The ability of kappa opioid agonists to modulate dopamine-mediated behavior and Fos immunoreactivity was assessed in adult rats. It was predicted that kappa agonist treatment would block the unconditioned and conditioned behaviors produced by cocaine (an indirect dopamine agonist). In the initial experiments, cocaine-induced locomotor activity was assessed after either acute or chronic injections of the kappa receptor agonist U-50,488 (5 mg/kg, SC). As expected, U-50,488 decreased cocaine-induced activity, while leaving baseline activity levels unaffected. Interestingly, chronic treatment with U-50,488 did not induce behavioral tolerance. The conditioned effects of cocaine (20 mg/kg, IP) were assessed using the conditioned place preference (CPP) paradigm. As expected, rats showed a preference for the cocaine-paired compartment, an effect blocked by U-50,488 (5 mg/kg, SC). One hour after CPP testing, rats were killed and Fos immunoreactivity was assessed. Rats conditioned with cocaine, but not U-50,488, showed increased Fos activity in the anterior cingulate cortex, piriform cortex, lateral septal area, and olfactory tubercles. When considered together, these results suggest that U-50,488 was effective at blocking the unconditioned and conditioned effects of cocaine, as well as cocaine-induced neuronal activity (as measured by Fos induction).

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The behavioral effects of repeated methylphenidate (MPH) treatment were assessed in young rats. In 4 experiments, rats (starting at Postnatal Day 10 or 16) were pretreated on 5 consecutive days with saline or MPH (2.5-20.0 mg/kg i.p.). Sensitization was assessed after 1 or 7 abstinence days, with rats receiving a test day challenge injection of either a low dose of MPH (2.5 mg/kg) or the same dose of MPH as given during pretreatment. Results show that a test day injection of 2.5 mg/kg MPH produced a sensitized locomotor response in rats pretreated with 2.5-20.0 mg/kg MPH. This MPH-induced locomotor sensitization was evident only after 1 abstinence day. Various pretreatment doses of MPH (5, 10, 15, or 20 mg/kg) were capable of sensitizing the stereotyped sniffing of young rats, but only rats pretreated and tested with the highest dose (20 mg/kg) of MPH showed an augmented stereotyped sniffing response that was still robust after 7 abstinence days. Results indicate that young rats are capable of exhibiting sensitization after an extended abstinence period, which contrasts with previous research suggesting that psychostimulant treatment does not produce long-term sensitization in young rats.

In the present study, we examined whether exposing rats to a high-dose regimen of manganese chloride (Mn) during the postnatal period would depress presynaptic dopamine functioning and alter nonassociative and associative behaviors. To this end, rats were given oral supplements of Mn (750 μg/day) on postnatal days (PD) 1-21. On PD 90, dopamine transporter (DAT) immunoreactivity and [ 3 H]dopamine uptake were assayed in the striatum and nucleus accumbens, while in vivo microdialysis was used to measure dopamine efflux in the same brain regions. The effects of postnatal Mn exposure on nigrostriatal functioning were evaluated by assessing rotorod performance and amphetamine-induced stereotypy in adulthood. In terms of associative processes, both cocaineinduced conditioned place preference (CPP) and sucrose-reinforced operant responding were examined. Results showed that postnatal Mn exposure caused persistent declines in DAT protein expression and [ 3 H]dopamine uptake in the striatum and nucleus accumbens, as well as long-term reductions in striatal dopamine efflux. Rotorod performance did not differ according to exposure condition, however Mn-exposed rats did exhibit substantially more amphetamine-induced stereotypy than vehicle controls. Mn exposure did not alter performance on any aspect of the CPP task (preference, extinction, or reinstatement testing), nor did Mn affect progressive ratio responding (a measure of motivation). Interestingly, acquisition of a fixed ratio task was impaired in Mn-exposed rats, suggesting a deficit in procedural learning. In sum, these results indicate that postnatal Mn exposure causes persistent declines in various indices of presynaptic dopaminergic functioning. Mninduced alterations in striatal functioning may have long-term impact on associative and nonassociative behavior.

The behavioral effects of repeated methylphenidate (MPH) treatment were assessed in the adult rat. Protein kinase A (PKA) and adenylyl cyclase (basal and DA-stimulated) activity in the dorsal striatum (i.e., caudate-putamen) were measured to determine whether MPH-induced alterations in these enzymes correlate with the occurrence of behavioral sensitization. In two experiments, adult rats were injected (i.p.) on 5 consecutive pre-exposure days with saline or MPH (5, 10, 15, or 20 mg/kg). Sensitization was tested after a single abstinence day, with rats receiving a challenge injection of MPH prior to either a 40- or 150-min testing session (additional control groups received saline on the test day). Immediately after the 40-min testing session, rats were killed and tissue from the dorsal striatum was dissected for later analysis of PKA and adenylyl cyclase activity. Results showed that repeated MPH treatment sensitized the stereotyped sniffing, but not the locomotor activity, of adult rats. PKA activity was significantly depressed in rats treated with MPH (10 or 20 mg/kg) during both the pre-exposure and test day phases. DA-stimulated adenylyl cyclase activity was reduced after chronic MPH treatment, while basal adenylyl cyclase values were enhanced. Thus, the present study showed that MPH was able to sensitize the stereotyped behaviors of adult rats, an action that corresponded with drug-induced changes in dorsal striatal DA signal transduction mechanisms.