Antagonist MDL Research Articles

The psychedelic drug DOI reduces heroin motivation by targeting 5-HT2A receptors in a heroin and alcohol co-use model

There has been a recent renewed interest in the potential use of psychedelic drugs as therapeutics for certain neuropsychiatric disorders, including substance use disorders. The psychedelic drug 2,5-dimethoxy-4-iodoamphetamine (DOI) has demonstrated therapeutic efficacy in preclinical models of opioid use disorder (OUD). Alcohol is commonly co-used in individuals with OUD, but preclinical models that recapitulate this comorbidity are lacking. We developed a polydrug model wherein male and female rats were allowed to self-administer intravenous heroin and oral alcohol (or saccharin control solution) over weeks of behavioral training, and then we conducted a series of progressive ratio tests to assess the animals' motivational state for heroin and alcohol. In this model, motivation for heroin is higher than alcohol, and DOI (0.4 mg/kg) administered prior to testing significantly reduced heroin motivation measured as the animals’ break point, or maximum effort the animal is willing to expend to obtain a single infusion of heroin. The 5-HT2A receptor antagonist MDL 100,907 (0.3 mg/kg), but not the 5-HT2C receptor antagonist SB-242084 (0.5 mg/kg), blocked the therapeutic effect of DOI on heroin motivation. No significant effects on alcohol break points were observed, nor did MDL 100,907 or SB-242084 have any effect on break points on their own. These data support the view that psychedelic drugs like DOI may have therapeutic effects on opioid use in individuals with OUD and comorbid alcohol use, by acting as a 5-HT2A receptor agonist.

5-HT2A receptors are involved in the pharmaco-toxicological effects of the synthetic cannabinoids JWH-018 and 5F-PB22: In vivo studies in mice

Over the last years, Synthetic Cannabinoids (SCs) have been among the largest and most frequently seized groups of Novel Psychoactive Substances (NPS). These substances have been frequently detected in biological samples from patients involved in several intoxication and death cases. Their serious adverse effects have been related to their action as potent agonist of cannabinoid CB1 receptors. However, evidence concerning the potential interaction between SCs and serotoninergic mechanisms has emerged. Therefore, this study aims to evaluate the involvement of 5-HT2A receptors in the effects induced by acute systemic administration of 1-pentyl-3-(1-naphthoyl)indole (JWH-018; 1 mg/kg) and quinolin-8-yl 1-pentyfluoro-1H-indole-3-8-carboxylate (5F-PB22; 1 mg/kg). Sensorimotor (visual, acoustic, and tactile) responses, pain threshold (acute mechanical and thermal nociception), core temperature, breath rate and motor performance (stepping activity) have been assessed in CD-1 male mice. The present results pointed out that both substances deeply alter sensorimotor responses, nociceptive threshold, core temperature, breath rate and motor activity in mice. Noteworthy, pretreatment with the selective 5-HT2A receptors antagonist MDL100907 (0.1 mg/kg) at least partially prevented sensorimotor disruption, antinociception and hypothermic effects. Conversely, the respiratory and motor impairment was not prevented. Thus, it states the relevance of serotoninergic 5-HT2A mechanisms on pharmaco-toxicological effects induced by SCs.

The G protein biased serotonin 5-HT2A receptor agonist lisuride exerts anti-depressant drug-like activities in mice.

