2A Receptor Function Research Articles

Neuronal ablation of p-Akt at Ser473 leads to altered 5-HT1A/2A receptor function

The serotonergic system regulates a wide range of behavior, including mood and impulsivity, and its dysregulation has been associated with mood disorders, autism spectrum disorder, and addiction. Diabetes is a risk factor for these conditions. Insulin resistance in the brain is specifically associated with susceptibility to psychostimulant abuse. Here, we examined whether phosphorylation of Akt, a key regulator of the insulin signaling pathway, controls serotonin (5-HT) signaling. To explore how impairment in Akt function regulates 5-HT homeostasis, we used a brain-specific rictor knockout (KO) mouse model of impaired neuronal phosphorylation of Akt at Ser473. Cortical 5-HT1A and 5-HT2A receptor binding was significantly elevated in rictor KO mice. Concomitant with this elevated receptor expression, the 5-HT1A receptor agonist 8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) led to an increased hypothermic response in rictor KO mice. The increased cortical 5-HT1A receptor density was associated with higher 5-HT1A receptor levels on the cortical cell surface. In contrast, rictor KO mice displayed significantly reduced head-twitch response (HTR) to the 5-HT2A/C agonist 2,5-dimethoxy-4-iodoamphetamine (DOI), with evidence of impaired 5-HT2A/C receptor signaling. In vitro, pharmacological inhibition of Akt significantly increased 5-HT1A receptor expression and attenuated DOI-induced 5-HT2A receptor signaling, thereby lending credence to the observed in vivo cross-talk between neuronal Akt signaling and 5-HT receptor regulation. These data reveal that defective central Akt function alters 5-HT signaling as well as 5-HT-associated behaviors, demonstrating a novel role for Akt in maintaining neuronal 5-HT receptor function.

Differential regulation of 5-HT 2A receptor mRNA expression following withdrawal from a chronic escalating dose regimen of d-amphetamine

Several lines of evidence indicate that psychostimulant withdrawal can induce negative emotional symptoms, such as anhedonia and dysphoria, which may be due in part, to dysfunction of the serotonin (5-HT) system, including alterations in 5-HT receptors. For example, changes in 5-HT 2A receptor function in prefrontal cortex (PFC) have been reported in association with psychostimulant withdrawal. However, it is not known if alterations in 5-HT 2A receptor mRNA expression occur in the PFC or other limbic-associated areas following withdrawal from chronic psychostimulant treatment. The goal of the current study was to determine the effects of chronic, escalating doses of d-amphetamine ( d-AMPH) and withdrawal on the expression of 5-HT 2A receptors in the cortex, caudate putamen, NAc and hippocampus of rat brain. Animals were treated three times a day for 4 days with escalating doses of d-AMPH (1–10 mg/kg). Twenty-four hours after the final dose of d-AMPH, animals were sacrificed and the tissue processed for in situ hybridization histochemistry. Chronic, escalating doses of d-AMPH, followed by a 24 h withdrawal period, significantly decreased 5-HT 2A receptor mRNA expression in the prefrontal, motor and cingulate cortices, while 5-HT 2A receptor mRNA expression in the NAc, caudal CPu and hippocampus were significantly increased. These data indicate that region-specific changes in 5-HT 2A receptor mRNA expression occur in limbic system and associated areas following chronic d-AMPH treatment, supporting the notion that alterations in the 5-HT system may contribute to the negative emotional aspects of psychostimulant withdrawal.

Nucleus accumbens and effort-related functions: behavioral and neural markers of the interactions between adenosine A 2A and dopamine D 2 receptors

Nucleus accumbens dopamine (DA) is a critical component of the brain circuitry regulating work output in reinforcement-seeking behavior and effort-related choice behavior. Moreover, there is evidence of an interaction between DA D 2 and adenosine A 2A receptor function. Systemic administration of adenosine A 2A antagonists reverses the effects of D 2 antagonists on tasks that assess effort related choice. The present experiments were conducted to determine if nucleus accumbens is a brain locus at which adenosine A 2A and DA D 2 antagonists interact to regulate effort-related choice behavior. A concurrent fixed ratio 5 (FR5)/chow feeding procedure was used; with this procedure, rats can choose between completing an FR5 lever-pressing requirement for a preferred food (i.e., high carbohydrate operant pellets) or approaching and consuming a freely available food (i.e., standard rodent chow). Rats trained with this procedure spend most of their time pressing the lever for the preferred food, and eat very little of the concurrently available chow. Intracranial injections of the selective DA D 2 receptor antagonist eticlopride (1.0, 2.0, 4.0 μg) into nucleus accumbens core, but not a dorsal control site, suppressed FR5 lever-pressing and increased consumption of freely available chow. Either systemic or intra-accumbens injections of the adenosine A 2A receptor antagonist MSX-3 reversed these effects of eticlopride on effort-related choice. Intra-accumbens injections of eticlopride also increased local expression of c-Fos immunoreactivity, and this effect was attenuated by co-administration of MSX-3. Adenosine and DA systems interact to regulate instrumental behavior and effort-related processes, and nucleus accumbens is an important locus for this interaction. These findings may have implications for the treatment of psychiatric symptoms such as psychomotor slowing, anergia and fatigue.

