A Second Space Age Spanning Omics, Platforms, and Medicine Across Orbits.

The recent acceleration of commercial, private, and multi-national spaceflight has created an unprecedented level of activity in low Earth orbit (LEO), concomitant with the highest-ever number of crewed missions entering space and preparations for exploration-class (>1 year) missions. Such rapid advancement into space from many new companies, countries, and space-related entities has enabled a"Second Space Age." This new era is also poised to leverage, for the first time, modern tools and methods of molecular biology and precision medicine, thus enabling precision aerospace medicine for the crews. The applications of these biomedical technologies and algorithms are diverse, encompassing multi-omic, single-cell, and spatial biology tools to investigate human and microbial responses to spaceflight. Additionally, they extend to the development of new imaging techniques, real-time cognitive assessments, physiological monitoring, and personalized risk profiles tailored for astronauts. Furthermore, these technologies enable advancements in pharmacogenomics (PGx), as well as the identification of novel spaceflight biomarkers and the development of corresponding countermeasures. In this review, we highlight some of the recent biomedical research from the National Aeronautics and Space Administration (NASA), Japan Aerospace Exploration Agency (JAXA), European Space Agency (ESA), and other space agencies, and also detail the commercial spaceflight sector's (e.g. SpaceX, Blue Origin, Axiom, Sierra Space) entrance into aerospace medicine and space biology, the first aerospace medicine biobank, and the myriad upcoming missions that will utilize these tools to ensure a permanent human presence beyond LEO, venturing out to other planets and moons.

Impact of Serine Racemase Deletion on Nicotine Discrimination.

INTRODUCTION: The high comorbidity between schizophrenia and cigarette smoking points to a possible shared heritable factor predisposing individuals with schizophrenia to nicotine addiction. The N-methyl-D-aspartate (NMDA) receptor has been highly implicated in both schizophrenia and nicotine addiction. METHODS: In the present study, we used mice with a null mutation on the serine racemase gene (srr), an established risk gene for schizophrenia, which encodes the enzyme to produce the NMDA receptor co-agonist D-serine, to model the pathology of schizophrenia and to determine whether NMDA receptor hypofunction reduced the ability of srr-/- mice to identify nicotine's subjective effects. Established nicotine discrimination procedures were used to train srr-/- and wild-type (WT) mice to discriminate 0.4 mg/kg nicotine under a 10-response fixed-ratio (FR10) schedule of food reinforcement. RESULTS: Results show that WT mice reliably acquired 0.4 mg/kg nicotine discrimination in about 54 training session, whereas srr-/- mice failed to acquire robust 0.4 mg/kg nicotine discrimination even after extended (>70) training sessions. These results show that NDMA receptor hypofunction in srr-/- mice decreased sensitivity to the interoceptive effects of nicotine. CONCLUSIONS: Projected to humans, NMDA receptor hypofunction caused by mutations to the serine racemase gene in schizophrenia may reduce sensitivity to nicotine's subjective effects leading to increased nicotine consumption to produce the same effects as those unaffected by schizophrenia. IMPLICATIONS: There is high comorbidity between schizophrenia and nicotine dependence as well as possible shared genetic risk factors between the two. The serine racemase knockout mouse (srr-/-) with NMDA receptor hypofunction has been developed a model for schizophrenia. We found that srr-/- mice were unable to acquire 0.4 mg/kg nicotine discrimination, whilst wild-type mice readily discriminated nicotine. These results show that decreased NMDA receptor function present in srr-/- mice and patients with schizophrenia may result in reduced sensitivity to nicotine's interoceptive effects, leading to increased nicotine consumption to produce the same subjective effects as those unaffected by schizophrenia.

Reinforcing and adverse observable effects of nicotine and minor tobacco alkaloids in squirrel monkeys.

