On the central mediation of anorexigenic drug effects

Effects of indorenate on food intake: a comparison with fenfluramine and amphetamine

Indorenate (TR3369, 5-methoxytryptamine b-methylcarboxylate HCl) is a 5-HT1-like receptor agonist with hypotensive activity. Here, we describe that indorenate also decreases food intake (ED50 26.1 mg/kg) without an appreciable effect in water intake (the estimated ED50 for water was 589.8 mg/kg). The anorectic activity of indorenate was compared to the effects of amphetamine and other serotonin agonists; the effect of indorenate was smaller than those of the other compounds; however, the effect of indorenate was specific to food, whereas all the other drugs also produced significant decrements in water intake. The serotonin antagonists cinanserin, cyproheptadine, methergoline and methysergide effectively prevented the decrease in food intake produced by indorenate and fenfluramine. Haloperidol, a dopaminergic antagonist, was ineffective in preventing the effect of indorenate although it prevented the anorectic effect of amphetamine. The present results suggest the participation of serotoninergic, but not dopaminergic mechanisms, in the decrease in food intake produced by indorenate.

Effects of haloperidol on anorexia induced by l-norephedrine and d-amphetamine in adult rats

Although amphetamine anorexia has been linked to activation of dopaminergic receptors within the lateral aspects of the hypothalamus, the receptor type by which phenylpropanolamine (PPA: the racemic mixture of d- and l-norephedrine) induces anorexia has not been identified. In the present experiment, separate groups of adult male rats were pretreated (IP) with either 0.9% saline or haloperidol (either 0.4 or 0.8 mg/kg) 45 minutes prior to treatment (IP) with either saline or 20 mg/kg l-NEP (the active enantiomer of PPA) and were then allowed 180 minutes access to food and water. Treatment with 20 mg/kg l-NEP induced comparable reductions in food intake of approximately 30% in rats pretreated with either dose of haloperidol or saline. In a sub-experiment, it was demonstrated that 1.0 mg/kg d-amphetamine sulfate reduced food intake by 25%, but this anorexic action was completely attenuated by 0.8 mg/kg haloperidol given 45 minutes prior to feeding. These results add to a growing body of literature that documents important differences between the mechanisms by which amphetamine and PPA produce their anorexic actions.

Food intake suppressant effect of baclofen in rats

1. Baclofen given intraperitoneally (i.p.) to rats caused a dose-dependent decrease in food intake. 2. Bicuculline or picrotoxin (GABAA-antagonist) and methergoline (5-HT antagonist) decreased the anorectic effect of baclofen. 3. Pimozide (dopamine receptor blocker), phenoxybenzamine and propranolol (alpha and beta adrenergic blockers) did not diminish the baclofen effect, but even increased the anorexia induced by the drug. 4. It can be postulated that, at least partially, GABAA receptor mechanism, GABA-5HT receptor complex and/or 5-HT mechanism may be involved in baclofen induced anorexia.

Behavioural microanalysis of the role of dopamine in amphetamine anorexia

A microstructural analysis paradigm was used to study amphetamine anorexia. Doses above 0.40 mg/kg significantly reduced food intake by reducing eating time; in contrast, eating rate was increased at these doses. Examination of the frequency distribution of interresponse times (IRTs) revealed a significant shift to shorter IRTs at doses as low as 0.125 mg/kg. Pimozide blocked amphetamine anorexia at 0.5 and 1.0 mg/kg, suggesting that at both doses amphetamine anorexia has a dopaminergic substrate. However, the atypical neuroleptic thioridazine did not antagonize amphetamine. Furthermore, effects of amphetamine were additive with those of apomorphine, administered at a dose known to suppress feeding by inhibiting mesolimbic DA neurons. These results provide evidence against an involvement of the mesolimbic DA system in amphetamine anorexia.

Neuropharmacology of drugs affecting food intake

The importance of the central monoamines NE, DA and 5-HT in ingestive behavior has inevitably resulted in considerable effort being expended in attempting to implicate these monoamines in the mechanism of action of anorectic drugs. The statements that amphetamine-induced anorexia is unlikely to be due to central serotoninergic systems and that central noradrenergic and dopaminergic systems are not implicated in the appetite suppressant effect of fenfluramine are in all probability correct. However, to attribute the ability of drugs to decrease food intake unequivocally to a specific effect on central monoaminergic systems is almost certainly an oversimplification, due to the fact that other putative neurotransmitters, such as GABA and peptides, play a critical role in eating. This can be achieved either directly or by modulating the release of other transmitters. An added complication in attempting to correlate a specific neurochemical process to a behavioral effect, such as anorexia, is the complexity of the central actions of the drug. At best, a predominant but not an exclusive process can be identified. Perhaps the in-built constraint of attempting to correlate a specific neurochemical effect to the desired action of a drug is accountable for the absence of a second generation of centrally acting anorectic drugs. Dramatic progress has been made in elucidating the factors involved in ingestive behavior over the last 5-10 years. This information should, and must, provide the catalyst for more efficacious anorectic drugs because obesity represents one of the few major diseases for which adequate drug therapy does not exist.

