Hippocampal lesions are without effect on olfactory memory formation in the context of pregnancy block.

Female mice which have mated and are subsequently exposed to male urine odours (pheromones), which differ from those of the male that mated (stud male), undergo hormonal changes resulting in a block to pregnancy. Since the stud male's odour blocks the pregnancy of females other than those he mated, this would suggest that a memory specific for this male's odour is established at the time of mating. Hippocampal lesions, when made prior to mating, did not disrupt memory formation to the odour of the stud male. This male's odour did not block pregnancy, while changing odour to that of the strange male did block pregnancy. In order to establish the functional effectiveness of these hippocampal lesions, olfactory discriminations were examined in a modified T-maze. The odour discrimination was urine-soiled bedding taken from a cage of different strain males versus same strain bedding. After 35 trials, neither control nor lesioned females were performing above chance level (50% correct responses), indicating the difficulty in cognitively learning this discrimination. These female mice were therefore trained to distinguish a novel odour (butyl acetate) from familiar strain urine-soiled bedding. Hippocampal-lesioned females were slower to acquire this discrimination and did not perform above chance level after 35 trials, in contrast to the 70% success of sham-lesioned females. Hence, hippocampal lesions, while producing a deficit in olfactory learning in the maze test, are without effect on the formation or retrieval of the specific olfactory memory formed for the stud male at mating, as revealed from pregnancy block tests.