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In typical laboratory settings, the defensive reactions of animals appear to be limited tofreezing, fleeing, and attacking. However, in the present investigations, rats.tested in the presence of movable ma-> terial incorporated it into a striking and adaptive behavioural sequence. Rats shocked once through a stationary prod buried this shock ^source, even when the shocks-test interval was 20 days. This burying behaviour occurred at a variety of shock intensities and seemed to be controlled sp cifically by the relation between the shock and the prod; rats shocked through a grid did not bury the prod, and rats shocked by one of two identical prods buried only the shock-prod. Both the position and bright ness of the prod seemed to control the burying behaviour. When either of these cues was changed prior to the test, burying behaviour was disrupted compared to control conditions in which these cues were unaltered. Although burying was a directed and consistent response of rats to prod shock, it was not a simple, reflexive behaviour; rats could adapt their burying behaviour to changes in both the kind and disposition of burying materials. Thus, the usual assumption that the rat's defensive reper^toire is limited to a few simple behaviours appears to have been shaped by the constraints of standard testing environments rather than by the actual propensities of the rat. These results were discussed in terms of their implications for a "biological" approach to aversive learning.

Behavioral paradigms that have been designed to mimic forms of learning that are important for the survival of animals in the wild, rather than to minimize the contributions of adaptive predispositions, may prove to be particularly useful for studying the behavioral effects of drugs. In the present experiments, the propensity of rats to bury sources of aversive stimulation was disrupted in a dose-dependent fashion by a single injection of the anxiolytic drug, diazepam. This suggested that the conditioned defensive burying paradigm could prove to be a valuable addition to the paradigms available for studying anxiolytic effects. Supporting this view were two additional observations. First, the relative potencies of diazepam, chlordiazepoxide, and pentobarbital in the burying paradigm compared favorably with their relative potencies in clinical settings. Second, the effects of anxiolytics on conditioned burying appeared to be dissociable from the effects of other drugs that disrupt this behavior.

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Although current models of hippocampal function stress its well-known role in cognitive functions, historically it has also been viewed as a neural mediator of emotion. Here, we review recent evidence from intrahippocampal infusion studies in animals that support a distinctive role of the hippocampus in anxiety, independent of its roles in learning and memory. Specifically, gamma-aminobutyric acid type A receptor agonists, both direct and indirect, reliably inhibit a number of animals' untrained anxiety reactions when microinfused into the hippocampus, whereas gamma-aminobutyric acid type A receptor antagonists do not. Intrahippocampal infusions of glutamatergic, serotonergic and cholinergic compounds also produce statistically reliable antianxiety effects, but the results vary as a function of specific anxiety reactions, and to some extent specific intrahippocampal targets. One hypothesis that may accommodate some of this variability is that anxiety is functionally segregated within the hippocampus, with ventral subregions more involved in anxiety-related processes, and dorsal subregions more involved with cognitive processes. Another possibility is that different hippocampal functions (e.g. memory and anxiety) are mediated by different neurotransmitter systems and/or different receptor subtypes within the hippocampus. Although there is some evidence that supports the latter hypothesis, the evidence for the former is not conclusive. Overall, however, the data clearly suggest that the hippocampus is importantly and directly involved in the mediation of untrained anxiety reactions in animals.

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