Immunohistochemical estimation of brain choline acetyltransferase and somatostatin related to the impairment of avoidance learning induced by thiamine deficiency

We have found that thiamine-deficient (TD) rats show significant impairment of avoidance learning on the 25th day after the start of TD diet, as measu red by passive- avoidance task. Administration of physostigmine (0.1 mg/kg, i.p.) from the 14th day after the start of TD diet improved the impairment of avoidance learning to the pair-fed (PF) control level by the 25th day. However, the recovery effect of physostigmine did not occur on the 25th day when the treatment was begun on the 21st day. To ascertain the correlation between the cholinergic neuronal function in rat brain and the avoidance l earning impairment induced by TD, the immunohistochemical distribution of b rain choline acetyltransferase (ChAT) was determined by fluorescence intens ity using two-dimensional microphotometry. The intensity of the ChAT fluore scence started to decrease in the cortex and hippocampus on the 14th day an d showed a marked decrease in the cortex, hippocampus and thalamus on the 2 5th day of TD feeding in comparison with PF controls. The intensity of the somatostatin (SST) fluorescence was unchanged on the 14th day of TD feeding , but on the 25th day, SST was significantly decreased in comparison with P F controls. Furthermore, physostigmine treatment from 14th day after the st art of TD diet reversed SST fluorescence intensity to the control level by the 25th day. These results suggest that the impairment of avoidance learni ng induced by TD may involve not only cholinergic but also somatostatinergi c systems. (C) 2000 Elsevier Science Inc.