Behavioral effects and central nervous system levels of the broadly available κ-agonist hallucinogen salvinorin A are affected by P-glycoprotein modulation in vivo

Profile of a short-acting κ-antagonist, LY2795050, on self-grooming behaviors, forced swim test and locomotor activity: sex comparison in mice

BACKGROUND

Novel short-acting κ(kappa)-opioid receptor selective antagonists are translational tools to examine the impact of the κ-receptor/dynorphin system in assays related to central nervous system dysfunction (e.g., substance use disorders, anhedonia and depression). The effects of such compounds have been compared in males and females under very limited conditions.

AIMS

The goal of this study was to examine potential sex differences in the effects of a κ-agonist and a short-acting κ-antagonist in an ethologically relevant test of anhedonia, the "splash test" of self-grooming, and also in the forced swim test and in locomotor activity.

METHODS

We examined the dose-dependence of grooming deficits caused by the κ-agonist U50,488 (0.1-3.2 mg/kg intraperitoneal (i.p.)) in gonadally intact adult male and female C57BL/6J mice. We then compared the effects of the short-acting κ-antagonist LY2795050 ((3-chloro-4-(4-(((2S)-2-pyridin-3-ylpyrrolidin-1-yl)methyl) phenoxy)benzamide)); 0.032-0.1 mg/kg i.p.) in blocking grooming deficits caused by U50,488 (3.2 mg/kg). The effects of LY2795050 were also studied in the forced swim test (FST). The effects of LY2795050 in blocking the locomotor depressant effects of U50,488 (10 mg/kg) were also studied.

RESULTS

U50,488 produced dose-dependent grooming deficits in male and female mice, and LY2795050 prevented these effects. In contrast, LY2795050 decreased immobility in the FST in males at a dose of 0.1 mg/kg, but not in females, up to a dose of 0.32 mg/kg. Also, LY2795050 (0.32 mg/kg) prevented and also reversed the locomotor-depressant effects of U50,488 (10 mg/kg), in males and females.

CONCLUSIONS

This study further implicates the κ-receptor system in ethologically relevant aspects of anhedonia, and confirms sexual dimorphism in some behavioral effects of novel κ-antagonists.

Pharmacokinetics and Pharmacodynamics of Salvinorin A and Salvia divinorum: Clinical and Forensic Aspects

Salvia divinorum Epling and Játiva is a perennial mint from the Lamiaceae family, endemic to Mexico, predominantly from the state of Oaxaca. Due to its psychoactive properties, S. divinorum had been used for centuries by Mazatecans for divinatory, religious, and medicinal purposes. In recent years, its use for recreational purposes, especially among adolescents and young adults, has progressively increased. The main bioactive compound underlying the hallucinogenic effects, salvinorin A, is a non-nitrogenous diterpenoid with high affinity and selectivity for the κ-opioid receptor. The aim of this work is to comprehensively review and discuss the toxicokinetics and toxicodynamics of S. divinorum and salvinorin A, highlighting their psychological, physiological, and toxic effects. Potential therapeutic applications and forensic aspects are also covered in this review. The leaves of S. divinorum can be chewed, drunk as an infusion, smoked, or vaporised. Absorption of salvinorin A occurs through the oral mucosa or the respiratory tract, being rapidly broken down in the gastrointestinal system to its major inactive metabolite, salvinorin B, when swallowed. Salvinorin A is rapidly distributed, with accumulation in the brain, and quickly eliminated. Its pharmacokinetic parameters parallel well with the short-lived psychoactive and physiological effects. No reports on toxicity or serious adverse outcomes were found. A variety of therapeutic applications have been proposed for S. divinorum which includes the treatment of chronic pain, gastrointestinal and mood disorders, neurological diseases, and treatment of drug dependence. Notwithstanding, there is still limited knowledge regarding the pharmacology and toxicology features of S. divinorum and salvinorin A, and this is needed due to its widespread use. Additionally, the clinical acceptance of salvinorin A has been hampered, especially due to the psychotropic side effects and misuse, turning the scientific community to the development of analogues with better pharmacological profiles.