There is now evidence from multiple Phase II clinical trials that psychedelic drugs can exert long-lasting anxiolytic, anti-depressant, and anti-drug abuse (nicotine and ethanol) effects in patients. Despite these benefits, the hallucinogenic actions of these drugs at the serotonin 2A receptor (5-HT2AR) limit their clinical use in diverse settings. Activation of the 5-HT2AR can stimulate both G protein and β-arrestin (βArr) -mediated signaling. Lisuride is a G protein biased agonist at the 5-HT2AR and, unlike the structurally-related lysergic acid diethylamide (LSD), the drug does not typically produce hallucinations in normal subjects at routine doses. Here, we examined behavioral responses to lisuride, in wild-type (WT), βArr1-knockout (KO), and βArr2-KO mice. In the open field, lisuride reduced locomotor and rearing activities, but produced a U-shaped function for stereotypies in both βArr lines of mice. Locomotion was decreased overall in βArr1-KOs and βArr2-KOs relative to wild-type controls. Incidences of head twitches and retrograde walking to lisuride were low in all genotypes. Grooming was decreased in βArr1 mice, but was increased then decreased in βArr2 animals with lisuride. Serotonin syndrome-associated responses were present at all lisuride doses in WTs, but they were reduced especially in βArr2-KO mice. Prepulse inhibition (PPI) was unaffected in βArr2 mice, whereas 0.5mg/kg lisuride disrupted PPI in βArr1 animals. The 5-HT2AR antagonist MDL100907 failed to restore PPI in βArr1 mice, whereas the dopamine D2/D3 antagonist raclopride normalized PPI in WTs but not in βArr1-KOs. Clozapine, SCH23390, and GR127935 restored PPI in both βArr1 genotypes. Using vesicular monoamine transporter 2 mice, lisuride reduced immobility times in tail suspension and promoted a preference for sucrose that lasted up to 2days. Together, it appears βArr1 and βArr2 play minor roles in lisuride's actions on many behaviors, while this drug exerts anti-depressant drug-like responses without hallucinogenic-like activities.

LSD-stimulated behaviors in mice require \u03b2-arrestin 2 but not \u03b2-arrestin 1

Recent evidence suggests that psychedelic drugs can exert beneficial effects on anxiety, depression, and ethanol and nicotine abuse in humans. However, their hallucinogenic side-effects often preclude their clinical use. Lysergic acid diethylamide (LSD) is a prototypical hallucinogen and its psychedelic actions are exerted through the 5-HT2A serotonin receptor (5-HT2AR). 5-HT2AR activation stimulates Gq- and β-arrestin- (βArr) mediated signaling. To separate these signaling modalities, we have used βArr1 and βArr2 mice. We find that LSD stimulates motor activities to similar extents in WT and βArr1-KO mice, without effects in βArr2-KOs. LSD robustly stimulates many surrogates of psychedelic drug actions including head twitches, grooming, retrograde walking, and nose-poking in WT and βArr1-KO animals. By contrast, in βArr2-KO mice head twitch responses are low with LSD and this psychedelic is without effects on other surrogates. The 5-HT2AR antagonist MDL100907 (MDL) blocks the LSD effects. LSD also disrupts prepulse inhibition (PPI) in WT and βArr1-KOs, but not in βArr2-KOs. MDL restores LSD-mediated disruption of PPI in WT mice; haloperidol is required for normalization of PPI in βArr1-KOs. Collectively, these results reveal that LSD’s psychedelic drug-like actions appear to require βArr2.

5-HT1A receptors mediate the analgesic effect of rosavin in a mouse model of oxaliplatin-induced peripheral neuropathic pain

Oxaliplatin, a third-generation platinum derivative, is the mainstay of current antineoplastic medications for advanced colorectal cancer therapy. However, peripheral neuropathic complications, especially cold allodynia, undermine the life-prolonging outcome of this anti-cancer agent. Rosavin, a phenylpropanoid derived originally from Rhodiola rosea, exhibits a wide range of therapeutic properties. The present study explored whether and how rosavin alleviates oxaliplatin-induced cold hypersensitivity in mice. In the acetone drop test, cold allodynia behavior was observed from days 3 to 5 after a single injection of oxaliplatin (6 mg/kg, i.p.). Cold allodynia was significantly attenuated following rosavin treatment (10 mg/kg, i.p.). Specific endogenous 5-HT depletion by three consecutive pretreatments with para-chlorophenylalanine (150 mg/kg/day, i.p.) abolished the analgesic action of rosavin; this effect was not observed following pretreatment with naloxone (opioid receptor antagonist, 10 mg/kg, i.p.). Furthermore, 5-HT1A receptor antagonist WAY-100635 (0.16 mg/kg, i.p.), but not 5-HT3 receptor antagonist MDL-72222 (1 mg/kg, i.p.), blocked rosavin-induced analgesia. These results suggest that rosavin may provide a novel approach to alleviate oxaliplatin-induced cold allodynia by recruiting the activity of 5-HT1A receptors.

