Met5-enkephalin-Arg6-Phe7 inhibition of noradrenaline and acetylcholine release from peripheral organs

The role of the endogenous neurotransmitters associated with neuropathic pain and in the opioid crisis: The innate pain-relieving system

Neuropathic pain is a chronic pain caused by central and peripheral nerve injury, long-term diabetes or treatment with chemotherapy drugs, and it is dissimilar to other chronic pain conditions. Chronic pain usually seriously affects the quality of life, and its drug treatment may result in increased costs of social and medical care. As in the USA and Canada, in Europe, the demand for pain-relieving medicines used in chronic pain has also significantly increased, but most European countries are not experiencing an opioid crisis. In this review, the role of various endogenous transmitters (noradrenaline, dopamine, serotonin, met- and leu-enkephalins, β-endorphin, dynorphins, cannabinoids, ATP) and various receptors (α₂, μ, etc.) in the innate pain-relieving system will be discussed. Furthermore, the modulation of pain processing pathways by transmitters, focusing on neuropathic pain and the role of the sympathetic nervous system in the side effects of excessive opioid treatment, will be explained.

Ovarial steroids and methionine-enkephalin modulation of adrenergic transmission in rabbit oviduct

1. The effect of methionine-enkephalin on the [3H]-noradrenaline ([3H]-NA) overflow and on contractions evoked by field electrical stimulation (FES) and by exogenous NA were studied in vitro in the isthmic part of oviduct of rabbits (untreated and treated with estradiol or progesterone). 2. The evoked tritium overflow (which reflected [3H]-NA overflow) was determined by liquid scintillation spectrometry. 3. Field electrical stimulation of 4 Hz (trains of 40 pulses, 0.3 msec) evoked guanethidine-sensitive contractions. 4. In all groups of animals methionine-enkephalin dose dependently decreased FES-evoked contractions but not those evoked by exogenous NA. 5. The amount of tritium overflow evoked by 4 Hz stimulation (600 pulses, 1 msec) was significantly lower in tissues from estradiol treated (1.16 +/- 0.19%) compared with those obtained in tissues from untreated (1.82 +/- 0.22%) and progesterone treated (2.07 +/- 0.21%) rabbits. Methionine-enkephalin, 1 microM, decreased the evoked tritium overflow in the isthmus from untreated rabbits by 36.1 +/- 3.6%, in estradiol treated by 22.8 +/- 2.9% and in progesterone treated by 52.3 +/- 4.5%. 6. The results suggest that the methionine-enkephalin effect on FES-evoked contractions could be due to a prejunctional effect on the adrenergic terminals and that there is a hormonal dependence of the opioid effect on [3H]-NA overflow.

Cardiovascular activities of intravenous methionine-enkephalin-Arg6-Phe7 and methionine-enkephalin-Arg6-Gly7-Leu8 in the conscious dog

The preproenkephalin A molecule from the adrenal medulla contains the opioid peptides methionine-enkephalin (Met-ENK), leucine-enkephalin (Leu-ENK), methionine-enkephalin-Arg6-Phe7 (heptapeptide), and methionine-enkephalin-Arg6-Gly7-Leu8 (octapeptide). In the conscious, chronically instrumented dog, Met-ENK and Leu-ENK simultaneously increase heart rate and systemic arterial pressure following intravenous administration. In 19 of 23 dogs, heptapeptide produced a response identical to Met-ENK and Leu-ENK, which was inhibited by naloxone but unaffected by the dipeptidyl carboxypeptidase inhibitor SQ20881. However, in four dogs, heptapeptide produced only a fall in systemic pressure associated with an increase in heart rate despite characteristic Met-ENK responses in the same dogs; naloxone did not appear to alter this hypotensive response. Octapeptide produced slight increases in systemic pressure and heart rate. These data suggest that heptapeptide may possess intrinsic cardiovascular activity at opiate receptors; however, in certain dogs, non-opiate mechanisms, perhaps histamine release, may predominate.