Adverse pharmacokinetic interactions between illicit substances and clinical drugs

Adverse pharmacokinetic interactions between illicit substances and clinical drugs are of a significant health concern. Illicit substances are taken by healthy individuals as well as by patients with medical conditions such as mental illnesses, acquired immunodeficiency syndrome, diabetes mellitus and cancer. Many individuals that use illicit substances simultaneously take clinical drugs meant for targeted treatment. This concomitant usage can lead to life-threatening pharmacokinetic interactions between illicit substances and clinical drugs. Optimal levels and activity of drug-metabolizing enzymes and drug-transporters are crucial for metabolism and disposition of illicit substances as well as clinical drugs. However, both illicit substances and clinical drugs can induce changes in the expression and/or activity of drug-metabolizing enzymes and drug-transporters. Consequently, with concomitant usage, illicit substances can adversely influence the therapeutic outcome of coadministered clinical drugs. Likewise, clinical drugs can adversely affect the response of coadministered illicit substances. While the interactions between illicit substances and clinical drugs pose a tremendous health and financial burden, they lack a similar level of attention as drug-drug, food-drug, supplement-drug, herb-drug, disease-drug, or other substance-drug interactions such as alcohol-drug and tobacco-drug interactions. This review highlights the clinical pharmacokinetic interactions between clinical drugs and commonly used illicit substances such as cannabis, cocaine and 3, 4-Methylenedioxymethamphetamine (MDMA). Rigorous efforts are warranted to further understand the underlying mechanisms responsible for these clinical pharmacokinetic interactions. It is also critical to extend the awareness of the life-threatening adverse interactions to both health care professionals and patients.

Kappa Opioid Receptor Agonist Mesyl Sal B Attenuates Behavioral Sensitization to Cocaine with Fewer Aversive Side-Effects than Salvinorin A in Rodents

The acute activation of kappa opioid receptors (KOPr) produces antinociceptive and anti-cocaine effects, however, their side-effects have limited further clinical development. Mesyl Sal B is a potent and selective KOPr analogue of Salvinorin A (Sal A), a psychoactive natural product isolated from the plant Salvia divinorum. We assessed the antinociceptive, anti-cocaine, and side-effects of Mesyl Sal B. The anti-cocaine effects are evaluated in cocaine-induced hyperactivity and behavioral sensitization to cocaine in male Sprague Dawley rats. Mesyl Sal B was assessed for anhedonia (conditioned taste aversion), aversion (conditioned place aversion), pro-depressive effects (forced swim test), anxiety (elevated plus maze) and learning and memory deficits (novel object recognition). In male B6.SJL mice, the antinociceptive effects were evaluated in warm-water (50 °C) tail withdrawal and intraplantar formaldehyde (2%) assays and the sedative effects measured with the rotarod performance task. Mesyl Sal B (0.3 mg/kg) attenuated cocaine-induced hyperactivity and behavioral sensitization to cocaine without modulating sucrose self-administration and without producing aversion, sedation, anxiety, or learning and memory impairment in rats. However, increased immobility was observed in the forced swim test indicating pro-depressive effects. Mesyl Sal B was not as potent as Sal A at reducing pain in the antinociceptive assays. In conclusion, Mesyl Sal B possesses anti-cocaine effects, is longer acting in vivo and has fewer side-effects when compared to Sal A, however, the antinociceptive effects are limited.

The C-2 derivatives of salvinorin A, ethoxymethyl ether Sal B and β-tetrahydropyran Sal B, have anti-cocaine properties with minimal side effects

RATIONALE

Kappa-opioid receptor (KOPr) agonists have pre-clinical anti-cocaine and analgesic effects. However, side effects including sedation, dysphoria, aversion, anxiety and depression limit their therapeutic development. The unique structure of salvinorin A has been used to develop longer acting KOPr agonists.

OBJECTIVES

We evaluate two novel C-2 analogues of salvinorin A, ethoxymethyl ether Sal B (EOM Sal B) and β-tetrahydropyran Sal B (β-THP Sal B) alongside U50,488 for their ability to modulate cocaine-induced behaviours and side effects, pre-clinically.

METHODS

Anti-cocaine properties of EOM Sal B were evaluated using the reinstatement model of drug seeking in self-administering rats. EOM Sal B and β-THP Sal B were evaluated for effects on cocaine-induced hyperactivity, spontaneous locomotor activity and sucrose self-administration. EOM Sal B and β-THP Sal B were evaluated for aversive, anxiogenic and depressive-like effects using conditioned place aversion (CPA), elevated plus maze (EPM) and forced swim tests (FSTs), respectively.

