To clarify the role of delta-opioid receptors on modulation of hippocampal electrical activity and behavior, deltorphin (DT), a naturally occurring heptapeptide that selectively binds to delta-opioid receptors, was intravenously (IV) administered to rabbits. For this purpose, at 8-day intervals, the effects of IV administration of normal saline and IV infusion of synthetic DT (1 mg/kg b.wt. for 2 min) on the spontaneous behavior in neutral environment, both in absence of any external stimulus and after the introduction of a stuffed predator, were examined in seven adult male rabbits, on separate and successive occasions. During each session of experimental procedure, hippocampal EEG was also recorded by telemetry. Behavioral activity showed an increase in alert and reactive immobility after the peptide injection in comparison with that observed during control period (saline administration). Under DT treatment the frequency of hippocampal electrical activity decreased, and a reduction in rhythmicity of electrical pattern was also observed in presence of stressful stimulus. These findings show that DT may affect neural and behavioral elements related to the control of attentional and emotional processes, suggesting a modulating role of delta-opioid receptors
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