Alteration of the disruptive effect of fenfluramine on food consumption in the rat by repeated post-session administration of d-amphetamine

The purpose of this study was to determine whether repeated treatment (15 days) with d-amphetamine (AMP) or fenfluramine (FEN), administered after a daily 3 h feeding session (e. g. post-session), would result in tolerance or crosstolerance to the decrement in food consumption induced by treatment with either drug before feeding (e. g. pre-session). Groups of male rats were treated IP with 0.5 ml saline, 1.0, 2.0, or 4.0 mg/kg AMP, or 2.5, 5.0, or 10.0 mg/kg FEN prior to a 3 h feeding session. For the next 15 sessions, the respective groups were treated post-session with saline (0.5 ml), AMP (4.0 mg/kg), or FEN (10 mg/kg). Following the 15 day postsession phase, each group again received this pre-session treatment. The initial pre-session treatment with all dosages of these two drugs produced a significant decrease in food consumption. Tolerance to the food intake suppressant effect of FEN, but not AMP, resulted from repeated post-session treatment with the same agent. Repeated post-session treatment with AMP resulted in a significant decrement in the suppressant activity of FEN on food intake, whereas the corresponding post-session treatments with FEN did not alter the pre-session effects of AMP, except for an enhancement seen with higher AMP doses.     

The discriminative stimulus and subjective effects of d-amphetamine,phenmetrazine and fenfluramine in humans

The discriminative stimulus (DS) and subjective effects of d-amphetamine (AMP), phenmetrazine (PMT) and fenfluramine (FFL) were studied in a group of normal healthy adults. Subjects (N=27) were trained to discriminate between placebo and 10 mg AMP (PO). Fourteen of the subjects (discriminators) reliably learned the discrimination, whereas the other 13 did not. Nearly all discriminators labelled AMP as a stimulant, and AMP, relative to placebo, increased ratings of drug liking and general activity level, and produced typical stimulant-like subjective effects, as measured by the Profile of Mood States, the Addiction Research Center Inventory, and a series of visual analog scales. The discrimination accuracy of discriminators increased as a function of hour after drug ingestion, as did analog ratings of how certain subjects were that their discrimination responses were correct. Discriminators were tested with doses of PMT (25 and 50 mg) and FFL (20 and 40 mg) to determine whether the DS properties of these drugs would substitute for those of AMP. Both doses of PMT consistently substituted for AMP, and PMT produced subjective effects very similar to those of AMP. Conversely, neither dose of FFL consistently substituted for AMP, and FFL produced essentially no subjective effects. These findings are consistent with results from discrimination studies with other species, and provide further evidence of the validity of this procedure for studying the DS properties of drugs in humans. Offprint requests to: L.D. Chait     

Differential effects of chlordiazepoxide and d-amphetamine on responding maintained by a DRL schedule of reinforcement

Rats pressed a lever and obtained food pellets on a schedule of differential reinforcement of low rate (DRL) which required that responses were spaced at least 15 sec apart in order for them to produce reinforcement. When responding had stabilised at slow and steady rates the effects of d-amphetamine and chlordiazepoxide were assessed. Low doses of both drugs increased response rates while higher doses decreased them. Reinforcement frequency showed a dose related decrease after both drugs. When interresponse times (IRTs) were analysed it was found that both drugs shifted the peak of the distribution towards shorter IRTs but that chlordiazepoxide also produced a specific increase in the percentage of responses after very short IRTs (bursts). When IRTs were divided into those following a reinforced response (hit) and those following a non-reinforced response (miss) it was found that bursts normally followed only misses and chlordiazepoxide consistently increased the number of bursts following misses only. Amphetamine did not affect bursts in any consistent way.     

Effect of two weeks' treatment with chlorimipramine and nortriptyline,alone or in combination with alcohol,on learning and memory

Mature male Wistar rats were given d-amphetamine sulphate (200 mg/l in the drinking water for a period of three weeks. The drug was then withdrawn and the rats were killed 12, 24, 36 and 48 h later. Pronounced behavioural depression was observed 12 h after the withdrawal of amphetamine; 24 h after withdrawal, behaviour was substantially normal but depression recurred at 36 h. Recovery appeared to be complete after 48 h. Fluorimetric determinations showed that noradrenaline and 5-hydroxytryptamine concentrations were reduced by the chronic administration of amphetamine in the cortex, hippocampus, thalamus/hypothalamus and mid-brain. Noradrenaline concentrations were also reduced in the pons/medulla. Twelve and 36 h after withdrawal, there was a further reduction in noradrenaline concentrations in the cortex, hippocampus and mid-brain; noradrenaline concentrations in the striatum were also reduced. 5-Hydroxytryptamine concentrations in the cortex and striatum were lower 12 and 36 h after the withdrawal than during chronic amphetamine treatment; 36 h after withdrawal, concentrations in the hippocampus and the pons/medulla were also lower than during drug treatment. There were also changes in the concentrations of 5-hydroxyindoleacetic acid and normetanephrine.     

