Accumulating evidence indicates that imidazoline I2 receptor agonists enhance the antinociceptive effects of opioids and therefore may be suitable for combination therapy with opioids for pain treatment. However, little is known of the effects of I2 receptor agonists on other behavioral effects of opioids. This study used schedule-controlled responding and dose-addition analyses to examine interactions between the μ opioid receptor agonist morphine and two imidazoline I2 receptor agonists, 2-BFI and BU224. In 8 rats responding under a fixed ratio 10 schedule of food presentation, morphine (3.2–17.8 mg/kg), 2-BFI (3.2–17.8 mg/kg), and BU224 (5.6–17.8 mg/kg) each dose-dependently decreased responding. The addition of fixed proportions of 2-BFI or BU224 shifted the morphine dose–effect curves leftward. The interactions between morphine and 2-BFI or BU224 were infra-additive when the same proportions of morphine and I2 receptor agonists were mixed; however, the interaction between morphine and I2 receptor agonists was additive when the drugs were mixed at other proportions. These results provide quantitative evidence that I2 receptor agonists do not enhance the response rate-decreasing effect of morphine and suggest that the enhancement of morphine antinociception is selective. Together, these results further support the therapeutic potential of combining I2 receptor agonists and opioids for pain control.

The polyamine agmatine modulates a variety of behavioral effects including the abuse-related effects of opioids and has been proposed as a potential medication candidate for the treatment of opioid abuse. However, little is known of the effects of agmatine on the abuse-related effects of other drugs of abuse. This study examined the effects of agmatine on the rewarding effects of methamphetamine in rats using a conditioned place preference paradigm. Methamphetamine (0.1-1.0mg/kg) dose-dependently increased the time spent in methamphetamine-paired side (place preference). Agmatine, at doses that did not produce place preference or aversion (10-32mg/kg), significantly decreased the development of methamphetamine-induced place preference when agmatine was administered in combination with methamphetamine during place conditioning. Agmatine also significantly decreased the expression of methamphetamine-induced place preference when an acute injection of agmatine was given immediately before test session. These doses of agmatine do not alter the motor activity in rats, suggesting that the observed attenuation of methamphetamine-induced place preference was not due to general behavioral disruption. Together, these data suggests that agmatine attenuates the rewarding effects of methamphetamine and may be able to modulate the abuse liability of methamphetamine.

Currently available analgesics cannot meet the increasing clinical needs and new analgesics with better therapeutic profiles are in great demand. The imidazoline I₂ receptor is an emerging drug target for analgesics. However, few studies have examined the effects of selective I₂ receptor ligands on the antinociceptive activity of opioids. This study examined the antinociceptive effects of the opioids morphine (0.1-10 mg/kg) and tramadol (3.2-56 mg/kg), the nonselective I₂ receptor ligand agmatine (10-100 mg/kg), and the selective I₂ receptor ligands 2-(2-benzofuranyl)-2-imidazoline hydrochloride (2-BFI; 1-10 mg/kg) and 2-(4, 5-dihydroimidazol-2-yl) quinoline hydrochloride (BU224; 1-10mg/kg), alone and in combination, in a warm water tail withdrawal procedure in rats. Morphine and tramadol but not agmatine, 2-BFI or BU224 increased tail withdrawal latency in a dose-related manner at 48°C water. Agmatine and 2-BFI but not BU224 dose-dependently enhanced the antinociceptive effects of morphine and tramadol, shifting the dose-effect curves of morphine and tramadol leftward. The enhancement of agmatine and 2-BFI on morphine and tramadol antinociception was prevented by BU224. These results, combined with the fact that BU224 and 2-BFI share similar behavioral effects under other conditions, suggest that BU224 has lower efficacy than 2-BFI at I₂ receptors, and that the enhancement of opioid antinociception by I₂ receptor ligands depends on their efficacies.

ABSTRACT Several noncoding microRNAs (miR or miRNA) have been shown to regulate the expression of drug‐metabolizing enzymes and transporters. Xenobiotic drug‐induced changes in enzyme and transporter expression may be associated with the alteration of miRNA expression. Therefore, this study investigated the impact of 19 xenobiotic drugs (e.g. dexamethasone, vinblastine, bilobalide and cocaine) on the expression of ten miRNAs (miR‐18a, ‐27a, ‐27b, ‐124a, ‐148a, ‐324‐3p, ‐328, ‐451, ‐519c and −1291) in MCF‐7, Caco‐2, SH‐SY5Y and BE(2)‐M17 cell systems. The data revealed that miRNAs were differentially expressed in human cell lines and the change in miRNA expression was dependent on the drug, as well as the type of cells investigated. Notably, treatment with bilobalide led to a 10‐fold increase of miR‐27a and a 2‐fold decrease of miR‐148a in Caco‐2 cells, but no change of miR‐27a and a 2‐fold increase of miR‐148a in MCF‐7 cells. Neuronal miR‐124a was generally down‐regulated by psychoactive drugs (e.g. cocaine, methadone and fluoxetine) in BE(2)‐M17 and SH‐SY5Y cells. Dexamethasone and vinblastine, inducers of drug‐metabolizing enzymes and transporters, suppressed the expression of miR‐27b, ‐148a and −451 that down‐regulate the enzymes and transporters. These findings should provide increased understanding of the altered gene expression underlying drug disposition, multidrug resistance, drug–drug interactions and neuroplasticity. Copyright © 2011 John Wiley & Sons, Ltd.

