Serotonin 2A Receptor Activation Research Articles

5-HT2ARs Mediate Therapeutic Behavioral Effects of Psychedelic Tryptamines.

Psychedelic compounds have displayed antidepressant potential in both humans and rodents. Despite their promise, psychedelics can induce undesired effects that pose safety concerns and limit their clinical scalability. The rational development of optimized psychedelic-related medicines will require a full mechanistic understanding of how these molecules produce therapeutic effects. While the hallucinogenic properties of psychedelics are generally attributed to activation of serotonin 2A receptors (5-HT2ARs), it is currently unclear if these receptors also mediate their antidepressant effects as several nonhallucinogenic analogues of psychedelics with antidepressant-like properties have been developed. Moreover, many psychedelics exhibit promiscuous pharmacology, making it challenging to identify their primary therapeutic target(s). Here, we use a combination of pharmacological and genetic tools to demonstrate that activation of 5-HT2A receptors is essential for tryptamine-based psychedelics to produce antidepressant-like effects in rodents. Our results suggest that psychedelic tryptamines can induce hallucinogenic and therapeutic effects through activation of the same receptor.

Acute serotonin 2A receptor activation impairs behavioral flexibility in mice

Serotonin 2A (5-HT2A) receptors are the primary site of action of hallucinogenic drugs and the target of atypical antipsychotics. 5-HT2A receptors are also implicated in executive function, including behavioral flexibility. Previous studies showed that 5-HT2A receptor blockade improved behavioral flexibility in rodent models related to autism spectrum disorder and schizophrenia. The current study instead was conducted to examine the impact of acute 5-HT2A receptor activation on behavior flexibility in the control C57BL/6 J strain. Because of the therapeutic potential of serotonergic hallucinogens and the unknown impact of many of these compounds on cognition, the present study examined how the 5-HT2A/2C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the more selective 5-HT2A agonist 25CN-NBOH impacted behavioral flexibility in C57BL/6 J mice. Male mice were tested on a probabilistic spatial discrimination and reversal learning task after an intraperitoneal injection of vehicle, 2.5 mg/kg DOI, 1.0 mg/kg 25CN-NBOH, 1.0 mg/kg of the 5-HT2C receptor antagonist SER-082 or combined treatment with SER-082 (1.0 mg/kg) and 2.5 mg/kg DOI before testing of probabilistic reversal learning. All groups demonstrated comparable performance on the initial spatial discrimination, i.e. similar trials to criterion. DOI alone did not impair reversal learning, whereas 25CN-NBOH increased the number of trials to criterion during reversal learning. Because 5-HT2A and 5-HT2C receptors have been shown to functionally antagonize each other in several behavioral paradigms, we also tested whether blockade of 5-HT2C receptors would unmask 5-HT2A receptor activation by DOI and impair reversal learning. Mice treated with SER-082 in combination with DOI required significantly more trials to reach criterion. In an additional experiment, a dose response experiment with 25CN-NBOH revealed that the 1.0 mg/kg dose tested in reversal learning did not affect locomotor activity. Together, these findings indicate that activation of 5-HT2A receptors impairs probabilistic reversal learning and that 5-HT2A and 5-HT2C receptors exert opposing effects on behavioral flexibility in male mice.

The serotonin 2A receptor agonist 25CN-NBOH increases murine heart rate and neck-arterial blood flow in a temperature-dependent manner.

