Opiate Analgesics as Negative Modulators of Adult Hippocampal Neurogenesis: Potential Implications in Clinical Practice

Potential role of hippocampal neurogenesis in spinal cord injury induced post-trauma depression

It has been reported both in clinic and rodent models that beyond spinal cord injury directly induced symptoms, such as paralysis, neuropathic pain, bladder/bowel dysfunction, and loss of sexual function, there are a variety of secondary complications, including memory loss, cognitive decline, depression, and Alzheimer's disease. The large-scale longitudinal population-based studies indicate that post-trauma depression is highly prevalent in spinal cord injury patients. Yet, few basic studies have been conducted to address the potential molecular mechanisms. One of possible factors underlying the depression is the reduction of adult hippocampal neurogenesis which may come from less physical activity, social isolation, chronic pain, and elevated neuroinflammation after spinal cord injury. However, there is no clear consensus yet. In this review, we will first summarize the alteration of hippocampal neurogenesis post-spinal cord injury. Then, we will discuss possible mechanisms underlie this important spinal cord injury consequence. Finally, we will outline the potential therapeutic options aimed at enhancing hippocampal neurogenesis to ameliorate depression.

Addictive drugs modify neurogenesis, synaptogenesis and synaptic plasticity to impair memory formation through neurotransmitter imbalances and signaling dysfunction

Drug abuse changes neurophysiological functions at multiple cellular and molecular levels in the addicted brain. Well-supported scientific evidence suggests that drugs negatively affect memory formation, decision-making and inhibition, and emotional and cognitive behaviors. The mesocorticolimbic brain regions are involved in reward-related learning and habitual drug-seeking/taking behaviors to develop physiological and psychological dependence on the drugs. This review highlights the importance of specific drug-induced chemical imbalances resulting in memory impairment through various neurotransmitter receptor-mediated signaling pathways. The mesocorticolimbic modifications in the expression levels of brain-derived neurotrophic factor (BDNF) and the cAMP-response element binding protein (CREB) impair reward-related memory formation following drug abuse. The contributions of protein kinases and microRNAs (miRNAs), along with the transcriptional and epigenetic regulation have also been considered in memory impairment underlying drug addiction. Overall, we integrate the research on various types of drug-induced memory impairment in distinguished brain regions and provide a comprehensive review with clinical implications addressing the upcoming studies.

Tramadol and Tapentadol Induce Conditioned Place Preference with a Differential Impact on Rewarding Memory and Incubation of Craving

Tramadol and tapentadol, synthetic opioids commonly prescribed for moderate-to-severe pain, have a unique pharmacology that optimizes their analgesia and safety. However, they are not devoid of risks, presenting addictive, abuse, and dependence potential. While tramadol-reinforcing properties have been documented by various studies with human and animal models, including conditioned place preference (CPP) assays, no similar studies have been performed with tapentadol. In the present study, we performed CPP assays by intraperitoneally administering Wistar rats with a tramadol/tapentadol therapeutic dose. Animal permanence and the number of entries in the CPP compartments were recorded in the preconditioning phase and then 1 (T1), 7 (T7), and 14 (T14) days after conditioning. Both opioids induced a change in place preference (T1), suggesting that they have short-term reinforcing properties. However, only tramadol was associated with place preference retention (T7 and T14), with an increase in the number of entries in the opioid-paired compartment (T1 and T7), showing that it causes rewarding memory and incubation of craving. The results indicate that at therapeutic doses: (1) both drugs cause short-term rewarding effects and (2) as opposed to tramadol, tapentadol does not cause CPP retention, despite its higher central nervous system activity and stricter scheduling.

Evaluation of the Negative Effects of Opium Tincture on Memory and Hippocampal Neurons in the Presence of Chicory Extract

Background

Due to the high prevalence of addiction in society and the need to its attention, various methods are used for addiction withdrawal. The side effects of some methods restrict their use and increase the risk of recurrence. One of the Iranian useful methods is consumption of opium tincture (OT) that may cause brain structure and memory defects. Hence, this study aimed the effects of different doses of OT on memory and hippocampal neurons with the use of an antioxidant such as various concentrations chicory.

Materials and Methods

In the present study, 70 Wistar rats were randomly divided into 10 groups and the effect of various doses of chicory extract and OT were assessed on memory by the passive avoidance test. The neurons and astrocyte cells numbers in dentate gyrus were investigated, using histological examination.

