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

A PAM of the α1A-Adrenergic receptor rescues biomarker, long-term potentiation, and cognitive deficits in Alzheimer's disease mouse models without effects on blood pressure

α1-Adrenergic Receptors (ARs) regulate the sympathetic nervous system by the binding of norepinephrine (NE) and epinephrine (Epi) through different subtypes (α1A, α1B, α1D). α1A-AR activation is hypothesized to be memory forming and cognitive enhancing but drug development has been stagnant due to unwanted side effects on blood pressure. We recently reported the pharmacological characterization of the first positive allosteric modulator (PAM) for the α1A-AR with predictive pro-cognitive and memory properties. In this report, we now demonstrate the in vivo characteristics of Compound 3 (Cmpd-3) in two genetically-different Alzheimer's Disease (AD) mouse models. Drug metabolism and pharmacokinetic studies indicate sufficient brain penetrance and rapid uptake into the brain with low to moderate clearance, and a favorable inhibition profile against the major cytochrome p450 enzymes. Oral administration of Cmpd-3 (3-9 mg/kg QD) can fully rescue long-term potentiation defects and AD biomarker profile (amyloid β-40, 42) within 3 months of dosing to levels that were non-significant from WT controls and which outperformed donepezil (1 mg/kg QD). There were also significant effects on paired pulse facilitation and cognitive behavior. Long-term and high-dose in vivo studies with Cmpd-3 revealed no effects on blood pressure. Our results suggest that Cmpd-3 can maintain lasting therapeutic levels and efficacy with disease modifying effects with a once per day dosing regimen in AD mouse models with no observed side effects.

Hindbrain Administration of Oxytocin Reduces Food Intake, Weight Gain and Activates Catecholamine Neurons in the Hindbrain Nucleus of the Solitary Tract in Rats

Existing studies show that CNS oxytocin (OT) signaling is important in the control of energy balance, but it is unclear which neurons may contribute to these effects. Our goals were to examine (1) the dose-response effects of acute OT administration into the third (3V; forebrain) and fourth (4V; hindbrain) ventricles to assess sensitivity to OT in forebrain and hindbrain sites, (2) the extent to which chronic 4V administration of OT reduces weight gain associated with the progression of diet-induced obesity, and (3) whether nucleus tractus solitarius (NTS) catecholamine neurons are downstream targets of 4V OT. Initially, we examined the dose-response effects of 3V and 4V OT (0.04, 0.2, 1, or 5 μg). 3V and 4V OT (5 μg) suppressed 0.5-h food intake by 71.7 ± 6.0% and 60 ± 12.9%, respectively. 4V OT (0.04, 0.2, 1 μg) reduced food intake by 30.9 ± 12.9, 42.1 ± 9.4, and 56.4 ± 9.0%, respectively, whereas 3V administration of OT (1 μg) was only effective at reducing 0.5-h food intake by 38.3 ± 10.9%. We subsequently found that chronic 4V OT infusion, as with chronic 3V infusion, reduced body weight gain (specific to fat mass) and tended to reduce plasma leptin in high-fat diet (HFD)-fed rats, in part, through a reduction in energy intake. Lastly, we determined that 4V OT increased the number of hindbrain caudal NTS Fos (+) neurons (156 ± 25) relative to vehicle (12 ± 3). The 4V OT also induced Fos in tyrosine hydroxylase (TH; marker of catecholamine neurons) (+) neurons (25 ± 7%) relative to vehicle (0.8 ± 0.3%). Collectively, these findings support the hypothesis that OT within the hindbrain is effective at reducing food intake, weight gain, and adiposity and that NTS catecholamine neurons in addition to non-catecholaminergic neurons are downstream targets of CNS OT.