Background:Serotonin 2A receptors, the molecular target of psychedelics, are expressed by neuronal and vascular cells, both of which might contribute to brain haemodynamic characteristics for the psychedelic state.Aim:Aiming for a systemic understanding of psychedelic vasoactivity, here we investigated the effect of N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine – a new-generation agonist with superior serotonin 2A receptor selectivity – on brain-supplying neck-arterial blood flow.Methods:We recorded core body temperature and employed non-invasive, collar-sensor based pulse oximetry in anesthetised mice to extract parameters of local blood perfusion, oxygen saturation, heart and respiration rate. Hypothesising an overlap between serotonergic pulse- and thermoregulation, recordings were done under physiological and elevated pad temperatures.Results:N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine (1.5 mg/kg, subcutaneous) significantly increased the frequency of heart beats accompanied by a slight elevation of neck-arterial blood flow. Increasing the animal-supporting heat-pad temperature from 37°C to 41°C enhanced the drug’s effect on blood flow while counteracting tachycardia. Additionally, N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine promoted bradypnea, which, like tachycardia, quickly reversed at the elevated pad temperature. The interrelatedness of N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine’s respiro-cardiovascular effects and thermoregulation was further corroborated by the drug selectively increasing the core body temperature at the elevated pad temperature. Arterial oxygen saturation was not affected by N-(2-hydroxybenzyl)-2,5-dimethoxy-4-cyanophenylethylamine at either temperature.Conclusions:Our findings imply that selective serotonin 2A receptor activation modulates systemic cardiovascular functioning in orchestration with thermoregulation and with immediate relevance to brain-imminent neck (most likely carotid) arteries. As carotid branching is a critical last hub to channel cardiovascular output to or away from the brain, our results might have implications for the brain haemodynamics associated with psychedelia.

Effective connectivity changes in LSD-induced altered states of consciousness in humans

Psychedelics exert unique effects on human consciousness. The thalamic filter model suggests that core effects of psychedelics may result from gating deficits, based on a disintegration of information processing within cortico-striato-thalamo-cortical (CSTC) feedback loops. To test this hypothesis, we characterized changes in directed (effective) connectivity between selected CTSC regions after acute administration of lysergic acid diethylamide (LSD), and after pretreatment with Ketanserin (a selective serotonin 2A receptor antagonist) plus LSD in a double-blind, randomized, placebo-controlled, cross-over study in 25 healthy participants. We used spectral dynamic causal modeling (DCM) for resting-state fMRI data. Fully connected DCM models were specified for each treatment condition to investigate the connectivity between the following areas: thalamus, ventral striatum, posterior cingulate cortex, and temporal cortex. Our results confirm major predictions proposed in the CSTC model and provide evidence that LSD alters effective connectivity within CSTC pathways that have been implicated in the gating of sensory and sensorimotor information to the cortex. In particular, LSD increased effective connectivity from the thalamus to the posterior cingulate cortex in a way that depended on serotonin 2A receptor activation, and decreased effective connectivity from the ventral striatum to the thalamus independently of serotonin 2A receptor activation. Together, these results advance our mechanistic understanding of the action of psychedelics in health and disease. This is important for the development of new pharmacological therapeutics and also increases our understanding of the mechanisms underlying the potential clinical efficacy of psychedelics.

LSD Increases Primary Process Thinking via Serotonin 2A Receptor Activation.

Rationale: Stimulation of serotonin 2A (5-HT2A) receptors by lysergic acid diethylamide (LSD) and related compounds such as psilocybin has previously been shown to increase primary process thinking – an ontologically and evolutionary early, implicit, associative, and automatic mode of thinking which is typically occurring during altered states of consciousness such as dreaming. However, it is still largely unknown whether LSD induces primary process thinking under placebo-controlled, standardized experimental conditions and whether these effects are related to subjective experience and 5-HT2A receptor activation. Therefore, this study aimed to test the hypotheses that LSD increases primary process thinking and that primary process thinking depends on 5-HT2A receptor activation and is related to subjective drug effects.Methods: Twenty-five healthy subjects performed an audio-recorded mental imagery task 7 h after drug administration during three drug conditions: placebo, LSD (100 mcg orally) and LSD together with the 5-HT2A receptor antagonist ketanserin (40 mg orally). The main outcome variable in this study was primary index (PI), a formal measure of primary process thinking in the imagery reports. State of consciousness was evaluated using the Altered State of Consciousness (5D-ASC) rating scale.Results: LSD, compared with placebo, significantly increased primary index (p < 0.001, Bonferroni-corrected). The LSD-induced increase in primary index was positively correlated with LSD-induced disembodiment (p < 0.05, Bonferroni-corrected), and blissful state (p < 0.05, Bonferroni-corrected) on the 5D-ASC. Both LSD-induced increases in primary index and changes in state of consciousness were fully blocked by ketanserin.Conclusion: LSD induces primary process thinking via activation of 5-HT2A receptors and in relation to disembodiment and blissful state. Primary process thinking appears to crucially organize inner experiences during both dreams and psychedelic states of consciousness.

