Induced lameness in a hind limb causes prominent compensatory movements resembling lameness in the ipsilateral forelimb. Induced lameness in a forelimb causes slight compensatory movements resembling lameness in the ipsilateral and contralateral hind limbs.

Pituitary adenylyl cyclase activating polypeptide (PACAP) and its receptors (PAC1, VPAC1, and VPAC2) are localized in brain regions implicated in stress response, reward seeking and aversive responses, raising the possibility that PACAP may be involved in motivational effects of nicotine. To test this hypothesis, we used twobottle choice (TBC) and place conditioning paradigms and assessed if nicotine preference or conditioned place preference (CPP) or aversion (CPA) induced by nicotine would be altered in mice lacking PACAP compared to their wild-type controls. In the TBC paradigm, mice had access to two water bottles during the first week and then one of the water bottles was switched to nicotine solution (20, 40 and then 80 μg/mL). The volume of water and nicotine consumed was measured every day. In the place conditioning paradigm, mice were tested for baseline place preference on day 1, received conditioning with saline versus a low (0.25) or high (1 mg/kg) dose nicotine and, respectively, tested for CPP or CPA 24 h following the last conditioning. We discovered that mice lacking PACAP compared to their wild-type controls exhibited more preference for nicotine over water in the TBC paradigm, particularly at the two higher concentrations of nicotine. While the rewarding action of the low dose nicotine was not altered in mice lacking PACAP, the aversive effect of the high dose nicotine was blunted in these mice compared to their wild-type controls. The present results suggest that endogenous PACAP may play a functional role in nicotine preference and its aversive effect.

Previous preclinical studies have shown that nicotine pretreatment during adolescence increases the reinforcing actions of cocaine. However, little is known about the effect of prior nicotine administration on cocaine-induced conditioned place preference (CPP) and its reinstatement in adult mice. Besides, little information is available regarding the role of sex in this cross-talk between nicotine and cocaine. Thus, we examined if nicotine administration during adulthood would differentially alter cocaine-induced CPP, its extinction and reinstatement in male versus female mice and if the dose of nicotine was important in this regard. To this end, mice were pretreated with saline or nicotine (0.25 or 1 mg/ kg; twice daily for seven days) and then tested for place preference before and after single and repeated conditioning with cocaine (15 mg/kg). Mice were then exposed to extinction training and tested for reinstatement of CPP. Our results showed that male and female mice pretreated with saline and conditioned with cocaine each exhibited a robust CPP after a single cocaine conditioning. However, this response was blunted in mice pretreated with the lower but not higher dose of nicotine. Female mice pretreated with the lower dose nicotine also failed to show CPP after repeated conditioning. Reinstatement of cocaine-induced CPP was also blunted in these mice compared to their respective controls. Together, these results suggest that nicotine administration during adulthood exerts differential effects on cocaine-induced CPP and its reinstatement in male and female mice and the dose of nicotine is important in this regard.

Endogenous opioids have been implicated in cocaine reward. However, the role of each opioid peptide in this regard is unknown. Notably, the role of each peptide in extinction and reinstatement is not fully characterized. Thus, we assessed whether cocaine-induced conditioned place preference (CPP) and its extinction and reinstatement would be altered in the absence of beta-endorphin. We also examined if sex-related differences would exist in these processes. Male and female mice lacking beta-endorphin and their respective controls were tested for baseline place preference on day 1. On day 2, mice were treated with saline/cocaine (15 mg/kg) and confined to the vehicle- or drug-paired chamber for 30 min, respectively. In the afternoon, mice were treated with the alternate treatment and confined to the opposite chamber. Mice were then tested for CPP on day 3. Mice then received additional conditioning on this day as well as on day 4. Mice were then tested for CPP on day 5. Mice then received extinction training on day 9. On day 10, mice were tested for extinction and then reinstatement of CPP following a priming dose of cocaine (7.5 mg/kg). Male and female mice lacking beta-endorphin did not exhibit CPP following single conditioning with cocaine. On the other hand, only male mice lacking beta-endorphin failed to show CPP after repeated conditioning. Nonetheless, reinstatement of CPP was blunted in both male and female mice lacking beta-endorphin compared to controls. The present results suggest that beta-endorphin plays a functional role in cocaine-induced CPP and its reinstatement, and sex-related differences exist in the regulatory action of beta-endorphin on the acquisition but not reinstatement of cocaine CPP.