Formulation and Characterization of Conjugate Vaccines to Reduce Opioid Use Disorders Suitable for Pharmaceutical Manufacturing and Clinical Evaluation.

This study focused on formulating conjugate vaccines targeting oxycodone and heroin for technology transfer, good manufacturing practice (GMP), and clinical evaluation. Lead vaccines used the highly immunogenic carrier protein keyhole limpet hemocyanin (KLH), which poses formulation problems because of its size. To address this barrier to translation, an oxycodone-based hapten conjugated to GMP-grade subunit KLH (OXY-sKLH) and adsorbed on alum adjuvant was studied with regard to carbodiimide coupling reaction time, buffer composition, purification methods for conjugates, conjugate size, state of aggregation, and protein/alum ratio. Vaccine formulations were screened for post-immunization antibody levels and efficacy in reducing oxycodone distribution to the brain in rats. While larger conjugates were more immunogenic, their size prevented characterization of the haptenation ratio by standard analytical methods and sterilization by filtration. To address this issue, conjugation chemistry and vaccine formulation were optimized for maximal efficacy, and conjugate size was measured by dynamic light scattering prior to adsorption to alum. An analogous heroin vaccine (M-sKLH) was also optimized for conjugation chemistry, formulated in alum, and characterized for potency against heroin in rats. Finally, this study found that the efficacy of OXY-sKLH was preserved when co-administered with M-sKLH, supporting the proof of concept for a bivalent vaccine formulation targeting both heroin and oxycodone. This study suggests methods for addressing the unique formulation and characterization challenges posed by conjugating small molecules to sKLH while preserving vaccine efficacy.

Effect of nicotine and alpha-7 nicotinic modulators on visceral pain-induced conditioned place aversion in mice.

BACKGROUND: Preclinical assays of affective and sensorial aspects of nociception play a key role in research on both the neurobiology of pain and the development of novel analgesics. Therefore, we investigated the effects of nicotine and alpha-7 nicotinic acetylcholine receptor (nAChR) modulators in the negative affective and sensory components of visceral pain in mice. METHODS AND RESULTS: Intraperitoneal acetic acid (AA) administration resulted in a robust stretching behaviour and conditioned place aversion (CPA) in mice. We observed a dose-dependent reduction in AA-induced stretching and CPA by the nonselective nAChRs agonist nicotine. Mecamylamine, a nonselective nAChRs agonist, was able to block its effects; however, hexamethonium, a peripherally restricted nonselective nicotinic antagonist, was able to block nicotine's effect on stretching behaviour but not on CPA. In addition, systemic administration of α7 nAChR full agonists PHA543613 and PNU282987 was failed to block stretching and CPA behaviour induced by AA. However, the α7 nAChR-positive allosteric modulator PNU120596 blocked AA-induced CPA in a dose-dependent manner without reducing stretching behaviours. CONCLUSIONS: Our data revealed that while nonselective nAChR activation induces antinociceptive properties on the sensorial and affective signs of visceral pain in mice, α7 nAChRS activation has no effect on these responses. In addition, nonselective nAChR activation-induced antinociceptive effect on stretching behaviour was mediated by central and peripheral mechanisms. However, the effect of nonselective nAChR activation on CPA was mediated centrally. Furthermore, our data suggest a pivotal role of allosteric modulation of α7 nAChRS in the negative affective, but not sensory, component of visceral pain. SIGNIFICANCE: The present results suggest that allosteric modulation of α7 nAChR may provide new strategies in affective aspects of nociception.

Safety and efficacy of an oxycodone vaccine: Addressing some of the unique considerations posed by opioid abuse.

