Effects of opiate antagonists and putative ? agonists on unpunished and punished operant behavior in the rat

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Abstract
The effects of naloxone and diprenorphine, opiate antagonists with different receptor-binding properties, and the putative κ-receptor agonists, ketocyclazocine and ethyl-ketocyclazocine (EKC), were studied on food-reinforced responding in rats. Behavior was maintained under a multiple-component 1-min variable-interval schedule in which 12-min periods of unpunished responding alternated with 4-min periods in which each response was punished by a brief electric footshock. Daily sessions were 1 h. Naloxone (0.01–10 mg/kg) decreased unpunished responding only slightly; punished responding was decreased significantly to 66% of control by 10 mg/kg. Diprenorphine (0.01–10 mg/kg) did not affect unpublished responding and increased punished responding dose-dependently to as much as 190% of control. EKC (0.01–1.0 mg/kg) decreased unpunished and punished responding dose-dependently and comparably, whereas ketocyclazocine (0.01–1.0 mg/kg)decreased unpublished responding but did not significantly affect punished responding. Diprenorphine was more potent than naloxone in blocking the decreases in responding produced by the κ agonists. Differences in the behavioral effects of naloxone and diprenorphine appear to reflect the different receptor-binding properties of the two opiate antagonists.