BACKGROUND: Central oxytocin (OT) may be a neurotransmitter or neuromodulator and play an important role in learning and memory, sexual behaviour, pain modulation and opiate tolerance and dependence. To research the interactions between oxytocinergic and opioidergic system in hippocampus has some significance.OBJECTIVE: To investigate the effects of OT administered intracerebroventricularly on evoked discharge of left dorsal hippocampal CA1 neurons in rats and the possible interactions between oxytocinergic and opioidergic system.DESIGN: A randomised controlled study.SETTING: Department of Physiology of Guangdong Medical College; Department of Physiology and Pathology of Medical College of Wuhan University.MATERIALS: The study was conducted in the Physiology Department of Medical College of Wuhan University from September 2002 to September 2003. A total of 36 male Sprague-Dawley rats were selected and randomly divided into six groups: control (NS), OT groups (0.2 mg/L, 2 mg/L and 20 mg/L), [d (CH2)5-OVT]+OT (2 mg/L), naloxone+OT (2 mg/L), with 6 rats in each group.METHODS: Single-unit recording was performed with glass microelectrode. The glass microelectrode was inserted by a micromanipulator into hippocampal CA1. The electrical activity was amplified by a microelectrode amplifier and then recorded by the biological experimental system,monitored at the same time with oscilloscope. When recording the neural discharge, electrical stimulation of the sciatic nerves was performed once 5minutes through a double stainless electrode. 5 μL oxytocin in dosage of 0.2, 2 and 20 mg/L were injected slowly into lateral ventricle via microlitre syringe. [d(CH2)5-OVT]+OT (2 mg/L) group: 2.5 μL [d(CH2)5-OVT](80 mg/L) was injected into lateral ventricle and then 2.5 μL oxytocin (2 mg/L). Naloxone+OT (2 mg/L) group: 2.5 μL naloxone (400 mg/L) was injected into lateral ventricle and then 2.5 μL oxytocin (2 mg/L). According to frequency of discharge, effect of oxytocin at various dosages on discharge induced by neurons in hippocampal CA1 area and [d (CH2)5-OVT]and naloxone on oxytocin was assayed. MAIN OUTCOME MEASURE: Changes of discharge frequency after stimulation.RESULTS: Data of totally 36 rats were entered the final analysis. ① OT (0.2 mg/L, 2 mg/L and 20 mg/L) administered by intracerebroventricularly could decrease the evoked discharge of hippocampal CA1 neurons in a dose-dependent manner. ② The inhibitory effects of OT (2 mg/L) could be blocked by pretreated intracerebroventricularly injection of [d (CH2)5-OVT](80 mg/L, 2.5 μL). ③ Intracerebroventricular injection of naloxone (400 mg/L, 2.5 μL) could attenuate the effects of OT (2 mg/L) significantly.CONCLUSION: OT can inhibit the electrical activities of hippocampal CA1 neurons to external electrical signal through activating the oxytocin receptor. Moreover, central opioid receptor is involving in the inhibitory effects of OT.