Tramadol and another atypical opioid meperidine have exaggerated serotonin syndrome behavioural effects, but decreased analgesic effects, in genetically deficient serotonin transporter (SERT) mice

Serotonin Transporter (5-Hydroxytryptamine Transporter, SERT, SLC6A4) and Sodium-dependent Reuptake Inhibitors as Modulators of Pain Behaviors and Analgesic Responses

The serotonin transporter (5-hydroxytryptamine transporter [5-HTT], Serotonin Transporter (SERT), SLC6A4) modulates the activity of serotonin via sodium-dependent reuptake. Given the established importance of serotonin in the control of pain, 5-HTT has received much interest in studies of pain states and as a pharmacological target for serotonin reuptake inhibitors (SRIs). Animal models expressing varying levels of 5-HTT activity show marked differences in pain behaviors and analgesic responses, as well as many serotonin-related physiological effects. In humans, functional nucleotide variations in the SLC6A4 gene, which encodes the serotonin transporter 5-HTT, are associated with certain pathologic pain conditions and differences in responses to pharmacological therapy. These findings collectively reflect the importance of 5-HTT in the intricate physiology and management of pain, as well as the scientific and clinical challenges that need to be considered for the optimization of 5-HTT-related analgesic therapies. PERSPECTIVE: The serotonin transporter 5-HTT/SCL6A4 is sensitive to pharmacological SRIs. Experimental studies on the physiological functions of serotonin, as well as genetic mouse models and clinical phenotype/genotype correlations of nucleotide variation in the human 5-HTT/SCL6A4 gene, provide new insights for the use of SRIs in chronic pain management.

Cross state-dependent memory retrieval between tramadol and ethanol: involvement of dorsal hippocampal GABAA receptors

RATIONALE

Tramadol and ethanol, as psychoactive agents, are often abused. Discovering the molecular pathways of drug-induced memory creation may contribute to preventing drug addiction and relapse.

OBJECTIVE

The tramadol- and ethanol-induced state-dependent memory (SDM) and cross-SDM retrieval between tramadol and ethanol were examined in this study. Moreover, because of the confirmed involvement of GABAA receptors and GABAergic neurotransmission in memory retrieval impairment, we assessed cross-SDM retrieval between tramadol and ethanol with a specific emphasis on the role of the GABAA receptors. The first hypothesis of this study was the presence of cross-SDM between tramadol and ethanol, and the second hypothesis was related to possible role of GABAA receptors in memory retrieval impairment within the dorsal hippocampus. The cannulae were inserted into the hippocampal CA1 area of NMRI mice, and a step-down inhibitory avoidance test was used to evaluate state dependence and memory recovery.

RESULTS

The post-training and/or pre-test administration of tramadol (2.5 and 5 mg/kg, i.p.) and/or ethanol (0.5 and 1 g/kg, i.p.) induced amnesia, which was restored after the administration of the drugs 24 h later during the pre-test period, proposing ethanol and tramadol SDM. The pre-test injection of ethanol (0.25 and 0.5 g/kg, i.p.) with tramadol at an ineffective dose (1.25 mg/kg) enhanced tramadol SDM. Moreover, tramadol injection (1.25 and 2.5 mg/kg) with ethanol at the ineffective dose (0.25 g/kg) promoted ethanol SDM. Furthermore, the pre-test intra-CA1 injection of bicuculline (0.0625, 0.125, and 0.25 μg/mouse), a GABAA receptor antagonist, 5 min before the injection of tramadol (5 mg/kg) or ethanol (1 g/kg) inhibited tramadol- and ethanol-induced SDM dose-dependently.

CONCLUSION

The findings strongly confirmed cross-SDM between tramadol and ethanol and the critical role of dorsal hippocampal GABAA receptors in the cross-SDM between tramadol and ethanol.

Pronounced Declines in Meperidine in the US: Is the End Imminent?

