Administration Of DOI Research Articles

Selective Activation of D3 Dopamine Receptors Ameliorates DOI-Induced Head Twitching Accompanied by Changes in Corticostriatal Processing.

D3 receptors, a key component of the dopamine system, have emerged as a potential target of therapies to improve motor symptoms across neurodegenerative and neuropsychiatric conditions. In the present work, we evaluated the effect of D3 receptor activation on the involuntary head twitches induced by 2,5-dimethoxy-4-iodoamphetamine (DOI) at behavioral and electrophysiological levels. Mice received an intraperitoneal injection of either a full D3 agonist, WC 44 [4-(2-fluoroethyl)-N-[4-[4-(2-methoxyphenyl)piperazin 1-yl]butyl]benzamide] or a partial D3 agonist, WW-III-55 [N-(4-(4-(4-methoxyphenyl)piperazin-1-yl)butyl)-4-(thiophen-3-yl)benzamide] five minutes before the intraperitoneal administration of DOI. Compared to the control group, both D3 agonists delayed the onset of the DOI-induced head-twitch response and reduced the total number and frequency of the head twitches. Moreover, the simultaneous recording of neuronal activity in the motor cortex (M1) and dorsal striatum (DS) indicated that D3 activation led to slight changes in a single unit activity, mainly in DS, and increased its correlated firing in DS or between presumed cortical pyramidal neurons (CPNs) and striatal medium spiny neurons (MSNs). Our results confirm the role of D3 receptor activation in controlling DOI-induced involuntary movements and suggest that this effect involves, at least in part, an increase in correlated corticostriatal activity. A further understanding of the underlying mechanisms may provide a suitable target for treating neuropathologies in which involuntary movements occur.

Effects of a psychedelic 5-HT2A receptor agonist on anxiety-related behavior and fear processing in mice.

Psychedelic-assisted psychotherapy gained considerable interest as a novel treatment strategy for fear-related mental disorders but the underlying mechanism remains poorly understood. The serotonin 2A (5-HT2A) receptor is a key target underlying the effects of psychedelics on emotional arousal but its role in fear processing remains controversial. Using the psychedelic 5-HT2A/5-HT2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) and 5-HT2A receptor knockout (KO) mice we investigated the effect of 5-HT2A receptor activation on emotional processing. We show that DOI administration did not impair performance in a spontaneous alternation task but reduced anxiety-like avoidance behavior in the elevated plus maze and elevated zero maze tasks. Moreover, we found that DOI did not block memory recall but diminished fear expression in a passive avoidance task. Likewise, DOI administration reduced fear expression in an auditory fear conditioning paradigm, while it did not affect retention of fear extinction when administered prior to extinction learning. The effect of DOI on fear expression was abolished in 5-HT2A receptor KO mice. Administration of DOI induced a significant increase of c-Fos expression in specific amygdalar nuclei. Moreover, local infusion of the 5-HT2A receptor antagonist M100907 into the amygdala reversed the effect of systemic administration of DOI on fear expression while local administration of DOI into the amygdala was sufficient to suppress fear expression. Our data demonstrate that activation of 5-HT2A receptors in the amygdala suppresses fear expression but provide no evidence for an effect on retention of fear extinction.

Delivery of the 5-HT2A Receptor Agonist, DOI, Enhances Activity of the Sphincter Muscle during the Micturition Reflex in Rats after Spinal Cord Injury.

