The role of opiate mechanisms in the development of tolerance to the anorectic effects of amphetamines

Is khat use disorder a valid diagnostic entity?

AIMS

This study aimed to validate the presence of a khat use disorder syndrome using DSM-5 criteria and to examine its relationship with increased experience of harms.

DESIGN

Cross-sectional, purposive sample of current khat-chewers, recruited from khat markets and cafes.

SETTING

Participants were recruited from the general community and from Adama Science and Technology University in Ethiopia.

PARTICIPANTS

A total of 400 current khat consumers aged 16 and above were recruited between September 2014 and January 2015 MEASURES: Survey comprising current clinical symptoms (using a modified Alcohol Use Disorder and Associated Disabilities Interview Schedule-IV), and validated measures of health, psychological distress, quality of life and academic functioning.

FINDINGS

A third [35.5% 95% confidence interval (CI) = 31.0-40.3] of respondents reported daily khat use and a quarter (25.4% 95% CI = 21.4-30.0) using three times or more per week. Using DSM-5 criteria, 10.5% (95% CI = 7.9-13.9) were categorized as experiencing mild, 8.8% (95% CI = 6.4-12.0) moderate and 54.5% (95% CI = 49.6-59.3) severe khat use disorder. Confirmatory factor analysis demonstrated a good fit of symptoms to a single underlying construct, consistent with other substance use disorders. Individuals categorized as experiencing khat use disorder demonstrated significantly greater frequency [odds ratio (OR) = 45.29; 95% CI = 10.97-19.01) and quantity of khat use (OR = 2.35; 95% CI = 1.29-4.29). They also demonstrated increased financial problems associated with use, greater problems with academic functioning and higher rates of self-reported mental health problems, higher psychological distress and poorer quality of life. Treatment access was poor, with only one-third (32.9%) of individuals with khat use disorder reporting life-time access, near-exclusively related to help-seeking from friends and relatives.

CONCLUSION

The construct of a substance use disorder syndrome for khat using DSM-5 criteria appears valid and performs in a manner consistent with other substances of dependence. Individuals with khat use disorder experience substantial problems in association with khat use. Despite this, there are low levels of help-seeking for these problems.

The role of serotonin, vasopressin, and serotonin/vasopressin interactions in aggressive behavior

Aggression control has been investigated across species and is centrally mediated within various brain regions by several neural systems that interact at different levels. The debate over the degree to which any one system or region affects aggressive responding, or any behavior for that matter, in some senses is arbitrary considering the plastic and adaptive properties of the central nervous system. Nevertheless, from the reductionist point of view, the compartmentalization of evolutionarily maladaptive behaviors to specific regions and systems of the brain is necessary for the advancement of clinical treatments (e.g., pharmaceutical) and novel therapeutic methods (e.g., deep brain stimulation). The general purpose of this chapter is to examine the confluence of two such systems, and how their functional interaction affects aggressive behavior. Specifically, the influence of the serotonin (5HT) and arginine vasopressin (AVP) neural systems on the control of aggressive behavior will be examined individually and together to provide a context by which the understanding of aggression modulation can be expanded from seemingly parallel neuromodulatory mechanisms, to a single and highly interactive system of aggression control.

Increased drinking after intra-striatal injection of the dopamine D2/D3 receptor agonist quinpirole in the rat

RATIONALE

Dopamine D2 receptor hyperactivity has been implicated in the development of psychogenic polydipsia in schizophrenic patients. Repeated treatment with dopamine agonists, including the D2/D3 agonist quinpirole, has been shown to induce hyperdipsia in a number of animal models. Despite these observations, obtained with systemic administrations, little attempt has been made to investigate where in the brain dopamine agonists act to induce hyperdipsia.

OBJECTIVE

The present study investigates the effects of repeated intra-caudate infusions of quinpirole on the intake of water by rats tested under free-drinking conditions.

MATERIALS AND METHODS

Rats with bilateral cannulae placed into the anterior, central or posterior caudate received quinpirole microinfusions (1 μg/side) for five consecutive days in their home cage. Water intake was measured 15 and 60 min after the treatment.

RESULTS

When injected in the central caudate, quinpirole increased water intake, and this effect progressively increased over sessions, indicating the development of sensitization. When injected in the posterior caudate, the dipsogenic effect of quinpirole was less intense and did not undergo sensitization. The infusion of quinpirole in the anterior caudate did not affect drinking.

CONCLUSION

The present study shows that caudate D2/3 receptors play an important role in the development of quinpirole-induced hyperdipsia, an animal model of psychotic polydipsia.

The effects of clozapine on quinpirole-induced non-regulatory drinking and prepulse inhibition disruption in rats

RATIONALE

The biological underpinnings of schizophrenic polydipsia are poorly understood.

OBJECTIVES

This study is aimed at fulfilling the requisites of an experimental model of this syndrome through the quinpirole (QNP) induction of non-regulatory drinking in rats.

