Negative patterning discrimination learning requires the hippocampal formation in rats

The possible mechanism of the negative patterning discrimination was tested in rats with hippocampal lesion (HL). Rats were trained to press the lever for light [L+] and/or tone [T+] and to withdraw it for simultaneously presented [LT-] cue. Animals received a brain surgery to produce HL by the local injection of kainic acid and colchicin or saline for sham operation. When the food intake limited about 80% of control, HL-rats attenuated the withdrawal of lever-press for [LT-] but not for [L+] and [T+] in the retention test. On the other hand, when subjects were allowed freely to access to food in home cages, HL-rats presented less number of lever press for three cues although sham rats discriminated lever pressings for [L+] and [T+] or [LT-]. The learning deficit caused by HL was recovered with the activation of brain serotonergic system, the injection of clomiplamin or the porta-caval anastomosis. These results suggest that the hippocampal formation is necessary to produce the memory of the negative pattern learning and brain serotonergic system may relate to control an appetite.