β2 subunit containing nicotinic acetylcholine receptors (β2*nAChRs; *denotes assembly with other subunits) are critical for nicotine self administration and nicotine-associated dopamine (DA) release that supports nicotine reinforcement. The α6 subunit assembles with β2 on DA neurons where α6β2*nAChRs regulate nicotine-stimulated DA release at neuron terminals. Using local infusion of alpha-conotoxin MII (α-CTX MII), an antagonist with selectivity for α6β2*nAChRs, the purpose of these experiments was to determine if α6β2*nAChRs in the nucleus accumbens (NAc) shell are required for motivation to self-administer nicotine. Long Evans rats lever pressed for 0.03 mg/kg i.v. nicotine accompanied by light+tone cues (NIC) or for light+tone cues unaccompanied by nicotine (CUEonly). Following extensive training, animals were tested under a progressive ratio (PR) schedule that required an increasing number of lever presses for each nicotine infusion and/or cue delivery. Immediately prior to each PR session, rats received micro-infusions of α-CTX MII (0, 1, 5, or 10 pmols/side) into the NAc shell or the overlying anterior cingulate cortex. α-CTX MII dose-dependently decreased break points and number of infusions earned by NIC rats following infusion into the NAc shell but not the anterior cingulate cortex. Concentrations of α-CTX MII that were capable of attenuating nicotine self administration did not disrupt locomotor activity. There was no effect of infusion on lever pressing in CUEonly animals and NAc infusion α-CTX MII did not affect locomotor activity in an open field. These data suggest that α6β2*nAChRs in the NAc shell regulate motivational aspects of nicotine reinforcement but not nicotine-associated locomotor activation.

Neuroinflammation induced by activated microglia and astrocytes can be elicited by drugs of abuse. Methamphetamine administration activates glial cells and increases proinflammatory cytokine production, and there is recent evidence of a linkage between glial cell activation and drug abuse-related behavior. We have previously reported that ibudilast (AV411; 3-isobutyryl-2-isopropylpyrazolo-[1,5-a]pyridine), which inhibits phosphodiesterase (PDE) and pro-inflammatory activity, blocks reinstatement of methamphetamine-maintained responding in rats, and that ibudilast and AV1013, an amino analog of ibudilast, which has similar glial-attenuating properties but limited PDE activity, attenuate methamphetamine-induced locomotor activity and sensitization in mice. The present study's objective was to determine whether co-administered ibudilast, AV1013, or minocycline, which is a tetracycline derivative that also suppresses methamphetamine-induced glial activation, would attenuate active methamphetamine i.v. self-administration in Long-Evans hooded rats. Rats were initially trained to press a lever for 0.1 mg/kg/inf methamphetamine according to a FR1 schedule during 2-h daily sessions. Once stable responding was obtained, twice daily ibudilast (1, 7.5, 10 mg/kg), AV1013 (1, 10, 30 mg/kg), or once daily minocycline (10, 30, 60 mg/kg), or their corresponding vehicles, were given i.p. for three consecutive days during methamphetamine (0.001, 0.03, 0.1 mg/kg/inf) self-administration. Ibudilast, AV1013, and minocycline all significantly (p<0.05) reduced responding maintained by 0.03 mg/kg/inf methamphetamine that had maintained the highest level of infusions under vehicle conditions. These results suggest that targeting glial cells may provide a novel approach to pharmacotherapy for treating methamphetamine abuse.

Stress and renewed contact with drug (a “slip”) have been linked to persisting relapse of methamphetamine abuse. Human brain microglial activation has been linked with methamphetamine abuse, and inhibitors of glial cell activation, certain phosphodiesterase (PDE) inhibitors, and glial cell derived neurotrophic factor (GDNF) have been reported to modulate drug abuse effects. Our objective was to determine whether the glial cell attenuator, 3-isobutyryl-2-isopropylpyrazolo-[1,5-a]pyridine (AV411, ibudilast), a non-selective PDE inhibitor and promoter of GDNF, could reduce stress- and methamphetamine prime-induced reinstatement of methamphetamine-seeking behavior. Male Long-Evans hooded rats were trained to lever press reinforced with 0.1 mg/kg i.v. methamphetamine infusion according to fixed-ratio 1 (FR1) reinforcement schedules during daily, 2-h experimental sessions. After performance had stabilized, lever pressing was extinguished for 12 consecutive sessions and doses of 0 (vehicle), 2.5 and 7.5 mg/kg AV411 were then administered intraperitoneally b.i.d. on the last two days of extinction and then once on the testday to separate groups of 12 rats. During testing, the rats were given 15 min of intermittent footshock or a 1 mg/kg i.p. methamphetamine prime followed by a 2-h reinstatement test session. AV411 significantly reduced response levels of footshock-induced (2.5 and 7.5 mg/kg) and prime-induced (7.5 mg/kg) reinstatement of extinguished methamphetamine-maintained responding. AV411 has properties consistent with the ability to attenuate relapse precipitated by stress and methamphetamine “slips” during abstinence. These results thus reinforce interest in atypical neurobiological mechanisms which could be exploited for developing novel medications for treating drug abuse disorders.

Background There are no approved pharmacotherapies for preventing psychomotor stimulant relapse. The operant reinstatement model has been suggested as a screen for identifying candidate medications. The present study examined if the anxiolytic buspirone could attenuate reinstatement of extinguished responding in Long-Evans rats that previously self-administered intravenous cocaine or methamphetamine. Methods Rats were trained in 2-h daily sessions to self-administer 0.5 mg/kg cocaine or 0.1 mg/kg methamphetamine infusions followed by 12 days of instrumental extinction. Reinstatement was evoked by 17 mg/kg i.p. cocaine primes or response-contingent cocaine-paired cues in cocaine-reinforced rats, and by 1 mg/kg i.p. methamphetamine primes or response-contingent methamphetamine-paired cues in methamphetamine-reinforced rats. Results Buspirone (1 and 3 mg/kg) significantly (p<0.05) attenuated cocaine cue but not cocaine prime reinstatement. Buspirone (1 and 3 mg/kg) also significantly attenuated methamphetamine cue reinstatement. Buspirone (3 mg/kg) significantly attenuated methamphetamine prime reinstatement. During all reinstatement tests, 3 mg/kg buspirone reduced levels of inactive lever pressing relative to those of vehicle, significantly so during the cocaine cue-induced reinstatement tests. Conclusions Given the complexity of buspirone's neuropharmacology consisting of serotonin 5-HT1A receptor partial agonist activity, and dopamine D2, D3 and D4 receptor antagonist effects, it is uncertain which of these activities or their combination is responsible for the present results. Overall, these results suggest that buspirone may reduce the likelihood of relapse to cocaine and methamphetamine use under some conditions, although this speculation must be interpreted with caution given buspirone's similar potency to attenuate inactive-lever responding.

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