METHODS

In a first experiment, clozapine (10 and 40 mg/kg p.o.) was substituted for haloperidol during the last 5 days of 10 days QNP (0.5 mg/kg i.p.) administration and water intake measured at 5 h. In a second experiment, animals treated with QNP alone or in combination with clozapine were assessed for water intake and prepulse inhibition (PPI). Expression of genes coding for the dopaminergic D2 receptor, as well as for the early genes BDNF (brain-derived neurotrophic factor) and c-Fos in prefrontal cortex, hippocampus, and striatum was also evaluated.

RESULTS

Clozapine prevented QNP-induced drinking at 10 and 40 mg/kg, but only at 40 mg/kg when it was substituted for haloperidol. In the second experiment, QNP-treated rats showed both non-regulatory drinking and PPI disruption. Both these effects were prevented by clozapine 40 mg/kg. QNP-reduced BDNF expression in the hippocampus and increased c-Fos in the prefrontal cortex. This effect was prevented by clozapine. Given by itself, clozapine reduced the expression of both D2 receptors and BDNF in the prefrontal cortex and striatum.

CONCLUSIONS

The present study lends further support to the hypothesis that non-regulatory drinking induced by QNP in rats is a robust and reliable pharmacological effect that might model psychotic polydipsia also in its sensitivity to clozapine.

Haloperidol both prevents and reverses quinpirole-induced nonregulatory water intake, a putative animal model of psychogenic polydipsia

RATIONALE

Polydipsia is a severe complication of long-term schizophrenia and, despite its unknown pathogenesis, is empirically treated with typical or atypical antipsychotics. In the rat, nonregulatory water intake is induced by repeated administration of amphetamine-like compounds or by the D2/3 agonist, quinpirole.

OBJECTIVE

This study is aimed at determining the potential activity of antipsychotic compounds with different affinities for D2 receptors in preventing and/or reversing quinpirole-induced polydipsia.

MATERIALS AND METHODS

Male Sprague-Dawley rats were treated with five injections of quinpirole (0.5 mg/kg i.p.) to induce polydipsia. The oral effects of haloperidol, olanzapine, clozapine, and ST2472 on QNP-induced polydipsia were analyzed in the following two schedules. In the preventive schedule, haloperidol (0.2, 0.4, and 0.8 mg/kg), olanzapine (1.5, 3, and 6 mg/kg), ST2472 (1 and 2 mg/kg), and clomipramine (5, 10, and 20 mg/kg) were given in combination with quinpirole from day 1 to day 5. In the reversal schedule, rats showing quinpirole-induced polydipsia on the third day received haloperidol (0.4 mg/kg), olanzapine (1.5 and 3 mg/kg), clozapine (10, 20, and 40 mg/kg), ST2472 (1, 2, 5, and 10 mg/kg), and clomipramine (5, 10, and 20 mg/kg) before quinpirole on days 4 and 5.

RESULTS

Haloperidol both prevented and reversed quinpirole-induced polydipsia, whereas olanzapine and ST2472 only reversed it. Clomipramine prevented but did not reverse quinpirole-induced polydipsia, and clozapine did not reverse it either.

CONCLUSIONS

We suggest that, once developed, polydipsia is governed by dopaminergic D2 mechanisms. In contrast, either an increase in the serotoninergic tone or an inhibition of D2 receptors can modulate the development of quinpirole-induced excessive drinking.

A review of the neuropharmacological properties of khat

BACKGROUND

The psychostimulant khat (Catha edulis Forsk), is a herbal drug cultivated and chewed as a recreational and socializing drug in East Africa and the Arabian Peninsula for centuries. Due to increasing air transportation and the loosening of customs restrictions, it is now readily available in the Western Countries mainly used by immigrants from khat growing areas causing a concern to policy-makers.

OBJECTIVE

We conducted this review to further gain an insight to the neuropharmacological effects of khat.

METHODOLOGY

PubMed search engine with key terms 'khat' or 'qat' or 'mirra' or'qaad/jaad' or 'cathinone' was used to obtain articles relevant to khat chewing. In total 284 English written articles published from 1959 to 2007 were screened.

RESULTS

Most of the studies focused on cathinone, the postulated active psychostimulant alkaloid in khat. There were few studies which investigated the entire plant extract in either in vitro or animal studies. In the majority of the studies it was reported that both cathinone and cathine, another psychoactive constituent, have actions that are similar to those of amphetamine.

CONCLUSIONS

It seems that the well investigated khat alkaloids have many features similar to amphetamines; however there is a need for a more thorough examination of khat itself in well designed in vitro, animal and human studies with a range of comparator drugs before confirming the claim that khat is a "natural amphetamine".

Khat chewing from the pharmacological point of view: an update

Khat chewing is deeply rooted in the every day life of people living in the Horn of Africa and in South Arabia, where Catha edulis is endemic. Considered little more than an exotic habit producing just mild pharmacological effects, systematic investigations on its active principles have instead lead to the isolation and chemical characterization of cathinone, a compound structurally related to amphetamine. Three decades of intense experimental and clinical research on khat have depicted a consistently clear picture of its pharmacological and toxicological effects.

