2.6. Conditioned place preference (CPP)
The potential of the five α-aminovalerophenone derivatives to induce rewarding effects was determined using a place conditioning paradigm (unbiased), as described by (Duart-Castells et al., 2019), with minor modifications. The apparatus consisted of three distinct compartments with differences in visual and tactile cues (two compartments communicated by a central corridor). CPP was performed in three different phases: preconditioning, conditioning and post-conditioning test. During the preconditioning phase (Day 0), mice were placed in the middle of the corridor and had free access to all the apparatus compartments for 15 min. The mean time spent in each compartment was video-monitored and recorded (Smart 3.0, Panlab, Barcelona, Spain).
During the conditioning phase (Day 1-4, sessions 1-8) mice received the corresponding i.p. injection using the same doses used previously in the HLA experiments (saline, pentedrone, N-ethyl-pentedrone, N,N-diethyl-pentedrone, α-PVP, or α-PpVP) and immediately placed into one of the two conditioning compartments for 20 min (sessions 1, 3, 5 and 7). On the alternate sessions (2, 4, 6 and 8), mice were given a saline injection and placed in the other compartment for 20 min. Two conditioning sessions per day were performed, separated by a 5-hours period. Control groups received a saline injection in every session. Sessions were counterbalanced as much as possible between compartments. Animals that spent >70% of the total session time in one compartment during the preconditioning phase were excluded due to the high preference.
Finally, the post-conditioning test was conducted as the preconditioning phase. A preference score was calculated as the difference between the time spent in the drug-paired compartment in the post-conditioning test minus the time spent in the same compartment in the preconditioning phase.