Effects of Imipramine and Bupropion on the Duration of Immobility of ACTH-Treated Rats in the Forced Swim Test: Involvement of the Expression of 5-HT<sub>2A</sub> Receptor mRNA

We examined the effect of chronic administration of imipramine and bupropion, monoamine reuptake inhibitors, on the duration of immobility in the forced swim test and serotonin (5-HT)(2A) receptor function in the form of 5-HT(2A) receptor mRNA levels in rats chronically treated with adrenocorticotropic hormone (ACTH). The immobility-decreasing effect of bupropion without imipramine did not influence the chronic ACTH treatment. The effect on the expression of 5-HT(2A) receptor mRNA of chronic ACTH treatment was decreased by bupropion, but not imipramine. These results suggest that bupropion has the effect of reducing immobility time in the forced swim test in the tricyclic antidepressant-resistant depressive model induced by chronic ACTH treatment in rats, and that decreased 5-HT(2A) receptor mRNA levels may be involved in this phenomenon.

Evidence that genetic variation in 5-HT transporter expression is linked to changes in 5-HT 2A receptor function

Variability in expression of the 5-HT transporter (5-HTT) gene in the human population has been associated with a range of behavioural phenotypes. The underlying mechanisms are unclear but may involve changes in 5-HT receptor levels and/or signalling. The present study used a novel 5-HTT overexpressing transgenic mouse to test the hypothesis that variability in 5-HTT expression may alter 5-HT 2A receptor function. In wildtype mice, the 5-HT 2 receptor agonist DOI increased regional brain mRNA expression of two immediate early genes ( c-fos and Arc), and induced head twitches, and both effects were abolished by pre-treatment with the 5-HT 2A receptor antagonist MDL 100907. In 5-HTT overexpressing mice, DOI induced a greater increase in both c-fos and Arc mRNA expression in cortical brain regions, and more head twitches, compared to wildtype mice. Autoradiographic and in situ hybridisation experiments showed that 5-HT 2A receptor binding sites and 5-HT 2A receptor mRNA did not differ between transgenic and wildtype mice. Finally, the transgenic mice had lower regional brain 5-HT levels compared to wildtype mice. This depletion of 5-HT may underpin the increase in 5-HT 2A receptor function because in wildtype mice 5-HT depletion using the 5-HT synthesis inhibitor, p-chlorophenylalanine, enhanced the head twitch response to DOI. These data demonstrate that elevated 5-HTT expression is accompanied by increased 5-HT 2A receptor function, an effect possibly mediated by decreased availability of synaptic 5-HT. Variation in levels of 5-HTT expression may therefore be a source of variability in 5-HT 2A receptor function, which may be an important modifier of 5-HTT-linked phenotypes.

Characterization of the antiparkinsonian effects of the new adenosine A 2A receptor antagonist ST1535: Acute and subchronic studies in rats