Simple SummarySpinal cord injury often disrupts connections between the brain and spinal cord leading to a plethora of health complications, including bladder dysfunction. Spinal cord injured patients are left with symptoms such as a leaky bladder (the inability to hold their urine), frequent urinary tract infections, and potential kidney failure. However, previous studies have shown that manipulation of serotoninergic receptors can improve urinary performance following spinal cord injury. In the current study, we sought to explore how stimulation of a specific serotonergic receptor subtype can significantly enhance bladder function in spinal cord injured rats. To do so, we utilized spinal cord injured female rats that underwent various bladder performance evaluations combined with pharmacological intervention of a specific serotonergic subtype. Additionally, the primary site of action was investigated to determine effects elicited during various administration routes (e.g., directly into the cord, into the femoral vein, or into the skin). Stimulation of this receptor subtype, regardless of delivery route, improved activity of the external urethral sphincter and detrusor-sphincter coordination in spinal cord injured rats. Collectively, the results of these experiments have the potential to provide vital guidance for the development of therapeutic strategies to alleviate urinary dysfunction following spinal cord injury.Traumatic spinal cord injury (SCI) interrupts spinobulbospinal micturition reflex pathways and results in urinary dysfunction. Over time, an involuntary bladder reflex is established due to the reorganization of spinal circuitry. Previous studies show that manipulation of serotonin 2A (5-HT2A) receptors affects recovered bladder function, but it remains unclear if this receptor regulates the activity of the external urethral sphincter (EUS) following SCI. To elucidate how central and peripheral serotonergic machinery acts on the lower urinary tract (LUT) system, we employed bladder cystometry and EUS electromyography recordings combined with intravenous or intrathecal pharmacological interventions of 5-HT2A receptors in female SCI rats. Three to four weeks after a T10 spinal transection, systemic and central blockage of 5-HT2A receptors with MDL only slightly influenced the micturition reflex. However, delivery of the 5-HT2A receptor agonist, DOI, increased EUS tonic activity and elicited bursting during voiding. Additionally, subcutaneous administration of DOI verified the enhancement of continence and voiding capability during spontaneous micturition in metabolic cage assays. Although spinal 5HT2A receptors may not be actively involved in the recovered micturition reflex, stimulating this receptor subtype enhances EUS function and the synergistic activity between the detrusor and sphincter to improve the micturition reflex in rats with SCI.

Effects of serotonergic hallucinogen R(−)‐2,5‐Dimethoxy‐4‐iodoamphetamine (DOI) on temporal discrimination in mice

R(−)‐2,5‐Dimethoxy‐4‐iodoamphetamine (DOI) and other compounds with agonist affinity for serotonergic 5‐HT2A receptors are hallucinogenic drugs of abuse, but also have therapeutic potential in numerous neuropsychiatric conditions, including major depressive disorder, post‐traumatic stress disorder, and in the treatment of migraine and cluster headaches. Thus, commonly abused hallucinogens could serve as lead compounds in the development of new therapeutic drugs. While there are already behavioral assays useful in the study of these compounds, it remains difficult to measure the primary effects of hallucinogenic drugs, which are subjective and unobservable. The drug‐elicited head twitch response (HTR) is a technically‐simple observational assay which yields dose‐dependent data, is sensitive to 5‐HT2A antagonism, and has a strong predictive validity for hallucinogenic effects in humans. However, it is not clear which aspect of the cluster of hallucinogenic effects is modeled by the HTR. Because hallucinogens and other psychoactive drugs have been shown to disrupt temporal discrimination in rodents and in humans, a model incorporating time perception may have better face validity than the HTR. In these studies, we used a temporal discrimination assay to measure the effects of DOI in male Swiss‐Webster mice in order to validate the procedure by determining whether reliable drug effects would be achievable in mice, would be dose‐dependent, and would be reversible by 5‐HT2A antagonism. In this procedure, a light‐tone compound stimulus was presented for either a short (2.7 sec) or long (27 sec) duration. Responses in one nose‐poke aperture were reinforced after passage of a short stimulus presentation, while responses in the other nose‐poke aperture were reinforced after passage of long stimulus presentation. Mice required a relatively large number of sessions (>60) to reach criterion performance, but once trained, administration of DOI dose‐dependently disrupted temporal discrimination. The doses of DOI required to alter temporal discrimination were relatively high, and are on the descending limb of dose‐effect functions derived in HTR studies. Nevertheless, these effects were at least partially mediated by 5‐HT2A agonism, as the selective 5‐HT2A antagonist M100907 dose‐dependently reversed the effects of DOI on temporal discrimination. Despite the large time investment required to train murine subjects, temporal discrimination appears to be a useful behavioral pharmacology assay to specifically study the effects of novel hallucinogen‐like drugs on time perception, and may be a valuable in the preclinical development of new therapeutic drugs with serotonergic mechanisms.Support or Funding InformationThese studies were funded in part by an NIGMS IDeA Program award (GM110702) and by the UAMS Translational Research Institute (RR029884), and MPT received a Summer Undergraduate Research Fellowships from the American Society of Pharmacology and Experimental Therapeutics to work on these studies.