The mast cell activator Compound 48/80 causes phasic urinary bladder smooth muscle contractions independent of histamine release

Inflammatory mediators released from mast cells cause both transient and phasic contractions of urinary bladder smooth muscle (UBSM). These contractions are viewed as initial disruptors of normal bladder function and can lead to profound bladder pathologies. However, the mechanisms responsible for each of these contractions has not been thoroughly investigated. Previously, we discovered that histamine-mediated contractions rapidly desensitize, whereas the mast cell activator compound 48/80 (10 µg/ml) caused both a long-lasting increase in the amplitude and frequency of phasic contractions in UBSM strips as well as a transient increase in baseline tension. Thus, we hypothesized that phasic contractions caused by compound 48/80 depended on the release of histamine from mast cells. Isometric contractility was performed with urothelium-intact UBSM strips from C57BL/6 mice in the presence or absence of the following drugs: the cholinergic antagonist atropine (2 µM); the 5HT2A receptor antagonist MDL 11939 (1 µM); the histamine H1 receptor antagonist fexofenadine (10 µM), the histamine H2 receptor antagonist roxatidine (25 µM), the H2 antagonist cimetidine (10 µM); or the mast cell stabilizer cromolyn (100 µM). None of the compounds tested significantly reduced the amplitude and duration of the spontaneous phasic contractions caused by compound 48/80. However, both MDL 11939 and cromolyn significantly reduced the baseline contraction. Our data suggest that responses to the mast cell activator compound 48/80 are a superimposition of 2 components: (1) a slowly desensitizing increase in baseline tension caused by release of 5-HT from mast cells, and (2) large-amplitude phasic contractions that are histamine- and 5HT-independent. However, the mechanism responsible for increases in the amplitude of phasic contractions remains unclear.

Lysergic acid diethylamide (LSD) promotes social behaviour through 5-HT2A and ampa in the medial prefrontal cortex (MPFC)

IntroductionAutism Spectrum Disorder and Social Anxiety Disorder are mental illnesses characterized by a dysfunction in social behavior (SB); a phenomenon largely mediated by the medial prefrontal cortex (mPFC). Clinical studies have demonstrated that lysergic acid diethylamide (LSD), a partial agonist of the 5-HT2A receptor, can promote SB. However, its mechanism of action on SB is unknown.ObjectivesTo assess the effects of repeated LSD administration on social behavior in mice and to identify which mPFC receptors mediate LSD’s behavioral effects.MethodsEight-week-old C57BL/6J male mice received vehicle or repeated LSD (30 μg/kg/day i.p. for 7 days) as well the selective 5-HT2A receptor antagonist MDL, or the AMPA receptor antagonist NBQX. Twenty-four hours following the last injection, mice underwent the Direct Social Interaction Test and the Three-Chamber Test (TCT) to assess sociability and preference for social novelty. in vivo electrophysiological recordings were performed in mice treated with vehicle or LSD using multi-barrelled electrodes for microiontophoretic ejections of the selective 5-HT2A receptor agonist DOI or the selective AMPA receptor agonist quisqualate on mPFC pyramidal neurons.ResultsRepeated treatment with low doses of LSD increased the interaction time in the DSI as well as sociability and social novelty indices in the TCT. These pro-social effects were blocked by the intra-PFC administration of both 5-HT2A and AMPA antagonists. LSD also potentiated, in a current-dependent manner, the excitatory response of mPFC neurons to 5-HT2Aand AMPA agonists.ConclusionsRepeated, low doses of LSD increases social behavior via a mechanism of action that is mediated by 5-HT2A and AMPA in the mPFC.

Role of prefrontal cortical 5-HT2A receptors and serotonin transporter in the behavioral deficits in post-pubertal rats following neonatal lesion of the ventral hippocampus