RESULTS

EOM Sal B (0.1, 0.3 mg/kg, intraperitoneally (i.p.)) dose dependently attenuated drug seeking, and EOM Sal B (0.1 mg/kg, i.p.) and β-THP Sal B (1 mg/kg, i.p.) attenuated cocaine-induced hyperactivity. No effects on locomotor activity, open arm times (EPM) or swimming behaviours (FST) were seen with EOM (0.1 or 0.3 mg/kg, i.p.) or β-THP Sal B (1 or 2 mg/kg, i.p.). However, β-THP Sal B decreased time spent in the drug-paired chamber.

CONCLUSION

EOM Sal B is more potent than Sal A and β-THP Sal B in reducing drug-seeking behaviour with fewer side effects. EOM Sal B showed no effects on sucrose self-administration (0.1 mg/kg), locomotor, depressive-like, aversive-like or anxiolytic effects.

22-azidosalvinorin A exhibits antidepressant-like effect in mice

The increasing cases of depression has made the searches for new drugs and understanding of the underligning neurobiology of this psychiatric disorder a necessity. Here, we modified the structure of salvinorin A (a known halucinogen) and investigated antidepressant-like activity of its four derivatives; 22-methylsulfanylsalvinorin A(SA1), 2-O-cinnamoylsalvinorin B (CSB), 22-azidosalvinorin A (SA2), and 2-O-(4^'-azidophenylsulfonyl)salvinorin B (SA3). Prior to behavioural tests (Irwin test, open field test - OFT, forced swimming test - FST and tail suspension test - TST), SA1 was prepared by reacting salvinorin B and methylthioacetic acid with 89% yield; CSB was obtained from the reaction of salvinorin B and cinnamic acid with 92% yield; SA2 was obtained from the reaction of salvinorin B and azidoacetic acid with 81% yield; and SA3 was prepared by reacting salvinorin B with 4-azidophenylsulfonyl chloride with 80% yield. Oral treatment of mice with these derivatives (1-1000mg/kg) did not elicit toxic sign or death. Unlike SA, SA1, CSB and SA3, treatment with SA2 (5, 10 and 20mg/kg) decreased the immobility (TST and FST) and swimming time (FST) without altering locomotor activity in OFT. A decrease in the immobility time in TST and FST confirmed antidepressant-like property of SA2. Although p-chlorophenylalanine (serotonin depletor) or WAY100635 (selective 5-HT_1A receptor antagonist) did not attenuate effect of SA2, alpha-methyl-para-tyrosine (catecholamine depletor) and prazosin (selective α₁-receptor antagonist) attenuated this effect. SA2 mildly inhibited monoamine oxidase and showed affinity for α₁A, α₁B, α₁D and κ-opioid receptor subtypes. In summary, SA2 induced monoamine-mediated antidepressant-like effect.

Kappa Opioids, Salvinorin A and Major Depressive Disorder

Opioids are traditionally associated with pain, analgesia and drug abuse. It is now clear, however, that the opioids are central players in mood. The implications for mood disorders, particularly clinical depression, suggest a paradigm shift from the monoamine neurotransmitters to the opioids either alone or in interaction with monoamine neurons. We have a special interest in dynorphin, the last of the major endogenous opioids to be isolated and identified. Dynorphin is derived from the Greek word for power, dynamis, which hints at the expectation that the neuropeptide held for its discoverers. Yet, dynorphin and its opioid receptor subtype, kappa, has always taken a backseat to the endogenous b-endorphin and the exogenous morphine that both bind the mu opioid receptor subtype. That may be changing as the dynorphin/ kappa system has been shown to have different, often opposite, neurophysiological and behavioral influences. This includes major depressive disorder (MDD). Here, we have undertaken a review of dynorphin/ kappa neurobiology as related to behaviors, especially MDD. Highlights include the unique features of dynorphin and kappa receptors and the special relation of a plant-based agonist of the kappa receptor salvinorin A. In addition to acting as a kappa opioid agonist, we conclude that salvinorin A has a complex pharmacologic profile, with potential additional mechanisms of action. Its unique neurophysiological effects make Salvinorina A an ideal candidate for MDD treatment research.

