Hippocampal lesions impair retention of discriminative responding based on energy state cues

Abstract

The present research investigated the hypothesis that the hippocampus is involved with the control of appetitive behavior by interoceptive "hunger" and "satiety" signals. Rats were trained to solve a food deprivation intensity discrimination problem in which stimuli produced by 0-hr and 24-hr food deprivation served as discriminative cues for the delivery of sucrose pellets. For Group 0+, sucrose pellets were delivered at the conclusion of each 4-min session that took place under 0-hr food deprivation, whereas no pellets were delivered during sessions that took place when the rats had been food deprived for 24 hr. Group 24+ received the reverse discriminative contingency (i.e., they received sucrose pellets under 24-hr but not under 0-hr food deprivation). When asymptotic discrimination performance was achieved (indexed by greater incidence of food magazine approach behavior on reinforced compared with nonreinforced sessions), half of the rats in each group received hippocampal lesions, and the remaining rats in each group were designated as sham- or nonlesioned controls. Following recovery from surgery, food deprivation discrimination performance was compared for lesioned and control rats in both Groups 0+ and 24+. Discriminative responding was impaired for rats with hippocampal lesions relative to their controls. This impairment was based largely on elevated responding to nonreinforced food deprivation cues. In addition, hippocampal damage was associated with increased body weight under conditions of ad libitum feeding. The results suggest that the inhibition of appetitive behavior by energy state signals may depend, in part, on the hippocampus.

Figure 1. Experiment 2 histology

Photomicrographs of cresyl violet-stained coronal sections taken from a representative rat in the Control (CON) and Complete Hippocampal (CHip) Groups in Experiment 2. Approximate stereotaxic coordinates of the coronal sections shown on the right are with reference to bregma.

Figure 2. Food deprivation intensity discrimination training

The data shown depict mean photobeam interruptions throughout each four-trial block of 4-min training sessions. All data were collected prior to surgical treatment. The left panel depicts data from 24-h food deprivation training sessions, while the right panel depicts data from sessions when the rats were 0-h food deprived. Data for rats trained with sucrose pellets delivered at the end of sessions under 24-h food, and not delivered under 0-h food deprivation are designated Group 24+ (filled symbols) whereas rats trained with the reversed deprivation level-sucrose pellet contingency are designated Group 0+ (open symbols). Error bars represent S.E.M.

Figure 3. Deprivation intensity discrimination testing

Leftmost two panels show mean photobeam interruption over the last two-trial block of training (pre-surgery) under 24-h and 0-h food deprivation for rats that were to subsequently receive lesions of the complete hippocampus (Hippocampal) and the combined operated and unoperated controls (Control). The two rightmost panels of show data collected for the Hippocampal and Control rats after surgery. Error bars represent S.E.M.

Figure 4. Body weight under 24- and 0-h food deprivation

Mean body weight for rats with Hippocampal lesions (filled symbols) and Controls (open symbols) under alternating 24-h (left panel) and 0-h (right panel) food deprivation. Error bars represent S.E.M.