Hippocampal theta activity after systemic administration of a non-peptide delta-opioid agonist in freely-moving rats: relationship to D1 dopamine receptors.

Hippocampal theta activity was acquired and processed off-line from digitized EEG recordings after subcutaneous (s.c.) administration of the non-opioid delta agonist BW 373U86 (0.5-2.5 mg/kg) in freely-moving rats. Relative theta power spectral analysis, implemented by a signal processing software, showed that BW 373U86 induced a dose-dependent increase in the slow component of theta band (Type 2 theta), while movement-related fast theta band (Type 1 theta) failed to show significant changes. Moreover, the increase in relative Type 2 theta power showed a maximal change at 1 mg/kg of BW 373U86, while higher doses, although effective in increasing relative Type 2 theta, induced locomotion and irregularly increased Type 1 hippocampal theta activity. The administration of 10.0 mg/kg of the delta antagonist Naltrindole (NLI) 30 min before BW 373U86, abolished hippocampal Type 2 theta increase. The rise of relative Type 2 theta power induced by BW 373U86 (1-2.5 mg/kg) was greatly attenuated by 0.1 mg/kg of the selective dopamine (DA) D1 antagonist SCH 23390. Administration of 0.1 mg/kg of SCH 23390 alone did not modify hippocampal Type 2 theta. These results indicate that delta receptors modulate the expression of hippocampal Type 2 theta and dopamine, through D1 receptors, exerts a permissive role on this influence.