Neonatal ventral hippocampal-lesioned (NVHL) rats have been shown to display neurochemical and behavioral abnormalities at adulthood, analogous to some of those seen in schizophrenia. Serotonergic neurotransmission is implicated the pathophysiology and treatment of schizophrenia. In this study, we evaluated possible role of serotonergic transmission is the behaviors of NVHL-lesioned rats. Bilateral lesions to the ventral hippocampus (VH) in rat pups were made using the excitotoxin ibotenic acid. We investigated 5-HT2A-receptor and SERT binding sites in cortical and subcortical areas in post-pubertal NVHL and sham-lesioned rats, using quantitative receptor autoradiography. We compared a 5-HT-dependent behavior in NVHL and sham animals, the wet-dog shake response (WDSr) to a 5-HT2A receptor agonist DOI. In addition, we studied prepulse inhibition (PPI) of startle responses in NVHL and Sham-lesioned animals treated with antipsychotic drugs haloperidol, risperidone and clozapine and 5-HT2A antagonists ketanserin or MDL100907. The WDSr elicited by DOI was enhanced in post-pubertal NVHL rats compared to sham-lesioned controls. Moreover, post-pubertal NVHL rats exhibited PPI deficits which was reversed by atypical antipsychotics, ketanserin and MDL100907. A significant increase in 5-HT2A-like receptor binding was observed in the medial prefrontal cortex (mPFC) in post-pubertal NVHL rats without any significant change in the striatum and ventral pallidum. A significant increase in SERT-like binding was also observed in the mPFC and striatum of NVHL rats at pre-pubertal period; however, at post-pubertal age, the binding remained elevated in mPFC only. These data suggest that increased prefrontal cortical 5-HT transmission may play a role in the behavioral deficits observed in this neurodevelopmental model of schizophrenia.

5-HT2A receptors modulate dopamine D2-mediated maternal effects

Serotonin 5-HT2A receptors are expressed throughout the mesolimbic and mesocortical dopamine pathways, and manipulation of this receptor system has a profound impact on dopamine functions and dopamine-mediated behaviors. It is highly likely that 5-HT2A receptors may also modulate the D2-mediated maternal effects. The present study investigated this issue and also explored the possible behavioral mechanisms. We tested the effects of two D2 drugs (an agonist quinpirole: 0.5, 1.0 mg/kg, and a potent D2 antagonist haloperidol: 0.05, 0.10 mg/kg, sc) and their combinations with two 5-HT2A drugs (a selective 5-HT2A agonist TCB-2: 2.5 mg/kg, and 5-HT2A antagonist MDL100907, 1.0 mg/kg, sc) on maternal behavior in Sprague-Dawley postpartum females. Individually, TCB-2 (2.5 mg/kg, sc) and quinpirole (0.5 and 1.0 mg/kg, sc) reduced pup preference and disrupted home-cage maternal behavior. In contrast, haloperidol (0.10 mg/kg, sc) only disrupted home-cage maternal behavior, but did not suppress pup preference. MDL100907 (1.0 mg/kg, sc) by itself had no effect on either pup preference or maternal behavior. When administered in combination, pretreatment of TCB-2 did not alter quinpirole's disruption of pup preference and home-cage maternal behavior (possibly due to the floor effect), however, it did enhance haloperidol's disruption of pup retrieval in the home cage. MDL100907 had no effect both quinpirole's and haloperidol's disruption of pup preference and home-cage maternal behavior. Interestingly, haloperidol attenuated TCB-2's disruptive effect on pup preference. These findings suggest that activation of 5-HT2A receptors tends to enhance D2-mediated maternal disruption, whereas blockade of 5-HT2A receptors is less effective. They also suggest that 5-HT2A receptors may have a direct effect on maternal behavior independent of their interaction with D2 receptors. The possible behavioral and neural mechanisms by which 5-HT2A- and D2-mediated maternal effects and their interaction are discussed.

The putative lithium-mimetic ebselen reduces impulsivity in rodent models.

Deficits in impulse control feature in many psychiatric conditions including bipolar disorder, suicidality and addictions. Lithium lowers impulsivity in clinical populations and decreases pathological gambling in experimental medicine studies, but suffers from adverse effects, poor compliance and a low therapeutic index. Recently we identified that the neuroprotective agent ebselen, which is reportedly safe in humans, inhibited inositol monophosphatase (IMPase), a candidate lithium mechanism. Ebselen also reduced 5-HT receptor (5-HT2A) function which predicts impulsivity lowering properties. Here we investigated the effect of ebselen in rat models of impulsive behaviour. Ebselen was tested in two models of impulsivity with human analogues: the five-choice serial reaction time task (5-CSRTT) and rodent gambling task (rGT). The main outcome measures were premature responses (5-CSRTT and rGT) and choice behaviour (rGT), which model motor impulsivity and choice impulsivity, respectively. At doses that decreased 5-HT2A receptor function (DOI-induced wet dog shakes), ebselen decreased premature responding in the 5-CSRTT both in the absence and presence of cocaine. The 5-HT2A receptor antagonist MDL 100,907 also reduced premature responding in the 5-CSRTT although not in the presence of cocaine. In the rGT ebselen showed a tendency to reduce premature responding but had no effect on choice behaviour. These findings suggest that ebselen preferentially reduces motor impulsivity over choice impulsivity, and that inhibition of 5-HT2A receptor function is a contributing mechanism. Collectively, these data support the repurposing of ebselen as an anti-impulsive treatment and fast-tracking to clinical trials in patient groups characterised by poor impulse control.

