Effect of enkephalin and endorphin analogs on receptors in the mouse vas deferens

A novel opioid structure which accepts protonated as well as non-protonated nitrogen: a family of pure, delta receptor selective antagonists

Conventional opioids including opioid peptides require an "opioid" nitrogen which exists in protonated state while interacting with the receptor. In the present paper we demonstrate that the Tyr-Pro-Gly-Phe-Leu-Thr hexapeptide sequence accepts N-terminal substituents such as N-t-Boc, N-phenylacetyl and N-diphenylacetyl where the N cannot become protonated, as well as "traditional" substitutions such as N,N-diallyl, where protonation is likely under physiological conditions. The opioid peptides bearing these substituents are pure antagonists of medium affinity (Ke values in the mouse vas deferens bioassay against [Met5]-enkephalin are in the 3 x 10(-7)-4 x 10(-6) M range) with a high delta receptor preference (50-350-fold delta over mu selectivity ratios).

Evidence that N-acetylation regulates the behavioral activity of alpha-MSH in the rat and human central nervous system

alpha-MSH immunoreactive peptides were fractionated and characterized in rat and human brain and rat pituitary by reversed phase high pressure liquid chromatographic techniques. alpha-MSH and deacetylated alpha-MSH were two major naturally existing peptides in both brain and pituitary gland. Subsequent experiments examined the roles of these two peptides in neuronal function. The alpha-MSH was clearly more effective than deacetylated alpha-MSH in improving performance on a visual discrimination task after intraperitoneal administration and in inducing excessive grooming after intraventricular administration. The difference in behavioral potency may be explained by the fact that alpha-MSH was much more resistant to peptidase degradation than was deacetylated alpha-MSH. N-acetylation of alpha-MSH may be an effective regulatory process for modulating the behavioral potency of the secretory product of alpha-MSH-containing pituitary cells and neurons.

Analgesia after peripheral administration of enkephalin and endorphin analogues

Several analogues of Met-enkephalin and beta-endorphin were tested for their analgesic properties after systemic injection. The latencies of mice to flick their tails away from a source of heat revealed that analogues of the opiate peptides can cause analgesia when injected by this route. In particular, compounds specifically designed to be more lipophilic or to possess additional binding sites were shown to be potent analgesic after peripheral administration.

Activity of VIP, somatostatin and other peptides in the mouse vas deferens assay

Non-opiate peptides such as vasoactive intestinal peptide (VIP) and somatostatin were tested for their effects on electrically induced contractions of the vas deferens. VIP(ED50 = 2.7 X 10(-8) M) and to a lesser extent somatostatin (ED50 = 5.2 X 10(-8) M) were found to be in the same general range of activity as enkephalin and the endorphins in this system. Human pancreatic polypeptide (HPP) exerted a biphasic effect, inhibiting the contractions at high concentrations but enhancing them at lower concentrations. A number of other natural occurring brain peptides were ineffective at concentrations of 1 X 10(-6) M. Several somatostatin analogues were tested and their activity on the vas deferens was found to more closely parallel their potency to inhibit the release of gastric acid than of growth hormone. In contrast to the brain opiates, however, the inhibitory effects of VIP, somatostatin and its analogues, and HPP were not reversed by the opiate antagonist naloxone. The results suggest that the vas deferens can be readily used for evaluation of analogues of VIP, somatostatin, and other peptides.

Endorphin analogs with potent and long-lasting analgesic effects

The analgesic effects of intracerebroventricular administration of alpha, gamma, and beta-endorphin and their D-Ala2-analogs were examined in the rat using the tail-flick test. Analgesia was produced by all substances. The actions of D-Ala2-alpha and -beta-endorphin were considerably greater than the parent forms, whereas D-Ala2-gamma-endorphin was approximately equivalent to the parent compound. The marked analgesia was dose dependent and prolonged for all analogs. Since these effects were reversed by the opiate antagonist naloxone, it was concluded that opiate receptors mediate the action of these analogs. It is suggested that these analogs may be useful in behavioral tests when a longer duration of action is desirable.