The most prevalent psychoactive chemical in tobacco smoke is nicotine, which has been shown to maintain tobacco consumption as well as cause acute adverse effects at high doses, like nausea and emesis. Recent studies in laboratory animals have suggested that many non-nicotine constituents of tobacco smoke (e.g., minor tobacco alkaloids) may also contribute to tobacco's overall reinforcing and adverse effects. Here, we used intravenous (IV) self-administration (n = 3) and observation (n = 4) procedures in squirrel monkeys to, respectively, compare the reinforcing and adverse observable effects of nicotine and three prominent minor tobacco alkaloids, nornicotine, anatabine, and myosmine. In self-administration studies, male squirrel monkeys were trained to respond under a second-order fixed-interval schedule of reinforcement and dose-effects functions for nicotine and each of the minor tobacco alkaloids nornicotine, anatabine, and mysomine were determined. Observation studies were conducted in a different group of male squirrel monkeys to quantify the ability of nicotine, nornicotine, anatabine, and mysomine to produce adverse overt effects, including hypersalivation, emesis, and tremors. Results show that nicotine and to a lesser extent nornicotine were readily self-administered, whereas anatabine and myosmine were not. In observation studies, all minor tobacco alkaloids produced adverse observable effects that were either comparable or more pronounced than nicotine. Collectively, the present results showing that nicotine and the minor tobacco alkaloids nornicotine, anatabine, and myosmine produce differential reinforcing and acute adverse observable effects in monkeys provides further evidence that these constituents may differently contribute to the psychopharmacological and adverse effects of tobacco consumption.

Investigation of monoclonal antibody CSX-1004 for fentanyl overdose.

The opioid crisis in the United States is primarily driven by the highly potent synthetic opioid fentanyl leading to >70,000 overdose deaths annually; thus, new therapies for fentanyl overdose are urgently needed. Here, we present the first clinic-ready, fully human monoclonal antibody CSX-1004 with picomolar affinity for fentanyl and related analogs. In mice CSX-1004 reverses fentanyl antinociception and the intractable respiratory depression caused by the ultrapotent opioid carfentanil. Moreover, toxicokinetic evaluation in a repeat-dose rat study and human tissue cross-reactivity study reveals a favorable pharmacokinetic profile of CSX-1004 with no safety-related issues. Using a highly translational non-human primate (NHP) model of respiratory depression, we demonstrate CSX-1004-mediated protection from repeated fentanyl challenges for 3-4 weeks. Furthermore, treatment with CSX-1004 produces up to a 15-fold potency reduction of fentanyl in NHP respiration, antinociception and operant responding assays without affecting non-fentanyl opioids like oxycodone. Taken together, our data establish the feasibility of CSX-1004 as a promising candidate medication for preventing and reversing fentanyl-induced overdose.

Effects of cannabinoid agonists and antagonists in male rats discriminating the synthetic cannabinoid AM2201.

The synthetic forms of delta-9-tetrahydrocannabinol (Δ9-THC), dronabinol or nabilone, have been approved to treat several indications. However, due to safety concerns their clinical utility remains limited. Consequently, there is a need for developing cannabinoid (CB) ligands that display better behavioral pharmacological profiles than Δ9-THC. Here, we utilized drug discrimination methods to compare the interoceptive effects of CB ligands that vary in potency, efficacy, and selectivity at the CB receptors, including two ligands, AM411 and AM4089, that show CB1 partial agonist-like actions in vitro. Male rats were trained to discriminate 0.1 mg/kg AM2201 from saline under a fixed-ratio (FR) 10 response schedule of food reinforcement. After establishing AM2201's discriminative-stimulus effects, pretreatment tests with the CB1 antagonist/inverse agonist rimonabant blocked AM2201's effects, whereas the peripherally-restricted antagonist AM6545 had no effect. Next, the generalization profiles of AM411 and AM4089 with CB1 full agonists (JWH-018, CP-55,940, AM8936), partial agonist (Δ9-THC), and non-cannabinoids (fentanyl, atropine) were compared. The CBs either fully (AM2201, CP-55,940, JWH-018, AM8936, Δ9-THC) or partially (AM411, AM4089) substituted for AM2201, whereas fentanyl and atropine did not produce AM2201-like effects. All CB drugs were more potent than Δ9-THC and correlation analysis confirmed that the relative behavioral potencies of CBs corresponded strongly with their relative affinities at the CB1 but not CB2 receptors. Together, our results further demonstrate that AM411 and AM4089 exhibit better pharmacological profiles compared to Δ9-THC, in that they are more potent and display in vivo partial agonist-like actions that are centrally mediated via CB1 receptors.

Complex 33-beam simulated galactic cosmic radiation exposure impacts cognitive function and prefrontal cortex neurotransmitter networks in male mice.

