Acute effects of para-chloroamphetamine on testosterone and markers of apoptosis in seminiferous epithelium of prepubertal male rats.

Serotonin is a neurotransmitter that affects the secretion of gonadotropins and testosterone. In prepubertal male rats, serotonin has a stimulating role in testosterone secretion. Here, we used prepubertal male rats to study the effects of para-chloroamphetamine (pCA) on circulating testosterone and gonadotropins and markers of apoptosis in germ cells from day 1 to day 5 post-treatment. The intraperitoneal administration of pCA induced a significant reduction in concentrations of hypothalamic serotonin and circulating testosterone, but gonadotropins were not affected. In the seminiferous epithelium of pCA-treated rats, increased the number of germ cells positive to markers of apoptosis, concomitantly with alterations in morphometry and the presence of multinucleated germ cells. Levels of testosterone were reduced starting from 1 day after pCA was administered. The time window between the administration of the pCA and collection of samples was sufficient to detect changes in testosterone levels, in contrast with a previous work where no changes were found. There was a possible relationship between the reduction of testosterone and an increase in the number of germ cells positive to apoptosis markers. However, the mechanism that links pCA-testosterone-germ cell positive to markers of apoptosis is unknown. Our outcomes support the view that pCA exposure during the prepubertal stage has an acute impact on testosterone levels and affects the structure and physiology of seminiferous epithelium.

p-Chloroamphetamine decreases serotonin and induces apoptosis in granulosa cells and follicular atresia in prepubertal female rats.

Amphetamine derivatives negatively impact serotonin (5-HT) production, which triggers apoptosis in different tissues, depending on the receptor they bind. 5-HT in the ovary stimulates estradiol secretion, a survival factor of granulosa cells. The effect of amphetamine derivatives on the serotonergic system of the ovary and follicular development is unknown. Therefore, in this study, we investigated the effects of p-chloroamphetamine (pCA), derived from amphetamines, on estradiol production, follicular development, apoptosis of granulosa cells, and serotonin 5-HT7 receptor (R5-HT7) expression. Female rats (30 days old) were injected with 10 mg/kg of pCA intraperitoneally and were euthanized 48 or 120 h after treatment. The concentration of 5-HT in the hypothalamus decreased at 48 and 120 h after treatment and in the ovary at 120 h. The serum concentration of estradiol decreased at all times studied. Follicular atresia, TUNEL-positive (apoptotic) granulosa cells and Bax expression were elevated by pCA, but none of these effects was associated with R5-HT7 expression. These results suggest that pCA induces the dysregulation of the serotonergic system in the hypothalamus and the ovary, negatively impacting estradiol production and follicular development.

5-HT2A receptor-mediated PKCδ phosphorylation is critical for serotonergic impairments induced by p-chloroamphetamine in mice.

p-Chloroamphetamine (PCA), an amphetamine derivative, has been shown to induce serotonergic toxicity. However, the precise mechanism of serotonergic toxicity induced by PCA remains unclear. In this study, PCA treatment (20 mg/kg, i.p.) did not significantly change 5-HT1A receptor gene expression, but significantly increased 5-HT2A receptor gene expression. Furthermore, 5-HT2A receptor antagonist MDL11939, but not 5-HT1A receptor antagonist WAY100635, significantly attenuated PCA-induced serotonergic impairments. We investigated whether PCA activated a specific isoform of protein kinase C (PKC), since previous evidence indicated the involvement of PKC in neurotoxicity induced by amphetamines. We observed that PCA treatment significantly increased the expression levels of PKCδ among all PKC isoforms. MDL11939 treatment significantly attenuated PCA-induced phosphorylation of PKCδ. However, PCA-induced increase in 5-HT2A receptor gene expression was not altered by rottlerin (a pharmacological inhibitor of PKCδ) in mice, suggesting that 5-HT2A receptor is an upstream molecule for the activation of PKCδ. Rottlerin or PKCδ knockout significantly attenuated serotonergic behaviors. However, MDL11939 did not show any additional effects against the attenuation caused by PKCδ knockout in mice, suggesting that PKCδ gene is a molecular target for 5-HT2A receptor-mediated serotonergic effects. Our results suggest that 5-HT2A receptor mediates PCA-induced serotonergic impairments via activation of PKC.δ.

Impaired serotonin communication during juvenile development in rats diminishes adult sperm quality.

