Challenges in the implementation of EAACI guidelines on allergen immunotherapy: A global perspective on the regulation of allergen products.

Regulatory approaches for allergen immunotherapy (AIT) products and the availability of high-quality AIT products are inherently linked to each other. While allergen products are available in many countries across the globe, their regulation is very heterogeneous. First, we describe the regulatory systems applicable for AIT products in the European Union (EU) and in the United States (US). For Europe, a depiction of the different types of relevant procedures, as well as the committees involved, is provided and the fundamental role of national agencies of the EU member states in this complex and unique network is highlighted. Furthermore, the regulatory agencies from Australia, Canada, Japan, Russia, and Switzerland provided information on the system implemented in their countries for the regulation of allergen products. While AIT products are commonly classified as biological medicinal products, they are made available by varying types of procedures, most commonly either by obtaining a marketing authorization or by being distributed as named patient products. Exemptions from marketing authorizations in exceptional cases, as well as import of allergen products from other countries, are additional tools applied by countries to ensure availability of needed AIT products. Several challenges for AIT products are apparent from this analysis and will require further consideration.

Macrophages-common culprit in obesity and asthma.

Macrophages are essential innate immune cells that also regulate local metabolism. Endogenous or exogenous stimuli may polarize macrophages toward phenotypes that serve distinct innate immunological metabolic functions. IFN-γ or lipopolysaccharide (LPS) polarizes macrophages toward the M1, or "classically activated" phenotype that participates in defense against intracellular pathogens. IL-4, IL-13, or chitin polarizes macrophages toward the M2, or "alternatively activated" phenotype, which defends against multicellular nematodes and fungi. As macrophages polarize in local environments, M1 and M2 macrophages may coexist in different organs and may differentially affect asthma and obesity, two comorbid diseases where polarized macrophages contribute to their pathogenesis. While M1 macrophages are considered beneficial in asthma and contribute to the pathology of obesity, M2 macrophages contribute to the pathology of asthma, but limit metabolic syndrome associated with obesity. Here, we discuss the roles for M1 and M2 macrophages in asthma and obesity, and propose a model by which M1-mediated inflammation in adipose tissue enhances M2-mediated inflammation in the asthmatic lung.

Diagnostic exome sequencing in children: A survey of parental understanding, experience and psychological impact.

Clinical exome sequencing (CES) is increasingly being used as an effective diagnostic tool in the field of pediatric genetics. We sought to evaluate the parental experience, understanding and psychological impact of CES by conducting a survey study of English-speaking parents of children who had diagnostic CES. Parents of 192 unique patients participated. The parent's interpretation of the child's result agreed with the clinician's interpretation in 79% of cases, with more frequent discordance when the clinician's interpretation was uncertain. The majority (79%) reported no regret with the decision to have CES. Most (65%) reported complete satisfaction with the genetic counseling experience, and satisfaction was positively associated with years of genetic counselor (GC) experience. The psychological impact of CES was greatest for parents of children with positive results and for parents with anxiety or depression. The results of this study are important for helping clinicians to prepare families for the possible results and variable psychological impact of CES. The frequency of parental misinterpretation of test results indicates the need for additional clarity in the communication of results. Finally, while the majority of patients were satisfied with their genetic counseling, satisfaction was lower for new GCs, suggesting a need for targeted GC training for genomic testing.

Allergen manufacturing and quality aspects for allergen immunotherapy in Europe and the United States: An analysis from the EAACI AIT Guidelines Project.

Adequate quality is essential for any medicinal product to be eligible for marketing. Quality includes verification of the identity, content and purity of a medicinal product in combination with a specified production process and its control. Allergen products derived from natural sources require particular considerations to ensure adequate quality. Here, we describe key aspects of the documentation on manufacturing and quality aspects for allergen immunotherapy products in the European Union and the United States. In some key parts, requirements in these areas are harmonized while other fields are regulated separately between both regions. Essential differences are found in the use of Reference Preparations, or the requirement to apply standardized assays for potency determination. As the types of products available are different in specific regions, regulatory guidance for such products may also be available in one specific region only, such as for allergoids in the European Union. Region-specific issues and priorities are a result of this. As allergen products derived from natural sources are inherently variable in their qualitative and quantitative composition, these products present special challenges to balance the variability and ensuring batch-to-batch consistency. Advancements in scientific knowledge on specific allergens and their role in allergic disease will consequentially find representation in future regulatory guidelines.

Accurate quantification of 5 German cockroach (GCr) allergens in complex extracts using multiple reaction monitoring mass spectrometry (MRM MS).

