Carol A. Paronis

Smoking, Stress, and Negative Affect: Correlation, Causation, and Context Across Stages of Smoking

This transdisciplinary review of the literature addresses the questions, Do stress and negative affect (NA) promote smoking? and Does smoking genuinely relieve stress and NA? Drawing on both human and animal literatures, the authors examine these questions across three developmental stages of smoking--initiation, maintenance, and relapse. Methodological and conceptual distinctions relating to within- and between-subjects levels of analyses are emphasized throughout the review. Potential mechanisms underlying links between stress and NA and smoking are also reviewed. Relative to direct-effect explanations, the authors argue that contextual mediator-moderator approaches hold greater potential for elucidating complex associations between NA and stress and smoking. The authors conclude with recommendations for research initiatives that draw on more sophisticated theories and methodologies.

Δ(9)-Tetrahydrocannabinol acts as a partial agonist/antagonist in mice.

&Dgr;9-Tetrahydrocannabinol (THC) has been characterized as a partial agonist at cannabinoid CB1 receptors in vitro; however, it often produces the same maximum effects in vivo as other cannabinoid agonists. This study was carried out to determine whether THC would antagonize the hypothermic effects of another cannabinoid agonist, AM2389, in mice. Male mice were injected with 1–100 mg/kg THC, 0.01–0.1 mg/kg AM2389, or a combination of 30 mg/kg THC and 0.1–1.0 mg/kg AM2389, and rectal temperature was recorded for up to 12 h after injection. THC reduced the temperature by 5.6°C at a dose of 30 mg/kg; further increases in the dose did not produce larger effects, indicating a plateau in the THC dose–effect function. AM2389 reduced temperature by 9.0°C at a dose of 0.1 mg/kg. One hour pretreatment with 30 mg/kg THC attenuated the hypothermic effects of 0.1 mg/kg AM2389; a 10-fold higher dose, 1.0 mg/kg AM2389, was required to further decrease temperature, reflecting a five-fold rightward shift of the lower portion of the AM2389 dose–effect function following THC pretreatment. These results indicate that, in an assay of mouse hypothermia, THC exerts both agonist and antagonist effects following acute administration, and mark the first demonstration of partial agonist/antagonist effects of THC in vivo.

Evaluation of the reinforcing effects of monoamine reuptake inhibitors under a concurrent schedule of food and I.V. drug delivery in rhesus monkeys

Most medications prescribed for attention-deficit-hyperactivity disorder are psychomotor stimulants with reinforcing effects in laboratory animals (eg methylphenidate). The present studies were conducted to evaluate the reinforcing effects of the recently approved medication atomoxetine in monkeys trained to ‘choose’ between automated deliveries of either an i.v. injection or food. Rhesus monkeys were trained to lever-press under concurrent schedules of reinforcement; responses on one lever resulted in an injection of either saline or drug, and responses on the alternative lever resulted in food delivery. Data were collected on four measures: response rates, percentage of total responses occurring on the injection-lever (% ILR), number of injections earned, and number of food pellets earned. Dose–effect functions were determined for cocaine (0.003–0.3 mg/kg/inj), methylphenidate (0.003–0.1 mg/kg/inj), amphetamine (0.003–0.1 mg/kg/inj), atomoxetine (0.01–0.3 mg/kg/inj), and desipramine (0.03–1.0 mg/kg/inj) using a double alternation schedule of saline and drug availability. Results indicate that the distribution of behavior changed according to the drug and dose available for self-injection. Saline availability was typically associated with high rates of food-maintained responding. The % ILR increased from 3±1% when saline was available to >90% when >0.03 mg/kg/inj of cocaine, methylphenidate or d-amphetamine was available. However, no dose of atomoxetine or desipramine maintained self-administration behavior on the injection-lever. The number of food pellets earned per session decreased as the dose of each drug increased, indicative of behavioral activity with all five drugs. The reinforcing effects of cocaine, methylphenidate, and d-amphetamine in these studies are consistent with previous findings in nonhuman primates and with their documented abuse liability. The absence of reinforcing effects of atomoxetine support the view that, like desipramine, it has no evident abuse potential.

