Do adenosinergic substrates mediate methylxanthine effects upon reinforcement thresholds for electrical brain stimulation in the rat?

β-Carbolines found in cigarette smoke elevate intracranial self-stimulation thresholds in rats

Identifying novel constituents that contribute to tobacco addiction is essential for developing more effective treatments and informing FDA regulation of tobacco products. While preclinical data indicate that monoamine oxidase (MAO) inhibitors can have abuse liability or potentiate the addiction-related effects of nicotine, most of these studies have used clinical MAO inhibitors (e.g., tranylcypromine) that are not present in cigarette smoke. The primary goal of this study was to evaluate the abuse potential of the β-carbolines harmane, norharmane, and harmine - MAO inhibitors that are found in cigarette smoke - in an intracranial self-simulation (ICSS) model in rats. A secondary goal was to evaluate the ability of norharmane to influence nicotine's acute effects on ICSS. None of the β-carbolines lowered ICSS thresholds at any dose studied when administered alone, suggesting a lack of abuse liability. Rather, all three β-carbolines produced dose-dependent elevations in ICSS thresholds, indicating aversive/anhedonic effects. Harmane and harmine also elevated ICSS response latencies, suggesting a disruption of motor function, albeit with reduced potency compared to their ICSS threshold-elevating effects. Norharmane (2.5 mg/kg) modestly attenuated the effects of nicotine on ICSS thresholds. Our findings indicate that these β-carbolines produced only aversive/anhedonic effects in an ICSS model when administered alone, and that norharmane unexpectedly attenuated nicotines acute effects on ICSS. Future work evaluating the addiction-related effects of nicotine combined with these and other MAO inhibitors present in smoke may be useful for understanding the role of MAO inhibition in tobacco addiction and informing FDA tobacco regulation.

Mechanisms of the psychostimulant effects of caffeine: implications for substance use disorders

BACKGROUND

The psychostimulant properties of caffeine are reviewed and compared with those of prototypical psychostimulants able to cause substance use disorders (SUD). Caffeine produces psychomotor-activating, reinforcing, and arousing effects, which depend on its ability to disinhibit the brake that endogenous adenosine imposes on the ascending dopamine and arousal systems.

OBJECTIVES

A model that considers the striatal adenosine A2A-dopamine D2 receptor heteromer as a key modulator of dopamine-dependent striatal functions (reward-oriented behavior and learning of stimulus-reward and reward-response associations) is introduced, which should explain most of the psychomotor and reinforcing effects of caffeine.

HIGHLIGHTS

The model can explain the caffeine-induced rotational behavior in rats with unilateral striatal dopamine denervation and the ability of caffeine to reverse the adipsic-aphagic syndrome in dopamine-deficient rodents. The model can also explain the weaker reinforcing effects and low abuse liability of caffeine, compared with prototypical psychostimulants. Finally, the model can explain the actual major societal dangers of caffeine: the ability of caffeine to potentiate the addictive and toxic effects of drugs of abuse, with the particularly alarming associations of caffeine (as adulterant) with cocaine, amphetamine derivatives, synthetic cathinones, and energy drinks with alcohol, and the higher sensitivity of children and adolescents to the psychostimulant effects of caffeine and its potential to increase vulnerability to SUD.

CONCLUSIONS

The striatal A2A-D2 receptor heteromer constitutes an unequivocal main pharmacological target of caffeine and provides the main mechanisms by which caffeine potentiates the acute and long-term effects of prototypical psychostimulants.