Background: Once a widely used analgesic in the United States (US), meperidine offered an alternative opioid to other opioids as a pain reliever and was widely assumed to be safer with acute pancreatitis. However, within the last two decades meperidine, has gone from a frequently used drug to being used only when patients exhibit atypical reactions to opioids (e.g., morphine and hydromorphone), to being taken off the World Health Organization List of Essential Medications and receiving strong recommendations for overall avoidance. The aim of this study was to identify changes in meperidine distribution in the US, and regional disparities as reported to the Drug Enforcement Administration’s Automation of Reports and Consolidated Orders System (DEA ARCOS) and Medicaid. Methods: Data related to meperidine distribution was obtained through ARCOS (2001−2021) and Medicaid public use files (2016−2021). Heat maps were used to visualize regional disparities in distribution by state. States outside a 95% confidence interval were statistically significant. Results: Meperidine distribution between 2001 and 2021 decreased by 97.4% (R = −0.97, p < 0.0001). There was a 34-fold state-level difference in meperidine distribution between Arkansas (16.8 mg/10 persons) and Connecticut (0.5 mg/10 persons) in 2020. Meperidine distribution in 2020 was elevated in Arkansas, Mississippi, and Alabama. In 2021, meperidine distribution was highest in Arkansas (16.7 mg/10 persons) and lowest in Connecticut (0.8 mg/10 persons). Total prescriptions of meperidine as reported by Medicaid decreased by 73.8% (R = −0.67, p = 0.045) between 2016 and 2021. Conclusion: We observed a decrease in the overall distribution of meperidine in the past two decades, with a similar recent decline in prescribing it to Medicaid enrollees. The shortage of some parenteral formulations is an important contributor to these declines, however, the most likely explanation for this global decline in use is related to an increased recognition of safety concerns related to important drug interactions and a neurotoxic metabolite. This data may reflect plans to phase out the use of this opioid, especially in the many situations where safer and more preferred opioids are available.

Declines and pronounced regional disparities in meperidine use in the United States

There have been increasing concerns about adverse effects and drug interactions with meperidine. The goal of this study was to characterize meperidine use in the United States. Meperidine distribution data were obtained from the Drug Enforcement Administration's Automated of Reports and Consolidated Orders System. The Medicare Part D Prescriber Public Use File was utilized to capture overall trends in national prescriptions in this observational report. Nationally, meperidine distribution decreased by 94.6% from 2001 to 2019. In 2019, Arkansas, Alabama, Oklahoma, and Mississippi saw significantly greater distribution when compared with the US state average of 9.27 mg per 10 persons (SD = 6.82). Meperidine distribution showed an 18-fold difference between the highest state (Arkansas = 36.8 mg) and lowest state (Minnesota = 2.1 mg). Five of the six states with the lowest distribution were in the Northeast. Meperidine distribution per state was correlated with the prevalence of adult obesity (r(48) = +0.48, p < .001). Family medicine and internal medicine physicians accounted for 28.9% and 20.5%, respectively, of meperidine total daily supply (TDS) in 2017. Interventional pain management (5.66) and pain management (3.48) physicians accounted for the longest TDS per provider. The use of meperidine declined over the last two decades. Meperidine varied by geographic region with south-central states, and those with more obesity, showing greater distribution. Primary care doctors continue to account for the majority of meperidine daily supply. Increasing knowledge of meperidine's undesirable adverse effects like seizures and serious drug-drug interactions is likely responsible for these pronounced reductions.

Low-Dose Tramadol as an Off-Label Antidepressant: A Data Mining Analysis from the Patients' Perspective

