Reversal by naloxone of the effects of chronic administration of drugs of dependence on rat liver and brain tryptophan metabolism

Tryptophan metabolism in alcoholism

Acute and chronic alcohol (ethanol) intake and subsequent withdrawal exert major effects on tryptophan (Trp) metabolism and disposition in human subjects and experimental animals. In rats, activity of the rate-limiting enzyme of Trp degradation, liver Trp pyrrolase (TP), is enhanced by acute, but inhibited after chronic, ethanol administration, then enhanced during withdrawal. These changes lead to alterations in brain serotonin synthesis and turnover mediated by corresponding changes in circulating Trp availability to the brain. A low brain-serotonin concentration characterizes the alcohol-preferring C57BL/6J mouse strain and many alcohol-preferring rat lines. In this mouse strain, liver TP enhancement causes the serotonin decrease. In man, acute ethanol intake inhibits brain serotonin synthesis by activating liver TP. This may explain alcohol-induced depression, aggression and loss of control in susceptible individuals. Chronic alcohol intake in dependent subjects may be associated with liver TP inhibition and a consequent enhancement of brain serotonin synthesis, whereas subsequent withdrawal may induce the opposite effects. The excitotoxic Trp metabolite quinolinate may play a role in the behavioural disturbances of the alcohol-withdrawal syndrome. Some abstinent alcoholics may have a central serotonin deficiency, which they correct by liver TP inhibition through drinking. Further studies of the Trp and serotonin metabolic status in long-term abstinence in general and in relation to personality characteristics, alcoholism typology and genetic factors in particular may yield important information which should facilitate the development of more effective screening, and preventative and therapeutic strategies in this area of mental health.

Behavioural studies in humans: anxiety, stress and smoking

Numerous observers have reported that smokers smoke more under stressful conditions. The most frequent explanation is that nicotine reduces anxiety, an intervening variable identified as a negative reinforcer for smoking behaviour. The conditions under which anxiety reduction occurs in response to smoking, however, have not been well defined, nor are underlying mechanisms well understood. There are several possible explanations, including Schachter's theory based on stress-induced changes in urinary pH and the hypothesis of endogenous opioid involvement. The work of Collins and his associates in animals has shown that genetic variations in corticosteroid responsiveness to nicotine are associated with differences in sensitivity to nicotine. Research in our laboratory has extended to humans Collins' findings that sensitivity to nicotine is inversely related to corticosteroid activity. We also found that the combination of a psychological stressor and smoking produced additive effects on cortisol release in humans. These findings suggest a novel way of explaining the interaction between smoking and stress, in that increased nicotine intake in the context of stress may in part reflect behavioural compensation for diminished sensitivity to nicotine when corticosteroid activity is enhanced by the stressor.

Effect of naloxone on behavioral changes induced by subchronic administration of ethanol in rats

Endogenous opioid peptides appear to be involved in acute behavioral effects induced by single doses of ethanol. However, its role in repeated ethanol exposure has not been studied. In the present study ethanol was given to rats at the doses of 2 and 4 g/kg by a stomach gauge for 15 days, and its effects on spontaneous motility, open-field activity, and active avoidance behavior recorded on the 3rd, the 6th and the 15th days. Then the effect of naloxone (0.5 and 2 mg/kg by intraperitoneal route) was tested against a challenge ethanol dose, administrated by oral route, on the 16th day. Control animals received tap water and saline instead of ethanol or naloxone, respectively. Both doses of ethanol induced a decrease in spontaneous motility that was antagonized by naloxone. Open-field ambulations were increased by the high dose of ethanol, low-dose lacking effect; naloxone did not modify these ethanol effects. The low dose of ethanol shortened latency time in shuttlebox, the high dose causing escape and freezing responses; none of these effects were modified by naloxone. Therefore, endogenous opioid peptides appear to play a limited role in the chronic effects of ethanol in rats; particularly its effects in tests inducing an increase in the level of anxiety were resistant to naloxone.

Serotonin and alcohol intake, abuse, and dependence: findings of animal studies

Despite a relatively large body of literature on the role of the neurotransmitter serotonin (5-hydroxytryptamine, or 5-HT) in the regulation of alcohol intake, the functional significance of serotonergic neurotransmission and its relationship to alcohol intake, abuse, and dependence remains to be fully elucidated. In part two of this review, the experimental (animal) data is summarized along two lines: the effects of serotonergic manipulations on the intake of alcohol, and the effects of acute and chronic alcohol intake, as well as the withdrawal of chronic alcohol, on the serotonergic system. It is concluded that serotonin mediates ethanol intake as a part of its larger role in behavior modulation, such that increases in serotonergic functioning decrease ethanol intake, and decreased serotonergic functioning increases ethanol intake. Ethanol produces transient increases in serotonergic functioning that activate the mesolimbic dopaminergic reward system. The results are discussed in light of recent theories describing the regulatory role of serotonin in general behavior.