Effects of caffeine and its metabolite paraxanthine on intracranial self-stimulation in male rats

Caffeine is the most widely used psychostimulant in the world, though preclinical studies suggest weaker evidence for abuse-related effects than stimulants with high abuse liability, such as amphetamine or cocaine. Intracranial self-stimulation (ICSS) is 1 procedure used to assess the abuse liability of drugs, and previous studies have produced mixed results regarding whether caffeine produces an abuse-related facilitation of ICSS. This study assessed both caffeine and its main metabolite in humans, paraxanthine, using a frequency-rate ICSS procedure and compared their effects to those of amphetamine and cocaine. Male Sprague-Dawley rats were implanted with intracranial electrodes targeting the medial forebrain bundle and trained to respond under a fixed-ratio 1 schedule for brain stimulation that varied across a range of frequencies (56-158 Hz in 0.05 log increments). Data analysis focused on 3 dependent measures: reinforced responding (defined as responses that produced brain stimulation), nonreinforced responding (defined as responses that occurred during each 0.5 s brain stimulation and that did not produce additional stimulation), and total responding (reinforced plus nonreinforced responding). Both amphetamine and cocaine produced robust increases in total, reinforced, and nonreinforced responses. Caffeine also increased total, reinforced, and nonreinforced responses, but the caffeine dose-effect curve had an inverted-U shape, and peak ICSS facilitation was less than that produced by amphetamine or cocaine. Paraxanthine increased only total responses and nonreinforced responses. These results suggest that paraxanthine has low abuse liability and does not mediate abuse-related effects of caffeine.

Pharmacology of the beta-carboline FG-7,142, a partial inverse agonist at the benzodiazepine allosteric site of the GABA A receptor: neurochemical, neurophysiological, and behavioral effects

Given the well-established role of benzodiazepines in treating anxiety disorders, beta-carbolines, spanning a spectrum from full agonists to full inverse agonists at the benzodiazepine allosteric site for the GABA(A) receptor, can provide valuable insight into the neural mechanisms underlying anxiety-related physiology and behavior. FG-7,142 is a partial inverse agonist at the benzodiazepine allosteric site with its highest affinity for the alpha1 subunit-containing GABA(A) receptor, although it is not selective. FG-7,142 also has its highest efficacy for modulation of GABA-induced chloride flux mediated at the alpha1 subunit-containing GABA(A) receptor. FG-7,142 activates a recognized anxiety-related neural network and interacts with serotonergic, dopaminergic, cholinergic, and noradrenergic modulatory systems within that network. FG-7,142 has been shown to induce anxiety-related behavioral and physiological responses in a variety of experimental paradigms across numerous mammalian and non-mammalian species, including humans. FG-7,142 has proconflict actions across anxiety-related behavioral paradigms, modulates attentional processes, and increases cardioacceleratory sympathetic reactivity and neuroendocrine reactivity. Both acute and chronic FG-7,142 treatment are proconvulsive, upregulate cortical adrenoreceptors, decrease subsequent actions of GABA and beta-carboline agonists, and increase the effectiveness of subsequent GABA(A) receptor antagonists and beta-carboline inverse agonists. FG-7,142, as a partial inverse agonist, can help to elucidate individual components of full agonism of benzodiazepine binding sites and may serve to identify the specific GABA(A) receptor subtypes involved in specific behavioral and physiological responses.

Theophylline inhibits tolerance and sensitization induced by morphine: a conditioned place preference paradigm study in female mice

The effect of theophylline on reward properties of morphine was examined in the present study. A biased conditioned place preference paradigm was used to study the effects of theophylline on the development of conditioned place preference by morphine in sensitized and tolerant female mice. Subcutaneous injection of morphine (0.5-10 mg/kg) induced conditioned place preference in mice, while intraperitoneal administration of theophylline (2.5-100 mg/kg) did not induce conditioned place preference or conditioned place aversion. Theophylline (2.5-100 mg/kg) in combination with morphine (5 mg/kg), during conditioning sessions, decreased the acquisition of morphine conditioned place preference dose independently. Administration of theophylline (2.5-100 mg/kg) before testing also caused a significant reduction of the expression of morphine-induced conditioned place preference in a dose-independent manner. Administration of morphine (12.5, 25 or 50 mg/kg) daily, for 3 days, produced tolerance to conditioned place preference induced by the drug (5 mg/kg). Administration of theophylline (2.5 and 10 mg/kg) 1 h before morphine (12.5, 25 mg/kg), during development of tolerance, abolished morphine tolerance. A higher dose of theophylline (100 mg/kg), however, did not alter morphine tolerance. In addition, theophylline (2.5, 10 and 100 mg/kg) failed to reduce tolerance to a higher dose of morphine (50 mg/kg). Daily administration of morphine (5 mg/kg) for 3 days followed by a 5-day interval caused sensitization to morphine place conditioning. When theophylline was administered (2.5, 10 and 100 mg/kg) 1 h before morphine (5 mg/kg), during development of sensitization, inhibition of morphine-induced sensitization was demonstrated. The effect of theophylline was dose independent. It is concluded that while theophylline has no effect by itself, it reduced both the acquisition and expression of morphine conditioned place preference. In addition, theophylline reduced the acquisition of morphine conditioned place preference in morphine-sensitized and morphine-tolerant mice.

