Influence of chronic administration of antidepressant drugs on mRNA for galanin, galanin receptors, and tyrosine hydroxylase in catecholaminergic and serotonergic cell-body regions in rat brain

Activity of locus coeruleus (LC) neurons and release of the peptide galanin (GAL), which is colocalized with norepinephrine (NE) in LC neurons, has been implicated in depression and, conversely, in antidepressant action. The present study examined the influence of chronic administration (for 14days, via subcutaneously-implanted minipump) of antidepressant (AD) drugs representing three different classes (tricyclic [desipramine], selective serotonin reuptake inhibitor [SSRI] [paroxetine], and monoamine oxidase inhibitor [MAOI] [phenelzine]) on mRNA for GAL, GAL receptors (GalR1, GalR2, and GalR3), and tyrosine hydroxylase (TH), the rate-limiting enzyme for NE synthesis, in four brain regions--LC, A1/C1, dorsal raphe (DRN), and ventral tegmentum (VTA) of rats. Consistent with previous findings that chronic administration of AD drugs decreases activity of LC neurons, administration of AD drugs reduced mRNA for both GAL and TH in LC neurons. GAL and TH mRNA in LC neurons was highly correlated. AD drugs also reduced GAL and TH mRNA in A1/C1 and VTA but effects were smaller than in LC. The largest change in mRNA for GAL receptors produced by AD administration was to decrease mRNA for GalR2 receptors in the VTA region. Also, mRNA for GalR2 and GalR3 receptors was significantly (positively) correlated in all three predominantly catecholaminergic brain regions (LC, A1/C1, and VTA). Relative to these three brain regions, unique effects were seen in the DRN region, with the SSRI elevating GAL mRNA and with mRNA for GalR1 and GalR3 being highly correlated in this brain region. The findings show that chronic administration of AD drugs, which produces effective antidepressant action, results in changes in mRNA for GAL, GAL receptors, and TH in brain regions that likely participate in depression and antidepressant effects.

Neuroprotective role for galanin in Alzheimer's disease

Galanin (GAL) and GAL receptors (GALR) are overexpressed in degenerating brain regions associated with cognitive decline in Alzheimer's disease (AD). The functional consequences of GAL plasticity in AD are unclear. GAL inhibits cholinergic transmission in the hippocampus and impairs spatial memory in rodent models, suggesting that GAL overexpression exacerbates cognitive impairment in AD. By contrast, gene expression profiling of individual cholinergic basal forebrain (CBF) neurons aspirated from AD tissue revealed that GAL hyperinnervation positively regulates mRNAs that promote CBF neuronal function and survival. GAL also exerts neuroprotective effects in rodent models of neurotoxicity. These data support the growing concept that GAL overexpression preserves CBF neuron function, which may in turn delay the onset of symptoms of AD. Further elucidation of GAL activity in selectively vulnerable brain regions will help gauge the therapeutic potential of GALR ligands in the treatment of AD.

The galanin peptide family: Receptor pharmacology, pleiotropic biological actions, and implications in health and disease

The galanin peptide family consists of the "parental" galanin, galanin-message-associated peptide (GMAP) which derives from the same peptide precursor gene product as galanin, galanin-like peptide (GALP) encoded by a different gene, and the recently discovered peptide alarin which is encoded by a splice variant of the GALP gene. The galanin receptor family currently comprises 3 members, GalR1, GalR2, and GalR3, which are all G-protein-coupled receptors. This review will provide an overview of the comprehensive, pharmacological characterization of endogenous and synthetic galanin receptor ligands and their interactions with the galanin receptors, a summary of the various (pleiotropic) biological actions of galanin and GALP (and alarin), and briefly discuss the implications of pathological changes for health and disease and potential clinical therapeutics. Since its discovery more than 20 years ago, a large number of putative physiological functions have been ascribed to galanin, and active research still continues to validate these functions and determine their importance for physiology and pathology. Since the more recent identification of GALP, considerable research has identified functions for this peptide in the central nervous system (CNS), but the identity of its preferred, native receptor is still unknown. Little is known of the role of alarin apart from evidence of its expression and a vasoactive action in the skin. The wide range of functions of the galanin peptide family indicates an essential role for galanin signaling in "mind and body homeostasis" and a potential therapeutic efficacy in a variety of human diseases, particularly epilepsy, Alzheimer's disease, and diabetes.