5-HT2A antagonist MDL100907 showed inconsistent effect on decision-making during reward-guided behavior

5-HT2A antagonist MDL100907 showed inconsistent effect on decision-making during reward-guided behavior

Further investigation of the effects of 5-hydroxytryptamine, 8-OH-DPAT and DOI to mediate contraction and relaxation responses in the intestine and emesis in Suncus murinus

5-HT receptors are implicated in many gastrointestinal disorders. However, the precise role of 5-HT in mediating GI responses in Suncus murnius is still unclear. Therefore in this study, the effects of 5-HT and its agonists were investigated in Suncus. The involvement of 5-HT2C receptors in mediating emesis was also investigated. The ability of 5-HT and its agonists/antagonists at 5-HT1A and 5-HT2 to modify GI motility was investigated in vitro and in vivo.WAY100635 (a 5-HT1A antagonist) inhibited the contraction response to 5-HT in the proximal segments without affecting the maximum response; whilst enhancing the contraction to 5-HT (>30.0nM) in the distal intestine. The selective 5-HT2A and 5-HT2B receptor antagonists MDL-100907 and RS-127445 attenuated 5-HT-induced contractions (<10.0µM) in the distal segments. RS-127445 also attenuated 5-HT-induced contractions in the central segments. The selective 5-HT2C receptor antagonist SB-242084, attenuated the responses to 5-HT (> 3.0nM) in the proximal and central but not the distal regions. 8-OH-DPAT-induced relaxation was resistant to the antagonism by 5-HT1A/7 antagonists. DOI in the presence of 5-HT1A/2A/2B/2C antagonists induced greater contraction responses (>1.0µM) in most tissues, whilst RS-127445, or SB-242084, reduced the responses to DOI (< 1.0µM) in some tissues. SB-242084 also suppressed emesis-induced by motion and intragastric CuSO4.In conclusion, within different regions of intestine, 5-HT2 receptors are differently involved in contraction and emetic responses and that 8-OH-DPAT induces relaxation via non-5-HT1A/7 receptors. Suncus could provide a model to investigate these diverse actions of 5-HT.

Activation of 5-HT2A receptor disrupts rat maternal behavior

Serotonin 5-HT2A receptor is widely distributed in the central nervous system and plays an important role in sensorimotor function, emotion regulation, motivation, executive control, learning and memory. We investigated its role in rat maternal behavior, a naturalistic behavior encompassing many psychological functions that the 5-HT2A receptor is involved in. We first showed that activation of 5-HT2A receptor by TCB-2 (a highly selective 5-HT2A agonist, 1, 2.5 or 5.0 mg/kg) disrupted maternal behavior dose-dependently, and this effect was reduced by pretreatment with a 5-HT2A receptor antagonist MDL 100907, but exacerbated by pretreatment with a 5-HT2C receptor antagonist SB242084 and a 5-HT2C receptor agonist MK212, indicating that the maternal disruptive effect of 5-HT2A activation is receptor-specific and can be modulated by 5-HT2C receptor bidirectionally. We then microinjected TCB-2 into two brain regions important for the normal expression of maternal behavior: the medial prefrontal cortex (mPFC) and the medial preoptic area (mPOA) and found that only acute intra-mPFC infusion of TCB-2 suppressed pup retrieval, whereas intra-mPOA had no effect. Finally, using c-Fos immunohistochemistry, we identified that the ventral bed nucleus of stria terminalis (vBNST), the central amygdala (CeA), and the dorsal raphe (DR) were additionally involved in the maternal-disruptive effect of TCB-2. These findings suggest that the 5-HT2A receptor in the mPFC and other maternally related regions is required for the normal expression of maternal behavior through its intrinsic action or interactions with other receptors (e.g. 5-HT2C). Functional disruption of this neuroreceptor system might contribute to postpartum mental disorders (e.g. depression and psychosis) that impair the quality of maternal care.

