Fig. 1.

Timeline and experimental design. (A) Timeline of the procedures across post-natal days (PNDs) for Experiments 1 and 2 where rats were given 2 conditioning sessions with their assigned unconditioned stimulus (CS+) in their initially non-preferred side of the conditioning place preference (CPP) apparatus, 2 conditioning sessions in the absence of their unconditioned stimulus (CS−) on their initially preferred side, and 2 conditioning sessions in the alternate environment (Alt) with exposure to the unconditioned stimuli that they had not received during CS+ conditioning sessions. (B) Conditioning procedures in the initially preferred, initially non-preferred, and alternate environments. Two conditioning sessions took place in each environment for 10 min each, occurring over 2 or 4 consecutive days. One session took place in the initially preferred side of the apparatus (CS−), during which the rat was alone and received a saline vehicle (S; Sal) injection. Another session took place in the initially non-preferred side (CS+), during which the rat received exposure to the assigned unconditioned stimulus (US). US conditions included either nicotine (N; Nic) or saline (S; Sal) experienced either during isolation (Iso) or in the presence of an age-, sex- and weight-matched social partner (Soc). The final session took place in the alternate environment (Alt) in a separate location from the CPP conditioning chambers. In the alternate environment, rats received whichever US they had not received in their initially non-preferred side in order to control for total reward exposure for Fos and Zif268 protein expression.