Spermatogenesis and steroidogenesis are testicular functions regulated by gonadotrophins as well as other factors, including serotonin. Testicular serotonin acts as an autocrine regulator of testosterone secretion, but studies on its role in spermatogenesis and sperm quality are scarce. Here, we analyzed the effects of intratesticular inhibition of serotonin synthesis on gonadotrophins, testosterone, and sperm quality. Both testicles of 30-day-old rats were injected once with saline solution (SS) or distinct concentrations of p-chloroamphetamine (PCA) (0.03, 0.06, or 0.12 mg). At 65 days of age, rats were euthanized and sperm density, motility, membrane integrity, mitochondrial function, and abnormalities were evaluated in gametes from the vas deferens. Inhibition of synthesis of intratesticular serotonin by PCA diminished the concentrations of testosterone and follicle-stimulating hormone (FSH) but luteinizing hormone (LH) levels were unaltered. Sperm density was not modified in animals injected with the different concentrations of PCA. In contrast, the percentage of sperm with abnormalities increased and the sperm membrane integrity decreased in animals injected at higher PCA concentrations. The functionality of sperm mitochondria in PCA-injected animals decreased only at the highest PCA dose. Our results indicate that testicular serotonin plays a role in testosterone synthesis and in the normal development of sperm, and blocking its effects disrupts the hormonal communication between the testis and hypophysis. ABBREVIATIONS: SS: saline solution; PCA: p-chloroamphetamine; FSH: follicle-stimulating hormone; LH: luteinizing hormone; TPH: tryptophan hydroxylase; MAO: monoamine oxidase; AC: absolute control group; PI: propidium iodide; FLICA: fluorescence inhibitor of caspase; 3ß-HSD: 3ß-hydroxysteroid dehydrogenase; 17-KSR: 17-ketosteroid reductase; DHT: 5-dihydrotestosterone.

Development and validation of a high-throughput calcium mobilization assay for the orphan receptor GPR88.

BACKGROUND: GPR88 is an orphan G protein-coupled receptor highly expressed in the striatum and is implicated in basal ganglia-associated disorders. However, the receptor functions of GPR88 are still largely unknown due to the lack of potent and selective ligands appropriate for central nervous system investigation. Development of a high-throughput screening assay for GPR88 should facilitate the discovery of novel ligands to probe GPR88 functions. METHODS: In this paper, we describe the development of a CHO-Gαqi5-GPR88 cell-based calcium mobilization assay. The assay takes advantage of functional coupling of GPR88 with the promiscuous Gαqi5 protein and consequent mobilization of intracellular calcium, which can be measured in a 384-well format with a Fluorescent Imaging Plate Reader. RESULTS: The CHO-Gαqi5-GPR88 cell-based calcium mobilization assay was validated by the structure-activity relationship study of known GPR88 agonist (1R,2R)-2-PCCA analogues. The assay was automated and miniaturized to a 384-well format, and was deemed robust and reproducible with a Z'-factor of 0.72 and tolerated dimethyl sulfoxide to a final concentration of 2%. Screening a pilot neurotransmitter library consisting of 228 compounds yielded 10 hits, but none of the hits were confirmed as GPR88 agonists in follow-up assays. CONCLUSIONS: We have developed a high-throughput calcium mobilization assay for the orphan receptor GPR88. This calcium mobilization assay can be used to identify several different types of GPR88 ligands including agonists, competitive and noncompetitive antagonists, inverse agonists, and allosteric modulators. These ligands will serve as valuable tools to probe signaling mechanisms and in vivo functions of GPR88, and could expedite development of novel therapies for diseases potentially mediated by GPR88.

Effect of Substitution on the Aniline Moiety of the GPR88 Agonist 2-PCCA: Synthesis, Structure-Activity Relationships, and Molecular Modeling Studies.

GPR88, an orphan receptor richly expressed in the striatum, is implicated in a number of basal ganglia-associated disorders. In order to elucidate the functions of GPR88, an in vivo probe appropriate for CNS investigation is required. We previously reported that 2-PCCA was able to modulate GPR88-mediated cAMP production through a Gαi-coupled pathway. Early structure-activity relationship (SAR) studies suggested that the aniline moiety of 2-PCCA is a suitable site for diverse modifications. Aimed at elucidating structural requirements in this region, we have designed and synthesized a series of analogues bearing a variety of substituents at the phenyl ring of the aniline moiety. Several compounds (e.g., 5j, 5o) showed improved or comparable potency, but have lower lipophilicity than 2-PCCA (clogP 6.19). These compounds provide the basis for further optimization to probe GPR88 in vivo functions. Computational studies confirmed the SAR trends and supported the notion that 4'-substituents on the biphenyl ring exit through a largely hydrophobic binding site to the extracellular loop.