Dreamlike effects of LSD on waking imagery in humans depend on serotonin 2A receptor activation

Accumulating evidence indicates that the mixed serotonin and dopamine receptor agonist lysergic acid diethylamide (LSD) induces an altered state of consciousness that resembles dreaming. This study aimed to test the hypotheses that LSD produces dreamlike waking imagery and that this imagery depends on 5-HT2A receptor activation and is related to subjective drug effects. Twenty-five healthy subjects performed an audiorecorded guided mental imagery task 7h after drug administration during three drug conditions: placebo, LSD (100mcg orally) and LSD together with the 5-HT2A receptor antagonist ketanserin (40mg orally). Cognitive bizarreness of guided mental imagery reports was quantified as a standardised formal measure of dream mentation. State of consciousness was evaluated using the Altered State of Consciousness (5D-ASC) questionnaire. LSD, compared with placebo, significantly increased cognitive bizarreness (p<0.001). The LSD-induced increase in cognitive bizarreness was positively correlated with the LSD-induced loss of self-boundaries and cognitive control (p<0.05). Both LSD-induced increases in cognitive bizarreness and changes in state of consciousness were fully blocked by ketanserin. LSD produced mental imagery similar to dreaming, primarily via activation of the 5-HT2A receptor and in relation to loss of self-boundaries and cognitive control. Future psychopharmacological studies should assess the differential contribution of the D2/D1 and 5-HT1A receptors to cognitive bizarreness.

Involvement of serotonin 2A receptor activation in modulating medial prefrontal cortex and amygdala neuronal activation during novelty-exposure

The medial prefrontal cortex (PFC) plays a major role in executive function by exerting a top-down control onto subcortical areas. Novelty-induced frontal cortex activation is 5-HT2A receptor (5-HT2AR) dependent. Here, we further investigated how blockade of 5-HT2ARs in mice exposed to a novel open-field arena affects medial PFC activation and basolateral amygdala (BLA) reactivity. We used c-Fos immunoreactivity (IR) as a marker of neuronal activation and stereological quantification for obtaining the total number of c-Fos-IR neurons as a measure of regional activation. We further examined the impact of 5-HT2AR blockade on the striatal-projecting BLA neurons. Systemic administration of ketanserin (0.5mg/kg) prior to novel open-field exposure resulted in reduced total numbers of c-Fos-IR cells in dorsomedial PFC areas and the BLA. Moreover, there was a positive correlation between the relative time spent in the centre of the open-field and BLA c-Fos-IR in the ketanserin-treated animals. Unilateral medial PFC lesions blocked this effect, ascertaining an involvement of this frontal cortex area. On the other hand, medial PFC lesioning exacerbated the more anxiogenic-like behaviour of the ketanserin-treated animals, upholding its involvement in modulating averseness. Ketanserin did not affect the number of activated striatal-projecting BLA neurons (measured by number of Cholera Toxin b (CTb) retrograde labelled neurons also being c-Fos-IR) following CTb injection in the ventral striatum. These results support a role of 5-HT2AR activation in modulating mPFC and BLA activation during exposure to a novel environment, which may be interrelated. Conversely, 5-HT2AR blockade does not seem to affect the amygdala-striatal projection.

