Behavioral impairments, brain lesions and monoaminergic activity in the rat following recovery from a bout of thiamine deficiency.

Learning impairments were measured in rats following recovery from a subacute bout of thiamine deficiency. Behavioral training was carried out in an automated T-maze, beginning with paired run spatial delayed non-matching to sample (PR-1), then light-dark discrimination (LD), light-dark discrimination reversal (LD-R), spatial discrimination (SD), spatial discrimination-reversal (SD-R), and finally retraining on the original paired run task (PR-2). Comparable deficits were observed for PR-1 and PR-2, thus demonstrating long-lasting impairment on delayed non-matching to sample. Experimentals performed as well as controls on LD and LD-R. Two experimental animals were unable to perform above chance on the simple SD task. The remaining 15 experimental animals were equivalent to controls on several measures of SD and SD-R performance (errors to criterion, number of animals reaching criterion, correct responses in last 60 trials) although they were significantly worse than controls on both PR-1 and PR-2. Taken together, these results indicate an impairment of representational memory (PR-1, PR-2) with a spared capacity for dispositional memory (LD, LD-R, SD, SD-R) as defined by Thomas and Spafford (Behav. Neurosci., 1984, 98: 394-404). Histological analyses of left hemispheres revealed a high incidence (94%) of thalamic lesions, specifically within the intralaminar nuclei and ventral parts of the mediodorsal nucleus; and an absence of detectable changes in other structures, including the mammillary bodies, hippocampus, cortex, and locus coeruleus. In the right hemispheres, assays of monoamines and metabolites in 17 brain regions showed significant reduction only for norepinephrine in entorhinal cortex. All animals that were selectively impaired on the paired-run task had both the medial thalamic lesions and reduction in entorhinal norepinephrine.