Unimpaired spatial working memory following mammillothalamic tract damage in rats: Implications for the neuroanatomy of memory
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
In humans, damage to the mammillothalamic tract (MTT) as a result of localised strokes, tumours or alcohol abuse has consistently been implicated in the severe anterograde amnesia evident in these patients. This small neural pathway, which connects the mammillary bodies (MB) to the anterior thalamic nuclei (ATN), is thought to provide one important link in a larger extended hippocampal circuit involved in encoding and retrieval of episodic memory. Brain damage in clinical cases is, however, typically diffuse and contributions from additional sites of pathology cannot be ruled out. There are also inconsistencies within a limited animal literature on MTT lesions. The current study made MTT lesions in female rats and used multiple „episodic - like‟ memory tasks relevant to the proposed importance of this pathway. The project also intended to test whether enrichment reduces any impairments after MTT lesions. None of the lesions resulted in complete bilateral disconnection of the MTT, but many had moderate to large bilateral (n = 6) (81% to 50%), or unilateral MTT damage (n = 4). Rats with bilateral lesions were compared to controls (n = 14, including 4 other lesion rats in which no lesion occurred). The severe working memory deficit in the water maze expected for rats with MTT lesion was not found and only a slight deficit in reference memory in the water maze was observed (so enrichment was not implemented). Although none of the bilateral MTT lesions were complete, they are also often incomplete in clinical cases and previous research has shown that lesions to the ATN in excess of 50% are sufficient to induce severe behavioural deficits in rats. Therefore, if the MTT is critical to memory then substantial but not total bilateral disconnection should be sufficient to induce profound deficits in rats, at least on spatial working memory. Taken together these findings suggest a less crucial role for the MTT in memory than previously suggested. Future research needs to resolve the inconsistencies observed in the animal literature by repeating the present study, using larger MTT lesions and both male and female rats.