The Fabric of Meaning and Subjective Effects in LSD-Induced States Depend on Serotonin 2A Receptor Activation

A core aspect of the human self is the attribution ofpersonal relevance to everyday stimuli enabling us to experience our environment as meaningful [1]. However, abnormalities in the attribution of personal relevance to sensory experiences are also critical features of many psychiatric disorders[2, 3]. Despite their clinical relevance, the neurochemical and anatomical substrates enabling meaningful experiences are largely unknown. Therefore, we investigated the neuropharmacology of personal relevance processing in humans by combining fMRI and the administration of the mixed serotonin (5-HT) and dopamine receptor (R) agonist lysergic acid diethylamide (LSD), well known to alter the subjective meaning of percepts, with and without pretreatment with the 5-HT2AR antagonist ketanserin. General subjective LSD effects were fully blocked by ketanserin. In addition, ketanserin inhibited the LSD-induced attribution of personal relevance to previously meaningless stimuli and modulated the processing of meaningful stimuli in cortical midline structures. These findings point tothe crucial role of the 5-HT2AR subtype andcortical midline regions in the generation and attribution of personal relevance. Our results thus increase our mechanistic understanding of personal relevance processing and reveal potential targets for the treatment of psychiatric illnesses characterized by alterations in personal relevance attribution.

Cane Toad Skin Extract-Induced Upregulation and Increased Interaction of Serotonin 2A and D2 Receptors via Gq/11 Signaling Pathway in CLU213 Cells.

Recent evidences show that activation of serotonin 2A receptors (5-HT2A R) by agonists is significant in improving therapeutic activity of disease conditions, such as obsessive-compulsive disorder (OCD). Though the exact molecular mechanism is still not well understood, it is thought to involve agonist-driven, enhanced expression of 5-HT2A R in certain areas of brain, such as the pre-frontal cortex (PFC). Several other reports have also demonstrated association of OCD with lower dopamine receptor (D2 R) availability, primarily in the striatum of the brain along with dysfunction of 5-HT2A R-D2 R heteromer regulation. We thus hypothesized that compound(s) interacting with this molecular mechanism could be developed as drugs for long-term beneficial effects against OCD. In the present study, we have obtained experimental evidence in cultured neuronal cells (CLU213) that aqueous extract (AE, 50 μg/mL, P < 0.05) of the Australian cane toad skin significantly increased the levels of 5-HT2A R and D2 R protein and mRNA expression. AE was also found to enhance the interaction between 5-HT2A R and D2 R and formation of expression of 5-HT2A R-D2 R heteromer using co-immunoprecipitation and Western blot. Further investigation showed the involvement of classical signaling pathway (Gq/11 -PLCβ) along with c-FOS transcription factor preferentially in 5-HT2A -mediated agonist activation. These results obtained demonstrated that AE upregulates 5-HT2A R by a mechanism that appears to involve Gq/11 -PLCβ signaling pathway and c-FOS transcription factor activation. We indicate this enhanced 5-HT2A R and D2 R expression and their interaction to induce increased 5-HT2A R-D2 R heteromer formation by exposure to AE might provide a molecular mechanism to develop potential novel drug candidates to ameliorate OCD symptoms. J. Cell. Biochem. 118: 979-993, 2017. © 2016 Wiley Periodicals, Inc.

Inhibition of alpha oscillations through serotonin-2A receptor activation underlies the visual effects of ayahuasca in humans