The brain galanin receptors: targets for novel antidepressant drugs

Our present view that the mood disorders involve dysfunction of monoaminergic system is a result of important clinical and preclinical observations over the past 40 years. The therapeutic efficacy of drugs such as the tricyclic antidepressants (TCAs), monoamine oxidase inhibitors, selective serotonin reuptake inhibitors (SSRIs) and lately of SNRIs (serotonin and norepinephrine reuptake inhibitors) helped to shape our view that mood regulation involves the monoaminergic systems in some way. It is thus little surprising when the neuropeptide, galanin, is discovered to coexist with norepinephrine (NE) in locus coeruleus (LC) neurons and with serotonin (5-HT) in the dorsal raphe nucleus (DRN) neurons, a link between galanin mediated signaling and mood regulation is sought. Galanin receptors are expressed in brain structures that are involved in the regulation of mood such as frontal cortex, amygdala, hypothalamus, LC, DRN and hippocampus. It is almost an accident of research fate that the potent effects of galanin on cognitive performance and seizure threshold have led galanin research to focus on the hippocampus where the neuropeptide is present in cholinergic and noradrenergic afferents and where the receptor density is much lower than in the monoaminergic nuclei. Hopefully it is not too late to report on the recent inroads into the roles of galanin and of galanin receptor subtypes 2 and 3 (GalR2 and GalR3) in mood regulation in animal models as well as in human patients with major depression. A body of existing data suggests that GalR2 signaling leads to antidepressant-like, anticonvulsant and neurogenesis-promoting effects, a spectrum of activities that are commonly associated with efficacious antidepressants. Similarly, GalR3 antagonists exhibit anxiolytic and antidepressant-like activity, another clinically useful combination for the treatment of mood disorders. Since both GalR2 and GalR3 are G-protein coupled receptors (GPCRs), a favorite target class for drug development, we believe that the pace of developing galaninergic antidepressants will increase significantly from now on.

Expression of galanin receptor messenger RNAs in different regions of the rat gastrointestinal tract

Galanin effects are mediated by three G-protein-coupled receptors: galanin receptor 1 (GalR1), GalR2 and GalR3. We quantified mRNA levels of GalR1, GalR2 and GalR3 in the rat stomach, small and large intestine using real-time RT-PCR. All three GalR mRNAs were detected throughout the gut at different levels. GalR1 and GalR2 mRNA levels were higher in the large than in the small intestine. GalR2 mRNA was most abundant in the stomach. GalR3 mRNA levels were generally quite low. The differential regional distribution of GalRs suggests that the complex effects of galanin in the gut are the result of activating multiple receptor subtypes, whose density, subtype and signaling vary along the gastrointestinal tract.

Galanin and Its Receptors in Neurological Disorders

Galanin is a highly inducible neuropeptide, showing distinct up-regulation after pathological disturbance within the nervous system. Significant increase in galanin expression is observed after peripheral nerve injury, in the basal forebrain in Alzheimer's disease (AD), during neuronal development, and after stimulation with estrogen, while seizure activity deplete galanin in the hippocampus. A wide distribution of galanin and its receptors is seen in the nervous system, often in co-localization with classical neurotransmitters and other neuromodulators. Galanin acts predominantly as an inhibitory, hyperpolarizing neuromodulator on neurotransmitter and glucose-induced insulin release and stimulates growth hormone and prolactin secretion. Galanin has been implicated in several higher order physiological functions including cognition, feeding, nociception, mood regulation, and neuroendocrine modulation. The effects of galanin are mediated via three G protein-coupled receptors with different functional coupling. Moderate to low pharmacological effects are seen by galanin under physiological conditions, in contrast to its dramatic effects on the nervous system after neuronal disturbance. This pathophysiological heavy function of the galaninergic system renders it an interest for disorders such as AD, depression, and epilepsy in terms of side effects. Some properties of the galaninergic system are of particular importance in the context of neurodegeneration. Galanin is highly inducible, 10- to 100-fold, upon nerve injury, whereas most neuropeptides are induced 1.5- to 2-fold. Galanin is strongly neurotrophic during development as well as subsequent to injury. Whereas other neurotrophic neuropeptides like VIP and PACAP activate cAMP synthesis, galanin suppresses its synthesis, yet it is a strong neurotrophic as well as neuroprotective agent. As we delineate which galanin receptor subtype mediates neuroprotective and neurotrophic effects and which mediates synaptic inhibition, pharmacological use of receptor- selective galaninergic ligands for treatment in neurodegenerative diseases are coming closer.

Regulation of feeding by galnon

Galanin is a neuropeptide that has been implicated in multiple bioactivities, inter alia eating disorders. In this study, we have examined the effects of galnon, a novel low molecular weight galanin receptor ligand. Previous studies have shown that galnon acts as a systemically active, blood-brain barrier crossing agonist on galanin signaling both in vitro and in vivo, inhibiting pentylenetetrazole-induced seizures. Here, intracerebroventricular (10-20 microg) and intraperitoneal (1.5-5 mg/kg) administration of galnon induced a strong, dose-dependent reduction of food intake in rats and mice. This reduction in feeding occurred without reducing general activity and was shown to be attenuated by an intracerebroventricular administration of M35, a peptide galanin antagonist. These data demonstrate that galnon is a promising tool for studies of the involvement of galanin in feeding disorders and other behavioral processes.

Novel G protein-coupled receptors as pain targets

G protein-coupled receptors (GPCRs) and their ligands play a number of important roles in the modulation of acute and chronic pain. Indeed, opioid and cannabinoid ligands are of established therapeutic value for pain management, and further exploitation of the specific GPCR subtypes (delta-opioid, CB1 and CB2) for these ligands may yield more selective, potent analgesics with favorable side effects. More recent identification of a number of other GPCRs involved in pain pathways (eg, sensory neuron specific receptors) and selective ligands that modulate pain transmission, has highlighted further therapeutic opportunities. A further challenge to understanding pain modulation and an additional dimension for targeting analgesia is the discovery of GPCR heteromerization and accessory and regulatory proteins, such as regulator of G protein-signaling proteins, involved in expression and regulation of GPCR.