Different regulation of rat 5-HT(2A) and rat 5-HT(2C) receptors in NIH 3T3 cells upon exposure to 5-HT and pipamperone

Serotonin 2A receptor clustering in peripheral lymphocytes is altered in major depression and may be a biomarker of therapeutic efficacy

BACKGROUND

In a previous report, we showed that the clustering of serotonin (5HT) transporter (SERT) protein on cell membranes of peripheral lymphocytes predicts responsivity to antidepressant medication in two subpopulations of naïve depression patients (Rivera-Baltanas et al., J Affect Disord, 2012, 137, 46-55). In this study, we extended this idea to 5-HT2A receptor clusters in a similar patient population.

METHODS

We collected blood samples from a subset of patients from our previous study on SERT clustering (20 untreated and newly diagnosed depression patients, and 20 matched control subjects). Blood samples were collected at the time of diagnosis and after 8 weeks of pharmacological treatment and at analogous times in control subjects. We used the Hamilton scale to quantify the level of depression in patients both before and after treatment. We then used immunocytochemistry to assess 5-HT2A receptor clusters in lymphocytes at the same time points.

RESULTS

We found that both the size and number of 5-HT2A receptor clusters were increased in naïve depression patients compared to control subjects. Interestingly, there were individual differences in the distribution of 5-HT2A receptor cluster size that allowed us to differentiate the depression patients into two subgroups: a D-I group and a D-II group. After 8 weeks of pharmacological treatment, patients in both groups showed an improvement of symptoms, but patients in the D-II group had a much better outcome with many of them showing remission of symptoms. Furthermore, although treatment decreased cluster number and size in both D-I and D-II groups, only the D-II patients showed an increase in the number of clusters within the modal peak. Importantly, the same patients that belonged in the D-I or D-II groups in the present report were also assigned to the same groups in our previous study on SERT clustering.

LIMITATIONS

The data should be replicated within a proper clinical trial.

CONCLUSIONS

5-HT2A receptor clusters in peripheral lymphocytes are altered in major depression, partially reversed by antidepressant treatment, and may be considered a putative biomarker of therapeutic efficacy in major depression.

The effect of prolonged exposure to morphine on canine cerebral 5-HT2A receptors measured with (123)I-R91150 SPECT

Down-stream neuronal alterations, including changes in the 5-HT-2A receptor system, play an important role in the etiology and treatment of depression. The present study examined the effect of prolonged opioid treatment on cerebral 5-HT2A receptors. Cerebral 5-HT2A receptor availability was estimated in seven healthy five-year-old female neutered Beagle dogs pre and post 10-day morphine treatment (oral sustained release morphine 20mg twice daily for 10 days) with (123)I-R-91150, a 5-HT2A selective radioligand, and SPECT. 5-HT2A receptor binding indices (BI) for the frontal, parietal, temporal and occipital cortex and the subcortical region were calculated. Statistical analysis was performed using a linear mixed-effect model with treatment as fixed effect and dog as random effect. Morphine treatment significantly (P≤0.05) lowered 5-HT2A BIs in the right and left frontal cortex, the right and left temporal cortex, the right and left parietal cortex, and the subcortical region. The decreased cerebral 5-HT2A receptor availability following prolonged morphine exposure provides further evidence for an interaction between the opioid and serotonergic system.

5-HT2A receptors in the feline brain: 123I-5-I-R91150 kinetics and the influence of ketamine measured with micro-SPECT

Subanesthetic doses of ketamine can be used as a rapid-acting antidepressant in patients with treatment-resistant depression. Therefore, the brain kinetics of (123)I-5-I-R91150 (4-amino-N-[1-[3-(4-fluorophenyl)propyl]-4-methylpiperidin-4-yl]-5-iodo-2-methoxybenzamide) and the influence of ketamine on the postsynaptic serotonin-2A receptor (5-hydroxytryptamine-2A, or 5-HT2A) status were investigated in cats using micro-SPECT.