In previous studies we have observed that, in comparison with wild type mice, Tg-CYP2D6 mice have increased serum levels of bufotenine [5-hydroxy-N,N-dimethyltryptamine] following the administration of 5-MeO-DMT. Furthermore, following the injection of 5-MeO-DMT, harmaline was observed to increase serum levels of bufotenine and 5-MeO-DMT in both wild-type and Tg-CYP2D6 mice. In the present investigation, 5-MeO-DMT-induced stimulus control was established in wild-type and Tg-CYP2D6 mice. The two groups did not differ in their rate of acquisition of stimulus control. When tested with bufotenine, no 5-MeO-DMT-appropriate responding was observed. In contrast, the more lipid soluble analog of bufotenine, acetylbufotenine, was followed by an intermediate level of responding. The combination of harmaline with 5-MeO-DMT yielded a statistically significant increase in 5-MeO-DMT-appropriate responding in Tg-CYP2D6 mice; a comparable increase occurred in wild-type mice. In addition, it was noted that harmaline alone was followed by a significant degree of 5-MeO-DMT-appropriate responding in Tg-CYP2D6 mice. It is concluded that wild-type and Tg-CYPD2D6 mice do not differ in terms of acquisition of stimulus control by 5-MeO-DMT or in their response to bufotenine and acetylbufotenine. In both groups of mice, harmaline was found to enhance the stimulus effects of 5-MeO-DMT.

Pain remains a significant clinical challenge and currently available analgesics are not adequate to meet clinical needs. Emerging evidence suggests the role of imidazoline I2 receptors in pain modulation primarily from studies of the non-selective imidazoline receptor ligand, agmatine. However, little is known of the generality of the effect to selective I2 receptor ligands. This study examined the antinociceptive effects of two selective I2 receptor ligands 2-BFI and BU224 (>2000-fold selectivity for I2 receptors over α2 adrenoceptors) in a hypertonic (5%) saline-induced writhing test and analyzed their interaction with morphine using a dose-addition analysis. Morphine, 2-BFI and BU224 but not agmatine produced a dose-dependent antinociceptive effect. Both composite additive curve analyses and isobolographical plots revealed a supra-additive interaction between morphine and 2-BFI or BU224, whereas the interaction between 2-BFI and BU224 was additive. The antinociceptive effect of 2-BFI and BU224 was attenuated by the I2 receptor antagonist/α2 adrenoceptor antagonist idazoxan but not by the selective α2 adrenoceptor antagonist yohimbine, suggesting an I2 receptor-mediated mechanism. Agmatine enhanced the antinociceptive effect of morphine, 2-BFI and BU224 and the enhancement was prevented by yohimbine, suggesting that the effect was mediated by α2 adrenoceptors. Taken together, these data represent the first report that selective I2 receptor ligands have substantial antinociceptive activity and produce antinociceptive synergy with opioids in a rat model of acute pain. These data suggest that drugs acting on imidazoline I2 receptors may be useful either alone or in combination with opioids for the treatment of pain.

5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT) belongs to a group of naturally-occurring psychoactive indolealkylamine drugs. It acts as a nonselective serotonin (5-HT) agonist and causes many physiological and behavioral changes. 5-MeO-DMT is O-demethylated by polymorphic cytochrome P450 2D6 (CYP2D6) to an active metabolite, bufotenine, while it is mainly inactivated through the deamination pathway mediated by monoamine oxidase A (MAO-A). 5-MeO-DMT is often used with MAO-A inhibitors such as harmaline. Concurrent use of harmaline reduces 5-MeO-DMT deamination metabolism and leads to a prolonged and increased exposure to the parent drug 5-MeO-DMT, as well as the active metabolite bufotenine. Harmaline, 5-MeO-DMT and bufotenine act agonistically on serotonergic systems and may result in hyperserotonergic effects or serotonin toxicity. Interestingly, CYP2D6 also has important contribution to harmaline metabolism, and CYP2D6 genetic polymorphism may cause considerable variability in the metabolism, pharmacokinetics and dynamics of harmaline and its interaction with 5-MeO-DMT. Therefore, this review summarizes recent findings on biotransformation, pharmacokinetics, and pharmacological actions of 5-MeO-DMT. In addition, the pharmacokinetic and pharmacodynamic drug-drug interactions between harmaline and 5-MeO-DMT, potential involvement of CYP2D6 pharmacogenetics, and risks of 5-MeO-DMT intoxication are discussed.

