Cocaine-like subjective effects of nicotine are not blocked by the D1 selective antagonist ecopipam (SCH 39166)

Interactions between the intrarenal dopaminergic and the renin-angiotensin systems in the control of systemic arterial pressure

Systemic arterial hypertension is one of the leading causes of morbidity and mortality in the general population, being a risk factor for many cardiovascular diseases. Although its pathogenesis is complex and still poorly understood, some systems appear to play major roles in its development. This review aims to update the current knowledge on the interaction of the intrarenal renin-angiotensin system (RAS) and dopaminergic system in the development of hypertension, focusing on recent scientific hallmarks in the field. The intrarenal RAS, composed of several peptides and receptors, has a critical role in the regulation of blood pressure (BP) and, consequently, the development of hypertension. The RAS is divided into two main intercommunicating axes: the classical axis, composed of angiotensin-converting enzyme, angiotensin II, and angiotensin type 1 receptor, and the ACE2/angiotensin-(1-7)/Mas axis, which appears to modulate the effects of the classical axis. Dopamine and its receptors are also increasingly showing an important role in the pathogenesis of hypertension, as abnormalities in the intrarenal dopaminergic system impair the regulation of renal sodium transport, regardless of the affected dopamine receptor subtype. There are five dopamine receptors, which are divided into two major subtypes: the D1-like (D1R and D5R) and D2-like (D2R, D3R, and D4R) receptors. Mice deficient in any of the five dopamine receptor subtypes have increased BP. Intrarenal RAS and the dopaminergic system have complex interactions. The balance between both systems is essential to regulate the BP homeostasis, as alterations in the control of both can lead to hypertension.

Dopamine D1R Receptor Stimulation as a Mechanistic Pro-cognitive Target for Schizophrenia

Decades of research have highlighted the importance of optimal stimulation of cortical dopaminergic receptors, particularly the D1R receptor (D1R), for prefrontal-mediated cognition. This mechanism is particularly relevant to the cognitive deficits in schizophrenia, given the abnormalities in cortical dopamine (DA) neurotransmission and in the expression of D1R. Despite the critical need for D1R-based therapeutics, many factors have complicated their development and prevented this important therapeutic target from being adequately interrogated. Challenges include determination of the optimal level of D1R stimulation needed to improve cognitive performance, especially when D1R expression levels, affinity states, DA levels, and the resulting D1R occupancy by DA, are not clearly known in schizophrenia, and may display great interindividual and intraindividual variability related to cognitive states and other physiological variables. These directly affect the selection of the level of stimulation necessary to correct the underlying neurobiology. The optimal mechanism for stimulation is also unknown and could include partial or full agonism, biased agonism, or positive allosteric modulation. Furthermore, the development of D1R targeting drugs has been complicated by complexities in extrapolating from in vitro affinity determinations to in vivo use. Prior D1R-targeted drugs have been unsuccessful due to poor bioavailability, pharmacokinetics, and insufficient target engagement at tolerable doses. Newer drugs have recently become available, and these must be tested in the context of carefully designed paradigms that address methodological challenges. In this paper, we discuss how a better understanding of these challenges has shaped our proposed experimental design for testing a new D1R/D5R partial agonist, PF-06412562, renamed CVL-562.

Hormones, nicotine, and cocaine: clinical studies

Nicotine and cocaine each stimulate hypothalamic-pituitary-adrenal and -gonadal axis hormones, and there is increasing evidence that the hormonal milieu may modulate the abuse-related effects of these drugs. This review summarizes some clinical studies of the acute effects of cigarette smoking or IV cocaine on plasma drug and hormone levels and subjective effects ratings. The temporal covariance between these dependent measures was assessed with a rapid (2 min) sampling procedure in nicotine-dependent volunteers or current cocaine users. Cigarette smoking and IV cocaine each stimulated a rapid increase in LH and ACTH, followed by gradual increases in cortisol and DHEA. Positive subjective effects ratings increased immediately after initiation of cigarette smoking or IV cocaine administration. However, in contrast to cocaine's sustained positive effects (<20 min), ratings of "high" and "rush" began to decrease within one or two puffs of a high-nicotine cigarette while nicotine levels were increasing. Peak nicotine levels increased progressively after each of three successive cigarettes smoked at 60 min intervals, but the magnitude of the subjective effects ratings and peak ACTH and cortisol levels diminished. Only DHEA increased consistently after successive cigarettes. The possible influence of neuroactive hormones on nicotine dependence and cocaine abuse and the implications for treatment of these addictive disorders are discussed.