Ayahuasca is an Amazonian psychotropic plant tea typically obtained from two plants, Banisteriopsis caapi and Psychotria viridis. It contains the psychedelic 5-HT2A and sigma-1 agonist N,N-dimethyltryptamine (DMT) plus β-carboline alkaloids with monoamine-oxidase (MAO)-inhibiting properties. Although the psychoactive effects of ayahuasca have commonly been attributed solely to agonism at the 5-HT2A receptor, the molecular target of classical psychedelics, this has not been tested experimentally. Here we wished to study the contribution of the 5-HT2A receptor to the neurophysiological and psychological effects of ayahuasca in humans. We measured drug-induced changes in spontaneous brain oscillations and subjective effects in a double-blind randomized placebo-controlled study involving the oral administration of ayahuasca (0.75mg DMT/kg body weight) and the 5-HT2A antagonist ketanserin (40mg). Twelve healthy, experienced psychedelic users (5 females) participated in four experimental sessions in which they received the following drug combinations: placebo+placebo, placebo+ayahuasca, ketanserin+placebo and ketanserin+ayahuasca. Ayahuasca induced EEG power decreases in the delta, theta and alpha frequency bands. Current density in alpha-band oscillations in parietal and occipital cortex was inversely correlated with the intensity of visual imagery induced by ayahuasca. Pretreatment with ketanserin inhibited neurophysiological modifications, reduced the correlation between alpha and visual effects, and attenuated the intensity of the subjective experience. These findings suggest that despite the chemical complexity of ayahuasca, 5-HT2A activation plays a key role in the neurophysiological and visual effects of ayahuasca in humans.

Activation of serotonin2A receptors in the medial septum-diagonal band of Broca complex enhanced working memory in the hemiparkinsonian rats

Serotonin2A (5-HT2A) receptors are highly expressed in the medial septum-diagonal band of Broca complex (MS-DB), especially in parvalbumin (PV)-positive neurons linked to hippocampal theta rhythm, which is involved in cognition. Cognitive impairments commonly occur in Parkinson's disease. Here we performed behavioral, electrophysiological, neurochemical and immunohistochemical studies in rats with complete unilateral 6-hydroxydopamine lesions of the medial forebrain bundle (MFB) to assess the importance of dopamine (DA) depletion and MS-DB 5-HT2A receptors for working memory. The MFB lesions resulted in working memory impairment and decreases in firing rate and density of MS-DB PV-positive neurons, peak frequency of hippocampal theta rhythm, and DA levels in septohippocampal system and medial prefrontal cortex (mPFC) compared to control rats. Intra-MS-DB injection of high affinity 5-HT2A receptor agonist TCB-2 enhanced working memory, increased firing rate of PV-positive neurons and peak frequency of hippocampal theta rhythm, elevated DA levels in the hippocampus and mPFC, and decreased 5-HT level in the hippocampus in control and lesioned rats. Compared to control rats, the duration of the excitatory effect produced by TCB-2 on the firing rate of PV-positive neurons was markedly shortened in lesioned rats, indicating dysfunction of 5-HT2A receptors. These findings suggest that unilateral lesions of the MFB in rats induced working memory deficit, and activation of MS-DB 5-HT2A receptors enhanced working memory, which may be due to changes in the activity of septohippocampal network and monoamine levels in the hippocampus and mPFC.

Novelty-induced activity-regulated cytoskeletal-associated protein (Arc) expression in frontal cortex requires serotonin 2A receptor activation

Many psychiatric disorders are characterized by cognitive and emotional alterations that are related to abnormal function of the frontal cortex (FC). FC is involved in working memory and decision making and is activated following exposure to a novel environment. The serotonin 2A receptor (5-HT(2A)R) is highly expressed in the FC where its activation induces hallucinations, while blockade of 5-HT(2A)Rs contributes to the therapeutic effects of atypical antipsychotic drugs. The purpose of the present study was to investigate the involvement of 5-HT(2A)R in FC activation following exposure to a novel environment. As an output of FC activation we measured expression of activity-regulated cytoskeletal-associated protein (Arc). Novelty-exposure (open-field arena) robustly up-regulated FC Arc mRNA expression (∼160%) in mice compared to home-cage controls. This response was inhibited with the 5-HT(2A)R antagonists ketanserin and MDL100907, but not with the selective 5-HT(2C)R antagonist SB242084. Novelty-exposure also induced Arc mRNA expression in hippocampus (∼150%), but not in cerebellum or brainstem. Pretreatment with 5-HT(2A)R antagonist ketanserin did not repress the Arc induction in hippocampus, indicating that the involvement of 5-HT(2A)R in this response is restricted to the FC. Similarly, the novelty-induced stress as determined by increasing levels of plasma corticosterone, was not influenced by 5-HT(2A)R antagonism suggesting that Arc mRNA and stress are activated via distinct mechanisms. Taken together, our results demonstrate that the induction of Arc in the FC following exposure to a novel environment is dependent on the 5-HT(2A)R, and that the simultaneous release of corticosterone is regulated via another system independent of 5-HT(2A)R activation.