Among vaccines aimed at treating substance use disorders, those targeting opioids present several unique medication development challenges. 1) Opioid overdose is a common complication of abuse, so it is desirable for an opioid vaccine to block the toxic as well as the addictive effects of opioids. 2) It is important that an opioid vaccine not interfere with the action of opioid antagonists used to reverse opioid overdose or treat addiction. 3) Some opioids are immunosuppressive and chronic ongoing opioid use could interfere with vaccine immunogenicity. 4) Although antibody-bound oxycodone is unable to enter the brain because of its size, it might still be able to activate peripheral opioid receptors. To assess vaccine impact on opioid toxicity, rats vaccinated with oxycodone conjugated to keyhole limpet hemocyanin subunit dimer (OXY-dKLH) adsorbed to alum or controls vaccinated with dKLH were compared with regard to oxycodone-induced hotplate analgesia and oxycodone-induced respiratory depression and bradycardia. Vaccination shifted the dose-response curves to the right, representing protection, for each of these endpoints. Naloxone was equally effective in both OXY-dKLH and control groups, providing complete and rapid reversal of respiratory depression. The administration of a long-acting naltrexone formulation during vaccination did not impair vaccine immunogenicity in mice. Similarly, serum anti-oxycodone antibody titers were not altered by continuous morphine infusion during vaccination compared to opioid-naïve controls. Competitive ELISA assay showed negligible or low affinity of immune antiserum for endogenous opioids or opioid antagonists. In vitro receptor binding assays showed that antibody-bound oxycodone does not activate mu opioid receptors. These data support further study of OXY-dKLH as a potential treatment for oxycodone abuse and suggest that vaccination might also reduce the severity of oxycodone overdose.

Synthesis, Nicotinic Acetylcholine Receptor Binding, and in Vitro and in Vivo Pharmacological Properties of 2'-Fluoro-(substituted thiophenyl)deschloroepibatidine Analogues.

The synthesis, nAChR in vitro and in vivo pharmacological properties of 2'-fluoro-3'-(substituted thiophenyl)deschloroepibatidine analogues (5a-f, 6a-d, and 7a-c) are presented herein. All had subnanomolar affinity at α4ß2*-nAChRs. Contrary to lead structure epibatidine, a potent nAChR agonist, all were potent α4ß2- and α3ß4-AChR antagonists in an in vitro functional assay. In vivo, the compounds were also nAChR antagonists with various degrees of agonist activity. Compounds 5e, 5f, 6a, 6c, 6d, and 7c had no agonist effects in the tail-flick, hot-plate, hypothermia, or spontaneous activity tests, whereas 5a-d, 7a and 7b did not have agonist activity in the tail-flick and hot-plate tests but, like varenicline, were agonists in the hypothermia and spontaneous activity tests. Compound 6b had agonist activity in all four in vivo tests. All the compounds were antagonists of nicotine-induced antinociception in the tail-flick test, and all except 5c, 5d, 5f, and 6b were antagonists of nicotine-induced antinociception in the hot-plate test. Compound 7c, which had a Ki = 0.86 nM in the binding assay similar potency at α4ß2/α3ß4 with selectivity relative to α7 nAChRs, had an AD50 value of 0.001 µg/kg in the tail-flick test with no agonist activity in the in vitro or in vivo test had one of the more interesting profiles.

The frequency of early-activated hapten-specific B cell subsets predicts the efficacy of vaccines for nicotine dependence.

Therapeutic vaccines for nicotine addiction show pre-clinical efficacy. Yet, clinical evaluation of the first-generation nicotine vaccines did not meet expectations because only a subset of immunized subjects achieved effective serum antibody levels. Recent studies suggest that vaccine design affects B cell activation, and that the frequency of the hapten-specific B cell subsets contributes to vaccine efficacy against drugs of abuse. To extend this hypothesis to nicotine immunogens, we synthesized a novel hapten containing a carboxymethylureido group at the 2-position of the nicotine structure (2CMUNic) and compared its efficacy to the previously characterized 6CMUNic hapten. Haptens were conjugated to the keyhole limpet hemocyanin (KLH) carrier protein, and evaluated for efficacy against nicotine in mice using the clinically approved alum adjuvant. Using a novel fluorescent antigen-based magnetic enrichment strategy paired with multicolor flow cytometry analysis, polyclonal hapten-specific B cell subsets were measured in mice immunized with either 6CMUNic-KLH or 2CMUNic-KLH. The 6CMUNic-KLH showed significantly greater efficacy than 2CMUNic-KLH on nicotine distribution to serum and to the brain. The 6CMUNic-KLH elicited higher anti-nicotine serum antibody titers, and greater expansion of hapten-specific B cells than 2CMUNic-KLH. Within the splenic polyclonal B cell population, a higher number of hapten-specific IgM(high) and germinal centre B cells predicted greater vaccine efficacy against nicotine distribution. These early pre-clinical findings suggest that hapten structure affects activation of B cells, and that variations in the frequency of early-activated hapten-specific B cell subsets underlie individual differences in vaccine efficacy.