Monoaminergic Psychomotor Stimulants: Discriminative Stimulus Effects and Dopamine Efflux

The present studies were conducted to investigate the relationship between discriminative stimulus effects of indirectly acting monoaminergic psychostimulants and their ability to increase extracellular levels of dopamine (DA) in the nucleus accumbens (NAcb) shell. First, the behavioral effects of methamphetamine (MA), cocaine (COC), 1-[2-[bis(4-fluorophenyl-)methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR 12909), d-amphetamine, and methylphenidate were established in rats trained to discriminate intraperitoneal injections of 0.3 mg/kg MA from saline. In other studies, in vivo microdialysis was used to determine the effects of MA, COC, and GBR 12909 on extracellular DA levels in the NAcb shell. Results show that all drugs produced dose-related and full substitution for the discriminative stimulus effects of 0.3 mg/kg MA. In microdialysis studies, cumulatively administered MA (0.3–3 mg/kg), COC (3–56 mg/kg), and GBR 12909 (3–30 mg/kg) produced dose-dependent increases in DA efflux in the NAcb shell to maxima of approximately 1200 to 1300% of control values. The increase in DA levels produced by MA and COC was rapid and short-lived, whereas the effect of GBR 12909 was slower and longer lasting. Dose-related increases in MA lever selection produced by MA, COC, and GBR 12909 corresponded with graded increases in DA levels in the NAcb shell. Doses of MA, COC, and GBR 12909 that produced full substitution increased DA levels to approximately 200 to 400% of control values. Finally, cumulatively administered MA produced comparable changes in DA levels in both naive and 0.3 mg/kg MA-trained rats. These latter results suggest that sensitization of DA release does not play a prominent role in the discriminative stimulus effects of psychomotor stimulants.

Sensitization and tolerance to the discriminative stimulus effects of mu-opioid agonists

The discriminative stimulus effects of several μ-opioid agonists were examined under conditions of opioid sensitization or tolerance, i.e., before and after 1-week SC infusions of naloxone or μ-opioid agonists. Rats were trained to discriminate 3.0 mg/kg morphine from saline using a two-lever, discrete trial, shock-avoidance/escape procedure. The rats generalized completely to morphine, fentanyl, meperidine, buprenorphine, and etorphine, and partially to pentazocine. A 7-day infusion of naloxone (0.3 mg/kg per h) potentiated the discriminative stimulus effects of all of these drugs. The magnitude of the increased potency varied indirectly with the efficacy of the μ-opioid agonists; potency ratios (pre-infusion ED50/post-infusion ED50) ranged from 1.58 (etorphine) to 3.58 (pentazocine). Stimulus generalization to morphine, fentanyl, and meperidine also was examined following infusions of equieffective doses of each of these three drugs. Differences among drugs were generally small, and failed to reach statistical significance. Nonetheless, the induction of μ-opioid tolerance did seem to vary with the efficacy of the three μ-opioid agonists. Thus, meperidine (6.25 mg/kg per h), which has the lowest efficacy of the drugs infused, produced the greatest shift to the right of the stimulus-generalization curves of these three drugs; the post-meperidine PR ranged between 0.40 and 0.61. Fentanyl (0.1 mg/kg per h), a drug with a higher efficacy at μ-opioid receptors, did not produce tolerance to the discriminative stimulus effects of morphine, fentanyl, or meperidine; potency ratios ranged from 0.50 to 0.75. Potency ratios for buprenorphine, etorphine, fentanyl, meperidine, and morphine after 7-day morphine infusions (0.75 mg/kg per h) ranged from 0.38 (buprenorphine) to 0.80 (etorphine). Morphine induced significant tolerance only to the discriminative stimulus effects of fentanyl. Our results suggest that different cellular mechanisms underlie the development of tolerance and sensitization to the discriminative stimulus effects of μ-opioid agonists.