Astronauts will encounter extended exposure to galactic cosmic radiation (GCR) during deep space exploration, which could impair brain function. Here, we report that in male mice, acute or chronic GCR exposure did not modify reward sensitivity but did adversely affect attentional processes and increased reaction times. Potassium (K+)-stimulation in the prefrontal cortex (PFC) elevated dopamine (DA) but abolished temporal DA responsiveness after acute and chronic GCR exposure. Unlike acute GCR, chronic GCR increased levels of all other neurotransmitters, with differences evident between groups after higher K+-stimulation. Correlational and machine learning analysis showed that acute and chronic GCR exposure differentially reorganized the connection strength and causation of DA and other PFC neurotransmitter networks compared to controls which may explain space radiation-induced neurocognitive deficits.

Circuits and Biomarkers of the Central Nervous System Relating to Astronaut Performance: Summary Report for a NASA-Sponsored Technical Interchange Meeting.

Biomarkers, ranging from molecules to behavior, can be used to identify thresholds beyond which performance of mission tasks may be compromised and could potentially trigger the activation of countermeasures. Identification of homologous brain regions and/or neural circuits related to operational performance may allow for translational studies between species. Three discussion groups were directed to use operationally relevant performance tasks as a driver when identifying biomarkers and brain regions or circuits for selected constructs. Here we summarize small-group discussions in tables of circuits and biomarkers categorized by (a) sensorimotor, (b) behavioral medicine and (c) integrated approaches (e.g., physiological responses). In total, hundreds of biomarkers have been identified and are summarized herein by the respective group leads. We hope the meeting proceedings become a rich resource for NASA's Human Research Program (HRP) and the community of researchers.

Effects of the cannabinoid CB1-receptor neutral antagonist AM4113 and antagonist/inverse agonist rimonabant on fentanyl discrimination in male rats.

Evidence suggests the existence of a functional interaction between endogenous cannabinoid (CB) and opioid systems. Thus, targeting CB1 receptors might be a viable approach to develop new medications for opioid use disorders (OUD). The present studies were undertaken to evaluate the effects of the neutral CB1 antagonist AM4113 and the CB1 antagonist/inverse agonist rimonabant in male rats trained to discriminate 0.032 mg/kg fentanyl from saline under a 10-response fixed-ratio (FR-10) schedule of food reinforcement. Results show that the µ-opioid agonists (fentanyl, oxycodone, and morphine) substituted fully and dose-dependently for fentanyl, whereas pretreatment with the µ-opioid antagonist naltrexone antagonized fentanyl's discriminative-stimulus effects. In interaction studies, AM4113 (0.32 or 1.0 mg/kg) was more effective in blocking fentanyl discrimination at 10-fold lower doses that did not modify rates of food-maintained responding, whereas rimonabant (1.0-10 mg/kg) produced some attenuation of fentanyl's discriminative-stimulus effects at the highest dose tested which also significantly decreased response rates. These results extend our recent work showing that AM4113 can effectively block the behavioral effects of heroin without producing rimonabant-like adverse effects. Taken together, these data suggests that CB1 neutral antagonists effectively block the behavioral effects of structurally distinct morphinan (heroin) and phenylpiperidine-based (fentanyl) opioids and may provide a novel therapeutic option for the treatment of OUD.

Impact of spaceflight stressors on behavior and cognition: A molecular, neurochemical, and neurobiological perspective.

The response of the human body to multiple spaceflight stressors is complex, but mounting evidence implicate risks to CNS functionality as significant, able to threaten metrics of mission success and longer-term behavioral and neurocognitive health. Prolonged exposure to microgravity, sleep disruption, social isolation, fluid shifts, and ionizing radiation have been shown to disrupt mechanisms of homeostasis and neurobiological well-being. The overarching goal of this review is to document the existing evidence of how the major spaceflight stressors, including radiation, microgravity, isolation/confinement, and sleep deprivation, alone or in combination alter molecular, neurochemical, neurobiological, and plasma metabolite/lipid signatures that may be linked to operationally-relevant behavioral and cognitive performance. While certain brain region-specific and/or systemic alterations titrated in part with neurobiological outcome, variations across model systems, study design, and the conspicuous absence of targeted studies implementing combinations of spaceflight stressors, confounded the identification of specific signatures having direct relevance to human activities in space. Summaries are provided for formulating new research directives and more predictive readouts of portending change in neurobiological function.

Nonhuman primate models in the study of spaceflight stressors: Past contributions and future directions.

