Depletion of brain serotonin following intra-raphe injections

Abstract 

Objectives: These experiments investigated the effects of selective serotonin (5-HT) depletion on intravenous self-administration of d-amphetamine. Methods: Depletion of brain 5-HT levels was induced by injecting the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) into the dorsal and median raphe nuclei. Rats were then trained to self-administer d-amphetamine according to various schedule and access conditions via chronically indwelling intravenous catheters. Results: Large reductions of brain 5-HT did not alter responding for a training dose of 120 µg/kg d-amphetamine delivered according to a fixed ratio 1 schedule during 3-h sessions. When the dose of d-amphetamine was altered (0, 3.75, 7.5, 15, 30, 60 µg/kg per infusion) a characteristic inverted U-shaped dose response function was obtained. The 5-HT depleted rats showed increased responding for the lower doses of d-amphetamine, with a large significant increase in responding for the 7.5 µg/kg dose. In these same rats, the suppressive effect of 10 mg/kg fluoxetine on d-amphetamine (60 µg/kg) self-administration was prevented. The 5,7-DHT lesion also did not alter responding for d-amphetamine (120 µg/kg) in longer (8 h) daily access sessions. Responding for d-amphetamine delivered on a progressive ratio schedule, in which response requirements increased for each successive infusion of d-amphetamine, was also determined in 5-HT depleted rats. The number of d-amphetamine infusions was not different from the number of infusions earned by sham-lesioned rats across a range of doses of d-amphetamine (7.5–60 µg/kg). In a final experiment, spontaneous acquisition of self-administration of low doses of d-amphetamine (10 and 30 µg/kg) was measured in 5-HT depleted and control rats. Again, self-administration behaviour in the 5-HT depleted rats did not differ from controls. Conclusions: These results provide no evidence that reducing 5-HT function alters the primary reinforcing effects of self-administered amphetamine. The increase in self-administration of a low dose of amphetamine observed in experiment 1 probably involves some other process such as increased resistance to extinction.