Methamphetamine‐Induced Changes to Serotonergic Markers in the Frontal Cortex

Methamphetamine use is associated with a variety of negative health outcomes including psychosis. Alternations in the frontal cortex including changes in serotonin (5HT) 2A receptors are thought to contribute to psychosis‐like behaviors. Previously we have reported that methamphetamine self‐administration changes serotonin in the frontal cortex of rats. The purpose of this study was to investigate changes in serotonergic markers in the frontal cortex following methamphetamine self‐administration and how these changes relate to hallucination‐like behavior. Male and female rats were allowed to self‐administer either methamphetamine or saline and were sacrificed either one hour after the last self‐administration session or following extinction training. Serotonin transporters (SERT) and 5HT2A receptors were assessed in the frontal cortex via autoradiography. No changes in SERT autoradiography was observed in the orbitofrontal, prelimbic, or infralimbic cortices one hour after self‐administration or after extinction training. However, preliminary findings suggest time dependent changes in 5HT2A receptors in the frontal cortex. One hour after the last self‐administration session 5HT2A receptors were increased in the orbitofrontal cortex. Further, there was a trend towards an increase in prelimbic 5HT2A receptors in rats that self‐administered methamphetamine. However, no changes were found in male rats following extinction training. To assess the functioning of the 5HT2A receptors, the 5HT2A/2C agonist, DOI, was administered in a separate group of animals following extinction training. Preliminary results suggest that rats with a history of METH self‐administration have reduced hyperthermia following the administration of DOI. Efforts are underway to quantify DOI‐induced head twitches which are thought to reflect 5HT2A receptor function in the brain and hallucination‐like behavior. Overall, results of the current study suggest that methamphetamine self‐administration causes time dependent changes in 5HT2A receptors but not SERT in the frontal cortex. Ongoing studies will examine how methamphetamine‐induced changes in the frontal cortex may result in changes in behaviors associated with psychosis such as hallucinations.Support or Funding InformationFunding for this study was provided the NIH grant DA036012.

Quantitative Phosphoproteomics Unravels Biased Phosphorylation of Serotonin 2A Receptor at Ser280 by Hallucinogenic versus Nonhallucinogenic Agonists

The serotonin 5-HT2A receptor is a primary target of psychedelic hallucinogens such as lysergic acid diethylamine, mescaline, and psilocybin, which reproduce some of the core symptoms of schizophrenia. An incompletely resolved paradox is that only some 5-HT2A receptor agonists exhibit hallucinogenic activity, whereas structurally related agonists with comparable affinity and activity lack such a psychoactive activity. Using a strategy combining stable isotope labeling by amino acids in cell culture with enrichment in phosphorylated peptides by means of hydrophilic interaction liquid chromatography followed by immobilized metal affinity chromatography, we compared the phosphoproteome in HEK-293 cells transiently expressing the 5-HT2A receptor and exposed to either vehicle or the synthetic hallucinogen 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) or the nonhallucinogenic 5-HT2A agonist lisuride. Among the 5995 identified phosphorylated peptides, 16 sites were differentially phosphorylated upon exposure of cells to DOI versus lisuride. These include a serine (Ser280) located in the third intracellular loop of the 5-HT2A receptor, a region important for its desensitization. The specific phosphorylation of Ser280 by hallucinogens was further validated by quantitative mass spectrometry analysis of immunopurified receptor digests and by Western blotting using a phosphosite specific antibody. The administration of DOI, but not of lisuride, to mice, enhanced the phosphorylation of 5-HT2A receptors at Ser280 in the prefrontal cortex. Moreover, hallucinogens induced a less pronounced desensitization of receptor-operated signaling in HEK-293 cells and neurons than did nonhallucinogenic agonists. The mutation of Ser280 to aspartic acid (to mimic phosphorylation) reduced receptor desensitization by nonhallucinogenic agonists, whereas its mutation to alanine increased the ability of hallucinogens to desensitize the receptor. This study reveals a biased phosphorylation of the 5-HT2A receptor in response to hallucinogenic versus nonhallucinogenic agonists, which underlies their distinct capacity to desensitize the receptor.