Compulsive-like effects of repeated administration of quinpirole on drinking behavior in rats

We have previously reported that repeated administrations of quinpirole, a D2/D3 dopamine receptor agonist, facilitate instrumental behavior in rats given the choice between operant and free access to water (contrafreeloading: CFL). The goal of the present study was to investigate the effects of repeated daily administrations of quinpirole (0.5 mg/kg i.p.) on the appetitive versus the consummatory component of water-reinforced behavior, under two experimental conditions. Under one condition, the rats were given access to tap water according to an FR3 schedule of reinforcement. Under the second condition, the rats were given the choice between operant and free access to water. Five major findings were obtained. First, acutely quinpirole suppressed operant behavior and, therefore, water intake for at least 1h. Second, upon repeated administrations tolerance developed to the suppressant effect of quinpirole on instrumental behavior but only to a lesser extent to the antidipsic effect, dissociating the appetitive from the consummatory components of water-reinforced behavior. Third, in CFL conditions quinpirole induced a progressively larger preference for the operant access. Fourth, even when the rats were given the choice between free access to highly palatable saccharine (0.05 or 0.01%) solutions and operant access to tap water, quinpirole shifted the animals towards the operant access. Fifth, repeated quinpirole produced lasting consequences on drinking behavior, since after rehydration and under drug-free conditions quinpirole-pretreated rats ingested larger amounts of water than control rats. In conclusion, the repeated activation of D2/D3 receptors appears to induce the rats to perseverate in performing needless instrumental behavior.

Endogenous opioids and feeding behavior: a 30-year historical perspective

This invited review, based on the receipt of the Third Gayle A. Olson and Richard D. Olson Prize for the publication of the outstanding behavioral article published in the journal Peptides in 2002, examines the 30-year historical perspective of the role of the endogenous opioid system in feeding behavior. The review focuses on the advances that this field has made over the past 30 years as a result of the timely discoveries that were made concerning this important neuropeptide system, and how these discoveries were quickly applied to the analysis of feeding behavior and attendant homeostatic processes. The discoveries of the opioid receptors and opioid peptides, and the establishment of their relevance to feeding behavior were pivotal in studies performed in the 1970s. The 1980s were characterized by the establishment of opioid receptor subtype agonists and antagonists and their relevance to the modulation of feeding behavior as well as by the use of general opioid antagonists in demonstrating the wide array of ingestive situations and paradigms involving the endogenous opioid system. The more recent work from the 1990s to the present, utilizes the advantages created by the cloning of the opioid receptor genes, the development of knockout and knockdown techniques, the systematic utilization of a systems neuroscience approach, and establishment of the reciprocity of how manipulations of opioid peptides and receptors affect feeding behavior with how feeding states affect levels of opioid peptides and receptors. The role of G-protein effector systems in opioid-mediated feeding responses, which was the subject of the prize-winning article, is then reviewed.

Dissociation in the effects of the D2/D3 dopaminergic agonist quinpirole on drinking and on vasopressin levels in the rat

In the present study, we investigated the role of vasopressin in the development of quinpirole-induced hyperdipsia in the rat. We report that: (1), an acute intraperitoneal (i.p.) injection of 0.56 mg/kg of quinpirole increased plasma vasopressin (radioimmunoassay) at 15 min but not at 30 or 120 min; (2), nine daily injections of quinpirole (0.56 mg/kg, i.p.) progressively increased water intake and diuresis for a period of several hours after each treatment; (3), quinpirole hyperdipsia was associated with apparently normal levels of vasopressin (which might be considered inappropriately high in the presence of excessive drinking); (4), quinpirole reduced vasopressin and oxytocin, but not angiotensin, immunoreactivity in the supraoptic nucleus. These findings suggest that quinpirole hyperdipsia is a sound animal model of psychotic polydipsia.

Quinpirole- and amphetamine-induced hyperdipsia: influence of fluid palatability and behavioral cost