Amphetamine promotes cortical Up state: Role of adrenergic receptors

Cortical neurons oscillate synchronously between the Up and Down state during slow-wave sleep and general anesthesia. Using local-field-potential recording in the rat prefrontal cortex (PFC), we have shown that systemic administration of methylphenidate promotes PFC Up states and reduces PFC slow oscillation, suggesting a depolarizing effect of the drug on PFC neurons. Here, we report that systemic injection of d-amphetamine produced similar effects. Our evidence further suggests that norepinephrine (NE) plays a major role in the effects of d-amphetamine since they were mimicked by the NE reuptake inhibitors tomoxetine and nisoxetine and completely blocked by the α₁ receptor antagonist prazosin. The effects of d-amphetamine persisted, however, in the presence of α₂ or β receptor blockade. Experiments with α₁ subtype-selective antagonists further suggest that d-amphetamine's effects depend on activation of central, but not peripheral, α_1A receptors. Unexpectedly, the putative α₁ receptor agonist cirazoline failed to mimic the effects of d-amphetamine. Previous studies suggest that cirazoline is also an antagonist at α₂ receptors. Furthermore, it is a partial, not full, agonist at α_1B and α_1D receptors. Whether or not these properties of cirazoline contribute to its failure to mimic d-amphetamine's effects remains to be determined. Methylphenidate and d-amphetamine are two most common medications for attention-deficit/hyperactivity disorder (ADHD). Both, however, are associated with adverse effects including abuse potential and psychotomimetic effects. Further understanding of their mechanisms of action will help develop safer treatments for ADHD and offer new insights into drug addiction and psychosis.

Obesity: Current and potential pharmacotherapeutics and targets

Obesity is a global epidemic that contributes to a number of health complications including cardiovascular disease, type 2 diabetes, cancer and neuropsychiatric disorders. Pharmacotherapeutic strategies to treat obesity are urgently needed. Research over the past two decades has increased substantially our knowledge of central and peripheral mechanisms underlying homeostatic energy balance. Homeostatic mechanisms involve multiple components including neuronal circuits, some originating in hypothalamus and brain stem, as well as peripherally-derived satiety, hunger and adiposity signals that modulate neural activity and regulate eating behavior. Dysregulation of one or more of these homeostatic components results in obesity. Coincident with obesity, reward mechanisms that regulate hedonic aspects of food intake override the homeostatic regulation of eating. In addition to functional interactions between homeostatic and reward systems in the regulation of food intake, homeostatic signals have the ability to alter vulnerability to drug abuse. Regarding the treatment of obesity, pharmacological monotherapies primarily focus on a single protein target. FDA-approved monotherapy options include phentermine (Adipex-P®), orlistat (Xenical®), lorcaserin (Belviq®) and liraglutide (Saxenda®). However, monotherapies have limited efficacy, in part due to the recruitment of alternate and counter-regulatory pathways. Consequently, a multi-target approach may provide greater benefit. Recently, two combination products have been approved by the FDA to treat obesity, including phentermine/topiramate (Qsymia®) and naltrexone/bupropion (Contrave®). The current review provides an overview of homeostatic and reward mechanisms that regulate energy balance, potential therapeutic targets for obesity and current treatment options, including some candidate therapeutics in clinical development. Finally, challenges in anti-obesity drug development are discussed.

Hindbrain oxytocin receptors contribute to the effects of circulating oxytocin on food intake in male rats

Oxytocin (OT)-elicited hypophagia has been linked to neural activity in the nucleus of the solitary tract (NTS). Because plasma OT levels increase after a meal, we hypothesized that circulating OT acts at both peripheral and hindbrain OT receptors (OTRs) to limit food intake. To initially determine whether circulating OT inhibits food intake by acting at hindbrain OTRs, we pretreated rats with an OTR antagonist administered into the fourth ventricle (4V) followed by either central or systemic OT administration. Administration of the OTR antagonist into the 4V blocked anorexia induced by either 4V or i.p. injection of OT. However, blockade of peripheral OTRs also weakened the anorectic response to ip OT. Our data suggest a predominant role for hindbrain OTRs in the hypophagic response to peripheral OT administration. To elucidate central mechanisms of OT hypophagia, we tested whether OT activates NTS catecholaminergic neurons. OT (ip) increased the number of NTS cells expressing c-Fos, of which 10%-15% were catecholaminergic. Furthermore, electrophysiological studies in mice revealed that OT stimulated 47% (8 of 17) of NTS catecholamine neurons through a presynaptic mechanism. However, OT-elicited hypophagia did not appear to require activation of α1-adrenoceptors, and blockade of glucagon-like peptide-1 receptors similarly did not attenuate anorexia induced by OT. These findings demonstrate that OT elicits satiety through both central and peripheral OTRs and that although catecholamine neurons are a downstream target of OT signaling in the NTS, the hypophagic effect is mediated independently of α1-adrenoceptor signaling.

