Mu-delta opioid interactions. I: The delta peptide, DPDPE, increases morphine-induced EEG and EEG spectral power

The effects of the selective delta peptide, DPDPE ([2-D-penicillamine,5-D-penicillamine]enkephalin), on morphine-induced EEG and EEG power spectra were assessed. Adult female Sprague-Dawley rats were implanted with cortical EEG electrodes and permanent indwelling ICV and IV cannulae. Rats were pretreated with ICV sterile water or ICV DPDPE at 1.25, 2.5, or 5.0 nmol, 10 min before receiving IV morphine at 3 mg/kg. The treatments produced high voltage EEG bursts and associated behavioral stupor. The EEG data were further analyzed on a Pathfinder II computer and statistical analysis of EEG spectral parameters was performed by using a one-way ANOVA followed by Turkey's HSD. ICV DPDPE at 2.5 nmol and 5.0 nmol significantly increased EEG absolute global spectral power. Furthermore, ICV DPDPE at 2.5 and 5.0 nmol significantly increased the duration of morphine-induced high voltage EEG bursts. These data further indicate an in vivo interaction between mu and delta opioid receptors.