Motivated exploratory behaviour in the rat: the role of hippocampus and the histaminergic neurotransmission

Abstract

Exploration is one of most basic adaptive behavioural responses, giving the animal an important evolutionary advantage to survive in a changing environment. Inspection of novel environments might be come with motivated exploratory behaviour. In spite that this type of exploration in the rat is known for many years, little attention has been given to the intrinsic mechanisms or the brain structures that are involved in. In the present work the hippocampus, the neurotransmitter histamine, and the geometrical features of novel objects were examined in a model of conflictive and non-conflictive exploration in the rat which evaluates incentive-motivated exploration. Young adult intact rats were tested in a neutral non-conflictive behavioural activity detector (OVM), with (eOVM) or without (sOVM) novel objects. Three different objects were used: a box, a toy duck, and a tower. Results show that animals decrease its general motor activity (horizontal, ambulatory and non-ambulatory activity) in favor to exploration of the objects. Motivated exploration was not the same for all three objects. Rats explored significantly more the Tower and the Box objects than the Duck item. Behavioral patterns of hippocampus-implanted rats showed decreased scores in motor activity but maintained the difference in the relation of "without/with objects" exploration. When hippocampus-implanted rats were tested in a conflictive exploration device (the elevated asymmetric plus-maze), exploration of the No Wall arm, considered the most fear-inducing environment, was significantly more explored by the animal when the tower object was positioned at its end than when it was absent. Microinjection into the ventral hippocampus of histamine abolished this motivated exploratory response. Pre-treatment with pyrilamine, and not with ranitidine, was effective to block the inhibitory effect of histamine on the object motivated exploration. Results confirm that the hippocampus is involved on incentive motivated exploration, and suggest that histamine is part of an analyzing neuronal circuit of novelty incentivating behavioural responses in rats.