Principles and Role in Psychopharmacology Theoretical Background

Analyses of learning experiments demonstrated very early that in conditioning certain relationships are established between particular external events, e.g., stimuli and/or responses. Psychological learning theory distinguishes between classical and instrumental aversive conditioning based on the different ways (contingencies) by which the aversive event (the ► unconditioned stimulus; US) is related to the ► conditioned stimulus (CS) and the unconditioned response (UCR). Instrumental (aversive) conditioning refers to contingencies in which the behavior ofthe subject determines whether or not the US will occur ((Ogren 1985). In ► active avoidance paradigms, the rat has to perform a discrete response of a low probability, e.g., running from one side of a two-compartment box to the other when a discrete stimulus, e.g., the conditioned stimulus (CS) is presented in order to escape or avoid the US. In the passive avoidance task the animal learns to suppress a motor response to avoid exposure to the test area (context) associated with or predictive of the aversive event, such as a dark compartment of the passive avoidance apparatus that is normally preferred over the brightly illuminated compartment. Thereby a conflict situation is created.

Analyses of avoidance learning indicate that it involves different processes. Initially, presentation of the US results in a learned emotional state (e.g., ► conditioned fear), involving Pavlovian classical conditioning followed by the acquisition of the discrete, adaptive behavioral responses, the escape or avoidance response. The adaptive response requires for its association temporal contiguity between the US and the sensory stimulus or context used as the CS. Typical examples of the experimental sequence of passive avoidance and fear conditioning are presented in Fig. 1.

More recent theories on aversive learning have focused on the cognitive processes by which the animal acquires information about its experimental context. The cognitive interpretation of passive avoidance suggests that this paradigm is based on place learning involving the hippocampus (see below).

Passive Avoidance Tasks

Several varieties of passive avoidance tasks exist such as step-down or step-through avoidance. Passive avoidance is a learning task based on associative ► emotional learning (Ogren 1985), similar to contextual fear conditioning (Fendt and Fanselow 1999; LeDoux 2000). Unlike active avoidance tests, passive avoidance tests do not use an explicit CS. Instead, the training context serves as CS. Contextual stimuli are important for optimal ► retention performance. Changes in training context or internal stimuli, e.g., time of testing will impair retention performance.

Passive avoidance training

Door opened

Dark comp, entry and closing of door US

Door opened

Dark comp, entry and closing of door US

Bright comp, time Dark comp, time!

Retention test opened Dark comp entry i—f- ~m

H Retention \ latency

Bright comp, time Dark comp, time!

Contextual fear conditioning training

Contextual retention test

Contextual fear conditioning training

Conditioning context

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