Results

In passive avoidance test, the total time in dark compartment was significantly more in groups with 100 and 75 μl OT compared with control and normal saline groups (P < 0.001). Traffic number results showed that there was a significant difference between T100 and control groups (P > 0.05). Moreover, initial latency time was significantly shorter in groups with 75 and 100 μl of OT compared with control and normal saline groups (P < 0.05). However, the presence 250 mg/kg of chicory increases granular layer thickness of dentate gyrus and number of neurons.

Conclusion

The use of 250 mg/kg of chicory extract may be promising strategy for inducing neurogenesis and this dose could prevent neural damage.

Tapentadol: A Review of Experimental Pharmacology Studies, Clinical Trials, and Recent Findings

Tapentadol is an analgesic compound that acts centrally to attenuate pain. Previous studies have shown that tapentadol has dual mechanisms of action as a mu-opioid receptor agonist and noradrenaline re-uptake inhibition. Therefore, tapentadol provides a great advantage over classic opioids in pain management from nociceptive to neuropathic. Cumulative evidence from in vitro data suggests that tapentadol effect of norepinephrine re-uptake could be a new target that overcomes other classic opioids in chronic neuropathic pain. Compared to tramadol and other opioids, tapentadol is associated with fewer adverse effects than tramadol. Tapentadol is a new alternative to treat acute, chronic, and neuropathic pain. Thus, this review article was focused on understanding the studies that led to the development of tapentadol as a novel analgesic drug and its advantages over conventional opioids. Thus, tapentadol is a good alternative with fewer adverse effects and is available for human use.

Early life nociceptive stimulus and fentanyl exposure increase hippocampal neurogenesis and anxiety but do not affect spatial learning and memory

This study aimed to determine whether preemptive fentanyl administration in neonatal rats reduces the impact of a nociceptive stimulus initiated during the first day of life (P1) on hippocampal neurogenesis, behavior, and learning. At P1, Wistar rat pups received either a subcutaneous injection of fentanyl (F) before intraplantar injection of complete Freund’s adjuvant (CFA) (CFA + F group), an isolated injection of CFA (CFA group), or subcutaneous injection of fentanyl without CFA injection (F). Control animals received saline injections using the same route and volume as the treatment groups. Hippocampal neurogenesis was evaluated by 5′ –bromo-2′-deoxyuridine (BrdU) staining on P10 and P39 to assess neuronal proliferation and survival, respectively. Anxiety behavior in adulthood was assessed using an open field test (OF) and an elevated plus maze test (EPM). Spatial memory was assessed on a Morris water maze test (MWM), where the animals were trained for seven days, beginning on P81, and the probe trial was performed to evaluate memory retention. Although the CFA + F group showed an increased number of proliferative cells on P10, this finding did not persist on P39. The CFA + F group spent more time in the closed arms in the EPM, revealing more anxious behavior, although the early noxious experience, both with and without fentanyl, did not alter neurogenesis in adolescence and learning in adulthood. This study highlights that the impact of pain in early life pain combined with fentanyl on hippocampal neurogenesis on P10 did not persist on P39. In addition, this combined intervention during the first week of life was associated with higher anxiety levels.

Effect of tramadol on apoptosis and synaptogenesis in hippocampal neurons: The possible role of µ-opioid receptor

Tramadol is a synthetic opioid with centrally acting analgesic activity that alleviates moderate to severe pain and treats withdrawal symptoms of the other opioids. Like other opioid drugs, tramadol abuse has adverse effects on central nervous system components. Chronic administration of tramadol induces maladaptive plasticity in brain structures responsible for cognitive function, such as the hippocampus. However, the mechanisms by which tramadol induces these alternations are not entirely understood. Here, we examine the effect of tramadol on apoptosis and synaptogenesis of hippocampal neuronal in vitro. First, the primary culture of hippocampal neurons from neonatal rats was established, and the purity of the neuronal cells was verified by immunofluorescent staining. To evaluate the effect of tramadol on neuronal cell viability MTT assay was carried out. The western blot analysis technique was performed for the assessment of apoptosis and synaptogenesis markers. Results show that chronic exposure to tramadol reduces cell viability of neuronal cells and naloxone reverses this effect. Also, the level of caspase-3 significantly increased in tramadol-exposed hippocampal neurons. Moreover, tramadol downregulates protein levels of synaptophysin and stathmin as synaptogenesis markers. Interestingly, the effects of tramadol were abrogated by naloxone treatment. These findings suggest that tramadol can induce neurotoxicity in hippocampal neuronal cells, and this effect was partly mediated through opioid receptors.