Objectives: The purpose of this analysis was to assess, from the patients' perspective, the effectiveness and relative safety of tramadol as an off-label antidepressant and to determine if patients' views and experiences are consistent with the biomedical literature. Method: A data mining approach was used to analyze databases available at drugs.com. Results: Tramadol was reported to be an effective or very effective antidepressant by 94.6% of patients (123/130) who provided ratings submitted to User Reviews for Tramadol to Treat Depression (https://www.drugs.com/comments/tramadol/for-depression.html). When compared to 34 other antidepressants in the database titled Drugs Used to treat Depression (https://www.drugs.com/condition/depression.html), for which there were ≥100 individual reviews for each drug, tramadol was rated as being the most effective (effectiveness rating = 9.1/10). Phenelzine (effectiveness rating = 8.7/10) was the only other antidepressant having ≥100 individual reviews coupled with a very high (8.0-10.0) effectiveness rating. Eleven patients reported significant symptoms of withdrawal upon cessation of tramadol, and five patients reported loss or reduction of libido as a side effect. Most (57/72, 79.2%) patients who reported a dose consumed experienced relief from depression at low therapeutic doses (25-150 mg/day). Fourteen patients reported taking this antidepressant for 5-10 years, and four patients reported taking tramadol for 10 or more years. Results demonstrated that most patients' comments and beliefs are consistent with the biomedical literature. Conclusions: Patients' reviews coupled with a survey of the biomedical literature indicate that at low therapeutic doses in the absence of interactions with other drugs, adult patients found tramadol to be a generally safe, effective, and fast-acting medication for relief from depression.

Behavioral effects of citalopram, tramadol, and binary mixture in zebrafish (Danio rerio) larvae

Pharmaceuticals are emerging as environmentally problematic compounds. As they are often not appropriately removed by sewage treatment plants, pharmaceutical compounds end up in surface water environments worldwide at concentrations in the ng to μg L^-1 range. There is a need to further explore single compound and mixture effects using e.g. in vivo test model systems. We have investigated, for the first time, behavioral effects in larval zebrafish (Danio rerio) exposed to a binary mixture of an antidepressant drug (citalopram) and a synthetic opioid (tramadol). Citalopram and tramadol have a similar mode of action (serotonin reuptake inhibition) and are known to produce drug-drug interactional effects resulting in serotonin syndrome (SS) in humans. Zebrafish embryo-larvae were exposed to citalopram, tramadol and 1:1 binary mixture from fertilization until 144 h post-fertilization. No effects on heart rate, spontaneous tail coiling, or death/malformations were observed in any treatment at tested concentrations. Behavior (hypoactivity in dark periods) was on the other hand affected, with lowest observed effect concentrations (LOECs) of 373 μg L^-1 for citalopram, 320 μg L^-1 for tramadol, and 473 μg L^-1 for the 1:1 mixture. Behavioral EC₅₀ was calculated to be 471 μg L^-1 for citalopram, 411 μg L^-1 for tramadol, and 713 μg L^-1 for the 1:1 mixture. The results of this study conclude that tramadol and citalopram produce hypoactivity in 144 hpf zebrafish larvae. Further, a 1:1 binary mixture of the two caused the same response, albeit at a higher concentration, possibly due to SS.

The anaesthetist, opioid analgesic drugs, and serotonin toxicity: a mechanistic and clinical review

Most cases of serotonin toxicity are provoked by therapeutic doses of a combination of two or more serotonergic drugs, defined as drugs affecting the serotonin neurotransmitter system. Common serotonergic drugs include many antidepressants, antipsychotics, and opioid analgesics, particularly fentanyl, tramadol, meperidine (pethidine), and methadone, but rarely morphine and other related phenanthrenes. Symptoms of serotonin toxicity are attributable to an effect on monoaminergic transmission caused by an increased synaptic concentration of serotonin. The serotonin transporter (SERT) maintains low serotonin concentrations and is important for the reuptake of the neurotransmitter into the presynaptic nerve terminals. Some opioids inhibit the reuptake of serotonin by inhibiting SERT, thus increasing the plasma and synaptic cleft serotonin concentrations that activate the serotonin receptors. Opioids that are good inhibitors of SERT (tramadol, dextromethorphan, methadone, and meperidine) are most frequently associated with serotonin toxicity. Tramadol also has a direct serotonin-releasing action. Fentanyl produces an efflux of serotonin, and binds to 5-hydroxytryptamine (5-HT)_1A and 5-HT_2A receptors, whilst methadone, meperidine, and more weakly tapentadol, bind to 5-HT_2A but not 5-HT_1A receptors. The perioperative period is a time where opioids and other serotonergic drugs are frequently administered in rapid succession, sometimes to patients with other serotonergic drugs in their system. This makes the perioperative period a relatively risky time for serotonin toxicity to occur. The intraoperative recognition of serotonin toxicity is challenging as it can mimic other serious syndromes, such as malignant hyperthermia, sepsis, thyroid storm, and neuroleptic malignant syndrome. Anaesthetists must maintain a heightened awareness of its possible occurrence and a readiness to engage in early treatment to avoid poor outcomes.

