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Effects of Social Defeat on Hippocampal CA1 Neuronal and Sympathetic Nerve Activities in Conscious Rats

Social defeat stress has been linked to depression, autonomic disorders, and cardiovascular disorders. The hippocampus has been implicated in the pathogenesis of depression, and hippocampal neuronal activity is presumed to be involved in autonomic disorders induced by social defeat stress. Potential linkages among hippocampal neuronal activities, sympathetic nerve activities, and cardiovascular function have received less attention. In the present study, we measured time courses of changes in hippocampal CA1 neuronal activity (CA1NA), renal sympathetic neuronal activity (RSNA), and lumbar sympathetic nerve activity (LSNA), arterial pressure (AP), and heart rate (HR) in response to social defeat stress in rats. Wistar rats were chronically implanted with electrodes for the measurement of CA1NA, RSNA, LSNA; electroencephalography, cervical electromyography, and diaphragm electromyography were performed, and a catheter was used to measure AP. Social defeat was induced by introduction of a Long–Evans rat, which was heavier and more aggressive than a Wistar rat, into the Wistar rat’s home cage for 30 minutes. The intrusion of the Long–Evans rat sometimes resulted in fighting between the Long–Evans and Wistar rats. Following defeat, Wistar rats displayed a submissive posture and exhibited immobilization for several minutes. Fighting caused immediate reduction of CA1NA, which remained suppressed during the period of submissive posture and immobilized behavior. However, the AP, HR, RSNA, and LSNA increased in transient fashion during immobilized behavior. These findings showed that the defeat stress considerably influenced CA1NA, whereas it marginally influenced RSNA, LSNA, AP, and HR. Therefore, it is likely that social defeat stress suppressed hippocampal function in tonic fashion, which could not be measured on the basis of sympathetic nerve activity and cardiovascular function.