Monoamine oxidase A binding in the prefrontal and anterior cingulate cortices during acute withdrawal from heavy cigarette smoking

CONTEXT

Greater prefrontal cortex and anterior cingulate cortex monoamine oxidase A (MAO-A) binding is associated with depressed mood. Substances in cigarette smoke, such as harman, inhibit MAO-A, and cigarette withdrawal is associated with depressed mood. Dysphoria during cigarette withdrawal predicts relapse. It is unknown whether MAO-A binding increases during early cigarette withdrawal.

OBJECTIVES

To measure prefrontal and anterior cingulate cortex MAO-A binding during acute cigarette withdrawal and to assess the relationship with smoking severity, plasma levels of harman, and severity of depression.

DESIGN

Study via positron emission tomography of healthy control and cigarette-smoking individuals.

PATIENTS

Twenty-four healthy nonsmoking and 24 otherwise healthy cigarette-smoking individuals underwent positron emission tomography with harmine labeled with carbon 11. Healthy nonsmoking individuals underwent scanning once. Cigarette-smoking individuals underwent scanning after acute withdrawal and after active cigarette smoking. Cigarette smoking was heavy (≥25 cigarettes per day) or moderate (15-24 cigarettes per day).

SETTING

Tertiary care psychiatric hospital.

MAIN OUTCOME MEASURE

An index of MAO-A density, MAO-A V(T), was measured in the prefrontal and anterior cingulate cortices.

RESULTS

In heavy-smoking individuals, prefrontal and anterior cingulate cortex MAO-A V(T) was greater during withdrawal (23.7% and 33.3%, respectively; repeated-measures multivariate analysis of variance, F(1,22) = 25.58, P < .001). During withdrawal from heavy smoking, prefrontal and anterior cingulate cortex MAO-A V(T) was greater than in healthy controls (25.0% and 25.6%, respectively; multivariate analysis of variance, F(2,33) = 6.72, P = .004). The difference in MAO-A V(T) in the prefrontal cortex and anterior cingulate cortex between withdrawal and active, heavy smoking covaried with change in plasma harman levels in the prefrontal cortex and anterior cingulate cortex (multivariate analysis of covariance, F(1,10) = 9.97, P = .01). The change in MAO-A V(T) between withdrawal and active, heavy smoking also covaried with severity of depression (multivariate analysis of covariance, F(1,10) = 11.91, P = .006).

CONCLUSIONS

The increase in prefrontal and anterior cingulate cortex MAO-A binding and associated reduction in plasma harman level represent a novel, additional explanation for depressed mood during withdrawal from heavy cigarette smoking. This finding resolves a longstanding paradox regarding the association of cigarette smoking with depression and suicide and argues for additional clinical trials on the effects of MAO-A inhibitors on quitting heavy cigarette smoking.

Neuropsychological performance in patients with schizophrenia and controls as a function of cigarette smoking status

Schizophrenia is associated with many neurocognitive deficits, some of which are improved by nicotine and cigarette smoking. To better understand the relationship between smoking and cognitive function in schizophrenia, cross-sectional assessment of neuropsychological performance as a function of smoking status (smoker or non-smoker) and smoking history (current, former or never-smoker) in clinically stable outpatients with schizophrenia and controls was evaluated. Subjects (n=140) were divided into subgroups on the basis of self-report and biochemical verification of smoking history. Current smokers with schizophrenia (n=38), former smokers with schizophrenia (n=17), never-smokers with schizophrenia (n=12), control smokers (n=31), control former smokers (n=16), and control never-smokers (n=26) were administered a comprehensive neuropsychological battery. Smokers were studied under non-deprivation conditions. Comparison of neuropsychological performance in schizophrenia and control subjects revealed significant main effects of diagnosis. Analysis of the data as a function of smoking history demonstrated that never-smokers with schizophrenia performed the poorest on measures of sustained attention, processing speed and response inhibition, when compared to the other schizophrenia subgroups. Cigarette smoking did not alter neuropsychological performance in controls. Our findings suggest that smoking status and history differentially alters neuropsychological outcomes in schizophrenia compared to non-psychiatric controls, and that never-smokers may present with more severe neurocognitive impairments.