The arginylation branch of the N-end rule pathway positively regulates cellular autophagic flux and clearance of proteotoxic proteins.

The N-terminal amino acid of a protein is an essential determinant of ubiquitination and subsequent proteasomal degradation in the N-end rule pathway. Using para-chloroamphetamine (PCA), a specific inhibitor of the arginylation branch of the pathway (Arg/N-end rule pathway), we identified that blocking the Arg/N-end rule pathway significantly impaired the fusion of autophagosomes with lysosomes. Under ER stress, ATE1-encoded Arg-tRNA-protein transferases carry out the N-terminal arginylation of the ER heat shock protein HSPA5 that initially targets cargo proteins, along with SQSTM1, to the autophagosome. At the late stage of autophagy, however, proteasomal degradation of arginylated HSPA5 might function as a critical checkpoint for the proper progression of autophagic flux in the cells. Consistently, the inhibition of the Arg/N-end rule pathway with PCA significantly elevated levels of MAPT and huntingtin aggregates, accompanied by increased numbers of LC3 and SQSTM1 puncta. Cells treated with the Arg/N-end rule inhibitor became more sensitized to proteotoxic stress-induced cytotoxicity. SILAC-based quantitative proteomics also revealed that PCA significantly alters various biological pathways, including cellular responses to stress, nutrient, and DNA damage, which are also closely involved in modulation of autophagic responses. Thus, our results indicate that the Arg/N-end rule pathway may function to actively protect cells from detrimental effects of cellular stresses, including proteotoxic protein accumulation, by positively regulating autophagic flux.

Endogenous 5-HT outflow from chicken aorta by 5-HT uptake inhibitors and amphetamine derivatives.

Chemoreceptor cells aggregating in clusters in the chicken thoracic aorta contain 5-hydroxytryptamine (5-HT) and have voltage-dependent ion channels and nicotinic acetylcholine receptors, which are characteristics typically associated with neurons. The aim of the present study was to investigate the effects of 5-HT uptake inhibitors, fluvoxamine, fluoxetine and clomipramine (CLM), and amphetamine derivatives, p-chloroamphetamine (PCA) and methamphetamine (MET), on endogenous 5-HT outflow from the isolated chick thoracic aorta in vitro. 5-HT was measured by using a HPLC system with electrochemical detection. The amphetamine derivatives and 5-HT uptake inhibitors caused concentration-dependent increases in endogenous 5-HT outflow. PCA was about ten times more effective in eliciting 5-HT outflow than MET. The 5-HT uptake inhibitors examined had similar potency for 5-HT outflow. PCA and CLM increased 5-HT outflow in a temperature-dependent manner. The outflow of 5-HT induced by PCA or 5-HT uptake inhibitors was independent of extracellular Ca(2+) concentration. The 5-HT outflow induced by CLM, but not that by PCA, was dependent on the extracellular NaCl concentration. These results suggest that the 5-HT uptake system of 5-HT-containing chemoreceptor cells in the chicken thoracic aorta has characteristics similar to those of 5-HT-containing neurons in the mammalian central nervous system (CNS).

Prepubertal male rats with high rates of germ-cell apoptosis present exacerbated rates of germ-cell apoptosis after serotonin depletion.

Male germ-cell apoptosis occurs naturally and can be increased by exposure to drugs and toxic chemicals. Individuals may have different rates of apoptosis and are likely to also exhibit differential sensitivity to outside influences. Previously, we reported that p-chloroamphetamine (pCA), a substance that inhibits serotonin synthesis, induced germ-cell apoptosis in prepubertal male rats. Here, we identified prepubertal rats with naturally high or low rates of germ-cell apoptosis and evaluated gene expression in both groups. Bax and Shbg mRNA levels were higher in rats with high rates of germ-cell apoptosis. Rats were then treated with pCA and the neuro-hormonal response and gene expression were evaluated. Treatment with pCA induced a reduction in serotonin concentrations but levels of sex hormones and gonadotrophins were not changed. Rats with initially high rates of germ-cell apoptosis had even higher rates of germ-cell apoptosis after treatment with pCA. In rats with high rates of germ-cell apoptosis Bax mRNA expression remained high after treatment with pCA. On the basis of category, an inverse relationship between mRNA expression of Bax and Bcl2, Bax and AR and Bax and Hsd3b2 was found. Here we provide evidence that innate levels of germ-cell apoptosis could be explained by the level of mRNA expression of genes involved with apoptosis and spermatogenesis.