BACKGROUND: German cockroach (GCr) allergen extracts are complex and heterogeneous products, and methods to better assess their potency and composition are needed for adequate studies of their safety and efficacy. OBJECTIVE AND METHODS: The objective of this study was to develop an assay based on liquid chromatography and multiple reaction monitoring mass spectrometry (LC-MRM MS) for rapid, accurate, and reproducible quantification of 5 allergens (Bla g 1, Bla g 2, Bla g 3, Bla g 4, and Bla g 5) in crude GCr allergen extracts. RESULTS: We first established a comprehensive peptide library of allergens from various commercial extracts as well as recombinant allergens. Peptide mapping was performed using high-resolution MS, and the peptide library was then used to identify prototypic and quantotypic peptides to proceed with MRM method development. Assay development included a systematic optimization of digestion conditions (buffer, digestion time, and trypsin concentration), chromatographic separation, and MS parameters. Robustness and suitability were assessed following ICH (Q2 [R1]) guidelines. The method is precise (RSD < 10%), linear over a wide range (r > 0.99, 0.01-1384 fmol/µL), and sensitive (LLOD and LLOQ <1 fmol/µL). Having established the parameters for LC-MRM MS, we quantified allergens from various commercial GCr extracts and showed considerable variability that may impact clinical efficacy. CONCLUSIONS AND CLINICAL RELEVANCE: Our data demonstrate that the LC-MRM MS method is valuable for absolute quantification of allergens in GCr extracts and likely has broader applicability to other complex allergen extracts. Definitive quantification provides a new standard for labelling of allergen extracts, which will inform patient care, enable personalized therapy, and enhance the efficacy of immunotherapy for environmental and food allergies.

Mass spectrometry to complement standardization of house dust mite and other complex allergenic extracts.

In the United States, the Center for Biologics Evaluation and Research of the US Food and Drug Administration regulates biologics used for diagnosis and treatment of allergic diseases. The Code of Federal Regulations 21CFR680.3(e) states that when measured, the potency of an allergenic extract is assessed according to its allergenic activity. As of 2016, 19 allergenic extracts are standardized for potency in the United States. While these standardized extracts constitute a minority of those available, they treat the most prevalent allergies (e.g. grass and ragweed pollens, dust mites, and cat) and those that induce life-threatening anaphylaxis (e.g. Hymenoptera venom). Standardization for potency enhances safety and efficacy of immunotherapy by minimizing the risks of variations in allergen dosing when switching from one lot of manufactured extract to another, and by providing an objective measure of stability of each lot of allergenic extract over time. Allergenic extracts that have multiple immunodominant allergenic proteins are standardized with little or no information about compositional differences among extracts. Here, we propose application of mass spectrometry towards measurement of compositional differences among extracts that may affect the efficacy and safety of allergen immunotherapy. In addition, we discuss of house dust mite allergen extracts as a prototypical complex extract that may be standardized by mass spectrometry.

Allergen exposure chambers: harmonizing current concepts and projecting the needs for the future - an EAACI Position Paper.

BACKGROUND: Allergen exposure chambers (AECs) are clinical facilities allowing for controlled exposure of subjects to allergens in an enclosed environment. AECs have contributed towards characterizing the pathophysiology of respiratory allergic diseases and the pharmacological properties of new therapies. In addition, they are complementary to and offer some advantages over traditional multicentre field trials for evaluation of novel therapeutics. To date, AEC studies conducted have been monocentric and have followed protocols unique to each centre. Because there are technical differences among AECs, it may be necessary to define parameters to standardize the AECs so that studies may be extrapolated for driving basic immunological research and for marketing authorization purposes by regulatory authorities. METHODS: For this task force initiative of the European Academy of Allergy and Clinical Immunology (EAACI), experts from academia and regulatory agencies met with chamber operators to list technical, clinical and regulatory unmet needs as well as the prerequisites for clinical validation. RESULTS: The latter covered the validation process, standardization of challenges and outcomes, intra- and interchamber variability and reproducibility, in addition to comparability with field trials and specifics of paediatric trials and regulatory issues. CONCLUSION: This EAACI Position Paper aims to harmonize current concepts in AECs and to project unmet needs with the intent to enhance progress towards use of these facilities in determining safety and efficacy of new therapeutics in the future.

Understanding the Link Between Cannabinoids and Psychosis.

