Enkephalins and ACTH in the mammalian nervous system

Enkephalins and Pain Modulation: Mechanisms of Action and Therapeutic Perspectives

Enkephalins, a subclass of endogenous opioid peptides, play a pivotal role in pain modulation. Enkephalins primarily exert their effects through opioid receptors located widely throughout both the central and peripheral nervous systems. This review will explore the mechanisms by which enkephalins produce analgesia, emotional regulation, neuroprotection, and other physiological effects. Furthermore, this review will analyze the involvement of enkephalins in the modulation of different pathologies characterized by severe pain. Understanding the complex role of enkephalins in pain processing provides valuable insight into potential therapeutic strategies for managing pain disorders.

Disparate Effects of Stressors on Met-Enkephalin System Parameters and on Plasma Concentrations of Corticosterone in Young Female Chickens

The effects of stressors were examined on Met-enkephalin-related parameters and plasma concentrations of corticosterone in 14-week-old female chickens. Water deprivation for 24 h was accompanied by a tendency for increased plasma concentration of Met-enkephalin while plasma concentrations of corticosterone were elevated in water-deprived birds. Concentrations of Met-enkephalin were reduced in the anterior pituitary gland and adrenal gland in water-deprived pullets while proenkephalin (PENK) expression was increased in both tissues. There were changes in the plasma concentrations of Met-enkephalin and corticosterone in pullets subjected to either feed withholding or crowding. Concentrations of Met-enkephalin were increased in the anterior pituitary gland but decreased in adrenal glands in pullets subjected to crowding stress. The increase in the plasma concentrations of Met-enkephalin was ablated when the chickens were pretreated with naltrexone. However, naltrexone did not influence either basal or crowding on plasma concentrations of corticosterone. In vitro release of Met-enkephalin from the anterior pituitary or adrenal tissues was depressed in the presence of naltrexone. It was concluded that Met-enkephalin was part of the neuroendocrine response to stress in female chickens. It was concluded that stress influenced the release of both Met-enkephalin and corticosterone, but there was not complete parallelism.

Modification and Delivery of Enkephalins for Pain Modulation

Chronic pain negatively affects patient's quality of life and poses a significant economic burden. First line pharmaceutical treatment of chronic pain, including NSAIDs or antidepressants, is often inefficient to reduce pain, or produces intolerable adverse effects. In such cases, opioids are frequently prescribed for their potent analgesia, but chronic opioid use is also frequently associated with debilitating side effects that may offset analgesic benefits. Nonetheless, opioids continue to be widely utilized due to the lack of effective alternative analgesics. Since their discovery in 1975, a class of endogenous opioids called enkephalins (ENKs) have been investigated for their ability to relieve pain with significantly reduced adverse effects compared to conventional opioids. Their low metabolic stability and inability to cross biological membranes, however, make ENKs ineffective analgesics. Over past decades, much effort has been invested to overcome these limitations and develop ENK-based pain therapies. This review summarizes and describes chemical modifications and ENK delivery technologies utilizing ENK conjugates, nanoparticles and ENK gene delivery approaches and discusses valid lessons, challenges, and future directions of this evolving field.

Regulation of Cortico-Thalamic JNK1/2 and ERK1/2 MAPKs and Apoptosis-Related Signaling Pathways in PDYN Gene-Deficient Mice Following Acute and Chronic Mild Stress

The crosstalk between the opioidergic system and mitogen-activated protein kinases (MAPKs) has a critical role in mediating stress-induced behaviors related to the pathophysiology of anxiety. The present study evaluated the basal status and stress-induced alterations of cortico-thalamic MAPKs and other cell fate-related signaling pathways potentially underlying the anxiogenic endophenotype of PDYN gene-deficient mice. Compared to littermates, PDYN knockout (KO) mice had lower cortical and or thalamic amounts of the phospho-activated MAPKs c-Jun N-terminal kinase (JNK1/2) and extracellular signal-regulated kinase (ERK1/2). Similarly, PDYN-KO animals displayed reduced cortico-thalamic densities of total and phosphorylated (at Ser191) species of the cell fate regulator Fas-associated protein with death domain (FADD) without alterations in the Fas receptor. Exposure to acute restraint and chronic mild stress stimuli induced the robust stimulation of JNK1/2 and ERK1/2 MAPKs, FADD, and Akt-mTOR pathways, without apparent increases in apoptotic rates. Interestingly, PDYN deficiency prevented stress-induced JNK1/2 and FADD but not ERK1/2 or Akt-mTOR hyperactivations. These findings suggest that cortico-thalamic MAPK- and FADD-dependent neuroplasticity might be altered in PDYN-KO mice. In addition, the results also indicate that the PDYN gene (and hence dynorphin release) may be required to stimulate JNK1/2 and FADD (but not ERK1/2 or Akt/mTOR) pathways under environmental stress conditions.