METHODS

This study was conducted on 6 cats using the radioligand (123)I-5-I-R91150, a 5-HT2A receptor antagonist, as the imaging probe. Anesthesia was induced and maintained with a continuous-rate infusion of propofol (8.4 ± 1.2 mg kg(-1) followed by 0.22 mg kg(-1) min(-1)) 75 min after tracer administration, and acquisition of the first image began 15 min after induction of anesthesia. After this first acquisition, propofol (0.22 mg kg(-1) min(-1)) was combined with ketamine (5 mg kg(-1) followed by 0.023 mg kg(-1) min(-1)), and the second acquisition began 15 min later. Semiquantification, with the cerebellum as a reference region, was performed to calculate the 5-HT2A receptor binding indices (parameter for available receptor density) in the frontal and temporal cortices. The binding indices were analyzed with Wilcoxon signed ranks statistics.

RESULTS

The addition of ketamine to the propofol continuous-rate infusion resulted in decreased binding indices in the right frontal cortex (1.25 ± 0.22 vs. 1.45 ± 0.16; P = 0.028), left frontal cortex (1.34 ± 0.15 vs. 1.49 ± 0.10; P = 0.028), right temporal cortex (1.30 ± 0.17 vs. 1.45 ± 0.09; P = 0.046), and left temporal cortex (1.41 ± 0.20 vs. 1.52 ± 0.20; P = 0.046).

CONCLUSION

This study showed that cats can be used as an animal model for studying alterations of the 5-HT2A receptor status with (123)I-5-I-R91150 micro-SPECT. Furthermore, an interaction between ketamine and the 5-HT2A receptors resulting in decreased binding of (123)I-5-I-R91150 in the frontal and temporal cortices was demonstrated. Whether the decreased radioligand binding resulted from a direct competition between ketamine and (123)I-5-I-R91150 or from a decreased affinity of the 5-HT2A receptor caused by ketamine remains to be elucidated.

The influence of morphine on cerebral 5-HT2A availability in dogs: a SPECT study

The opioid and serotonergic systems are closely involved in pain processing and mood disorders. The aim of this study was to assess the influence of systemic morphine on cerebral serotonin 2A receptor (5-HT(2A)) binding in dogs using SPECT with the 5-HT(2A) radioligand (123)I-5I-R91150.

METHODS

5-HT(2A) binding was estimated with and without morphine pretreatment in 8 dogs. The 5-HT(2A) binding indices in the frontal, parietal, temporal, and occipital cortex and in the subcortical region were obtained by semiquantification.

RESULTS

A significantly decreased 5-HT(2A) binding index was found in the morphine group for the right (morphine, 1.41 ± 0.06; control, 1.52 ± 0.10) and left (morphine, 1.44 ± 0.08; control, 1.55 ± 0.11) frontal cortices, with P = 0.012 and P = 0.040, respectively. No significant differences were noted for the other regions.

CONCLUSION

Morphine decreased the frontocortical 5-HT(2A) availability, confirming an interaction between the 5-HTergic and the opioid systems. Whether this interaction is caused by decreased receptor density due to direct internalization or is the result of indirect actions, such as increased endogenous serotonin release, remains to be elucidated.

Differences between human wild-type and C23S variant 5-HT2C receptors in inverse agonist-induced resensitization

The aim of this study was to analyze functional properties of the naturally occurring C23S variant of the human 5-HT2C receptor. In HEK293 cells transiently expressing the unedited forms of the variant receptor (VR) or the wild-type receptor (WTR), surface expression was determined by [3H]mesulergine binding to membrane fragments. Function was examined by an aequorin luminescence-based Ca2+ assay. Surface expression of the VR was 116% of that of the WTR. The 5-HT-induced increase in cytosolic Ca2+ ([Ca2+]i), and its inhibition by the inverse agonist SB 206553 did not differ between VR- or WTR-expressing cells. Preexposure of VR- or WTR-expressing cells to 0.5 μM 5-HT (3 min-4.5 h) led to a practically identical time course and extent in the reduction of the 5-HT-induced increase in [Ca2+]i. In contrast, prolonged preexposure to the inverse agonist SB 206553 (1 μM) elevated the 5-HT-induced increase in [Ca2+]i for both isoreceptors. A preexposure time of 4.5 h was necessary to significantly elevate the Ca2+ response of the WTR, but the VR produced this elevation within 1 h with virtually no further effect after 4.5 h of preexposure. In conclusion, prolonged preexposure to 5-HT caused equally rapid and strong desensitization of both isoreceptors. The different time course of SB 206553-induced resensitization of the two isoreceptors might be therapeutically relevant for drugs exhibiting inverse agonist properties at 5-HT2C receptors, such as atypical antipsychotics and certain antidepressants.

