Inverse relationship between onset and duration of EEG effects of six peripherally administered peptides

Electrical activity in the rat brain after the peripheral injection of equimolar (0.1 mM) doses of Metenkephalin, Leu-enkephalin, beta-endorphin, gamma-endorphin, DSIP, and alpha-MSH was assessed by power spectral analysis. The mean onset of EEG activity for each peptide varied between 13.7 and 20.7 minutes and lasted between 27.8 and 40.8 minutes. The significant (p less than 0.001) interaction between duration and latency revealed that the longer the latency, the shorter the duration. Similar findings were observed after injection of the same peptides at a fixed dose of 80 microgram/kg body weight. Thus, the results demonstrate that peripherally injected peptides can exert EEG effects that last longer the sooner they start.