Salvia divinorum: An overview of the usage, misuse, and addiction processes

Salvia divinorum, a sage plant with leaves that can produce a psychoactive high, has been used for hundreds of years for its psycho-mimetic effects in religious rituals in South America. Salvia has now become popular mainly with adolescents and young adults for the short-lived relatively pleasant experiences many consider a "legal high" and its ready availability through Internet purchases. The main (psycho)active compound in salvia is Salvinorin A, a potent κ-opioid agonist and although the short and long-term effects have not been examined in sufficient detail, it is widely believed to have low addictive potential and low toxicity. Recent findings, however, seem to suggest that Salvinorin A can precipitate psychiatric symptoms and negatively affect cognition. Its ready availability and increasingly widespread use requires clinicians to have knowledge and awareness of its effects.

Salvinorin A, a kappa-opioid receptor agonist hallucinogen: pharmacology and potential template for novel pharmacotherapeutic agents in neuropsychiatric disorders

Salvinorin A is a potent hallucinogen, isolated from the ethnomedical plant Salvia divinorum. Salvinorin A is a selective high efficacy kappa-opioid receptor (KOPr) agonist, and thus implicates the KOPr system and its endogenous agonist ligands (the dynorphins) in higher functions, including cognition and perceptual effects. Salvinorin A is the only selective KOPr ligand to be widely available outside research or medical settings, and salvinorin A-containing products have undergone frequent non-medical use. KOPr/dynorphin systems in the brain are known to be powerful counter-modulatory mechanisms to dopaminergic function, which is important in mood and reward engendered by natural and chemical reinforcers (including drugs of abuse). KOPr activation (including by salvinorin A) can thus cause aversion and anhedonia in preclinical models. Salvinorin A is also a completely new scaffold for medicinal chemistry approaches, since it is a non-nitrogenous neoclerodane, unlike other known opioid ligands. Ongoing efforts have the goal of discovering novel semi-synthetic salvinorin analogs with potential KOPr-mediated pharmacotherapeutic effects (including partial agonist or biased agonist effects), with a reduced burden of undesirable effects associated with salvinorin A.

Studies toward the Development of Antiproliferative Neoclerodanes from Salvinorin A

The success rate for central nervous system (CNS) drug candidates in the clinic is relatively low compared to the industry average across other therapeutic areas. Penetration through the blood-brain barrier (BBB) to reach the therapeutic target is a major obstacle in development. The rapid CNS penetration of salvinorin A has suggested that the neoclerodane nucleus offers an excellent scaffold for developing antiproliferative compounds that enter the CNS. The Liebeskind-Srogl reaction was used as the main carbon-carbon bond-forming step toward the synthesis of quinone-containing salvinorin A analogues. Quinone-containing salvinorin A analogues were shown to have antiproliferative activity against the MCF7 breast cancer cell line, but show no significant activity at the κ-opioid receptors. In an in vitro model of BBB penetration, quinone-containing salvinorin A analogues were shown to passively diffuse across the cell monolayer. The analogues, however, are substrates of P-glycoprotein, and thus further modification of the molecules is needed to reduce the affinity for the efflux transporter.

Sex differences in sensitivity to the depressive-like effects of the kappa opioid receptor agonist U-50488 in rats

BACKGROUND

Dynorphin, an endogenous ligand at kappa opioid receptors (KORs), produces depressive-like effects and contributes to addictive behavior in male nonhuman primates and rodents. Although comorbidity of depression and addiction is greater in women than men, the role of KORs in female motivated behavior is unknown.

METHODS

In adult Sprague-Dawley rats, we used intracranial self-stimulation to measure effects of the KOR agonist (±)-trans-U-50488 methanesulfonate salt (U-50488) (.0-10.0 mg/kg) on brain stimulation reward in gonadally intact and castrated males and in females at estrous cycle stages associated with low and high estrogen levels. Pharmacokinetic studies of U-50488 in plasma and brain were conducted. Immunohistochemistry was used to identify sex-dependent expression of U-50488-induced c-Fos in brain.

RESULTS

U-50488 dose-dependently increased the frequency of stimulation (threshold) required to maintain intracranial self-stimulation responding in male and female rats, a depressive-like effect. However, females were significantly less sensitive than males to the threshold-increasing effects of U-50488, independent of estrous cycle stage in females or gonadectomy in males. Although initial plasma concentrations of U-50488 were higher in females, there were no sex differences in brain concentrations. Sex differences in U-50488-induced c-Fos activation were observed in corticotropin releasing factor-containing neurons of the paraventricular nucleus of the hypothalamus and primarily in non-corticotropin releasing factor-containing neurons of the bed nucleus of the stria terminalis.