Influence of the antipsychotic drug pipamperone on the expression of the dopamine D4 receptor

The dopamine D4 receptor is a G protein-coupled receptor that binds with high affinity various antipsychotics. The receptor may be involved in attention/cognition, and in genetic studies a polymorphic repeat sequence in its coding sequence has been associated with attention deficit/hyperactivity disorder. We developed an inducible episomal expression system based on the reverse tetracycline transactivator and Epstein-Barr viral sequences. In HEK293rtTA cells expressing the dopamine D4 receptor from this episomal expression vector, addition of doxycycline in combination with sodium butyrate and trichostatin A induces high levels of receptor expression, resulting in 1970 +/- 20 fmol/mg membrane protein. Addition of the dopamine D4 receptor and serotonin 5-HT2A receptor antagonist pipamperone to these cells further increased the expression of the dopamine receptor, reaching 3800 +/- 60 fmol/mg membrane protein. This up-regulation was not restricted to the dopamine D4 receptor but was also found for the serotonin 5-HT2A receptor. We further provide evidence that the increase in receptor expression is not due to increased mRNA synthesis. As pipamperone could rescue the expression of a folding mutant of the dopamine D4 receptor (M345), we propose that pipamperone acts as a pharmacological chaperone for correct receptor folding thereby resulting in an increased dopamine D4 receptor expression. This study describes a strong and inducible expression system for proteins, difficult to express in other heterologous expression systems. This study also demonstrates that pipamperone, an antipsychotic, acts as a pharmacological chaperone and by doing so, increases the expression level of the dopamine D4 receptor. The fact that ligands can also act as pharmacological chaperones is a fairly new additional element in the regulation of receptor expression levels with potential great impact in drug treatment.

Regulation of 5-HT2A/C receptors and DOI-induced behaviors by protein kinase Cgamma

Protein kinase Cgamma (PKCgamma) null mutant mice demonstrate increased behavioral impulsivity and ethanol consumption. Pharmacological studies have shown that 5-HT(2A/C) receptors modulate impulsivity and ethanol consumption in rodents and that PKC can regulate 5-HT(2A/C) receptors. To determine whether PKCgamma plays a selective role in 5-HT(2A/C) receptor regulation, biochemical and behavioral experiments were performed in PKCgamma mutant and wild-type mice. DOI-stimulated phosphoinositol hydrolysis and [(125)I]-DOI saturation binding in the PFC, and quantitative autoradiography of [(125)I]-DOI binding sites in 15 brain regions were analyzed. DOI-induced head twitch responses (HTR) were measured in naive mice after an acute 2.5 mg/kg injection of DOI. Results indicated that DOI-induced HTR was significantly greater in mutant mice compared to wild-type mice. Results of the phosphoinositol hydrolysis, membrane binding, and autoradiography experiments indicated that in mutant mice, increased HTR was associated with increased 5-HT(2A/C) receptor function in the PFC, but not increased receptor number or affinity suggesting that PKCgamma regulates receptor function but not receptor number. These data support a role for 5-HT(2A/C) receptors in the PFC in mediating some of the behavioral differences observed between PKCgamma mutant and wild-type mice.

Effects of clozapine and 2,5-dimethoxy-4-methylamphetamine [DOM] on 5-HT2A receptor expression in discrete brain areas

Activation of 5-HT2A receptors has been shown to be an essential component of the discriminative stimulus effects of indoleamine and phenethylamine hallucinogens. The objective of the present study was to determine the neuroanatomical location of the 5HT2A receptors which may be responsible for the stimulus effects of the phenethylamine hallucinogen [-]2,5-dimethoxy-4-methylamphetamine (DOM). It was hypothesized that brain areas containing altered 5-HT2A receptor expression in the context of a similar alteration in DOM-induced stimulus control might be important in mediating the stimulus effects of DOM. Fisher 344 rats were treated with either clozapine (25 mg/kg/day) or DOM (2 mg/kg/day) for 7 days, and the consequences of these drug treatment regimens on DOM-induced stimulus control and on 5-HT2A receptor expression in several brain areas were determined. Chronic administration of clozapine was associated with a wide-spread decrease in levels of 5-HT2A/2C receptors. Conversely, treatment with DOM had varied effects including a neuroanatomically selective decrease in 5-HT2A/2C receptor levels that was restricted to the olfactory nucleus. Both chronic treatment with DOM and clozapine decreased the stimulus effects of DOM. The present findings suggest a role for the olfactory nucleus in producing the stimulus effects of DOM.

