Comparison of the analgetic activity of opioid peptides administered intracisternally and intravenously to mice

: Some of synthetic analogs of enkephalins, having protected bonds on the side of N- and C-terminals, were found to exhibit high analgetic activity when injected intracisternally. The analgetic action of enkephalin analogs was unchanged upon intravenous injection, which is accounted for by their resistance to proteolysis and the capacity to penetrate the blood-brain barrier. Based on an analysis of the drug ED50 ratios derived during intravenous and intracisternal injections an assumption might be made about low capacity of the test opioid peptides to reach opiate receptors in the central nervous system after systemic drug administration.