimbic acetylcholine signaling is active during this process.

ß2 nAChR Activation on VTA DA Neurons Is Sufficient for Nicotine Reinforcement in Rats.

Mesolimbic nicotinic acetylcholine receptor (nAChRs) activation is necessary for nicotine reinforcement behavior, but it is unknown whether selective activation of nAChRs in the dopamine (DA) reward pathway is sufficient to support nicotine reinforcement. In this study, we tested the hypothesis that activation of ß2-containing (ß2*) nAChRs on VTA neurons is sufficient for intravenous nicotine self-administration (SA). We expressed ß2 nAChR subunits with enhanced sensitivity to nicotine (referred to as ß2Leu9'Ser) in the VTA of male Sprague Dawley (SD) rats, enabling very low concentrations of nicotine to selectively activate ß2* nAChRs on transduced neurons. Rats expressing ß2Leu9'Ser subunits acquired nicotine SA at 1.5 µg/kg/infusion, a dose too low to support acquisition in control rats. Saline substitution extinguished responding for 1.5 µg/kg/inf, verifying that this dose was reinforcing. ß2Leu9'Ser nAChRs also supported acquisition at the typical training dose in rats (30 µg/kg/inf) and reducing the dose to 1.5 µg/kg/inf caused a significant increase in the rate of nicotine SA. Viral expression of ß2Leu9'Ser subunits only in VTA DA neurons (via TH-Cre rats) also enabled acquisition of nicotine SA at 1.5 µg/kg/inf, and saline substitution significantly attenuated responding. Next, we examined electrically-evoked DA release in slices from ß2Leu9'Ser rats with a history of nicotine SA. Single-pulse evoked DA release and DA uptake rate were reduced in ß2Leu9'Ser NAc slices, but relative increases in DA following a train of stimuli were preserved. These results are the first to report that ß2* nAChR activation on VTA neurons is sufficient for nicotine reinforcement in rats.

Relapse-like behavior and nAChR sensitization following intermittent access nicotine self-administration.

Many tobacco smokers consume nicotine intermittently, but the underlying mechanisms and neurobiological changes associated with intermittent nicotine intake are unclear. Understanding intermittent nicotine intake is a high priority, as it could promote therapeutic strategies to attenuate tobacco consumption. We examined nicotine intake behavior and neurobiological changes in male rats that were trained to self-administer nicotine during brief (5 min) trials interspersed with longer (15 min) drug-free periods. Rats readily adapted to intermittent access (IntA) SA following acquisition on a continuous access (ContA) schedule. Probabilistic analysis of IntA nicotine SA suggested reduced nicotine loading behavior compared to ContA, and nicotine pharmacokinetic modeling revealed that rats taking nicotine intermittently may have increased intake to maintain blood levels of nicotine that are comparable to ContA SA. After IntA nicotine SA, rats exhibited an increase in unreinforced responses for nicotine-associated cues (incubation of craving) and specific alterations in the striatal proteome after 7 days without nicotine. IntA nicotine SA also induced nAChR functional upregulation in the interpeduncular nucleus (IPN), and it enhanced nicotine binding in the brain as determined via [11C]nicotine positron emission tomography. Reducing the saliency of the cue conditions during the 5 min access periods attenuated nicotine intake, but incubation of craving was preserved. Together, these results indicate that IntA conditions promote nicotine SA and nicotine seeking after a nicotine-free period.

Nicotine Self-Administration Induces Plastic Changes to Nicotinic Receptors in Medial Habenula.

Chronic nicotine upregulates nicotinic acetylcholine receptors (nAChRs) throughout the brain, and reducing their activity may promote somatic and affective states that lead to nicotine seeking. nAChRs are functionally upregulated in animal models using passive nicotine administration, but whether/how it occurs in response to volitional nicotine intake is unknown. The distinction is critical, as drug self-administration (SA) can induce neurotransmission and cellular excitability changes that passive drug administration does not. In this study, we probed the question of whether medial habenula (MHb) nAChRs are functionally augmented by nicotine SA. Male rats were implanted with an indwelling jugular catheter and trained to nose poke for nicotine infusions. A saline SA group controlled for non-specific responding and nicotine-associated visual cues. Using patch-clamp whole-cell recordings and local application of acetylcholine, we observed robust functional enhancement of nAChRs in MHb neurons from rats with a history of nicotine SA. To determine whether upregulated receptors are generally enhanced or directed to specific cellular compartments, we imaged neurons during recordings using two-photon laser scanning microscopy (2PLSM). nAChR activity at the cell soma and on proximal and distal dendrites was examined by local nicotine uncaging using a photoactivatable nicotine (PA-Nic) probe and focal laser flash photolysis. Results from this experiment revealed strong nAChR enhancement at all examined cellular locations. Our study demonstrates nAChR functional enhancement by nicotine SA, confirming that volitional nicotine intake sensitizes cholinergic systems in the brain. This may be a critical plasticity change supporting nicotine addiction.