Nicotine's opioid and anti-opioid interactions: proposed role in smoking behavior

National study of discontinuation of long-term opioid therapy among veterans

Veterans have high rates of chronic pain and long-term opioid therapy (LTOT). Understanding predictors of discontinuation from LTOT will clarify the risks for prolonged opioid use and dependence among this population. All veterans with at least 90 days of opioid use within a 180-day period were identified using national Veteran's Health Affairs (VHA) data between 2009 and 2011. Discontinuation was defined as 6 months with no opioid prescriptions. We used Cox proportional hazards analysis to determine clinical and demographic correlates for discontinuation. A total of 550,616 veterans met criteria for LTOT. The sample was primarily male (93%) and white (74%), with a mean age of 57.8 years. The median daily morphine equivalent dose was 26 mg, and 7% received high-dose (>100mg MED) therapy. At 1 year after initiation, 7.5% (n=41,197) of the LTOT sample had discontinued opioids. Among those who discontinued (20%, n=108,601), the median time to discontinuation was 317 days. Factors significantly associated with discontinuation included both younger and older age, lower average dosage, and having received less than 90 days of opioids in the previous year. Although tobacco use disorders decreased the likelihood of discontinuation, co-morbid mental illness and substance use disorders increased the likelihood of discontinuation. LTOT is common in the VHA system and is marked by extended duration of use at relatively low daily doses with few discontinuation events. Opioid discontinuation is more likely in veterans with mental health and substance use disorders. Further research is needed to delineate causes and consequences of opioid discontinuation.

Association of smoking with μ-opioid receptor availability before and during naltrexone blockade in alcohol-dependent subjects

Persons with a history of alcohol dependence are more likely to use tobacco and to meet criteria for nicotine dependence compared with social drinkers or non-drinkers. The high levels of comorbidity of nicotine and alcohol use and dependence are thought to be related to interactions between nicotinic, opioid and dopamine receptors in mesolimbic regions. The current study examined whether individual differences in regional μ-opioid receptor (MOR) availability were associated with tobacco use, nicotine dependence and level of nicotine craving in 25 alcohol-dependent (AD) subjects. AD subjects completed an inpatient protocol, which included medically supervised alcohol withdrawal, monitored alcohol abstinence, transdermal nicotine maintenance (21 mg/day) and Positron Emission Tomography (PET) imaging using the MOR agonist [(11) C]-carfentanil (CFN) before (basal scan) and during treatment with 50 mg/day naltrexone (naltrexone scan). Subjects who had higher scores on the Fagerström Nicotine Dependence Test had significantly lower basal scan binding potential (BPND ) across mesolimbic regions, including the amygdala, cingulate, globus pallidus, thalamus and insula. Likewise, the number of cigarettes per day was negatively associated with basal scan BPND in mesolimbic regions. Higher nicotine craving was significantly associated with lower BPND in amygdala, globus pallidus, putamen, thalamus and ventral striatum. Although blunted during naltrexone treatment, the negative association was maintained for nicotine dependence and cigarettes per day, but not for nicotine craving. These findings suggest that intensity of cigarette smoking and severity of nicotine dependence symptoms are systematically related to reduced BPND across multiple brain regions in AD subjects.

Effects of chronic khat use on cardiovascular, adrenocortical, and psychological responses to stress in men and women

BACKGROUND

Khat is a psychostimulant plant widely used in Africa and its use has been growing rapidly in Europe and North America.

OBJECTIVES

We investigated effects of chronic khat (Catha edulis) use on cardiovascular, adrenocortical, and psychological responses to acute stress.

METHODS

Chronic khat users and nonusers were compared on physiological measures and mood reports in a cross-sectional, mixed design. Measurements were conducted during 24-hour ambulatory monitoring and during a laboratory session. A total of 152 participants (58 women) were recruited by flyers posted around Sana'a University campus and the surrounding community in Sana'a, Yemen. Salivary cortisol and self-report measures were collected during a 24-hour ambulatory period prior to a lab testing session. In addition, blood pressures (BP), salivary cortisol, and mood measures were assessed during rest and in response to acute mental stress.