The Altered Migration and Distribution of Systemically Administered Mesenchymal Stem Cells in Morphine-Treated Recipients

Mesenchymal stem cells (MSCs) have the ability to migrate to the site of injury or inflammation, and to contribute to the healing process. Since patients treated with MSCs are often users of analgesic drugs, to relieve their uncomfortable pain associated with the tissue disorder, there is a possibility of negative effects of these drugs on the migration of endogenous and exogenous MSCs. Therefore, we tested the impact of acute and chronic treatment with morphine on the migration and organ distribution of exogenous adipose tissue-derived MSCs in mouse models. Firstly, we showed that the incubation of MSCs with morphine significantly reduced the expression of adhesive molecules CD44 (HCAM), CD54 (ICAM-1) and CD106 (VCAM-1) on MSCs. Using a model of systemic administration of MSCs labeled with vital dye PKH26 and by the application of flow cytometry to detect living CD45^-PKH26⁺ cells, we found a decreased number of labeled MSCs in the lung, spleen and bone marrow, and a significantly increased number of MSCs in the liver of morphine-treated recipients. A skin allograft model was used to study the effects of morphine on the migration of exogenous MSCs to the superficial wound. Intraperitoneally administered MSCs migrated preferentially to the wound site, and this migration was significantly decreased in the morphine-treated recipients. The present results showed that morphine significantly influences the distribution of exogenous MSCs in the body, and decreases their migration to the site of injury.

Repeated Administration of Clinically Relevant Doses of the Prescription Opioids Tramadol and Tapentadol Causes Lung, Cardiac, and Brain Toxicity in Wistar Rats

Tramadol and tapentadol, two structurally related synthetic opioid analgesics, are widely prescribed due to the enhanced therapeutic profiles resulting from the synergistic combination between μ-opioid receptor (MOR) activation and monoamine reuptake inhibition. However, the number of adverse reactions has been growing along with their increasing use and misuse. The potential toxicological mechanisms for these drugs are not completely understood, especially for tapentadol, owing to its shorter market history. Therefore, in the present study, we aimed to comparatively assess the putative lung, cardiac, and brain cortex toxicological damage elicited by the repeated exposure to therapeutic doses of both prescription opioids. To this purpose, male Wistar rats were intraperitoneally injected with single daily doses of 10, 25, and 50 mg/kg tramadol or tapentadol, corresponding to a standard analgesic dose, an intermediate dose, and the maximum recommended daily dose, respectively, for 14 consecutive days. Such treatment was found to lead mainly to lipid peroxidation and inflammation in lung and brain cortex tissues, as shown through augmented thiobarbituric acid reactive substances (TBARS), as well as to increased serum inflammation biomarkers, such as C reactive protein (CRP) and tumor necrosis factor-α (TNF-α). Cardiomyocyte integrity was also shown to be affected, since both opioids incremented serum lactate dehydrogenase (LDH) and α-hydroxybutyrate dehydrogenase (α-HBDH) activities, while tapentadol was associated with increased serum creatine kinase muscle brain (CK-MB) isoform activity. In turn, the analysis of metabolic parameters in brain cortex tissue revealed increased lactate concentration upon exposure to both drugs, as well as augmented LDH and creatine kinase (CK) activities following tapentadol treatment. In addition, pneumo- and cardiotoxicity biomarkers were quantified at the gene level, while neurotoxicity biomarkers were quantified both at the gene and protein levels; changes in their expression correlate with the oxidative stress, inflammatory, metabolic, and histopathological changes that were detected. Hematoxylin and eosin (H & E) staining revealed several histopathological alterations, including alveolar collapse and destruction in lung sections, inflammatory infiltrates, altered cardiomyocytes and loss of striation in heart sections, degenerated neurons, and accumulation of glial and microglial cells in brain cortex sections. In turn, Masson's trichrome staining confirmed fibrous tissue deposition in cardiac tissue. Taken as a whole, these results show that the repeated administration of both prescription opioids extends the dose range for which toxicological injury is observed to lower therapeutic doses. They also reinforce previous assumptions that tramadol and tapentadol are not devoid of toxicological risk even at clinical doses.