Effect of naloxone on cigarette smoking

Several recent studies have tried to link endogenous opioid peptides (endorphins) with smoking reinforcement. Karras and Kane (1980) showed a one-third decrease in smoking after administration of the opiate antagonist naloxone. However, Nemeth-Coslett and Griffiths (1986) failed to replicate this result with use of a wide range of doses. The current study was an attempt to replicate Karras and Kane's results using 10 male chronic smokers. Each subject received naloxone (10 mg sc) or placebo in a double-blind, cross-over design (cross-over interval 6-14 days) after overnight abstinence from tobacco use. Naloxone reduced the number of cigarettes smoked by 16% in the first 1/2 hour smoking period, with a 10% reduction in carbon monoxide level 1 hour later. Naloxone had no effect on subjective and physiological variables and did not cause any signs of a tobacco withdrawal syndrome. These results are consistent with those of Karras and Kane, and suggest a role for endorphins in smoking reinforcement. Inconsistent results across studies may be due to methodological factors, such as differences in smoking deprivation time and postnaloxone administration testing intervals because naloxone has a short half-life.

Naloxone-induced increase in blood and brain ethanol concentrations in rats

Although a reduction in blood ethanol concentration has been proposed to mediate the ethanol antagonist activity of naloxone observed in clinical and experimental situations, an increase in this variable as well as in brain ethanol concentration has been found in rats treated with naloxone (0.5 and 2.0 mg/kg, i.p.) ten min after intragastric administration of ethanol (1 and 2 g/kg). This effect disappeared either when naloxone was administered 50 min after ethanol or when ethanol was given intraperitoneally. On the other hand, naloxone induced a slight but significant slowing in intestinal transit rate. These results suggest that naloxone may facilitate gastrointestinal absorption of ethanol when administered soon after an oral load of this drug. Therefore, mechanisms other than a pharmacokinetic interaction appear to be involved in the antagonist action of naloxone.

Antagonism of the stimulant and depressant effects of ethanol in rats by naloxone

The action of naloxone (0.5 and 2 mg/kg IP) on the behavioural effects of a low (2 g/kg PO) and a high dose (4 g/kg PO) of ethanol was studied in rats. Ethanol at the low dose increased spontaneous motility, enhancing open-field external ambulations and reducing shuttle-box latency. All these effects were antagonized by naloxone. Ethanol at the high dose produced by hypomotility, decreasing open-field external ambulations and impairing shuttle-box performance. In this case, naloxone also reduced the ethanol effect, but its action was less consistent. Therefore, although mechanisms other than a specific opioid receptor blockade by naloxone must be considered, an involvement of opioid peptides in the effects of ethanol cannot be discounted.

Effects of nicotine on exploratory behavior in rats: correlation with regional brain monoamine levels

The results of a dose-response study of the effects of nicotine on exploratory behavior in male rats is reported. Nicotine at 0.5 mg/kg elevated locomotor activity without significantly changing other parameters of exploration. Low-dose nicotine (0.2 mg/kg) did not produce any effect on exploration measures, while high-dose nicotine (0.8 mg/kg) produced a state of ataxia in animals and decreased most exploration measures in general. Additional, nicotine at high doses seems to reduce the animal's state of fear/anxiety, while at low dose the drug seems to increase the animal's level of curiosity in a novel environment. Biochemically, nicotine has been found to accelerate dopamine synthesis and norepinephrine turnover, and to decrease serotonin turnover. More importantly, the amino acid precursors tyrosine and tryptophan were found to be the neurochemical measures most related to the behavioral changes produced by nicotine.

Effects of naloxone and other opiate antagonists on blood-ethanol concentration in acutely-ethanol-intoxicated rats

The effects of naloxone hydrochloride (NAL) and other opiate antagonists on blood-ethanol concentration (BEC) in acutely-ethanol-intoxicated rats were examined. Using a 1 mg/kg body wt. dose of NAL, the maximum decrease in BEC was found to occur at 30 min. At 30 min after administration of various doses of NAL, it was found that BEC was decreased maximally by a 2 mg/kg dose, whereas the first significant decrease was caused by a 10 micrograms/kg dose. BEC was also decreased by naltrexone (1 mg/kg), but not by a 4 mg/kg dose of any of four Mr compounds (Mr 1452, Mr 1453, Mr 2266 and Mr 2267). It is suggested that pharmacokinetic antagonism of acute alcohol intoxication by naloxone and naltrexone is unrelated to the property of opiate antagonism, but may involve the ability of certain such antagonists to interact with hepatic NAD+-dependent oxidative metabolism.