Serotonin gating of cortical and thalamic glutamate inputs onto principal neurons of the basolateral amygdala

The basolateral amygdala (BLA) is a key site for crossmodal association of sensory stimuli and an important relay in the neural circuitry of emotion. Indeed, the BLA receives substantial glutamatergic inputs from multiple brain regions including the prefrontal cortex and thalamic nuclei. Modulation of glutamatergic transmission in the BLA regulates stress- and anxiety-related behaviors. Serotonin (5-HT) also plays an important role in regulating stress-related behavior through activation of both pre- and postsynaptic 5-HT receptors. Multiple 5-HT receptors are expressed in the BLA, where 5-HT has been reported to modulate glutamatergic transmission. However, the 5-HT receptor subtype mediating this effect is not yet clear. The aim of this study was to use patch-clamp recordings from BLA neurons in an ex vivo slice preparation to examine 1) the effect of 5-HT on extrinsic sensory inputs, and 2) to determine if any pathway specificity exists in 5-HT regulation of glutamatergic transmission. Two independent input pathways into the BLA were stimulated: the external capsule to mimic cortical input, and the internal capsule to mimic thalamic input. Bath application of 5-HT reversibly reduced the amplitude of evoked excitatory postsynaptic currents (eEPSCs) induced by stimulation of both pathways. The decrease was associated with an increase in the paired-pulse ratio and coefficient of variation of eEPSC amplitude, suggesting 5-HT acts presynaptically. Moreover, the effect of 5-HT in both pathways was mimicked by the selective 5-HT1B receptor agonist CP93129, but not by the 5-HT1A receptor agonist 8-OH DPAT. Similarly the effect of exogenous 5-HT was blocked by the 5-HT1B receptor antagonist GR55562, but not affected by the 5-HT1A receptor antagonist WAY 100635 or the 5-HT2 receptor antagonists pirenperone and MDL 100907. Together these data suggest 5-HT gates cortical and thalamic glutamatergic inputs into the BLA by activating presynaptic 5-HT1B receptors.

Effect of a hallucinogenic serotonin 5-HT2A receptor agonist on visually guided, hippocampal-dependent spatial cognition in C57BL/6J mice.

By acting on serotonin 5-HT2A receptors (5-HT2A Rs), serotonergic psychedelic drugs induce perceptual and visual hallucinations by increasing neuronal excitability and altering visual-evoked neuronal responses. The present study was designed to examine whether the perceptual alterations induced by a serotonergic psychedelic drug would affect the integrity of hippocampal-dependent, visually guided spatial cognition. phenylalkylamine hallucinogen TCB-2 is a selective agonist of 5-HT2A Rs. Mice received TCB-2 (1.0 mgkg-1 , i.p.), and spatial behaviors and hippocampal electrophysiological responses were measured with water maze tasks and in vivo single-unit recording, respectively. TCB-2 did not affect visual cue approach behavior in the visible platform water maze, but increased the latency of trained mice to initiate goal-directed swimming during a probe test in the hidden platform Morris water maze, which could be prevented by 5-HT2A R antagonist MDL 11,939. Interestingly, TCB-2 did not affect the efficiency of the swim path or the proper use of distal visual cues during the probe test. Hippocampal place cell activity is considered to represent spatial and context-specific episodic memory. Systemic TCB-2 did not affect previously established place fields of CA1 neurons in mice exploring a familiar environment, or the remapping of place cells when the mice explored a novel environment. However, TCB-2 impaired the long-term stability of place fields for the novel environment initially encoded under the influence of TCB-2, which could be prevented by 5-HT2A R antagonist MDL 11,939. Our data indicate that hallucinogenic 5-HT2A R agonist delays the initiation of spatial search behavior, but does not impair the use of visual cues to guide goal-directed spatial behavior. Moreover, activation of 5-HT2A Rs does not impair the coding and retrieval of spatial information, but impairs the long-term stability of new formed place fields of CA1 neurons. © 2017 Wiley Periodicals, Inc.

Blockade of Serotonin 5-HT2A Receptors Suppresses Behavioral Sensitization and Naloxone-Precipitated Withdrawal Symptoms in Morphine-Treated Mice.

