Individual differences in cocaine-induced locomotor activity of male Sprague-Dawley rats are not explained by plasma corticosterone levels.

Humans differ in their initial response to, and subsequent abuse of, addictive drugs like cocaine. Rodents also exhibit marked individual differences in responsiveness to cocaine. Previously, we classified male Sprague-Dawley rats as either low or high cocaine responders (LCRs or HCRs, respectively), based on their acute low-dose cocaine-induced locomotor activity, and found that with repeated drug exposure LCRs exhibit greater cocaine locomotor sensitization, reward and reinforcement than HCRs. Differential cocaine-induced increases in striatal dopamine help to explain the LCR/HCR phenotypes. Differential levels of stress and/or anxiety could also contribute but have not been explored. Here we measured open-field activity and plasma corticosterone levels both pre- and post-cocaine treatment in LCRs, HCRs, and saline-treated controls. The three groups did not differ in baseline locomotor activity or corticosterone levels. Importantly, LCR/HCR differences in corticosterone levels were also not observed following acute cocaine (10mg/kg, i.p.), when cocaine induced approximately 3.5-fold greater locomotor activity in HCRs than LCRs. Additionally, there were no LCR/HCR differences in plasma corticosterone levels following 5 days of once-daily cocaine, during which time LCRs developed locomotor sensitization such that their cocaine-induced locomotor activity no longer differed from that of HCRs. Likewise, there were no group activity differences in any of four concentric zones within the open-field chamber. In summary, neither plasma corticosterone levels nor thigmotaxis-type anxiety appears to be a factor that contributes to the observed cocaine-induced LCR/HCR behavioral differences.

Subjective and objective probability effects on P300 amplitude revisited.

Does objective probability affect P300 size independently and in addition to subjective probability? The latter was manipulated by the number of stimuli presented and classification task. Five groups saw target and frequent stimuli. Two saw these with p=.2 or .067, with two different button presses. Three groups saw two additional nontarget stimuli each with p=.067. One group pressed a different button for each stimulus. A second group pressed one button for the three oddballs, another for the frequent. A third critical group pressed one button for the target and another for other stimuli. In this group, P300 was larger for targets versus nontargets, and larger for nontargets versus frequents. Although nontargets were classified with frequents, their actual low probability distinguished them from frequents, and their subjective probability distinguished them from targets. Therefore, actual and subjective probability effects were independently found.