Indices of dentate gyrus neurogenesis are unaffected immediately after or following withdrawal from morphine self-administration compared to saline self-administering control male rats

Opiates - including morphine - are powerful analgesics with high abuse potential. In rodents, chronic opiate exposure or self-administration negatively impacts hippocampal-dependent function, an effect perhaps due in part to the well-documented opiate-induced inhibition of dentate gyrus (DG) precursor proliferation and neurogenesis. Recently, however, intravenous (i.v.) morphine self-administration (MSA) was reported to enhance the survival of new rat DG neurons. To reconcile these disparate results, we used rat i.v. MSA to assess 1) whether a slightly-higher dose MSA paradigm also increases new DG neuron survival; 2) how MSA influences cells in different stages of DG neurogenesis, particularly maturation and survival; and 3) if MSA-induced changes in DG neurogenesis persist through a period of abstinence. To label basal levels of proliferation, rats received the S-phase marker bromodeoxyuridine (BrdU, i.p.) 24 -h prior to 21 days (D) of i.v. MSA or saline self-administration (SSA). Either immediately after SA (0-D) or after 4 weeks in the home cage (28-D withdrawal), stereology was used to quantify DG proliferating precursors (or cells in cell cycle; Ki67+ cells), neuroblast/immature neurons (DCX+ cells), and surviving DG granule cells (BrdU+ cells). Analysis revealed the number of DG cells immunopositive for these neurogenesis-relevant markers was similar between MSA and SSA rats at the 0-D or 28-D timepoints. These negative data highlight the impact experimental parameters, timepoint selection, and quantification approach have on neurogenesis results, and are discussed in the context of the large literature showing the negative impact of opiates on DG neurogenesis.

Non-nociceptive roles of opioids in the CNS: opioids' effects on neurogenesis, learning, memory and affect

Mortality due to opioid use has grown to the point where, for the first time in history, opioid-related deaths exceed those caused by car accidents in many states in the United States. Changes in the prescribing of opioids for pain and the illicit use of fentanyl (and derivatives) have contributed to the current epidemic. Less known is the impact of opioids on hippocampal neurogenesis, the functional manipulation of which may improve the deleterious effects of opioid use. We provide new insights into how the dysregulation of neurogenesis by opioids can modify learning and affect, mood and emotions, processes that have been well accepted to motivate addictive behaviours.

Salmeterol, a β2 Adrenergic Agonist, Promotes Adult Hippocampal Neurogenesis in a Region-Specific Manner

Neurogenesis persists in the subgranular zone of the hippocampal formation in the adult mammalian brain. In this area, neural progenitor cells (NPCs) receive both permissive and instructive signals, including neurotransmitters, that allow them to generate adult-born neurons which can be functionally integrated in the preexisting circuit. Deregulation of adult hippocampal neurogenesis (ahNG) occurs in several neuropsychiatric and neurodegenerative diseases, including major depression, and represents a potential therapeutic target. Of interest, several studies suggested that, both in rodents and in humans, ahNG is increased by chronic administration of classical monoaminergic antidepressant drugs, suggesting that modulation of this process may participate to their therapeutic effects. Since the established observation that noradrenergic innervations from locus coeruleus make contact with NPC in the dentate gyrus, we investigated the role of beta adrenergic receptor (β-AR) on ahNG both in vitro and in vivo. Here we report that, in vitro, activation of β₂-AR by norepinephrine and β₂-AR agonists promotes the formation of NPC-derived mature neurons, without affecting NPC survival or differentiation toward glial lineages. Additionally, we show that a selective β₂-AR agonist able to cross the blood-brain barrier, salmeterol, positively modulates hippocampal neuroplasticity when chronically administered in adult naïve mice. Indeed, salmeterol significantly increased number, maturation, and dendritic complexity of DCX⁺ neuroblasts. The increased number of DCX⁺ cells was not accompanied by a parallel increase in the percentage of BrdU⁺/DCX⁺ cells suggesting a potential prosurvival effect of the drug on neuroblasts. More importantly, compared to vehicle, salmeterol promoted ahNG, as demonstrated by an increase in the actual number of BrdU⁺/NeuN⁺ cells and in the percentage of BrdU⁺/NeuN⁺ cells over the total number of newly generated cells. Interestingly, salmeterol proneurogenic effects were restricted to the ventral hippocampus, an area related to emotional behavior and mood regulation. Since salmeterol is commonly used for asthma therapy in the clinical setting, its novel pharmacological property deserves to be further exploited with a particular focus on drug potential to counteract stress-induced deregulation of ahNG and depressive-like behavior.