Differential regulation of rat peripheral 5-HT(2A) and 5-HT(2B) receptor systems: influence of drug treatment

Most studies of 5-HT(2) receptor regulation have been carried out on the central nervous system (CNS) (which expresses 5-HT(2A) and 5-HT(2C) receptors); very few in vitro studies have addressed the peripheral receptors 5-HT(2A) and 5-HT(2B). The aim of this investigation was to compare the possible short- and long-term processes regulating these peripheral receptors in the rat. The in vitro contractile response elicited by serotonin (5-HT, 10 micro M) in the rat gastric fundus (5-HT(2B) receptor system) was rapid and followed by a partial fade to a steady state, in contrast with the rat thoracic aorta response (5-HT(2A) receptor system), which was more stable, slower and sustained. To characterize drug-receptor interactions, cumulative concentration/response curves (CCRCs) for 5-HT were constructed ex vivo for rat tissues treated with drugs acting at these receptors. Rats were examined 4 or 24 h after a single, i.p. administration of (+/-)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(+/-)DOI, 1 or 2.5 mg/kg], clozapine, cyproheptadine or rauwolscine (10 mg/kg), 48 h after a single i.p. administration of (+/-)DOI (2.5 mg/kg), clozapine or cyproheptadine (10 mg/kg) or 24 h after the last of with 15 daily i.p. administrations of (+/-)DOI (1 or 2.5 mg/kg), clozapine, cyproheptadine or rauwolscine (10 mg/kg). In the aorta, E(max) (the maximum response elicited by 5-HT) was unchanged 4 h after a single dose of any of the drugs tested. However, 24 h after a single dose, E(max) was lower in animals treated with (+/-)DOI (2.5 mg/kg), clozapine or cyproheptadine than in controls, whilst 48 h after a single dose of (+/-)DOI (2.5 mg/kg), clozapine or cyproheptadine there was no difference in E(max) between experimental and control animals. After chronic treatment with (+/-)DOI (2.5 mg/kg), clozapine and cyproheptadine, E(max) was lower than in controls. In the gastric fundus, E(max) 4 h after a single dose of each drug was lower than in controls, and the response recovered by 24 or 48 h. Following chronic treatment, E(max) was significantly lower than in controls for each drug used. These findings suggest first, that regulation of peripheral 5-HT(2) receptors (5-HT(2A) and 5-HT(2B)) is a functionally significant phenomenon in vivo, and occurs after administration of both agonists and antagonists. Second, the kinetics of peripheral 5-HT(2) receptor regulation were similar in both in vivo and ex vivo experiments. The 5-HT(2B) receptors in rat gastric fundus are more sensitive to drug-induced regulation than the 5-HT(2A) rat aortic receptors. Finally, long-term regulation of both receptors stabilizes short-term desensitization for longer.

5-HT2A and 5-HT2C receptors and their atypical regulation properties

The 5-HT(2A) and 5-HT(2C) receptors belong to the G-protein-coupled receptor (GPCR) superfamily. GPCRs transduce extracellular signals to the interior of cells through their interaction with G-proteins. The 5-HT(2A) and 5-HT(2C) receptors mediate effects of a large variety of compounds affecting depression, schizophrenia, anxiety, hallucinations, dysthymia, sleep patterns, feeding behaviour and neuro-endocrine functions. Binding of such compounds to either 5-HT(2) receptor subtype induces processes that regulate receptor sensitivity. In contrast to most other receptors, chronic blockade of 5-HT(2A) and 5-HT(2C) receptors leads not to an up- but to a (paradoxical) down-regulation. This review deals with published data involving such non-classical regulation of 5-HT(2A) and 5-HT(2C) receptors obtained from in vivo and in vitro studies. The underlying regulatory processes of the agonist-induced regulation of 5-HT(2A) and 5-HT(2C) receptors, commonly thought to be desensitisation and resensitisation, are discussed. The atypical down-regulation of both 5-HT(2) receptor subtypes by antidepressants, antipsychotics and 5-HT(2) antagonists is reviewed. The possible mechanisms of this paradoxical down-regulation are discussed, and a new hypothesis on possible heterologous regulation of 5-HT(2A) receptors is proposed.