Opioid-induced inhibition of the human 5-HT and noradrenaline transporters in vitro: link to clinical reports of serotonin syndrome

Background and Purpose

Opioids may inhibit the 5‐HT transporter (SERT) and the noradrenaline transporter (NET). NET inhibition may contribute to analgesia, and SERT inhibition or interactions with 5‐HT receptors may cause serotonergic toxicity. However, the effects of different opioids on the human SERT, NET and 5‐HT receptors have not been sufficiently studied.

Experimental Approach

We determined the potencies of different opioids to inhibit the SERT and NET in vitro using human transporter‐transfected HEK293 cells. We also tested binding affinities at 5‐HT_1A, 5‐HT_2A and 5‐HT_2C receptors. Additionally, we assessed clinical cases of the serotonin syndrome associated with each opioid reported by PubMed and a World Health Organization database.

Key Results

Dextromethorphan, l(R)‐methadone, racemic methadone, pethidine, tramadol and tapentadol inhibited the SERT at or close to observed drug plasma or estimated brain concentrations in patients. Tapentadol was the most potent NET inhibitor. Pethidine, tramadol, l(R)‐methadone, racemic methadone, dextromethorphan and O‐desmethyltramadol also inhibited the NET. 6‐Monoacetylmorphine, buprenorphine, codeine, dihydrocodeine, heroin, hydrocodone, hydromorphone, morphine, oxycodone and oxymorphone did not inhibit the SERT or NET. Fentanyl interacted with 5‐HT_1A receptors and methadone, pethidine and fentanyl with 5‐HT_2A receptors, in the low micromolar range. Opioids most frequently associated with the serotonin syndrome are tramadol, fentanyl, tapentadol, oxycodone, methadone and dextromethorphan.

Conclusions and Implications

Some synthetic opioids interact with the SERT and NET at potentially clinically relevant concentrations. SERT inhibition by tramadol, tapentadol, methadone, dextromethorphan and pethidine may contribute to the serotonin syndrome. Direct effects on 5‐HT_1A and/or 5‐HT_2A receptors could be involved with methadone and pethidine.

Cortical excitability in tramadol dependent patients: A transcranial magnetic stimulation study

BACKGROUND

Addiction to tramadol, a widely used analgesic, is becoming increasingly common. Tramadol can also induce seizures even after a single clinical dose. We tested whether the epileptogenicity of tramadol was associated with any changes in cortical excitability and inhibitory transmission using transcranial magnetic stimulation (TMS).

METHODS

The study included 16 tramadol dependent patients and 15 age and sex matched healthy volunteers. Clinical evaluation was conducted using an addiction severity index. TMS assessment of excitability was conducted on the motor cortex since the response to each TMS pulse at that site is easily measured in terms of the amplitude of the twitches it evokes in contralateral muscles. Measures included resting and active motor threshold (RMT and AMT respectively), motor evoked potential (MEP) amplitude, cortical silent period (CSP) duration, transcallosal inhibition (TCI), and short interval intracortical inhibition and facilitation (SICI and ICF respectively). Urinary level of tramadol was measured immediately before assessing cortical excitability in each patient.

RESULTS

RMT and AMT were significantly lower, the duration of the CSP was shorter and SICI was reduced in patients compared with the control group. These findings are suggestive of increased neural excitability and reduced GABAergic inhibition following exposure to tramadol. Also there were negative correlations between the severity of tramadol dependence and a number of cortical excitability parameters (AMT, RMT, and CSP with P=0.002, 0.005, and 0.04 respectively).

CONCLUSIONS

The results provide evidence for hyperexcitability of the motor cortex coupled with inhibitory deficits in tramadol dependent patients.