Effects of d-amphetamine and ethanol alone and in combination on schedule-controlled responding of pigeons

Key pecking by pigeons was maintained under either a 5-min fixed-interval or a 30-response fixed-ratio schedule of food delivery. d-Amphetamine (0.1–1.0 mg/kg) either increased or did not affect overall rates of responding under the fixed-interval schedule; the lowest dose of ethanol (0.5 g/kg) did not affect or slightly decreased response rates, whereas higher doses (1.0–2.0 g/kg) substantially decreased rates. Combinations of low noneffective ethanol doses with most doses of d-amphetamine increased rates of responding under the fixed-interval schedule above those obtained with d-amphetamine alone; decreases produced by the higher doses of ethanol were attenuated by most doses of d-amphetamine. Doses of d-amphetamine (0.1–1.0 mg/kg) and ethanol (0.5–1.5 g/kg) alone generally had no effect on responding maintained under the fixed-ratio schedule; higher doses of these drugs decreased responding. The effects of dose combinations other than the highest ones generally differed little from those obtained with ethanol alone; the effects of high doses of each drug were antagonized by low to moderate doses of the other. Combinations of ethanol with d-amphetamine can result in higher rates of responding than are obtained with either drug alone. Further, effects of the drugs alone and in combination depend on the schedule under which behavior is maintained.     

Intraspecies aggression in rats: Effects of d-amphetamine and chlordiazepoxide

d-Amphetamine sulfate and chlordiazepoxide hydrochloride, administered to either the dominant or subordinate rat, altered several components of fighting behavior in a dose-dependent biphasic manner. Stereotypic sequences of attack, threat, defense, and submission were generated between pairs of previously isolated Sprague-Dawley rats by extinction of a food-reinforced response. Low doses of amphetamine (0.05, 0.1 mg/kg) and chlordiazepoxide (2.5, 5.0 mg/kg) given i.m. to the dominant rat 30 min prior to 15 min tests increased attack bites and leaps and the display of aggressive postures and threats, whereas higher doses of both drugs (0.5, 1.0 mg/kg amphetamine; 20 mg/kg chlordiazepoxide) suppressed attacks and threats. Amphetamine and chlordiazepoxide, administered to the subordinate rat, caused a more prolonged display of submissive-supine and defensive-upright postures; chlordiazepoxide (10.0, 20.0 mg/kg) prolonged immobile crouching whereas amphetamine (0.5, 1.0 mg/kg) greatly reduced this response. Drugged subordinate rats also provoked more attacks and threats by the non-drugged opponents. The multi-response analysis of fighting reveals that various elements of aggressive and defensive-submissive behavior patterns are differentially sensitive to drug action. The results indicate that amphetamine and chlordiazepoxide can facilitate or inhibit attack or defense depending on the dose level and which of the opponents was injected, but do not reverse dominance-subordination relationships.     

Some effects of chlordiazepoxide and d-amphetamine on response force during punished responding in rats

Rats were reinforced with water on a continuous reinforcement schedule and were also punished with electric shock for every fifth response applied to a silent, isometric, force-sensing manipulandum. Oral doses of chlordiazepoxide (3.0, 9.0, 27.0 mg/kg) increased both conventional rate and force of punished responding. In contrast, d-amphetamine (0.8, 1.6, 3.2 mg/kg, by gavage) further decreased conventional rate and force of response, but this latter drug increased the rate of recorded responses that were lower than the 15-g force criterion for response consequences. The results for chlordiazepoxide are viewed in terms of its anxiolytic properties, while the d-amphetamine data appear to support a theory of amphetamine effects based on the concept of stereotyped behaviors.     