Tapentadol for neuropathic pain: a review of clinical studies

Neuropathic pain (NP) is an enormous burden for patients, caregivers and society. NP is a pain state that may develop after injury of the peripheral or central nervous system because of a wide range of diseases and traumas. A NP symptom component can be found also in several types of chronic pain. Many NP patients are substantially disabled for years. Due to its chronicity, severity and unpredictability, NP is difficult to treat. Tapentadol is a central-acting oral analgesic with combined opioid and noradrenergic properties, which make it potentially suitable for a wide range of pain conditions, particularly whenever a NP component is present or cannot be excluded. In randomized controlled trials, tapentadol has proved to be effective in relieving NP in diabetic peripheral neuropathy and in chronic low back pain. In observational studies, tapentadol reduced NP in chemotherapy-induced peripheral neuropathies, blood and solid cancers, and the NP component in neck pain and Parkinson's disease. This narrative review aims to provide clinicians with a broad overview of tapentadol effects on NP.

The Impact of Morphine on the Characteristics and Function Properties of Human Mesenchymal Stem Cells

Morphine is an analgesic drug therapeutically administered to relieve pain. However, this drug has numerous side effects, which include impaired healing and regeneration after injuries or tissue damages. It suggests negative effects of morphine on stem cells which are responsible for tissue regeneration. Therefore, we studied the impact of morphine on the properties and functional characteristics of human bone marrow-derived mesenchymal stem cells (MSCs). The presence of μ-, δ- and κ-opioid receptors (OR) in untreated MSCs, and the enhanced expression of OR in MSCs pretreated with proinflammatory cytokines, was demonstrated using immunoblotting and by flow cytometry. Morphine modified in a dose-dependent manner the MSC phenotype, inhibited MSC proliferation and altered the ability of MSCs to differentiate into adipocytes or osteoblasts. Furthermore, morphine rather enhanced the expression of genes for various immunoregulatory molecules in activated MSCs, but significantly inhibited the production of the vascular endothelial growth factor, hepatocyte growth factor or leukemia inhibitory factor. All of these observations are underlying the selective impact of morphine on stem cells, and offer an explanation for the mechanisms of the negative effects of opioid drugs on stem cells and regenerative processes after morphine administration or in opioid addicts.

Effects of tapentadol on pain, motor symptoms and cognitive functions in Parkinson's disease

Background

Pain is a common and undertreated non-motor symptom in patients with Parkinson's disease (PD). Opioids have been seldom used in PD because they could worsen cognitive and motor functions.

Objective

We aimed to assess efficacy and tolerability of tapentadol in PD patients.

Methods

We retrospectively reviewed 21 PD patients treated with tapentadol extended release (ER) for chronic pain. Patients were evaluated before treatment and at 3 and 6 months during treatment for pain intensity (current, 24-hour average, and minimum and worst) with a 0-10 Numerical Rating Scale and the painDETECT questionnaire; for motor symptom severity with the Unified PD Rating Scale part III and the Hoehn and Yahr scale; for cognitive functions with Mini-Mental Status Examination, Corsi's Block-Tapping test, Digit Span test, Digit-Symbol Substitution test, FAS test, Rey's Auditory Verbal Learning test, Trail-Making test A and B and the 9-Hole Peg test; for anxiety and depression with the Hospital Anxiety and Depression Scale; and for the quality of life with the Short Form-12. Data were analyzed by 1-way analysis of variance and paired t-test, and by Friedman's and Wilcoxon's tests. Statistical significance was taken in all cases as P<0.05.

