Effects of acute intravenous administration of met-enkephalin in the rabbit: a computer analysis

Intravenous administration of doses of Met-enkephalin ranging from 33.3 to 5000 micrograms/kg was followed by modifications in the EEG pattern in the rabbit. The changes were a prevalence of strongly synchronized EEG patterns, lasting about 90 min, in the absence of any behavioural sign of drowsiness. A transient fall in arterial blood pressure, lasting 15 sec, was observed immediately following the injection. Quantification of the EEG effects showed a remarkable increase of EEG total power. Such an increase is statistically significant. It was less marked in the posterior leads and appeared to be dose-related as regards its intensity, duration, and latency from injection. Qualitatively such a spectral profile matches the one typical of physiological drowsiness. As regards EEG effects, no tolerance to the peptide developed if the administration was repeated after 90 min. The specific antagonist of opiates both prevented and reversed the hypersynchronization of the tracing. These findings demonstrate that Met-enkephalin crosses the blood-brain barrier in an amount adequate to producing changes in the functional organization of the brain, resulting in EEG patterns corresponding to functional depression.