Role of adenosine A2 receptors in brain stimulation reward under baseline conditions and during cocaine withdrawal in rats

The present experiments tested the hypothesis that adenosine A2 receptors are involved in central reward function. Adenosine receptor agonists or antagonists were administered to animals that had been trained to self-stimulate in a rate-free brain stimulation reward (BSR) task that provides current thresholds as a measure of reward. The adenosine A(2A) receptor-selective agonists 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido adenosine hydrochloride (CGS 21680) (0.1-1.0 mg/kg) and 2-[(2-aminoethylamino)carbonylethyl phenylethylamino]-5'-N-ethylcarboxamido adenosine (APEC) (0.003-0.03 mg/kg) elevated reward thresholds without increasing response latencies, a measure of performance. Specifically, CGS 21680 had no effect on response latency, whereas APEC shortened latencies. Bilateral infusion of CGS 21680 (3, 10, and 30 ng/side), directly into the nucleus accumbens, elevated thresholds but shortened latencies. The highly selective A(2A) antagonist 8-(3-chlorostyryl)caffeine (0.01-10.0 mg/kg) and the A2-preferring antagonist 3,7-dimethyl-1-propargylxanthine (DMPX) (0.3-10.0 mg/kg) did not alter thresholds or latencies, but DMPX (1.0, 10.0 mg/kg) blocked the threshold-elevating effect of APEC (0.03 mg/kg). In another study, repeated administration of cocaine (eight cocaine injections of 15 mg/kg, i.p., administered over 9 hr) produced elevations in thresholds at 4, 8, and 12 hr after cocaine. DMPX (3 and 10 mg/kg), administered before both the 8 and 12 hr post-cocaine self-stimulation tests, reversed the threshold elevation produced by cocaine withdrawal. These results indicate that stimulating adenosine A(2A) receptors diminishes BSR without producing performance deficits, whereas blocking adenosine receptors reverses the reward impairment produced by cocaine withdrawal or by an A(2A) agonist. These findings indicate that adenosine, via A(2A) receptors, may inhibit central reward processes, particularly during the neuroadaptations associated with chronic drug-induced neuronal activation.

Central adenosine A(2A) receptors: an overview

Recent advances in molecular biology, biochemistry, cell biology and behavioral pharmacology together with the development of more selective ligands to the various adenosine receptors have increased our understanding of the functioning of central adenosine A(2A) receptors. The A(2A) receptor is one of four adenosine receptors found in the brain. Its expression is highest in striatum, nucleus accumbens and olfactory tubercles, although it also occurs in neurons and microglia in most other brain regions. The receptor has seven transmembrane domains and couples via Gs to adenyl cyclase stimulation. Antagonistic interactions between A(2A) receptors and dopamine D(2) receptors have been described, as stimulation of the A(2A) receptor leads to a reduction in the affinity of D(2) receptors for D(2) receptor agonists. The A(2A) receptor is thought to play a role in a number of physiological responses and pathological conditions. Indeed, A(2A) receptor antagonists may be useful for the treatment of acute and chronic neurodegenerative disorders such as cerebral ischemia or Parkinson's disease. A(2A) receptor agonists may treat certain types of seizures or sleep disorders. This review discusses the characteristics, distribution, pharmacochemical properties and regulation of central A(2A) receptors, as well as A(2A) receptor-mediated behavioural responses and their potential role in various neuropsychiatric disorders.

