Novelty reward as a measure of anhedonia.

A decrease in sensitivity to pleasurable stimuli, anhedonia, is a major symptom of depression in humans. Several animal models have been developed to simulate this symptom (e.g. drug withdrawal, learned helplessness) using reward-sensitive procedures such as intracranial self-stimulation and progressive ratio responding as a measure of reward function. Recently, we introduced the use of another procedure, novel-object place conditioning in rats, to measure reward function in an associative learning situation. Withdrawal from chronic nicotine blocked a place preference conditioned by access to novel objects. This blockade was not due to impairment of object interaction, general activity, novelty detection, environmental familiarization, or expression of learning. Consequently, nicotine withdrawal directly reduced the rewarding properties of novelty. It is proposed that the novel-object place conditioning procedure could be usefully extended to other experimental situations and to genetically altered mice, so as to better understand the processes underlying changes in reward function.