β2* nAChR sensitivity modulates acquisition of cocaine self-administration in male rats

Abstract

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 β2 nAChR subunit (β2Leu9'Ser) in the ventral tegmental area of male Sprague Dawley rats. Compared to control rats, β2Leu9'Ser rats acquired (fixed ratio) intravenous cocaine self-administration faster and with greater likelihood. By contrast, β2Leu9'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, β2Leu9'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 β2* (β2-containing) nAChRs in acquisition of cocaine SA, suggesting that mesolimbic acetylcholine signaling is active during this process.

Figure 1 –

Expression of β2Leu9′Ser nAChR subunits in rat VTA. A. Adeno associated virus for Cre-dependent expression of β2Leu9′Ser subunits. B. β2Leu9′Ser subunits incorporate into natively-expressed β2-containing nAChRs such as α4β2 receptors. C. Fluorescence microscopy image of GFP reporter and mCherry-Cre expression in a rat microinjected in VTA. The corresponding anatomical location from the rat brain atlas is shown at right. D. High-magnification image of GFP & Cre expression in VTA from the image in C. E-G. Epibatidine binding to membranes from control rats and rats expressing β2Leu9′Ser nAChR subunits. Total epibatidine (E) was determined, followed by competition with 150 nM cytisine, revealing cytosine-sensitive (F) and cytisine-resistant (G) components.

Figure 2 –

Modulation of Cocaine self-administration acquisition by VTA expression of β2Leu9′Ser nAChR subunits. A. Cocaine SA paradigm. Stereotaxic AAV injection surgery and jugular catheter surgery were conducted, then rats were allowed to acquire cocaine SA (FR1) at a dose of 0.375 mg/kg/inf. PR sessions were run for 5 days after SA acquisition. B. Raster plot of cocaine infusions showing the acquisition history of a representative +Cre rat during FR1 (20 infusion cap) training. C. Summary of cocaine SA acquisition in β2Leu9′Ser +Cre and −Cre rats. D. Kaplan-Meier curve plotting percent of +Cre and −Cre rats acquiring cocaine SA as a function of SA session #. E-F. Histogram of days to acquire (bin size = 2 days) in +Cre (E) and −Cre (F) rats.

Figure 3 –

Analysis of cocaine SA acquisition in β2Leu9′Ser +Cre and −Cre rats. A. For each rat in the indicated group that acquired cocaine SA, infusions per day were aligned such that day 0 represents the first day when each rat earned 20 infusions. B. For rats in the indicated group that acquired cocaine SA, summary data is shown for infusions on the day immediately before the first day when each rat earned 20 infusions. C. For rats that did not acquire cocaine SA, the mean number of infusions per day across all training sessions is shown. D-E. For each rat in the +Cre (D) and −Cre (E) group that acquired cocaine SA, active and inactive responses per day were aligned such that day 0 represents the first day when each rat earned 20 infusions. F. Mean (± SEM) active and inactive responses are shown for each +Cre and −Cre rat for FR1 SA sessions with an infusion cap of 40.

Figure 4 –

Cocaine PR self-administration in rats expressing β2Leu9′Ser nAChR subunits in VTA. A. Raster plot (top) and cumulative response plot for the final PR session is shown for the same rat whose data is depicted in Fig. 2F. Red vertical tick marks indicate infusions, blue vertical ticks indicate unrewarded responses on the active lever, and black vertical tick marks indicate responses on the inactive lever. Inset: a magnified view of active responses and infusions is shown for an early part of the session. B. Mean (± SEM) number of infusions achieved during each PR session for the indicated groups. C. Geometric mean (± 95% CI) breakpoint achieved during each PR session for the indicated groups.

Figure 5 –

Food self-administration in rats expressing β2Leu9′Ser nAChR subunits in VTA. A. Food SA paradigm. Stereotaxic AAV injection surgery was conducted, then rats were allowed to self-administer food for 7 days (FR1). PR sessions were run for 5 days after FR1 food SA. B. Mean (± SEM) number of food rewards earned during each FR1 session for the indicated groups. C. Mean (± SEM) number of food rewards earned during each PR session for the indicated groups.