Daily administration of moderate doses of amphetamine or of the dopaminergic D2 agonist quinpirole is associated with the development of excessive, non-regulatory drinking. Here we compared the influence of manipulating fluid palatability and behavioral cost on the development of this drinking augmentation. Experiment 1 was based on the phenomenon of contrafreeloading (CFL): animals work for a resource even though the same resource is freely available. The effects of 15 daily injections of amphetamine (1.0 and 1.7 mg/kg i.p. ) or quinpirole (0.1 and 0.56 mg/kg i.p.) were evaluated in mildly water-deprived rats. For the first 6 days the rats obtained water by lever pressing (FR3) only; over the following 9 days water was also freely available (CFL). Initially, 0.56 mg/kg quinpirole reduced lever pressing for water. A complete recover of responding was then obtained, and was followed by a progressive increment in the amount water obtained by lever pressing during the CFL phase (from 10 to 50%). Amphetamine did not affect percent CFL, but at the highest dose (1.7 mg/kg) reduced total water intake during the last 3 days of treatment. In experiment 2 the rats had free access to two bottles, one of which contained tap water, and the other contained either an ethanol (6%) or a sucrose (5%) solution. After habituation to this regimen, the rats received 10 daily i.p. injections of vehicle, amphetamine (1.0 or 3 mg/kg), or quinpirole (0.1 or 0.56 mg/kg). Quinpirole 0.56 mg/kg enhanced daily fluid intake under both sucrose and ethanol conditions, but selectively reduced ethanol preference. The higher amphetamine dose reduced fluid intake and sucrose preference. In conclusion, chronic exposure to a dopaminergic D2 agonist, but not to amphetamine, produced an increment of drinking that was resistant to manipulation of either palatability or the behavioral cost of the fluid.

Sensitization of anorexia and locomotion induced by chronic administration of ephedrine in rats

Repeated daily administration of the sympathomimetic agent ephedrine (EPH) leads to an augmentation (sensitization) of locomotor activity in rats. The present experiments examined the impact of repeated administration of the (-)- and (+)-EPH enantiomers on feeding in rats to assess whether the anorexic activity of EPH exhibits tolerance or sensitization during chronic exposure and whether the time course of these effects follows that observed in studies of locomotion. Adult male rats were injected once daily for 12 days with either vehicle or 5, 10 or 20 mg/kg (-)-EPH or with 10 or 20 mg/kg (+)-EPH. Horizontal locomotion and diet consumption were assessed for 60 min in an activity chamber. Suppression of feeding and the induction of locomotion were augmented over the first four days of administration of either 10 mg/kg or 20 mg/kg of the (-)-EPH enantiomer. In contrast, repeated administration of 20 mg/kg (+)-EPH resulted in augmentation of appetite suppression but not locomotion. These results confirm and extend the phenomenon of locomotor and feeding sensitization for ephedrine, but suggest that these effects may differ for the two enantiomers of ephedrine.

Long-lasting sensitization to the accelerating effects of amphetamine on the speed of an internal clock

Drinking in the rat occurs in bursts of rapid licking, a high frequency rhythmic behavior controlled by a neural clock located in the brain stem. We found that 3.0 mg/kg amphetamine increased the speed of licking and shifted to the left the frequency distribution of inter-lick intervals. Repeated amphetamine treatments result in long-lasting sensitization to this effect. Thus, it appears possible to produce enduring changes in the activity of a biological interval clock (or 'stopwatch') by manipulating catecholaminergic transmission. These findings may be important for an understanding of the neural basis of normal and pathological timing behavior.

Amphetamine reinstates polydipsia induced by chronic exposure to quinpirole, a dopaminergic D2 agonist, in rats

The hypothesis that the combined activation of D1 and D2 dopaminergic receptors is instrumental in inducing amphetamine (AMPH)-mediated hyperdipsia was tested in rats. The D1 agonist SKF-38393 (SKF) and the D2 agonist quinpirole (QNP) were i.p. injected, alone or in combination, to male rats for 10 days. After 2 days of wash-out, a single dose of AMPH (3 mg/kg) was administered. Intake of water and food and diuresis were daily measured at 2, 5 and 24 h. In two further experiments the higher dose of QNP (0.56 mg/kg) was given with two different doses of the D1 antagonist SCH-23390 (SCH), or, respectively, of the peripheral D2 antagonist domperidone (DMP). In a fourth experiment, the possibility that QNP, given alone or in combination with SKF, produces an AMPH-like internal state was evaluated by using a drug-discrimination paradigm. Results show that chronic administration of QNP produced a significant increase of 24 h water intake that was reinstated by AMPH. This QNP effect was only partially prevented by DMP, suggesting a main central mechanism of action. By itself D1 receptor manipulation did not affect water intake, but influenced QNP polydipsia that, accordingly, was enhanced by the lower dose of SKF (0.3 mg/kg) and inhibited by the lower dose of SCH (0.01 mg/kg). In rats trained to discriminate AMPH from solvent, QNP partially generalized for the AMPH stimulus, an effect that was potentiated by SKF. In conclusion, a D1-modulated sensitization of D2 dopaminergic mechanisms is probably involved in AMPH-induced hyperdipsia.