Mechanisms of tramadol-related neurotoxicity in the rat: Does diazepam/tramadol combination play a worsening role in overdose?

Poisoning with opioid analgesics including tramadol represents a challenge. Tramadol may induce respiratory depression, seizures and serotonin syndrome, possibly worsened when in combination to benzodiazepines. Our objectives were to investigate tramadol-related neurotoxicity, consequences of diazepam/tramadol combination, and mechanisms of drug-drug interactions in rats. Median lethal-doses were determined using Dixon-Bruce's up-and-down method. Sedation, seizures, electroencephalography and plethysmography parameters were studied. Concentrations of tramadol and its metabolites were measured using liquid-chromatography-high-resolution-mass-spectrometry. Plasma, platelet and brain monoamines were measured using liquid-chromatography coupled to fluorimetry. Median lethal-doses of tramadol and diazepam/tramadol combination did not significantly differ, although time-to-death was longer with combination (P=0.04). Tramadol induced dose-dependent sedation (P<0.05), early-onset seizures (P<0.001) and increase in inspiratory (P<0.01) and expiratory times (P<0.05). The diazepam/tramadol combination abolished seizures but significantly enhanced sedation (P<0.01) and respiratory depression (P<0.05) by reducing tidal volume (P<0.05) in addition to tramadol-related increase in respiratory times, suggesting a pharmacodynamic mechanism of interaction. Plasma M1 and M5 metabolites were mildly increased, contributing additionally to tramadol-related respiratory depression. Tramadol-induced early-onset increase in brain concentrations of serotonin and norepinephrine was not significantly altered by the diazepam/tramadol combination. Interestingly neither pretreatment with cyproheptadine (a serotonin-receptor antagonist) nor a benserazide/5-hydroxytryptophane combination (enhancing brain serotonin) reduced tramadol-induced seizures. Our study shows that diazepam/tramadol combination does not worsen tramadol-induced fatality risk but alters its toxicity pattern with enhanced respiratory depression but abolished seizures. Drug-drug interaction is mainly pharmacodynamic but increased plasma M1 and M5 metabolites may also contribute to enhancing respiratory depression. Tramadol-induced seizures are independent of brain serotonin.

Effects of LSD on grooming behavior in serotonin transporter heterozygous (Sert⁺/⁻) mice

Serotonin (5-HT) plays a crucial role in the brain, modulating mood, cognition and reward. The serotonin transporter (SERT) is responsible for the reuptake of 5-HT from the synaptic cleft and regulates serotonin signaling in the brain. In humans, SERT genetic variance is linked to the pathogenesis of various psychiatric disorders, including anxiety, autism spectrum disorders (ASD) and obsessive-compulsive disorder (OCD). Rodent self-grooming is a complex, evolutionarily conserved patterned behavior relevant to stress, ASD and OCD. Genetic ablation of mouse Sert causes various behavioral deficits, including increased anxiety and grooming behavior. The hallucinogenic drug lysergic acid diethylamide (LSD) is a potent serotonergic agonist known to modulate human and animal behavior. Here, we examined heterozygous Sert(+/-) mouse behavior following acute administration of LSD (0.32 mg/kg). Overall, Sert(+/-) mice displayed a longer duration of self-grooming behavior regardless of LSD treatment. In contrast, LSD increased serotonin-sensitive behaviors, such as head twitching, tremors and backwards gait behaviors in both Sert(+/+) and Sert(+/-) mice. There were no significant interactions between LSD treatment and Sert gene dosage in any of the behavioral domains measured. These results suggest that Sert(+/-) mice may respond to the behavioral effects of LSD in a similar manner to wild-type mice.

Brain serotonin content regulates the manifestation of tramadol-induced seizures in rats: disparity between tramadol-induced seizure and serotonin syndrome

BACKGROUND

Tramadol-induced seizures might be pathologically associated with serotonin syndrome. Here, the authors investigated the relationship between serotonin and the seizure-inducing potential of tramadol.

