Acquisition of a spatial conditioned place preference is impaired by amygdala lesions and improved by fornix lesions

This experiment examined the role of the lateral nucleus of the amygdala, the hippocampus and the dorsal striatum in the acquisition of a conditioned place preference (CPP). The CPP was established on a radial maze which was rotated before each daily trial so that only distal cues could be used to distinguish among locations on it. Each rat was assigned a paired location in which the arm contained food, and an unpaired location in which the arm was empty. A training trial consisted of 30-min confinements in the paired and unpaired locations on successive days. Groups of animals received between 1 and 4 training trials, after which they were tested for their preference between their two assigned locations with no food in either one. Normal animals acquired a CPP after 4 training trials, an instance of stimulus--reward learning. Electrolytic and neurotoxic lesions of the lateral nucleus of the amygdala prevented acquisition of the CPP in 4 training trials, suggesting that some function of this structure (or of a neural system that includes it) is required for acquisition and/or expression of stimulus--reward associations. Striatal lesions had no effect on the CPP. Fornix lesions facilitated acquisition of the CPP after 1-3 training trials, but no CPP was observed in animals with combined fornix and amygdala lesions, suggesting that exposure to the maze environment led to the processing of information in a hippocampal system that interfered with acquisition or expression of the amygdala-based CPP. Fornix lesions also increased the number of entries into maze arms on the test day, and concurrent amygdala lesions attenuated this increase. This finding suggests that the increase in arm entries may have been generated by the amygdala and suppressed by the hippocampus. Arm entries were interpreted as an instance of a particular type of exploratory behavior. Therefore, the present results suggest that a neural system including the lateral nucleus of the amygdala mediates both stimulus--reward learning and at least one form of exploration, possibly through its connections with the motor output system in ventral striatum. A hippocampal system appears to suppress or interfere with both of these behaviors in normal animals.