Self-administration of intravenous nicotine in male and female cigarette smokers

Although nicotine is the main addictive chemical in tobacco, there have been few studies of pure nicotine self-administration in humans. The goal of this study was to test the parameters of an intravenous (IV) nicotine self-administration model using nicotine doses presumed to be within the range of those of average intake from cigarette smoking. Six male and four female smokers participated in a double-blind, placebo-controlled, crossover study, which consisted of one adaptation and three experimental sessions. In each experimental session, subjects were randomly assigned to one of the three doses of nicotine (0.1, 0.4, or 0.7 mg). The lowest nicotine dose, 0.1 mg, was chosen to be approximately half the amount of nicotine inhaled from one puff of a cigarette. During each experimental session, subjects first sampled the assigned nicotine dose and placebo and then had the opportunity to choose between nicotine and placebo for a total of six choices over a 90-min period. Out of six options, the average (SEM) number of nicotine choices were 3.0 (0.48) for 0.1 mg, 4.7 (0.48) for 0.4 mg and 4.5 (0.46) for 0.7 mg, indicating a significant effect of nicotine dose on nicotine choice. Both the 0.4 and 0.7, but not the 0.1 mg, nicotine doses were preferred to placebo. These higher doses also produced increases in heart rate, blood pressure, and ratings of drug liking and high. Overall, these findings indicate that smokers chose both the 0.4 and the 0.7 mg nicotine doses over placebo. Our model may be useful in the evaluation of the effects of both behavioral and pharmacological manipulations on nicotine self-administration in humans.

Randomized controlled trials of the D1/D5 antagonist ecopipam for weight loss in obese subjects

OBJECTIVE

To evaluate the efficacy and safety of the selective dopamine D1/D5 antagonist ecopipam for the treatment of obesity.

RESEARCH METHODS AND PROCEDURES

Four randomized, double-blind, multicenter trials compared ecopipam (n=1667) and placebo (n=1118) in obese subjects including type 2 diabetic subjects. Subjects received oral ecopipam 10, 30, or 100 mg daily for 12 weeks (Phase 2) or 50 or 100 mg daily for 52 weeks (Phase 3) combined with a weight loss program. Primary efficacy variables were the proportion of subjects with>or=5% weight loss from baseline at 12 weeks (Phase 2) or the distribution of percentage weight loss from baseline at 52 weeks (Phase 3).

RESULTS

In the Phase 2 study, 26% of subjects administered ecopipam 100 mg vs. 6% of placebo subjects achieved>or=5% weight loss after 12 weeks (p<0.01). In the Phase 3 studies, ecopipam 100 mg produced a 3.1% to 4.3% greater weight loss than placebo at 52 weeks. More subjects administered ecopipam vs. placebo achieved a 5% to 10% or >10% weight loss in two non-diabetic phase 3 trials. Ecopipam-treated subjects also maintained more weight loss compared with placebo subjects at 52 weeks. Phase 3 studies were discontinued because of unexpected psychiatric adverse events (ecopipam 31% vs. placebo 15%), including depression, anxiety, and suicidal ideation.

DISCUSSION

Ecopipam was effective for achieving and maintaining weight loss in obese subjects, including type 2 diabetic subjects; however, the adverse effects on mood observed in the Phase 3 studies exclude its projected use in weight management.

Euphoriant effects of nicotine in smokers: fact or artifact?

RATIONALE

The claim that nicotine in cigarettes is euphoriant to smokers is largely based on two studies (Pomerleau and Pomerleau, Psychopharmacology, 108:460-465, 1992; Tobacco Control, 3:374, 1994) in which smokers were instructed to respond to sensations of rush, buzz, or high while smoking low-nicotine or regular cigarettes. However, the assumption that these sensations are pleasurable was not tested and may have biased the results.

OBJECTIVES

The aim of this study was to re-examine the claim that smoked nicotine is euphoriant to smokers.

METHODS

Study 1 surveyed the frequency and pleasantness of the smoking-related sensations of rush, buzz, and high in a sample of smokers. Study 2 replicated Pomerleau and Pomerleau (Psychopharmacology, 108:460-465, 1992) with two sets of instruction. One set, as in the original study, defined these sensations as pleasurable, whereas the other defined them as unpleasant.