Studies in rodents suggest that exposure to distinct spaceflight stressors (e.g., space radiation, isolation/confinement, microgravity) may have a profound impact on an astronaut's ability to perform both simple and complex tasks related to neurocognitive performance, central nervous system (CNS) and vestibular/sensorimotor function. However, limited information is currently available on how combined exposure to the spaceflight stressors will impact CNS-related neurocognitive and neurobiological function in-flight and, as well, terrestrial risk of manifesting neurodegenerative conditions when astronauts return to earth. This information gap has significantly hindered our ability to realistically estimate spaceflight hazard risk to the CNS associated with deep space exploration. Notwithstanding a significant body of work with rodents, there have been very few direct investigations of the impact of these spaceflight stressors in combination and, to our knowledge, no such investigations using nonhuman primate (NHP) animal models. In view of the widely-recognized translational value of NHP data in advancing biomedical discoveries, this research deficiency limits our understanding regarding the impact of individual and combined spaceflight stressors on CNS-related neurobiological function. In this review, we address this knowledge gap by conducting a systematic and comprehensive evaluation of existing research on the impact of exposure to spaceflight stressors on NHP CNS-related function. This review is structured to: a) provide an overarching view of the past contributions of NHPs to spaceflight research as well as the strengths, limitations, and translational value of NHP research in its own right and within the existing context of NASA-relevant rodent research; b) highlight specific conclusions based on the published literature and areas needed for future endeavors; c) describe critical research gaps and priorities in NHP research to facilitate NASA's efforts to bridge the key knowledge gaps that currently exist in translating rodent data to humans; and d) provide a roadmap of recommendations for NASA regarding the availability, validity, strengths, and limitations of various NHP models for future targeted research.

Differential Effects of Nicotine and Nicotine Withdrawal on Fear Conditioning in Male Rats.

BACKGROUND: Tobacco use is prevalent in individuals who are routinely exposed to stress. However, little is known about how nicotine affects responses to trauma. We examined in rats how nicotine exposure affects fear conditioning, a procedure often used to study stress-related psychiatric illness. METHODS: We examined 2 methods of nicotine exposure: self-administration, modeling voluntary use, and experimenter-programmed subcutaneous administration, modeling medicinal administration (nicotine patch). For self-administered nicotine, rats trained to self-administer nicotine i.v. were fear conditioned (via light cue preceding foot-shock) either immediately after a 12-hour self-administration session or 12 hours later during a period with somatic signs of nicotine withdrawal. For experimenter-delivered nicotine, rats were conditioned after 1-21 days of nicotine delivered by programmable (12 hours on) subcutaneous mini-pumps. Tests to evaluate acoustic startle responses to the conditioning environment (context-potentiated startle) and in the presence or absence of the light cue (fear-potentiated startle) occurred after a 10-day period. RESULTS: Rats fear conditioned immediately after nicotine self-administration showed reduced responses to the shock-associated context, whereas those trained during nicotine withdrawal showed exaggerated responses. Experimenter-programmed nicotine produced effects qualitatively similar to those seen with self-administered nicotine. CONCLUSIONS: Self-administration or experimenter-programmed delivery of nicotine immediately before exposure to aversive events can reduce conditioned fear responses. In contrast, exposure to aversive events during nicotine withdrawal exacerbates fear responses. These studies raise the possibility of developing safe and effective methods to deliver nicotine or related drugs to mitigate the effects of stress while also highlighting the importance of preventing withdrawal in nicotine-dependent individuals.

N-Methyl-d-aspartate receptor co-agonist availability affects behavioral and neurochemical responses to cocaine: insights into comorbid schizophrenia and substance abuse.

Both schizophrenia (SZ) and substance abuse (SA) exhibit significant heritability. Moreover, N-methyl-d-aspartate receptors (NMDARs) have been implicated in the pathophysiology of both SZ and SA. We hypothesize that the high prevalence of comorbid SA in SZ is due to dysfunction of NMDARs caused by shared risk genes. We used transgenic mice with a null mutation of the gene encoding serine racemase (SR), the enzyme that synthesizes the NMDAR co-agonist d-serine and an established risk gene for SZ, to recreate the pathology of SZ. We determined the effect of NMDAR hypofunction resulting from the absence of d-serine on motivated behavior by using intracranial self-stimulation and neurotransmitter release in the nucleus accumbens by using in vivo microdialysis. Compared with wild-type mice, SR-/- mice exhibited similar baseline intracranial self-stimulation thresholds but were less sensitive to the threshold-lowering (rewarding) and the performance-elevating (stimulant) effects of cocaine. While basal dopamine (DA) and glutamate release were elevated in the nucleus accumbens of SR-/- mice, cocaine-induced increases in DA and glutamate release were blunted. γ-Amino-butyric acid efflux was unaffected in the SR-/- mice. Together, these findings suggest that the impaired NMDAR function and a consequent decrease in sensitivity to cocaine effects on behavior are mediated by blunted DA and glutamate responses normally triggered by the drug. Projected to humans, NMDAR hypofunction due to mutations in SR or other genes impacting glutamatergic function in SZ may render abused substances less potent and effective, thus requiring higher doses to achieve a hedonic response, resulting in elevated drug exposure and increased dependence/addiction.