Antagonism of adenosine A 2A receptor function has been proposed as an effective therapy in the treatment of Parkinson's disease. Thus, the study of new adenosine receptor antagonists is of great importance for the potential use of these drugs in clinical practice. The present study evaluated effects of the new preferential adenosine A 2A receptor antagonist 2-butyl-9-methyl-8-(2 H-1,2,3-triazol-2-yl)-9 H-purin-6-ylamine (ST1535) in unilaterally 6-hydroxydopamine lesioned rats. Acute ST1535 dose-dependently potentiated contralateral turning behaviour induced by a threshold dose of l-3,4-dihydroxyphenylalanine ( l-DOPA) (3 mg/kg i.p.), a classical test for antiparkinson drug screening. Subchronic (18 days, twice a day) ST1535 (20 mg/kg i.p.) + l-DOPA (3 mg/kg i.p.) did not induce sensitization to turning behaviour or abnormal involuntary movements during the course of treatment, indicating a low dyskinetic potential of the drug. Moreover, while subchronic administration of a fully effective dose of l-DOPA (6 mg/kg i.p.) significantly increased GABA synthesizing enzyme glutamic acid decardoxylase (GAD67), dynorphin and enkephalin mRNA levels in the lesioned striatum, subchronic ST1535 (20 mg/kg i.p.) + l-DOPA (3 mg/kg i.p.) did not modify any of these markers, although it induced a similar number of contralateral rotations at the beginning of treatment. Finally, acute administration of ST1535 (20 mg/kg i.p.) proved capable of reducing jaw tremors in tacrine model of Parkinson's disease tremor. Results showed that ST1535, in association with a low dose of l-DOPA, displayed antiparkinsonian activity similar to that produced by a full dose of l-DOPA without exacerbating abnormal motor side effects. Moreover, in agreement to other well characterized adenosine A 2A receptor antagonists, ST1535 features antitremorigenic effects.

Cocaine-mediated supersensitivity of 5-HT 2A receptors in hypothalamic paraventricular nucleus is a withdrawal-induced phenomenon

We previously reported that treatment and withdrawal from cocaine increases: (1) 5-HT 2A receptor-mediated neuroendocrine responses, and (2) Gα q and Gα 11 G-protein levels in the hypothalamic paraventricular nucleus (PVN) at 48 h post-treatment. This study investigates changes in the initial 24 h of withdrawal to discern whether 5-HT 2A receptor supersensitivity is due to cocaine treatment or is induced during the withdrawal period. We report here increases in 5-HT 2A receptor-mediated neuroendocrine responses only 12 or 24 h post-treatment, but not during the initial 4 h withdrawal period. Levels of membrane- or cytosol-associated Gα q or Gα 11 proteins in PVN are not altered during the first 24 h of withdrawal. However, the density of 125I-(−)-1-(2,5 dimethoxy-4-iodophenyl)-2-amino-propane HCl (DOI)-labeled high-affinity 5-HT 2A receptors in PVN increased 35% in rats withdrawn from cocaine for 24 h. These findings demonstrate that cocaine-induced increases in 5-HT 2A receptor function in PVN represents a withdrawal-induced phenomena that: (1) is likely attributed to increased G-protein coupled/high-affinity conformational state of the 5-HT 2A receptor, and (2) occurs in the absence of changes in the levels of associated G proteins during the first 24 h.

Desensitization of 5-HT 2A receptor function by chronic administration of selective serotonin reuptake inhibitors

We have previously shown that chronic treatment with selective serotonin reuptake inhibitors (SSRIs), fluvoxamine and paroxetine, attenuated m-chlorophenylpiperazine ( mCPP)-induced hypolocomotion in rats. The effect of these SSRIs on the response to mCPP is thought to be caused by the desensitization of 5-HT 2C receptor function. In the present study, we investigated whether chronic administration of SSRI could reduce another pharmacological response to mCPP in rats, i.e., the induction of the secretion of corticosterone. The mCPP-induced increase in the serum concentration of corticosterone was not blocked by the 5-HT 2C antagonist SB242084, but was blocked by the 5-HT 2A antagonist ketanserin. Chronic treatment with fluvoxamine and paroxetine attenuated the response to mCPP, while these SSRIs had no effects in control rats. These results suggest that the desensitization of 5-HT 2A receptor function occurs in the same way as that of 5-HT 2C receptor function through chronic treatment with either fluvoxamine or paroxetine as a consequence of prolonged exposure to elevated levels of serotonin. The hypersensitivity of 5-HT 2A receptors is observed in depressed patients, and chronic treatment with many antidepressants such as tricyclic antidepressants have been reported to reduce 5-HT 2A receptor density and/or efficacy. The desensitization of 5-HT 2A receptor function might contribute to the therapeutic mechanism of action of these SSRIs, as seen with other classes of antidepressants.

