The effect of chronic cocaine on self-stimulation train-duration thresholds.

The effect of chronic cocaine treatment on brain stimulation reward was assessed by examining self-stimulation train-duration thresholds. Following a predrug, saline injection period, cocaine hydrochloride (10 or 15 mg/kg) was injected (IP) across 18 consecutive days of testing. Cocaine lowered thresholds across the entire period of drug administration, with the magnitude of cocaine's effect remaining stable during this time. The subjects returned to predrug, saline levels during a postdrug test conducted immediately following chronic cocaine treatment. In a final attempt to modify cocaine's effects, the subjects received 25 mg/kg cocaine HCl three times/day for three consecutive days. Subsequent testing at the original dosage levels revealed no change in the magnitude of cocaine's effect. It was concluded that cocaine's effect on brain stimulation reward does not show tolerance or sensitization with chronic use. Similar effects have been reported for morphine and amphetamine's effect on brain stimulation reward.