Drug-induced serotonin syndrome: a review

Serotonin syndrome, or serotonin toxicity (ST), is a clinical condition that occurs as a result of an iatrogenic drug-induced increase in intrasynaptic serotonin levels primarily resulting in activation of serotonin2A receptors in the central nervous system. The severity of symptoms spans a spectrum of toxicity that correlates with the intrasynaptic serotonin concentration. Although numerous drugs have been implicated in ST, life-threatening cases generally occur only when monoamine oxidase inhibitors are combined with either selective or nonselective serotonin re-uptake inhibitors. The triad of clinical features consists of neuromuscular hyperactivity, autonomic hyperactivity and altered mental status, which may present abruptly and progress rapidly. The awareness of ST is crucial not only in avoiding the unintentional lethal combination of therapeutic drugs but also in recognizing the clinical picture when it occurs so that treatment can be promptly initiated. In this review, the pathophysiology, clinical features, implicated drugs, diagnosis and treatment of ST are discussed.

Point:Counterpoint: Medullary pacemaker neurons are essential for both eupnea and gasping in mammals vs. medullary pacemaker neurons are essential for gasping, but not eupnea, in mammals

to the editor: Implicit in the commentaries concerning the possible role of medullary pacemakers in the neurogenesis of eupnea and gasping is one critical and recurring question: How are in vitro rhythms related to these classical patterns of automatic ventilatory activity? Suzue ([5][1]) considered

P.3.073 Bifeprunox: providing a new option for treating psychosis

Background: The disruption of prepulse inhibition of acoustic startle (PPI) is an animal model for some aspects of schizophrenia. Phencyclidine causes psychotomimetic symptoms in human and disrupts PPI in animals, however, the mechanism underlying this disruption remains unclear. The present experiment tested the hypothesis that serotonin 2A receptor blocking property of drugs reverses the phencyclidine-induced PPI disruption.Methods: The ED50 value of spiperone, haloperidol, chlorpromazine, clozapine, risperidone, olanzapine, seroquel, pipamperone, mianserin, or desipramine to reverse the phencyclidine- or apomorphine-induced PPI disruption in rats was determined. Then the correlation between the ED50 value and the affinity for the serotonin 2A, 2C, dopamine D2, or α-1 receptor of each drug was examined.Results: The ED50 value of the drugs to reverse the phencyclidine-induced PPI disruption was significantly correlated with the affinity for the serotonin 2A receptor, but not for the dopamine D2, serotonin 2C, or α-1 receptor of each drug. In contrast, the ED50 value of the drugs to reverse the apomorphine-induced PPI disruption was significantly correlated with the affinity for the dopamine D2 receptor, but not for the serotonin 2A receptor.Conclusions: An activation of serotonin 2A receptors would mediate the phencyclidine-induced PPI disruption.