Involvement of Nicotinic Receptor Subtypes in the Behavioral Effects of Nicotinic Drugs in Squirrel Monkeys.

Evidence suggests that the α4ß2, but not the α7, subtype of the nicotinic acetylcholine receptor (nAChR) plays a key role in mediating the behavioral effects of nicotine and related drugs. However, the importance of other nAChR subtypes remains unclear. The present studies were conducted to examine the involvement of nAChR subtypes by determining the effects of selected nicotinic agonists and antagonists in squirrel monkeys either 1) responding for food reinforcement or 2) discriminating the nicotinic agonist (+)-epibatidine (0.001 mg/kg) from vehicle. In food-reinforcement studies, nicotine, (+)-epibatidine, varenicline and cytisine all produced dose-dependent decreases in rates of food-maintained responding. The rate-decreasing effects of nicotine were antagonized by mecamylamine (nonselective), not appreciably altered by dihydro-ß-erythroidine (α4ß2 selective), and exacerbated by the nicotinic partial agonists, varenicline and cytisine. Results from discrimination studies show that non-nicotinic drugs did not substitute for (+)-epibatidine, and that except for lobeline, the nicotinic agonists produced either full [(+)-epibatidine, (-)-epibatidine, and nicotine] or partial (varenicline, cytisine, anabaseine, and isoarecolone) substitution for (+)-epibatidine. In interaction studies with antagonists differing in selectivity, (+)-epibatidine discrimination was substantively antagonized by mecamylamine, slightly attenuated by hexamethonium (peripherally restricted) or dihydro-ß-erythroidine, and not altered by methyllycaconitine (α7 selective). Varenicline and cytisine enhanced (+)-epibatidine's discriminative-stimulus effects. Correlational analysis revealed a close correspondence between relative behavioral potencies of nicotinic agonists in both studies and their published relative binding affinities at α4ß2 and α3ß4, but not α7 nAChR, subtypes. Collectively, these results are consistent with the idea that the α4ß2 and α3ß4, but not α7 nAChR subtypes play a role in the behavioral effects of nicotinic agonists.

Behavioral phenotyping and dopamine dynamics in mice with conditional deletion of the glutamate transporter GLT-1 in neurons: resistance to the acute locomotor effects of amphetamine.

RATIONALE: GLT-1 is the major glutamate transporter in the brain and is expressed predominantly in astrocytes but is also present in excitatory axon terminals. To understand the functional significance of GLT-1 expressed in neurons, we generated a conditional GLT-1 knockout mouse and inactivated GLT-1 in neurons using Cre-recombinase expressed under the synapsin 1 promoter, (synGLT-1 KO). OBJECTIVES: Abnormalities of glutamate homeostasis have been shown to affect hippocampal-related behaviors including learning and memory as well as responses to drugs of abuse. Here, we asked whether deletion of GLT-1 specifically from neurons would affect behaviors that assessed locomotor activity, cognitive function, sensorimotor gating, social interaction, as well as amphetamine-stimulated locomotor activity. METHODS/RESULTS: We found that the neuronal GLT-1 KO mice performed similarly to littermate controls in the behavioral tests we studied. Although performance in open field testing was normal, the acute locomotor response to amphetamine was significantly blunted in the synGLT-1 KO (40% of control). We found no change in amphetamine-stimulated extracellular dopamine in the medial shell of the nucleus accumbens, no change in electrically stimulated or amphetamine-induced dopamine release, and no change in dopamine tissue content. CONCLUSIONS: These results support the view that GLT-1 expression in neurons is required for amphetamine-induced behavioral activation, and suggest that this phenotype is not produced through a change in dopamine uptake or release. Although GLT-1 is highly expressed in neurons in the CA3 region of the hippocampus, the tests used in this study were not able to detect a behavioral phenotype referable to hippocampal dysfunction.

