Novelty seeking behavior in the rat is dependent upon the integrity of the noradrenergic system

Abstract

These experiments were designed to investigate the role of the noradrenergic system in promoting investigation of novelty in rats. Behavior was monitored in a hole board equipped with photoelectric cells strategically placed so that locomotor activity, rearing and investigation of each of the holes could be quantified independently. Specially designed computer software permitted recording of the sequence and cumulative duration of the visits to specific holes throughout the session. Dose-response curves of the sedative effect of the α2 adrenergic receptor agonist clonidine were establised, a sedative effect being defined as a decrease in overall horizontal displacements, rearings and hole visits. After a one week interval, the rats were rerun in the holeboard, with novel objects placed in four of the nine holes. Previous experiments had shown that rats spend significantly more time investigating holes containing objects than empty holes in this apparatus and this was replicated here. Doses of clonidine which were below threshold for inducing any sedative effect (10 μg/kg) totally eliminated preference for holes with objects while having no effect on total time investigating the holes. A subsequent experiment showed that the β receptor antagonist propranolol (10 mg/kg) produced a similar effect. These results suggest that the noradrenergic system is implicated in stimulus seeking behavior and the post-synaptic β receptors are involved in mediating the behavior.