Changes in β-endorphin level in the cingulate cortex in rats after peripheral loperamide and methylnaloxone administration at rest and during emotional stress

Involvement of Peripheral Opioid Receptors in the Realization of Food Motivation Into Eating Behavior

The involvement of peripheral opioid receptors in the mechanisms of eating behavior is still unclear. The aim of this work was to study the role of peripheral, predominantly gastric mu and delta opioid receptors in the realization of food motivation in conditions of different energy costs for eating behavior. Experiments were performed under a between-sessions progressive ratio schedule of reinforcement in food-deprived rats. The level of food motivation was calculated using a self-developed method. Food intake, motor activity, and metabolic rate were recorded in fed and hungry animals. Results showed that intragastric administration of the mu opioid receptor agonist DAMGO led to an increase in the level of food motivation in the light variant of operant feeding behaviors. Food consumption did not change. At high costs for feeding behavior, the administration of DAMGO did not alter food motivation; however, food consumption and motor activity were reduced. Intragastric administration of the delta opioid receptor agonist DADLE did not lead to changes in the level of food motivation and physical activity, but inhibition of feeding behavior was observed in all reinforcement schedules. Three regulatory pathways of eating behavior in difficult food conditions by peripheral, predominantly gastric opioid receptors are hypothesized: environmental-inhibitory afferentations and suppression of the realization of food motivation into behavior; homeostatic-inhibitory action on food motivation; and rewarding-suppression of the anticipatory reinforcement.

β-Endorphin Induction by Psychological Stress Promotes Leydig Cell Apoptosis through p38 MAPK Pathway in Male Rats

Psychological stress (PS) disturbs the reproductive endocrine system and promotes male infertility, but the underlying pathogenic mechanisms have not been extensively studied. This study aimed to uncover the mechanisms of PS-induced male reproductive related abnormalities subjected to a ‘terrified sound’ exposure. Male rats subjected to PS displayed slow growth, decreased sperm quality, abnormal levels of the reproductive endocrine hormones, decreased expression of the reproductive-related proteins androgen-binding protein (ABP) and bromodomain-containing protein (BRDT), increased apoptosis in the testis, and accompanied by elevated levels of β-endorphin (β-EP). These effects were reversed by naloxone. Furthermore, PS-induced β-EP could promote mu opioid receptor (MOR) activation and ensure intracellular p38 MAPK phosphorylation and then lead to Leydig cells (LCs) apoptosis. The current result showed that β-EP was a key factor to PS-induced male infertility.

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.

Naloxone effects on extinction of ethanol- and cocaine-induced conditioned place preference in mice

RATIONALE

Previous studies found that naloxone (NLX) facilitated choice extinction of ethanol conditioned place preference (CPP) using long (60 min) test sessions, but there is little information on the variables determining this effect.

OBJECTIVES

These studies examined repeated exposure to NLX during extinction of ethanol- or cocaine-induced CPP using both short and long tests.

METHODS

DBA/2J mice were injected with NLX (0 or 10 mg/kg) before three 10- or 60-min choice extinction tests (experiment 1). All mice received a final 60-min test without NLX. Post-test NLX was given in experiment 2. Experiment 3 tested whether NLX would affect a forced extinction procedure. Experiment 4 tested its effect on extinction of cocaine-induced CPP.

RESULTS

Pre-test (but not post-test) injections of NLX-facilitated choice extinction of ethanol CPP at both test durations. Pre-test NLX also facilitated forced extinction. However, pre-test NLX had no effect on choice extinction of cocaine CPP.

CONCLUSIONS

Extinction test duration is not critical for engaging the opioid system during ethanol CPP extinction (experiment 1). Moreover, NLX's effect does not depend on CPP expression during extinction, just exposure to previously conditioned cues (experiment 3). The null effect of post-test NLX eliminates a memory consolidation interpretation (experiment 2) and the failure to alter cocaine CPP extinction argues against alteration of general learning or memory processes (experiment 4). Overall, these data suggest that the endogenous opioid system mediates a conditioned motivational effect that normally maintains alcohol-induced seeking behavior, which may underlie the efficacy of opiate antagonists in the treatment of alcoholism.

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.

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 β-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.

Effect of peripheral D₂ dopamine receptor antagonist domperidone on metabolism, feeding behavior, and locomotor activity of rats

We studied the possibility of activation of the central dopaminergic system compartment by modulating activity of D2 dopamine receptors in the gastrointestinal tract with domperidone, an antagonist not crossing the blood-brain barrier. Intragastric administration of 0.1 mg/kg domperidone to rats was followed by a significant decrease in feeding behavior and stimulation of basal metabolism, but had no effect on locomotor activity of animals in a Phenomaster system. These effects are typical of psychostimulant agents that stimulate dopamine release from nerve endings in the nucleus accumbens and some regions of the brain cortex. Our results indicate that physiological functions associated with activity of the central dopaminergic system can be modulated through peripheral dopamine receptors.

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.

Loperamide effects on anxiety level and feeding behavior in rats. Role of vagal afferentation

We investigated the role of vagal afferentation in the interaction of the peripheral and central parts of the endogenous opioid system, in the mechanisms of sensorial satiation and anxiety in rats. It has been established that vagotomized rats spent less time in open arm of the plus-maze in comparison with sham-operated animals. Peripheral administration of μ-opioid receptor agonist loperamide was shown to reduce anxiety level in sham-operated rats and had no effect on this parameter in vagotomized animals. Testing in a PhenoMaster module system showed that loperamide administration suppressed feeding behavior in sham-operated animals and partially suppressed it in vagotomized animals. Vagotomy virtually completely blocked the anxiolytic effect of loperamide and partially blocked the anorexigenic effect of the μ-opioid receptor agonist.

ATP concentration in the cingulum bundle of rats during stimulation of the ventromedial hypothalamus

We measured the concentration of ATP in the posterior cingulum bundle of Sprague-Dawley rats during electrical stimulation of the negative emotiogenic zone in the ventromedial hypothalamus. Electrostimulation of the ventromedial hypothalamus in animals was accompanied by an increase in systolic and diastolic blood pressure, which illustrates the autonomic response to this treatment. Variations in BP of rats were followed by an increase in ATP content in the posterior cingulum bundle. After stimulation of the ventromedial hypothalamus, ATP concentration in the cingulum bundle reached the maximum levels of 1.3±0.3 μM (n=4) and 1.67±0.43 μM (n=10) and remained high for 42.6±7.5 sec. The enhanced release of ATP in the cingulate region of the brain is probably related to the involvement of this structure into emotional reactions. ATP plays a role of the major energy source and signal molecule, which provides the adequate biochemical and physiological processes and cell-to-cell interaction in CNS upon the exposure to negative emotiogenic factors.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).