Influence of a Nitric Oxide Synthase Inhibitor on the Anxiolytic, Stimulating, and Analgesic Effects of Long-Term Perinatal Caffeine Exposure in Rats.

We studied the effect of inducible NO synthase (iNOS) inhibitor aminoguanidine on the behavioral effects of chronic perinatal caffeine exposure. Administration of caffeine in the prenatal and early postnatal periods led to the development of anxiolytic, stimulating, and analgesic effects. Administration of aminoguanidine attenuated the anxiolytic and stimulating effects and potentiated the analgesic effect of perinatal administration of caffeine. Chronic perinatal administration of caffeine leads to significant changes in the level of anxiety, motor activity, and pain sensitivity, and inhibition of iNOS has a pronounced multidirectional effect on these effects.

Relationship between Adenosine Deaminase Activity in the Cerebral Cortex and the Stimulating Effect of Caffeine in Adult Wistar Rats of Different Ages.

The aim of this study was to test the hypothesis that the higher the activity of adenosinedeaminase (ADA) in the brain, the greater should be the motor activity of animals, and possibly the stronger the psychostimulant effect of caffeine. We studied the effect of caffeine (10 and 20 mg/kg) on the motor activity and ADA activity in the frontal cortex of the brain in 2- and 5-month-old rats with different levels of spontaneous motor activity. Total motor activity significantly decreased with age, which was accompanied by a decrease in ADA activity. Administration of caffeine in a dose of 10 mg/kg stimulated motor activity in both 2- and 5-month-old animals, while ADA activity decreased in 2-month-old rats and increased in 5-month-old animals. Administration of caffeine in a dose of 20 mg/kg did not change the motor activity, however, in 5-month-old animals it led to an even greater increase in ADA activity. Thus, the age-related decrease in motor activity can be due to a decrease in ADA activity. However, the effect of caffeine on motor activity is not directly related to ADA activity in the cerebral cortex.

Features of the Effects of Glutamatergic System Modulators in the Model of Hyperthermal Seizures in Rat Pups.

We studied the effect of lamotrigine, an anticonvulsant inhibiting the presynaptic release of glutamate, and LY341495, an antagonist of metabotropic glutamate 2/3 receptors, on the development of hyperthermic seizures and the content of LPO products in the brain of 8-10-day-old Wistar rats. Rat pups in the early postnatal period demonstrated pronounced seizures in response to thermal exposure, which was accompanied by an increase in the level of LPO products in the cerebral cortex. It was shown that the latency of generalized seizures increased after administration of both lamotrigine and LY341495. The most pronounced effect was observed in animals treated with lamotrigine. Both test substances prevented LPO intensification induced by hyperthermic exposure to varying degrees.

Effect of Nitric Oxide Synthase Inhibitor on the Learning and Spatial Memory in Rats Subjected to Long-Term Perinatal Administration of Caffeine.

We studied the effect of inducible nitric oxide synthase inhibitor aminoguanidine on learning and spatial memory in rats exposed to long-term administration to caffeine during the prenatal and early postnatal periods. The rats perinatally receiving caffeine demonstrated high learning ability in the Morris water maze. At the same time, the ability to remember the location of the hidden platform in the trial probe in these rats was reduced in comparison with that of the control group rats perinatally receiving water. Administration of aminoguanidine to rats under conditions of perinatal exposure to caffeine significantly improved the parameters of spatial learning and memory. Thus, inhibition of inducible nitric oxide synthase has a beneficial effect on the cognitive functions in offspring perinatally receiving caffeine.

The Development of Pathological Dependence after Intermittent Use of Sodium Glutamate, but Not Sucrose or Sodium Chloride Solutions.

The possibility of development of dependence was studied during the intermittent consumption of sucrose, sodium chloride, and sodium glutamate solutions. Rats were allowed to choose and consume solutions of sucrose, sodium chloride, and sodium glutamate for 28 days. On days 29-31 of the experiment, the animals were deprived of the preferred solutions. On days 32-33, the solutions of sucrose, sodium chloride, and sodium glutamate, but not water were provided again. The consumption of sucrose and sodium chloride solutions did not increase, but consumption of 0.5 and 1% sodium glutamate solutions increased after 3-days withdrawal. The consumption of 2% solution of sodium glutamate was the same before and after withdrawal. The observed effects of sodium glutamate deprivation probably indicate the development of pathological glutamate dependence.

