Effect of the endogenous analgesic dipeptide, kyotorphin, on transmitter release in sympathetic ganglia

Review of Kyotorphin Research: A Mysterious Opioid Analgesic Dipeptide and Its Molecular, Physiological, and Pharmacological Characteristics

Tyrosine-arginine (kyotorphin), an opioid analgesic dipeptide, was discovered more than 40 years ago in 1979. The evidence accumulated during this period has established the physiological significance of kyotorphin as a neuromodulating peptide, and pharmacological applications. Some of the following important findings have been discussed in this review: (1) kyotorphin is unevenly distributed in the brain; it is found in high concentrations in the pain pathway, which involves the regions associated with morphine analgesia; (2) kyotorphin is subcellularly localized in the synaptosome fraction or nerve-ending particles; (3) a specific synthetase generates kyotorphin from tyrosine and arginine; (4) kyotorphin may be also processed from calpastatin by a novel calcium-activated neutral protease or calpain; (5) kyotorphin preloaded into the synaptosome is released by high K⁺ depolarization in a Ca²⁺-dependent manner; (6) kyotorphin has a specific G protein coupled receptor, which mediates the activation of phospholipase C (PLC) and inhibition of adenylyl cyclase through G_i; (7) leucine-arginine works as a specific kyotorphin receptor antagonist; 8) membrane-bound aminopeptidase or excretion through a peptide transporter, PEPT2, may contribute to the inactivation of kyotorphin; and (9) kyotorphin causes increased Met-enkephalin release from brain and spinal slices. It is also known that the opening of plasma membrane Ca²⁺ channels through a conformational coupling of the InsP₃ receptor with the transient receptor potential C1, which is downstream of the kyotorphin receptor-mediated activation of G_i and PLC, could be a potential underlying mechanism of Met-enkephalin release. Considering these findings, translational research is an exciting domain that can be explored in the future. As kyotorphin is a small molecule, we could design function-added kyotorphin derivatives. These studies would include not only the brain-permeable kyotorphin derivatives but also hybrid kyotorphin derivatives conjugated with small compounds that have additional pharmacological actions. Further, since there are reports of kyotorphin being involved in either the etiology or treatment of Alzheimer's disease, epilepsy, inflammation, and chronic pain, studies on the beneficial effects of kyotorphin derivatives should also be expected in the future.

Blockade of analgesic effects following systemic administration of N-methyl-kyotorphin, NMYR and arginine in mice deficient of preproenkephalin or proopiomelanocortin gene

Kyotorphin is a unique biologically active neuropeptide (l-tyrosine-l-arginine), which is reported to have opioid-like analgesic actions through a release of Met-enkephalin from the brain slices. N-methyl-l-tyrosine-l-arginine (NMYR), an enzymatically stable mimetic of kyotorphin, successfully caused potent analgesic effects in thermal and mechanical nociception tests in mice when it was given through systemic routes. NMYR analgesia was abolished in μ-opioid receptor-deficient (MOP-KO) mice, and by intracerebroventricular (i.c.v.) injection of naloxone and of N-methyl l-leucine-l-arginine (NMLR), a kyotorphin receptor antagonist. In the Ca²⁺-mobilization assay using CHO cells expressing Gα_qi5 and hMOPr or hDOPr, however, the addition of kyotorphin neither activated MOPr-mechanisms, nor affected the concentration-dependent activation of DAMGO- or Met-Enkephalin-induced MOPr activation, and Met-enkephalin-induced DOPr activation. NMYR-analgesia was significantly attenuated in preproenkephalin (PENK)- or proopioimelanocortin (POMC)-KO mice. The systemic administration of arginine, which is reported to elevate the level of endogenous kyotorphin selectively in midbrain and medulla oblongata, pain-related brain regions, caused significant analgesia, and the analgesia was reversed by i.c.v. injection of NMLR or naloxone. In addition, PENK- and POMC-KO mice also attenuated the arginine-induced analgesia. All these findings suggest that NMYR and arginine activate brain kyotorphin receptor in direct and indirect manner, respectively and both compounds indirectly cause the opioid-like analgesia through the action of endogenous opioid peptides.

Opioid peptides inhibit excitatory but not inhibitory synaptic transmission in the rat dorsal motor nucleus of the vagus

Opioid peptides produce gastrointestinal inhibition and increase feeding when applied to the brainstem. The present studies were designed to determine the actions of opioid peptides on synaptic transmission within the dorsal motor nucleus of the vagus (DMV) and the localization of mu-opioid receptors. Whole-cell recordings were made from identified gastrointestinal-projecting DMV neurons in thin brainstem slices of the rat. Electrical stimulation of the nucleus of the tractus solitarius evoked EPSCs and IPSCs. In all neurons tested, methionine (Met)-enkephalin (0.003-30 microm) inhibited the peak amplitude of the EPSCs. The effect was prevented by naloxone (1 microm) as well as by naloxonazine (0.2 microm). An increase in the ratio of the evoked paired pulses indicated that the inhibition was attributable to actions at presynaptic receptors. This presynaptic inhibitory action was mimicked by [d-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (0.1 microm) and the analgesic dipeptide kyotorphin (10 microm) but not by cyclic[d-Pen(2), d-Pen(5)]-enkephalin (1 microm) and trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide methanesulfonate (1 microm). In contrast, the amplitude of evoked IPSCs was not altered either by Met-enkephalin or by any of the opioid receptor-selective agonists. Immunohistochemical studies revealed that nerve terminals apposing DMV neurons showed immunoreactivity to mu-opioid receptors colocalized with glutamate immunoreactivity but not glutamic acid decarboxylase immunoreactivity. These results suggest that within the DMV, mu-opioid receptors are present on the nerve terminals of excitatory but not inhibitory inputs to GI motoneurons. Such specificity may imply that the central inhibitory action of opioid peptides on gastrointestinal function targets selected pathways.