Results

Pain intensity decreased over the course of treatment. No differences were found in PD symptom severity and dopaminergic drug dosages between pretreatment and treatment evaluations. No decrement in cognitive neuropsychological performances was found and an improvement was observed in Digit Span test, Digit-Symbol Substitution test, and FAS test. The levels of anxiety, depression, and quality of life improved. Overall, tapentadol ER was well tolerated and most patients reported no or mild and short-lived gastroenterological and neurological side effects.

Conclusion

These results indicate the potential efficacy and tolerability of medium-high doses of tapentadol ER for the treatment of pain in PD.

Morphine regulates adult neurogenesis and contextual memory extinction via the PKCε/Prox1 pathway

We have previously reported that the miR-181a/Prox1/Notch1 pathway mediates the effect of morphine on modulating lineage-specific differentiation of adult neural stem/progenitor cells (NSPCs) via a PKCε-dependent pathway, whereas fentanyl shows no such effect. However, the role of the PKCε/Prox1 pathway in mediating drug-associated contextual memory remains unknown. The current study investigated the effect of PKCε/Prox1 on morphine-induced inhibition of adult neurogenesis and drug-associated contextual memory in mice, while the effect of fentanyl was tested simultaneously. By using BrdU labeling, we were able to examine the lineages of differentiated NSPCs in adult DG. PKCε knockout blocked morphine's effects on inducing in vivo astrocyte-preferential differentiation of NSPCs, but did not alter NSPC lineages upon fentanyl treatment. Inhibited adult neurogenesis further resulted in prolonged extinction and enhanced reinstatement of morphine-induced CPP, as well as prolonged extinction of space reference memory indicated by the Morris water maze paradigm. However, after fentanyl administration, no significant changes were found between wild-type and PKCε knockout mice, during either CPP or water maze tasks. When the lentivirus encoding Nestin-promoter-controlled Prox1 cDNA was injected into hippocampi of wildtype and PKCε knockout adult mice to modulate PKCε/Prox1 activity, similar effects were discovered in adult mice injected with lentivirus encoding Prox1, and more dramatic effects were found in PKCε knockout mice with concurrent Prox1 overexpression. In conclusion, morphine mediates lineage-specific NSPC differentiation, inhibits adult neurogenesis and regulates contextual memory retention via the PKCε/Prox1 pathway, which are implicated in the eventual context-associated relapse.

Conditioned Reward of Opioids, but not Psychostimulants, is Impaired in GABA-A Receptor δ Subunit Knockout Mice

Extrasynaptic δ subunit-containing γ-aminobutyric acid type A receptors (δ-GABA_A Rs) are emerging as targets for a number of neuropsychopharmacological drugs, including the direct GABA site agonist gaboxadol and neuroactive steroids. Among other regions, these δ-GABA_A Rs are functionally expressed in the ventral tegmental area (VTA), the cell body region of mesocorticolimbic dopamine (DA) system important for motivated behaviours, and in the target region, the nucleus accumbens. Gaboxadol and neurosteroids induce VTA DA neuron plasticity ex vivo, by inhibiting the VTA GABA neurons, and aversive place conditioning, which are absent in the δ-GABA_A R knockout mice (δ-KO). It is not known whether δ-GABA_A Rs are important for the effects of other drugs, such as opioids (that also inhibit GABA neurons) and stimulants (that primarily elevate monoamine levels). Here, we used δ-KO mice and conditioned place preference (CPP) test to study the rewarding effects of morphine (20 mg/kg), methamphetamine (1 mg/kg) and mephedrone (5 mg/kg). Morphine-induced nociception was also assessed using tail-flick and hot-plate tests. We found that the δ-KO mice failed to express morphine-induced CPP, but that they were more sensitive to morphine-induced analgesia in the tail-flick test. In contrast, stimulant-induced CPP in the δ-KO mice was similar to that in the wild-type controls. Thus, the conditioned rewarding effect by opioids, but not that of stimulants, was impaired in the absence of δ-GABA_A Rs. Further studies are warranted to assess the potential of δ-GABA_A R antagonists as possible targets for reducing morphine reward and potentiating morphine analgesia.