Long-term α1B-adrenergic receptor activation shortens lifespan, while α1A-adrenergic receptor stimulation prolongs lifespan in association with decreased cancer incidence

The α1-adrenergic receptor (α1AR) subtypes, α1AAR and α1BAR, have differential effects in the heart and central nervous system. Long-term stimulation of the α1AAR subtype prolongs lifespan and provides cardio- and neuro-protective effects. We examined the lifespan of constitutively active mutant (CAM)-α1BAR mice and the incidence of cancer in mice expressing the CAM form of either the α1AAR (CAM-α1AAR mice) or α1BAR. CAM-α1BAR mice have a significantly shortened lifespan when compared with wild-type (WT) animals; however, the effect was sex dependent. Female CAM-α1BAR mice lived significantly shorter lives, while the median lifespan of male CAM-α1BAR mice was not different when compared with that of WT animals. There was no difference in the incidence of cancer in either sex of CAM-α1BAR mice. The incidence of cancer was significantly decreased in CAM-α1AAR mice when compared with that in WT, and no sex-dependent effects were observed. Further study is warranted on cancer incidence after activation of each α1AR subtype and the effect of sex on lifespan following activation of the α1BAR. The implications of a decrease in cancer incidence following long-term α1AAR stimulation could lead to improved treatments for cancer.

Role of oxytocin signaling in the regulation of body weight

Obesity and its associated metabolic disorders are growing health concerns in the US and worldwide. In the US alone, more than two-thirds of the adult population is classified as either overweight or obese [1], highlighting the need to develop new, effective treatments for these conditions. Whereas the hormone oxytocin is well known for its peripheral effects on uterine contraction during parturition and milk ejection during lactation, release of oxytocin from somatodendrites and axonal terminals within the central nervous system (CNS) is implicated in both the formation of prosocial behaviors and in the control of energy balance. Recent findings demonstrate that chronic administration of oxytocin reduces food intake and body weight in diet-induced obese (DIO) and genetically obese rodents with impaired or defective leptin signaling. Importantly, chronic systemic administration of oxytocin out to 6 weeks recapitulates the effects of central administration on body weight loss in DIO rodents at doses that do not result in the development of tolerance. Furthermore, these effects are coupled with induction of Fos (a marker of neuronal activation) in hindbrain areas (e.g. dorsal vagal complex (DVC)) linked to the control of meal size and forebrain areas (e.g. hypothalamus, amygdala) linked to the regulation of food intake and body weight. This review assesses the potential central and peripheral targets by which oxytocin may inhibit body weight gain, its regulation by anorexigenic and orexigenic signals, and its potential use as a therapy that can circumvent leptin resistance and reverse the behavioral and metabolic abnormalities associated with DIO and genetically obese models.

Catecholamine modulation of prefrontal cortical cognitive function

The prefrontal cortex (PFC) utilizes working memory to guide behavior and to release the organism from dependence on environmental cues and is commonly disrupted in neuropsychiatric disorders, normal aging, or exposure to uncontrollable stress. This review posits that the PFC is very sensitive to changes in the neuromodulatory inputs it receives from norepinephrine (NE) and dopamine (DA) systems and that this sensitivity can lead to marked changes in the working-memory functions of the PFC. While NE and DA have important beneficial influences on processing in this area, very high levels of catecholamine release, for example, during exposure to uncontrollable stress, disrupt the cognitive functions of the PFC. This fresh understanding of the neurochemical influences on PFC function has led to new treatments for cognitive disorders such as Attention Deficit Hyperactivity Disorder (ADHD), and may help to elucidate the prevalence of PFC dysfunction in other mental disorders.