Sensitization of stress-induced feeding in rats repeatedly exposed to brief restraint: the role of corticosterone

Groups of male Wistar rats lived in cages capable of monitoring feeding and drinking continuously at 0.1-s intervals, 24 h per day. Intact animals were subjected to 20 min of restraint stress or to brief handling (Brief Pick-Up), daily or every third day, 6 h after the beginning of the 12-h light period. In both studies, food-intake increased in the first hour after restraint, peaking between 15 and 45 min. Smaller increases were seen following Brief Pick-Up. More interestingly, the amount of food eaten increased across test sessions, indicating sensitization of the response to stress. Drinking also increased following stress, occurring before feeding and diminishing after the first 15 min. In adrenalectomized animals implanted with slow-release pellets to replace basal diurnal levels of corticosterone (ADX animals), sensitization of the feeding response to restraint stress developed across test sessions, although in these animals, the acute increase in food-intake following restraint stress was attenuated. ADX animals subjected only to Brief Pick-Up showed no increases in food-intake. Daily injections of 3.0 mg/kg corticosterone given to such ADX animals were unable to mimic the effects of restraint on either food-intake or drinking, nor did they augment the effects of restraint in ADX animals. We conclude that sensitization to the effects of brief restraint stress on food-intake can occur independently of a stress-induced rise in plasma corticosterone.

Tolerance does not develop to the suppressant effects of (-)-norpseudoephedrine on ingestive behavior in the rat

-(-)-Norpseudoephedrine (NPE), the enantiomer of cathine and a structural analog of phenylpropanolamine, shows anorectic and antidipsic effects that have been referred to its structural analogies with amphetamine. When amphetamine is chronically administered to rats, its anorectic effects fade out, water intake is progressively increased, and the diuretic response to the drug remains stable. Our previous studies show that chronic administration of NPE does not produce the typical amphetamine hyperdipsic response. In the present study, designed to obtain a more detailed picture of the ingestive and diuretic effects of chronic exposure to NPE, we evaluated the effects of 11 daily administrations of three doses of NPE (17, 32, and 56 mg/kg IP) on food and water intake, as well as on diuresis, in rats maintained in conditions of free access to food and water. Results show that all three doses inhibited food intake at 2 h, whereas only the highest dose inhibited food intake at 5 h. No differences between groups were detected at 24 h. These responses remained unchanged throughout the 11 days of treatment, and substitution of NPE with a solvent injection caused no rebound feeding. NPE treatment did not modify the ingestive response to a challenge injection of amphetamine, 0.56 and 1.0 mg/kg IP, given 1 day apart. Although NPE inhibited water intake throughout the experiment, it did so significantly only during the first 2 h postinjection. Urine output in the NPE-treated groups increased significantly on the first day only. These findings make it unlikely that the anorectic effects of NPE depend on an amphetamine-like mechanism of action. In addition, the short-lasting anorectic and antidipsic effects of NPE and the lack of tolerance to them raise the possibility of a therapeutic use of this drug as an adjuvant in the therapy of eating disorders characterized by binge episodes.

Ventral tegmental area opioid mechanisms and modulation of ingestive behavior

In this paper we report on the effects of intra-VTA infusion of opioid agonists on rat ingestive behavior in a variety of experimental contexts. When the animals were tested outside of their home cages surrounded only by food-pellets (Experiment 1), the injection of the mu-opioid agonist DAMGO, but not the kappa-opioid agonist U-50,488H, into the ventral tegmental area facilitated food-related behaviors, decreasing the latency to feed and increasing the number of interactions with food. When, as in Experiment 2, gnawable objects and a drinking tube were also available, intra-VTA DAMGO gnawing and drinking behaviors, whereas the effects on feeding were negligible. These effects intra-VTA DAMGO increased were greatly enhanced in rats that underwent repeated treatments with amphetamine. On the other hand, when food-related behaviors were studied in a home-cage, where access to the food supply was achieved by entry into a tunnel, latency to feed and total food-intake were not enhanced in tests made during either the dark or the light phase (Experiment 3 and 4). This was true whether powdered standard lab chow or a highly palatable food was available. It appears that when a number of alternative incentive stimuli are available, increases in dopamine transmission such as that induced by intra-VTA DAMGO may ultimately have the effect of interfering with behavior normally directed primarily to one of these stimuli, by enhancing the salience of others. These effects bears some resemblance to the effects of tail-pinch and electrical brain stimulation of the medial forebrain bundle-lateral hypothalamic area on the responses to natural incentive stimuli.

Opioid peptides, adrenocorticotrophic hormone, and idiopathic (orthostatic) edema

The effect of dextroamphetamine sulfate (Dexedrine) on plasma opioid peptides, hormones, and other metabolites was studied in eight female subjects with idiopathic (orthostatic) edema and five healthy females. All subjects were given 20 mg of dextroamphetamine sulfate, a drug widely used in the treatment of this disorder, and blood samples were collected before and 30, 60, and 90 minutes after treatment. Patients with idiopathic (orthostatic) edema had significantly lower plasma sodium levels but higher blood urea nitrogen, aldosterone, and renin levels. D-amphetamine decreased aldosterone and renin levels in both groups. Plasma adrenocorticotropin levels were lower whereas met-enkephalin levels were higher in idiopathic (orthostatic) edema subjects compared to control subjects. D-amphetamine had no significant effect on plasma beta-endorphin, adrenocorticotrophic hormone, or enkephalins. Our data indicate that opioid peptides, especially enkephalins, and adrenocorticotrophic hormone may be involved in the pathogenesis of idiopathic (orthostatic) edema syndrome, but they seem uninvolved in the aldosterone- and renin-lowering action of amphetamine. It is possible that amphetamine is acting further down the chain, either directly on the adrenal and kidney or the microvasculature, rather than at hypothalamus-pituitary axis.