Mechanisms of morphine-venlafaxine interactions in diabetic neuropathic pain model.

BACKGROUND: we investigated the possible mechanisms involved in the interactions of venlafaxine (VFX), a selective serotonin and noradrenaline reuptake inhibitor, and morphine (MRF), an opioid receptor agonist, after acute and chronic VFX treatment in diabetic neuropathic pain model (DNPM). METHODS: The studies were performed on male rats. The changes in nociceptive thresholds were determined by using mechanical stimuli (the Randall-Selitto and the von Frey tests). Diabetes was induced by intramuscular administration of streptozotocin. In order to investigate the mechanism of interaction, animals were also pretreated with naloxone (NLX), a nonselective opioid antagonist, yohimbine (YOH), a nonselective α2-adrenergic antagonist, and p-chloroamphetamine (PCA), a neurotoxin that destroys serotonergic neurons. The µ-opioid receptors' density was determined with the use of radioligand binding assay. RESULTS: VFX potentiated antinociceptive action of MRF after acute administration of VFX and this effect was decreased by pretreatment of NLX, YOH and PCA. On the contrary, VFX administered for 21 days prior to MRF significantly decreased the analgesic action of MRF; this effect was augmented only after YOH pretreatment. Also, 21-days administration of VFX caused decreasing tendency in the number of µ-opioid receptors in the brain stem. CONCLUSIONS: The results of our study show that single administration of VFX potentiates antinociceptive action of morphine in DNPM. This effect is probably mediated by both, noradrenergic and serotonergic systems. On the other hand, 21-days administration of VFX significantly decreases analgesic action of MRF. Moreover, there is a possibility that VFX acts as an antagonist of N-methyl-d-aspartate receptors.

A neurostimulant para-chloroamphetamine inhibits the arginylation branch of the N-end rule pathway.

In the arginylation branch of the N-end rule pathway, unacetylated N-terminal destabilizing residues function as essential determinants of protein degradation signals (N-degron). Here, we show that a neurostimulant, para-chloroamphetamine (PCA), specifically inhibits the Arg/N-end rule pathway, delaying the degradation of its artificial and physiological substrates, including regulators of G protein signaling 4 (RGS4), in vitro and in cultured cells. In silico computational analysis indicated that PCA strongly interacts with both UBR box and ClpS box, which bind to type 1 and type 2 N-degrons, respectively. Moreover, intraperitoneal injection of PCA significantly stabilized endogenous RGS4 proteins in the whole mouse brain and, particularly, in the frontal cortex and hippocampus. Consistent with the role of RGS4 in G protein signaling, treatment with PCA impaired the activations of GPCR downstream effectors in N2A cells, phenocopying ATE1-null mutants. In addition, levels of pathological C-terminal fragments of TDP43 bearing N-degrons (Arg208-TDP25) were significantly elevated in the presence of PCA. Thus, our study identifies PCA as a potential tool to understand and modulate various pathological processes regulated by the Arg/N-end rule pathway, including neurodegenerative processes in FTLD-U and ALS.

Synthesis, pharmacological characterization, and structure-activity relationship studies of small molecular agonists for the orphan GPR88 receptor.

GPR88 is an orphan G-protein-coupled receptor (GPCR) enriched in the striatum. Genetic deletion and gene expression studies have suggested that GPR88 plays an important role in the regulation of striatal functions and is implicated in psychiatric disorders. The signal transduction pathway and receptor functions of GPR88, however, are still largely unknown due to the lack of endogenous and synthetic ligands. In this paper, we report the synthesis of a GPR88 agonist 2-PCCA and its pure diastereomers, which were functionally characterized in both transiently and stably expressing GPR88 HEK293 cells. 2-PCCA inhibited isoproterenol-stimulated cAMP accumulation in a concentration-dependent manner in cells expressing GPR88 but not in the control cells, suggesting that the observed cAMP inhibition is mediated through GPR88 and that GPR88 is coupled to Gαi. 2-PCCA did not induce calcium mobilization in GPR88 cells, indicating no Gαq-mediated response. A structure-activity relationship (SAR) study of 2-PCCA was also conducted to explore the key structural features for GPR88 agonist activity.

Label-free dopamine imaging in live rat brain slices.

