Brain stimulation reward in the lateral preoptic area: An examination of its substrate and functional connectivity to the ventral tegmental area.

Abstract

In order to determine the nature of the hypothesized connection between the lateral preoptic and the ventral tegmental areas, two major experiments were conducted. The first major study consisted of an in-depth mapping of the substrate for self-stimulation in the lateral preoptic area using dorsoventrally moveable electrodes which permitted the testing and characterization of multiple stimulation sites in each subject. Self-stimulation was obtained throughout the lateral preoptic area and compartments 'a' and 'b' of the medial forebrain bundle; the pattern of positive sites was consistent with anatomical descriptions of the trajectory of the medial forebrain bundle. An examination of the period/current tradeoff functions generated at positive self-stimulation sites suggested that the substrate in the lateral preoptic area has a homogeneous distribution that is less dense than that found in the lateral hypothalamus. The next experiment investigated the existence of a direct anatomical connection between the lateral preoptic area and the ventral tegmental area reward neurons using the behavioural adaptation of the collision test. The collision test is a double pulse, two electrode technique based on the axonal conduction failure that occurs when two separate sites in the same axon bundle are electrically stimulated. In this study, nine rats with a total of forty-four pairs of sites were examined. In seven sites the double-pulse effectiveness curves were consistent with the characteristics of transynaptic collision. The unique shapes of collision curves predicted by the transynaptic collision model permit the direction of conduction to be inferred from the matching collision profiles; six of the seven curves were suggestive of a caudorostral direction of conduction, that is, ventral tegmental area to lateral preoptic area. These results do not support the hypothesis that the lateral preoptic area is the location of the cell bodies of origin for the descending reward pathway that courses between the lateral hypothalamus and ventral tegmental area. However, the absence of axonal collision effects or transynaptic collision profiles consistent with a rostrocaudal direction of conduction can not be interpreted to suggest that other patterns of connectivity between the reward neurons in the lateral preoptic and ventral tegmental areas do not exist. (Abstract shortened by UMI.)