Mecamylamine - a nicotinic acetylcholine receptor antagonist with potential for the treatment of neuropsychiatric disorders

Mecamylamine (Inversine), the first orally available antihypertensive agent launched in the 1950s, is rarely used today for hypertension because of its widespread ganglionic side effects at antihypertensive doses (25 - 90 mg/day). However, more recent clinical studies suggest that mecamylamine is effective at much lower doses for blocking the central and peripheral effects of nicotine. Pharmacologically, mecamylamine has been well characterized as a nonselective and noncompetitive antagonist of nicotinic acetylcholine receptors (nAChRs). Because mecamylamine easily crosses the blood - brain barrier at relatively low doses (2.5 - 10 mg), it has been used by several research groups over the past two decades investigating the role of central nAChRs in the etiology and treatment of various neuropsychiatric disorders, including addiction disorders, Tourette's syndrome, schizophrenia and various cognitive and mood disorders. Two independent Phase II clinical trials recently confirmed mecamylamine's hypothesized antidepressant activity and suggest that it may be effective as an augmentation pharmacotherapy for SSRI treatment resistant major depression. These areas of investigation for mecamylamine are reviewed and recommendations for future research directions are proposed.

Mechanisms underlying the comorbidity of tobacco use in mental health and addictive disorders

We discuss potential explanations for the high prevalence of tobacco use and tobacco dependence (TD) in people with mental health and addictive (MHA) disorders. The biopsychosocial basis for this comorbidity is presented, integrating evidence from epidemiologic and clinical studies. We also review evidence that suggests a shared vulnerability related to biological, genetic, and environmental factors may be the most parsimonious mechanism to explain the association between TD and MHA disorders. Finally, we review the examples of various MHA disorders that are associated with TD, and suggest avenues for new investigation that could aid in the development of rationale and more effective treatments for tobacco and MHA disorder comorbidities.

Channelling of substrate promiscuity of the skeletal-muscle ADP-ribosyl cyclase isoform

The novel Ca2+-mobilizing second messengers cADPr (cyclic ADP-ribose) and NAADP (nicotinic acid-adenine dinucleotide phosphate) are both synthesized by ADP-ribosyl cyclases. Using HSR (heavy sarcoplasmic reticulum) fractions from rabbit skeletal muscle, NAADP-induced Ca2+ release was observed. In the present paper, we show in HSR membranes the formation of authentic cADPr, cGDPr (cyclic GDP-ribose) and NAADP. The cyclization reaction to form cADPr and cGDPr as well as the base-exchange reaction to form NAADP were strictly dependent on pH. Although the formation of cGDPr is optimized at pH 6, the synthesis of NAADP was most pronounced at a pH below 5. A novel regulation mechanism is provided for nicotinic acid, the co-substrate for NAADP synthesis. Nicotinic acid had virtually no influence on the cyclization reaction, but increased the affinity of NADP at an acidic pH and had the opposite effect at alkaline pH. Nicotinamide, the side product of cADPr synthesis, is an inhibitor of the cyclization reaction (IC50, 0.7+/-0.1 mM) and was 30-fold more potent at suppressing the base-exchange reaction. Although the synthesis of NAADP was highly sensitive to nicotinamide inhibition, this was not via a competition with the nicotinic-acid-binding site. In contrast with the ecto-ADP-ribosyl cyclase (CD38), the cyclization and base-exchange reaction of the skeletal muscle isoform was inhibited by Cu2+ and Zn2+, while other bivalent cations such as Ca2+, Mg2+ and Mn2+ had virtually no effect. These findings allow for the prediction of a novel ADP-ribosyl cyclase isoform in skeletal muscle HSR, other than CD38. Hence the enzymic prerequisite for cADPr- and NAADP-mediated Ca2+ signalling is present.