Further delineation between typical and atypical dopamine uptake inhibitors: effects on food-maintained behavior and food consumption.

The present studies compared the acute effects of benztropine analogs (4-Cl-BZT, JHW 007, AHN 1-055), which are atypical dopamine uptake inhibitors, with those of the standard dopamine uptake inhibitors GBR 12909 and cocaine, on the reinforcing efficacy of food and food intake in male Sprague-Dawley rats. Repeated drug effects of JHW 007 on food intake were also determined. The number of ratios completed under a progressive-ratio schedule of food delivery was used as an index of reinforcing efficacy. Food intake was determined by measuring powdered laboratory-chow consumption during daily 40 min food-availability time periods. Under the progressive-ratio schedule, cocaine and GBR 12909 dose-dependently increased the number of ratios completed. JHW 007 decreased ratios completed, whereas neither 4-Cl-BZT nor AHN 1-055 increased ratios completed with a magnitude that approximated any of the increases produced by cocaine or GBR 12909. Acute administration of each drug dose-dependently decreased food intake; however, the benztropine analogs were more potent than cocaine and GBR 12909. A reduction in food intake emerged after repeated administration of a low dose of JHW 007. Future studies that compare JHW 007 with standard anorectic drugs (e.g. phentermine) and continue investigation of the repeated drug effects under similar experimental procedures are clearly warranted.

Influence of experimental history on nicotine self-administration in squirrel monkeys.

RATIONALE: Methods for establishing robust long-term self-administration of intravenous (i.v.) nicotine, the primary psychoactive agent in tobacco, are not well-established in laboratory animals. OBJECTIVE: Here, we examine the use of a fading procedure to establish robust and consistent i.v. nicotine self-administration under second-order schedule conditions in squirrel monkeys. METHODS: First, self-administration behavior was developed in two groups of male squirrel monkeys using a second-order fixed-interval 5-min schedule with fixed-ratio 5 units (FI 5-min (FR5: S)). Comparable performances were maintained by i.v. cocaine (0.032 mg/kg/injection (inj); group A, n = 3) and the combination of food delivery (20-30 % condensed milk) and 0.01 mg/kg/inj i.v. nicotine (group B, n = 3). Subsequently, the concentration of condensed milk was gradually reduced to zero in the second group and self-administration behavior was maintained by i.v. nicotine alone. Next, self-administration of a range of doses of i.v. nicotine (0.001-0.032 mg/kg/inj) and, in additional experiments, the minor tobacco alkaloid anatabine (0.01-0.18 mg/kg/inj) was studied in both groups. RESULTS: Results show that nicotine and anatabine had reinforcing effects in both groups. However, optimal doses of nicotine and anatabine maintained significantly higher rates of i.v. self-administration behavior in subjects trained with the fading procedure than in subjects provided with a history of cocaine-maintained responding. CONCLUSION: These results illustrate conditions under which robust i.v. nicotine self-administration can be established in squirrel monkeys and the influence of prior experimental history in the expression of reinforcing effects of nicotine and anatabine.

Nicotinic effects of tobacco smoke constituents in nonhuman primates.

RATIONALE: Recent studies in rodents suggest that non-nicotine constituents of tobacco smoke (e.g., minor tobacco alkaloids) may promote tobacco consumption-either through their own pharmacological effects or by augmenting the effects of nicotine. However, there is scant information on the behavioral pharmacology of minor tobacco alkaloids in primate species. OBJECTIVE: The present studies were conducted to determine whether the minor tobacco alkaloids nornicotine, anabasine, anatabine, myosmine, and cotinine exhibit nicotine-like behavioral effects in squirrel monkeys. METHODS: Initial experiments were conducted to determine the effects of nicotine (0.032-1.0 mg/kg) and the minor tobacco alkaloids nornicotine (1-1.8 mg/kg), anabasine (0.1-1.0 mg/kg), anatabine (10-32 mg/kg), myosmine (0.32-1.8 mg/kg), and cotinine (10-180 mg/kg) on food-maintained performance (n = 4). Next, the ability of tobacco alkaloids to substitute for the α4ß2-selective nicotinic agonist (+)-epibatidine in drug discrimination experiments was evaluated in a separate group of monkeys (n = 4). RESULTS: Results show that nicotine and each minor tobacco alkaloid except cotinine (a) produced dose-related decreases in food-maintained responding; (b) substituted for (+)-epibatidine and, in additional experiments, produced additive effects when combined with nicotine; (c) induced emesis or tremor at doses that reduced food-maintained responding and had (+)-epibatidine-like discriminative-stimulus effects; and (d) based on correlation with reported receptor binding affinities, likely produced their behavioral effects through α4ß2 receptor mechanisms. CONCLUSION: Selected minor tobacco alkaloids have nicotinic-like effects that may contribute to tobacco consumption and addiction.