Effects of chlordiazepoxide,ripazepam and d-amphetamine on conditioned acceleration of timing behaviour in rats

The lever-pressing behaviour of three rats was maintained by a schedule in which food reinforcement was obtained by any response which was emitted at least 15 s after the previous response (DRL 15 s). When performance on this schedule had stabilised, the animals were presented intermittently with 1-min periods of a white noise stimulus, the termination of which was accompanied by the delivery of a mild electric footshock. This procedure led to reliable increases in response rates during the stimulus although responding at other times continued to be appropriate to the DRL 15-s schedule. Administration of the benzodiazepine chlordiazepoxide (1, 3, 10, 17 and 30 mg/kg) and of ripazepam (1, 3, 10, 30 and 56 mg/kg), a non-benzodiazepine reported to have anxiolytic properties, increased response rates on the DRL baseline while decreasing the acceleration of responding produced by the preshock stimulus. Baseline response rates were also increased by d-amphetamine (0.25, 0.5, 1.0 and 2.0 mg/kg) and at the higher doses this drug completely abolished the accelerated responding during the preshock stimulus. Although the effects of chlordiazepoxide and ripazepam are consistent with the suggestion that these drugs may attenuate the behavioural effects of aversive stimuli, in this experiment the behavioural effects of d-amphetamine were similar in many respects.     

Interactions of chlordiazepoxide and anorectic agents on rate and duration parameters of feeding in the rat

d-Amphetamine (0.5 and 1.0 mg/kg) and dl-fenfluramine (2.0 and 4.0 mg/kg) reduced food intake in a short exposure feeding test, and their effects were counteracted by chlordiazepoxide, particularly at 5.0 mg/kg. Chlordiazepoxide reduced latency to eat, extended the duration of feeding and depressed the rate of feeding. Antagonism occurred in combination with d-amphetamine in relation to latency and duration, but in this experiment d-amphetamine did not affect feeding rate. Antagonism also occurred in combination with fenfluramine, with latency and duration measures, but only at the lower chlordiazepoxide dose. Instead, chlordiazepoxide (10.0 mg/kg) enhanced fenfluramine's effects to reduce feeding duration and feeding rate.     

Effects of morphine alone and in combination with naloxone or d-amphetamine on shock-maintained behavior in the squirrel monkey

Key-pressing behavior in the squirrel monkey was maintained under an 8-min fixed-interval (FI) schedule of electric-shock delivery. The acute i.m. administration of morphine prior to a daily session decreased response rates at doses of 1.0–3.0 mg/kg but had little systematic effect on rate at doses of 0.03–0.3 mg/kg. When naloxone was administered concomitantly with morphine prior to a session, 0.01 mg/kg naloxone required a three-fold increase in the dose of morphine necessary to obtain decreased response rates, 0.1 mg/kg naloxone required a 30-fold increase in morphine, and 1.0 mg/kg required more than a 30-fold increase in morphine. Moreover, the administration of naloxone with morphine resulted in increased rates of responding at certain combinations of doses of the two drugs. The administration of d-amphetamine (0.03 or 0.1 mg/kg) alone increased mean response rates under the FI schedule; when combined with 0.03–0.3 mg/kg morphine the increases in responding were greater than obtained with d-amphetamine alone. The negative slope of the linear regression lines relating the effects of morphine to control rates of responding engendered under the FI schedule was decreased when morphine was combined with naloxone, but not with d-amphetamine. These results show that naloxone, but not d-amphetamine, can antagonize the response-rate decreasing effect of morphine when responding in the squirrel monkey is maintained by response-produced electric shock.     

Blockage of progesterone-induced release of luteinizing hormone and prolactin byd-amphetamine and fenfluramine in rats

Subcutaneous administration ofd-amphetamine at various doses (1.25, 2.5, and 5 mg/kg) decreased plasma luteinizing-hormone levels in ovariectomized rats primed with estradiol and injected with progesterone. In these animals prolactin levels decreased after injection of 0.6 and 1.25 mg/kg ofd-amphetamine. No significant hormone modifications were detected in ovariectomized and ovariectomized estradiol-primed rats after injection of 2.5 mg/kg ofd-amphetamine. Fenfluramine at doses of 25 mg/kg induced decreases of plasma LH and prolactin levels in ovariectomized estradiol-and progesterone-treated rats. A low dose of fenfluramine, 2.5 mg/kg, had no effect.It is concluded thatd-amphetamine and fenfluramine are able to alter the facilitatory actions of progesterone on luteinizing hormone and prolactin release in ovariectomized estradiol-primed rats.National Scientific Research Council of Argentina (CONICET) investigator     

Interaction effects of d-amphetamine treatment and ambient temperature on rat's food intake

The thermoregulatory theory of hunger posits that rats placed in a cold environment should increase the amount of food intake, while rats placed in a hot environment should decrease their food intake. d-Amphetamine causes hyperthermia among rats kept at warm ambient temperature, and results in hypothermia among animals kept in a cold environment. d-Amphetamine-caused-hyperthermia should therefore result in decreased eating behavior, and d-amphetamine-caused hypothermia should result in increased eating behavior. One must take into account the fact that d-amphetamine is an anorexic drag. The interaction between (a) ambient temperature, (b) body temperature, and (c) food intake were tested on groups of rats injected with various doses of d-amphetamine (1.5–15 mg/kg) and placed in ambient temperatures ranging from 4–37C. No increase in food intake was revealed under any dosage or temperature condition. The decrease in food intake found with d-amphetamine treated animals could not be explained in the thermoregulatory theory of hunger. Our data indicate that d-amphetamine anorexic effects and thermal effects are mediated by different mechanisms.     