Buprenorphine and methoclocinnamox: agonist and antagonist effects on respiratory function in rhesus monkeys

Buprenorphine and methoclocinnamox are partial micro-opioid receptor agonists with potential use in the treatment of opioid abuse. The ability of these drugs to suppress respiration as well as their ability to antagonize the respiratory suppressant effects of morphine and heroin were tested in rhesus monkeys. Frequency (f), minute volume (V(e)) tidal volume (V(t)) in monkeys breathing air or 5% CO(2) in air were recorded using a pressure-displacement plethysmograph. Buprenorphine (0.001-10 mg/kg) produced a dose-dependent decrease in respiratory parameters that plateaued at a dose of 1 mg/kg in both air and 5% CO(2). Methoclocinnamox (0. 032-1 mg/kg) also produced dose-dependent respiratory depression that plateaued at a dose of 0.3 mg/kg in air, and was directly related to dose in 5% CO(2). Respiratory suppression produced by buprenorphine 1 and 10 mg/kg lasted for 3 and 7 days, respectively, whereas the suppression produced by the largest dose of methoclocinnamox (1 mg/kg, the solubility limit) lasted less than 24 h. Buprenorphine and methoclocinnamox antagonized morphine- and heroin-induced respiratory depression, and this antagonist effect was observed concomitantly with, as well as following, the mu-opioid receptor agonist effects of buprenorphine and methoclocinnamox. The mu-opioid receptor antagonist effects of buprenorphine (10 mg/kg) and methoclocinnamox (1 mg/kg) lasted for 2 weeks. These results suggest that buprenorphine and methoclocinnamox have a wide margin of safety in clinical use and that these two compounds have a prolonged, insurmountable, mu-opioid receptor antagonist effect after the disappearance of their agonist effects.

Clocinnamox dose-dependently antagonizes morphine-analgesia and [3H]DAMGO binding in rats.

Clocinnamox is a long-lasting, nonequilibrium, mu-opioid receptor antagonist in mice and monkeys. The present studies examined the in vivo and ex vivo effects of clocinnamox in rats. Under control conditions, morphine dose-dependently increased tail-withdrawal latencies from 50 degrees C water, with a mean ED50 of 7.3 +/- 1.1 mg/kg. Clocinnamox antagonized the antinociceptive effects of morphine. 1.0 mg/kg clocinnamox displaced the morphine dose-response curve 4-fold to the right of the control curve and 10 mg/kg clocinnamox eliminated morphines antinociceptive effects at doses up to 1000 mg/kg for at least seven days. There was a gradual recovery of the antinociceptive response to morphine; however, the morphine dose-response curve did not return to its original position by five weeks after 10 mg/kg clocinnamox. Whole brain membranes were prepared from separate groups of rats for determination of binding parameters of [3H][D-Ala2, N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO). Clocinnamox dose-dependently decreased [3H]DAMGO binding ex vivo and the decreased binding was a result of changes in Bmax. The control Bmax for [3H]DAMGO was 234 +/- 8 fmol/mg protein; in membranes prepared from rats pretreated with 10 mg/kg clocinnamox, the Bmax value for [3H]DAMGO was 54 +/- 2 fmol/mg protein. The Bmax values for [3H]DAMGO binding after an injection of 10 mg/kg clocinnamox returned towards control values gradually, four weeks after clocinnamox the Bmax was 178 +/- 10 fmol/mg protein. These results suggest that clocinnamox is a long-lasting, nonequilibrium mu-opioid receptor antagonist in rats.

Buprenorphine and opioid antagonism, tolerance, and naltrexone-precipitated withdrawal.

