Locomotor activity correlates with modifications of hippocampal mossy fibre synaptic transmission

The hippocampus has long been implicated in memory formation. Although accumulating evidence suggests involvement of the hippocampus in other brain functions including locomotor regulation and emotional processes, cellular and synaptic bases underlying these functions remain largely unknown. We here report that environmental manipulations in mice unveiled the association of locomotor activity with the hippocampal mossy fibre (MF) synaptic transmission. Electrophysiological recordings of synaptic responses were made using hippocampal slices prepared from mice whose behaviour had been analysed. Environmental enrichment induced parallel decreases in open-field locomotor activity and MF synaptic facilitation. Facilitation induced by paired-pulse stimulation at relatively long intervals (>or=200 ms) was selectively reduced while the basal synaptic efficacy and high-frequency transmission were unaffected. Social isolation caused a change in behaviour in an elevated plus-maze, but neither the open-field activity nor the MF synaptic transmission was significantly altered. Effects of dopamine, a neurotransmitter essential for locomotor regulation, on the MF synapse were also examined using these mice. Environmental manipulations did not cause significant changes in potentiation of the MF synaptic transmission induced by dopamine. However, analysis of behavioural and electrophysiological results in individual subjects revealed that locomotor activity negatively correlates with magnitude of the dopamine-induced potentiation. These results suggest that the MF synapse plays important roles in the regulation of locomotor activity. We propose that the MF synapse can serve as the synaptic model for certain forms of locomotor regulation, with potential importance for investigation of the pathophysiology of psychiatric diseases using animal models.