Gpm6b deficiency impairs sensorimotor gating and modulates the behavioral response to a 5-HT2A/C receptor agonist

The neuronal tetraspan proteins, M6A (Gpm6a) and M6B (Gpm6b), belong to the family of proteolipids that are widely expressed in the brain. We recently reported Gpm6a deficiency as a monogenetic cause of claustrophobia in mice. Its homolog proteolipid, Gpm6b, is ubiquitously expressed in neurons and oligodendrocytes. Gpm6b is involved in neuronal differentiation and myelination. It interacts with the N-terminal domain of the serotonin transporter (SERT) and decreases cell-surface expression of SERT. In the present study, we employed Gpm6b null mutant mice (Gpm6b−/−) to search for behavioral functions of Gpm6b. We studied male and female Gpm6b−/− mice and their wild-type (WT, Gpm6b+/+) littermates in an extensive behavioral test battery. Additionally, we investigated whether Gpm6b−/− mice exhibit changes in the behavioral response to a 5-HT2A/C receptor agonist. We found that Gpm6b−/− mice display completely normal sensory and motor functions, cognition, as well as social and emotionality-like (anxiety, depression) behaviors. On top of this inconspicuous behavioral profile, Gpm6b−/− mice of both genders exhibit a selective impairment in prepulse inhibition of the acoustic startle response. Furthermore, in contrast to WT mice that show the typical locomotion suppression and increase in grooming activity after intraperitoneal administration of DOI [(±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride], Gpm6b−/− mice demonstrate a blunted behavioral response to this 5-HT2A/C receptor agonist. To conclude, Gpm6b deficiency impairs sensorimotor gating and modulates the behavioral response to a serotonergic challenge.

Pharmacological modulation of abnormal involuntary DOI-induced head twitch response in male DBA/2J mice: I. Effects of D2/D3 and D2 dopamine receptor selective compounds

Because of the complexity and heterogeneity of human neuropsychiatric disorders, it has been difficult to identify animal models that mimic the symptoms of these neuropathologies and can be used to screen for antipsychotic agents. For this study we selected the murine 5HT2A/2C receptor agonist-induced head twitch response (HTR) induced by the administration of 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), which has been proposed as an animal model of symptoms associated with a variety of behavioral and psychiatric conditions. We investigated the DOI-induced HTR in male DBA/2J mice using a panel of D2-like (D2, D3 and D4) and D2 dopamine receptor selective compounds. When DBA/2J mice were administered a daily dose of DOI (5 mg/kg), tolerance to the DOI occurs. However, administrations of the same dose of DOI every other day (48 h) or on a weekly basis did not lead to tolerance and the ability to induce tolerance after daily administration of DOI remains intact after repeated weekly administration of DOI. Subsequently, a panel of D2-like dopamine receptor antagonists was found to effectively inhibit the DOI-induced HTR in DBA/2J mice. However, the benzamide eticlopride, which is a high affinity D2-like antagonist, was a notable exception. SV 293, SV-III-130s and N-methylbenperidol, which exhibit a high affinity for D2 versus the D3 dopamine receptor subtypes (60- to 100-fold binding selectivity), were also found to inhibit the HTR in DBA/2J mice. This observation suggests a functional interaction between dopaminergic and serotonergic systems through D2 dopamine receptors and the 5-HT2A serotonin receptors in vivo.

The response of juxtacellular labeled GABA interneurons in the basolateral amygdaloid nucleus anterior part to 5-HT2A/2C receptor activation is decreased in rats with 6-hydroxydopamine lesions

Here we report that juxtacellular labeled GABA interneurons in the basolateral amygdaloid nucleus anterior part (BLA) of rats with 6-hydroxydopamine lesions of the substantia nigra pars compacta (SNc) showed a more burst-firing pattern, while having no change in the firing rate. In sham-operated and the lesioned rats, systemic administration of 5-HT2A/2C receptor agonist DOI produced excitation, inhibition and unchanged in the firing rate of the interneurons, and the mean response of DOI was excitatory. However, cumulative dose producing excitation in the lesioned rats was higher than that of sham-operated rats. The local administration of DOI in the BLA also produced three types of responses in two groups of rats. Furthermore, the local administration of DOI excited the interneurons in sham-operated rats, whereas the mean firing rate of the interneurons in the lesioned rats was not affected at the same dose. The excitatory effect of the majority of the interneurons after systemic and local administration of DOI was not reversed by the selective 5-HT2C receptor antagonist SB242084, and the inhibitory effect of DOI in all the interneurons examined was reversed by GABAA receptor antagonist picrotoxinin. The SNc lesion in rats did not change the density of GAD67/5-HT2A receptor co-expressing neurons in the BLA. These results indicate that the SNc lesion changes the firing activity of BLA GABA interneurons. Moreover, DOI regulated the firing activity of the interneurons mainly through activation of 5-HT2A receptor, and the lesion led to a decreased response of the interneurons to DOI, which attributes to dysfunction of 5-HT2A receptor on these interneurons.

