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.