CONCLUSIONS

These data suggest that the role of KORs in motivated behavior of rats is sex-dependent, which has important ramifications for the study and treatment of mood-related disorders, including depression and drug addiction in people.

From local to global-fifty years of research on Salvia divinorum

ETHNOPHARMACOLOGICAL RELEVANCE

In 1962 ethnopharmacologists, Hofmann and Wasson, undertook an expedition to Oaxaca, Mexico. These two researchers were the first scientists to collect a flowering specimen of Salvia divinorum allowing the identification of this species. While the species' traditional use is confined to a very small region of Mexico, since Hofmann and Wasson's expedition 50 years ago, Salvia divinorum has become globally recognized for its main active constituent, the diterpene salvinorin A, which has a unique effect on human physiology. Salvinorin A is a kappa-opioid agonist and the first reported psychoactive diterpene.

METHODS

This review concentrates on the investigation of Salvia divinorum over the last 50 years including ethnobotany, ethnopharmacology, taxonomy, systematics, genetics, chemistry and pharmacodynamic and pharmacokinetic research. For the purpose of this review, online search engines were used to find relevant research. Searches were conducted between October 2011 and September 2013 using the search term "Salvia divinorum". Papers were excluded if they described synthetic chemical synthesis of salvinorin A or analogues.

RESULTS

Ethnobotanically there is a comprehensive body of research describing the traditional Mazatec use of the plant, however, the modern ethnobotanical use of this plant is not well documented. There are a limited number of botanical investigations into this plant and there are still several aspects of the botany of Salvia divinorum which need further investigation. One study has investigated the phylogenetic relationship of Salvia divinorum to other species in the genus. To date the main focus of chemistry research on Salvia divinorum has been salvinorin A, the main active compound in Salvia divinorum, and other related diterpenoids. Finally, the effects of salvinorin A, a KOR agonist, have primarily been investigated using animal models.

CONCLUSIONS

As Salvia divinorum use increases worldwide, the emerging cultural use patterns will warrant more research. More botanical information is also needed to better understand this species, including germination, pollination vector and a better understanding of the endemic environment of Salvia divinorum. As well there is a gap in the genetic knowledge of this species and very little is known about its intra-species genetics. The terpenes in Salvia divinorum are very well documented, however, other classes of constituents in this species warrant further investigation and identification. To date, the majority of the pharmacology research on Salvia divinorum has focused on the effects of salvinorin A using animal models. Published human studies have not reported any harmful effects when salvinorin A is administered within the dose range of 0.375-21µg/kg but what are the implications when applied to a larger population? More data on the toxicology and safety of Salvia divinorum are needed before larger scale clinical trials of the potential therapeutic effects of Salvia divinorum and salvinorin A are undertaken.

Salvinorin A: A Mini Review of Physical and Chemical Properties Affecting Its Translation from Research to Clinical Applications in Humans

Salvinorin A is a potent and selective agonist of kappa opioid receptors in the brain. Recent studies in several animal models have revealed that Salvinorin A has anti-addiction, anti-depression properties and exhibits pronounced neuroprotective effects against hypoxia/ischemia induced brain damage, and have raised interest in potential clinical applications in several acute pathologies involving oxygen deficiency in the brain. This review focuses on the chemical and physical properties of Salvinorin A and their impact on development of a rational formulation to enable its translation from a research compound to a novel therapeutic agent.

LC-MS/MS quantification of salvinorin A from biological fluids

A facile method for quantifying the concentration of the powerful and widely available hallucinogen salvinorin A (a selective kappa opioid agonist) from non-human primate cerebrospinal fluid (CSF) and human plasma has been developed using liquid chromatography-tandem mass spectrometry (LC-MS/MS) in positive electrospray ionization (ESI) mode. With CSF solid phase extraction can be avoided completely by simply diluting each sample to 10 % (v/v) acetonitrile, 1 % (v/v) formic acid and injecting under high aqueous conditions for analyte focusing. Extensive plasma sample preparation was investigated including protein precipitation, SPE column selection, and plasma particulate removal. Human plasma samples were centrifuged at 21,000 × gravity for 4 minutes to obtain clear particulate-free plasma, from which 300 μl was spiked with internal standard and loaded onto a C₁₈ SPE column with a 100 mg mL^-1 loading capacity. Guard columns (C_18, hand packed 1 mm × 20 mm) were exchanged after backpressure increased above 4600psi, about 250 injections. A shallow acetonitrile/water gradient was used, 29 to 33% CH₃CN over 8 minutes to elute salvinorin A. Reduction of chemical noise was achieved using tandem mass spectrometry with multiple reaction monitoring while sensitivity increases were observed using a 50 μL injection volume onto a small bore analytical column (C_18, 1 mm ID × 50 mm) thus increasing peak concentration. Limits of quantification were found to be 0.0125 ng mL^-1 (CSF) and 0.05 ng mL^-1 (plasma) with interday precision and accuracy below 1.7 % and 9.42 % (CSF) and 3.47 % and 12.37 % (plasma) respectively. This method was used to determine the concentration of salvinorin A from an in vivo Rhesus monkey study and a trial of healthy human research participants, using behaviorally active doses.

