Centrally acting drugs act as conditioned stimuli in a conditioned suppression of drinking task

Abstract

An experiment was conducted to test whether centrally acting drugs could act as conditioned stimuli (CS) in a classical conditioning paradigm in which electric shock acted as the unconditioned stimulus (US) and suppression of drinking was used as an indicator of a conditioned response (CR). Thirsty rats were allowed to drink water during daily classical conditioning sessions which took place in their home cages. The CS was either a drug injected before the session or a “cocktail” of sensory stimuli (light+tone+vibration) turned on at the beginning of the session. Part way through some sessions the animals received electric foot shock as the US. Two different drugs and the sensory cocktail were used as CSs in a discriminated classical conditioning paradigm in which one drug or stimulus (the CS+) predicted the subsequent occurrence of shock, and the other two conditions acted as CS− stimuli and predicted absence of shock. After an average of 5.7 pairings of the CS+ with shock, conditioned suppression of drinking was observed; the CR occurred only during tests preceded by the CS+ drug or stimulus. At one time or another during the experiment, pentobarbital, phencyclidine, morphine, and pentylenetetrazol were employed as the CS+. Each acquired the ability to elicit a CR, although pentobarbital was noticeably less effective than the other three drugs. All conditioning trials took place in hanging metal cages, but the CR generalized into plastic cages with sawdust floors. Each rat received three successive phases of conditioning with a different CS+ condition employed in each phase; each phase of conditioning was followed by extinction of the CR. Speed of acquisition and speed of extinction of the CR remained relatively uniform across successive phases. The results suggest that centrally acting drugs can act as conditioned stimuli in a classical conditioning paradigm when suppression of drinking is used as the CR.