Suppressive effects of isorhynchophylline on 5-HT 2A receptor function in the brain: Behavioural and electrophysiological studies

Isorhynchophylline is a major oxindole alkaloid found in Uncaria species which have long been used in traditional Chinese medicine. Here, we investigated the effects of isorhynchophylline and isorhynchophylline-related alkaloids on 5-hydroxytryptamine (5-HT) receptor-mediated behavioural responses in mice and 5-HT-evoked current responses in Xenopus oocytes expressing 5-HT 2A or 5-HT 2C receptors. Isorhynchophylline dose-dependently inhibited 5-HT 2A receptor-mediated head-twitch but not 5-HT 1A receptor-mediated head-weaving responses evoked by 5-methoxy-N,N-dimethyltryptamine. Pretreatment with reserpine, a monoamine-depleting agent, enhanced the head-twitching, but did not influence the effect of isorhynchophylline on the behavioural response. Isocorynoxeine, an isorhynchophylline-related alkaloid in which the configuration of the oxindole moiety is the same as in isorhynchophylline, also reduced the head-twitch response in reserpinized mice over the same dose range as isorhynchophylline, while both rhynchophylline and corynoxeine, stereoisomers of isorhynchophylline and isocorynoxeine, did not. None of the alkaloids tested had an effect on meta-chlorophenylpiperazine-induced hypolocomotion, a 5-HT 2C receptor-mediated behavioural response. In experiments in vitro, isorhynchophylline and isocorynoxeine dose-dependently and competitively inhibited 5-HT-evoked currents in Xenopus oocytes expressing 5-HT 2A receptors, but had less of a suppressive effect on those in oocytes expressing 5-HT 2C receptors. These results indicate that isorhynchophylline and isocorynoxeine preferentially suppress 5-HT 2A receptor function in the brain probably via a competitive antagonism at 5-HT 2A receptor sites and that the configuration of the oxindole moiety of isorhynchophylline is essential for their antagonistic activity at the 5-HT 2A receptor.

Increased 5-HT 2A receptor expression and function following central glucocorticoid receptor knockdown in vivo

Central glucocorticoid receptor function may be reduced in depression. In vivo modelling of glucocorticoid receptor underfunctionality would assist in understanding its role in depressive illness. The role of glucocorticoid receptors in modulating 5-HT 2A receptor expression and function in the central nervous system (CNS) is presently unclear, but 5-HT 2A receptor function also appears altered in depression. With the aid of RNAse H accessibility mapping, we have developed a 21-mer antisense oligodeoxynucleotide (5′-TAAAAACAGGCTTCTGATCCT-3′, termed GRAS-5) that showed 56% reduction in glucocorticoid receptor mRNA and 80% down-regulation in glucocorticoid receptor protein in rat C6 glioma cells. Sustained delivery to rat cerebral ventricles in slow release biodegradable polymer microspheres produced a marked decrease in glucocorticoid receptor mRNA and protein in hypothalamus (by 39% and 80%, respectively) and frontal cortex (by 26% and 67%, respectively) 5 days after a single injection, with parallel significant up-regulation of 5-HT 2A receptor mRNA expression (13%) and binding (21%) in frontal cortex. 5-HT 2A receptor function, determined by DOI-head-shakes, showed a 55% increase. These findings suggest that central 5-HT 2A receptors are, directly or indirectly, under tonic inhibitory control by glucocorticoid receptor.

The atypical antipsychotic drug clozapine enhances chronic PCP-induced regulation of prefrontal cortex 5-HT2 A receptors

The ability of antipsychotic drugs to affect 5-HT 2A receptor function has been widely suggested to contribute to their therapeutic properties. We have compared the ability of the antipsychotic drugs clozapine and haloperidol, alone and in combination with chronic phencyclidine (PCP), to modulate 5-HT 2A receptor binding and mRNA. Acute (i.p. 45 min) and chronic (21-day) clozapine (osmotic minipump (OMP); 20 mg/kg/day) produced widespread decreases in 5-HT 2A receptor binding (−60%–80%), measured using [ 3H]ketanserin autoradiography. Conversely, 5-HT 2A mRNA levels, determined using in-situ hybridisation, were modestly increased by chronic clozapine treatment (+10%–30%). Chronic PCP treatment, at a dose (2.58 mg/kg i.p. intermittently for 28 days) that reproduces many of the neurochemical deficits of schizophrenia, decreased 5-HT 2A receptor binding in the prefrontal cortex (PFC; −16%), consistent with the changes in post-mortem brain tissue from schizophrenic patients. Combined chronic PCP (i.p.) and clozapine (OMP) treatment down-regulated 5-HT 2A receptor binding in many areas, similar to the effects of clozapine treatment alone and clozapine further enhanced the effects of PCP in the prefrontal cortex. In contrast 5-HT 2A mRNA was not altered. Haloperidol treatment alone (1 mg/kg/day; OMP) and in combination with PCP (i.p.), generally produced no changes in 5-HT 2A receptor protein or mRNA. Hence chronic PCP treatment, as employed here, mimics the decreased 5-HT 2A receptor binding observed in the PFC of schizophrenic patients. Clozapine’s enhancement of the natural response of PCP to down-regulate PFC 5-HT 2A receptors may contribute to it’s improved therapeutic profile against negative symptoms and cognitive deficits.

