Selective inhibition of M5 muscarinic acetylcholine receptors attenuates cocaine self-administration in rats

Abstract

Cocaine use disorder (CUD) remains a debilitating health problem in the United States for which there are no Food and Drug Administration-approved treatment options. Accumulating anatomical and electrophysiological evidence indicates that the muscarinic acetylcholine receptor (mAChR) subtype 5 (M₅ ) plays a critical role in the regulation of the mesolimbic dopaminergic reward circuitry, a major site of action for cocaine and other psychostimulants. In addition, M₅ knockout mice exhibit reduced cocaine self-administration behaviors with no differences in sugar pellet-maintained responding relative to wild-type mice. These findings suggest that selective inhibition of M₅ mAChR may provide a novel pharmacological approach for targeting CUD. Recently, we reported the synthesis and characterization of ML375, a selective negative allosteric modulator (NAM) for the rat and human M₅ mAChR with optimized pharmacokinetic properties for systemic dosing in rodents. In the present study, male Sprague-Dawley rats were trained to self-administer intravenous cocaine (0.1-0.75 mg/kg/infusion) under a 10-response fixed ratio or a progressive ratio schedule of reinforcement. Under both schedules of reinforcement, ML375 produced dose-related reductions in cocaine self-administration. ML375 also modestly reduced sugar pellet-maintained responding on the 10-response, fixed ratio schedule but had no effect under a progressive ratio schedule of reinforcement. Further, ML375 did not affect general motor output as assessed by a rotarod test. Collectively, these results provide the first demonstration that selective inhibition of M₅ using the M₅ NAM ML375 can attenuate both the reinforcing effects and the relative strength of cocaine and suggest that M₅ NAMs may represent a promising, novel treatment approach for CUD.

Keywords: cocaine self-administration; muscarinic receptor; negative allosteric modulation.

Figure 1

A, The highest dose of ML375 administered reduces cocaine self-administration (0.5 mg/kg) under a FR10 schedule of reinforcement and this reduction persists 24 hours post injection, n=7. B, ML375 reduces the number of sugar pellets earned under the same schedule but the effect does not persist 24 hours post injection, n =7. C, The cumulative reduction in cocaine infusions after treatment with ML375 (30 mg/kg, triangle symbols) is significant from 45 minutes after the start of the session and continuing to the end of the self-administration session, and this effect is similar 24 hours post injection. D, ML375 (10 mg/kg, square symbols) reduces the cumulative number of sugar pellets 30 minutes after the start of the session to 60 minutes, while the highest dose (30 mg/kg) reduces the number of sugar pellets from 30 minutes to the end of the session. All comparisons are to sessions with vehicle pretreatment (circle symbols). ^ ( 10 mg/kg 15 min PT) *( 30 mg/kg 15 min & 24 hr post inj), p < 0.05.

Figure 2

A, ML375 (30 mg/kg) reduces the number of reinforcers earned during cocaine self-administration sessions at the lower doses of cocaine, n=8. B, Response rates (resp/sec) during cocaine self-administration sessions were reduced after treatment with ML375 compared to vehicle. Vehicle pretreatment (black squares), ML375 pretreatment (open squares), 24 hr post ML375 pretreament (gray squares). *( 15 min PT), & (24 hr post inj), p < 0.05.

Figure 3

Representative individual cocaine dose-response curves after vehicle and ML375 (30 mg/kg) administration. Note the variable cocaine dose that elicited maximum responding, and the individual sensitivities to ML375.

Figure 4

A, ML375 (30 mg/kg, open squares) reduces breakpoints of the lower doses of cocaine compared to vehicle (black squares) sessions, n=7. This effect is not seen 24 hr post ML375 ( 30 mg/kg) injection (gray squares). B, ML375 does not reduce breakpoints for sugar pellets compared to vehicle at either 10 or 30 mg/kg, n=10. *, p < 0.05.

Figure 5

ML375 does not impair performance on the rotarod test in rats. Latency to fall off the rotarod turning at 20 rpm was not affected by ML375. Data are represented as mean ± SEM of 7–8 rats per treatment group.