Central and peripheral contributions to the enhancement of amphetamine anorexia by desmethylimipramine (DMI)

Intrahypothalamic administration of amphetamine to rats increased food intake, but pre-treatment with the alpha-receptor antagonist phentolamine unmasked an anorexic effect commensurate with that seen after peripheral amphetamine administration. Pretreatment with systemic DMI increased anorexia after peripheral or central amphetamine administration, but the enhancement of centrally-induced anorexia was small. It is concluded that enhancement of the anorexic effect of peripherally administered amphetamine by DMI is primarily a peripheral phenomenon, with interactions within the central nervous system making a relatively minor contribution.

Determination of the course of brainstem catecholamine fibers mediating amphetamine anorexia

Previous studies suggest that brainstem catecholamine (CA) fibers which mediate amphetamine (AMPH)-induced anorexia ascend through the midlateral medical forebrain bundle and perifornical region and terminate in the perifornical hypothalamic region (PFH) at the level of the hypothalamic ventromedial nucleus. Through studies of wire-knife cuts (KCs) placed in the lower brainstem, the present paper further delineates the course of fibers mediating AMPH feeding suppression, as they ascend through the medullary, pontine and midbrain tegmentum. The results indicate that the crucial CA fibers ascend through the ventrolateral medulla just dorsal to the nucleus of the seventh cranial nerve, 1.1-1.9 mm lateral to midline. In their rostral course, these fibers apparently maintain a relatively straight position in the ventral pons and then enter the ventrolateral midbrain just dorsal to the medial lemniscus, between 0.7 and 1.1 mm lateral to midline. These medullary fibers, possibly originating from the norepinephrine/epinephrine-containing ventrolateral cell group (A1/C1), then appear to join additional fibers from the scattered dopamine-containing neurons positioned in the caudal midbrain (A8 CA cell group). Together, these dopamine, epinephrine and norepinephrine systems are believed to ascend into the medial aspect of the medial forebrain bundle on their way to the PFH at the level of the ventromedial nucleus.

The effect of d-amphetamine and haloperidol alone and in combination on milk drinking in rats

The effects of d-amphetamine (0.5-4.0 mg/kg IP) and haloperidol (0.015-1.0 mg/kg IP) alone and in combination on intake of a sweetened milk solution were determined in rats. Experimental sessions consisted of a 15-min access to the milk solution once a day, 7 days a week. d-Amphetamine was administered 15 min prior to the session and haloperidol was administered 60 min prior to the session. The interaction of these compounds was determined by repeated determination of the dose-response function for d-amphetamine in combination with different doses of haloperidol. When given alone, each drug produced a dose-dependent decrease in milk intake. In combination with haloperidol, the decrement caused by d-amphetamine was shifted to lower doses in a dose-dependent manner. Isobolographic analysis of the interaction indicates that the intermediate doses (0.03-0.06 mg/kg) of haloperidol were additive and the highest doses (0.12-0.25 mg/kg) of haloperidol were infraadditive with d-amphetamine. Dose-dependent neuropharmacological actions of these compounds may account for dose-dependent interactions.

Microstructural analysis of the involvement of beta-receptors in amphetamine anorexia

Rats were trained to take food by pushing the door of the pellet dispenser in an operant chamber. Log survivor analysis of the inter-response time frequency distribution was used to determine whether or not an animal was eating, at any time during a thirty minute session. This information was used to compute eating time, eating rate, and the mean length of bouts of eating and gaps between eating bouts. Video-recordings confirmed that the method discriminated eating from not eating with an accuracy of approximately ninety percent. Amphetamine (0.5 mg/kg) significantly reduced total food intake and eating time, and increased gap length; propranolol (5 mg/kg) significantly increased eating time and bout length. Following propranolol pretreatment, amphetamine significantly reduced eating time and bout length but also significantly increased eating rate; as a result there was no significant decrease in total food intake. The possible mediation of these effects by beta-adrenergic and dopaminergic systems is discussed.

Anorexigenic effects of two amines obtained from Catha edulis Forsk. (Khat) in rats

The anorexigenic effects of cathine (phenylpropanolamine) and cathinone (alpha-aminopropiophenone), both amines obtained from Catha edulis Forsk. (Khat) were investigated by acute and chronic experiments in rats. Amphetamine was included for comparison purposes. Both khat amines reduced food intake when administered acutely and body weight when given chronically. Cathinone was more effective than cathine, and both were less active than amphetamine. Partial or total cross-tolerance was observed among the 3 drugs.

