Effects of perifornical hypothalamic microinjections of phenylpropanolamine and amphetamine on latency to feed and mash intake in rats

Dual roles in feeding for AMPA/kainate receptors: receptor activation or inactivation within distinct hypothalamic regions elicits feeding behavior

We have previously shown that hypothalamic injections of glutamate, or agonists of its ionotropic receptors (iGluRs), elicit intense feeding responses in satiated rats [Brain Res. 613 (1993) 88, Brain Res. 630 (1993) 41]. While attempting to clarify the role of the AMPA and kainate (KA) receptor subtypes in glutamatergic feeding systems, we discovered that lateral hypothalamic (LH) injection of high doses of the competitive AMPA/KA receptor antagonist, NBQX (10 and 30 nmol), elicited a pronounced feeding response. We questioned whether this effect was due to inactivation of AMPA or possibly KA receptors. To determine whether other AMPA/KA antagonists can also elicit feeding, we tested whether injection of CNQX, another AMPA/KA receptor antagonist, also stimulates eating and whether these feeding stimulatory effects were due to antagonists' actions in the LH or in other hypothalamic sites. Here we report that NBQX and CNQX elicit feeding in a dose dependent manner and are most effective when injected into the perifornical hypothalamus (PFH), or into the paraventricular nucleus (PVN) and, to a lesser extent, into the LH of satiated rats. In contrast, AMPA was most effective in stimulating feeding when injected into the LH, confirming previous reports. These data suggest that either activation or inactivation of AMPA/KA receptors in distinct but overlapping hypothalamic sites may be sufficient to induce feeding behavior, indicating a broadened role for glutamate in hypothalamic feeding mechanisms.

Effects of systemic phenylpropanolamine and fenfluramine on serotonin activity within rat paraventricular hypothalamus

Phenylpropanolamine (PPA) anorexia has been linked to activation of alpha 1-adrenergic receptors within rat paraventricular hypothalamus (PVN) by studies documenting that intra-PVN injection of PPA and other alpha 1-adrenergic agonists suppress food intake. The present experiments examine the hypothesis that PPA may suppress appetite indirectly via release of serotonin (5-HT) within the PVN. In Experiment 1, we compare the effects of PPA (20 mg/kg, IP) and of d,l-fenfluramine (FEN: 7.5 mg/kg, IP), relative to a vehicle treatment, on extracellular levels of 5-HT and the 5-HT metabolite 5-HIAA in adult male rats prepared with concentric microdialysis probes aimed at either the PVN or the perifornical hypothalamus (PFH). Injection of either vehicle or 20 mg/kg PPA had no significant effect on extracellular 5-HT within the PFH or the PVN. In contrast, a subsequent injection of 7.5 mg/kg FEN produced significant increases in 5-HT and significant decreases in 5-HIAA within the PVN and the PFH. In Experiment 2, the 5-HT1a autoreceptor agonist 8-OH-DPAT (0.25 mg/kg, SC) was used to suppress presynaptic release of 5-HT prior to systemic injection of either fenfluramine (5 mg/kg, IP) or PPA (5, 10, 20, and 30 mg/kg, IP). The anorexic action of FEN, but not PPA, was reduced by pretreatment with 8-OH-DPAT. These results suggest that the anorexic action of PPA is not mediated by an indirect effect of PPA on presynaptic release of 5-HT.

Effects of chronic phenylpropanolamine infusion and termination on body weight, food consumption and water consumption in rats

The present study determined the effect of chronic PPA infusion and withdrawal on weight regulation. Male Sprague-Dawley rats received PPA (0, 90 or 180 mg/kg) via miniosmotic pumps for 2 weeks. Body weight and food and water consumption were measured daily before, during, and for 2 weeks after PPA infusion. Additionally, body weight was measured once 6 weeks after the last day of drug administration. PPA infusion produced dose-dependent reductions in body weight and food consumption throughout drug administration. During the first week of PPA termination, food consumption returned to control levels; however, body weights of drug-treated animals remained below those of controls throughout the 6-week post-drug period. PPA depressed water intake during the first week of drug administration, but tolerance to this effect developed by the second week of administration. These results suggest chronic PPA infusion produces persistent appetite suppression and weight loss and that discontinuation of PPA does not result in hyperphagia or rapid weight gain. These findings may have clinical significance for the many individuals who wish to lose weight but have difficulty reducing intake without pharmacologic assistance.