Chronic fluvoxamine treatment changes 5-HT2A/2C receptor-mediated behavior in olfactory bulbectomized mice

AimsOlfactory bulbectomy (OBX) in rodents represents a valuable experimental model of depression. This study was designed to shed further light on the impact of putative serotonergic neuronal degeneration in OBX mice and to assess the effect of a widely used antidepressant on serotonergic related behavioral changes induced by OBX. Main methodsAdult male ddY mice were subject to bilateral OBX or sham surgery. The serotonin (5-HT)2A/2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) enhanced a head-twitch response (HTR) in OBX mice. Effects of 5-HT2A, 5-HT2C antagonists and fluvoxamine were observed in OBX mice following DOI administration. Key findingsThe HTR elicited by the administration of DOI (0.5mg/kg and 1mg/kg, i.p.) was increased about twofold in OBX mice when compared with controls on the 14th day after the surgery. The injection of ketanserin (0.025mg/kg, i.p.), a 5-HT2A receptor antagonist, inhibited the enhancement of the DOI-induced HTR after OBX. Likewise, the administration of SB 242084 (1mg/kg, s.c.), a 5-HT2C receptor antagonist, also inhibited the DOI-induced HTR in OBX mice. Chronic but not acute treatment with the antidepressant fluvoxamine, a selective serotonin reuptake inhibitor (SSRI), suppressed the enhancement of DOI-induced HTR after OBX. SignificanceThese findings indicate that OBX, and the subsequent degeneration of neurons projecting from the olfactory bulb, caused a supersensitivity of 5-HT2A/2C receptors which may be involved in symptoms of depression.

Induction of the plasticity-associated immediate early gene Arc by stress and hallucinogens: role of brain-derived neurotrophic factor

Exposure to stress and hallucinogens in adulthood evokes persistent alterations in neurocircuitry and emotional behaviour. The structural and functional changes induced by stress and hallucinogen exposure are thought to involve transcriptional alterations in specific effector immediate early genes. The immediate early gene, activity regulated cytoskeletal-associated protein (Arc), is important for both activity and experience dependent plasticity. We sought to examine whether trophic factor signalling through brain-derived neurotrophic factor (BDNF) contributes to the neocortical regulation of Arc mRNA in response to distinct stimuli such as immobilization stress and the hallucinogen 2,5-dimethoxy-4-iodoamphetamine (DOI). Acute exposure to either immobilization stress or DOI induced Arc mRNA levels within the neocortex. BDNF infusion into the neocortex led to a robust up-regulation of local Arc transcript expression. Further, baseline Arc mRNA expression in the neocortex was significantly decreased in inducible BDNF knockout mice with an adult-onset, forebrain specific BDNF loss. The induction of Arc mRNA levels in response to both acute immobilization stress or a single administration of DOI was significantly attenuated in the inducible BDNF knockout mice. Taken together, our results implicate trophic factor signalling through BDNF in the regulation of cortical Arc mRNA expression, both under baseline conditions and following stress and hallucinogen exposure. These findings suggest the possibility that the regulation of Arc expression via BDNF provides a molecular substrate for the structural and synaptic plasticity observed following stimuli such as stress and hallucinogens.

Effects of LPS and serotonergic drugs on hygienic behavior in mice

Hygienic self-grooming is a behavioral adaptation for removing litter particles and pathogenic agents from animal fur and skin. We studied contribution of brain serotonin system into mechanisms regulating hygienic behavior in intact mice and mice with LPS(lipopolysaccharide)-induced sickness. A spot of fluorescent dye was applied on the back of a mouse, and the decrease in its fluorescence served as an index of fur cleaning efficiency estimated using original classifier algorithm. Agonist of 5-HT1A receptor (8-OH-DPAT) or 5-HT2A/2C receptor (DOI) attenuated fur cleaning at a dose of 1mg/kg but not of 0.2mg/kg. MAO-A inhibitor clorgyline decreased hygienic self-grooming at a dose of 10 but not of 5mg/kg. SSRI paroxetine had no effect while fluoxetine diminished hygienic behavior at the higher dose used (20mg/kg). Inhibitory effect of LPS treatment (50μg/kg) on fur cleaning was not altered by administration of p-MPPI (5-HT1A receptor antagonist, 1mg/kg) or DOI (1mg/kg) while 8-OH-DPAT (1mg/kg) produced additive effect. The results suggest the involvement of 5-HT1A and 5-HT2A/2C brain serotonin receptors and MAO-A in the inhibition of hygienic behavior in mice. However, LPS-induced depression of fur cleaning appeared to be mediated via different mechanisms and enhanced by 5-HT1A receptor activation.

