Endomorphin-1 and endomorphin-2: pharmacology of the selective endogenous μ-opioid receptor agonists
Introduction
It is well known that complex opioid systems modulate a number of physiological processes, including pain, reward, stress, immune responses, gastrointestinal, respiratory, and neuroendocrine functions, and cardiovascular control (Olson et al., 1998). These diverse effects are mediated by endogenous opioid peptides through the activation of specific membrane-bound opioid receptors. These receptors normally interact with opioid peptides, which are distributed in many regions of the mammalian brain. Since the demonstration of receptors for morphine, the μ-opioid receptors, over 25 years ago, neuroscientists have searched for substances produced by the nervous system that activate these receptors. The search led to the discovery of the enkephalins and the endorphins in the 1970s. These compounds display relatively low selectivity and efficacy at the μ-opioid receptors (Zadina et al., 1994), and the recently described endogenous peptides endomorphins-1 and -2 are the likely ligands for the morphine-preferring μ-opioid receptors in many brain regions. They exhibit the highest affinity and specificity for the μ-receptors of any compound found so far in the mammalian nervous system. Their μ characteristics have been supported by a variety of in vivo and in vitro studies. The endomorphin family includes two peptides that differ in one amino acid: endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) (Fig. 1), with molecular masses of 665.7 and 626.7 kDa, respectively. These endogenous ligands differ from conventional endogenous opioid receptor ligands in their N-terminal sequence, peptide length, and C-terminal amidation. The discovery of the endomorphins has opened up new possibilities in opioid research. In the 3 years that have elapsed since the original report of the isolation of endomorphins, close to 90 studies (until February 2000) have appeared on the binding profile, the release, the transduction mechanism, the distribution, and the in vitro and in vivo effects of these peptides. The purpose of this review is to summarize the wealth of information that has become available through the use of molecular biological, immunocytochemical, physiological, and pharmacological techniques, and to point to the perspectives for novel strategies, especially in pain therapy.
Section snippets
Identity of endomorphins
The route to the identification of the endomorphins was interesting in itself, because it involved an unusual interplay between pharmacology and combinatorial chemistry (Zadina et al., 1997). From a structural point of view, it has been established that naturally occurring opioid peptides consist of two components, a biologically important N-terminal tri- or tetrapeptide fragment (message sequence) and the remaining C-terminal fragment (address sequence) (Yamazaki et al., 1993). Endorphins,
Conformational analysis
To identify structural attributes unique to endomorphin-1, potential sites of recognition, and a possible correlation between the biological properties and the conformational preferences, a few conformational analysis studies have been performed by means of multidimensional NMR and molecular modeling techniques Fiori et al., 1999, Podlogar et al., 1999, Paterlini et al., 2000. The first spectroscopic results, from experiments in dimethylsulfoxide and in water, indicate that endomorphin-1 exists
Binding characteristics
Zadina et al. (1997) demonstrated that endomorphins-1 and -2 have high affinities (Ki=360 and 690 pM, respectively) and selectivities (4000- and 13,400-fold preference, respectively, over the δ-receptor, and 15,000- and 7600-fold preference, respectively, over the κ-receptor) for the μ-receptor of synaptic plasma membranes prepared from rat brain. They have equal affinities, but greater selectivities, for the μ-receptor than those of a μ-selective analog of enkephalin [[d-Ala(2), N
Transduction mechanisms
Recent cloning and expression studies have revealed that the μ-opioid receptors belong to the superfamily of 7-transmembrane domain receptors that are coupled to the inhibitory Gi/Go class of G-proteins (Chen et al., 1993). The binding of opioids to μ-opioid receptors produces coordinated changes at a cellular level, i.e., the closure of voltage-sensitive Ca2+ channels (Porzig, 1990), the activation of K+ channels (North, 1989), and a reduction in cyclic AMP (cAMP) formation (Childers, 1991).
Isolation and distribution of the endomorphins
In studies using radioimmunoassay and immunocytochemistry, endomorphin-1 and/or endomorphin-2 immunoreactivity has been isolated and localized within the bovine, human, rat, monkey, mice, and guinea pig nervous systems. The concentrations of the peptides (2.1 pmol/g) are below those of enkephalin, but similar to those of the less abundant opioids, β-endorphin and dynorphin, in bovine brain extracts (Zadina et al., 1997). The tetrapeptides, however, were found in much higher amounts (151 pmol/g)
Endomorphins and antinociception
Agonists that exert activity at the μ-opioid receptors are well known to be the most effective means of alleviating severe pain in a wide range of conditions. Their analgesic efficacy stems from their activity at the supraspinal, spinal, and peripheral levels. A number of studies have been performed to investigate the roles of endomorphins in pain transmission and in antinociception both in vitro and in vivo.