The dual antagonist effects of the mixed-action μ-opioid partial agonist/κ-opioid antagonist buprenorphine have not been previously compared in behavioral studies, and it is unknown whether they are comparably modified by chronic exposure. To address this question, the dose-related effects of levorphanol, trans-(−)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide (U50,488), heroin, and naltrexone on food-maintained behavior in rhesus monkeys were studied after acute and chronic treatment with buprenorphine (0.3 mg/kg/day). In acute studies, the effects of levorphanol and U50,488 were determined at differing times after buprenorphine (0.003–10.0 mg/kg i.m.). Results show that buprenorphine produced similar, dose-dependent rightward shifts of the levorphanol and U50,488 dose-response curves that persisted for ≥24 h after doses larger than 0.1 mg/kg buprenorphine. During chronic treatment with buprenorphine, the effects of levorphanol, U50,488, heroin, and naltrexone were similarly determined at differing times (10 min to 48 h) after intramuscular injection. Overall, results show that buprenorphine produced comparable 3- to 10-fold rightward shifts in the U50,488 dose-response curve under both acute and chronic conditions, but that chronic buprenorphine produced larger (10- to ≥30-fold) rightward shifts in the heroin dose-effect function than observed acutely. Naltrexone decreased operant responding in buprenorphine-treated monkeys, and the position of the naltrexone dose-effect curve shifted increasingly to the left as the time after daily buprenorphine treatment increased from 10 min to 48 h. These results suggest that the μ-antagonist, but not the κ-antagonist, effects of buprenorphine are augmented during chronic treatment. In addition, the leftward shift of the naltrexone dose-effect function suggests that daily administration of 0.3 mg/kg buprenorphine is adequate to produce opioid dependence.

Acute dependence on, but not tolerance to, heroin and morphine as measured by respiratory effects in rhesus monkeys.

Acute dependence on and tolerance to heroin and morphine were assessed in rhesus monkeys using measures of respiration. Respiratory frequency (f) and minute volume (V(e)) were measured in monkeys breathing air or 5% CO(2) in air using a pressure-displacement plethysmograph. Cumulative doses of naltrexone (0.0001-1.0 mg/kg, i.m) did not alter these parameters in untreated monkeys. Twenty-four hours after a cumulative dose of heroin (1 mg/kg, i.m.), naltrexone produced an increase in both f and V(e) when monkeys were breathing air or 5% CO(2). Following 1 to 3 days of treatment with heroin (0.5 mg/kg/day, i.m.) or morphine (16 mg/kg/day, i.m.), naltrexone produced an increase in f and V(e) that was related to the dose of naltrexone and the number of days of agonist administration. Two days following termination of heroin administration, naltrexone-induced respiratory stimulation declined and had disappeared completely by the fifth day. In tolerance studies, heroin (0.032-0.5 mg/kg, i.m.) and morphine (1-16 mg/kg, i. m.) were injected cumulatively each day for three consecutive days. These drugs suppressed f and V(e) to nearly the same extent on day 3 as they had on day 1 of administration. These results suggest that dependence to morphine and heroin can be measured under conditions of acute 1 to 3 day administration conditions in primates using f and V(e) as reliable and quantitative indicators of opioid withdrawal. Under these conditions, tolerance does not occur.

Effects of the δ opioid agonist AZD2327 upon operant behaviors and assessment of its potential for abuse.

AZD2327 is a brain-penetrant agonist at δ opioid receptors which has antidepressant and anxiolytic properties in a wide array of animal models. As part of the preclinical safety pharmacology assessment, a number of studies were conducted in order to characterize its behavioral effects and its potential for abuse, in order to enable testing in humans. AZD2327 produced only modest effects when tested in a multiple fixed-ratio differential reinforcement of low rate schedule in rats, and did not enhance the rate-suppressing effects of ethanol in the procedure. In a suppressed responding test, AZD2327 only reduced rates of unpunished responding. In drug discrimination studies, AZD2327 produced partial or no generalization from known drugs of abuse. In primates trained to self-administer cocaine, substitution with AZD2327 did not result in appreciable self-administration of AZD2327, indicating that it does not behave as a positive reinforcer under the present conditions. Following termination of repeated administration of AZD2327, no signs of physical dependence (withdrawal) were noted. Overall, the data suggest that AZD2327 does not possess a high potential for abuse, and appears to have only subtle behavioral effects as measured by operant behaviors.