We report that rats learning a spatial memory task in the Morris water maze
show elevated expression of the signal transduction receptor for BDNF and
the synaptic associated protein synapsin I in the hippocampus. Nuclease pro
tection assays showed maximal levels of TrkB and synapsin I mRNAs in the hi
ppocampus by the time that asymptotic learning performance had been reached
(Day 6). Increases in synapsin I mRNA were matched by changes in synapsin
I protein as revealed by western blot analysis. Synapsin I is a downstream
effector for the BDNF tyrosine kinase cascade pathway which has important r
oles in synaptic remodeling and function. Therefore, parallel changes in Tr
kB and synapsin I mRNAs suggest a role of the BDNF system in synaptic funct
ion or adaptation. Levels of TrkB mRNA in the hippocampus were attenuated a
fter learning acquisition (Day 20), but synapsin I mRNA was still elevated,
suggesting that the BDNF system may participate in events secondary to lea
rning, such as strengthening of neural circuits. TrkB and synapsin I mRNAs
showed an increasing trend in the cerebellum of learning rats and no change
s were observed in the caudal cerebral cortex. The selectivity of the chang
es in trkB and synapsin I, affecting the hippocampus, is in agreement with
the role of this structure in processing of spatial information. Behavioral
regulation of neurotrophins may provide a molecular basis for the enhanced
cognitive function associated with active lifestyles, and guide developmen
t of strategies to promote neural healing after CNS injury or disease. (C)
2001 Elsevier Science B.V. All rights reserved.