Impact of baseline prepulse inhibition on nicotine-induced locomotor sensitization in rats

The rats having high locomotor reactivity to a novel environment (LRNE) are known to be more vulnerable to develop locomotor sensitization, which reflects the initial neuroplastic changes in brain systems related to addictive behaviours. The present study aimed to investigate whether sensorimotor gating level, measured by prepulse inhibition (PPI) of acoustic startle reflex, also reflects vulnerability for nicotine sensitization. A batch of rats was assigned into three groups according to their baseline PPI values. The highest 1/3 and the lowest 1/3 proportions were selected and defined as high-inhibitory (HI) and low-inhibitory (LI) groups. LRNE was measured in the rats, then they were treated with nicotine (1 mg/kg, tartrate salt, subcutaneously) or saline and locomotor activity (LMA) was immediately recorded for 15 min. This procedure was performed daily for 5 successive days. After a 3-day drug-free period, all rats were challenged with nicotine (1 mg/kg) on 9th day and with saline on 12th day. Same sensitization protocol was applied in another batch of rats, except assigning them into the high-responder (HR) and low-responder (LR) groups according to LRNE levels. There was no significant difference between HI and LI rats in LRNE. Although the acute effect of nicotine on LMA was higher in HI rats, a locomotor sensitization developed and expressed only in LI rats. In the following experiments, nicotine stimulated LMA both in HR and LR rats, but induced and expressed locomotor sensitization only in HR rats. The present study shows that acute locomotor stimulant effect and locomotor sensitization developing effects of nicotine are associated with the baseline PPI and LRNE levels. But these two factors are independent from each other.