Transcriptional Regulatory Role of NELL2 in Preproenkephalin Gene Expression

Preproenkephalin (PPE) is a precursor molecule for multiple endogenous opioid peptides Leu-enkephalin (ENK) and Met-ENK, which are involved in a wide variety of modulatory functions in the nervous system. Despite the functional importance of ENK in the brain, the effect of brain-derived factor(s) on PPE expression is unknown. We report the dual effect of neural epidermal growth factor (EGF)-likelike 2 (NELL2) on PPE gene expression. In cultured NIH3T3 cells, transfection of NELL2 expression vectors induced an inhibition of PPE transcription intracellularly, in parallel with downregulation of protein kinase C signaling pathways and extracellular signal-regulated kinase. Interestingly, these phenomena were reversed when synthetic NELL2 was administered extracellularly. The in vivo disruption of NELL2 synthesis resulted in an increase in PPE mRNA level in the rat brain, suggesting that the inhibitory action of intracellular NELL2 predominates the activation effect of extracellular NELL2 on PPE gene expression in the brain. Biochemical and molecular studies with mutant NELL2 structures further demonstrated the critical role of EGF-like repeat domains in NELL2 for regulation of PPE transcription. These are the first results to reveal the spatio-specific role of NELL2 in the homeostatic regulation of PPE gene expression.

Immunomodulatory Role of Neuropeptides in the Cornea

The transparency of the cornea along with its dense sensory innervation and resident leukocyte populations make it an ideal tissue to study interactions between the nervous and immune systems. The cornea is the most densely innervated tissue of the body and possesses both immune and vascular privilege, in part due to its unique repertoire of resident immune cells. Corneal nerves produce various neuropeptides that have a wide range of functions on immune cells. As research in this area expands, further insights are made into the role of neuropeptides and their immunomodulatory functions in the healthy and diseased cornea. Much remains to be known regarding the details of neuropeptide signaling and how it contributes to pathophysiology, which is likely due to complex interactions among neuropeptides, receptor isoform-specific signaling events, and the inflammatory microenvironment in disease. However, progress in this area has led to an increase in studies that have begun modulating neuropeptide activity for the treatment of corneal diseases with promising results, necessitating the need for a comprehensive review of the literature. This review focuses on the role of neuropeptides in maintaining the homeostasis of the ocular surface, alterations in disease settings, and the possible therapeutic potential of targeting these systems.

Endogenous Opioids and Their Role in Stem Cell Biology and Tissue Rescue

Opioids are considered the oldest drugs known by humans and have been used for sedation and pain relief for several centuries. Nowadays, endogenous opioid peptides are divided into four families: enkephalins, dynorphins, endorphins, and nociceptin/orphanin FQ. They exert their action through the opioid receptors (ORs), transmembrane proteins belonging to the super-family of G-protein-coupled receptors, and are expressed throughout the body; the receptors are the δ opioid receptor (DOR), μ opioid receptor (MOR), κ opioid receptor (KOR), and nociceptin/orphanin FQ receptor (NOP). Endogenous opioids are mainly studied in the central nervous system (CNS), but their role has been investigated in other organs, both in physiological and in pathological conditions. Here, we revise their role in stem cell (SC) biology, since these cells are a subject of great scientific interest due to their peculiar features and their involvement in cell-based therapies in regenerative medicine. In particular, we focus on endogenous opioids’ ability to modulate SC proliferation, stress response (to oxidative stress, starvation, or damage following ischemia–reperfusion), and differentiation towards different lineages, such as neurogenesis, vasculogenesis, and cardiogenesis.

Opioids and Ocular Surface Pathology; A Literature Review of New Treatments Horizons

This review discusses the role of opioids in the corneal surface and the different pathways and therapeutic methods of management. A literature review was performed using PubMed database. For the database search, the main searching words “opioid” and “topical opioid treatment” were used with the descriptors “cornea”, “ocular surface”, “neuropathic corneal pain”, “corneal sensitivity” and “naltrexone”; original scientific articles and reviews were included to achieve the purpose of the review. The endogenous opioid system has relevant functions in the organism, and in daily use, opioids are used as painkillers. However, these drugs may be employed for other indications as opioid pathways have a wide spectrum. The corneal surface for topical treatment is easily accessible, hence sparing the side effects of systemic opioids. Instillation of opioid antagonist substances, such as naltrexone, increases corneal healing rates and stimulates the division of corneal epithelium cells without deleterious effects. The natural modulation of endogenous opioids controls different forms of pain, including inflammatory and neuropathic pain, both in the ocular surface and in the central nervous system. There are diverse methods in controlling pain using opioids, especially in refractory forms. This review attempts to collect the literature about corneal surface and opioid pathways to provide an overview image and a possible direction of the news treatments.