The increasing prescription of opioids is fueling an epidemic of addiction and overdose deaths. Morphine is a highly addictive drug characterized by a high relapse rate – even after a long period of abstinence. Serotonin (5-HT) neurotransmission participates in the development of morphine dependence, as well as the expression of morphine withdrawal. In this study, we examined the effect of blockade of 5-HT2A receptors (5-HT2ARs) on morphine-induced behavioral sensitization and withdrawal in male mice. 5-HT2AR antagonist MDL 11,939 (0.5 mg/kg, i.p.) suppressed acute morphine (5.0 mg/kg, s.c.)-induced increase in locomotor activity. Mice received morphine (10 mg/kg, s.c.) twice a day for 3 days and then drug treatment was suspended for 5 days. On day 9, a challenge dose of morphine (10 mg/kg) was administered to induce the expression of behavioral sensitization. MDL 11,939 (0.5 mg/kg, i.p.) pretreatment suppressed the expression of morphine-induced behavioral sensitization. Another cohort of mice received increasing doses of morphine over a 7-day period to induce morphine-dependence. MDL 11,939 (0.5 mg/kg, i.p.) prevented naloxone-precipitated withdrawal in morphine-dependent mice on day 7. Moreover, chronic morphine treatment increased 5-HT2AR protein level and decreased the phosphorylation of extracellular signal-regulated kinases in the prefrontal cortex. Together, these results by the first time demonstrate that 5-HT2ARs modulate opioid dependence and blockade of 5-HT2AR may represent a novel strategy for the treatment of morphine use disorders.Highlights(i)Blockade of 5-HT2A receptors suppresses the expression of morphine-induced behavioral sensitization.(ii)Blockade of 5-HT2A receptors suppresses naloxone-precipitated withdrawal in morphine-treated mice.(iii)Chronic morphine exposure induces an increase in 5-HT2A receptor protein level and a decrease in ERK protein phosphorylation in prefrontal cortex.

Serotonergic mechanisms of trigeminal meningeal nociception: Implications for migraine pain

Serotonergic mechanisms play a central role in migraine pathology. However, the region-specific effects of serotonin (5-HT) mediated via multiple types of receptors in the nociceptive system are poorly understood. Using extracellular and patch-clamp recordings, we studied the action of 5-HT on the excitability of peripheral and central terminals of trigeminal afferents. 5-HT evoked long-lasting TTX-sensitive firing in the peripheral terminals of meningeal afferents, the origin site of migraine pain. Cluster analysis revealed that in majority of nociceptive fibers 5-HT induced either transient or persistent spiking activity with prevailing delta and theta rhythms. The 5-HT3-receptor antagonist MDL-72222 or 5-HT1B/D-receptor antagonist GR127935 largely reduced, but their combination completely prevented the excitatory pro-nociceptive action of 5-HT. The 5-HT3 agonist mCPBG activated spikes in MDL-72222-dependent manner but the 5HT-1 receptor agonist sumatriptan did not affect the nociceptive firing. 5-HT also triggered peripheral CGRP release in meninges, which was blocked by MDL-72222.5-HT evoked fast membrane currents and Ca2+ transients in a fraction of trigeminal neurons. Immunohistochemistry showed expression of 5-HT3A receptors in fibers innervating meninges. Endogenous release of 5-HT from degranulated mast cells increased nociceptive firing. Low pH but not histamine strongly activated firing. 5-HT reduced monosynaptic inputs from trigeminal Aδ- and C-afferents to the upper cervical lamina I neurons and this effect was blocked by MDL-72222. Consistent with central inhibitory effect, 5-HT reduced CGRP release in the brainstem slices. In conclusion, 5-HT evokes powerful pro-nociceptive peripheral and anti-nociceptive central effects in trigeminal system transmitting migraine pain.

Effects of Antipsychotic Drugs Haloperidol and Clozapine on Visual Responses of Retinal Ganglion Cells in a Rat Model of Retinitis Pigmentosa.