Abnormal error processing in depressive states: a translational examination in humans and rats.

Depression has been associated with poor performance following errors, but the clinical implications, response to treatment and neurobiological mechanisms of this post-error behavioral adjustment abnormality remain unclear. To fill this gap in knowledge, we tested depressed patients in a partial hospital setting before and after treatment (cognitive behavior therapy combined with medication) using a flanker task. To evaluate the translational relevance of this metric in rodents, we performed a secondary analysis on existing data from rats tested in the 5-choice serial reaction time task after treatment with corticotropin-releasing factor (CRF), a stress peptide that produces depressive-like signs in rodent models relevant to depression. In addition, to examine the effect of treatment on post-error behavior in rodents, we examined a second cohort of rodents treated with JDTic, a kappa-opioid receptor antagonist that produces antidepressant-like effects in laboratory animals. In depressed patients, baseline post-error accuracy was lower than post-correct accuracy, and, as expected, post-error accuracy improved with treatment. Moreover, baseline post-error accuracy predicted attentional control and rumination (but not depressive symptoms) after treatment. In rats, CRF significantly degraded post-error accuracy, but not post-correct accuracy, and this effect was attenuated by JDTic. Our findings demonstrate deficits in post-error accuracy in depressed patients, as well as a rodent model relevant to depression. These deficits respond to intervention in both species. Although post-error behavior predicted treatment-related changes in attentional control and rumination, a relationship to depressive symptoms remains to be demonstrated.

Examination of the metabolite hydroxybupropion in the reinforcing and aversive stimulus effects of nicotine in rats.

BACKGROUND: Preclinical studies with bupropion in rodent models of nicotine dependence have generated equivocal findings with regard to translating the clinical efficacy of the antidepressant as a smoking cessation agent. OBJECTIVE: Given that rats are poor metabolizers of bupropion, the present experiments examined (2S,3S)-hydroxybupropion, the major active metabolite, on the positive reinforcing and aversive stimulus properties of nicotine in rats. METHODS: In male hooded Lister rats, (2S,3S)-hydroxybupropion (1.0-10.0 mg/kg IP) was tested on intravenous nicotine (0.03 mg/kg/inf) self-administration behaviour for three sessions (n = 8), and in another experiment, the same doses of (2S,3S)-hydroxybupropion were tested in a conditioned taste aversion procedure to assess the aversive stimulus properties of nicotine, a function implicated in the regulation of nicotine intake. RESULTS: (2S,3S)-hydroxybupropion attenuated nicotine intake in a manner similar to that produced by mecamylamine pretreatment (1.0 mg/kg SC). This effect on nicotine-taking was specific since these doses had no effect on responding maintained by sucrose presented orally (200 µl of 5 % w/v). (2S,3S)-hydroxybupropion (1, 3 and 10 mg/kg IP) pretreatment failed to modify the aversive effects produced by a small dose of nicotine (0.1 mg/kg SC). CONCLUSIONS: These results demonstrate this metabolite to specifically modify the positive reinforcing effects of nicotine without affecting its aversive motivational effects. We propose that the clinical efficacy of bupropion may be due to a combination of effects produced by bupropion and/or its active metabolite (2S,3S)-hydroxybupropion involving the inhibition of reuptake of dopamine and noradrenaline in reward centres of the brain and the noncompetitive antagonism of neuronal nicotinic receptors.

A Double-Blind, Placebo-Controlled Trial to Evaluate the Safety, Tolerability, and Pharmacokinetics of Single, Escalating Oral Doses of JDTic.