Zimeldine: a review of its effects on ethanol consumption

This review evaluates the literature and describes an extensive series of experiments which examined the effects of zimeldine , its metabolite norzimeldine and other serotonin and norepinephrine reuptake inhibitors on voluntary ethanol consumption in rats. The results of these experiments indicate that drugs which specifically inhibit serotonin reuptake are capable of decreasing voluntary ethanol consumption. The behavioral mechanism through which these drugs exert their effects seems to be extinction of the primary reinforcing properties of alcohol. These effects seem to be partially attenuated both by drugs which modulate the norepinephrine system as well as by the serotonin postsynaptic receptor blocker methergoline. The data presented in this review are discussed in terms of the involvement of the serotonin and norepinephrine systems in the mechanism of action of these drugs. In addition, several alternative hypotheses concerning the nature of the phenomenon are offered. Finally, the implications of these data for the possible development of a treatment procedure for problem drinkers is discussed.

Does naloxone always act as an opiate antagonist?

Evidence for the ability of the opiate antagonist naloxone to block a variety of metabolic effects exerted by morphine and non-opiate drugs is reviewed. Naloxone prevents or reverses the following effects in the rat: (a) the chronic morphine-induced increase in liver [NADPH]; (b) the consequent chronic morphine-induced inhibition of liver tryptophan pyrrolase activity; (c) the resultant chronic morphine-induced enhancement of brain 5-hydroxytryptamine synthesis; (d) the similar effects on liver and brain tryptophan metabolism exerted chronically by other drugs of dependence (ethanol, nicotine and phenobarbitone); (e) the acute ethanol-induced increase in the hepatic [NADH]/[NAD] ratio. Naloxone also (f) inhibits basal and stimulated lipolysis in fed and 24hr-starved rats. This leads to prevention of (g) the consequent increase in the availability of circulating free tryptophan, and (h) the resultant tryptophan-mediated decrease in liver 5-aminolaevulinate synthase activity. The question of how many of these effects involve changes in endogenous opiates or at opiate receptors is not clearly understood at present and thus merits investigation. However, because most of the above effects are explained on biochemical grounds, and in view of evidence from behavioural and pharmacological studies [see (1)], the possibility must be considered that many of the actions of naloxone may be unrelated to its opiate-receptor-antagonistic properties.

Rapid liquid chromatographic determination of 5-hydroxyindoles and dihydroxyphenylacetic acid in cerebrospinal fluid of the rat

A stainless steel guide was implanted in the anterior third ventricle of the anesthetized rat and an internal needle shorter than the guide was used to continuously collect cerebrospinal fluid (CSF) at a constant outflow of 1 microliter/min. Five microliter samples were injected directly into a liquid chromatographic column. The mobile phase was adjusted for selective separation of 5-hydroxytryptophan (5-HTP), serotonin (5-HT), dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindolacetic acid (5-HIAA). Electrochemical detection with a limit of 0.05 pmol was used, 5-HTP and 5-HT concentrations were in the 10(-8) M range in controls while DOPAC and 5-HIAA were in the 10(-7) and 10(-6) M range. Brain aromatic amino acid decarboxylase inhibition with high doses of benserazide corresponded to an increased CSF level of 5-HTP. Monoamine oxidase inhibition with tranylcypromine resulted in a diminution of DOPAC and 5-HIAA. L-Tryptophan loading associated with monoamine oxidase inhibition induced an increase in CSF level of serotonin. These pharmacologically induced changes in serotonin and dopamine metabolite levels exemplify the usefulness of these CSF determinations as indices of brain function.

Mechanisms of elevation of rat brain tryptophan concentration by various doses of salicylate

1 The roles of inhibition of liver tryptophan pyrrolase activity and of displacement of tryptophan from its binding sites on serum proteins have been investigated in relation to the increase in rat brain tryptophan concentration after administration of various doses of sodium salicylate. 2 The elevation of brain tryptophan concentration by sodium salicylate (0.5 mg/kg) was caused by inhibition of liver pyrrolase activity, whereas that by doses of the drug of 50 mg/kg and above was achieved mainly by tryptophan displacement. Both tryptophan displacement of pyrrolase inhibition caused the increase in brain tryptophan concentration by sodium salicylate at 10 mg/kg. 3 The smallest dose of salicylate capable of displacing serum-protein-bound tryptophan was 2.5 mg/kg.

The mechanism of the antagonism by naloxone of acute alcohol intoxication

Naloxone lowers blood-ethanol concentration and causes a simultaneous reversal of the disturbances in the redox states of the hepatic nicotinamide-adenine dinucleotide (phosphate) couples in acutely-ethanol-intoxicated rats. It is suggested that these effects of naloxone form the basis of its antagonism of acute alcohol intoxication.

Rapid reversal by naloxone of the chronic effects of morphine on rat liver and brain tryptophan metabolism

The chronic morphine-induced inhibition of rat liver tryptophan pyrrolase activity and the resultant increases in tryptophan availability to the brain and brain 5-hydroxytryptamine (5-HT) synthesis are reversed within 10 min after naloxone administration. The possible involvement of hepatic tryptophan metabolism in morphine dependence is briefly discussed.