DISSOCIATIONS IN HIPPOCAMPAL 5-HYDROXYTRYPTAMINE RELEASE IN THE RAT FOLLOWING PAVLOVIAN AVERSIVE-CONDITIONING TO DISCRETE AND CONTEXTUAL STIMULI

The experiments examined the release of 5-hydroxytryptamine using in v ivo microdialysis methods in the hippocampus of freely moving rats fol lowing Pavlovian aversive conditioning to discrete and contextual stim uli. Differential conditioning was achieved by manipulating the interv al between the offset of a discrete auditory 'clicker' stimulus and th e onset of a mild foot-shock reinforcer (0.5 mA, 0.5 s). Foot-shock oc curred either simultaneously with the last second of the discrete audi tory stimulus (in short-trace subjects) or 60 s later (long-trace subj ects), In this way, subjects were preferentially conditioned to the di screte stimulus and background 'contextual' stimuli respectively. Duri ng conditioning subjects also received two identical unpaired visual s timuli. At test, dialysates were collected and behavioural measures ta ken as all animals experienced (i) the aversive and two other 'neutral ' environments, and (ii) the discrete unconditioned and conditioned st imuli presented in both aversive and neutral environments. Exposure to the aversive environment, but not to either of the two neutral enviro nments, was associated with significantly increased hippocampal 5-hydr oxytryptamine release in long-trace subjects. There was also a small b ut non-significant increase in 5-hydroxytryptamine release in short-tr ace animals. In contrast, hippocampal 5-hydroxytryptamine release was unaffected by presentation of either of the discrete stimuli under all conditions. The last result was obtained despite robust behavioural r esponses (freezing) to the discrete conditioned stimulus. These data d o not agree with the hypothesis that aversive cues generally activate 5-hydroxytryptamine function in the hippocampus. Rather, they suggest a degree of specificity whereby 5-hydroxytryptamine release in the hip pocampus was determined primarily by other qualitative properties of t he conditioned aversive stimulus, namely whether the aversive cue was discrete or contextual, as well as by the magnitude of conditioning.