Response to novelty and its rapid habituation in locus coeruleus neurons of the freely exploring rat.

Activity of single units of the noradrenergic nucleus locus coeruleus was recorded in rats during active exploration of a novel environment. Novelty was controlled by the placement of objects in given holes in a hole board. The basic protocol included a habituation session in which the holes were empty and an object session in which a novel object was placed in one of the two holes. During the habituation session, when the whole environment was unfamiliar, there was a phasic response the first time the rat visited any hole, which habituated after one visit. During the second session, when one of the holes contained an object, the cell fired when the rat encountered the novel object. There was no response to empty holes in this session. The neuronal response was markedly diminished or entirely absent on the second and subsequent visits to object-containing holes, indicative of rapid habituation. In some rats it was possible to run a second object session, when a new object was introduced into a previously empty hole. Visits to this hole elicited a robust response, which again habituated after one single visit. The results show that the responses of locus coeruleus to novelty or change, which has been demonstrated in formal learning situations, occurs in freely behaving rats while they are learning about a new environment. Moreover, the response to novelty and change in the environment is short-lived, rapidly habituating after one or two encounters with the stimulus.