The temporal structure of motivation IV: A reexamination of extinction effects in intracranial reward

Abstract

Adult male Sprague-Dawley rats were stereotaxically implanted with unipolar electrodes and trained to press a panel to receive contingent electrical stimulation of the anterior medial forebrain bundle. A chronic design was employed in which rats had continuous access to brain stimulation. Over a 2-week period, overall rates of response per 24 hr stabilized, typically displaying a discrete episodic organization and occurring exclusively during the dark period of the lighting cycle. Normal patterns of response and response patterns after the discontinuation of stimulation were studied on a minute-by-minute basis to determine the behavioral basis and time course of experimental extinction. Separate mechanisms for the initiation and maintenance of responding were identified. The response maintenance mechanism (reflecting a priming process) extinguished within a few minutes of the onset of the extinction schedule, although rats continued to initiate episodes at high rates for at least 4 days. Thus, two response mechanisms may be empirically dissociated. Neuroleptics have been claimed to produce an anhedonic state similar to experimental extinction, and the influence of the neuroleptic drug fluspirilene was therefore examined for purposes of comparison. Unlike normal extinction neuroleptic treatment diminished response initiation but not response maintenance. The two experiments suggest it is possible to doubly dissociate two mechanisms controlling local aspects of intracranial reinforcement in the rat, and that at least some of the behavioral mechanisms underlying experimental extinction and neuroleptic incentive reduction are orthogonal.