An assessment of the involvement of paraventricular hypothalamic alpha 2-adrenoceptors in phenylpropanolamine anorexia

Systemic injection of phenylpropanolamine (PPA), an alpha 1-adrenergic receptor agonist with some activity at alpha 2-adrenergic receptors, suppresses food intake in rats. However, only limited information is available as to the effect of intracranial PPA injections on food and water intake. In Experiment 1, microinjection of PPA (80-240 nM) into the hypothalamic paraventricular nucleus (PVN) induced a dose-dependent suppression of feeding (ED50 = 181 nM) but was without significant effect on water intake. Experiment 2 evaluated the effect of systemic PPA on paraventricular hypothalamic norepinephrine (NE) levels. Rats were treated with either vehicle or 20 mg/kg (IP) PPA prior to a 100-min period in which extracellular NE within the PVN was monitored via an indwelling microdialysis probe. Systemic injection of PPA suppressed extracellular NE level within PVN by approximately 70%, an action consistent with stimulation by PPA of a presynaptic alpha 2-adrenergic autoreceptor. Experiment 3 evaluated whether the alpha 2-adrenergic activity of PPA contributes to its feeding-suppressive action. Unlike prior results using the alpha 1-antagonist benoxathian, PVN microinjection of the alpha 2-antagonist rauwolscine in Experiment 3 of the present study failed to block systemically induced PPA anorexia. These results further support the contention that PVN alpha 1-adrenergic receptors suppress feeding and suggest that PPA's alpha 2-adrenergic effects do not modulate the anorexic action of PPA.

Effects of intra-PVN injections of d- and l-norephedrine on feeding in rats

Phenylpropanolamine (PPA) is thought to inhibit feeding by activation of alpha 1-adrenergic receptors within the paraventricular hypothalamus (PVN). Systemic injections of the PPA component enantiomers, d- and l-norephedrine (NEP), result in differential suppression of feeding (l-NEP more potent than d-NEP). Whether the norephedrine racemates induce differential anorexia subsequent to injection into the PVN is unknown. In the present study, adult male rats received intra-PVN injections of the d- and l-norephedrine enantiomers (0, 80, 160, and 240 nmol). Significantly greater anorexia was obtained for l-NEP relative to d-NEP. These results document a stereospecific effect of the norephedrine enantiomers within the PVN in inhibiting food intake and suggest that the interaction of these enantiomers with PVN alpha 1-adrenoceptors may mediate the similar difference in potency noted for systemic injections of d- and l-norephedrine.

Effects on food and water intake of the alpha 1-adrenoceptor agonists amidephrine and SK&F-89748

Activation of alpha-1 adrenoceptors, via systemic injection of agonists such as cirazoline and phenylpropanolamine (PPA), reliably suppresses food intake in rats. These effects are thought to result from stimulation of central alpha 1-adrenoceptors within the rat paraventricular hypothalamic nucleus (PVN), based on studies in which direct injections of cirazoline, methoxamine, phenylephrine and PPA into PVN suppress food intake. Because relatively few alpha 1-agonists have been tested to date using the systemic route of exposure, the present study examined the effects of the alpha 1-adrenoceptor agonists amidephrine and SKF-89748 on food and water intake. Adult male rats received systemic injections (IP) of either amidephrine (0.025, 0.05, 0.01 mg/kg) or of SK&F 89748 (0.01, 0.02, and 0.04 mg/kg). Amidephrine markedly suppressed food intake (ED50 = 0.49 mg/kg) and water intake (ED50 = 0.50 mg/kg), while SK&F 89748 marginally suppressed food intake (ED50 = 0.37 mg/kg) and was less potent in suppressing water intake (ED50 = 0.76 mg/kg). These results document that systemic injection of the alpha 1-adrenoceptor agonists amidephrine and SK&F 89748 induces anorexia with amidephrine exerting greater potency than SK&F 89748. These results further support the hypothesis that stimulation of alpha 1-adrenoceptors suppresses food intake.