Prenatal cocaine exposure potentiates paroxetine-induced desensitization of 5-HT 2A receptor function in adult male rat offspring

This study investigated the ability of prenatal exposure to cocaine to alter serotonin 2A (5-HT 2A) receptor function and paroxetine-induced desensitization of 5-HT 2A receptor function in rat offspring. Following exposure to saline or (−)cocaine (15 mg/kg, s.c., b.i.d.), during gestational days 13–20, adult male offspring were treated with either saline or paroxetine (10 mg/kg/day, i.p. 14 days). Eighteen hours post-treatment, changes in the stimulation of oxytocin, adrenocorticotropic hormone (ACTH) and corticosterone by (−)4-iodo-2,5-dimethoxyphenylisopropylamine (DOI, 0.5 or 2.0 mg/kg, s.c.) and in 5-HT 2A receptor densities were determined. Prenatal cocaine exposure did not alter 5-HT 2A receptor-mediated neuroendocrine responses or 5-HT 2A receptor densities. In contrast, paroxetine treatment reduced cortical 5-HT 2A receptors (18–25%) and desensitized 5-HT 2A receptor-mediated oxytocin responses in both offspring groups. Furthermore, in cocaine offspring, paroxetine produced an inhibition of 5-HT 2A receptor-mediated increase in plasma ACTH levels and a greater attenuation of the oxytocin responses to (−)DOI. Paroxetine-induced reductions in body weight gain (−8.8%) were comparable in both offspring groups. These data, demonstrating that prenatal exposure to cocaine potentiates paroxetine-induced desensitization of 5-HT 2A receptor function, may be clinically relevant with respect to treating mood disorders in adults exposed in utero to cocaine.

Serotonin 5-HT 2A receptor gene polymorphism, 5-HT 2A receptor function and personality traits in healthy subjects: a negative study

Background: Central serotonin-2A (5-HT 2A) receptor dysfunction is regarded as an important factor in the etiology of affective disorders. The relations between some personality traits and the vulnerability of affective disorders are also implicated. Moreover, there are several reports which describe the association between 5-HT 2A receptor gene polymorphisms and mental disorders. We therefore examined the relationship between personality traits, the 5-HT 2A receptor function, and 5-HT 2A receptor gene polymorphisms. Methods: 5-HT-induced intraplatelet calcium (Ca) mobilization, 5-HT 2A receptor gene polymorphisms (A-1438G, T102C, T516C, C1340T, C1354T), and Temperament and Character Inventory (TCI) scores were examined in 133 healthy subjects. Results: Neither 5-HT-induced Ca mobilization nor 5-HT 2A receptor gene polymorphisms (A-1438G, T102C) appear to be associated with seven personality dimensions including Harm Avoidance. There was no significant difference in the Ca response among the subjects with −1438A/A, A/G and G/G genotypes. Since the appearance of the other types of the 5-HT 2A receptor gene polymorphisms (T516C, C1340T and C1354T) was quite rare in our sample, we were unable to examine the relationship between these polymorphisms, and the TCI score or the Ca response. Limitations: Our failure to find a significant association may reflect the false negative results due to the small sample size and low statistical power. Further studies in depressed patients may clarify the complicated relationship between personality traits and the vulnerability of affective disorders. Conclusions: Personality traits detected by TCI may not be directly related to the 5-HT 2A receptor function or 5-HT 2A receptor gene polymorphism which may be involved in the vulnerability of affective disorders.

Stimulus generalization by fenfluramine in a quipazine–ketanserin drug discrimination is not dependent on indirect serotonin release