A previous investigation in our laboratory found that the stimulus effects of the 5-HT2A agonist, LSD, are potentiated by 5-HT1A receptor agonists including the prototypic agonist, 8-OH-DPAT. Also suggestive of behaviorally relevant interactions between 5-HT1A and 5-HT2A receptors are behavioral analyses of locomotor activity, head-twitch response, forepaw treading and production of the serotonin syndrome; in some instances effects are augmented, in other, diminished. These observations led us in the present investigation to test the hypothesis that stimulus control by 8-OH-DPAT [0.2 mg/kg; 15 min pretreatment time] is modulated by 5-HT2A ligands. Stimulus control was established with 8-OH-DPAT in a group of 10 rats. A two-lever, fixed ratio 10, positively reinforced task with saline controls was employed. As shown previously, stimulus control by 8-OH-DPAT and the generalization of 8-OH-DPAT to the 5-HT1A partial agonist, buspirone, was completely blocked by the selective 5-HT1A antagonist, WAY-100635. In contrast, antagonism by the selective 5-HT2A antagonist, M100907 [0.1 mg/kg; 30 min pretreatment time], of 8-OH-DPAT and of the generalization of 8-OH-DPAT to buspirone was statistically significant but less than complete. In light of our previous conclusions regarding the interactions of 5-HT1A agonists with LSD-induced stimulus control, the present data suggest that the interaction between 5-HT1A and 5-HT2A receptors is bidirectional in drug discrimination studies.

N,N-dipropyltryptamine (DPT) is a synthetic tryptamine hallucinogen which has been used psychotherapeutically in humans, but has been studied preclinically only rarely. In the present studies, DPT was tested in a drug-elicited head-twitch assay in mice, and in rats trained to discriminate lysergic acid diethylamide (LSD), N,N-dimethyl-4-phosphoryloxytryptamine (psilocybin), or 3,4-methylenedioxymethamphetamine (MDMA). A separate group of rats was also trained to recognize DPT itself as a discriminative stimulus, and in all cases, the behavioral effects of DPT were challenged with the selective serotonin (5-HT)2A antagonist M100907, the 5-HT1A selective antagonist WAY-100635, or their combination. In the head-twitch assay, DPT elicited dose-dependent effects, producing a biphasic dose–effect curve. WAY-100635 produced a parallel rightward shift in the dose–effect curve for head twitches, indicative of surmountable antagonism, but the antagonist effects of M100907 were functionally insurmountable. DPT produced partial to full substitution when tested in rats trained to discriminate LSD, psilocybin or MDMA, and served as a discriminative stimulus. In all cases, the antagonist effects of M100907 were more profound than were those of WAY-100635. DPT is thus active in two rodent models relevant to 5-HT2 agonist activity. The effectiveness with which M100907 antagonizes the behavioral actions of this compound strongly suggest that the 5-HT2A receptor is an important site of action for DPT, but the modulatory actions of WAY-100635 also imply a 5-HT1A-mediated component to the actions of this compound.

Induction of mRNA or protein for immediate–early genes, such as c-fos, is used to identify brain areas, specific cell types, and neuronal circuits that become activated in response to various stimuli including psychoactive drugs. The objective of the present study was to identify the cell types in the prefrontal cortex in which lysergic acid diethylamide (d-LSD) induces c-Fos expression. Systemic administration of d-LSD resulted in a dose-dependent increase in c-Fos immunoreactivity. Although c-Fos-positive cells were found in all cortical layers, they were most numerous in layers III, IV, and V. d-LSD-induced c-Fos immunoreactivity was found in cells co-labeled with anti-neuron-specific enolase or anti-oligodendrocyte Oligo1. The Oligo1-labeled cells had small, round bodies and nuclear diameters characteristic of oligodendrocytes. Studies using confocal microscopy confirmed colocalization of c-Fos-labeled nuclei in NeuN-labeled neurons. Astrocytes and microglia labeled with glial fibrillary acidic protein antibody and OX-42 antibody, respectively, did not display LSD-induced c-Fos expression. Pyramidal neurons labeled with anti-neurofilament antibody also did not show induction of c-Fos immunoreactivity after systemic d-LSD administration. The present study demonstrates that d-LSD induced expression of c-Fos in the prefrontal cortex occurs in subpopulations of neurons and in oligodendrocytes, but not in pyramidal neurons, astrocytes, and microglia.