The role of opioid mechanisms in the anorectic effects of stimulants: U50,488H enhances amphetamine inhibition of free feeding in rats

The influences of the kappa-opiate agonist U50,488H (U50; 4 mg/kg IP), the neuroleptic haloperidol (HAL; 0.3 mg/kg IP), and MK-801 (0.2 mg/kg IP), a noncompetitive antagonist for NMDA receptors, were compared on the effects of nine days of d-amphetamine (AMPH) treatment (3 mg/kg IP) on food and water intake and urine output. AMPH prevented feeding stimulation produced by U50 during the first 2 h, whereas U50 inhibited the hyperphagic phase that rats showed between 2 and 5 h after AMPH administration. Tolerance did not develop to the first 2-h suppression of feeding; in contrast, the late hyperphagic phase slowly recovered across the nine days of treatment. Combined administration of the two drugs barely affected water intake but considerably increased urine output. Unlike U50, HAL left the late hyperphagic response to AMPH unchanged and delayed the development of hyperdipsia. In our study MK-801 had one effect only: It significantly reduced amphetamine diuresis. These results suggest that by inhibiting the late hyperphagic response U50 enhances the anorectic effects of AMPH, but that dopamine probably has no direct role in this interaction.

Long-continuous observation of the effects of methamphetamine on wheel-running and drinking in mice

1. Effects of methamphetamine (MAP) on wheel-running and drinking in mice, housed under 12-hr light-dark schedule (light period; 06:00-18:00), were investigated through long-continuous observation. 2. MAP (1, 2 and 4 mg/kg, s.c.) acutely increased the wheel-running and drinking for 2-4 hr in a dose-dependent manner after the administration at 11:00, midpoint of light period. 3. MAP administered at 11:00 sub-acutely suppressed the spontaneous increment during dark period (18:00-06:00) in both the behaviors. 4. Many factors, such as the time-of-day and interval of the administration as well as the dose administered, affected the behavioral suppression induced by MAP. 5. In addition to these findings, the wheel-running and drinking during the light period increased even on the days without MAP administration. 6. These results suggest that MAP have not only acute stimulant and sub-acute depressant effects, but also long-lasting effects.

Tolerance to the anorectic effect of dexfenfluramine in rats: role of serotonin, cholecystokinin, and neuropeptide Y

Tolerance to the anorectic effect of dexfenfluramine (DFEN) is shown in rats treated either chronically with low doses or once with a high dose of the agent. Rats given DFEN regimens that result in complete loss of DFEN anorexia showed no change in the anorexia caused by peripheral injection of cholecystokinin (CCK). The orexigenic effects of metergoline and neuropeptide Y were also unaltered as a function of DFEN pretreatment. Both the magnitude and duration of tolerance to a test dose of DFEN seemed to depend, in part, upon contingent (situational) factors, and were independent of whether brain serotonin (5-HT) measures were either unaffected or decreased by the DFEN pretreatment (low and high doses, respectively).

Environment-specific reinstatement of amphetamine-mediated hyperdipsia by morphine and (-)-norpseudoephedrine

In a study designed to determine whether environmental and pharmacological stimuli have the ability to take control of amphetamine-mediated hyperdipsia, rats were injected with d,l-amphetamine (AMPH; 4 mg/kg, IP) alone or in combination with (-)-norpseudoephedrine (NPE; 10 mg/kg, IP) and then returned to the home cage or transferred to a distinct environment (test cage). Water intake was measured hourly for 3 h, in the absence of food. AMPH treatment lasted for 10 days, followed by a 6-day extinction phase during which AMPH, but not NPE, injections were discontinued. Subsequently, all animals received challenge injections: NPE (10 mg/kg) on day 17; AMPH (4 mg/kg) on day 19; and morphine (MOR; 1 mg/kg) on day 21. AMPH-mediated hyperdipsia developed in 50% of animals and had an early onset in the home cage. NPE prevented the AMPH effects. Discontinuation of AMPH treatment promptly normalized drinking in the home cage but increased it further in the test cage. Within 6 days of AMPH discontinuation, hyperdipsia completely disappeared. It was reinstated, in the test cage alone, by a challenge injection of NPE or MOR. We suggest that hyperdipsia is a primary AMPH effect, which in some way is counter-acted by a distinct environment. This appears to elicit a compensatory mechanism that is revealed in the absence of AMPH and is reinstated in a nonspecific way by pharmacological stimuli.

