Endomorphin-1 (EM1) and endomorphin-2 (EM2) are two endogenous ligands that belong to the opioid peptide family and have the highest affinity and selectivity for the µ-opioid receptor (MOR). The neuroanatomical distribution, ultrastructural features and neural circuitry of EM-containing neuronal structures have been morphologically demonstrated. In addition, the modulation effects of the EMs in different areas reflect their potential endogenous roles in many major physiological processes, including their remarkable roles in the transmission and modulation of noxious information. The distinguished antinociceptive property of the EMs in acute and chronic pain, including neuropathic pain, cancer pain and inflammatory pain, has been revealed and investigated for therapeutic purposes. However, EMs exert adverse effects in the gastrointestinal, urinary, cardiovascular, and respiratory systems, which impede the development of EMs as new analgesics. Numerous studies have synthesized and investigated EM analogues and demonstrated that these EM derivatives had improved pharmacological properties, supporting their therapeutic perspectives. In the present review, the results of previous studies, particularly morphological and pharmacological studies, were summarized. Finally, EM modifications and their potential clinical implications were described. Applying this knowledge about EMs may provide information for further investigations in clinical application.
As the two endogenous ligands with the highest affinity and a remarkable selectivity for the µ-opioid receptor (MOR) of all known mammalian opioids in the opioid peptide family, endomorphin-1 (EM1) and endomorphin-2 (EM2) were first discovered and isolated from the bovine brain by Zadina et al. in 1997 [1-4]. EM1 (Tyr-Pro-Trp-Phe-NH2) differs from EM2 (Tyr-Pro-Phe-Phe-NH2) by one amino acid [1]. Since the endomorphins (EMs) were isolated in 1997, they have been shown to exist in the rodent, primate and human central nervous system (CNS) and peripheral nervous system (PNS) [5-7].
Previous studies strongly suggest that EM1 was mainly distributed throughout the brain and the upper brainstem, whereas EM2 was principally detected in the spinal cord and lower brainstem [3, 7-9]. Furthermore, EM2 shows immunoreactivities in the PNS and peripheral tissues, such as primary afferent fibres and the nervous cells from which they originated in the somatic and visceral sensory ganglia [10], gastrointestinal tract [11, 12] and immune system [13, 14]. EMs were prominently present in many, but not all, regions in which MORs have been reported to be concentrated [15, 16].
The neuroanatomical distribution of EMs and MORs provides a basis for the existence of the EMs-related neural circuitry [17-24]. The EMs were also reported to be co-localized with other neurotransmitters in different areas of the living body in animals [10-12, 25-27]. The intricate landscape of the EMs in the nervous system reflects their potential endogenous roles in many major physiological processes; of these processes, the EMs may play a role in the perception of pain and have analgesic effects due to their binding to the MOR, one of the dominant antinociceptive targets among the opioid receptors [28-30].
There are three classic opioid receptors, i.e., mu (µ)-opioid receptor (MOR), delta (δ)-opioid receptor (DOR) and kappa (κ)-opioid receptor (KOR), in the CNS and PNS. Each opioid receptor has natural specific ligands with a high affinity [31, 32]. Opiates act on these receptors and have been widely studied and used in clinical practice due to their potent analgesic effects. However, the traditional opiates induce serious adverse effects; for instance, morphine, which is an exogenous ligand of the MOR, easily induces tolerance, physical dependence and adverse effects in the gastrointestinal, urinary, cardiovascular, respiratory, and motor systems [33-35].
The discovery of the endogenous opioid EMs raises the possibility that EMs might be a good alternative to the exogenous opioid morphine. The results of in vivo and in vitro studies have revealed that EMs exerted a more potent analgesia in acute and neuropathic pain than other opiates, such as morphine [36-38]. Compared with other opiates, EMs had remarkably fewer side effects [39, 40]. However, EMs are associated with undesirable effects in certain animal experiments, such as tolerance and addiction, which restricted their development as therapeutic analgesics [41-43], and their low membrane permeability and susceptibility to enzymatic degradation also prohibit their direct clinical application [44, 45]. Therefore, many attempts have been made to remodel the EMs to identify possible solutions, indicating their potential value in clinical application [46-52].