Endogenous Opioid Dependence Induced in Rats by Periodic Intake of 5% Ethanol Solution.

We studied the possibility of formation of endogenous opioid dependence in rats during periodic intake of 5% ethanol solution. In the control group, both drinking bottles contained water. In the experimental group, the second bottle was filled with 5% ethanol solution for 12 h per day; in the following 12 h, these rats were deprived of food and ethanol. This regimen was maintained over 8 days. The rats were subdivided into alcohol- and water-preferring subgroups. Ethanol deprivation followed by naloxone injection evoked the signs of opiate withdrawal syndrome in both subgroups. These findings suggest that periodic voluntary intake of a weak ethanol solution over 8 days led to the formation of endogenous opioid dependence in rats irrespective of amount of the consumed alcohol.

Physiology and Pharmacology of Positive Reinforcement.

An original concept of a two-stage mechanism of positive reinforcement is proposed. The first stage, "virtual" reinforcement, is formed in parallel with the action result acceptor when the result is still not achieved. At this stage, the importance of the planned result and the probability of its achievement are assessed. The greater are these indices, the stronger is "virtual" reinforcement. Hypothetically, the "virtual" reinforcement is mediated by dopamine release from nerve terminals in the mesencephalon. The "real" reinforcement (the second stage) occurs after achievement of the result. Probably, an important role in the mechanisms of the "real" reinforcement is given to endogenous opioids, cannabinoids, and GABA. Based on the advanced hypothesis on interaction between the central and peripheral subdivisions of the corresponding neurochemical systems, the review focuses on possibility of pharmacological intervention into the mechanisms of positive reinforcement by modifying activity of the peripheral opioid and dopamine receptors with the ligands that cannot cross blood-brain barrier.

Influence of Caffeine Consumption by Pregnant Rats on Behavior and Learning in Their Offspring.

We studied the effect of long-term prenatal administration of caffeine on the behavior and learning of rats in postnatal ontogeny. Experiments were carried out on male rats born by females receiving caffeine solution as the only source of fluid throughout gestation. The control group consisted of pups obtained from females receiving drinking water throughout pregnancy. It was found that long-term caffeine intake by female rats during pregnancy determined increased locomotor activity of the offspring. Rat pups born from mothers treated with caffeine during pregnancy faster reached the underwater platform in the Morris maze, i.e. demonstrated better spatial memory formation than control animals.

Method for Studies of Orientation and Exploratory Behavior in Humans. Effects of Emotional Stress.

We developed a new experimental model for studies of orientation and exploratory behavior in humans. This method allows analysis of variants of orientation and exploratory behavior in humans, including risky behavior, and cognitive and individual typological parameters. The model allows distinguishing examinees with high and low orientation and exploratory activity, which helps to determine their psychoemotional status. Surprisingly, individuals in emotional strain demonstrated longer orientation and exploratory behavior. This allows them to achieve the results more rapidly during purposeful behavior under similar conditions. This model provides the tool for evaluation of the electrophysiological, autonomic, and biochemical mechanisms of orientation and exploratory behavior of humans.

Effect of Peripheral µ-, δ-, and κ-Opioid Ligands on the Development of Tolerance to Ethanol-Induced Analgesia.

We studied the rate of development of tolerance to the ethanol-induced analgesia under the effect of µ-, δ-, and κ-opioid agonists and antagonists not crossing the blood-brain barrier and rapidly inactivated by gastric and duodenal proteolytic enzymes. Activation of gastric κ-opioid receptors eliminated the analgesic effect of ethanol and accelerated the development of tolerance to ethanol-induced analgesia. In contrast, activation of gastric µ-opioid receptors decelerated the development of this tolerance. Activation of gastric δ-opioid receptors produced no effect on examined tolerance. µ-Opioid receptor antagonist decelerated and δ-opioid receptor antagonist accelerated the development of tolerance to ethanol-induced analgesia. Thus, the state of gastric opioid receptors affects the manifestation of ethanol-induced analgesia and the development of tolerance to this effect.

New Experimental Model of Rat Risk Behavior. Effects of Nicotine and Phenazepam.

