In addition, α-endorphin attenuated extinction of passive avoidance behavior in a non-opioid fashion. They found that both β- and α-endorphin attenuate extinction of active avoidance (“pole jump”) behavior in rats, effects that were not blocked by the opiate antagonist naltrexone ( De Wied et al., 1978a). In 1978, De Wied and coworkers first reported on biological effects of endorphins that apparently are not mediated through opiate receptors.

David De Wied, in Progress in Brain Research, 1992 The hypothesis This effect is not dependent on changes in blood volume, blood pressure, or blood glucose and suggests that the ability of carbamazepine to produce antidiuresis in patients with neurogenic (pituitary) diabetes insipidus is due to an independent action on the kidney. Carbamazepine inhibits vasopressin secretion by diminishing the sensitivity of the osmoregulatory system ( 55). The inhibitory effect of alcohol may also be mediated in part by endogenous opiates because it is also due to upward resetting of the osmostat and can be partly reversed by treatment with naaloxone ( 101). However, it would appear to be due to the agonist properties of these opiates because it can be blocked by naloxone ( 58, 71). The mechanism of the resetting has not been completely defined. In the case of morphine and butorphanol, the inhibition is due to an increase in the osmotic threshold for vasopressin release and is independent of changes in blood volume or pressure ( 71, 91). They include oxilorphan and butorphanol, the κ-agonists U50488 ( 100), leu-morphin, and U62066E, as well as low doses of morphine. Inhibitors such as the dopaminergic antagonists fluphenazine, haloperidol, and promethazine probably act by suppressing the emetic center because they inhibit the vasopressin response to nausea but not to osmotic or hemodynamic stimuli. It is probably of little physiologic significance because sodium depletion and large doses of the hormone are required to consistently elicit this response. The mechanism of this feedback effect is unknown, but it appears to be totally independent of any of the other recognized osmotic and nonosmotic stimulus. In high doses, vasopressin can also stimulate its own secretion ( 44). The stimulatory effects of chlorpropamide and clofibrate are still controversial and a mechanism of action has not been proposed. This effect is independent of changes in water balance and appears to result from an increase in sensitivity of the osmoregulatory system ( 56). Lithium, which antagonizes the antidiuretic effect of vasopressin, also increases secretion of the hormone. Vincristine may have a direct toxic effect on the neurohypophysis or peripheral neurons involved in the regulation of vasopressin secretion. Those that stimulate such as histamine, bradykinin, prostaglandin, α-endorphin, and high doses of morphine have not been studied sufficiently to define their mechanisms of action but most if not all probably work by decreasing blood pressure or producing nausea. Many drugs and hormones also influence vasopressin secretion ( Table 2). Figure 1 compares human and bovine β-endorphin (1–31).

Human β-endorphin also differs from that of other mammalian species in that a tyrosine rather than a histidine residue is present at position 27. In most mammalian species, the C-terminal amino acid is glycyl-glutamine, but in the human it is glutamic acid. β-Endorphin is an opiate by virtue of its N-terminal pentapeptide sequence and a pleiotropic hormone by virtue of various regions of its molecular sequence, including the C-terminal di- or tetrapeptide region that is involved in biological activity but not analgesia. Other opioid peptides, Leu-enkephalin, and dynorphins (derived from prodynorphin) differ from the endorphins and Met-enkephalin in a leucine, not a methionine residue at position 5. These peptides contain the N-terminal pentapeptide opioid sequence Tyr-Gly-Gly-Phe-Met, which is also the sequence of Met-enkephalin, an opioid peptide derived from proenkephalin. The opioid peptides derived from proopiomelanocortin (POMC) are β-endorphin(1–31), α-endorphin (β-endorphin 1–16), and γ-endorphin (β-endorphin 1–17) (see Fig.

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