In the present review, previous studies exploring the EMs are summarized with a focus on the following aspects: (1) the morphological features of EM-immunoreactive (-IR) structures and EMergic connections that are closely related to the transmission of noxious information and modulation of functions; (2) the involvement of EMs in the transmission and modulation of noxious information; (3) the neural mechanisms underlying the side effects of opioid substances as revealed by investigations of the EMs; and (4) examination and prospects of EMs and their modifications in clinical implications.
EMs in the CNS. EM1 and EM2, which were first discovered and isolated from bovine extracts, are two endogenous opioid peptides that not only exist in the bovine brain but are also distributed throughout the human, rodent and primate CNS and PNS as revealed by immunoassay and immunohistochemistry [1, 5-12].
Fig.1 illustrates areas of CNS distribution where both EM1 and EM2 have been identified. EM1- and EM2-containing neuronal cell bodies were identified primarily in both the hypothalamus and nucleus tractus solitarii (NTS) using light and electron microscopy or immunohistochemistry [7, 53, 54]. Both EM1- and EM2-IR terminals are abundant in areas related to the processing of nociceptive information, such as the bed nucleus of the stria terminals (BNST), periaqueductal grey (PAG), locus coeruleus (LC), parabrachial nucleus (PBN) and NTS [7-9, 53, 55].
However, there are also important differences in the neuroanatomical distribution of the two peptides. Compared with EM2, EM1 was more prevalent throughout the brain and the upper brainstem [7, 9]. In addition to those regions in which EM1 was as abundant as EM2, EM1-containing fibres were also rich in the amygdala, nucleus accumbens (Nac), cerebral cortex, diagonal band, thalamus and hypothalamus [3, 7, 9]. Compared to EM1, EM2 was mainly observed in the superficial laminae of the spinal cord and the spinal trigeminal nucleus, which was supported by immunohistochemistry findings and immuno-electron microscopic examinations [2, 4, 7, 55, 56], with a modest density in the substantia nigra, nucleus raphe magnus, ventral tegmental area (VTA), pontine nuclei, Nac and amygdala [7, 9, 53, 55]. EM2 was sparse in the cerebral cortex, whereas EM1 was abundant [7, 55].
In addition, EM-IR fibres were prominently present in many, but not all, regions in which MORs are reported to be concentrated. Notably, there were negligible EM1- and EM2-immunoreactivities in the striatum, which is a region known to express high levels of MORs [3, 7-9, 15, 53]. Thus, although the EMs are specific MOR ligands, they are likely not exclusive ligands of the MOR in the CNS.
Regarding the differences in the distribution of the EMs, two distinct EM precursors or two different processing pathways involving the same precursor have been proposed [57], but further studies are required. In addition, the morphological features of the EMs are related to their functions. Therefore, it is necessary to determine whether the different expression patterns of the EMs reflect their distinct functions, whether EM1 and EM2 function at overlapping sites and whether one peptide can compensate for the down-regulated expression of the other peptide.
EMs in the PNS and peripheral tissues. Since the 1970s, a substantial number of published reports have shed light on the EMs in the PNS and peripheral tissues [6, 10-13, 58]. The avidin-biotin-peroxidase method was performed in the brainstem, spinal cord and sensory ganglia from rats and identified a dense aggregation of EM2-IR fibres in the dorsal root, and EM2-IR neuronal perikarya were found in the dorsal root ganglion (DRG) and trigeminal ganglion (TG) [8]. Therefore, it was hypothesized that EM2 was synthesized in primary sensory neurons in the ganglia and then transported to the superficial dorsal horn. Subsequently, mechanical (deafferentation by dorsal rhizotomy) and chemical (exposure to the primary afferent neurotoxin capsaicin) methods of disrupting spinal primary sensory afferents were use