Effects of abused drugs on thresholds and breaking points of intracranial self-stimulation in rats

The present study aimed to compare the effects of various abused drugs on threshold current intensities and the breaking points of intracranial self-stimulation. Effects of morphine (1-10 mg/kg, s.c.), d-amphetamine (0.3-3.2 mg/kg, i.p.), nicotine (0.1-3.2 mg/kg, s.c.), ethanol (0.6-2.4 g/kg, p.o.), caffeine (1-20 mg/kg, i.p.) and phencyclidine (0.3-5.6 mg/kg, i.p.) were studied in male Wistar rats trained to lever-press for electrical stimulation of ventral tegmental area. Morphine and d-amphetamine were the only two drugs that both decreased threshold currents and increased the maximal ratio of reinforced and non-reinforced responses. Nicotine (1 mg/kg) and ethanol (1.2 g/kg) lowered threshold currents while both decreases and increases in threshold current were seen after administration of low (5 mg/kg) and high (20 mg/kg) doses of caffeine, respectively. Nicotine, ethanol and caffeine had no effects in the progressive ratio procedure. Effects of phencyclidine did not reach levels of statistical significance in either procedure although high doses of phencyclidine disrupted performance in the progressive ratio procedure.

Parametric changes in response equilibrium during an intra-cranial self stimulation (ICSS) task: can reward value be assessed independently of absolute threshold?

Traditional ICSS methodologies have attempted to evaluate changes in the rewarding value of brain stimulation by assessing the lowest value of the stimulation that will support responding. However, orderly changes in suprathreshold indicants of hedonic magnitude such as titration point have been shown. In the present experiments, rats were trained to respond on two ICSS autotitration schedules in which every response on one lever produced stimulation of the medial forebrain bundle, and every Xth response decreased either the stimulation current or the stimulation frequency. At any time, a response on a second "reset" lever restored the stimulation current or frequency available on the stimulation lever to its starting level and operationally defined changes in "reward value". In order to study this titration point measure, two response requirements (responses/stepdown; step size) and two stimulation parameters (initial stimulation level; train duration) were systematically varied. Under both current and frequency titration schedules, data indicated that response rate and titration point remained stable over repeated trials and multiple testing days--parameters being constant. Across all conditions, compared to the frequency titration schedule, subjects responding under the current titration schedule showed significantly higher titration points and lower rates of responding. Indicating the independence of rate and titration point data, parametric manipulations did not affect titration point and rate data concurrently. Results support the conclusion that titration point is a relative measure of "reward value" that is generally independent of response rate, but that is affected by manipulations that alter the amount of stimulation available between "resets". Additional work is needed in order to determine the relationship between the magnitude of stimulation needed to maintain minimal responding and that needed to maintain response equilibrium in an autotitration task.

Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects

Caffeine is the most widely consumed central-nervous-system stimulant. Three main mechanisms of action of caffeine on the central nervous system have been described. Mobilization of intracellular calcium and inhibition of specific phosphodiesterases only occur at high non-physiological concentrations of caffeine. The only likely mechanism of action of the methylxanthine is the antagonism at the level of adenosine receptors. Caffeine increases energy metabolism throughout the brain but decreases at the same time cerebral blood flow, inducing a relative brain hypoperfusion. Caffeine activates noradrenaline neurons and seems to affect the local release of dopamine. Many of the alerting effects of caffeine may be related to the action of the methylxanthine on serotonin neurons. The methylxanthine induces dose-response increases in locomotor activity in animals. Its psychostimulant action on man is, however, often subtle and not very easy to detect. The effects of caffeine on learning, memory, performance and coordination are rather related to the methylxanthine action on arousal, vigilance and fatigue. Caffeine exerts obvious effects on anxiety and sleep which vary according to individual sensitivity to the methylxanthine. However, children in general do not appear more sensitive to methylxanthine effects than adults. The central nervous system does not seem to develop a great tolerance to the effects of caffeine although dependence and withdrawal symptoms are reported.