Modulation of feeding by hypothalamic paraventricular nucleus alpha 1- and alpha 2-adrenergic receptors

Noradrenergic receptor populations within the paraventricular hypothalamus (PVN) modulate feeding. Satiated rats exhibit enhanced feeding subsequent to activation of alpha 2-adrenergic receptors within the PVN induced by exogenous infusion of either norepinephrine (NE) or clonidine (CLON). The feeding-stimulatory effect of alpha 2-adrenergic agents presumably reflects an inhibitory action on receptors located on medial hypothalamic "satiety" cells. Adrenergic receptors of the alpha 1-subclass have been identified within the PVN which are excitatory and which may function to suppress food intake. Microinjection into rat PVN of various alpha 1-adrenergic agonists including cirazoline, methoxamine, phenylpropanolamine and phenylephrine suppress feeding; an effect that is reversed by pretreatment with alpha 1-adrenergic receptor antagonists. The present review argues that alpha 1- and alpha 2-adrenoceptors within brain and specifically within the PVN are organized in an antagonistic fashion and that the effects of various adrenergic agonists on feeding may reflect the degree to which these agonists act at alpha 1- and alpha 2-adrenoceptors as well the relative balance of these receptors and their activity within the PVN.

Reversal of cirazoline- and phenylpropanolamine-induced anorexia by the α1-receptor antagonist prazosin

Phenylpropanolamine (PPA) is a phenethylamine anorectic drug that exerts direct agonist effects predominantly on alpha 1-adrenoceptors, with some alpha 2-adrenergic activity. Microinjections of PPA, as well as the alpha 1-adrenergic receptor agonists cirazoline, methoxamine, and 1-phenylephrine, into rat paraventricular nucleus (PVN) suppress feeding. The present study further evaluates the alpha 1-adrenergic basis of PPA-induced anorexia by examining the effects of systemic injections of the alpha 1-adrenergic antagonist prazosin (PRAZ, 2 and 5 mg/kg, IP) on the anorexia induced by systemic injections of PPA (5, 10, and 20 mg/kg, IP), as well as cirazoline (0.05, 0.1, and 0.2 mg/kg, IP). Although neither PRAZ dose alone altered food intake in the present study, 2 mg/kg PRAZ effectively reversed the feeding-suppressive effects of both PPA and cirazoline. These results strongly support the hypothesis that alpha 1-adrenoceptor stimulation mediates the anorexia induced by drugs such as PPA and cirazoline.

Effects of the α1-adrenergic agonist cirazoline on locomotion and brown adipose tissue thermogenesis in the rat

Anorexia is induced by injection of alpha 1-adrenergic receptor agonists into the hypothalamic paraventricular nucleus (PVN) in rats. Of the agonists tested to date, cirazoline is the most potent when administered either into the PVN or systemically. The present experiments assess the effects of systemically administered cirazoline, at doses that suppress food intake, on dopamine and norepinephrine systems as evident in locomotion and stereotypy and in the induction of brown adipose tissue (BAT) thermogenesis. In Experiment 1, adult male rats were treated with either vehicle (0) or 0.05, 0.1, 0.2 or 0.4 mg/kg cirazoline (IP) prior to 30 minutes assessment of horizontal and vertical locomotion and stereotypy in Omnitech activity chambers. Horizontal activity and stereotypy were significantly suppressed at 0.05 mg/kg cirazoline but these effects waned at higher cirazoline doses. In Experiment 2, interscapular BAT temperature in adult male rats was monitored for 30 minutes after injection (IP) of either vehicle or 0.4 mg/kg cirazoline. Cirazoline, at 0.4 mg/kg did not influence BAT temperature whereas a positive control treatment (phenylpropanolamine: 40 mg/kg) rapidly increased BAT temperature during a 15 minute period after injection. These results suggest that cirazoline-induced anorexia is not the result of competing motor responses and that this drug, at a dose that produces maximal suppression of feeding, does not alter BAT thermogenesis.