Long-term α1A-adrenergic receptor stimulation improves synaptic plasticity, cognitive function, mood, and longevity

The role of α(1)-adrenergic receptors (α(1)ARs) in cognition and mood is controversial, probably as a result of past use of nonselective agents. α(1A)AR activation was recently shown to increase neurogenesis, which is linked to cognition and mood. We studied the effects of long-term α(1A)AR stimulation using transgenic mice engineered to express a constitutively active mutant (CAM) form of the α(1A)AR. CAM-α(1A)AR mice showed enhancements in several behavioral models of learning and memory. In contrast, mice that have the α(1A)AR gene knocked out displayed poor cognitive function. Hippocampal brain slices from CAM-α(1A)AR mice demonstrated increased basal synaptic transmission, paired-pulse facilitation, and long-term potentiation compared with wild-type (WT) mice. WT mice treated with the α(1A)AR-selective agonist cirazoline also showed enhanced cognitive functions. In addition, CAM-α(1A)AR mice exhibited antidepressant and less anxious phenotypes in several behavioral tests compared with WT mice. Furthermore, the lifespan of CAM-α(1A)AR mice was 10% longer than that of WT mice. Our results suggest that long-term α(1A)AR stimulation improves synaptic plasticity, cognitive function, mood, and longevity. This may afford a potential therapeutic target for counteracting the decline in cognitive function and mood associated with aging and neurological disorders.

Noradrenergic modulation of ephedrine-induced hypophagia

The hypophagic action of the sympathomimetic amine ephedrine (EPH) in the rat may reflect actions on central dopaminergic (DA) and noradrenergic (NE) systems. EPH indirectly facilitates DA and NE activity and acts as a partial agonist at alpha(1)-adrenergic receptors. Two approaches were used to assess the possible contribution of NE and DA pathways to EPH-induced hypophagia. In the first, regression analyses of published archival data were computed to characterize the relation between the hypophagic potency values of (-)-(EPH) and related sympathomimetic drugs, including (+)-amphetamine, aminorex, mazindol, and phentermine (data derived from Blosser JC et al., 1987) and the most potent action of these drugs on facilitating NE activity or DA activity in rat brain (data derived from Rothman RB et al., 2001). In the NE analyses, the ED(50) values for these drugs for the inhibition of eating in rats were significantly related (r = 0.91, P = 0.03) to the potency of each drug in facilitating NE activity (either release or inhibition of [(3)H]NE reuptake), whereas in the DA analyses the correlation between ED(50) values and DA activity for these drugs was also significant (r = 0.98, P = 0.003). The regression analyses are thus supportive of a role for NE or DA in the hypophagic capacity of EPH. Although an earlier study noted that administration of the putative DA antagonist pimozide in rats attenuated EPH hypophagia, pimozide exerts similar potency in antagonizing DA receptors and alpha(1)-adrenergic receptors. To clarify the role of alpha(1)-adrenoceptors in EPH-induced hypophagia, adult male rats were pretreated with the alpha(1)-adrenergic receptor antagonist prazosin (0.0.5 and 2 mg/kg) prior to the administration of (-)-EPH (0, 5, 10, or 20 mg/kg, IP). Prazosin pretreatment at 2.0 mg/kg significantly attenuated the hypophagia, but not the hypodipsia, induced by administration of 10 mg/kg and by 20 mg/kg (-)-EPH. Collectively, these results confirm a critical contribution of of alpha(1)-adrenoceptors to the hypophagic action of (-)-EPH in rats.

Stress, norepinephrine and depression

Experimental and clinical evidence implicates stress as a major predisposing factor in depression and other severe psychiatric disorders. In this review, evidence is presented to show how the impact of stress on the central sympathetic system leads to changes in the endocrine, immune and neurotransmitter axes which underlie the main clinical symptoms of depression. Thus it can be shown that the noradrenergic system is dysfunctional in depression, a situation which reflects the chronic hypersecretion of glucocorticoids and inflammatory mediators within the brain in addition to an enhanced activity of the locus ceruleus. With regard to the actions of antidepressants in modulating the stress response and alleviating depression it is now evident that, irrespective of the presumed specificity of the antidepressants for the noradrenergic or serotonergic systems, they all normalize noradrenergic function. This action is due partly to the regulation of tyrosine hydroxylase activity in the locus ceruleus but also enhances neuronal sprouting which counteracts the neurodegenerative effects of chronic stress.

