Adrenoceptors and the pharmacology of affective illness: a unifying theory

Based on recent clinical and preclinical research, it is theorized that antimanic and antidepressant effects of clinically available drugs can be produced through their actions on alpha-1 adrenoreceptor-mediated neurotransmission in the central nervous system. The theory suggests that final effects on alpha-1 mediated neurotransmission may be produced not only by drugs which have direct effects on the alpha-1 receptor or its second messenger, but also by drugs having effects on neurotransmitter systems such as acetylcholine, GABA, and serotonin, among others, which modulate the activity of central norepinephrine neurons or, via feedback mechanisms, by drugs having effects on adrenergic receptors other than the alpha-1 receptor itself.     

Epidermal growth factor and the nervous system

Various growth factors and their receptors are present in the nervous system. This review focuses on the presence of epidermal growth factor (EGF) and its receptors in the central nervous system (CNS). Evidence indicates that EGF in the CNS is the result of local synthesis, by intrinsic and blood-derived macrophages, glial cells and neurons, and uptake from the peripheral blood through the circumventricular organs and probably also through the blood-brain barrier. Evidence is accumulating suggesting that EGF regulates a variety of CNS functions in a specific manner. EGF influences CNS growth, differentiation and maintenance (actions proposed to promote neural regeneration and cell survival following a variety of insults). EGF also induces neuromodulatory actions, affects the neuroendocrine system, and suppresses food intake and gastric acid secretion. Acute and chronic pathological processes, e.g., various cancers, stimulate the production and release of EGF in various cell systems. Monitoring of EGF by the CNS may participate in several neurological manifestations (e.g., appetite suppression, neuroendocrine alterations) frequently accompanying acute and chronic disease. EGF and transforming growth factor-alpha (TGF-alpha, a factor that binds to the EGF receptor with high affinity and induces the same biological signals as EGF) also may be involved in the promotion of malignancy in the CNS and in the neuropathogenesis of degenerative disorders. Thus evidence is accumulating concerning the neurophysiological or neuropathophysiological significance of EGF in the nervous system.     

Functional Heterogeneity of Arcuate Nucleus Pro-Opiomelanocortin Neurons: Implications for Diverging Melanocortin Pathways

Arcuate nucleus (ARC) pro-opiomelanocortin (POMC) neurons are essential regulators of food intake, energy expenditure, and glucose homeostasis. POMC neurons integrate several key metabolic signals that include neurotransmitters and hormones. The change in activity of POMC neurons is relayed to melanocortin receptors in distinct regions of the central nervous system. This review will summarize the role of leptin and serotonin receptors in regulating the activity of POMC neurons and provide a model in which different melanocortin pathways regulate energy and glucose homeostasis.     

Towards the therapeutic use of intranasal neuropeptide administration in metabolic and cognitive disorders

The nose provides an effective way for delivering neuropeptides to the central nervous system, bypassing the blood-brain barrier and avoiding systemic side effects. Thereby intranasal neuropeptide administration enables the modulation of central nervous signaling pathways of body weight regulation and cognitive functions. Central nervous control of energy homeostasis is assumed to rely on hypothalamic neuropeptidergic pathways that are triggered by the peripheral adiposity signals insulin and leptin conveying the amount of body fat to the brain. Melanocortins, including alpha-melanocyte stimulating hormone (alpha-MSH), are essential for inducing anorexigenic/catabolic effects, i.e. for inhibiting caloric intake and increasing energy expenditure. Insulin, in addition to its function as an adiposity signal, also influences memory formation. Here we present a series of studies on the intranasal administration of MSH/ACTH(4-10), a melanocortin receptor agonist, and of insulin. Prolonged administration of MSH/ACTH(4-10) induced weight loss in normal-weight, but not in overweight humans. Intranasal insulin reduced body fat and improved memory functions in the absence of adverse peripheral side effects. Our results may contribute to the future development of therapeutic strategies in disorders like obesity and cognitive impairments that derive from dysfunctions of central nervous neuropeptidergic pathways.     

Effect of intraventricular injection of 5,7-dihydroxytryptamine on regional tryptophan hydroxylase of rat brain

5,6-DIHYDROXYTRYPTAMINE has been shown to cause selective degeneration of serotonergic neurons in the central nervous sytem (BAUMGARTEN, LACHENMAYER and SCHLOSSBERGER, 1972b). This degeneration is accompanied by depletion of serotonin (BAUMGARTEN et al., 1971; 1972a) and loss of tryptophan hydroxylase activity (VICTOR, BAUMGARTEN and LOVENBERG, 1973) in certain regions of the brain. In the current experiments, the effect of 5,7-dihydroxytryptamine (another dihydroxylated tryptamine derivative) on tryptophan hydroxylase activity has been examined. Since tryptophan hydroxylase is the rate-limiting enzyme in serotonin biosynthesis and has a similar distribution to that of serotonin in the brain, it is used as a biochemical marker of serotonergic neurons, Recent experiments also indicate that 5,7-dihydroxytryptamine causes morphological damage to serotonergic neurons of the central nervous system (BAUMGARTEN and LACHENMAYER, 1972).     