Endogenous opiates and behavior: 2012

This paper is the thirty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2012 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The 2-methoxy methyl analogue of salvinorin A attenuates cocaine-induced drug seeking and sucrose reinforcements in rats

κ Opioid receptor activation by traditional arylacetamide agonists and the novel neoclerodane diterpene κ opioid receptor agonist Salvinorin A (Sal A) results in attenuation of cocaine-seeking behavior in pre-clinical models of addiction. However, adverse effects such as sedation, depression and aversion limit their clinical utility. The Sal A analogue, 2-methoxy-methyl salvinorin B (MOM Sal B) is a longer acting Sal A analogue with high affinity for κ opioid receptors. In this study, we tested MOM Sal B for its ability to modulate cocaine-seeking behavior in rats. MOM Sal B (0.3mg/kg) successfully attenuated cocaine-seeking but also attenuated sucrose reinforcement. No change in activity was observed in either cocaine-induced hyperactivity or spontaneous open field activity tests but increased immobility and decreased swimming times in the forced swim test were observed. This study indicates that κ opioid receptor activation by more potent Sal A analogues modulates cocaine-seeking behavior non-selectively without causing sedation, suggesting an improved side effects profile. However, pro-depressive effects are seen, which may limit the therapeutic potential of this compound. Future studies with Sal A analogues having affinities at other opioid receptors are warranted as they have the potential to identify compounds having effective anti-addiction properties.

A method for conducting functional MRI studies in alert nonhuman primates: initial results with opioid agonists in male cynomolgus monkeys

Functional MRI (fMRI) has emerged as a powerful technique for assessing neural effects of psychoactive drugs and other stimuli. Several experimental approaches have been developed to use fMRI in anesthetized and awake animal subjects, each of which has its advantages and complexities. We sought to assess whether one particular method to scan alert postanesthetized animals can be used to assess fMRI effects of opioid agonists. To date, the use of fMRI as a method to compare pharmacological effects of opioid drugs has been limited. Such studies are important because mu and kappa opioid receptor agonists produce distinct profiles of behavioral effects related both to clinically desirable endpoints (e.g., analgesia) and to undesirable effects (e.g., abuse potential). This study sought to determine whether we could use our fMRI approach to compare acute effects of behaviorally equipotent (3.2 μg/kg) intravenous doses of fentanyl and U69,593 (doses that do not affect cardiorespiratory parameters). Scans were acquired in alert male cynomolgus macaques acclimated to undergo fMRI scans under restraint, absent excessive stress hormone increases. These opioid agonists activated bilateral striatal and nucleus accumbens regions of interest. At the dose tested, U69,593 induced greater left nucleus accumbens BOLD activation than fentanyl, while fentanyl activated left dorsal caudate nucleus more than U69,593. Our results suggest that our fMRI approach could be informative for comparing effects of opioid agonists.

Neoclerodanes as atypical opioid receptor ligands

The neoclerodane diterpene salvinorin A is the major active component of the hallucinogenic mint plant Salvia divinorum Epling and Játiva (Lamiaceae). Since the finding that salvinorin A exerts its potent psychotropic actions through the activation of opioid receptors, the site of action of morphine and related analogues, there has been much interest in elucidating the underlying mechanisms behind its effects. These effects are particularly remarkable because (1) salvinorin A is the first reported non-nitrogenous opioid receptor agonist and (2) its effects are not mediated through the previously investigated targets of psychotomimetics. This Perspective outlines our research program, illustrating a new direction to the development of tools to further elucidate the biological mechanisms of drug tolerance and dependence. The information gained from these efforts is expected to facilitate the design of novel agents to treat pain, drug abuse, and other central nervous system disorders.