The purpose of this study was to determine if animals trained to discriminate a serotonin 2A (5-HT 2A) receptor agonist from a 5-HT 2A receptor antagonist would also be sensitive to alterations in serotonin neurotransmission brought about by 5-HT reuptake inhibitors and releasers. Previous work from our laboratory has shown that the quipazine–ketanserin discrimination is mediated solely by the 5-HT 2A receptor, thus providing a behavioral continuum of 5-HT 2A receptor function. Rats were trained to discriminate quipazine (0.35 mg/kg) from ketanserin (1.0 mg/kg) on a variable interval-30 schedule of reinforcement. Following acquisition, substitution tests were conducted with the training drug, quipazine, and agents that have been shown to alter the synaptic levels of 5-HT, including fenfluramine, norfenfluramine, 5-methoxy-6-methyl-2-aminoindan (MMAI) and fluoxetine. All compounds substituted, except fluoxetine. Antagonist tests with mianserin and MDL 100,907 indicated that fenfluramine's and MMAI's substitution for quipazine was mediated by the 5-HT 2A receptor. Animals were pretreated with PCPA to determine whether 5-HT release or direct agonism mediated the discriminative stimulus effects of fenfluramine and MMAI. PCPA blocked the substitution of MMAI but not of fenfluramine for quipazine. Analysis of 3H-IP formation in cells showed that norfenfluramine dose-dependently stimulated phosphoinositide hydrolysis to levels similar to that of serotonin and quipazine. These results indicate that fenfluramine's substitution for quipazine in rats trained on a quipazine–ketanserin discrimination are due to direct agonism at the 5-HT 2A receptor likely mediated by norfenfluramine, an active metabolite.

Effect of acute lipopolysaccharide administration on (±)-1-(2,5-dimethoxy-4-iodophenyl)-2 aminopropane-induced wet dog shake behavior in rats: comparison with body weight change and locomotor activity

1. Several reports have shown that serotonin (5-HT) 2A receptor density and its function are altered after physiological or pharmacological stress. To examine whether an acute administration of lipopolysaccharide (LPS), a bacterial endotoxin, affected 5-HT 2A receptor function, wet dog shakes of male Wistar rats were observed after a subcutaneous injection of DOI, a 5-HT 2A receptor agonist following LPS treatment. Body weight change and locomotor activity were also observed. 2. DOI (1 mg/kg)-induced WDS significantly decreased after 400 or 1000 μg/kg LPS treatment compared with that of control rats 1 and 3 hr after injection, and WDS completely recovered 8 hr after LPS treatment. Treatment with 10 mg/kg indomethacin (IND) or 1 mg/kg naltrexone (NLTX) canceled the effect of 400 μg/kg LPS on DOI-induced WDS. 3. Body weight decrease was significantly greater in LPS-treated rats compared with control rats 3, 5 and 8 hr after treatment. Treatment with IND (10 mg/kg) significantly recovered the reduction in body weight induced by 400 μg/kg LPS. Treatment with NLTX (1 mg/kg) also prevented the LPS effect on body weight decrease. 4. Eight hr after treatment with LPS (400 μg/kg), the rats showed significant attenuation of locomotor activity. IND (10mg/kg) treatment abolished the inhibitory effect of LPS on locomotor activity, and NLTX (1 mg/kg) also improved the decrease in locomotion 8 hr after LPS treatment. 5. Plasma tumor necrosis factor (TNF)-α concentration dramatically increased 1 hr after the injection of 400 μg/kg LPS, and returned almost to the basal level 3 hr later. Next, rats were injected with 50 μg/kg TNF-α intraperitoneally, and body weight change and DOI-induced WDS was determined 3 hr after TNF-α injection. Body weight loss was significantly greater in rats treated with TNF-α. On the other hand, DOI-induced WDS was not altered when rats were treated with TNF-α. 6. These results suggest that acute treatment with LPS inhibited 5-HT 2A receptor-mediated behavior via cyclooxygenase and opioid receptor activation, but that the inhibition of the WDS by LPS appears to be independent of TNF-α production.

Modulation of adenosine concentration by opioid receptor agonists in rat striatum

There is evidence that adenosine and morphine interact in the striatum. However, little is known about the precise role of the opioid receptor subtypes implicated in the modulation of adenosine tissue concentration and in adenosine receptor expression and function. We sought to evaluate, in the absence of withdrawal symptoms, the effects of the short-term administration of selective μ-, δ- or κ-opioid receptor agonists on adenosine concentration and on adenosine A 2A receptor function in rat striatum. Adenosine A 2A receptor was chosen because the neuronal sub-population expressing this receptor coexpresses enkephalin, suggesting that adenosine A 2A receptor may be regulated by opioid receptor agonists. Oxymorphone hydrochloride (μ-opioid receptor agonist, 6 mg/kg/day), +[-(5α,7α,8β)-(−)- N-methyl- N(7-(1-pyrrolidinyl)1-oxaspiro (4.5)dec-8-yl) benzenacetamide] (U69593) (κ-opioid receptor agonist, 0.75 mg/kg/day), and (+)-4[(α R)-α-((2 S,5 R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]- N, N-diethylbenzamide) (SNC80) (δ-opioid receptor agonist, 9 mm/kg/day), or vehicle, were administered i.p 3× daily during 5 days to groups of rats ( n=6). We also investigated the effects of opioid receptor agonists on adenosine uptake by striatal cell extracts. We found that administration of μ- or δ-opioid receptor agonists significantly decreased adenosine uptake in striatal cell extracts and increased adenosine concentration (mean+24% and +45% for μ- and δ-opioid receptor agonist, respectively, relative to controls). None of the receptor agonists tested induced obvious modifications of adenosine A 2A receptor function. However, the δ-opioid receptor agonist induced an increase in adenosine A 2A mRNA expression (mean 44%). We conclude that μ and δ receptor agonists inhibit adenosine uptake by striatal cell extracts and increase adenosine concentrations in rat striatum.

