Serotonergic neurons projecting to hippocampus activate locomotion

Abstract

Although the role of brain serotonergic neurons in locomotion has been extensively studied, their influence may vary depending upon the terminal areas. Thus, using microdialysis and microinjection techniques, we examined the relationship between serotonin (5-HT) levels in striatum, hippocampus or prefrontal cortex (PFC) and motor activity in rats. The systemic injection (10 mg/kg i.p.) of monoamine oxidase inhibitor, tranylcypromine (TC), significantly elevated 5-HT levels in the striatum, hippocampus and PFC accompanied by a parallel increase in motor activity of the rats. This effect was mimicked by microinfusions of TC (1.0 mM) or 5-HT (1.0 mM) into the hippocampus and to some extent into PFC (the response delayed in time), but not into striatum. The increase in motor activity produced by local infusions of TC either into the hippocampus or PFC could be prevented by pretreatment with 10 μM tetrodotoxin infused into the hippocampus. However, tetrodotoxin infused to PFC failed to prevent hyperlocomotion produced by intrahippocampal infusion of TC, although the response was delayed in time. Thus, we conclude that serotonergic neurons projecting to the hippocampus are involved in locomotor activity and PFC serotonergic fibers may facilitate hippocampal control of locomotion.