Effects of the Nanoparticle-Based Vaccine, SEL-068, on Nicotine Discrimination in Squirrel Monkeys.

A key feature of addiction to nicotine likely resides in its ability to produce subjective effects that, in turn, may be reflected in its discriminative-stimulus properties. Vaccination against such effects of nicotine offers an intriguing therapeutic approach for smoking cessation, but a reliably effective and immunologically safe vaccine remains to be identified. Here we report on the ability of SEL-068, a nanoparticle-based vaccine that targets nicotine, to modify the discriminative-stimulus effects of nicotine in a primate species. Results indicate that squirrel monkeys vaccinated with SEL-068 failed to acquire 0.1 mg/kg nicotine discrimination but readily learned to discriminate 0.001 mg/kg of the nicotinic full agonist (+)-epibatidine ((+)-EPI). After (+)-EPI training, doses of nicotine ⩾ 0.32 mg/kg, which produced behaviorally adverse actions, still failed to substitute for the (+)-EPI training stimulus in immunized monkeys, whereas (+)-EPI and the partial agonist varenicline engendered, respectively, complete and partial substitution in all monkeys with potency comparable to their potency in non-immunized subjects. In other subjects, nicotine was trained as a discriminative-stimulus and then replaced by (+)-EPI. Subsequent vaccination with SEL-068 led to a threefold and long-lasting (>30 weeks) decrease in the potency of nicotine but not (+)-EPI or varenicline. Collectively, our results show that SEL-068 can block the development of nicotine discrimination and attenuate nicotine's effects in nicotine-experienced monkeys without altering the discriminative-stimulus properties of other nicotinic drugs. The difference in the vaccine's effects in naive and nicotine-experienced subjects provides important insight into the conditions under which immunotherapy may be effective in combating nicotine addiction.

Methamphetamine-like discriminative-stimulus effects of nicotinic agonists.

Nicotine was recently shown to engender d-methamphetamine (MA)-like discriminative-stimulus effects in rats, which may be indicative of shared psychomotor stimulant properties. To further investigate such overlapping discriminative-stimulus effects, nicotinic agonists varying in efficacy and selectivity were studied in squirrel monkeys that discriminated a moderate intramuscular dose of MA (0.1 mg/kg) from vehicle. These included α4ß2-selective ligands that may vary in efficacy from relatively high [nicotine, (+)- and (-)-epibatidine] to relatively low [isoarecolone, varenicline, (-)-cytisine, (-)-lobeline] and the α7-selective ligands anabaseine and anabasine. Results show that nicotine, (+)-epibatidine, and (-)-epibatidine substituted fully for MA, whereas the highest doses of other nicotinic agonists produced intermediate levels of MA-like effects (isoarecolone, anabaseine, anabasine, and varenicline) or did not substitute for MA [(-)-cytisine and (-)-lobeline]. The relative potencies of nicotinic agonists, based on effective dose50 (ED50) values, corresponded more closely with their relative affinities at α4ß2 than at α7 receptors. Regardless of selectivity or efficacy, nicotinic agonists also were observed to produce untoward effects, including salivation and emesis during or after experimental sessions. In pretreatment studies, the α4ß2-selective antagonist dihydro-ß-erythroidine hydrobromide (DHßE) (0.032 and 0.1 mg/kg) and the partial agonists varenicline (0.0032-0.1 mg/kg) and (-)-cytisine (0.032 and 0.1 mg/kg) surmountably antagonized (>10-fold rightward shift) nicotine's MA-like effects but were ineffective in blocking nicotine's emetic effects. Overall, our results show that 1) MA-like discriminative-stimulus effects of nicotinic agonists likely are mediated through α4ß2 nicotinic acetylcholine receptor actions, and 2) nicotinic α4ß2 partial agonists, like the nicotinic antagonist DHßE, can reduce MA-like behavioral effects of nicotine.