A family of Drosophila serotonin receptors with distinct intracellular signalling properties and expression patterns.

Biogenic amines such as serotonin elicit or modulate a wide range of behaviours by interacting with multiple receptor subtypes. We have isolated cDNA clones encoding three distinct Drosophila serotonin receptors which belong to the G protein-coupled receptor family. When expressed in mammalian cells, these receptors activate different intracellular effector systems. The 5HT-dro1 receptor stimulates adenylate cyclase while the 5HT-dro2A and the 5HT-dro2B receptors inhibit adenylate cyclase and activate phospholipase C. Expression of all three receptors starts in late embryos and is restricted to distinct populations of cells in the central nervous system. The 5HT-dro2A receptor is predominantly expressed in midline motor neurons (VUM neurons) that innervate larval muscles thus suggesting a role for this receptor in motor control.     

Prenatal diagnosis: a chance? risk? dilemma?

In research on congenital metabolic disorders, a biochemist can choose between the theoretical and the practical approach. The diagnosis of metabolic diseases relies on 1) the determination of the presence of metabolites under normal conditions that are direct substrates of the defective enzyme (e.g., the Gm2 ganglioside in the brain tissue of a patient with Tay-Sachs disease); 2) the determination of the lack or insufficiency of the direct product of the defective enzyme (e.g., aryl sulfatase A in the cells of patients with metachromatic leukodystrophy), hormone (hypothyroidism), or receptor (congenital hypercholesterolemia); 3) determination of substance whose reduction was established by experimentation, but the cause of the decrease is not known (ceruloplasmin in Wilson's disease); and 4) DNA analysis. Metabolic impairment of genetic origin is not treatable. The disease can be prevented by 1) removing the inappropriate metabolite (e.g., copper accumulation can be avoided by giving penicillamine or zinc salts); 2) limiting those substances in the critical phase of childhood that are components of the defective enzyme (e.g. gluten reduction in colic and protein in phenylketonuria); 3) supplementing the insufficient metabolite (e.g., phosphate in hypophosphatemia by sound for 12 hours a day); 4) protecting the patients (e.g. from light in porphyria); and 5) treatment by substances (giving coagulation factor VIII in hemophilia and thyroid hormones in hypothyroidism). There is a dilemma in subjecting patients to a diagnosis of progression to Huntington's chorea 20 years in advance or informing them about the high risk of hereditary disease for the next child (25% for the recessive and 50% for the dominant mode). Ethical committees have usually opted for a recommendation of selective abortion in clear-cut cases. Increasingly refined diagnostic methods have magnified the responsibility of the biochemist.     

Serotonin reciprocally regulates melanocortin neurons to modulate food intake

The neural pathways through which central serotonergic systems regulate food intake and body weight remain to be fully elucidated. We report that serotonin, via action at serotonin1B receptors (5-HT1BRs), modulates the endogenous release of both agonists and antagonists of the melanocortin receptors, which are a core component of the central circuitry controlling body weight homeostasis. We also show that serotonin-induced hypophagia requires downstream activation of melanocortin 4, but not melanocortin 3, receptors. These results identify a primary mechanism underlying the serotonergic regulation of energy balance and provide an example of a centrally derived signal that reciprocally regulates melanocortin receptor agonists and antagonists in a similar manner to peripheral adiposity signals.     

Glucagon-like peptide 1 (7-36) amide (GLP-1) and exendin-4 stimulate serotonin release in rat hypothalamus

Glucagon-like peptide 1 (7-36) amide (GLP-1) and exendin-4 are gastrointestinal hormones as well as neuropeptides involved in glucose homeostasis and feeding regulation, both peripherally and at the central nervous system (CNS), acting through the same GLP-1 receptor. Aminergic neurotransmitters play a role in the modulation of feeding in the hypothalamus and we have previously found that peripheral hormones and neuropeptides, which are known to modulate feeding in the central nervous system, are able to modify catecholamine and serotonin release in the hypothalamus. In the present paper we have evaluated the effects of GLP-1 and exendin-4 on dopamine, norepinephrine, and serotonin release from rat hypothalamic synaptosomes, in vitro. We found that glucagon-like peptide 1 (7-36) amide and exendin-4 did not modify either basal or depolarization-induced dopamine and norepinephrine release; on the other hand glucagon-like peptide 1 (7-36) amide and exendin-4 stimulated serotonin release, in a dose dependent manner. We can conclude that the central anorectic effects of GLP-1 agonists could be partially mediated by increased serotonin release in the hypothalamus, leaving the catecholamine release unaffected.     