We described a new model of risk behavior in rats that allows selection of animals predisposed to risk behavior in the absence of other biological motivations. Phenazepam administration did not change the intensity of risk behavior in "risky" animals, but stimulated risk behavior in rats that were not predisposed to it. Nicotine inhibited risk behavior in "risky" animals and strengthened it in "cautious" rats. In the intermediate group, the drugs did not induce significant changes. A similar effect of the drugs was observed in the previous models. More complex effect of nicotine on the risk behavior may be explained by the absence of severe food motivation typical for the early models.

Activation of Peripheral κ-Opioid Receptors Normalizes Caffeine Effects Modified in Nicotine-Dependent Rats during Nicotine Withdrawal.

The study examined the effect of peripheral (intragastric) ICI-204,448, an agonist of gastric κ-opioid receptors, on the psychostimulating and anxiolytic effects of caffeine in nicotinedependent rats at the stage of nicotine withdrawal. In these rats, the effects of caffeine (10 mg/kg) were perverted. In nicotine-dependent rats, caffeine produced an anxiolytic effect accompanied by pronounced stimulation of motor activity, in contrast to anxiogenic effect induced by caffeine in intact rats without nicotine dependence. During nicotine withdrawal, nicotine-dependent rats demonstrated enhanced sensitivity to nicotine. Intragastric administration of κ-opioid receptor agonist ICI-204,448 normalized the effect of caffeine in nicotinedependent rats. We have previously demonstrated that activation of peripheral κ-opioid receptors inhibited central κ-opioid activity and eliminated manifestations of nicotine withdrawal syndrome in nicotine-dependent rats, e.g. metabolism activation, stimulation of motor activity, and enhancement of food consumption. In its turn, inhibition of central κ-opioid structures activates the brain adenosine system, which can attenuate the caffeine-induced effects in nicotine-dependent rats.

Anxiolytic, Psychostimulant, and Analgesic Effects of Various Volumes of Ethanol Solution in Different Concentrations, but in the Same Dose.

We studied the effect of ethanol (dose 2 g/kg) in various concentrations (5, 13, and 40%) and different volumes (40, 15.5, and 5 ml/kg) on the level of anxiety, locomotor activity, and pain sensitivity in rats. Administration of 40 ml/kg water to animals was followed by a significant increase in the time spent in the open arms of the elevated plus maze. Administration of water in a volume of 5 or 15.5 ml/kg had little effect on the level of anxiety. The greater was the volume of intragastrically administered ethanol, the stronger was the anxiolytic effect. The psychostimulant and analgesic effects of ethanol were more pronounced after administration of medium volumes and intermediate concentrations of ethanol-containing solutions. Since administration of these solutions cannot produce maximum blood concentration of ethanol, we believe that the observed effects are mainly related to the direct effect of ethanol on the stomach tissue.

Antagonists of Metabotropic Glutamate Receptors Prevent the Development of Audiogenic Seizures.

Pretreatment with mGluR1 antagonist AIDA (1 mg/kg) nearly completely prevented the onset of tonic-clonic seizures and increased generation of NO in the cerebral cortex of rats with genetically determined audiogenic reaction to acoustic stimulation. Administration of mGluR5 antagonist MPEP (10 mg/kg) before audiogenic exposure was followed by a significant decrease in the degree of seizure and partially prevented increased generation of NO due to acoustic stimulation. These data indicate that mGlu receptors and NO play an important role in the pathogenetic mechanisms of audiogenic seizures.

Effect of peptide agonists of peripheral opioid receptors on operant feeding behavior and food motivation in rats.

We studied the effect of intragastric administration of peptide agonists of µ-opioid receptors (DAMGO) and δ-opioid receptors (DADLE) on food consumption and food motivation during operant feeding behavior of different intensity and effectiveness. To obtain one food granule, trained rats should press a lever 1 time (day 1), 2 times (day 2), 4 times (day 3), 8 times (day 4), 16 times (day 5), or 32 times (day 6). Activation of δ-opioid receptors in the stomach was followed by suppression of feeding behavior at low energy expenditure. The level of food motivation under these conditions practically did not differ from the control. Activation of µ-opioid receptors in the stomach suppressed energy-consuming feeding behavior, which was accompanied by an increase in the level of food motivation. It can be hypothesized that protein metabolites exhibiting µ-opioid activity probably provide afferent signals into CNS via the vagus nerve to terminate energy expenditure under adverse conditions (although food motivation is not satisfied). Food motivation under these conditions probably contributes to the behavior aimed towards the search for more available food.

