Pergolide interactions with nicotine and pilocarpine in rats on the radial-arm maze

The effects of pergolide on memory and oxidative stress in a rat model of Parkinson's disease

One of the most widely used animal models of Parkinson's disease (PD) involves injecting 6-hydroxydopamine (6-OHDA) directly into the substantia nigra (SN). Some recent reports speculated that dopaminergic drugs may exert brain antioxidant activity, which could explain some of their protective actions. In this way, the aim of the present study was to examine the effects of low-dose pergolide on memory deficits and brain oxidative stress in a 6-OHDA-induced rat model of PD. Right-unilateral lesions of the SN were produced with 6-OHDA. Two weeks after neurosurgery, pergolide (0.3 mg/kg/day) was injected intraperitoneally in the 6-OHDA + pergolide and sham-operated + pergolide groups, while sham-operated and 6-OHDA alone groups received saline. Radial-8-arm maze and Y-maze were used for memory assessment. We also determined some enzymatic antioxidant defenses like superoxide dismutase or glutathione peroxidase and a lipid peroxidation marker [malondialdehyde (MDA)], from the temporal lobe. A reduced number of working/reference memory errors was observed in 6-OHDA + pergolide group, compared to sham-operated rats. Additionally, post hoc analysis showed significant differences between 6-OHDA and 6-OHDA + pergolide groups in both Y-maze and radial-arm-maze tasks. We also noted a significant decrease of MDA level in the 6-OHDA + pergolide group, compared to sham-operated rats. Significant correlations were also found between behavioral parameters and MDA levels. Our data suggest that pergolide facilitates spatial memory and improves brain oxidative balance, after a 6-OHDA-induced model of PD. This could be useful for further investigations and clinical applications of pergolide.

Dopaminergic and cholinergic involvement in the discriminative stimulus effects of nicotine and cocaine in rats

RATIONALE

Previous work has demonstrated asymmetrical cross-generalization between the discriminative stimulus effects of nicotine and cocaine: nicotine fully substitutes for cocaine, whereas cocaine only partially substitutes for nicotine. The factors responsible for the similarities and differences between the two drugs remain unclear.

OBJECTIVE

The study tested the involvement of dopaminergic and/or cholinergic mechanisms in the discriminative stimulus effects of nicotine and cocaine.

METHODS

One set of rats was trained to discriminate cocaine (8.9 mg/kg) from saline, and two other sets of rats were trained to discriminate nicotine (0.1 mg/kg) from saline.

RESULTS

In cocaine-trained rats, among the cholinergic agonists studied only nicotine (0.01-0.56 mg/kg) produced full, dose-related substitution; nornicotine (1-5.6 mg/kg) substituted only partially, and lobeline (2.71-15.34 mg/kg) and pilocarpine (0.26-2.55 mg/kg) failed to engender any cocaine-appropriate responding. The nicotinic antagonist mecamylamine (1-5.6 mg/kg) failed to block cocaine's discriminative stimulus effects. The dopamine antagonist cis-flupentixol (0.48 mg/kg) blocked the substitution of nicotine for cocaine. In nicotine-trained rats, the dopamine uptake blockers cocaine, bupropion and nomifensine (0.2-26.1 mg/kg) each substituted only partially for nicotine, and cis-flupentixol (0.48-0.86 mg/kg) antagonized the discriminative stimulus effects of nicotine.

CONCLUSIONS

Nicotine fully substitutes for cocaine because of its effects on dopamine transmission, and not because the discriminative stimulus effects of cocaine incorporate a cholinergic component. Substitution of nicotine for cocaine may depend more on nicotine-induced dopamine release than does the nicotine-trained discriminative stimulus; there may be differential dopaminergic involvement after acute and repeated treatment with nicotine or cocaine.

Chronic haloperidol administration does not block acute nicotine-induced improvements in radial-arm maze performance in the rat

Nicotine has been found to improve cognitive performance in a variety of tasks including the radial maze. Nicotine has also been shown to promote the release of a variety of neurotransmitters including dopamine (DA). DA has been found to be important for nicotine's reinforcing effects. DA involvement with nicotine's cognitive effects is unclear. In the current study, the effects of acute nicotine injections (0, 0.1, 0.2, or 0.4 mg/kg) were examined on radial-arm maze performance in rats given chronic infusions the DA antagonist haloperidol (0, 0.2, or 0.6 mg/kg/day). Chronic haloperidol infusion was not found to attenuate the memory improvement caused by acute nicotine injection. In fact, the dose-related nicotine-induced memory improvement was clearer in the haloperidol-treated groups than in controls. This is similar to the effect of nicotine we saw in human subjects given chronic doses of haloperidol. Our previous studies demonstrated significant nicotinic-DA interactions with regard to memory function. The current results suggest that in the DA-nicotinic relationship DA stimulation is not necessary for the memory improvement caused by nicotine.

Chronic nicotine working and reference memory effects in the 16-arm radial maze: interactions with D1 agonist and antagonist drugs

Chronic nicotine infusion has been found in a series of studies in our laboratory to significantly improve choice accuracy of rats in the eight-arm radial maze. The current study was designed to compare the effects of chronic nicotine infusion on working and reference memory in a 16-arm radial maze. Nicotine was administered to female Sprague-Dawley rats at approximately 5 mg/kg per day SC via osmotic minipumps. Controls received saline infusions. Chronic nicotine infusion significantly lowered the number of working memory errors compared to controls, whereas the number of reference memory errors was not significantly affected. The modest nicotine-induced reduction in working memory errors was seen as a main effect over the 4 weeks of infusion, but the clearest effect was seen in weeks 3-4 of nicotine administration. For the 2 weeks after withdrawal, the nicotine effect was no longer evident. Acute D1 challenges were given with the D1 agonist dihydrexidine (0, 0.25, 0.5 and 1 mg/kg) and the D1 antagonist SCH 23390 (0, 0.005, 0.015 and 0.05 micrograms/kg) during weeks 3-4 of chronic nicotine administration and weeks 1-2 after withdrawal from nicotine. Dihydrexidine caused a modest dose-related increase in reference memory errors but not working memory errors in the nicotine-treated, but not the control rats. The D1 antagonist SCH 23390 caused a modest though significant decrease in reference memory errors but not working memory errors in the control, but not the nicotine-treated rats. The behavioral specificity of chronic nicotine infusion was demonstrated with selective improvement in working memory function. Pharmacological interactions were seen with chronic nicotine treatment increasing responsivity to D1 agonist and decreasing responsivity to a D1 antagonist with regard to reference memory. The mechanisms of this interaction are still undiscovered.