Involvement of serotonin 2A receptors in phencyclidine-induced disruption of prepulse inhibition of the acoustic startle in rats

Background: The disruption of prepulse inhibition of acoustic startle (PPI) is an animal model for some aspects of schizophrenia. Phencyclidine causes psychotomimetic symptoms in human and disrupts PPI in animals, however, the mechanism underlying this disruption remains unclear. The present experiment tested the hypothesis that serotonin 2A receptor blocking property of drugs reverses the phencyclidine-induced PPI disruption. Methods: The ED 50 value of spiperone, haloperidol, chlorpromazine, clozapine, risperidone, olanzapine, seroquel, pipamperone, mianserin, or desipramine to reverse the phencyclidine- or apomorphine-induced PPI disruption in rats was determined. Then the correlation between the ED50 value and the affinity for the serotonin 2A, 2C, dopamine D 2, or α-1 receptor of each drug was examined. Results: The ED 50 value of the drugs to reverse the phencyclidine-induced PPI disruption was significantly correlated with the affinity for the serotonin 2A receptor, but not for the dopamine D 2, serotonin 2C, or α-1 receptor of each drug. In contrast, the ED 50 value of the drugs to reverse the apomorphine-induced PPI disruption was significantly correlated with the affinity for the dopamine D 2 receptor, but not for the serotonin 2A receptor. Conclusions: An activation of serotonin 2A receptors would mediate the phencyclidine-induced PPI disruption.

Calcium-mediated intracellular messengers modulate the serotonergic effects on axonal excitability

We carried out experiments to investigate the mechanisms of serotonin-induced axonal excitability changes using isolated dorsal columns from young (seven to 11-day-old) Long–Evan's hooded rats. Conducting action potentials were activated by submaximal (50%) and supramaximal constant current electrical stimuli and recorded with glass micropipette electrodes. In experiment 1, to study Ca 2+-mediated mechanisms, we superfused the preparations with Ringer solutions containing varying Ca 2+ concentrations. Following superfusion with Ca 2+-free Ringer solution for 4 h, we tested initial responses to serotonin agonists. Studies then were repeated after preparations had been washed for 1 h with Ringer solution containing 1.5 mM Ca 2+ and 1.5 mM Mg 2+. After 4 h superfusion of Ca 2+-free Ringer solution, quipazine (a serotonin 2A agonist, 100 μM) did not induce significant axonal excitability changes (amplitude change of 1.4±1.3%, percentage of predrug control level, ±S.D., n=6). A 100 μM concentration of 8-hydroxy-dipropylaminotetralin (a serotonin 1A agonist) reduced response amplitudes by 36.3±4.2% (±S.D., P<0.0005, n=7) and prolonged latencies by 22.3±4.3% (±S.D., P<0.0005, n=7). Application of serotonin (100 μM) decreased amplitudes by 6.6±5.0% (±S.D., P<0.05, n=6). Extracellular calcium concentration ([Ca 2+] e) was measured at various depths in the dorsal column with ion-selective microelectrodes. Four hours' superfusion with Ca 2+-free Ringer solution reduced [Ca 2+] e to less than 0.1 mM in dorsal columns. In 1.5 mM Ca 2+ Ringer solution, quipazine increased the amplitudes by 38.3±5.8% ( P<0.0005, n=6). Likewise, serotonin increased the amplitudes by 13.8±4.9% ( P<0.005, n=6). In contrast however, 8-hydroxy-dipropylaminotetralin still reduced amplitudes by 35.0±6.4% ( P<0.0005, n=7) and prolonged latencies by 24.1±4.5% ( P<0.0005, n=7). In experiment 2, we investigated calcium-dependent and cAMP-mediated protein kinase signalling pathways to evaluate their role as intracellular messengers for serotonin 2A receptor activation. Two protein kinase inhibitors, 50 μM H7 (an inhibitor of protein kinase C and c-AMP dependent protein kinase) and 100 μM d-sphingosine (an inhibitor of protein kinase A and C) effectively eliminated the excitatory effects of the serotonin 2A agonist. 100 μM cadmium (a Ca 2+ channel blocker) also blocked the effects of quipazine. Neither these protein kinase inhibitors nor cadmium alone affected action potential amplitudes. These results suggest that replacing Ca 2+ with Mg 2+ blocks the excitatory effects of quipazine but does not prevent the inhibitory effects of 8-hydroxy-dipropylaminotetralin, and calcium-mediated protein kinase mechanisms modulate axonal excitability changes induced by serotonin and its agonist.