Peripheral administration of a µ-opioid receptor agonist DAMGO suppresses the anxiolytic and stimulatory effects of caffeine.

We studied the possibility of modulation of the stimulatory and anxiolytic effects of caffeine by activation of µ-opioid receptors in the gastrointestinal tract. Caffeine in a dose of 10 mg/kg (but not in a dose of 100 mg/kg) had a strong anxiolytic and psychostimulant effect. This effect was manifested in a significant increase in the time spent in the open arms of the elevated plus-maze, elevation of locomotor activity, and stimulation of metabolism. Administration of DAMGO to animals receiving caffeine in a dose of 10 mg/kg abolished the anxiolytic and psychostimulant effects of caffeine. By contrast, administration of DAMGO to rats receiving caffeine in a dose of 100 mg/kg had the anxiolytic effect. Activation of peripheral µ-opioid receptors is followed by the inhibition of the central µ-opioid system. We observed a decrease in the number of µ-opioid receptors in the midbrain and cerebral cortex and inhibition of ß-endorphin release from nerve ending of the cingulate cortex in rats. These changes are probably followed by activation of the adenosine system in the brain. Caffeine dose should be increased to achieve the effect. Therefore, the anxiolytic and stimulatory effects of caffeine in a dose of 10 mg/kg are abolished under these conditions. By contrast, the anxiolytic effect of caffeine in a dose of 100 mg/kg (not observed under normal conditions) develops after this treatment.

Method for determining the level of food motivation in rats.

A new method for the quantitative evaluation of the level of food motivation was developed. This method takes into account not only the information, but also the energy component of operant feeding behavior of different intensity and effectiveness with a simultaneous study of metabolism by means of indirect calorimetry. Our experiments showed that an increase in the number of lever pressing episodes (from 1 to 8) to obtain one food granule during operant feeding behavior is accompanied by a progressive decrease in the level of food motivation. The level of food motivation remains practically unchanged with an increase in the ratio of pressing episodes to 16 and 32 (despite consumption of food).

Effect of peripheral opioid receptor agonists on depressive activity of ethanol.

We studied the effect of peripheral µ- and κ-opioid receptor agonists (not crossing the bloodbrain barrier) on locomotor activity and metabolism in rats after acute administration of ethanol. Intraperitoneal injection of ethanol in a single dose of 2 g/kg had a strong depressive effect manifested in a decrease in horizontal locomotor activity and suppression of metabolism. µ-Opioid receptor agonist DAMGO and κ-opioid receptor agonist ICI 204,448 partly abolished the effect of ethanol on locomotor activity of rats. ICI 204,448 was most potent in this respect. In contrast to µ-opioid receptor agonist DAMGO, κ-opioid receptor agonist ICI 204,448 prevented metabolism suppression induced by ethanol. Our results indicate that ICI 204,448 significantly inhibits the depressive effect of ethanol. DAMGO showed only partial effectiveness under these experimental conditions.

Effect of activated peripheral κ-opioid receptors on the action of nicotine and its withdrawal in nicotine-dependent rats.

We studied the possibilities of modulating the effects of nicotine and its withdrawal in nicotine-dependent rats by peripheral injection of κ-opioid receptor agonist ICI 204,448. Injection of nicotine to rats previously treated with nicotine for 14 days reduced motor activity, suppressed metabolism, and increased food intake. In rats receiving ICI 204,448 after chronic administration of nicotine, food intake did not differ from that in control animals receiving isotonic NaCl solution. ICI 204,448 had virtually no effect on suppression of motor activity and metabolism. The rats receiving the last injection of nicotine 24 h prior to the experiment demonstrated an increase in metabolism, locomotor activity, and food intake. In these animals, ICI 204,448 completely abolished the effects of nicotine withdrawal. It was found that peripheral administration of compound ICI 204,448 did not significantly inhibit the effect of nicotine in nicotine-dependent rats, but abolished symptoms of nicotine withdrawal. It can be hypothesized that nicotine withdrawal syndrome is related to inhibition of dopamine release in the nucleus accumbens probably caused by enhanced κ-opioid activity in presynaptic terminals. Activation of peripheral κ-opioid receptors apparently suppressed (via vagal afferent pathways) central κ-opioid activity and reduced nicotine withdrawal symptoms in nicotine-dependent subjects.