RESULTS

Khat users exhibited enhanced evening and attenuated morning cortisol levels, reflecting a blunted diurnal pattern of adrenocortical activity compared to nonusers. Khat users reported greater negative affect during the ambulatory period and during the laboratory session. In addition, they exhibited attenuated BP responses to stress.

CONCLUSIONS AND SCIENTIFIC SIGNIFICANCE

These novel results demonstrate altered adrenocortical activity and increased dysphoric mood among khat users. The extent to which these associations are due to effects of chronic khat use per se or instead reflect predisposing risk factors for khat use is yet to be determined.

Stress response dysregulation and stress-induced analgesia in nicotine dependent men and women

Alterations in the stress response and endogenous pain regulation mechanisms may contribute directly and indirectly to maintenance of nicotine dependence and relapse. We examined the extent to which nicotine dependence alters endogenous pain regulatory systems, including the hypothalamic-pituitary-adrenocortical axis, cardiovascular activity, and stress-induced analgesia. Smokers and nonsmokers attended a laboratory session that included assessment of hormonal and cardiovascular responses to stress. Smokers smoked at their regular rate prior to the session. The hand cold pressor and heat thermal pain tests were completed twice, once after acute stress (public speaking and math tasks) and the other after rest. While smokers and nonsmokers exhibited significant hormonal and cardiovascular responses to stress, smokers exhibited blunted stress responses relative to nonsmokers. They also exhibited diminished stress-induced analgesia. Results demonstrate altered stress response and diminished stress-induced analgesia among chronic smokers, and suggest that these dysregulated physiological responding may contribute to altered endogenous pain regulation.

Acute HPA axis response to naltrexone differs in female vs. male smokers

BACKGROUND

Both opioid antagonist administration and cigarette smoking acutely increase hypothalamic-pituitary-adrenal (HPA) axis activity as measured by adrenocorticotropic hormone (ACTH) and cortisol levels. However, male and female smokers may differ in their response to the opioid antagonist naltrexone, which may be partially mediated by sex differences in HPA axis function. Smokers, as a group, have frequently been shown to have HPA axis dysfunction, which may have relevance to the course and maintenance of nicotine dependence. The purpose of this study was to examine possible sex differences in HPA axis function by comparing stress-hormone response to naltrexone within healthy male and female smokers. Additionally, exploratory analyses compared the combined effects of naltrexone and cigarette smoking on hormonal responsivity between the sexes.

METHOD

Thirty-eight healthy smokers (22 men) were tested in two separate morning sessions after 12h of smoking abstinence. For women, self-reports of menstrual cycle information were obtained prior to each session (date of last menstruation, cycle length, reproductive phase, etc.). Each participant received 50mg naltrexone or placebo capsule (in random order) and plasma levels of ACTH and cortisol were assessed at regular intervals for several hours. A subgroup of 12 participants underwent a similar, additional session in which they smoked a single cigarette three hours after naltrexone administration.

RESULTS

Naltrexone significantly increased ACTH and cortisol levels in women, but not men (DrugxSexxTime, p<0.05). A post hoc analysis suggested that women at an estimated 'high estrogen' phase had a greater cortisol response (DrugxEstrogen level, p<0.05) than those at an estimated 'low estrogen' phase. Exploratory analyses showed that smoking a single cigarette potentiated naltrexone-induced increases in ACTH (p<0.05) and cortisol (p<0.01) in all participants.

CONCLUSION

The findings support the hypothesis that women are more sensitive to opioid antagonism at the level of the HPA axis. Although further studies are needed to examine mechanisms underlying these responses, both results may have clinical implications for the use of naltrexone as a treatment for nicotine dependence.

Blunted opiate modulation of hypothalamic-pituitary-adrenocortical activity in men and women who smoke

OBJECTIVE

To examine the extent to which nicotine dependence alters endogenous opioid regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis functions. Endogenous opiates play an important role in regulating mood, pain, and drug reward. They also regulate the HPA functions. Previous work has demonstrated an abnormal HPA response to psychological stress among dependent smokers.

