The relative salience of morphine and contextual cues as conditioned stimuli

Two experiments were conducted to delineate further the properties of conditioning when morphine is used as a conditioned stimulus (CS) in the conditioned suppression of drinking paradigm. Experiment 1 used a test for overshadowing designed to compare the relative salience of contextual cues (metal box) and morphine induced cues (6 mg/kg, IP) as CSs when each was paired with a foot shock unconditioned stimulus (US) in water deprived rats. Six groups (six rats each) were exposed to conditioning procedures during which the conditioning context was present 19 h (groups 1 and 2), 90 min (groups 3 and 4), or 5 min (groups 5 and 6) before shock onset, and morphine (in groups 1, 3, and 5) or saline (in groups 2, 4, and 6) was injected 10 min before shock. Subsequently, the magnitude of suppression of drinking in response to morphine, to the metal box, and to morphine plus the metal box was measured. Only group 1 (19 h group) suppressed drinking in response to morphine, while groups 3–6 suppressed drinking whenever tested in the metal box. The results indicate that morphine cues acted as a CS that elicited suppression of drinking in group 1, and that contextual cues present up to 90 min before morphine cues overshadowed morphine. Experiment 2 showed that expression of the conditioned response to morphine was blocked by naloxone.