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Sensitization to cocaine in pigeons: interaction with an operant contingency.

A 2-part study with pigeons investigated the role of an explicit operant contingency in determining how cocaine interacts with locomotor activity. In Part 1, pigeons pecked on a fixed-ratio-20 schedule of food presentation. In Part 2, different pigeons were studied without opportunity to peck for food. After determination of cocaine's initial effects, pigeons were exposed to daily administrations of a locomotion-increasing dose of cocaine. Locomotor sensitization was evident in the pigeons of Part 2, and tolerance developed to cocaine's effects on key pecking in the pigeons of Part 1. Locomotor sensitization was generally not evident in the pigeons of Part 1. These results suggest that explicitly conditioned operant behavior may compete with behavior sensitized by prolonged exposure to cocaine.