METHODS

Two groups of rats received pretreatment to modulate brain levels of serotonin and one group was treated as a sham control (n = 6 per group). Serotonin modulation groups received either para-chlorophenylalanine or benserazide + 5-hydroxytryptophan. Serotonin, dopamine, and histamine levels in the posterior hypothalamus were then measured by microdialysis, while simultaneously infusing tramadol until seizure onset. In another experiment, seizure threshold with tramadol was investigated in rats intracerebroventricularly administered with either a serotonin receptor antagonist (methysergide) or saline (n = 6).

RESULTS

Pretreatment significantly affected seizure threshold and serotonin fluctuations. The threshold was lowered in para-chlorophenylalanine group and raised in benserazide + 5-hydroxytryptophan group (The mean ± SEM amount of tramadol needed to induce seizures; sham: 43.1 ± 4.2 mg/kg, para-chlorophenylalanine: 23.2 ± 2.8 mg/kg, benserazide + 5-hydroxytryptophan: 59.4 ± 16.5 mg/kg). Levels of serotonin at baseline, and their augmentation with tramadol infusion, were less in the para-chlorophenylalanine group and greater in the benserazide + 5-hydroxytryptophan group. Furthermore, seizure thresholds were negatively correlated with serotonin levels (correlation coefficient; 0.71, P < 0.01), while intracerebroventricular methysergide lowered the seizure threshold (P < 0.05 vs. saline).

CONCLUSIONS

The authors determined that serotonin-reduced rats were predisposed to tramadol-induced seizures, and that serotonin concentrations were negatively associated with seizure thresholds. Moreover, serotonin receptor antagonism precipitated seizure manifestation, indicating that tramadol-induced seizures are distinct from serotonin syndrome.

Serotonergic modulation of pain and analgesic responses: a study in rats with constitutionally altered serotonin transporters

BACKGROUND

A role of the serotonin (5HT) transporter, a key regulator of serotonergic transmission, in the physiology, pharmacology and genetics of pain responses has been proposed recently. The present study aimed to explore the impact of constitutive differences in the activity of the serotonin transporter, and 5HT homeostasis in general, on the modulation on pain sensitivity and analgesic responses to drugs that utilize 5HT mechanisms.

METHODS

A novel genetic animal model, Wistar-Zagreb 5HT rats, obtained by selective breeding of animals for extreme activity of the platelet serotonin transporter was used. As a consequence of breeding, two sublines of this model, termed high-5HT and low-5HT, differ in both central and peripheral serotonin homeostasis. Thermal pain sensitivity of 5HT sublines was assessed at baseline and following administration of analgesic drugs, as determined by paw withdrawal latency to radiant heat stimulation.

RESULTS

Animals from 5HT sublines show differences in both basal pain sensitivity and analgesic responses. Rats with the low-5HT phenotype displayed decreased baseline paw withdrawal latencies (hyperalgesia) in comparison to their high-5HT counterpart (25%; p < 0.001). They also showed better analgesic response to acute and prolonged treatment with tramadol (p = 0.027) and clomipramine (p = 0.019), respectively, whereas administration of fluvoxamine did not produce an analgesic effect in either 5HT subline.

CONCLUSIONS

These findings support the idea that functionality of the serotonin transporter is one of the physiological/genetic determinants of individual differences in pain responses and modulation. They also validate Wistar-Zagreb 5HT rats, with constitutionally up-regulated/down-regulated serotonin transporter, as a potential new genetic model for studying serotonergic modulation of pain responses.

Mutations in monoamine oxidase (MAO) genes in mice lead to hypersensitivity to serotonin-enhancing drugs: implications for drug side effects in humans

A possible side effect of serotonin-enhancing drugs is the serotonin syndrome, which can be lethal. Here we examined possible hypersensitivity to two such drugs, the serotonin precursor 5-hydroxy-L-tryptophan (5-HTP) and the atypical opioid tramadol, in mice lacking the genes for both monoamine oxidase A (MAOA) and MAOB. MAOA/B-knockout (KO) mice displayed baseline serotonin syndrome behaviors, and these behavioral responses were highly exaggerated following 5-HTP or tramadol versus baseline and wild-type (WT) littermates. Compared with MAOA/B-WT mice, baseline tissue serotonin levels were increased ∼2.6-3.9-fold in MAOA/B-KO mice. Following 5-HTP, serotonin levels were further increased ∼4.5-6.2-fold in MAOA/B-KO mice. These exaggerated responses are in line with the exaggerated responses following serotonin-enhancing drugs that we previously observed in mice lacking the serotonin transporter (SERT). These findings provide a second genetic mouse model suggestive of possible human vulnerability to the serotonin syndrome in individuals with lesser-expressing MAO or SERT polymorphisms that confer serotonergic system changes.