Dual Enkephalinase Inhibitors and Their Role in Chronic Pain Management

PURPOSE OF REVIEW

Dual enkephalinase inhibitors (DENKIs) are pain medications that indirectly activate opioid receptors and can be used as an alternative to traditional opioids. Understanding the physiology of enkephalins and their inhibitors and the pharmacology of these drugs will allow for proper clinical application for chronic pain patients in the future.

RECENT FINDINGS

DENKIs can be used as an alternative mode of analgesia for patients suffering from chronic pain by preventing the degradation of endogenous opioid ligands. By inhibiting the two major enkephalin-degrading enzymes (neprilysin and aminopeptidase N), DENKIs can provide analgesia with less adverse effects than nonendogenous opioids. The purpose of this paper is to review the current literature investigating DENKIs and explore their contribution to chronic pain management.

A Rationale for Hypoxic and Chemical Conditioning in Huntington's Disease

Neurodegenerative diseases are characterized by adverse cellular environments and pathological alterations causing neurodegeneration in distinct brain regions. This development is triggered or facilitated by conditions such as hypoxia, ischemia or inflammation and is associated with disruptions of fundamental cellular functions, including metabolic and ion homeostasis. Targeting intracellular downstream consequences to specifically reverse these pathological changes proved difficult to translate to clinical settings. Here, we discuss the potential of more holistic approaches with the purpose to re-establish a healthy cellular environment and to promote cellular resilience. We review the involvement of important molecular pathways (e.g., the sphingosine, δ-opioid receptor or N-Methyl-D-aspartate (NMDA) receptor pathways) in neuroprotective hypoxic conditioning effects and how these pathways can be targeted for chemical conditioning. Despite the present scarcity of knowledge on the efficacy of such approaches in neurodegeneration, the specific characteristics of Huntington’s disease may make it particularly amenable for such conditioning techniques. Not only do classical features of neurodegenerative diseases like mitochondrial dysfunction, oxidative stress and inflammation support this assumption, but also specific Huntington’s disease characteristics: a relatively young age of neurodegeneration, molecular overlap of related pathologies with hypoxic adaptations and sensitivity to brain hypoxia. The aim of this review is to discuss several molecular pathways in relation to hypoxic adaptations that have potential as drug targets in neurodegenerative diseases. We will extract the relevance for Huntington’s disease from this knowledge base.

Effects of varying intensities of heat stress on neuropeptide Y and proopiomelanocortin mRNA expression in rats

The aim of the present study was to investigate the effects of varying intensities of heat stress on the mRNA expression levels of neuropeptide Y (NPY), proopiomelanocortin (POMC) and stress hormones in rats. To establish a rat model of heat stress, the temperature and time were adjusted in a specialized heating chamber. Sprague-Dawley (SD) rats were randomly divided into four groups; control (CN; temperature, 24±1˚C); moderate strength 6 h (MS6; temperature, 32±1˚C time, 6 h), moderate strength 24 h (MS24; temperature, 32±1˚C; time, 24 h) and high strength 6 h (HS6; temperature, 38±1˚C; time, 6 h) groups. SD rats were exposed to heat for 14 consecutive days. The levels of heat stress-related factors, including corticotropin-releasing hormone (CRH), cortisol (COR), epinephrine (EPI) and heat shock protein 70 (HSP70), were measured in the rat blood using ELISA. In addition, the weight of the spleen, thymus, hypophysis and hypothalamus were determined. The mRNA expressions levels of NPY and POMC were detected using quantitative PCR. The results showed that the CRH, COR and HSP70 levels were increased in the three heat stress groups compared with the CN group. Notably, the levels of CRH, EPI and HSP70 were increased in the HS6 group compared with the CN and MS6 groups (P<0.05). Furthermore, the weights of the hypophysis and hypothalamus in the HS6 group were significantly lower compared with the CN group (P<0.05). In addition, NPY and POMC expression levels were downregulated in the MS24 group compared with the CN group. The mRNA expression levels of NPY and POMC were altered in response to different intensities of heat stress. Therefore, their levels were downregulated and upregulated following long-time and moderate-time heat exposure, respectively. The results of the present study suggested that the reduced mRNA expression levels of NPY may be partially responsible for the heat-induced injuries in rats following long-time heat exposure.