Endomorphins and the cardiovascular system
It is well known that opioids exhibit vasodilator actions through both central and peripheral sites (Bowman & Rand, 1980). The endogenous opioid system has been shown to play roles in regulation of the vascular smooth muscle tone, regional blood flow, and blood pressure in normal and hypertensive states, and morphine decreases the systemic arterial pressure in anesthetized cats and rats (Olson et al., 1998). This activity may be mediated by an action on the autonomic nervous system or by
Gastrointestinal functions
μ-Opioid receptors are also present in the peripheral nervous system, including the enteric nervous system, and opioids have long been recognized as having inhibitory effects on the gastrointestinal transit (Olson et al., 1998). There is little evidence as concerns the possible roles of the endomorphins in the gastrointestinal function.
The respiratory system
The opioid system significantly influences the respiratory system, although no data are available on the respiratory effects of centrally administered endomorphins. In the airways, opioids have been shown to modulate the release of mediators from the parasympathetic and sensory nerve fibers Bartho et al., 1987, Belvisi et al., 1990. The endomorphins induce a concentration-dependent inhibition of electrical-field stimulation-induced tachykinin-mediated contractions of the guinea pig bronchus (
Inflammation
Neurogenic inflammation is mediated via sensory peptides released from the peripheral terminals of the sensory nerves, and can be modulated by opioid peptides locally released at the site of injury. Khalil et al. (1999) used a vacuum-induced blister model in anesthetized rats to examine the effects of endomorphin-1 (0.01–10 μM) on the inflammatory responses. Local perfusion of endomorphin-1 significantly inhibited the inflammatory responses to both electrical stimulation of the sciatic nerve
The immune system
Endogenous opioid peptides possess a variety of immunomodulatory properties (Olson et al., 1998). It has been shown that the μ3-opioid receptor mediates inhibition by morphine of the activation of human peripheral blood monocytes and granulocytes (Stefano et al., 1993). Similar to other opioid peptides (Stefano et al., 1993), the endomorphins do not release NO from human monocytes and granulocytes, supporting the binding results (see Section 4; Rialas et al., 1998). The very surprising result
Miscellaneous
The central mechanisms regulating food intake are complex and still not understood completely. Opioid agonists have been shown to stimulate food intake, raising the possibility that opioid receptors are involved in the regulation of feeding (Olson et al., 1998). Intracerebroventricular injection of the endomorphins (0.03–30 nmol) enhanced the food intake for 4 hr in non-food-deprived mice in a dose-related manner (Asakawa et al., 1998). It is also well known that opioid agonists generally
Tolerance to endomorphins
Some in vivo data suggest the development of acute tolerance (or tachyphylaxis) against both endomorphins Stone et al., 1997, Higashida et al., 1998, Horvath et al., 1999. The development of acute spinal tolerance to pain sensitivity was observed 30 min following pretreatment with endomorphin-2 Stone et al., 1997, Horvath et al., 1999. Interestingly, endomorphin-1 required a longer pretreatment time (90 min) before tolerance was observed (Stone et al., 1997), which suggests that it is possible
Endomorphin analogs
One practical difficulty with the endomorphins is the short duration of their effects. One solution to this problem is to use stable analogs. Several analogs of the endomorphins have been synthesized to date. The first were d-Ala-endomorphin-2 [Tyr-d-Ala-Phe-Phe-NH2 (TAPP)] and d-Pro-endomorphin-2, where the l-Pro at Position 2 was substituted with d-Ala or d-Pro, respectively Champion & Kadowitz, 1998, Champion & Kadowitz, 1999. Next came β-MePhe4-endomorphins-1 and -2, where the endomorphins
Metabolism
The metabolic fate of the endomorphins either in vitro or in the organism remains relatively unknown. The endomorphin-induced [35S]GTPγS binding does not change in the presence of peptidase inhibitors Alt et al., 1998, Narita, 1998, implying that enzymatic peptide cleavage does not play a significant role under the conditions of these assays (Alt et al., 1998). In contrast, endomorphin-2 is an excellent substrate of different endopeptidases. It has been shown that DPP IV, a membrane-bound Ser
Conclusions
The substantial number of studies discussed in this review have clearly demonstrated that both endomorphins display potent bioactivities in vitro and in vivo. Their high affinities and selectivities for μ-opioid receptors and their potent antinociceptive and vasodilator activities have been firmly proven, although (as with all investigations) many questions remain to be addressed. At present, we possess some data on their catabolism, but we do not know the enzymes that take part in their
Acknowledgements
This work was supported by ETT 590/96. I am grateful to Drs. Ulrike Holzer-Petsche and György Benedek for their critical reading of the manuscript and helpful advice. I am indebted to Ildikó Dobos for technical contributions in the manuscript.
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