Animal studies suggest that kappa opioid receptor antagonists (KORAn) potentially could treat a wide variety of addictive and depressive disorders. We assessed the KORAn JDTic for safety, tolerability, and pharmacokinetics in a double-blind, placebo-controlled, randomized trial evaluating single oral doses in healthy adult males. Predose and postdose safety assessments included orthostatic vital signs; 6-lead continuous telemetry monitoring (approximately 16 h predose to 24 h postdose); 12-lead electrocardiograms (ECGs); clinical chemistry, hematology, coagulation, and urinalysis; psychomotor functioning (using the Wayne Saccadic Fixator (WSF)); and adverse events. As a potential indicator of JDTic effects on affect, the POMS Standard instrument was administered predose and daily postdose Days 1-6. At 1 mg, 2 of the 6 JDTic (and 0/6 placebo) subjects experienced a single, asymptomatic event of multiple beats of nonsustained ventricular tachycardia (NSVT). Their events were temporally similar with respect to time postdose (and the postdose timing of an NSVT event in a monkey). These events triggered a study stopping rule. No differences were observed between the placebo and JDTic subjects with respect to clinical chemistry, hematology, coagulation, urinalysis, orthostatic vital signs, WSF, or 12-lead ECG parameters. Plasma JDTic levels were below the lower limit of quantitation (0.1 nM) in all subjects. There were no significant differences in POMS scores between the placebo and JDTic groups. Although the evidence is circumstantial, it suggests that NSVT is a potential JDTic toxicity in humans. Given the therapeutic potential of KORAn, further investigation is needed to determine whether a significant JDTic human cardiac effect indeed exists, and if so, whether it is specific to JDTic or represents a KORAn class effect.

Anti-nociception mediated by a κ opioid receptor agonist is blocked by a δ receptor agonist.

BACKGROUND AND PURPOSE: The opioid receptor family comprises four structurally homologous but functionally distinct sub-groups, the µ (MOP), δ (DOP), κ (KOP) and nociceptin (NOP) receptors. As most opioid agonists are selective but not specific, a broad spectrum of behaviours due to activation of different opioid receptors is expected. In this study, we examine whether other opioid receptor systems influenced KOP-mediated antinociception. EXPERIMENTAL APPROACH: We used a tail withdrawal assay in C57Bl/6 mice to assay the antinociceptive effect of systemically administered opioid agonists with varying selectivity at KOP receptors. Pharmacological and genetic approaches were used to analyse the interactions of the other opioid receptors in modulating KOP-mediated antinociception. KEY RESULTS: Etorphine, a potent agonist at all four opioid receptors, was not anti-nociceptive in MOP knockout (KO) mice, although etorphine is an efficacious KOP receptor agonist and specific KOP receptor agonists remain analgesic in MOP KO mice. As KOP receptor agonists are aversive, we considered KOP-mediated antinociception might be a form of stress-induced analgesia that is blocked by the anxiolytic effects of DOP receptor agonists. In support of this hypothesis, pretreatment with the DOP antagonist, naltrindole (10 mg·kg(-1) ), unmasked etorphine (3 mg·kg(-1) ) antinociception in MOP KO mice. Further, in wild-type mice, KOP-mediated antinociception by systemic U50,488H (10 mg·kg(-1) ) was blocked by pretreatment with the DOP agonist SNC80 (5 mg·kg(-1) ) and diazepam (1 mg·kg(-1) ). CONCLUSIONS AND IMPLICATIONS: Systemic DOP receptor agonists blocked systemic KOP antinociception, and these results identify DOP receptor agonists as potential agents for reversing stress-driven addictive and depressive behaviours mediated through KOP receptor activation. LINKED ARTICLES: This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.

A thermostable bacterial cocaine esterase rapidly eliminates cocaine from brain in nonhuman primates.

A long-acting, thermostable bacterial cocaine esterase (CocE) has been identified that rapidly degrades cocaine with a K(M) of 1.33+0.085 µM. In vivo evaluation of CocE has shown protection against convulsant and lethal effects of cocaine in rodents, confirming the therapeutic potential of CocE against cocaine overdose. However, the current study is the first to evaluate the effects of CocE on cocaine brain levels. Positron emission tomogrpahy neuroimaging of [(11)C]cocaine was used to evaluate the time course of cocaine elimination from brain in the presence and absence of CocE in nonhuman primates. Systemic administration of CocE eliminated cocaine from the rhesus-monkey brain approximately three times faster than control conditions via peripheral actions through attenuating the input function from blood plasma. The efficiency of this process is sufficient to alleviate or prevent adverse central nervous system effects induced by cocaine. Although the present study used tracer doses of cocaine to access brain clearance, these findings further support the development of CocE for the treatment of acute cocaine toxicity.