The changing legal landscape surrounding cannabis is likely a contributing factor to the increasing rates of cannabis use worldwide. Accordingly, consequences of cannabis use translate into sizeable public health implications. While the medicinal purposes of cannabis are often sensationalized, information regarding its harmful effects is largely confined to the scientific community. Therefore, educational campaigns informing the public about the links between cannabinoids and psychosis are urgently needed.

The nexus between atopic disease and autoimmunity: a review of the epidemiological and mechanistic literature.

There has been considerable interest in defining the relationship between the expression of allergic and autoimmune diseases in populations of patients. Are patients with autoimmune disease 'protected' from developing allergic (immunoglobulin E-mediated) diseases? Does the establishment of an atopic phenotype reduce the risk of the subsequent development of autoimmune diseases? Although there are clinical studies addressing this question, methodological problems, particularly in identification of atopic subjects, limits their usefulness. Moreover, an immune-based explanation of the observed epidemiological findings has relied on a paradigm that is currently undergoing increased scrutiny and modification to include newly defined effector cell subsets and the interaction between genetic and environmental factors, such as early endotoxin or mycobacterial exposure. To address this question, we reviewed a series of clinical reports that addressed coincidence or co-prevalence of atopy with four autoimmune diseases: psoriasis, rheumatoid arthritis, multiple sclerosis and type I diabetes mellitus. We present a model whereby active T helper type 1 (Th1) inflammation may suppress the development of atopy, and atopy may suppress the severity but not necessarily the onset of autoimmunity, and then discuss our model in the context of mechanisms of adaptive immunity with particular reference to the Th1/Th2 paradigms. Because the ultimate goal is to ameliorate or cure these diseases, our discussion may help to predict or interpret unexpected consequences of novel therapeutic agents used to target autoimmune or atopic diseases.

Marked decrease of LSD-induced stimulus control in serotonin transporter knockout mice.

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.

The stimulus effects of 8-OH-DPAT: evidence for a 5-HT2A receptor-mediated component.

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.

Psilocybin-induced stimulus control in the rat.

Although psilocybin has been trained in the rat as a discriminative stimulus, little is known of the pharmacological receptors essential for stimulus control. In the present investigation rats were trained with psilocybin and tests were then conducted employing a series of other hallucinogens and presumed antagonists. An intermediate degree of antagonism of psilocybin was observed following treatment with the 5-HT(2A) receptor antagonist, M100907. In contrast, no significant antagonism was observed following treatment with the 5-HT(1A/7) receptor antagonist, WAY-100635, or the DA D(2) antagonist, remoxipride. Psilocybin generalized fully to DOM, LSD, psilocin, and, in the presence of WAY-100635, DMT while partial generalization was seen to 2C-T-7 and mescaline. LSD and MDMA partially generalized to psilocybin and these effects were completely blocked by M-100907; no generalization of PCP to psilocybin was seen. The present data suggest that psilocybin induces a compound stimulus in which activity at the 5-HT(2A) receptor plays a prominent but incomplete role. In addition, psilocybin differs from closely related hallucinogens such as 5-MeO-DMT in that agonism at 5-HT(1A) receptors appears to play no role in psilocybin-induced stimulus control.

Hallucinogen-like actions of 5-methoxy-N,N-diisopropyltryptamine in mice and rats.

Few studies have examined the effects of 5-methoxy-N,N-diisopropyltryptamine (5-MeO-DIPT) in vivo. In these studies, 5-MeO-DIPT was tested in a drug-elicited head twitch assay in mice where it was compared to the structurally similar hallucinogen N,N-dimethyltryptamine (N,N-DMT) and challenged with the selective serotonin (5-HT)2A antagonist M100907, and in a lysergic acid diethylamide (LSD) discrimination assay in rats where its subjective effects were challenged with M100907 or the 5-HT 1A selective antagonist WAY-100635. Finally, the affinity of 5-MeO-DIPT for three distinct 5-HT receptors was determined in rat brain. 5-MeO-DIPT, but not N,N-DMT, induced the head twitch responses in the mouse, and this effect was potently antagonized by prior administration of M100907. In rats trained with LSD as a discriminative stimulus, there was an intermediate degree (75%) of generalization to 5-MeO-DIPT and a dose-dependent suppression of response rates. These interoceptive effects were abolished by M100907, but were not significantly attenuated by WAY-100635. Finally, 5-MeO-DIPT had micromolar affinity for 5-HT 2A and 5-HT 2C receptors, but much higher affinity for 5-HT 1A receptors. 5-MeO-DIPT is thus effective in two rodent models of 5-HT2 agonist activity, and has affinity at receptors relevant to hallucinogen effects. The effectiveness with which M100907 antagonizes the behavioral actions of this compound, coupled with the lack of significant antagonist effects of WAY-100635, strongly suggests that the 5-HT 2A receptor is an important site of action for 5-MeO-DIPT, despite its apparent in vitro selectivity for the 5-HT 1A receptor.

