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The level of expression of the c-Fos protein in neurons was used as a measure of the activation of transcription in the hippocampus of common voles (Microtus arvalis Pall.) after rapid spatial training. Stained Fos-positive cells were counted on 20 brain sections along the rostrocaudal axis of the hippocampus. Voles were trained to find the exit to their home cages through one of the arms of a modified eight-arm radial maze (using a 2-h series of six trials on one day). Animals were initially trained to leave the home cage via an arm not connected to the maze. Voles of the "active" control group were passed through the isolated arm into the home cage six times on the experimental day. Animals for the "passive" control for c-Fos levels were collected from their home cages. Significant increases in c-Fos expression in voles trained in the maze and the active control group, as compared with passive controls, were seen in all areas studied (hippocampal fields CA1 and CA3 and the dentate fascia). At the same time, a significant increase in the number of c-Fos-positive neurons in voles trained in the maze, as compared with the active controls, was noted only in the caudal hippocampus, no differences being seen in the rostral part. The greatest levels of activation were seen in the dentate fascia and field CA3. These results provide evidence for the heterogeneous functioning of the hippocampus along the rostrocaudal axis during training of voles to solve a spatial task.

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