Comparative pharmacology and toxicology of tramadol and tapentadol

Moderate-to-severe pain represents a heavy burden in patients' quality of life, and ultimately in the society and in healthcare costs. The aim of this review was to summarize data on tramadol and tapentadol adverse effects, toxicity, potential advantages and limitations according to the context of clinical use. We compared data on the pharmacological and toxicological profiles of tramadol and tapentadol, after an extensive literature search in the US National Library of Medicine (PubMed). Tramadol is a prodrug that acts through noradrenaline and serotonin reuptake inhibition, with a weak opioid component added by its metabolite O-desmethyltramadol. Tapentadol does not require metabolic activation and acts mainly through noradrenaline reuptake inhibition and has a strong opioid activity. Such features confer tapentadol potential advantages, namely lower serotonergic, dependence and abuse potential, more linear pharmacokinetics, greater gastrointestinal tolerability and applicability in the treatment of chronic and neuropathic pain. Although more studies are needed to provide clear guidance on the opioid of choice, tapentadol shows some advantages, as it does not require CYP450 system activation and has minimal serotonergic effects. In addition, it leads to less side effects and lower abuse liability. However, in vivo and in vitro studies have shown that tramadol and tapentadol cause similar toxicological damage. In this context, it is important to underline that the choice of opioid should be individually balanced and a tailored decision, based on previous experience and on the patient's profile, type of pain and context of treatment.

SIGNIFICANCE

This review underlines the need for a careful prescription of tramadol and tapentadol. Although both are widely prescribed synthetic opioid analgesics, their toxic effects and potential dependence are not completely understood yet. In particular, concerning tapentadol, further research is needed to better assess its toxic effects.

l-Acetylcarnitine: A Mechanistically Distinctive and Potentially Rapid-Acting Antidepressant Drug

Current therapy of mood disorders has several limitations. Although a high number of drugs are clinically available, as of today, nearly two-thirds of individuals do not achieve full symptomatic remission after treatment with conventional antidepressants. Moreover, several weeks of drug treatment are usually required to obtain clinical effects, a limitation that has considerable clinical implications, ranging from high suicide risk to reduced compliance. The characteristic lag time in classical antidepressant effectiveness has given great impulse to the search for novel therapeutics with more rapid effects. l-acetylcarnitine (LAC), a small molecule of growing interest for its pharmacological properties, is currently marketed for treatment of neuropathic pain. Recent preclinical and clinical data suggested that LAC may exert antidepressant effects with a more rapid onset than conventional drugs. Herein, we review data supporting LAC antidepressant activity and its distinctive mechanisms of action compared with monoaminergic antidepressants. Furthermore, we discuss the unique pharmacological properties of LAC that allow us to look at this molecule as representative of next generation antidepressants with a safe profile.

Chronic pain and adult hippocampal neurogenesis: translational implications from preclinical studies

Adult hippocampal neurogenesis (ahNG) occurs in the human brain. Adult generated neurons have been proposed to functionally contribute to relevant hippocampal functions such as learning and memory, mood regulation, and stress response. Learning, environmental enrichment, and physical exercise exert positive effects on ahNG. In parallel, these proneurogenic stimuli have been shown to ameliorate cognitive performance and/or depressive-like behavior in animal models. Conversely, aging, social isolation, and chronic stress exert negative effects on ahNG. Interestingly, reduction of hippocampal neurogenesis is suggested to potentially contribute to cognitive decline and mood alterations associated with aging and several neuropsychiatric disorders. Clinical observation demonstrates that patients affected by chronic pain often exhibit increased anxiety and depression, impaired cognitive flexibility, and memory capacities. As of today, our understanding of the molecular and cellular events that may underlie the comorbidity of chronic pain, depression, and cognitive impairment is limited. Herein we review recent preclinical data suggesting that chronic pain may induce profound changes in hippocampal plasticity, including reduced ahNG. We discuss the possibility that deregulated hippocampal neurogenesis in chronic pain may, at least in part, contribute to cognitive and mood alterations. Based on this hypothesis, the mechanisms underlying chronic pain-associated changes in hippocampal neurogenesis and related functions need to be addressed experimentally. One interesting feature of ahNG is its susceptibility to pharmacological modulation. Again, based on preclinical data we discuss the possibility that, at least in principle, distinct analgesic drugs commonly used in chronic pain states (typical and atypical opiates, α2δ ligands, and acetyl-l-carnitine) may differentially impact ahNG and that this aspect could be taken into account to reduce and/or prevent the potential risk of cognitive and emotional side effects in the clinical setting.