In vivo pharmacological interactions between a type II positive allosteric modulator of α7 nicotinic ACh receptors and nicotinic agonists in a murine tonic pain model.

BACKGROUND AND PURPOSE: The α7 nicotinic ACh receptor subtype is abundantly expressed in the CNS and in the periphery. Recent evidence suggests that α7 nicotinic ACh receptor (nAChR) subtypes, which can be activated by an endogenous cholinergic tone comprising ACh and the α7 agonist choline, play an important role in chronic pain and inflammation. In this study, we evaluated whether type II α7 positive allosteric modulator PNU-120596 induces antinociception on its own and in combination with choline in the formalin pain model. EXPERIMENTAL APPROACH: We assessed the effects of PNU-120596 and choline and the nature of their interactions in the formalin test using an isobolographic analysis. In addition, we evaluated the interaction of PNU-120596 with PHA-54613, an exogenous selective α7 nAChR agonist, in the formalin test. Finally, we assessed the interaction between PNU-120596 and nicotine using acute thermal pain, locomotor activity, body temperature and convulsing activity tests in mice. KEY RESULTS: We found that PNU-120596 dose-dependently attenuated nociceptive behaviour in the formalin test after systemic administration in mice. In addition, mixtures of PNU-120596 and choline synergistically reduced formalin-induced pain. PNU-120596 enhanced the effects of nicotine and α7 agonist PHA-543613 in the same test. In contrast, PNU-120596 failed to enhance nicotine-induced convulsions, hypomotility and antinociception in acute pain models. Surprisingly, it enhanced nicotine-induced hypothermia via activation of α7 nAChRs. CONCLUSIONS AND IMPLICATIONS: Our results demonstrate that type II α7 positive allosteric modulators produce antinociceptive effects in the formalin test through a synergistic interaction with the endogenous α7 agonist choline.

Effects of the kappa opioid receptor antagonist, norbinaltorphimine, on stress and drug-induced reinstatement of nicotine-conditioned place preference in mice.

RATIONALE: Several studies implicate stress as a risk factor for the development and maintenance of drug addictive behaviors and drug relapse. Kappa opioid receptor (KOR) antagonists have been shown to attenuate behavioral responses to stress and stress-induced reinstatement of cocaine and ethanol seeking and preference. OBJECTIVES: In the current study, we determined whether the selective KOR antagonist, norbinaltorphimine (nor-BNI), would block stress-induced reinstatement of nicotine preference. METHODS: Adult Institute of Cancer Research mice were conditioned with 0.5 mg/kg nicotine, injected subcutaneously (s.c.) for 3 days and tested in the nicotine-conditioned place preference (CPP) model. After 3 days extinction, nor-BNI (10 mg/kg, s.c.) was administered 16 h prior to a priming dose of nicotine (0.1 mg/kg, s.c.), and mice were tested in the CPP model for nicotine-induced reinstatement of CPP. A separate group of mice was subjected to a 2-day modified forced swim test (FST) paradigm to induce stress after 3 days extinction from CPP. Mice were given vehicle or nor-BNI (10 mg/kg, s.c.) 16 h prior to each FST session. RESULTS: Nor-BNI pretreatment significantly attenuated stress-induced reinstatement of nicotine-CPP, but had no effect on nicotine-primed reinstatement. CONCLUSIONS: Blockade of KORs by selective antagonists attenuates stress-induced reinstatement of nicotine-CPP. Overall, the kappa opioid system may serve as a therapeutic target for suppressing multiple signaling processes which contribute to maintenance of smoking, smoking relapse, and drug abuse in general.

Effects of the specific α4ß2 nAChR antagonist, 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine, on nicotine reward-related behaviors in rats and mice.