Diurnal changes in paraventricular hypothalamic alpha1 and alpha2-adrenoceptors and food intake in rats

The prominent feeding rhythm evident in rats may reflect circadian variation in activity of feeding-relevant adrenoceptors within the hypothalamic paraventricular nucleus (PVN). In the present study, separate groups of rats were sacrificed at six time points (ZT0, ZT4, ZT8, ZT12, ZT16, ZT20) over a diurnal cycle. Food intakes were recorded during the 4-h period prior to sacrifice in each group. Brain sections were incubated with either an alpha1-adrenoceptor ligand (3H)-prazosin [(3H)-PRZ] or an alpha2-adrenoceptor ligand (3H) para-aminoclonidine [(3H)-PAC] prior to autoradiography analyses. Binding of (3H)-PRZ within the PVN varied as a function of the diurnal cycle, with significantly greater binding evident during the light phase of ZT0 (first 4 h of the light phase) and at ZT4, compared to nadir binding during the dark phase at ZT16 (first 4 h of the dark phase). Binding of (3H)-PAC within the PVN also varied as a function of the diurnal cycle, with significantly greater binding evident during the first 8 h of the dark phase (ZT16 and ZT20) than during the light phase. Food intake and alpha1-adrenergic binding were inversely related across the diurnal cycle. These results support the hypothesis that PVN adrenergic systems may be organized in an antagonistic fashion so as to modulate feeding in the rat.

Lateral hypothalamic NMDA receptor subunits NR2A and/or NR2B mediate eating: immunochemical/behavioral evidence

Cells within the lateral hypothalamic area (LHA) are important in eating control. Glutamate or its analogs, kainic acid (KA) and N-methyl-D-aspartate (NMDA), elicit intense eating when microinjected there, and, conversely, LHA-administered NMDA receptor antagonists suppress deprivation- and NMDA-elicited eating. The subunit composition of LHA NMDA receptors (NMDA-Rs) mediating feeding, however, has not yet been determined. Identifying this is important, because distinct second messengers/modulators may be activated by NMDA-Rs with differing compositions. To begin to address this, we detected LHA NR2A and NR2B subunits by immunoblotting and NR2B subunits by immunohistochemistry using subunit-specific antibodies. To help determine whether NMDA-Rs mediating feeding might contain these subunits, we conducted behavioral studies using LHA-administered ifenprodil, an antagonist selective for NR2A- and/or NR2B-containing NMDA-Rs at the doses we used (0.001-100 nmol). Ifenprodil maximally suppressed NMDA- and deprivation-elicited feeding by 63 and 39%, respectively, but failed to suppress KA-elicited eating, suggesting its actions were behaviorally specific. Collectively, these results suggest that LHA NMDA-Rs, some of which contribute to feeding control, are composed of NR2A and/or NR2B subunits, and implicate NR2A- and/or NR2B-linked signal transduction in feeding behavior.

Alpha-1 noradrenergic receptor stimulation impairs prefrontal cortical cognitive function

BACKGROUND

Many neuropsychiatric disorders are associated with high levels of noradrenergic turnover, and most antipsychotic medications have alpha-1 adrenoceptor blocking properties, yet little is known about alpha-1 influences on higher cortical function.

METHODS

The alpha-1 adrenergic agonist, phenylephrine, was infused into the prefrontal cortex (PFC) of rats (0.1 microgram/0.5 microL) performing a spatial working memory task, delayed alternation. The phenylephrine response was challenged with coinfusion of the alpha-1 adrenergic antagonist, uripidil (0.01 microgram), or with a dose of lithium chloride (4 mEq/kg, i.p., 18 hours) known to suppress phosphotidylinositol (PI) turnover, the second messenger pathway coupled to alpha-1 adrenoceptors.

RESULTS

Phenylephrine infusions in PFC markedly impaired delayed alternation performance. The phenylephrine response was reversed by coinfusion of uripidil, or by pretreatment with lithium, consistent with actions at alpha-1 adrenoceptors coupled to a PI pathway.

CONCLUSIONS

These findings demonstrate that alpha-1 adrenoceptor stimulation in the PFC impairs cognitive function. Excessive stimulation of alpha-1 adrenoceptors may contribute to PFC deficits (e.g., distractibility, impulsivity) in disorders such as mania, dementia, and anxiety associated with high noradrenergic turnover.