The 5-HT1A receptor and the stimulus effects of LSD in the rat.

RATIONALE: It has been suggested that the 5-HT1A receptor plays a significant modulatory role in the stimulus effects of the indoleamine hallucinogen lysergic acid diethylamide (LSD). OBJECTIVE: The present study sought to characterize the effects of several compounds with known affinity for the 5-HT1A receptor on the discriminative stimulus effects of LSD. METHODS: Twelve male Fischer 344 rats were trained in a two-lever, fixed-ratio (FR) 10, and food-reinforced task with LSD (0.1 mg/kg, i.p.; 15-min pretreatment) as a discriminative stimulus. Combination and substitution tests with the 5-HT(1A) agonists, 8-OH-DPAT, buspirone, gepirone, and ipsapirone, with LSD-induced stimulus control were then performed. The effects of these 5-HT1A ligands were also tested in the presence of the selective 5-HT1A receptor antagonist, WAY-100,635 (0.3 mg/kg, s.c.; 30-min pretreatment). RESULTS: In combination tests, stimulus control by LSD was increased by all 5-HT1A receptor ligands with agonist properties. Similarly, in tests of antagonism, the increase in drug-appropriate responding caused by stimulation of the 5-HT1A receptor was abolished by administration of WAY-100,635. CONCLUSION: These data, obtained using a drug discrimination model of the hallucinogenic effects of LSD, provide support for the hypothesis that the 5-HT1A receptor has a significant modulatory role in the stimulus effects of LSD.

The stimulus properties of LSD in C57BL/6 mice.

RATIONALE: Drug-induced stimulus control has proven to be a powerful tool for the assessment of a wide range of psychoactive drugs. Although a variety of species has been employed, the majority of studies have been in the rat. However, with the development of techniques which permit the genetic modification of mice, the latter species has taken on new importance. Lysergic acid diethylamide [LSD], the prototypic indoleamine hallucinogen, has not previously been trained as a discriminative stimulus in mice. OBJECTIVE: To demonstrate the feasibility of LSD-induced stimulus control in the mouse and to provide a preliminary characterization of the stimulus properties of LSD in that species. METHODS: Male C57BL/6 mice were trained using a left or right nose-poke operant on a fixed ratio 10, water reinforced task following the injection of lysergic acid diethylamide [LSD, 0.17 or 0.30 mg/kg, s.c.; 15 min pretreatment] or vehicle. RESULTS: Stimulus control was established in 6 of 16 mice at a dose of LSD of 0.17 mg/kg after 39 sessions. An increase in dose to 0.30 mg/kg for the remaining mice resulted in stimulus control in an additional 5 subjects. In the low dose group, subsequent experiments demonstrated an orderly dose-effect relationship for LSD and a rapid offset of drug action with an absence of LSD effects 60 min after injection. When LSD [0.17 mg/kg] was administered in combination with the selective 5-HT2A antagonist, M100907, LSD-appropriate responding was significantly but incompletely reduced to approximately 50%; concurrently, response rates declined significantly. In mice trained with a dose of LSD of 0.30 mg/kg, full generalization to the phenethylamine hallucinogen, [-]-2,5-dimethoxy-4-methylamphetamine [DOM] was observed. CONCLUSIONS: The present data demonstrate the feasibility of LSD-induced stimulus control in the mouse. The general features of stimulus control by LSD in the mouse closely resemble those observed in the rat but the present data suggest that there may be significant differences as well.

Serotonergic/glutamatergic interactions: potentiation of phencyclidine-induced stimulus control by citalopram.

Previous investigations in our laboratory have found that the stimulus effects of the hallucinogenic serotonergic agonists DOM and LSD are potentiated by phencyclidine [PCP], a non-competitive NMDA antagonist. Also suggestive of behaviorally significant serotonergic/glutamatergic interactions is our finding that stimulus control by both PCP and LSD is partially antagonized by the mGlu2/3 agonist, LY 379268. These observations coupled with the fact that the stimulus effects of LSD and DOM are potentiated by selective serotonin reuptake inhibitors [SSRIs] led us in the present investigation to test the hypothesis that stimulus control by PCP is potentiated by the SSRI, citalopram. Stimulus control was established with PCP [3.0 mg/kg; 30 min pretreatment time] in a group of 12 rats. A two-lever, fixed ratio 10, positively reinforced task with saline controls was employed. Potentiation by citalopram of an intermediate dose of PCP was observed. In an attempt to establish the mechanism by which citalopram might interact with PCP, subsequent experiments examined the effects on that interaction of antagonists at serotonergic receptors. It was found that the selective 5-HT2C-selective antagonists, SDZ SER 082 and SB 242084, significantly, albeit only partially, blocked the effects of citalopram on PCP. In agreement with our previous conclusions regarding the interaction of citalopram with DOM, the present data suggest that potentiation of the stimulus effects of PCP by citalopram are mediated in part by agonist activity at 5-HT2C receptors.