The effect of propranolol phentolamine and pimozide on drug-induced anorexia in the mouse

Pimozide was a potent antagonist of (+)amphetamine, diethylpropion, mazindol and phentermine anorexia in the mouse. Phentolamine and propranolol produced no such antagonism, but either potentiated or had no effect on the drug-induced anorexia. Although the mechanism of action of the four anorectic agents appears to involve dopamine receptor agonist activity, an antagonist or partial agonist effect at noradrenergic receptors may also be involved.

Synergistic action of p-chloroamphetamine and fluoxetine on food and water consumption patterns in the rat

The distribution of eating, drinking and body weight changes during the 24 hr day were examined following brain 5-HT depletion with p-chloroamphetamine (PCA). Following a baseline period, measurements of food and water intakes and body weights were recorded 1, 2, 3, 6, 12, 20 and 24 hr following PCA, 5.0 mg/kg, or saline. Other animals were pretreated with fluoxetine, 10.0 mg/kg, prior to either PCA or saline in an attempt to block the PCA effects. The results indicate acute hypophagia, hypodipsia, and body weight losses. These decreases were not influenced by the time of day when PCA was administered. Pretreatment with fluoxetine enhanced rather than blocked these effects. No long term changes in ingestive behavior were seen. These results are discussed with respect to the possible role of 5-HT in the control of ingestive behavior.

Role of brain monoamines in the anorectic activity of mazindol and d-amphetamine in the rat

The interaction of mazindol and d-amphetamine with brain monoamines was studied in rats. At each dose used, both compounds markedly counteracted the decrease of brain noradrenaline induced by 6-hydroxydopamine while only at high doses they did significantly reduce the effect of 6-hydroxydopamine on brain dopamine. Unlike d-amphetamine, mazindol significantly counteracted the decrease of brain serotonin induced by fenfluramine. The anorectic effect of mazindol and of d-amphetamine was markedly reduced by an electrolytic lesion at the level of the ventral noradrenergic bundle but not by an electrolytic lesion of the nucleus raphe medianus. An intrastriatal injection of 6-hydroxydopamine significantly reduced the effect of mazindol but not that of d-amphetamine. The results indicate that both compounds may block noradrenaline uptake in the brain while their effect on dopamine uptake is less evident. Ulike d-amphetamine, mazindol appears to inhibit serotonin uptake also. In addition, the integrity of the noradrenergic neurons in the brain appears to be an important condition for these drugs to exert their anorectic effect.

Role of dopamine in the anorexigenic effect of DITA; comparison with d-amphetamine

The effects of d,1-alpha-methyltyrosine (alphaMT), haloperidol, phenoxybenazmine, propranolol, methysergide and cyproheptadine on the anorexigenic activities of DITA and d-amphetamine were studied in male mice. The pretreatment of mice with methysergide (10 mg/kg, s.c.), cyproheptadine (5 mg/kg, i.p.), phenoxybenzamine (10 mg/kg, i.p.), and propranolol (5 mg/kg, i.p.) failed to alter the anorexigenic effect of DITA and d-amphetamine. On the other hand, alphaMT (32 mg/kg, i.p.) and haloperidol (0.5 mg/kg, i.p.) significantly antagonized the anorexigenic effect of DITA and d-amphetamine. Our data indicate that the anorexigenic activities of DITA and d-amphetamine are mediated mainly through the dopaminergic system.

Neuroleptic-induced deficits in food and water regulation: similarities to the lateral hypothalamic syndrome

The role of central dopaminergic mechanisms in the regulation of food and water intake was assessed by examining the effects of haloperidol and pimozide on various measures of feeding and drinking in rats. Haloperidol (0.20 mg/kg) or pimozide (0.45 mg/kg) did not significantly affect 1-hr water intake in response to 24 hrs of water deprivation, nor did they influence 2-hr food intake after 24 hrs food deprivation. However both pimozide and haloperidol significantly reduced drinking in response to injections of hypertonic saline. In addition, animals pretreated with these drugs drank less than controls in the absence of food (a measure of "non-prandial" drinking), and drank less than controls when the water was adulterated with quinine (a measure of "finickiness"). These drugs also significantly reduced food intake in response to injections of insulin and attenuated amphetamine anorexia. These deficits are similar to those observed after electrolytic lesions of the lateral hypothalamus or after 6-hydroxydopamine lesions of the substantia nigra. Because haloperidol and pimozide block central dopaminergic receptor sites, the present findings are consistent with the hypothesis that part of the lateral hypothalamic syndrome is the result of damage to the dopaminergic nigro-neostriatal projection. Finally, the data suggest that the changes in feeding and drinking induced by haloperidol and pimozide reflect genuine homeostatic deficits rather than being due to a neuroleptic-induced motor dysfunction.