Persisting sensitization of the behavioural response to formalin-induced injury in the rat through activation of serotonin 2A receptors

Serotonin potentiates pain induced by intraplantar injection of other inflammatory mediators through the 5-hydroxytryptamine 2A receptor, and 5-hydroxytryptamine 2A antagonists dose-dependently block pain induced by formalin. The present study examined the temporal characteristics of 5-hydroxytryptamine-mediated potentiation of pain in the rat. Simultaneous injection of the 5-hydroxytryptamine 2 receptor agonist α-methyl-5-hydroxytryptamine (10 μg) plus prostaglandin E 2 (0.1 μg) produced 10 min of lifting and licking of the injected paw. When α-methyl-5-hydroxytryptamine was injected before prostaglandin E 2, the response decreased as the interval increased, and by 1 h, it was not different from injection of prostaglandin E 2 alone. When prostaglandin E 2 was injected prior to α-methyl-5-hydroxytryptamine, sensitization to the latter persisted for more than 3 h. It was blocked by the 5-hydroxytryptamine 2 antagonist ketanserin (5 μg), injected locally 5 min before α-methyl-5-hydroxytryptamine. Ketanserin (5 μg) or the 5-hydroxytryptamine 1A/2A antagonist spiperone (0.3 μg), injected locally 5 min before α-methyl-5-hydroxytryptamine and prostaglandin E 2, reduced pain by 80–90%, but only by 40–50% when injected 2 min after. Formalin produced a biphasic pain response. The 5-hydroxytryptamine 2 antagonists, ketanserin (50 μg), spiperone (3 μg) and ritanserin (50 μg) injected 5 min before formalin reduced the pain response by 70, 90 and 74%, respectively, but only by 40, 30 and 24% when injected at the beginning of the second phase. Pretreatment of the paw with 2 μg indomethacin did not alter the response to α-methyl-5-hydroxytryptamine plus prostaglandin E 2, indicating that the sensitization was not due to prostaglandin synthesis. These data show that 5-hydroxytryptamine sensitizes tissue to the pain-producing actions of other inflammatory mediators, and that the sensitization produces a persisting change in tissue that can be blocked by pretreatment with an antagonist, but not reversed after it has occurred. The data imply that 5-hydroxytryptamine 2A antagonists may act prophylactically to prevent the evolution of pain in injured tissue, but do not reduce already-present pain.

P-22-17 - Neuroleptic treatment of psychotic symptoms in HIV seropositive patients

Background: The disruption of prepulse inhibition of acoustic startle (PPI) is an animal model for some aspects of schizophrenia. Phencyclidine causes psychotomimetic symptoms in human and disrupts PPI in animals, however, the mechanism underlying this disruption remains unclear. The present experiment tested the hypothesis that serotonin 2A receptor blocking property of drugs reverses the phencyclidine-induced PPI disruption.Methods: The ED50 value of spiperone, haloperidol, chlorpromazine, clozapine, risperidone, olanzapine, seroquel, pipamperone, mianserin, or desipramine to reverse the phencyclidine- or apomorphine-induced PPI disruption in rats was determined. Then the correlation between the ED50 value and the affinity for the serotonin 2A, 2C, dopamine D2, or α-1 receptor of each drug was examined.Results: The ED50 value of the drugs to reverse the phencyclidine-induced PPI disruption was significantly correlated with the affinity for the serotonin 2A receptor, but not for the dopamine D2, serotonin 2C, or α-1 receptor of each drug. In contrast, the ED50 value of the drugs to reverse the apomorphine-induced PPI disruption was significantly correlated with the affinity for the dopamine D2 receptor, but not for the serotonin 2A receptor.Conclusions: An activation of serotonin 2A receptors would mediate the phencyclidine-induced PPI disruption.