Serotonin and energy balance: molecular mechanisms and implications for type 2 diabetes

The neurotransmitter serotonin is an important regulator of energy balance. In the brain, serotonergic fibres from midbrain raphe nuclei project to key feeding centres, where serotonin acts on specific receptors to modulate the activity of various downstream neuropeptide systems and autonomic pathways and thus affects ingestive behaviour and energy expenditure. Serotonin, released by intestinal enterochromaffin cells, also appears to regulate energy homeostasis through peripheral mechanisms. Serotonergic effects on energy balance lead to secondary effects on glucose homeostasis, based on a well-established link between obesity and insulin resistance. However, serotonergic pathways may also directly affect glucose homeostasis through regulation of autonomic efferents and/or action on peripheral tissues. Several serotonergic compounds have been evaluated for clinical use in the treatment of obesity and type 2 diabetes; results of these trials are discussed here. Finally, future directions in the elucidation of serotonergic metabolic regulation are discussed.     

The Mitochondrial Permeability Transition as a Target for Neuroprotection

Mitochondria serve as checkpoints and amplifiers on cell death pathways. In the central nervous system, mitochondrial involvement seems essential for normal expression of cell death phenotypes, and interference with these pathways thus seems a reasonable approach to neuroprotection. We have been involved in examining the potential involvement of the mitochondrial permeability transition (mPT) as one of several possible mechanisms by which mitochondria may be drawn into these death cascades. This possibility, though still controversial, is supported by evidence that factors that may stimulate mPT induction are associated with some forms of cell death (e.g., in stroke) and are modulated by diseases of the central nervous system (e.g., Huntington's). Evidence of neuroprotection seen with compounds such as N-Met-Val cyclosporine also support this possibility.     

Development and sensitivity to serotonin of Drosophila serotonergic varicosities in the central nervous system

Serotonin is a classical small-molecule neurotransmitter with known effects on developmental processes. Previous studies have shown a developmental role for serotonin in the fly peripheral nervous system. In this study, we show that serotonin can modulate the development of serotonergic varicosities within the fly central nervous system. We have developed a system to examine the development of serotonergic varicosities in the larval CNS. We use this method to describe the normal serotonergic development in the A7 abdominal ganglion. From first to third instar larvae, the volume of the neuropil and number of serotonergic varicosities increase substantially while the varicosity density remains relatively constant. We hypothesize that serotonin is an autoregulator for serotonergic varicosity density. We tested the sensitivity of serotonergic varicosities to serotonin by adding neurotransmitter at various stages to isolated larval ventral nerve cords. Addition of excess exogenous serotonin decreases native varicosity density in older larvae, and these acute effects are reversible. The effects of serotonin appear to be selective for serotonergic varicosities, as dopaminergic and corazonergic varicosities remain qualitatively intact following serotonin application.     

Neural control of the anorexia-cachexia syndrome

The anorexia-cachexia syndrome is a debilitating clinical condition characterizing the course of chronic diseases, which heavily impacts on patients' morbidity and quality of life, ultimately accelerating death. The pathogenesis is multifactorial and reflects the complexity and redundancy of the mechanisms controlling energy homeostasis under physiological conditions. Accumulating evidence indicates that, during disease, disturbances of the hypothalamic pathways controlling energy homeostasis occur, leading to profound metabolic changes in peripheral tissues. In particular, the hypothalamic melanocortin system does not respond appropriately to peripheral inputs, and its activity is diverted largely toward the promotion of catabolic stimuli (i.e., reduced energy intake, increased energy expenditure, possibly increased muscle proteolysis, and adipose tissue loss). Hypothalamic proinflammatory cytokines and serotonin, among other factors, are key in triggering hypothalamic resistance. These catabolic effects represent the central response to peripheral challenges (i.e., growing tumor, renal, cardiac failure, disrupted hepatic metabolism) that are likely sensed by the brain through the vagus nerve. Also, disease-induced changes in fatty acid oxidation within hypothalamic neurons may contribute to the dysfunction of the hypothalamic melanocortin system. Ultimately, sympathetic outflow mediates, at least in part, the metabolic changes in peripheral tissues. Other factors are likely involved in the pathogenesis of the anorexia-cachexia syndrome, and their role is currently being elucidated. However, available evidence shows that the constellation of symptoms characterizing this syndrome should be considered, at least in part, as different phenotypes of common neurochemical/metabolic alterations in the presence of a chronic inflammatory state.     