In the P23H rat model of retinitis pigmentosa, the dopamine D2 receptor antagonists sulpiride and eticlopride appear to improve visual responses of retinal ganglion cells (RGCs) by increasing light sensitivity of RGCs and transforming abnormal, long-latency ON-center RGCs into OFF-center cells. Antipsychotic drugs are believed to mediate their therapeutic benefits by blocking D2 receptors. This investigation was conducted to test whether haloperidol (a typical antipsychotic drug) and clozapine (an atypical antipsychotic drug) could similarly alter the light responses of RGCs in the P23H rat retina. Extracellular recordings were made from RGCs in isolated P23H rat retinas. Responses of RGCs to flashes of light were evaluated before and during bath application of a drug. Both haloperidol and clozapine increased light sensitivity of RGCs on average by ∼0.3 log unit. For those ON-center RGCs that exhibit an abnormally long-latency response to the onset of a small spot of light, both haloperidol and clozapine brought out a short-latency OFF response and markedly reduced the long-latency ON response. The selective serotonin 5-HT2A antagonist MDL 100907 had similar effects on RGCs. The effects of haloperidol on light responses of RGCs can be explained by its D2 receptor antagonism. The effects of clozapine on light responses of RGCs on the other hand may largely be due to its 5-HT2A receptor antagonism. Overall, the results suggest that antipsychotic drugs may be useful in improving vision in patients with retinitis pigmentosa.

Characterization of the discriminative stimulus effects of lorcaserin in rats.

Lorcaserin is approved by the Food and Drug Administration for treating obesity and is under consideration for treating substance use disorders; it has agonist properties at serotonin (5-HT)2C receptors and might also have agonist properties at other 5-HT receptor subtypes. This study used drug discrimination to investigate the mechanism(s) of action of lorcaserin. Male Sprague-Dawley rats discriminated 0.56 mg/kg i.p. lorcaserin from saline while responding under a fixed-ratio 5 schedule for food. Lorcaserin (0.178-1.0 mg/kg) dose-dependently increased lorcaserin-lever responding. The 5-HT2C receptor agonist mCPP and the 5-HT2A receptor agonist DOM each occasioned greater than 90% lorcaserin-lever responding in seven of eight rats. The 5-HT1A receptor agonist 8-OH-DPAT occasioned greater than 90% lorcaserin-lever responding in four of seven rats. The 5-HT2C receptor selective antagonist SB 242084 attenuated lorcaserin-lever responding in all eight rats and the 5-HT2A receptor selective antagonist MDL 100907 attenuated lorcaserin-lever responding in six of seven rats. These results suggest that, in addition to agonist properties at 5-HT2C receptors, lorcaserin also has agonist properties at 5-HT2A and 5-HT1A receptors. Because some drugs with 5-HT2A receptor agonist properties are abused, it is important to fully characterize the behavioral effects of lorcaserin while considering its potential for treating substance use disorders.

Modification of 5-methoxy-N,N-dimethyltryptamine-induced hyperactivity by monoamine oxidase A inhibitor harmaline in mice and the underlying serotonergic mechanisms.

5-Methoxy-N,N-dimethyltryptamine (5-MeO-DMT) and harmaline are indolealkylamine (IAA) drugs often abused together. Our recent studies have revealed the significant effects of co-administered harmaline, a monoamine oxidase inhibitor (MAOI), on 5-MeO-DMT pharmacokinetics and thermoregulation. This study was to delineate the impact of harmaline and 5-MeO-DMT on home-cage activity in mouse models, as well as the contribution of serotonin (5-HT) receptors. Home-cage activities of individual animals were monitored automatically in the home cages following implantation of telemetry transmitters and administration of various doses of IAA drugs and 5-HT receptor antagonists. Area under the effect curve (AUEC) of mouse activity values were calculated by trapezoidal rule. High dose of harmaline (15mg/kg, ip) alone caused an early-phase (0-45min) hypoactivity in mice that was fully attenuated by 5-HT1A receptor antagonist WAY-100635, whereas a late-phase (45-180min) hyperactivity that was reduced by 5-HT2A receptor antagonist MDL-100907. 5-MeO-DMT (10 and 20mg/kg, ip) alone induced biphasic effects, an early-phase (0-45min) hypoactivity that was completely attenuated by WAY-100635, and a late-phase (45-180min) hyperactivity that was fully suppressed by MDL-100907. Interestingly, co-administration of MAOI harmaline (2-15mg/kg) with a subthreshold dose of 5-MeO-DMT (2mg/kg) induced excessive hyperactivities at late phase (45-180min) that could be abolished by either WAY-100635 or MDL-100907. Co-administration of MAOI with 5-MeO-DMT provokes excessive late-phase hyperactivity, which involves the activation of both 5-HT1A and 5-HT2A receptors.