RESULTS

Study 1 found that whereas rush and high were perceived as pleasant, buzz was unpleasant to most smokers. Study 2 found that under both sets of instructions, smokers reported more sensations when smoking the regular, as compared to the low-nicotine cigarette. Additionally, the sensations of rush, buzz, and high were rated as more pleasant under the pleasant instructions as compared to the unpleasant instructions. Finally, in the pleasant instructions condition, many participants reported having pressed the button to indicate a pleasurable sensation despite having actually experienced that sensation as unpleasant.

CONCLUSIONS

Our results suggest that the findings of Pomerleau and Pomerleau (Psychopharmacology, 108:460-465, 1992; Tobacco Control, 3:374, 1994) may have been biased by the experimental instructions and cannot be taken as evidence that smoked nicotine is euphoriant to smokers.

Adrenergic blocker carvedilol attenuates the cardiovascular and aversive effects of nicotine in abstinent smokers

The cardiovascular response to nicotine is mediated mainly by noradrenergic activation. Whether noradrenergic activation mediates other effects of nicotine has not been well documented in humans. In this study, we examined the effects of an alpha and beta-adrenergic receptor blocker: carvedilol, on cardiovascular and subjective responses to nicotine lozenge and on the ability of nicotine lozenge to suppress tobacco withdrawal symptoms in overnight abstinent smokers. Fifteen smokers, nine men and six women, participated in a double-blind, placebo-controlled, crossover study. In each of the three experimental sessions, participants were treated orally with a single 25 or 50 mg dose of carvedilol or placebo. Two hours and 10 min following the medication treatment, participants received a single 4 mg nicotine lozenge. Carvedilol treatment attenuated the nicotine-induced heart rate, systolic and diastolic blood pressure increases. Carvedilol also attenuated the self-report rating of 'bad effects' in response to nicotine. Carvedilol, alone or in combination with nicotine lozenge, did not affect tobacco withdrawal symptoms. Carvedilol treatment did not affect performance on the Stroop Test. These results support the effectiveness of carvedilol for attenuating the cardiovascular effects of nicotine. Attenuation of the rating of 'bad effects' by carvedilol suggests that noradrenergic activation may also mediate the aversive effects of nicotine.

Lack of effect of acute dopamine precursor depletion in nicotine-dependent smokers

RATIONALE

Nicotine increases dopamine (DA) release but its role in nicotine dependence remains unclear.

OBJECTIVE

To assess the role of DA in nicotine craving and self-administration using acute phenylalanine/tyrosine depletion (APTD).

METHODS

Fifteen nicotine-dependent men ingested, a minimum of 3days apart, a nutritionally balanced amino acid (AA) mixture (BAL), a mixture deficient in the catecholamine precursors, phenylalanine and tyrosine, and APTD followed by the immediate DA precursor, L-DOPA. Beginning 3h after ingestion of the AA mixture, subjects smoked 4 cigarettes. Craving, mood, and other aspects of subjective state were assessed with self-report scales. Smoking puff topography was measured with a computerized flowmeter.

RESULTS

APTD did not change smoking puff topography, cigarette craving, or subjective effects of smoking.

CONCLUSIONS

The findings suggest that in nicotine-dependent smokers craving for cigarettes, subjective effects of nicotine, and the self-administration of freely available cigarettes are largely unrelated to acute changes in DA neurotransmission.

Does nicotine do what we think it does? A meta-analytic review of the subjective effects of nicotine in nasal spray and intravenous studies with smokers and nonsmokers

We conducted a meta-analysis of placebo-controlled laboratory studies of the subjective effects of nicotine. A total of 15 studies (11 with nasal spray, four with intravenous administration) with smokers and six studies (all with nasal spray) with never-smokers were included. Studies of other routes of administration (e.g., smoked tobacco) were not included because of insufficient numbers of available effect sizes. Meta-analysis results indicated that nicotine increased vigor for smokers but increased fatigue for never-smokers. Nicotine increased head rush for both smokers and never-smokers. In studies of smokers only, nicotine also increased ratings of drug high and drug liking. Contrary to expectations, nicotine decreased relaxation and increased tension/jitteriness for both smokers and never-smokers. Dose-response relationships were most clearly observed for head rush and drug high. Considerable variability was found across studies for a given nicotine dose and route of administration. Implications of the current findings about the role of subjective effects in nicotine reinforcement and self-administration are discussed along with commentary on methodological issues and recommendations for future studies.