Physiological and environmental aspects of drinking stimulated by chronic exposure to amphetamine in rats

1. To examine whether the hyperdipsic response to chronic administration of d,l-amphetamine (AMPH) is associated with modification of salt appetite, rats were allowed to choose between tap water and a 1.7% NaCl solution. 2. Under AMPH rats preferred water to saline throughout the experiment. 3. By testing rats in a distinct test cage environmental influences on AMPH-mediated hyperdipsia were also evaluated. 4. In the test cage hyperdipsia was suppressed, but preference for tap water was preserved. 5. Finally, the role of alpha 2-adrenoceptors in the drinking response to AMPH was evaluated by studying the effects of clonidine and yohimbine on water intake. 6. We conclude that AMPH-induced preference for tap water over saline is unrelated to hyperdipsia but, being also induced by yohimbine, it may depend on noradrenergic mechanisms.

Enhancement of the prophagic but not of the antidipsogenic effect of U-50, 488H after chronic amphetamine

Two groups of rats were treated with seven daily injections of either saline or d-amphetamine (3 mg/kg IP). On the 2 days following the last injection, rats were tested according to a counterbalanced experimental design, each animal receiving, immediately prior to the beginning of the dark phase, saline on one day and the highly selective kappa-opioid agonist trans- +/- 3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzene- acetamide methanesulfonate hydrate [U-50,488H (U50)] on the other. A microcomputer-controlled data acquisition system was used for the structural analysis of the feeding and drinking responses to amphetamine and U50. U50 enhanced feeding and depressed drinking in the first hour. The increased food intake was probably the result of the effect of U50 on the development of satiation and duration of satiety. Chronic amphetamine potentiated the prophagic effect but not the antidipsogenic effect of U50. The structural analysis demonstrated that the characteristics of the prophagic effect of U50 were amplified but not changed.

The α1-blocker dapiprazole inhibits diuresis but not drinking and feeding induced by U-50,488H

To further explore the interaction between opiates and catecholamines in the control of water balance, we studied the effects of the alpha 1-adrenoceptor antagonist dapiprazole on the modifications in drinking and diuresis produced by U-50,488H (a selective kappa-opiate agonist), morphine, naloxone, and amphetamine in rats. Because animals were maintained in a free-feeding paradigm and water intake is also controlled by feeding (prandial drinking), food intake was also measured. At doses administered (3-6 mg/kg, IP), dapiprazole had no effect on basal food and water intake or on diuresis. Nor did it modify changes in feeding and drinking produced by U-50,488H, morphine, naloxone, and amphetamine. It did, however, antagonize the diuretic effect of both U-50,488H and amphetamine. In addition, suppression of diuresis was obtained by combining doses of dapiprazole and morphine or naloxone that were devoid of antidiuretic effects when administered independently. A further experiment showed that diuresis produced by water load was also prevented by dapiprazole. alpha 1-Adrenoceptors thus appear to play a role in the regulation of water balance in a condition of free access to water, inhibiting diuresis without affecting drinking.

The kappa-opioid U-50,488H suppresses the initiation of nocturnal spontaneous drinking in normally hydrated rats

The effect of a systemic (IP) treatment with 1.0, 3.0 and 9.0 mg/kg U-50,488H (U50), a highly selective kappa-agonist, on spontaneous, nocturnal ingestive behavior of the rat was studied using a microcomputer controlled data acquisition system. The latency to initiate drinking was increased and drinking behavior was suppressed in the first hour after injection in a dose-dependent manner. The consummatory indices of drinking were not affected. After this period of adipsia, a phase of polydipsia, that was probably due to the diuretic effect of U50, was evident. This prophagic effect of U50 was evident only at the dose of 3 mg/kg and was accompanied by an increased duration of feeding episodes but not by a reduced latency to feed. These results suggest that kappa-receptors play a pivotal role in modulating spontaneous drinking in the normally hydrated rat and that this control is mainly exerted on the motivational aspect of drinking.

Opiatergic modulation of preparatory and consummatory components of feeding and drinking

We present data here indicating that stimulation of kappa but not mu opiate receptors influences motivational and consummatory aspects of feeding and drinking. To differentiate mu and kappa mechanisms controlling preparatory (appetitive) and consummatory components of ingestive behavior, the effects of morphine (MORPH), compound U50488H (U50) and naloxone (NAL) were studied in rats trained to negotiate a straight runway using food or water as a reinforcer. At doses that increase feeding and drinking in conditions of free access to food and water (i.e., 1-2 mg/kg IP), MORPH affected neither food- nor water-maintained runway performance. Since 1 mg/kg of NAL is also devoid of effects, mu-opiate mechanisms are probably not involved in food- or water-maintained behavior. Pharmacological manipulation of kappa-opiate mechanisms had complex effects. At 5 mg/kg, NAL accelerated satiation, depressing food intake, without affecting running. U50 did not increase food intake, but accelerated running for food, an effect that was antagonized by a high dose of NAL (5 mg/kg). These findings suggest that motivational and consummatory components of food-maintained runway performance are both activated by kappa-opiate mechanisms. NAL also reduced water intake but had minimal influences on running. In contrast, U50 depressed both water intake and runway performance; rather than being antagonized, these effects were slightly enhanced by NAL. The combined antidipsic and diuretic effects of U50 suggest that kappa-opiate mechanisms play a dissipatory role in water balance. However, the similar antidipsic effects of U50 and NAL, and the fact that NAL did not antagonize the antidipsic effects of U50, suggest that U50 may reduce drinking by mechanisms other than kappa-opiate agonism.