Background Although man’s first encounters with hallucinogens predate written history, it was not until the rise of the sister disciplines of organic chemistry and pharmacology in the nineteenth century that scientific studies became possible. Mescaline was the first to be isolated and its chemical structure determined. Since then, additional drugs have been recovered from their natural sources and synthetic chemists have contributed many more. Given their profound effects upon human behavior and the need for verbal communication to access many of these effects, some see humans as ideal subjects for study of hallucinogens. However, if we are to determine the mechanisms of action of these agents, establish hypotheses testable in human subjects, and explore the mechanistic links between hallucinogens and such apparently disparate topics as idiopathic psychosis, transcendental states, drug abuse, stress disorders, and cognitive dysfunction, studies in animals are essential. Stimulus control by hallucinogens has provided an intuitively attractive approach to the study of these agents in nonverbal species. Objective The intent of this review is to provide a brief account of events from the time of the first demonstration of hallucinogen-induced stimulus control to the present. In general, the review is limited to lysergic acid diethylamide (LSD) and the hallucinogenic derivatives of phenethylamine and tryptamine. Results The pharmacological basis for stimulus control by LSD and hallucinogenic phenethylamines and tryptamines is serotonergic in nature. The 5-HT 2A receptor appears to be the primary site of action with significant modulation by other serotonergic sites including 5-HT 2C and 5-HT 1A receptors. Interactions with other neurotransmitters, especially glutamate and dopamine, are under active investigation. Most studies to date have been conducted in the rat but transgenic mice offer interesting possibilities. Conclusions Hallucinogen-induced stimulus control provides a unique behavioral tool for the prediction of subjective effects in man and for the elucidation of the pharmacological mechanisms of the action of these agents.

Rationale: Based upon extensive studies in the rat, it has been suggested that stimulus control by LSD is mediated by 5-HT2A receptors, with serotonergic receptors of the 5-HT1A and 5-HT2C subtypes playing modulatory roles. In genetically modified mice lacking the serotonin transporter (SERT), 5-HT2A receptor density is decreased and, at a functional level, the head-twitch response following the administration of DOI, an index of activation of 5-HT2A receptors, is reduced. Taken together, these studies led us to hypothesize that the efficacy of LSD in establishing stimulus control is diminished or abolished in mice lacking the serotonin transporter.Objective: Determine the efficacy of LSD for establishing stimulus control in SERT knockout (KO) mice.Methods: SERT KO mice and wildtype (WT) littermates were trained in a visual discrimination on a progressive fixed ratio (FR) water-reinforced task and subsequently trained on a FR10 schedule with LSD (0.17 or 0.30 mg/kg) or vehicle. To control for general deficiencies in drug discrimination, mice were trained with pentobarbital (15 or 30 mg/kg) or vehicle.Results: The visual stimulus exerted control in both genotypes. LSD-induced stimulus control in 90% of WT mice but only 31% of SERT KO mice. In contrast, pentobarbital-induced stimulus control in 80% of WT mice and 54% of knockout mice.Conclusions: Although SERT KO mice exhibited stimulus control by the non-serotonergic drug, pentobarbital, the efficacy of LSD in these animals was markedly decreased, suggesting that reduced density of 5-HT1A and/or 5-HT2A receptors underlies the absence of stimulus control by LSD. (C) 2007 Elsevier Inc. All rights reserved.

It has been observed that agents with agonist activity at 5-HT2A receptors prevent neurotoxicity induced by the non-competitive NMDA antagonist, dizocilpine (MK-801). Subsequent behavioral studies reported complete antagonism by LSD and DOM of the stimulus effects of the related NMDA antagonist, phencyclidine [PCP]. The present study sought to extend those observations to include other psychoactive drugs. Male F-344 rats were trained in a 2-lever, fixed-ratio 10, food-reinforced task with PCP (3.0 mg/kg; IP; 30 min pretreatment) as a discriminative stimulus. Tests of generalization were then conducted using the training dose of PCP in combination with a range of doses of DOM, LSD, d-amphetamine, MDMA, psilocybin, buspirone, and GHB. All of the drugs tested in combination with PCP produced a statistically significant diminution of PCP-appropriate responding but for none was antagonism complete. These data, obtained using a stimulus control model of the hallucinogenic effects of PCP, fail to support the hypothesis that LSD and DOM completely antagonize stimulus control by PCP. Instead, the data suggest complex interactions between PCP-induced stimulus control and a variety of psychoactive drugs including GHB, an agent with no known affinity for serotonergic receptors.