Animal models of the serotonin syndrome: a systematic review

The serotonin syndrome (SS) is a potentially life-threatening disorder in humans which is induced by ingestion of an overdose or by combination of two or more serotonin (5-HT)-enhancing drugs. In animals, acute administration of direct and indirect 5-HT agonists also leads to a set of behavioral and autonomic responses. In the current review, we provide an overview of the existing versions of the animal model of the SS. With a focus on studies in rats and mice, we analyze the frequency of behavioral and autonomic responses following administration of 5-HT-enhancing drugs and direct 5-HT agonists administered alone or in combination, and we briefly discuss the receptor mediation of these responses. Considering species differences, we identify a distinct set of behavioral and autonomic responses that are consistently observed following administration of direct and indirect 5-HT agonists. Finally, we discuss the importance of a standardized assessment of SS responses in rodents and the utility of animal models of the SS in translational studies, and provide suggestions for future research.

Alterations in grooming activity and syntax in heterozygous SERT and BDNF knockout mice: the utility of behavior-recognition tools to characterize mutant mouse phenotypes

Serotonin transporter (SERT) and brain-derived neurotrophic factor (BDNF) are key modulators of molecular signaling, cognition and behavior. Although SERT and BDNF mutant mouse phenotypes have been extensively characterized, little is known about their self-grooming behavior. Grooming represents an important behavioral domain sensitive to environmental stimuli and is increasingly used as a model for repetitive behavioral syndromes, such as autism and attention deficit/hyperactivity disorder. The present study used heterozygous ((+/-)) SERT and BDNF male mutant mice on a C57BL/6J background and assessed their spontaneous self-grooming behavior applying both manual and automated techniques. Overall, SERT(+/-) mice displayed a general increase in grooming behavior, as indicated by more grooming bouts and more transitions between specific grooming stages. SERT(+/-) mice also aborted more grooming bouts, but showed generally unaltered activity levels in the observation chamber. In contrast, BDNF(+/-) mice displayed a global reduction in grooming activity, with fewer bouts and transitions between specific grooming stages, altered grooming syntax, as well as hypolocomotion and increased turning behavior. Finally, grooming data collected by manual and automated methods (HomeCageScan) significantly correlated in our experiments, confirming the utility of automated high-throughput quantification of grooming behaviors in various genetic mouse models with increased or decreased grooming phenotypes. Taken together, these findings indicate that mouse self-grooming behavior is a reliable behavioral biomarker of genetic deficits in SERT and BDNF pathways, and can be reliably measured using automated behavior-recognition technology.

Monoaminergic Antidepressants in the Relief of Pain: Potential Therapeutic Utility of Triple Reuptake Inhibitors (TRIs)