Role of lipids and lipid signaling in the development of cannabinoid tolerance

Cannabinoid agonists such as Delta9-tetrahydrocannabinol (THC) produce a wide range of pharmacological effects both in the central nervous system and in the periphery. One of the most striking features of cannabinoids such as THC is the magnitude to tolerance that can be produced upon repetitive administration of this substance to animals. Relatively modest dosing regimens are capable of producing significant tolerance, whereas greater than 100-fold tolerance can be obtained with aggressive treatments. While cannabinoid tolerance has been studied quite extensively to establish its relevance to the health consequences of marijuana use, it has also proven to be a valuable strategy in understanding the mechanism of action of cannabinoids. The discovery of the endocannabinoid system that contains two receptor subtypes, CB1 and CB2, associated signaling pathways, endocannabinoids (anandamide and 2-arachidonoylglycerol) and their synthetic and degradative pathways has provided a means of systematically evaluating the mechanism of cannabinoid tolerance. It is well known that the CB1 cannabinoid receptor is down-regulated in states of cannabinoid tolerance along with uncoupling from its second messenger systems. Endocannabinoid levels are also altered in selected brain regions during the development of tolerance. While it is reasonable to speculate that a likely relationship exists between receptor and endocannabinoid levels, at present, little is known regarding the biological signal that leads to alterations in endocannabinoid levels. It is also unknown to what degree synthetic and degradative pathways for the endocannabinoids are altered in states of tolerance. The discovery that the brain is abundant in fatty acid amides and glycerols raises the question as to what roles these lipids contribute to the endocannabinoid system. Some of these lipids also utilize the endocannabinoid metabolic pathways, produce similar pharmacological effects, and are capable of modulating the actions of anandamide and 2-arachidonoylglycerol. In addition, there are dopamine, glycine, and serotonin conjugates of arachidonic acid that may also contribute to the actions of endocannabinoids. A systematic examination of these lipids in cannabinoid tolerance might shed light on their physiological relevance to the endocannabinoid system.     

Activation and targeting of mitogen-activated protein kinases by G-protein-coupled receptors

Over the past decade, it has become apparent that many G-protein-coupled receptors (GPCRs) generate signals that control cellular differentiation and growth, including stimulation of Ras family GTPases and activation of mitogen-activated protein (MAP) kinase pathways. The mechanisms that GPCRs use to control the activity of MAP kinases vary between receptor and cell type but fall broadly into one of three categories: signals initiated by classical G protein effectors, e.g., protein kinase (PK)A and PKC, signals initiated by cross-talk between GPCRs and classical receptor tyrosine kinases, e.g., "transactivation" of epidermal growth factor (EGF) receptors, and signals initiated by direct interaction between beta-arrestins and components of the MAP kinase cascade, e.g., beta-arrestin "scaffolds". While each of these pathways results in increased cellular MAP kinase activity, emerging data suggest that they are not functionally redundant. MAP kinase activation occurring via PKC-dependent pathways and EGF receptor transactivation leads to nuclear translocation of the kinase and stimulates cell proliferation, while MAP kinase activation via beta-arrestin scaffolds primarily increases cytosolic kinase activity. By controlling the spatial and temporal distribution of MAP kinase activity within the cell, the consequences of GPCR-stimulated MAP kinase activation may be determined by the mechanism by which they are activated.     

Interaction between noradrenaline or adrenaline and the beta 1-adrenergic receptor in the nervous system triggers early metamorphosis of larvae in the ascidian,Ciona savignyi

Molecular mechanisms underlying the metamorphosis of larvae, e.g., ligand and receptor interaction, have to be determined and roles for the nervous system in marine invertebrates are not well understood. We report here that treatment of swimming larvae of the ascidian Ciona savignyi with noradrenaline or adrenaline promoted morphological changes in early metamorphosis, e.g., tail resorption. Antagonists of the beta-adrenergic receptor, propranolol, and the beta(1)-adrenergic receptor, metoprolol, inhibited the noradrenaline-induced tail resorption, while an antagonist of the alpha-adrenergic receptor, phentolamine, and of the beta(2)- adrenergic receptor, butoxamine, had no inhibitory effects. In addition, a selective agonist of the beta-adrenergic receptor, isoproterenol, the concentration of which was lower than the effective concentration of the neurotransmitters, facilitated tail resorption. Immunohistochemical studies, using an anti-dopamine-hydroxylase antibody, showed that neurotransmitters such as noradrenaline and adrenaline localized around the brain vesicle of the larvae during metamorphosis. The beta(1)-adrenergic receptor stained with antibodies was localized on the nervous system. Temporal expression of the beta(1)-adrenergic receptor was intense in the nervous system in the larvae competent for metamorphosis. We propose that interactions between noradrenaline or adrenaline and the beta(1)-adrenergic receptor in the nervous system mediate the process of metamorphosis of Ciona larvae.