METHODS

Smokers and nonsmokers (total n = 48 participants) completed two sessions during which a placebo or 50 mg of naltrexone was administered, using a double-blind design. Blood and saliva samples, cardiovascular and mood measures were obtained during a resting absorption period, after exposure to two noxious stimuli, and during an extended recovery period. Thermal pain threshold and tolerance were assessed in both sessions. Participants also rated pain during a 90-second cold pressor test.

RESULTS

Opioid blockade increased adrenocorticotropin, plasma cortisol, and salivary cortisol levels; these increases were enhanced by exposure to the noxious stimuli. These responses were blunted in smokers relative to nonsmokers. Smokers tended to report less pain than nonsmokers, and women reported more pain during both pain procedures, although sex differences in pain were significant only among nonsmokers.

CONCLUSIONS

We conclude that nicotine dependence is associated with attenuated opioid modulation of the HPA. This dysregulation may play a role in the previously observed blunted responses to stress among dependent smokers.

Tobacco/nicotine and endogenous brain opioids

Smoking is a major public health problem with devastating health consequences. Although many cigarette smokers are able to quit, equal numbers of others cannot! Standard medications to assist in smoking cessation, such as nicotine replacement therapies and bupropion, are ineffective in many remaining smokers. Recent developments in the neurobiology of nicotine dependence have identified several neurotransmitter systems that may contribute to the process of smoking maintenance and relapse. These include: especially dopamine, but also norepinephrine, 5-hydroxytryptamine, acetylcholine, endogenous opioids, gamma-aminobutyric acid (GABA), glutamate, and endocannabinoids. The present review examines the limited contribution of the endogenous opioid system to the complex effects of nicotine/tobacco smoking.

The κ-opioid receptor is involved in the stimulating effect of nicotine on adrenocortical activity but not in nicotine induced anxiety

The kappa (kappa) opioid system appears to interact with nicotine in the modulation of locomotion and addiction related processes. In this study we have investigated the possible implication of the kappa-opioid system in the effects of nicotine on anxiety and adrenocortical activity. In two different experiments, we analysed the possible interaction between nicotine (0.5 mg/kg i.p.) and either the kappa-opioid receptor antagonist nor-binaltorphimine (5 mg/kg i.p.) or the kappa-opioid receptor agonist U50,488H (1 mg/kg s.c.). Behavioural and endocrine experiments were performed in different groups of animals. Animals were exposed to the holeboard immediately followed by the plus-maze. Serum corticosterone levels were determined by radioimmunoassay. Nicotine induced an anxiogenic-like effect in the plus-maze and a significant decrease of holeboard activity. The anxiogenic-like effect in the plus-maze was not modified by any of the kappa-opioid receptor ligands. Nicotine also induced a significant increase in the corticosterone levels, and the kappa antagonist, which did not exert any effect per se, antagonised this effect. The kappa-agonist U50,488H induced a significant increase in corticosterone concentration when administered alone. We provide the first evidence for the involvement of the kappa-opioid receptor in the stimulatory effect of nicotine on adrenocortical activity.

Naltrexone alteration of acute smoking response in nicotine-dependent subjects

There are mixed results on the effects of opioid antagonists on acute nicotine response in humans. The present study examined the effects of a single dose of 50 mg oral naltrexone relative to placebo on smoking response in 22 chronic smokers during short-term nicotine abstinence, after acute smoking and subsequent smoking deprivation, and on smoking behavior in a choice paradigm. The results showed that naltrexone significantly reduced immediate postcigarette ratings of smoking craving and desire to smoke and increased light-headedness, dizziness, and head rush (ps < 0.05). Reductions in craving and smoking desire persisted during a subsequent 1 h nonsmoking interval. Naltrexone also was found to significantly reduce the total number of cigarettes smoked in the choice interval, which was supported by objective measures of both reduced CO and plasma nicotine levels (ps < 0.01). Exploratory analyses on potential individual difference factors revealed that smokers with the highest levels of craving during abstinence showed the most pronounced naltrexone attenuation of smoking response. The results support the continued exploration of naltrexone as an adjunct to smoking cessation, especially in identified smoker subgroups most sensitive to the effects of opioid antagonism.

