Context did not influence the expression of behavioral sensitization in preweanling rats, suggesting that deficits in associative or memory processes may be responsible for age-dependent differences in behavioral sensitization and conditioned activity.

Rationale Previous studies in rodents show that early exposure to methylphenidate alters later responsiveness to drugs of abuse. An interesting feature of these studies is that early methylphenidate treatment decreases the rewarding value of cocaine when measured by conditioned place preference (CPP), but the same treatment increases cocaine self-administration. Objective The goal of the present study was to examine the effects of early methylphenidate exposure on cocaine-induced responding using both reward paradigms. Methods Rats were treated with methylphenidate (0, 2, or 5 mg/kg) from postnatal day (PD) 11 to PD 20 and then cocaine-induced CPP or cocaine self-administration was measured in separate groups of rats in adulthood. The CPP procedure included eight days of acquisition training, eight days of extinction training, and a reinstatement test. Rats were conditioned with 0, 10 or 20 mg/kg cocaine. Reinstatement was assessed after a priming dose of cocaine (10 mg/kg). For the self-administration experiment, a jugular catheter was implanted and rats were trained to press a lever reinforced with cocaine (0.25 or 0.75 mg/kg/infusion) on a fixed ratio (FR) 1 schedule. Rats were gradually moved from an FR1 to an FR10 schedule and, after criterion was reached, rats were placed on a progressive ratio schedule for five days. Results Cocaine produced robust rewarding effects as determined by both the CPP and self-administration experiments; however, early methylphenidate exposure only enhanced the reinforcing effects of cocaine on the self-administration paradigm. Interestingly, this methylphenidate enhancement was only seen in male rats. Conclusions These data suggest that in males methylphenidate enhances the reinforcing value of cocaine, but not cocaine-associated cues.

In the present study, we examined the ability of post-training injections of cocaine to facilitate spatial memory performance using the Morris water maze (MWM). We also investigated the role that hippocampal protein kinase A (PKA) and extracellular signal-regulated kinase 1/2 (ERK) signaling may play in cocaine-mediated spatial memory consolidation processes. Male and female C57BL/6 mice were first trained in a MWM task (eight consecutive trials) then injected with cocaine (0, 1.25, 2.5, 5, or 20 mg/kg), and memory for the platform location was retested after a 24 hr delay. Cocaine had a dose-dependent effect on spatial memory performance because only the mice receiving 2.5 mg/kg cocaine displayed a significant reduction in latency to locate the platform. No sex differences in MWM performance were observed; however, females showed higher hippocampal levels of PKA when compared to males. A second experiment demonstrated that 2.5 mg/kg cocaine enhanced MWM performance only when administered within 2, but not 4 hr after spatial training. We also found that cocaine (2.5 mg/kg) increased ERK2 phosphorylation within the hippocampus and one of its downstream targets (ribosomal S6 kinase), a mechanism that may be responsible, at least in part, for the enhanced cocaine-mediated spatial memory performance. Overall, these data demonstrate that a low dose of cocaine (2.5 mg/kg) administered within 2-hr after training facilitates MWM spatial memory performance in C57BL/6 mice.

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