Effects on ingestive behavior in rats of the α1-adrenoceptor agonist cirazoline

Microinjections of various alpha 1-adrenoceptor agonists including phenylephrine and phenylpropanolamine into the paravenricular hypothalamic nucleus (PVN) suppress food intake in rats, suggesting that this receptor type might act in opposition to previously identified facilatory PVN alpha 2-adrenoceptors in the modulation of feeding. In the present experiments, we examine the effects on food and water intake of intra-PVN as well as systemic injection of cirazoline, a highly potent alpha 1-adrenoceptor agonist. In Experiment 1, intra-PVN microinjection of cirazoline (0, 3, 6, 12 and 24 nmol) suppressed food intake (ED50 = 23.4 nmol) without significant effects on water intake. In Experiment 2, systemic injection of cirazoline (0, 0.05, 0.1, 0.2, 0.4 mg/kg) also markedly suppressed food intake (ED50 = 0.05 mg/kg i.p.), with a less potent action on water intake (ED50 = 0.22 mg/kg i.p.). The results of this study as well as our previous investigations strongly support the notion that alpha 1-adrenoceptors within rat PVN act to reliably suppress food intake.

Suppression of feeding induced by phenylephrine microinjections within the paraventricular hypothalamus in rats

Rats were treated with the alpha-2 agonist clonidine (4, 20 and 50 nMol) and with the alpha-1 agonist 1-phenylephrine (50, 100, 200 and 400 nMol). Phenylpropanolamine (PPA) is a phenythylamine anorectic drug that exerts direct agonist effects predominantly on alpha-1 adrenergic receptors, with some alpha-2 adrenergic activity. We recently reported that injection of PPA significantly suppressed feeding in rats. Prior studies have noted that into the paraventricular hypothalamus (PVN) microinjections of the alpha-2 adrenergic receptor agonist clonidine into the PVN induced feeding behavior in satiated rats. However, the effect on feeding of administration of alpha-1 adrenergic agonists within the PVN remains unknown. In the present study, unilateral guide cannulae aimed at the PVN were surgically implanted in adult male rats. In an initial 60 min feeding test conducted under free-feeding ("non-deprived") conditions, each rat was found to eat significantly more food after injection of 25 nMol norepinephrine (NE) into the PVN. In subsequent tests, the feeding increased significantly to 4 nMol clonidine; however, feeding was suppressed by 50 nMol clonidine. Food intake after 20 nMol clonidine was not significantly different from that recorded after vehicle. In contrast, phenylephrine (100-400 nMol) reliably suppressed feeding behavior. In the final phase of the study, the rats ate significantly less food after injection of 160 nMol PPA into the PVN but consumed significantly more food after a final injection of 25 nMol NE into the PVN. These results suggest that the anorexic action of PPA may be linked to activation of alpha-1 adrenergic receptors within the PVN.

Enhancement of phenylpropanolamine anorexia by proglumide in rats

Although a number of studies have characterized the anorectic state induced by phenylpropanolamine (PPA), the mechanism by which this drug suppresses appetite remains elusive. PPA inhibits gastric emptying at doses that also suppress appetite as does the gut hormone cholecystokinin (CCK). To evaluate whether PPA anorexia results via an action on gut CCK activity, rats in the present study were treated (IP) with either 0.9% saline or 150 mg/kg proglumide, a CCK receptor antagonist, 30 minutes before a 15 min feeding trial and then injected (IP) 5 min prior to the trial with either 0.9% saline, 5 mg/kg PPA, 10 mg/kg PPA, 8 ug/kg CCK or 16 ug/kg CCK. Although 150 mg/kg proglumide antagonized CCK anorexia, this dose of proglumide significantly enhanced the anorectic action of PPA. These results suggest that PPA does not act via an endogenous CCK system to suppress feeding.

Reversal of phenylpropanolamine anorexia in rats by the alpha-1 receptor antagonist benoxathian

Phenylpropanolamine (PPA) is a phenethylamine anorectic drug that exerts direct agonist effects predominantly on alpha-1 adrenergic receptors, with some alpha-2 adrenergic activity. Direct injections of PPA as well as the alpha-1 agonist 1-phenylephrine into rat paraventricular nucleus (PVN) suppress feeding. In the present study, we evaluate the hypothesis that systemic PPA acts within the PVN on an alpha-1 receptor population to suppress feeding. Accordingly, adult male rats were prepared with a unilateral guide cannula aimed at the PVN. Microinjection of the alpha-1 adrenergic receptor antagonist benoxathian (0, 2.5, 5.0 or 10.0 nmol) into the PVN was found to have no effect on baseline feeding behavior. Microinjection of 10.0 nmol benoxathian into the PVN completely reversed the anorexia induced by 2.5, 5.0 or 10.0 mg/kg PPA (IP), yet did not alter the hypodipsia produced by PPA. These data strongly suggest that PPA anorexia is mediated by an alpha-1 adrenergic satiety mechanism within the PVN.