Effects of chronic nicotine treatment and withdrawal on hypothalamic proopiomelanocortin gene expression and neuroendocrine regulation

Considerable evidence suggest that some responses to smoking and nicotine are mediated by forebrain beta-endorphinergic opioid mechanisms. It has also been demonstrated that nicotine stimulates rat tuberoinfundibular dopaminergic activity. Since we have proposed that interactions between mediobasohypothalamic (MBH) dopaminergic and beta-endorphinergic mechanisms have a key role in neuroendocrine integration, we investigated the effects of chronic nicotine treatment and withdrawal on: (1) MBH concentrations of proopiomelanocortin (POMC, precursor for beta-endorphin biosynthesis) mRNA; (2) MBH concentrations of tyrosine hydroxylase (TH, rate limiting enzyme in catecholamine biosynthesis) mRNA; (3) corresponding serum prolacin, corticosterone, luteinizing hormone (LH), and testosterone concentrations. POMC and TH mRNA levels were measured by RNase protection/solution hybridization assay; serum hormone levels were measured by radioimmunoassay. Adult male rats received subcutaneous injections of either nicotine or saline during the dark period of each day on an increasing frequency (1-3 injections/day) and dosage (0.4-0.5 mg nicotine/kg body weight) schedule over 4 weeks. The rats were sacrificed after 4 weeks treatment and at 1, 3, 7, 14 and 21 days withdrawal. Chronic daily nicotine administration induced significant changes in serum corticosterone, serum prolactin, MBH TH mRNA, and MBH POMC mRNA concentrations that tended to persist through day 3 of withdrawal; serum prolactin and MBH POMC mRNA concentrations were suppressed whereas serum corticosterone and MBH TH mRNA concentrations were stimulated. None of the parameters were significantly different from control levels following 7 or more days of withdrawal from nicotine, except for a significant decrease of MBH POMC mRNA concentrations on day 21. Chronic daily nicotine or withdrawal did not significantly alter serum LH or testosterone concentrations. These results suggest that chronic nicotine inhibited POMC gene expression and thus, probably, biosynthesis of beta-endorphin and other opiomelanocortins. We hypothesize that suppression of forebrain beta-endorphin synthesis in response to long-term nicotine exposure produces a chronically opioid deficient condition which may play an important role in maintaining nicotine self-administration and in mediating some changes during the nicotine withdrawal syndrome.

The analgesia-enhancing component of ingested amniotic fluid does not affect nicotine-induced antinociception in naltrexone-treated rats

Ingestion of amniotic fluid and placenta by rats has been shown to enhance opioid-mediated antinociception but not affect the nonopioid-mediated antinociception produced by aspirin, suggesting specificity for opioid-mediated processes. However, enhancement by the active substance(s) in amniotic fluid and placenta (POEF, for placental opioid-enhancing factor) of antinociception produced by other nonopioid mechanisms has yet to be examined. The present experiments tested whether ingestion of amniotic fluid enhances the antinociception produced by nicotine injection. In Experiment 1A, enhancement of morphine-mediated antinociception by ingestion of amniotic fluid was demonstrated in a hot-plate assay. In Experiment 1B, rats pretreated with naltrexone were given an orogastric infusion of amniotic fluid or control (0.25 ml), then injected with nicotine (0, 0.075, 0.125, or 0.225 mg/kg subcutaneously), then tested for antinociception in a hot-plate assay. Amniotic fluid ingestion did not enhance the antinociception produced by various doses of nicotine. In Experiment 2, rats pretreated with naltrexone were given an orogastric infusion of amniotic fluid (0, 0.125, 0.25, or 0.50 ml) and then injected with 0.125 mg/kg nicotine. None of the doses of amniotic fluid enhanced the nicotine-induced antinociception. The findings of these experiments lend support to our contention that the enhancement by POEF of antinociception is specific to opioid-mediated processes.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.