Over 75% of depressed patients suffer from painful symptoms predicting a greater severity and a less favorable outcome of depression. Imaging, anatomical and functional studies have demonstrated the existence of common brain structures, neuronal pathways and neurotransmitters in depression and pain. In particular, the ascending serotonergic and noradrenergic pathways originating from the raphe nuclei and the locus coeruleus; respectively, send projections to the limbic system. Such pathways control many of the psychological functions that are disturbed in depression and in the perception of pain. On the other hand, the descending pathways, from monoaminergic nuclei to the spinal cord, are specifically implicated in the inhibition of nociception providing rationale for the use of serotonin (5-HT) and/or norepinephrine (NE) reuptake inhibitors (SSRIs, NRIs, SNRIs), in the relief of pain. Compelling evidence suggests that dopamine (DA) is also involved in the pathophysiology and treatment of depression. Indeed, recent insights have demonstrated a central role for DA in analgesia through an action at both the spinal and suprasinal levels including brain regions such as the periaqueductal grey (PAG), the thalamus, the basal ganglia and the limbic system. In this context, dopaminergic antidepressants (i.e., containing dopaminergic activity), such as bupropion, nomifensine and more recently triple reuptake inhibitors (TRIs), might represent new promising therapeutic tools in the treatment of painful symptoms with depression. Nevertheless, whether the addition of the dopaminergic component produces more robust effects than single- or dual-acting agents, has yet to be demonstrated. This article reviews the main pathways regulating pain transmission in relation with the monoaminergic systems. It then focuses on the current knowledge regarding the in vivo pharmacological properties and mechanism of action of monoaminergic antidepressants including SSRIs, NRIs, SNRIs and TRIs. Finally, a synthesis of the preclinical studies supporting the efficacy of these antidepressants in analgesia is also addressed in order to highlight the relative contribution of 5-HT, NE and DA to nociception.

Monoamine transporters: vulnerable and vital doorkeepers

Transporters of dopamine, serotonin, and norepinephrine have been empirically used as medication targets for several mental illnesses in the last decades. These protein-targeted medications are effective only for subpopulations of patients with transporter-related brain disorders. Since the cDNA clonings in early 1990s, molecular studies of these transporters have revealed a wealth of information about the transporters' structure-activity relationship (SAR), neuropharmacology, cell biology, biochemistry, pharmacogenetics, and the diseases related to the human genes encoding these transporters among related regulators. Such new information creates a unique opportunity to develop transporter-specific medications based on SAR, mRNA, DNA, and perhaps transporter trafficking regulation for a number of highly relevant diseases including substance abuse, depression, schizophrenia, and Parkinson's disease.

Functional interactions between 5-HT2A and presynaptic 5-HT1A receptor-based responses in mice genetically deficient in the serotonin 5-HT transporter (SERT)

BACKGROUND AND PURPOSE

Despite decreased presynaptic 5-HT(1A) and altered 5-HT(2A) receptor function in genetically-deficient serotonin (5-HT) transporter (SERT) mice, the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide maleate salt (WAY 100635) still induced head twitches in these mice, a well-established 5-HT(2A) receptor-mediated response.

EXPERIMENTAL APPROACH

Interactions between 5-HT(1A) and 5-HT(2A) receptors were assessed using the head-twitch response following 5-HT(1A) and 5-HT(2A) receptor agonists and antagonists in SERT wild-type (+/+), heterozygous (+/-), and knockout (-/-) mice. The role of brain 5-HT availability in WAY 100635 induced head twitches was also examined.

KEY RESULTS

WAY 100635 induced head twitches in a SERT gene-dose dependent manner, inducing 5-fold more head twitches in SERT -/- versus SERT +/+ mice. In SERT -/- mice, inhibition of 5-HT synthesis with p-chlorophenylalanine (PCPA) markedly depleted tissue 5-HT in all five brain areas examined and abolished WAY 100635 induced head twitches. Further, the selective 5-HT reuptake inhibitor fluvoxamine increased WAY 100635 induced head twitches in SERT +/+ and +/- mice. Head twitches following the 5-HT(2A) receptor agonist (+/-)-2,5-dimethoxy-4-iodophenyl-2-aminopropane (DOI) were robust in SERT +/+ and +/- mice but much reduced in SERT -/- mice. DOI-induced head twitches were decreased by the 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) in SERT +/+ and +/- mice. All drug-induced head twitches were blocked by the 5-HT(2A) receptor antagonist a-Phenyl-1-(2-phenylethyl)-4-piperidinemethanol (MDL 11,939).

CONCLUSIONS AND IMPLICATIONS

These data show that indirect activation of 5-HT(2A) receptors via blockade of presynaptic 5-HT(1A) receptors potentiated head-twitch responses, suggesting functional interactions between these receptors, interactions affected by altered 5-HT availability. Our findings strongly support the correlation of WAY 100635 induced head twitches with increased 5-HT availability, induced genetically or pharmacologically.