RATIONALE: Alleviating addiction to tobacco products could prevent millions of deaths. Investigating novel compounds selectively targeting α4ß2 nAChRs hypothesized to have a key role in the rewarding effects of nicotine may be a useful approach for future treatment. OBJECTIVES: The present study was designed to evaluate 2-fluoro-3-(4-nitrophenyl) deschloroepibatidine (4-nitro-PFEB), a potent competitive antagonist of neuronal α4ß2 nAChRs, in several animal models related to nicotine reward: drug discrimination, intracranial self-stimulation (ICSS), conditioned place preference, and limited access to self-administration. METHODS: Long Evans rats were trained in a two-lever discrimination procedure to discriminate 0.4 mg/kg nicotine (s.c.) from saline. Male Sprague-Dawley rats were stereotaxically implanted with electrodes and trained to respond for direct electrical stimulation of the medial forebrain bundle. ICR mice were evaluated using an unbiased place preference paradigm, and finally, male Wistar rats were implanted with intrajugular catheters and tested for nicotine self-administration under limited access (1 h/day). RESULTS: 4-Nitro-PFEB attenuated the discriminative stimulus effects of nicotine, but alone did not produce nicotine-like discriminative stimulus effects. Nicotine-induced facilitation of ICSS reward thresholds was reversed by 4-nitro-PFEB, which alone had no effect on thresholds. 4-Nitro-PFEB also blocked the conditioned place preference produced by nicotine, but alone had no effect on conditioned place preference. Finally, 4-nitro-PFEB dose-dependently decreased nicotine self-administration. CONCLUSIONS: These results support the hypothesis that neuronal α4ß2 nAChRs play a key role in mediating the rewarding effects of nicotine and further suggest that targeting α4ß2 nAChRs may yield a potential candidate for the treatment of nicotine dependence.

Simultaneous measurement of extracellular dopamine and dopamine transporter occupancy by cocaine analogs in squirrel monkeys.

Several classes of drugs bind to the dopamine transporter (DAT) with high affinity, but some are weaker positive reinforcers than cocaine, suggesting that affinity for and occupancy of the DAT is not the only determinant of a drug's reinforcing effectiveness. Other factors such as the rate of onset have been positively and strongly correlated with the reinforcing effects of DAT inhibitors in nonhuman primates. In the current studies, we examined the effects of acute systemic administration of cocaine and three cocaine analogs (RTI-150, RTI-177, and RTI-366) on binding to DAT in squirrel monkey brain using positron emission tomography (PET) neuroimaging. During the PET scan, we also measured drug effects on dopamine (DA) levels in the caudate using in vivo microdialysis. In general, our results suggest a lack of concordance between drug occupancy at DAT and changes in DA levels. These studies also indicate that acute cocaine administration decreases the availability of plasma membrane DAT for binding, even after cocaine is no longer blocking DA uptake as evidence by a return to basal DA levels.

Structurally distinct nicotine immunogens elicit antibodies with non-overlapping specificities.

Nicotine conjugate vaccine efficacy is limited by the concentration of nicotine-specific antibodies that can be reliably generated in serum. Previous studies suggest that the concurrent use of 2 structurally distinct nicotine immunogens in rats can generate additive antibody responses by stimulating distinct B cell populations. In the current study we investigated whether it is possible to identify a third immunologically distinct nicotine immunogen. The new 1'-SNic immunogen (2S)-N,N'-(disulfanediyldiethane-2,1-diyl)bis[4-(2-pyridin-3-ylpyrrolidin-1-yl)butanamide] conjugated to keyhole limpet hemocyanin (KLH) differed from the existing immunogens 3'-AmNic-rEPA and 6-CMUNic-BSA in linker position, linker composition, conjugation chemistry, and carrier protein. Vaccination of rats with 1'-SNic-KLH elicited high concentrations of high affinity nicotine-specific antibodies. The antibodies produced in response to 1'-SNic-KLH did not appreciably cross-react in ELISA with either 3'-AmNic-rEPA or 6-CMUNic-BSA or vice versa, showing that the B cell populations activated by each of these nicotine immunogens were non-overlapping and distinct. Nicotine retention in serum was increased and nicotine distribution to brain substantially reduced in rats vaccinated with 1'-SNic-KLH compared to controls. Effects of 1'-SNic-KLH on nicotine distribution were comparable to those of 3'-AmNic-rEPA which has progressed to late stage clinical trials as an adjunct to smoking cessation. These data show that it is possible to design multiple immunogens from a small molecule such as nicotine which elicit independent immune responses. This approach could be applicable to other addiction vaccines or small molecule targets as well.