A review of the physiological bases of the anorexic action of phenylpropanolamine (d,1-norephedrine)

Phenylpropanolamine (PPA) is a phenethylamine that induces a variety of effects including anorexia and weight loss. The present review compares the acute anorexic effects of PPA with those of amphetamine in animals and humans, describes the persistent chronic effects of PPA on feeding as well as differences in anorexic potency between the norephedrine enantiomers (d-norephedrine less than 1-norephedrine) and summarizes the generality of PPA anorexia. The review also describes the putative mechanisms by which PPA is thought to reduce feeding behavior including alteration of gastric emptying and interaction with central nervous system neurons.

Diurnal variation of phenylpropanolamine anorexia in rats

An examination of the effects of the diurnal cycle on phenylpropanolamine (PPA) anorexia was conducted using two groups of rats, differentiated on the basis of time (Night or Day) of drug injection (0, 10, 20, or 30 mg/kg PPA). The results demonstrate that PPA, over a range of doses, has greater anorectic potency during the dark phase of the diurnal cycle. Moreover, the effect of PPA on water intake was not influenced by the diurnal cycle, suggesting that the diurnal effect was limited to the inhibitory action of PPA on feeding. Other studies are cited which, combined with the present results, suggest that the diurnal cycle may modulate feeding control processes, and that the impact of an anorectic agent such as PPA on feeding behavior may be, in part, a function of the diurnal cycle.

Conditioned taste reactivity in rats after phenylpropanolamine, d-amphetamine or lithium chloride

That an aversive property of phenylpropanolamine (PPA) in part contributes to its anorexic capacity is suggested by the demonstration of conditioned taste avoidance to PPA doses ranging from 10-40 mg/kg. In order to further evaluate the putative aversive property of PPA, the present experiment compared the effects of PPA on multiple measures of aversion (chin rubs, gaping) in the taste reactivity (TRT) paradigm with those produced by the classic agent lithium chloride and by amphetamine. Male rats were infused via an intraoral cannula with 0.5 M sucrose followed by injection with either vehicle, 127 mg/kg lithium chloride (LiCl), 1.5 or 3.0 mg/kg amphetamine or by 10, 20 or 40 mg/kg PPA. LiCl and 40 mg/kg PPA induced significant chin rub responses during conditioning but only the aversive response induced by 40 mg/kg PPA persisted during extinction trials. In contrast, lower doses of PPA (10 mg/kg, 20 mg/kg) were not aversive in the TRT paradigm. These results suggest that an aversive component is not contributing to anorexia induced by PPA within the dose range of 10-20 mg/kg, but that higher doses may further suppress appetite via an aversive action.

Effects of paraventricular hypothalamic microinjections of phenylpropanolamine and d-amphetamine on mash intake in rats

The present experiment compared the effects of unilateral microinjections (40, 80 and 160 nmol/0.5 microliter) of phenylpropanolamine (PPA: d,l-norephedrine) and d-amphetamine sulfate within the paraventricular hypothalamus (PVN) on consumption of a palatable sweetened-mash diet in 15-hour food-deprived adult male rats. Intracranial microinjections were administered 5 minutes prior to each 30-minute feeding trial. PPA, at a dose of 160 nmol, suppressed feeding by 42%, whereas a similar dose of amphetamine suppressed feeding by 49%. Amphetamine or PPA doses of 40 and 80 nmol were without significant effect on feeding behavior. A relatively high dose of 160 nmol amphetamine was required to suppress feeding after injection into the PVN whereas much lower amphetamine concentrations are required to suppress feeding after injection into the perifornical hypothalamus. In contrast, PPA has some anorexic activity within the PVN but not within the perifornical hypothalamus.

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.