Early postnatal cocaine exposure causes sequential, dose-dependent, enduring but reversible supersensitivity in 5-HT 2A receptor-mediated function during development in male mice

This report investigated whether postnatal exposure to cocaine affects the index of 5-HT 2A receptor function during development by utilizing the ability of the 5-HT 2A/C agonist DOI to induce the head-twitch response (HTR) in mice. Thus, several groups of mice litters were treated with varying doses of cocaine (0, 0.5, 1, 5, and 10 mg/kg, IP) twice daily from postnatal days 5 to 14. Then, different groups of cocaine-exposed male mice pups along with their corresponding age-matched vehicle-exposed control groups were HTR tested once during development on the following postnatal test days: 15, 16, 18, 20, 30, 45, and 60. The HTR testing involved administration of DOI (0.5 mg/kg, IP) and counting the frequency of the behavior for the next 20 min. Cocaine exposure caused bell-shaped, dose-dependent, enduring but reversible increase in DOI-induced HTR frequency (mean ± SEM) during development. The developing pups were most sensitive to low and intermediate doses of cocaine (0.5–5 mg/kg). The greatest degree of increase in HTR frequency in response to DOI challenge occurred in the 1 mg/kg cocaine-exposure group on most test days. The onset of HTR supersensitivity varied from 48 h (5 mg/kg) to 144 h (0.5 mg/kg) following the termination of chronic cocaine exposure. Moreover, maximal supersensitivity for the latter doses of cocaine occurred 96 and 384 h postcocaine treatment, respectively. Other cocaine exposure groups attained their maxima sometime between the latter time periods. The duration of persistence of 5-HT 2A receptor supersensitivity varied with different doses of cocaine: the 10-mg/kg group was supersensitive up to 384 h postcocaine treatment, the 1- and 5-mg/kg groups up to 744 h; and the 0.5-mg/kg group up to 1104 h. Although developmentally cocaine-exposed pups exhibit some similarities (i.e., exquisite sensitivity and bell-shaped dose–response) in 5-HT 2A receptor adaptation to mature adult mice exposed to cocaine, they also differ from mature adult cocaine-exposed mice in the onset of appearance as well as the enduring persistence of the induced supersensitivity.

Evidence for an increase in functional platelet 5-HT 2A receptors in depressed patients using the new ligand [ 125I]-DOI

Abnormalities in the serotonergic system have been implicated in the pathophysiology of depressive disorders. Human platelets possess serotonin-2A (5-HT 2A) receptors, and previous research using LSD or ketanserin as ligands have indicated that their number is increased in depressed patients. Compared to other ligands previously used in platelet studies, DOI is highly selective for the 5-HT 2A receptor and binds to its high-affinity state, therefore labeling only the receptors that are biologically coupled to the G-protein. We determined the density (Bmax) and the affinity (Kd) of 5-HT 2A receptors labeled by [ 125I]-DOI in platelets from 21 untreated patients with major depression and 21 healthy volunteers. The density of the 5-HT 2A binding sites was found to be increased in platelets from female depressed patients as compared to controls. No changes were observed in the Kd. We did not find any relationship between the binding parameters and either the severity of the depressive episode or the suicidal tendencies of the patients. Our results show that the number of coupled platelet 5-HT 2A receptors is increased in depressed patients, indicating that platelet 5-HT 2A receptor function is enhanced in depression.