Unilateral lesion of the nigrostriatal pathway decreases the response of interneurons in medial prefrontal cortex to 5-HT 2A/2C receptor stimulation in the rat

The aim of the present study was to investigate changes in the firing rate and pattern of interneurons in the medial prefrontal cortex (mPFC), and effects of 5-HT 2A/2C receptor agonist DOI and antagonist ritanserin, and the selective 5-HT 2C receptor antagonist SB 242084 on the neuronal firing in rats with 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) by extracellular recording in vivo. The lesion of the SNc decreased the firing rate of the interneurons compared to sham-lesioned rats, and firing pattern of these interneurons changed toward a more burst-firing. Administration of DOI (20–320 μg/kg, i.v.) dose-dependently increased the firing rate of all interneurons examined in sham-lesioned and the 6-OHDA-lesioned rats. The excitation was significant at doses higher than 40 μg/kg and 320 μg/kg in sham-lesioned and the 6-OHDA-lesioned rats, respectively. This dose, which produced marked effect in the 6-OHDA-lesioned rats, was much higher than that of sham-lesioned rats. The local application of DOI (5 μg) in mPFC increased the firing rate of the interneurons in sham-lesioned rats, while having no effect on the firing rate in the 6-OHDA-lesioned rats. The excitatory effect of DOI in sham-lesioned and the 6-OHDA-lesioned rats was completely or partially reversed by ritanserin or SB 242084. The results of our study show that lesion of the SNc leads to a decrease in the firing rate of interneurons in mPFC and fire with a more burst pattern, and decreased response of the interneurons to DOI in rat.

Effects of acute systemic administration of serotonin2A/C receptor ligands in a delay-based decision-making task in rats

Serotonin (5-hydoxytryptamine; 5-HT) has been implicated in the regulation of impulsivity, and high levels of impulsive behavior are associated with certain neuropsychiatric disorders. An important aspect of impulsive behavior is the inability to tolerate delays in reward. This study investigated the effects of the 5-HT(2A/C) receptor agonist DOI [(+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropan hydrochloride] and the 5-HT(2A) receptor antagonist ketanserin on impulsive behavior measured in a delay-based decision-making task. Male Wistar rats were trained in a T-maze to choose a large but 10-s delayed food reward instead of a small immediate reward. After stable baseline performance (70% choice of large reward), the effects of acute systemic administration of 5-HT(2A/C) receptor ligands on waiting capacity were tested. Systemic administration of DOI (0.1, 0.3, and 0.5 mg/kg) impaired waiting capacity in a dose-dependent manner, whereas ketanserin had no effect. When combined with ketanserin, DOI did not impair waiting capacity. The data indicate that DOI-induced impairment of the ability to discount a delay in reward in a T-maze is probably regulated by 5-HT(2A) receptors. Furthermore, this study extends the existing findings of 5-HT2 receptor involvement in different tasks of delay aversion in rodents.

In vivo effects of activation and blockade of 5-HT 2A/2C receptors in the firing activity of pyramidal neurons of medial prefrontal cortex in a rodent model of Parkinson's disease

In the present study, we examined changes in the firing rate and firing pattern of pyramidal neurons in medial prefrontal cortex (mPFC), and the effects of 5-HT 2A/2C receptor agonist DOI and antagonist ritanserin on the neuronal firing in rats with 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta by using extracellular recording. The unilateral lesion of the nigrostriatal pathway significantly increased the mean firing rate of pyramidal neurons compared to sham-operated rats, and the firing pattern of these neurons also changed significantly towards a more bursty one. Systemic administration of DOI (20–320 μg/kg, i.v.) increased the mean firing rate of pyramidal neurons in sham-operated and the lesioned rats. The excitation was significant only at doses higher than 160 μg/kg and 320 μg/kg in sham-operated and the lesioned rats, respectively. In addition, the local application of DOI, 5 μg, in mPFC inhibited the firing rate of pyramidal neurons in sham-operated rats, while having no effect on firing rate in the lesioned rats. After treatment with GABA A receptor antagonist picrotoxinin, the local application of DOI, at the same dose, increased the mean firing rate of the neurons in sham-operated rats; however, DOI did not alter the firing activity of the neurons in the lesioned rats. These results indicate that the lesion of the nigrostriatal pathway leads to hyperactivity of pyramidal neurons in mPFC, and the decreased response of pyramidal neurons to DOI, suggesting dysfunction of 5-HT 2A and 5-HT 2C receptors on pyramidal neurons and GABAergic interneurons in the 6-OHDA-lesioned rats.