The alpha-1 adrenergic agonist, cirazoline, impairs spatial working memory performance in aged monkeys

The alpha-1 adrenergic agonist, cirazoline, was examined for effects on spatial working memory performance in aged rhesus monkeys. Cirazoline has additional high affinity for imidazoline receptors and has good brain penetrance when administered systemically. Spatial working memory was assessed using the variable delayed response task, a test dependent upon prefrontal cortical function in monkeys. Low doses of cirazoline (0.00001-0.001 mg/kg) impaired delayed response performance significantly. This impairment did not appear to result from nonspecific changes in behavior, because cirazoline had no significant effect on performance of control trials where the delay was "0" s, and had no significant effect on behavioral ratings. Impairment was reversed by pretreatment with the alpha-1 adrenergic antagonist, prazosin, consistent with drug actions at alpha-1 adrenergic receptors. In contrast, preliminary data suggest that higher cirazoline doses (0.001-0.01 mg/kg) occasionally produced improved performance that was not reversed by prazosin, but rather, by the imidazoline/alpha-2 adrenergic antagonist, idazoxan. The finding that alpha-1 adrenergic receptor stimulation impairs spatial working memory performance complements previous research demonstrating that alpha-2 adrenergic receptor stimulation improves working memory, and suggests that norepinephrine may have opposing actions at alpha-1 vs. alpha-2 receptors in the prefrontal cortex as it does in the hypothalamus and thalamus.

Investigation of the mechanisms underlying the hypophagic effects of the 5-HT and noradrenaline reuptake inhibitor, sibutramine, in the rat

1. Sibutramine is a novel 5-hydroxytryptamine (5-HT) and noradrenaline reuptake inhibitor (serotonin-noradrenaline reuptake inhibitor, SNRI) which is currently being developed as a treatment for obesity. Sibutramine has been shown to decrease food intake in the rat. In this study we have used a variety of monoamine receptor antagonists to examine the pharmacological mechanisms underlying sibutramine-induced hypophagia. 2. Individually-housed male Sprague-Dawley rats were maintained on reversed phase lighting with free access to food and water. Drugs were administered at 09 h 00 min and food intake was monitored over the following 8 h dark period. 3. Sibutramine (10 mg kg-1, p.o.) produced a significant decrease in food intake during the 8 h following drug administration. This hypophagic response was fully antagonized by the alpha 1-adrenoceptor antagonist, prazosin (0.3 and 1 mg kg-1, i.p.), and partially antagonized by the beta 1-adrenoceptor antagonist, metoprolol (3 and 10 mg kg-1, i.p.) and the 5-HT receptor antagonists, metergoline (non-selective; 0.3 mg kg-1, i.p.); ritanserin (5-HT2A/2C; 0.1 and 0.5 mg kg-1, i.p.) and SB200646 (5-HT2B/2C; 20 and 40 mg kg-1, p.o.). 4. By contrast, the alpha 2-adrenoceptor antagonist, RX821002 (0.3 and 1 mg kg-1, i.p.) and the beta 2-adrenoceptor antagonist, ICI 118,551 (3 and 10 mg kg-1, i.p.) did not reduce the decrease in food intake induced by sibutramine. 5. These results demonstrate that beta 1-adrenoceptors, 5-HT2A/2C-receptors and particularly alpha 1-adrenoceptors, are involved in the effects of sibutramine on food intake and are consistent with the hypothesis that sibutramine-induced hypophagia is related to its ability to inhibit the reuptake of both noradrenaline and 5-HT, with the subsequent activation of a variety of noradrenaline and 5-HT receptor systems.

Effects of the alpha 1a-adrenoceptor antagonist RS-17053 on phenylpropanolamine-induced anorexia in rats

Activation of alpha 1-Adrenergic receptors via systemic administration of drugs such as phenylpropanolamine (PPA) and cirazoline results in the suppression of feeding in rats. Whether PPA acts via activation of the three currently identified alpha 1-Adrenoceptor subtypes is unknown. The intent of the present study was thus to examine the effects of systemic administration of the novel alpha 1a-Adrenoceptor antagonist RS-17053 on PPA-induced anorexia. Adult male rats (n = 6 to 8 per group) were pretreated (IP) with either 0, 0.1, 0.5, 2.5, or 10.0 mg/kg RS-17053 or with 2.0 mg/kg of the prototypical alpha 1-Adrenoceptor antagonist prazosin. Five minutes later, each rat was treated (IP) with either 0, 5, 10 or 15 mg/kg PPA. Food and water intakes were recorded for a 30 min period starting 10 min after the the treatment injection. Rats pretreated with vehicle and then treated with PPA exhibited a dose-dependent suppression of feeding with a maximal effect evident at the 15 mg/kg dose of PPA. Pretreatment with 2.0 mg/kg prazosin reversed the anorexic activity of PPA. Pretreatment with RS-17053 (0.1-2.5 mg/kg) did not alter either baseline feeding or the anorexic action of PPA. These results suggest that PPA does not act via the alpha 1a-Adrenergic receptor subtype to suppress food intake.