Effects of chlordiazepoxide,oxazepam, chlorpromazine,andd-Amphetamine on sexual responses in male and female hamsters

The acute effects on sexual behavior of oxazepam (16–64 mg/kg), chlordiazepoxide (8–64 mg/kg), chlorpromazine (2–8 mg/kg), andd-amphetamine (0.8–3.2 mg/kg) were examined in intact male and female golden hamsters (Mesocricetus auratus). Intraperitoneal injections were given 45 min before the first behavioral test. In 10-min tests lordosis was observed in estrous females both before and after copulation, and mounts, intromissions, and ejaculations were observed in males. Dose-response related decrements in male sexual behavior were observed following chlorpromazine and chlordiazepoxide. All dose levels of oxazepam depressed male sexual behavior. The highest dose of chlordiazepoxide and oxazepam attenuated the onset of female sexual behavior, and all dose levels reduced postcopulatory lordosis durations. Amphetamine did not interrupt either male or female sexual behavior, and chlorpromazine disrupted male but not female behavior.     

The effects of d-amphetamine,chlordiazepoxide and alpha-flupenthixol on food-reinforced tracking of a visual stimulus by rats

Rats were trained to respond to one of two levers under a random ratio schedule of food reinforcement. Which of the levers was correct was redetermined before each response and signalled by a light. The effects of d-amphetamine (0.2–3.2 mg/kg), chlordiazepoxide (1–8 mg/kg), and the neuroleptic alpha-flupenthixol (0.03–0.33 mg/kg) on the efficiency of rats tracking this visual cue were examined. d-Amphetamine increased the proportion of responses made on the correct lever at low and intermediate doses, but reduced the proportion at 3.2 mg/kg. At the highest dose, chlordiazepoxide produced a small increase in this measure, together with a reduction in response rate, but alpha-flupenthixol had no effect, even at a dose reducing response rate. Low doses of amphetamine also increased switching between the levers, producing a proportionately greater increase in switching from the correct lever to the incorrect lever than vice versa. The results are interpreted as showing that d-amphetamine facilitates tracking performance as a result of its action of enhancing response switching, and supporting the hypothesis that facilitation of performance by amphetamine-like drugs depends on the effect of the drug on response output coinciding with task requirements.     

The effects of chronic treatment with d-amphetamine on food intake,body weight,locomotor activity and subcellular distribution of the drug in rat brain

Female Wistar rats were treated chronically with d-amphetamine sulphate in drinking water. The concentrations of amphetamine were 0.01%, 0.02%, 0.03%, 0.04% and 0.05% in the 1st, 2nd, 3rd, 4th, and 5th week of treatment. The consumed doses of amphetamine increased from 16 mg/kg on the first day up to 90 mg/kg on the 36th day of treatment. The effects of chronic treatment with amphetamine on food intake, body weight and locomotor activity of rats were determined. The rats developed tolerance to the overall toxicity and to the anorexigenic effect of maphetamine. No tolerance to the effects of the drug on body weight and locomotor activity was observed. The concentration of H³-d-amphetamine in brains of chronically treated rats is significantly higher than in controls. No difference in the pattern of distribution of radioactivity among the subcellular fractions of rat brain was observed between control and chronically treated groups. The relationship between developmen tof tolerance and the concentration of amphetamine in the brain is discussed.     

Discriminative stimulus properties of fenfluramine: Evidence for serotonergic involvement

Rats were trained to discriminate 3 mg/kg fenfluramine (FEN) from saline using a milk-reinforced (FR 10 schedule) two-lever operant task. To assess the involvement of the serotonin (5-HT) system in elicitation of the FEN cue, 5-HT compounds were tested for their ability to substitute for or to antagonize the the discriminative stimulus produced by FEN. Following acquisition, the FEN cue was dose-dependent, had a rapid onset (10 min) and a long duration (12 h), and was stereospecific. The putative 5-HT receptor antagonists methysergide and cinanserin antagonized the FEN discriminative stimulus, while chlordiazepoxide, an indirect inhibitor of 5-HT turnover, did not. The FEN cue was also antagonized by the selective 5-HT reuptake inhibitor fluoxetine. Norfenfluramine, p-fluoroamphetamine, and p-chloroamphetamine, compounds structurally and pharmacologically similar to FEN, substituted for FEN, whereas fluoxetine cinanserin, methysergide, and chlordiazepoxide did not. The 5-HT precursor 5-hydroxytryptophan partially generalized to the FEN cue. It was further shown that the discriminative stimulus properties of FEN are not based on its anorectic action. These results suggest that the cue properties of FEN might be partially mediated through an interaction with the 5-HT system.     