Dopaminergic neurotransmission has been investigated extensively, yet direct optical probing of dopamine has not been possible in live cells. Here we image intracellular dopamine with sub-micrometer three-dimensional resolution by harnessing its intrinsic mid-ultraviolet (UV) autofluorescence. Two-photon excitation with visible light (540 nm) in conjunction with a non-epifluorescent detection scheme is used to circumvent the UV toxicity and the UV transmission problems. The method is established by imaging dopamine in a dopaminergic cell line and in control cells (glia), and is validated by mass spectrometry. We further show that individual dopamine vesicles/vesicular clusters can be imaged in cultured rat brain slices, thereby providing a direct visualization of the intracellular events preceding dopamine release induced by depolarization or amphetamine exposure. Our technique opens up a previously inaccessible mid-ultraviolet spectral regime (excitation ~270 nm, emission < 320 nm) for label-free imaging of native molecules in live tissue.

A quantitative model of amphetamine action on the 5-HT transporter.

BACKGROUND AND PURPOSE: Amphetamines bind to the plasmalemmal transporters for the monoamines dopamine (DAT), noradrenaline (NET) and 5-HT (SERT); influx of amphetamine leads to efflux of substrates. Various models have been proposed to account for this amphetamine-induced reverse transport in mechanistic terms. A most notable example is the molecular stent hypothesis, which posits a special amphetamine-induced conformation that is not likely in alternative access models of transport. The current study was designed to evaluate the explanatory power of these models and the molecular stent hypothesis. EXPERIMENTAL APPROACH: Xenopus laevis oocytes and HEK293 cells expressing human (h) SERT were voltage-clamped and exposed to 5-HT, p-chloroamphetamine (pCA) or methylenedioxyamphetamine (MDMA). KEY RESULTS: In contrast to the currents induced by 5-HT, pCA-triggered currents through SERT decayed slowly in Xenopus laevis oocytes once the agonist was removed (consistent with the molecular stent hypothesis). However, when SERT was expressed in HEK293 cells, currents induced by 3 or 100 µM pCA decayed 10 or 100 times faster, respectively, after pCA removal. CONCLUSIONS AND IMPLICATIONS: This discrepancy in decay rates is inconsistent with the molecular stent hypothesis. In contrast, a multistate version of the alternative access model accounts for all the observations and reproduces the kinetic parameters extracted from the electrophysiological recordings. A crucial feature that explains the action of amphetamines is their lipophilic nature, which allows for rapid diffusion through the membrane.

Electrophysiological evidence for rapid 5-HT1A autoreceptor inhibition by vilazodone, a 5-HT1A receptor partial agonist and 5-HT reuptake inhibitor.

This study examined the effect of vilazodone, a combined serotonin (5-HT) reuptake inhibitor and 5-HT(1A) receptor partial agonist, paroxetine and fluoxetine on the sensitivity of 5-HT(1A) autoreceptors of serotonergic dorsal raphe nucleus neurons in rats. These effects were assessed by determining the intravenous dose of (±)-8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT) required to suppress the basal firing rate of these neurons by 50% (ID50) in anesthetized rats using in vivo electrophysiology. 5-HT uptake inhibition was determined by the ability of the compounds to reverse (±)-p-chloroamphetamine (PCA)-induced rat hypothalamic 5-HT depletion ex vivo. Acute vilazodone administration (0.63 and 2.1 µmol/kg, s.c.), compared with vehicle, significantly increased (2-3-fold) the ID50 of 8-OH-DPAT at 4 h, but not 24h after administration. Subchronic administration (3 days) significantly increased the ID50 value at 4 h (3-4-fold) and at 24 h (~2-fold). In contrast, paroxetine and fluoxetine at doses that were supramaximal for 5-HT uptake inhibition did not significantly alter the ID50 value of 8-OH-DPAT after acute or subchronic administration. Vilazodone antagonized the action of PCA 3.5 h and 5 h after a single dose (ID50 1.49 and 0.46 µmol/kg, s.c., respectively), but was inactive 18 h post-administration, corroborating the electrophysiological results at 24 h following acute administration. The results are consistent with the concept of rapid and, following repeated treatment, prolonged inhibition of 5-HT(1A) autoreceptors by vilazodone. This effect could occur by either direct interaction with, or desensitization of, these receptors, an effect which cannot be ascribed to vilazodone's 5-HT reuptake inhibiting properties.

Fluoxetine suppresses synaptically induced [Ca²âº]i spikes and excitotoxicity in cultured rat hippocampal neurons.