Catecholamine regulation of the prefrontal cortex

The catecholamines dopamine (DA) and norepinephrine provide an essential modulatory influence on the working memory and attentional functions of the prefrontal cortex (PFC). The following critique reviews evidence that (1) either insufficient or excessive DA D1 receptor stimulation is detrimental to PFC function, while DA stimulation of the D2 family of receptors may contribute to detrimental actions in PFC and (2) that norepinephrine has an important beneficial influence on PFC function through its actions at post-synaptic, alpha 2A adrenergic receptors, but impairs PFC function through actions at alpha 1 adrenergic receptors. Critical levels of catecholamine stimulation may be needed to optimize PFC cognitive function; high levels of catecholamine release during stress may serve to take the PFC 'off-line' to allow faster, more habitual responses mediated by the posterior and/or subcortical structures to regulate behavior. These studies have relevance to our understanding and treatment of disorders with prominent symptoms of PFC dysfunction.

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.

Conditioned taste aversion in rats induced by the alpha 1-adrenoceptor agonist cirazoline

Recent studies have indicated that alpha 1-adrenoceptor agonists such as phenylpropanolamine (PPA), cirazoline, amidephrine, and SK&F-89748 suppress food intake in rats. These compounds activate alpha 1-adrenoceptors within the paraventricular hypothalamic nucleus (PVN) and may excite efferent fibers that inhibit feeding. Studies of the effects of alpha 1-agonists suggest a specificity for feeding behavior, but no study to date has evaluated whether these agonists may suppress feeding behavior by the induction of malaise. Accordingly, the present experiment examined the ability of systemically administered cirazoline (0.1, 0.2, and 0.4 mg/kg, IP) to induce conditioned taste aversion (CTA) to a saccharin solution. Significant CTA was noted for 0.2 and 0.4 mg/kg cirazoline but not for 0.1 mg/kg cirazoline, compared to a vehicle treatment. The ED50 for cirazoline-induced aversion was computed to be 0.3 mg/kg, which contrasts with an ED50 value of 0.09 mg/kg for the effect of cirazoline on food intake (computed in other studies). More importantly, a 0.1 mg/kg dose of cirazoline, which is slightly greater than that of the ED50 value for suppression of feeding, did not induce significant CTA in the present study. These results suggest that malaise is not a prominent factor in the suppressive activity of cirazoline on food intake and advocate the use of cirazoline as an effective appetite suppressant.

Neurochemical mechanism of action of anorectic drugs

Studies with dexfenfluramine, an anorectic agent which releases 5-hydroxytryptamine (5-HT) from nerve terminals and inhibits its reuptake, have considerably increased our knowledge of the role of 5-HT in feeding control. 5-HT1B receptors mediate the satiating effect of dexfenfluramine, whereas the mechanism by which 5-HT uptake inhibitors such as fluoxetine and sertraline cause anorexia is not clear. Anorexia induced by (+)-amphetamine, phentermine, diethylpropion and phenylpropanolamine seems to be the result of their ability to increase the release of noradrenaline and/or dopamine from nerve terminals and inhibit their reuptake or, in the case of phenylpropanolamine, to stimulate directly alpha 1-adrenoceptors. It has been suggested that beta- and alpha 1-adrenoceptors and D1 dopamine receptors are involved in their effect on food intake. The difficulties of extrapolation across species limit our knowledge of the mechanism of the anorectic action in humans. Significant advances in the treatment of feeding pathology will be linked to identifying new receptor types and subtypes for neurotransmitters and quantifying and modelling eating disorders such as binge-eating and food craving.

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.