Dapiprazole, a selective alpha-1 adrenoceptor antagonist, inhibits diuresis but not polydipsia produced by amphetamine in rats

Chronic amphetamine administration has been found to produce diuresis and polydipsia in rats. We have found that dapiprazole acutely suppresses the diuretic, but not the ingestive, effects of amphetamine. To see whether diuresis is the physiological stimulus driving amphetamine-mediated polydipsia, we injected rats daily with d,l-amphetamine and the alpha-1 adrenergic antagonist dapiprazole. Throughout 19 days of treatment, dapiprazole completely prevented the increased urine output produced by amphetamine, but did not affect the development of polydipsia. This finding rules out a renal site of the primary action for amphetamine-mediated polydipsia and proposes water and electrolyte imbalance produced by chronic amphetamine administration as a model of the polydipsia and hyponatremia that develop in some psychotic patients.

Chronic systemic administration of amphetamine increases food intake to morphine, but not to U50-488H, microinjected into the ventral tegmental area in rats

Chronic intermittent administration of amphetamine (AMPH) sensitizes rats to the stimulatory effects on feeding produced by systemic injections of either morphine (MORPH) or the kappa-opiate receptor agonist, U50,488H (U50). Both morphine and the putative kappa-receptor endogenous ligand, dynorphin, have been reported to stimulate feeding when administered into the ventral tegmental area (VTA). To evaluate whether the VTA is the site where AMPH produces sensitization to the feeding effects of opiates, rats were given daily IP injections of either saline or AMPH (3 mg/kg). The amount of powdered food ingested during the 5 h following the injections was measured. After 9 days of AMPH or saline administration, twice weekly tests were begun of the effects of either saline, MORPH (1-10 nmol) or U50 (10 pmol to 10 nmol) injected into the VTA; AMPH administration was continued on intervening days. MORPH produced a statistically significant greater increase in food intake in rats chronically treated with AMPH than in saline treated rats. No statistically significant effects were produced by U50. However, when U50 was administered systemically to the same animals, food intake increased, and the effect was significantly greater in the AMPH-pretreated group. Thus the sensitization to the feeding effects of MORPH and U50 produced by chronic AMPH administration appears to involve different systems; the mesolimbic dopamine system appears to mediate sensitization to the effects of the predominately mu-receptor agonist, MORPH, but not of the kappa-receptor agonist, U50.

Pharmacological properties of the stimulant khat

The chewing of the stimulant leaf khat is a habit that is widespread in certain countries of East Africa and the Arabian peninsula. During the last decade, important progress has been made in understanding the pharmacological basis for the effects of khat. It is now known that the CNS action of this drug is due to the presence of the alkaloid cathinone, and the results of various in vitro and in vivo experiments indicate that this substance must be considered a natural amphetamine. It is the purpose of the present review to describe briefly the khat habit and to summarize the pharmacology of khat and of its active constituents.

Endogenous opiates: 1988

This paper is the eleventh installment in our annual review of the research during the past year involving the endogenous opiate system. It is concerned with nonanalgesic and behavioral studies of the opiate peptides that were published during 1988. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic functions; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical activity; locomotor activity; sex, pregnancy, and development; immunology and cancer; and other behavior.

Khat consumption: a pharmacological review

The present review deals with the considerable body of evidence gathered in the last ten years on the clinical and experimental pharmacology of Khat. Khat effects are generally agreed to be of amphetamine-like type. In particular, Khat ingestion, like amphetamine ingestion, produces sympathetic activation, anorexia, euphoria, increased intellectual efficiency and alertness. These effects are mainly mediated by phenylalkylamines, such as cathinone and cathine, because the pharmacological actions of these agents and those produced by amphetamine almost overlap. In infra-human species cathinone is an effective positive reinforcer (i.e., it maintains self-administration). However, it would be inappropriate to infer from cathinone and cathine effects assessed in animals a high potential of abuse for Khat in humans; apart from other reasons the bulk volume of Khat leaves, limits the ingestion of high quantities of the active principles. Accordingly, in habitual consumers Khat dependence is probably mild, because craving and tolerance to the sympathomimetic and neuroendocrine effects of Khat are present, but there is no definite abstinence syndrome. Therefore, in our opinion, policies restricting the use of Khat should be adopted with caution, lest they simply change the pattern of drug abuse and increase the spread of more dangerous drugs.