Effects of L-thyrosyl - L-arginine (kyotorphin) on the behavior of rats and goldfish

The effects of kyotorphin (KTP), a dipeptide (L-Tyr-L-Arg), on the level of sensory attention to stimuli of different modalities in rats and the exploratory behavior in goldfish were investigated. In both cases KTP was found to suppress the exploratory activity. When 5-HTP, a precursor of serotonin synthesis, is activated the inhibitory effects of KTP increased. It is assumed that the regulatory effect of KTP on the exploratory behavior of animals is mediated by the monoaminergic (neurotransmitting) brain systems, as distinct from its analgetic effects, which are mediated by the opioid brain systems.

L-DOPA induces concentration-dependent facilitation and inhibition of presynaptic acetylcholine release in the guinea-pig submucous plexus

Neurotransmitter- or neuromodulator-like actions of L-DOPA were investigated with intracellular recordings from submucous plexus neurons of the guinea-pig caecum. L-DOPA at 30 nM augmented the amplitude of fast EPSPs, but did not affect depolarizations elicited by puff application of acetylcholine (ACh). The augmenting effect of L-DOPA on the fast EPSPs was counteracted by L-DOPA methyl ester. The fast EPSPs were depressed by 10 microM L-DOPA, but transiently augmented after rinsing the drug. L-DOPA methyl ester did not affect the inhibitory action of L-DOPA on the fast EPSPs, but antagonized the potentiation following the inhibition. The depolarization elicited by exogenously applied ACh was inhibited by 10 microM L-DOPA. Intracellular Ca2+ concentrations ([Ca2+]i) of the neuronal soma were measured with fura-2 microfluorophotometry. The transient increase in the [Ca2+]i evoked by the somatic action potential (delta[Ca2+]AP) was facilitated by 30 nM L-DOPA, but decreased by the drug at 10 microM. It is concluded that L-DOPA at low concentrations enhances the delta[Ca2+]AP, increasing the neurotransmitter release, but at high dose diminishes the delta[Ca2+]AP, inhibiting the neurotransmission.

Kyotorphin synthetase activity in rat adrenal glands and spinal cord

Kyotorphin, an endogenous [Met5]enkephalin-releasing antinociceptive dipeptide (L-Tyr-L-Arg), is formed by kyotorphin synthetase from its constituent amino acids, L-Tyr and L-Arg, in the brain in an ATP-Mg(2+)-dependent manner. To elucidate the physiological role of kyotorphin in organs other than the brain, we examined the activity of kyotorphin synthetase in the rat adrenal glands and spinal cord. By Sephacryl S-300 gel-filtration chromatography of the soluble extracts from both the organs, the enzyme activity forming immunoreactive kyotorphin from L-Tyr and L-Arg in the presence of ATP and MgCl2 was detected in the fractions with the molecular mass of 200-300 kDa, being drastically reduced by the omission of ATP and MgCl2 from the reaction medium. The Km values of the partially purified adrenal and spinal kyotorphin synthetase for L-Tyr, L-Arg, ATP, and MgCl2 were close to those of the brain enzyme. The activity of adrenal kyotorphin synthetase was inhibited by some L-Arg analogues. NG-nitro-L-arginine methyl ester, alpha-methyl-L-ornithine and D-Arg, but not by NG-nitro-L-arginine and N-iminoethyl-L-ornithine. In the crude soluble extracts from the adrenal glands and spinal cord, kyotorphin was formed by kyotorphin synthetase, and also by the enzymatic processing of the precursor proteins, in the presence of physiological concentrations of L-Tyr and L-Arg in addition to ATP and MgCl2. Thus, kyotorphin synthetase resembling that in the brain is present in the rat adrenal glands and spinal cord. The present findings may predict a functional role of the L-Arg-kyotorphin pathway in these organs.

Methionine enkephalin presynaptically facilitates and inhibits bullfrog sympathetic ganglionic transmission

The effects of [Met5]enkephalin (ME) on the fast excitatory postsynaptic potential (EPSP) in bullfrog sympathetic ganglion cells were studied with intracellular recording in vitro. The variance and failure methods were used to calculate quantal content and quantal size of the fast EPSP in a low Ca2+/high Mg2+ solution. High concentrations of ME (1 and 10 microM) decreased the amplitude and the mean quantal content of the fast EPSP, whereas low concentrations (10 pM to 10 nM) of the peptide increased EPSP amplitude and quantal content. The mean quantal size of the EPSP was not changed by ME. The inhibitory effect of ME at high concentration (10 microM) was reversibly antagonized by the same concentration of naloxone; the facilitatory effect of ME at low concentration (1 nM) was not affected by 10 times higher concentration of naloxone (10 nM), but inhibited by 10 microM naloxone. A low concentration of naloxone (100 pM) itself increased the amplitude and the mean quantal content of the fast EPSP without changing the mean quantal size. The other concentrations of naloxone used in this study (1 pM to 10 microM) caused no significant change in the fast EPSP. ME (100 fM to 10 microM) and naloxone (1 pM to 10 microM) did not change the resting membrane potential or input resistance, the amplitude and duration of action potentials, and the sensitivity to the acetylcholine applied by iontophoresis. These results indicate that ME may act on preganglionic nerve terminals either to facilitate or to depress transmitter release; the inhibitory action is naloxone sensitive, whereas the facilitatory action is less sensitive to naloxone.