Immunohistochemical estimation of rat brain somatostatin on avoidance learning impairment induced by thiamine deficiency.

In rats, on the 25th day after the start of a thiamine-deficient (TD) diet, impairment of avoidance learning was significantly induced in proportion to the decrease somatostatin (SST) fluorescence intensity in the cortex, amygdala, thalamus, hypothalamus, and hippocampus, including the CA1, CA2, and dentate gyrus (DG). Only a single injection of thiamine HCl (0.5 mg/rat, subcutaneous) on the 14th day after the start of a TD diet improved the amnesia to the level of the pair-fed control and prevented the decrease in the SST level. Whereas these reversal effects of thiamine treatment were not found when the treatment was given on the 21st day after the start of a TD diet. These results indicate that, after a certain degree of thiamine deficiency, TD-induced behavioral effects might be reversible, but some neuronal fibers might be irreversibly damaged, probably due to the reduction of thiamine-dependent enzymes in brain mitochondria. The results also suggest the possibility that SST in the brain may be closely related to the avoidance learning impairment induced by TD.