In vivo evidence for serotonin 5-HT 2C receptor-mediated long-lasting excitability of lumbar spinal reflex and its functional interaction with 5-HT 1A receptor in the mammalian spinal cord

The purpose of the present study was to investigate the 5-HT 2C receptor-mediated effects on the spinal monosynaptic mass reflex activities and also its functional interactions with 5-HT 1A receptors in anesthetized, acutely spinalized mammalian adult spinal cord in vivo. Intravenous administration of (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) (0.1 mg/kg), an agonist of 5-HT 2A/2C receptors, significantly increased the excitability of spinal motoneurons as reflected by an increase in the spinal monosynaptic mass reflex amplitude to 150–200% of the control. 5-HT 2A/2C receptor-induced motoneuron excitability was slow, persistent and long-lasting for more than 2 h that was significantly inhibited by 5-HT 2C receptor specific antagonist SB 242084 administered 10 min prior to DOI. Simultaneous administration of DOI (0.1 mg/kg, i.v.) along with (±)-8-hydroxy dipropylaminotetraline hydrobromide (8-OH-DPAT) (0.1 mg/kg, i.v.) completely inhibited DOI-induced spinal monosynaptic mass reflex facilitation. In another separate study, administration of 8-OH-DPAT (0.1 mg/kg, i.v.) at the maximum response of DOI also inhibited the motoneuron's excitability; however, the inhibition lasted only for a period of 40–60 min after administration of 8-OH-DPAT, after which the spinal monosynaptic mass reflex amplitude reached its maximum level. These findings suggest that the 5-HT 2C receptor is primarily involved in the mediation of the long-lasting excitability of spinal motoneurons and possibly interacts with its functional counterpart, 5-HT 1A receptors in the mammalian adult spinal cord.

Marked decrease of LSD-induced stimulus control in serotonin transporter knockout mice

Rationale Based upon extensive studies in the rat, it has been suggested that stimulus control by LSD is mediated by 5-HT 2A receptors, with serotonergic receptors of the 5-HT 1A and 5-HT 2C subtypes playing modulatory roles. In genetically modified mice lacking the serotonin transporter (SERT), 5-HT 2A receptor density is decreased and, at a functional level, the head-twitch response following the administration of DOI, an index of activation of 5-HT 2A receptors, is reduced. Taken together, these studies led us to hypothesize that the efficacy of LSD in establishing stimulus control is diminished or abolished in mice lacking the serotonin transporter. Objective Determine the efficacy of LSD for establishing stimulus control in SERT knockout (KO) mice. Methods SERT KO mice and wildtype (WT) littermates were trained in a visual discrimination on a progressive fixed ratio (FR) water-reinforced task and subsequently trained on a FR10 schedule with LSD (0.17 or 0.30 mg/kg) or vehicle. To control for general deficiencies in drug discrimination, mice were trained with pentobarbital (15 or 30 mg/kg) or vehicle. Results The visual stimulus exerted control in both genotypes. LSD-induced stimulus control in 90% of WT mice but only 31% of SERT KO mice. In contrast, pentobarbital-induced stimulus control in 80% of WT mice and 54% of knockout mice. Conclusions Although SERT KO mice exhibited stimulus control by the non-serotonergic drug, pentobarbital, the efficacy of LSD in these animals was markedly decreased, suggesting that reduced density of 5-HT 1A and/or 5-HT 2A receptors underlies the absence of stimulus control by LSD.