Fluoxetine is a widely used antidepressant with an action that is primarily attributed to the inhibition of serotonin re-uptake into the synaptic terminals of the central nervous system. Fluoxetine also has blocking effects on various ion channels, including Ca(2+) channels. It remains unclear, however, how fluoxetine may affect synaptically induced [Ca(2+)](i) spikes. We investigated the effects of fluoxetine on [Ca(2+)](i) spikes, along with the subsequent neurotoxicity that is synaptically evoked by lowering extracellular Mg(2+) in cultured rat hippocampal neurons. Fluoxetine inhibited the synaptically induced [Ca(2+)](i) spikes in p-chloroamphetamine-treated and non-treated neurons, in a concentration-dependent manner. However, other selective serotonin reuptake inhibitors, such as paroxetine and citalopram, did not significantly affect the spikes. Pretreatment with fluoxetine for 5 min inhibited [Ca(2+)](i) increases induced by glutamate, α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, and N-methyl-d-aspartate. Fluoxetine also inhibited α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-induced currents. In addition, fluoxetine decreased the [Ca(2+)](i) responses induced by the metabotrophic glutamate receptor agonist (S)-3,5-dihydroxyphenylglycine or the ryanodine receptor agonist caffeine. Fluoxetine inhibited [Ca(2+)](i) responses induced by 20mM KCl. Fluoxetine decreased the release of FM1-43 induced by electric field stimulation. Furthermore, fluoxetine inhibited 0.1mM [Mg(2+)](o)-induced cell death. Collectively, our results suggest that fluoxetine suppresses the spikes and protects neurons against excitotoxicity, particularly in cultured rat hippocampal neurons, presumably due to both direct inhibition of presynaptic glutamate release and postsynaptic glutamate receptor-mediated [Ca(2+)](i) signaling. In addition to an indirect inhibitory effect via 5-HT levels, these data suggest a new, possibly direct inhibitory action of fluoxetine on synaptically induced [Ca(2+)](i) spikes and neuronal cell death.

Venlafaxine and neuropathic pain.

The possible mechanisms involved in the antinociceptive effect of venlafaxine (VFX), a selective serotonin and noradrenaline reuptake inhibitor, after a single administration and chronic treatment were investigated in a diabetic neuropathic pain (DNP) model. VFX produced a significant antihyperalgesic effect after a single and repeated administration. This effect was reversed by pretreatment with yohimbine (a relatively selective α(2)-adrenergic antagonist) and p-chloroamphetamine (a neurotoxin which destroys serotonergic neurons). Conversely, naloxone (a nonselective opioid antagonist) did not reverse the effect of VFX in a DNP model. It is concluded that both noradrenergic and serotonergic mechanisms participate in the antinociceptive effect of VFX in the DNP model. However, the noradrenergic mechanism probably plays a more important role.

The GPR88 receptor agonist 2-PCCA does not alter the behavioral effects of methamphetamine in rats.

GPR88 is a novel orphan G protein-coupled receptor that is primarily located at the striatum. Genetic knockout studies reveal phenotypes of increased dopamine D(2) receptor sensitivity in mice, suggesting that GPR88 receptors may be involved in the modulation of dopaminergic system. However, there is no study that examines the pharmacological effects of GPR88 receptor ligands in in vivo preparations. This study examined the effects of a GPR88 receptor agonist, (1R, 2R)-2-pyridin-2-yl-cyclopropane carboxylic acid ((2S, 3S)-2-amino-3-methyl-pentyl)-(4'-propylbiphenyl-4-yl)-amide (2-PCCA), on the motor activity in rats and on methamphetamine-induced hyperactivity and discriminative stimulus effects. 2-PCCA (0.1-3.2mg/kg) dose-dependently decreased the locomotor activity in rats and, when studied in combination with 1.0mg/kg methamphetamine, also dose-dependently decreased methamphetamine-induced hyperactivity. However, the dose of 2-PCCA that significantly attenuated methamphetamine-induced hyperactivity was also the dose that by itself markedly decreased the baseline locomotor activity. In rats discriminating 0.32mg/kg methamphetamine, 2-PCCA (1-3.2mg/kg) itself did not produce methamphetamine-like discriminative stimulus effects and, when studied in combination, did not alter the discriminative stimulus effects of methamphetamine. Together, these data have provided the first line of evidence that activation of GPR88 receptors does not alter the behavioral effects of methamphetamine. The potential implications of these findings are also discussed.