Food deprivation and stimulant self-administration in rats: differences between cocaine and d-amphetamine

The effects of food deprivation (24 h) on response rates of rats self-administering d-amphetamine and cocaine were compared. Food deprivation clearly increased rates of responding for both drugs but did so to a significantly greater extent for cocaine than for d-amphetamine. Consistent with other findings, the results suggest that the neural substrates underlying cocaine and d-amphetamine reinforcement are not identical.     

Independence of the effects of d-amphetamine and food deprivation or body weight on the food consumption of rats

Two experiments studied the combined effects of absolute doses (milligrams per rat) of d-amphetamine with either food deprivation (Experiment I) or body weight (Experiment II) on the food consumption of rats. In the first experiment, each of 16 rats was tested once under every combination of three levels of food deprivation (0, 12, and 24 h) and three levels of drug (0, 0.33, and 1.0 mg per rat). Food consumption was measured over 2.5-h sessions that began 30 min after injection. Increasing doses of d-amphetamine and decreasing levels of food deprivation both decreased food consumption, but the effects of the two variables were additive with nonsignificant interaction.In the second experiment, four groups of four rats each, differing in age and body weight, were studied. Under 48-h food deprivation, each rat received each of four doses of d-amphetamine (0, 0.27, 0.54, and 0.81 mg). Food consumption was measured over a 2.5-h period that began 30 min after injection. Increasing doses of d-amphetamine and decreased body weights decreased food consumption, and the combined effects of these two variables were additive. The lack of an interaction between dose and differing body weights in these experiments indicates that the generally-accepted mode of administering d-amphetamine, as milligrams per kilograms of body weight, does not equate rats of differing body weights.This reserch was supported by USPHS reserch grant MH-01604 from the National Institutes of Mental Health. We thank Smith, Kline and French Laboratory for supplying the d-amphetamine used in this study. Robert C. MacPhail is now at the Department of Pharmacology, The University of Chicago, Chicago, Illinois 60637.     

Behavioural studies on the effects of d-amphetamine and estradiol benzoate alone and in combination

The effects of d-amphetamine sulphate on grooming, rearing, and ambulatory behaviour in the T-maze was studied in rats. Low doses (0.25–2.0 mg/kg) produced a dose-dependent change in behaviour; ambulatory behaviour was increased while grooming and rearing behaviour was decreased. The results suggest that the behavioural changes are a direct effect of amphetamine rather than a secondary consequence of a competition between different types of behaviour.The effects of d-amphetamine sulphate and/or estradiol benzoate on grooming, rearing, and ambulatory behaviours in the T-maze and open field were also studied. Rats chronically treated with estradiol or oil were injected with amphetamine or saline just prior to evaluation in the maze or open field. Amphetamine treatment, irrespective of environment or hormone treatment, stimulated ambulatory behaviour while inhibiting grooming behaviour. Estradiol specifically antagonized the amphetamine induced reduction of grooming in the maze only. The results suggest that amphetamine has an independent action on T-maze behaviour whereas estradiol has an effect that depends on the environment for its manifestation.     

Feeding stimulated by very low doses of d-amphetamine administered systemically or by microinjection into the striatum

The effects of d-amphetamine over a wide range of doses (0.125–4.0 mg/kg IP) on rat unconditioned behaviour were examined in the presence of food and water (experiment 1), in their absence (experiment 2) and after microinjection (2.0 g in 0.5 l) directly into the striatum (experiment 3). In experiment 1 very low doses (0.125 and 0.25 mg/kg) stimulated the intake of food, but not water, and higher doses produced locomotor hyperactivity, rearing, stereotyped sniffing and anorexia. In experiment 2 all doses, including very low doses (0.125 and 0.25 mg/kg), significantly potentiated locomotor activity. In experiment 3, microinjection into the corpus striatum elicited substantial feeding, but not drinking, locomotor activity or stereotyped behaviour. The results suggest that a single graded facilitative mechanism underlies the effects on food intake and other behavioural effects of amphetamine, as implied by a general hypothesis of amphetamine action proposed in the literature, and that these effects may to a large extent by mediated by forebrain dopamine systems.