Serotonin 2A Receptor Modulation of D 1 Dopamine Receptor-Mediated Grooming Behavior

We have previously observed that intracerebroventricular infusion of a 5-HT 2A receptor antisense oligonucleotide for 8 days results in an increase in cortical 5-HT 2A receptor sites and an increase in central 5-HT 2A receptor function as measured by quantitation of 5-HT 2A receptor-mediated headshake behavior (28). Because lesioning serotonergic neurons or chronic administration of 5-HT 2A receptor antagonists does not result in an increase in 5-HT 2A receptor density or function in the brain, we have taken advantage of this unique upregulation of 5-HT 2A receptors following 5-HT 2A receptor antisense oligonucleotide infusion to study the modulation of D 1 receptor-mediated behaviors by 5-HT 2A receptors. Grooming behavior, elicited by acute injection of SKF 38393, was attenuated after chronic ICV infusion of a 5-HT 2A receptor antisense oligonucleotide. There was also a reduction in vacuous chewing behavior induced by SKF 38393, which did not reach statistical significance. Other oral behaviors (i.e., tongue protrusions and gnawing at the cage bottom) were not attenuated. An increase in the density of cortical, as well as striatal 5-HT 2A receptor sites was observed following chronic antisense oligonucleotide administration. There was no change in striatal D 1 dopamine receptors following 5-HT 2A receptor antisense oligonucleotide administration. SKF 38393-induced grooming behavior was also attenuated in naive rats pretreated acutely with the 5-HT 2 receptor agonist DOI. These results suggest a role for the 5-HT 2A receptor in the modulation of D 1 receptor function.

The mechanism by which the selective 5-HT 1A receptor antagonist S-(−)UH 301 produces head-twitches in mice

Electrophysiological studies indicate that certain 5-HT 1A receptor antagonists increase the basal firing rate of some but not all raphe neurons by antagonizing the inhibitory endogenous serotonin tone operating on the somatodendritic pulse-modulating presynaptic 5-HT 1A autoreceptors. This effect should enhance the synaptic concentration of 5-HT (5-hydroxytryptamine) in serotonergic terminal fields, which may then activate postsynaptic 5-HT receptors. However, in vivo microdialysis studies show that generally such 5-HT 1A antagonists by themselves do not increase the basal 5-HT release but potentiate the ability of serotonin reuptake blockers to increase the neuronal serotonin terminal output in the rat brain via the above mechanism. The purpose of the present study was to determine whether antagonism of the proposed endogenous serotonin tone on the 5-HT 1A autoreceptors can potentiate the activity of other postsynaptic serotonin receptors. To this end, we utilized the head-twitch response (HTR) in mice as an in vivo model of postsynaptic 5-HT 2A receptor function. The selective and silent 5-HT 1A receptor antagonist, S-(−)UH 301, by itself, in a dose-dependent manner, produced the HTR in normal but not in reserpinized animals. The 5-HT 2A antagonist, SR 46349B, completely prevented S-(−)UH 301-induced HTR. Pretreatment with S-(−)UH 301 also potentiated 5-hydroxytryptophan (5-HTP)-induced HTR both in normal and in the reserpinized mice. At low doses (0.06–0.25 mg/kg), the 5-HT 1A selective agonist, 8-OH DPAT, significantly but partially inhibited 5-HTP-induced HTR. However, further attenuation was not observed following the administration of larger doses of 8-OH DPAT. Depending upon the dose used, S-(−)UH 301 pretreatment not only antagonized but also broke through the inhibitory effect of 8-OH DPAT on 5-HTP-induced HTR. The selective (sertraline) and nonselective (cocaine) serotonin reuptake blockers potentiated the ability of 5-HTP to induce the head-twitch behavior in mice. Pretreatment with S-(−)UH 301 enhanced the potentiating effect of serotonin reuptake blockers on the 5-HTP-induced HTR. These results suggest that an endogenous 5-HT tone via the discussed mechanism controls the terminal field synaptic activity of serotonergic neurons in mice. In addition, disinhibition of pulse-modulating 5-HT 1A autoreceptors by S-(−)UH 301 can potentiate the synaptic effects of serotonin reuptake blockers as well as the serotonin precursor 5-HTP. However, a more firm general conclusion regarding antagonism of presynaptic 5-HT 1A receptors leading to indirect functional enhancement of other postsynaptic serotonergic receptors can only be made when the above hypothesis is further tested with other selective 5-HT 1A receptor antagonists (such as WAY 100 635), which we were unable to obtain. The present study is the first report to show that a selective 5-HT 1A antagonist by itself can produce a serotonin-mediated function via indirect stimulation of another serotonin receptor subtype in mice.