Beta-Endorphin: dissociation of receptor binding activity from analgesic potency.

Abstract

We previously reported the isolation, sequence determination, and synthesis of camel f3-endorphin (Ic-EP) (1, 2) and human f3-endorphin (/3h-EP) (3, 4) (see Fig. 1 MATERIALS AND METHODSSynthetic ic-EP and 1h-EP were obtained as described, and [Gln8,Gly31]-3h-EP-Gly-Gly-NH2 were synthesized as described (9). The radioreceptor binding assay was performed according to the procedure recently described (7,8 presence of 2.5 ,uM 13h-EP. Triplicate determinations were performed with a variation of less than 7%. The analgesic activity was assessed in mice by the tail-flick method (11). Male ICR mice weighing 25-28 g were used (Simonsen Lab, Gilroy, CA). The peptide was dissolved in sterilized saline and injected intracerebroventricularly in a volume of S ul according to the method described (12). To evaluate the tail-flick response, a control latency (To) was obtained from the mean of two latency determinations. The To value was usually 1.5-2.5 sec. Mice with a control latency of more than 3 sec were not used for the study. The test latencies (T1) were determined at 5, 10, 20, 30, 60, 90, and 120 min after the injection of the peptide for each animal. Percent analgesia was calculated as [(T1-To)/(T2 -To)] X 100, in which the cut-off time (T2) was 7 sec. Mice were considered to be analgetic if the percent analgesia was 50% or more, observed at 10, 20, or 30 min after injection. Groups of at least eight mice were injected with different doses of the peptide. The median antinociceptive dose (AD50) and 95% confidence limits were computed as described (13) The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.

Beta-endorphin: opiate receptor binding activities of six naturally occurring beta-endorphin homologs studied by using tritiated human hormone and naloxone as primary ligands--effects of sodium ion.

“…The inability to correlate the potency of various f3-endorphin (,B-EP) analogs and homologs in producing analgesia in vivo and inhibiting the binding of human [3H]-,B-EP in vitro has been documented in a number ofprevious studies (1)(2)(3). The affinity determined in vitro may not be simply related to that in vivo due to differences in assay conditions or the status of the tissue preparation.…”