Activation of central 5HT2A receptors reduces the craniofacial nociception of rats

We assessed the contribution of central 5HT2A receptors to the craniofacial tissue nociception in naïve male rats. First, we tested whether activation of central 5HT2A receptors affected nociceptive neural activities recorded from superficial laminae of the trigeminal subnucleus caudalis (Vc)/upper cervical spinal cord junction (Vc/C2) region. Two types of units, such as deep-nociceptive or skin–wide dynamic range (WDR) units were identified from extracellular recordings. Topical administration of 5HT2A receptor agonist, (±)-2,5-dimethoxy-4-iodoamphetamine (DOI) onto the Vc/C2 region significantly reduced deep-nociceptive unit discharges evoked by formalin injection into the masseter muscle. Noxious pinch stimulation to the facial skin-evoked skin–WDR unit discharges was significantly reduced by topical administration of 0.1 mg/rat DOI onto the Vc/C2 region. Second, we tested whether i.c.v. administration of DOI affected Fos-like immunoreactivity (-LI) evoked by formalin injection into the masseter muscle. Fos-LI was significantly induced mainly at the ventrolateral (vl) area of trigeminal subnucleus interpolaris (Vi)/Vc junction (vl-Vi/Vc) region and Vc/C2 region in vehicle-treated rats. Formalin-evoked Fos-LI was significantly reduced in laminae I–II of the Vc/C2, but not vl-Vi/Vc region after i.c.v. administration of DOI. Finally, orofacial nocifensive behavioral activities evoked by formalin injection into the masseter muscle were significantly reduced by intracisternal administration of DOI. These results suggest that 5HT2A receptors in the Vc/C2 region mediate antinociceptive effects in the craniofacial nociception.

Effects of the serotonin 5-HT 2A/2C receptor agonist DOI and of the selective 5-HT 2A or 5-HT 2C receptor antagonists EMD 281014 and SB-243213, respectively, on sleep and waking in the rat

The effects of the serotonin 5-HT 2A/2C receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and of the selective 5-HT 2A or 5-HT 2C receptor antagonists 7-{4-[2-(4-fluoro-phenyl)-ethyl]-piperazine-1-carbonyl}-1H-indole-3-carbonitrile HCl (EMD 281014) and 5-methyl-1-[[-2-[(2-methyl-3-pyridyl)oxy]-5-pyridyl]carbamoyl]-6-trifluoromethylindoline HCl (SB-243213), respectively, on spontaneous sleep were studied in adult rats implanted for chronic sleep recordings. Subcutaneous administration of DOI (0.35–0.7 mmol/kg) significantly increased waking and light sleep and reduced slow wave sleep, rapid-eye-movement (REM) sleep, and the number of REM periods. With subcutaneous EMD 281014 (1.2–4.8 mmol/kg) or SB-243213 (1.2–4.8 mmol/kg) a significant reduction in time spent in REM sleep was also seen. Pretreatment with EMD 281014 prevented the DOI-induced increase of waking and light sleep and the reduction of slow wave sleep. However, REM sleep remained suppressed. SB-243213 failed to reverse the changes of sleep and waking induced by DOI. Thus, on the basis of these results it appears that serotonin 5-HT 2A receptor mechanisms might be responsible for the DOI-induced effects on waking and slow wave sleep.

The activation of 5‐HT2A receptors in prefrontal cortex enhances dopaminergic activity

Atypical antipsychotics show preferential 5-HT 2A versus dopamine (DA) D2 receptor affinity. At clinical doses, they fully occupy cortical 5-HT2 receptors, which suggests a strong relationship with their therapeutic action. Half of the pyramidal neurones in the medial prefrontal cortex (mPFC) express 5-HT 2A receptors. Also, neurones excited through 5-HT 2A receptors project to the ventral tegmental area (VTA). We therefore hypothesized that prefrontal 5-HT 2A receptors can modulate DA transmission through excitatory mPFC-VTA inputs. In this study we used single unit recordings to examine the responses of DA neurones to local (in the mPFC) and systemic administration of the 5-HT 2A/2C agonist 1-[2,5-dimethoxy-4-iodophenyl-2-aminopropane] (DOI). Likewise, using microdialysis, we examined DA release in the mPFC and VTA (single/dual probe) in response to prefrontal and systemic drug administration. The local (in the mPFC) and systemic administration of DOI increased the firing rate and burst firing of DA neurones and DA release in the VTA and mPFC. The increase in VTA DA release was mimicked by the electrical stimulation of the mPFC. The effects of DOI were reversed by M100907 and ritanserin. These results indicate that the activity of VTA DA neurones is under the excitatory control of 5-HT 2A receptors in the mPFC. These observations may help in the understanding of the therapeutic action of atypical antipsychotics.