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.

A 5-HT(2C) receptor-mediated interaction between 2,5-dimethoxy-4-methylamphetamine and citalopram in the rat.

Previous studies conducted in our laboratory have shown that acute administration of the selective serotonin re-uptake inhibitor (SSRI), citalopram, potentiates the stimulus effects of the phenethylamine hallucinogen [-]-2,5-dimethoxy-4-methylamphetamine (DOM) in the rat while neither substituting for the DOM stimulus when administered alone nor altering brain levels of DOM. The present investigation was designed to determine the mechanism by which citalopram acts on DOM-induced stimulus control. To that end, we tested the following hypotheses: (a) citalopram blocks the transport of DOM by the serotonin transporter, (b) citalopram acts via the 5-HT(1A) receptor, and (c) citalopram acts via the 5-HT(2C) receptor. Hypothesis (a) was rejected on the basis of equilibrium saturation studies of [3H]citalopram binding, which revealed no significant affinity of DOM for the 5-HT transporter of rat brain membranes. Hypotheses (b) and (c) were tested in a group of 20 rats in which stimulus control was established with DOM (0.6 mg/kg; 75 min pretreatment time). A two-lever, fixed ratio 10 (FR10), positively reinforced task with saline controls was employed. Hypothesis (b), a role for the 5-HT(1A) receptor, was rejected on the basis of an absence of antagonism of the effects of citalopram on DOM by the selective 5-HT(1A) receptor antagonist, WAY-100635. In contrast, Hypothesis (c), a role for the 5-HT(2C) receptor, gained support from the observation of significant antagonism of the effects of citalopram on DOM by the selective 5-HT(2C) receptor antagonist, SB-242084.

Serotonergic/glutamatergic interactions: the effects of mGlu2/3 receptor ligands in rats trained with LSD and PCP as discriminative stimuli.

RATIONALE: On the basis of electrophysiological evidence, it has been proposed that both antagonism of NMDA receptors by drugs such as PCP and stimulation of 5- HT(2A) receptors by drugs such as LSD result in the release of glutamate. Furthermore, it has been observed that antagonists and agonists at mGlu(2/3) receptors increase and decrease, respectively, the release of glutamate. Taken together, these observations predict behaviorally significant interactions between ligands at mGlu(2/3) receptors and hallucinogens such as LSD and PCP. OBJECTIVE: The present study sought to test in rats the glutamate hypothesis of hallucinogenesis using drug-induced stimulus control as the dependent variable and selected glutamatergic and serotonergic receptor ligands as independent variables. METHODS: Male F-344 rats were trained in a two-lever, fixed ratio 10, food-reinforced task with either phencyclidine (PCP; 3.0 mg/kg; i.p.; 30 min pretreatment) or lysergic acid diethylamide (LSD; 0.1 mg/kg; IP; 15 min pretreatment) as discriminative stimuli. The interactions of PCP and the mGlu(2/3) selective ligands, LY341495 and LY379268, with stimulus control by LSD were determined. The effects of these drugs were compared with those of serotonergic antagonists known to antagonize the stimulus effects of LSD, specifically, pirenperone and M100907. RESULTS: Stimulus control by LSD was potentiated by both PCP and the mGlu(2/3) antagonist, LY341495. In tests of antagonism, stimulus control by LSD was significantly but incompletely diminished by the mGlu(2/3) agonist, LY379268; this result was in contrast with the complete antagonism of LSD by both pirenperone and M100907. In PCP-trained rats, LY341495 was without effect on stimulus control by an intermediate dose of PCP. In contrast, the training dose of PCP was significantly but incompletely antagonized by LY379268. CONCLUSIONS: These data, obtained using a stimulus control model of the hallucinogenic effects of PCP and LSD, provide support for the hypothesis that glutamate release is a factor in hallucinogenesis by both 5-HT(2) agonists and non-competitive NMDA antagonists.