Functional reorganization of the presynaptic dopaminergic terminal in parkinsonism.

Whether dopamine (DA) release is compensated during the presymptomatic phase of Parkinson's disease (PD) is controversial. Here we use in vivo voltammetry in the parkinsonian rat and an electrical stimulation protocol established to fatigue nigrostriatal dopaminergic (DAergic) neurons to investigate the plasticity of DA-release mechanisms. Amplitudes of evoked voltammetric signals recorded in intact rat striata decreased with repetitive, high-frequency stimulation (60 Hz, every 5 min/60 min). Strikingly, DA levels were maintained during an identical "fatiguing" protocol in 6-hydroxydopamine-lesioned (<40% denervation) striata in the absence of enhanced DA synthesis. In contrast, more severely lesioned striata (>55% denervation) also appeared to sustain DA release, however, this was demonstrated in the presence of enhanced synthesis. Sustained release was replicated in intact animals after irreversible blockade of the dopamine transporter (DAT) via RTI-76, implicating neuronal uptake as a trigger. We further demonstrate through kinetic analysis that lesions and compromised uptake target a "long-term" (time constant of minutes) presynaptic depression, which underlies the maintenance of release. Taken together, our findings identify a denervation-induced maintenance of DA release that was independent of activated synthesis and driven by altered uptake. This novel neuroadaptation may contribute to early preclinical normalization of function and help resolve discrepant findings regarding compensatory changes in DA release during progression of the parkinsonian state.

An investigation of bupropion substitution for the interoceptive stimulus effects of nicotine.

Although the exact mechanism that makes bupropion hydrochloride (Zyban) effective as a smoking cessation aid has not been fully elucidated, studies have found that bupropion and nicotine share behavioural and neurophysiological properties suggesting that bupropion might serve as a substitute for nicotine. In fact, bupropion prompts nicotine-appropriate responding in operant and Pavlovian drug discrimination studies with rats. A majority of the literature examining this substitution pattern has been done with an operant paradigm. The present research extended this literature by further characterising the behavioural and neuropharmacological properties underlying the substitution for a nicotine conditioned stimulus (CS). Examination of the dose-effect function and temporal dynamics of this substitution pattern showed that bupropion (20 mg/kg) produced conditioned responding similar to nicotine (0.4 mg base/kg) (ED(50) = 9.9 mg/kg) at 15 and 30 min after injection and partially substituted 5 and 60 min post-injection. Bupropion produced a pattern of conditioned responding similar to nicotine during a 60-min extinction test. Additionally, it has been hypothesised that bupropion and nicotine have an overlapping dopaminergic mechanism. We tested the effects of bupropion pretreatment, the nicotine dose-effect function and the ability of dopamine antagonist to block the substitution of bupropion for nicotine. Pretreatment with doses of bupropion that did not substitute for the nicotine stimulus (5 and 10 mg/kg) did not affect nicotine-conditioned responding; pretreatment with 20 mg/kg attenuated nicotine-evoked responding. Pretreatment with the dopamine antagonists SCH-23390 and eticlopride blocked the substitution. Finally, S,S-hydroxybupropion, the major metabolite of bupropion in humans, did not substitute for the nicotine CS.

Neutral antagonist activity of naltrexone and 6beta-naltrexol in naïve and opioid-dependent C6 cells expressing a mu-opioid receptor.