Altered alpha 1-adrenoceptor binding in intact and adrenalectomized obese Zucker rats (fa/fa)

While many autonomic and metabolic defects associated with genetic obesity in the Zucker rat are corrected by adrenalectomy (Adx), brain adrenoceptor function has not been examined in this context. Here, 3 weeks after Adx or sham surgery, brains of 11 weeks old lean (Fa/Fa) and obese (fa/fa) male Zucker rats were assayed for alpha 1-([3H]prazosin; [3H]PRZ) and alpha 2-adrenoceptor ([3H]paraminoclonidine; [3H]PAC) binding by autoradiography. By genotype, obese rats had 19-256% higher [3H]PRZ binding than lean rats in the amygdala (central [ACN], basolateral [ABL], basomedial [ABM] and medial [MAN] nuclei [n.]), hypothalamus (dorsomedial n. [DMN] and lateral [LH]) and somatosensory cortex. In the ABL and ACN, increased maximal binding (Bmax) in obese rats was associated with decreased affinity (increased Kd). Three weeks after surgery, sham-operated obese rats gained 27% more weight than lean rats but lean and obese Adx rats gained the same amount of weight. Adx reduced [3H]PRZ binding in both lean and obese rats by 37-70% in the amygdala (ABM, ACN, MAN) compared to sham-operated rats. But, Adx selectively reduced [3H]PRZ binding only in lean rats in the ABL, DMN, ventromedial hypothalamic n. (VMN) and ventroposteromedial thalamic n. In most areas, decreases in maximal binding (Bmax) associated with Adx were accompanied by decreases in Kd. Unlike [3H]PRZ binding, there was no consistent genotype difference in [3H]PAC binding although Adx was followed by increased binding in obese and decreased binding in lean rats in the ABL. In only the VMN, obese rats had a 21% higher alpha 2- to alpha 1-adrenoceptor ratio than lean rats.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Effects of adrenalectomy and deprivation condition on food intake after phenylpropanolamine or clonidine

alpha-Adrenergic receptors within the paraventricular hypothalamus (PVN) modulate feeding such that activation of alpha 2-adrenoceptors by drugs such as clonidine (CLON) increase feeding; whereas activation of alpha 1-adrenoceptors by drugs such as phenylpropanolamine (PPA) suppress feeding. Prior studies suggest that the feeding-stimulatory effect of alpha 2-adrenergic activation is a function of drug dose as well as the deprivation condition and adrenal status of the animal. Specifically, CLON's effects on feeding are greatest at low doses in food-satiated adrenally intact rats. Whether a similar profile is produced by alpha 1-adrenoceptor agonists such as PPA has not previously been explored. Thus, the present study provides a comparison of the effects on food intake of drug dose, deprivation condition, and adrenalectomy induced by these alpha 2- and alpha 1-adrenergic drugs. Accordingly, both adrenalectomized (ADX) as well as sham-control (SHAM) adult male rats underwent a series of 1-h feeding tests following administration of PPA (5, 10, 20 mg/kg, IP) as well as CLON (0.0125, 0.025, 0.05, 0.1 mg/kg, IP) under both deprived and nondeprived testing conditions. The results suggest that the deprivation condition, but not the surgical condition (ADX vs. SHAM), exerts the greatest overall effect on food intake following administration of alpha-adrenergic drugs.

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.

Microinjection of the alpha 1-agonist methoxamine into the paraventricular hypothalamus induces anorexia in rats

Adrenergic receptors within the paraventricular hypothalamus (PVN) play a prominent role in the control of food intake: stimulation of alpha 2-adrenoceptors induces food intake whereas stimulation of alpha 1-adrenoceptors suppresses food intake. This study further examines the role of PVN alpha 1-adrenoceptors by examining the effects on food and water intake of the alpha 1-adrenergic agonist methoxamine (100, 200, 400 nMol) microinjected into the rat paraventricular hypothalamus. Methoxamine suppressed food intake but not water intake. Doses of 100, 200, and 400 nMol methoxamine suppressed food intake by 47%, 64%, and 96%, respectively. These results further confirm the hypothesis that administration of alpha 1-agonists into the PVN acts to significantly suppress food intake; an action that is in opposition to the facilitory effects of alpha 2-adrenergic agonists on food intake.

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.