BACKGROUND AND PURPOSE: Adenylyl cyclase sensitization occurs on chronic agonist activation of mu-opioid receptors and is manifested by an increase in cAMP levels (overshoot) on challenge with antagonist. It has been proposed that a long lasting constitutively active receptor is formed on chronic mu-opioid exposure and that antagonists with inverse agonist activity rapidly return the receptor to a basal state causing a cAMP overshoot and a more severe withdrawal response in vivo. This hypothesis depends on an accurate characterization of neutral and inverse agonist properties of opioid antagonists. EXPERIMENTAL APPROACH: C6 glioma and HEK293 cells expressing mu-opioid receptors were used. Opioid antagonists were examined for their ability to induce a cAMP overshoot following chronic treatment with the agonist DAMGO ([D-Ala(2),N-Me-Phe(4),Glyol(5)]-enkephalin). The compounds were also characterized as agonists, inverse agonists or neutral antagonists by using assays for competitive binding, [(35)S]GTPgammaS (guanosine-5'-O-(3-[(35)S]thio)triphosphate) binding and changes in cell surface receptor expression. KEY RESULTS: Naltrexone, 6beta-naltrexol and naloxone were indistinguishable to the mu-opioid receptor in the opioid-naïve or dependent state and acted as neutral antagonists. The delta-opioid receptor inverse agonist RTI-5989-25 [(+)-N-[trans-4'-(2-methylphenyl)-2'-butenyl]-(3R,4R)-dimethyl-4-(3-hydroxyphenyl)piperidine], a 3,4-dimethyl-4-(3-hydroxyphenyl)-piperidine, was an inverse agonist at the mu-opioid receptor, and the peptide antagonist CTAP (H-D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH(2)) showed variable, assay-dependent properties. All the antagonists precipitated the same degree of cAMP overshoot in opioid-dependent cells. CONCLUSIONS AND IMPLICATIONS: Antagonists at the mu-opioid receptor may be neutral or show inverse agonist activity. Formation of a constitutively active mu-opioid receptor is not a requirement for the development or expression of adenylyl cyclase sensitization.

Effects of the monoamine uptake inhibitors RTI-112 and RTI-113 on cocaine- and food-maintained responding in rhesus monkeys.

Cocaine blocks uptake of the monoamines dopamine, serotonin and norepinephrine, and monoamine uptake inhibitors constitute one class of drugs under consideration as candidate "agonist" medications for the treatment of cocaine abuse and dependence. The pharmacological selectivity of monoamine uptake inhibitors to block uptake of dopamine, serotonin and norepinephrine is one factor that may influence the efficacy and/or safety of these compounds as drug abuse treatment medications. To address this issue, the present study compared the effects of 7-day treatment with a non-selective monoamine uptake inhibitor (RTI-112) and a dopamine-selective uptake inhibitor (RTI-113) on cocaine- and food-maintained responding in rhesus monkeys. Monkeys (N=3) were trained to respond for cocaine injections (0.01 mg/kg/inj) and food pellets under a second-order schedule [FR2(VR16:S)] during alternating daily components of cocaine and food availability. Both RTI-112 (0.0032-0.01 mg/kg/hr) and RTI-113 (0.01-0.056 mg/kg/h) produced dose-dependent, sustained and nearly complete elimination of cocaine self-administration. However, for both drugs, the potency to reduce cocaine self-administration was similar to the potency to reduce food-maintained responding. These findings do not support the hypothesis that pharmacological selectivity to block dopamine uptake is associated with behavioral selectivity to decrease cocaine- vs. food-maintained responding in rhesus monkeys.

Structure-activity relationship of trihexyphenidyl analogs with respect to the dopamine transporter in the on going search for a cocaine inhibitor.

A series of trihexyphenidyl (THP) analogs were used to search for a derivative that could serve as a cocaine inhibitor. A compound that blocks binding of the cocaine analog carboxyfluorotropane (CFT), allows dopamine uptake and exhibits low side effects could serve as a good candidate for that purpose. All analogs were tested for the extent to which they inhibit CFT binding, dopamine uptake and n-methyl scopolamine (NMS) binding. Several structure-function relationships emerged. Methylation/halogenation of THP's benzene ring enhanced the compound's ability to block CFT binding in comparison to its ability to block dopamine uptake (5a-e). Replacement of the cyclohexyl ring with a benzene ring tended to create compounds that had lower affinities to the dopamine transporter (7b compared to THP, 7d compared to 5h, 7c compared to 8c) and modification of THP's piperidine ring tended to enhance affinity to the dopamine transporter (5f-h, 8a, 8c). One analog (5f) that showed little muscarinic activity indicating that it would probably have few side effects was investigated for its effects as an in vivo cocaine inhibitor. However, it showed few antagonistic effects in vivo. Nevertheless, this work greatly elucidates the structure-function relationships required for potential cocaine inhibitors and so lays out promising directions for future research.