Galanin and galanin-like peptide differentially modulate neuronal activities in rat arcuate nucleus neurons

Sleep-promoting neurons remodel their response properties to calibrate sleep drive with environmental demands

Falling asleep at the wrong time can place an individual at risk of immediate physical harm. However, not sleeping degrades cognition and adaptive behavior. To understand how animals match sleep need with environmental demands, we used live-brain imaging to examine the physiological response properties of the dorsal fan-shaped body (dFB) following interventions that modify sleep (sleep deprivation, starvation, time-restricted feeding, memory consolidation) in Drosophila. We report that dFB neurons change their physiological response-properties to dopamine (DA) and allatostatin-A (AstA) in response to different types of waking. That is, dFB neurons are not simply passive components of a hard-wired circuit. Rather, the dFB neurons intrinsically regulate their response to the activity from upstream circuits. Finally, we show that the dFB appears to contain a memory trace of prior exposure to metabolic challenges induced by starvation or time-restricted feeding. Together, these data highlight that the sleep homeostat is plastic and suggests an underlying mechanism.

Unified Classification of Molecular, Network, and Endocrine Features of Hypothalamic Neurons

Peripheral endocrine output relies on either direct or feed-forward multi-order command from the hypothalamus. Efficient coding of endocrine responses is made possible by the many neuronal cell types that coexist in intercalated hypothalamic nuclei and communicate through extensive synaptic connectivity. Although general anatomical and neurochemical features of hypothalamic neurons were described during the past decades, they have yet to be reconciled with recently discovered molecular classifiers and neurogenetic function determination. By interrogating magnocellular as well as parvocellular dopamine, GABA, glutamate, and phenotypically mixed neurons, we integrate available information at the molecular, cellular, network, and endocrine output levels to propose a framework for the comprehensive classification of hypothalamic neurons. Simultaneously, we single out putative neuronal subclasses for which future research can fill in existing gaps of knowledge to rationalize cellular diversity through function-determinant molecular marks in the hypothalamus.

Leptin modulates nutrient reward via inhibitory galanin action on orexin neurons

Objective

Leptin modulates food reward via central leptin receptor (LepRb) expressing neurons. Food reward requires stimulation of midbrain dopamine neurons and is modulated by central leptin action, but the exact central mechanisms remain unclear. Stimulatory and inhibitory leptin actions on dopamine neurons have been reported, e.g. by indirect actions on orexin neurons or via direct innervation of dopamine neurons in the ventral tegmental area.

Methods

We showed earlier that LepRb neurons in the lateral hypothalamus (LHA) co-express the inhibitory acting neuropeptide galanin (GAL-LepRb neurons). We studied the involvement of GAL-LepRb neurons to regulate nutrient reward in mice with selective LepRb deletion from galanin neurons (GAL-LepRb^KO mice).

Results

We found that the rewarding value and preference for sucrose over fat was increased in GAL-LepRb^KO mice compared to controls. LHA GAL-LepRb neurons innervate orexin neurons, but not the VTA. Further, expression of galanin and its receptor GalR1 are decreased in the LHA of GAL-LepRb^KO mice, resulting in increased activation of orexin neurons.

Conclusion

We suggest galanin as an important mediator of leptin action to modulate nutrient reward by inhibiting orexin neurons.

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GAL-LepRb^KO shows ↓ galanin and ↓ GalR1 mRNA, ↑ body weight gain.

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GAL-LepRb^KO shows ↑ orexin/hypocretin neuronal activation.

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GAL-LepRb neurons innervate local orexin/hypocretin and noradrenergic locus coeruleus neurons.

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Leptin regulates natural reward and body weight via GAL-LepRb neurons.

Physiology, signaling, and pharmacology of galanin peptides and receptors: three decades of emerging diversity

Galanin was first identified 30 years ago as a "classic neuropeptide," with actions primarily as a modulator of neurotransmission in the brain and peripheral nervous system. Other structurally-related peptides-galanin-like peptide and alarin-with diverse biologic actions in brain and other tissues have since been identified, although, unlike galanin, their cognate receptors are currently unknown. Over the last two decades, in addition to many neuronal actions, a number of nonneuronal actions of galanin and other galanin family peptides have been described. These include actions associated with neural stem cells, nonneuronal cells in the brain such as glia, endocrine functions, effects on metabolism, energy homeostasis, and paracrine effects in bone. Substantial new data also indicate an emerging role for galanin in innate immunity, inflammation, and cancer. Galanin has been shown to regulate its numerous physiologic and pathophysiological processes through interactions with three G protein-coupled receptors, GAL1, GAL2, and GAL3, and signaling via multiple transduction pathways, including inhibition of cAMP/PKA (GAL1, GAL3) and stimulation of phospholipase C (GAL2). In this review, we emphasize the importance of novel galanin receptor-specific agonists and antagonists. Also, other approaches, including new transgenic mouse lines (such as a recently characterized GAL3 knockout mouse) represent, in combination with viral-based techniques, critical tools required to better evaluate galanin system physiology. These in turn will help identify potential targets of the galanin/galanin-receptor systems in a diverse range of human diseases, including pain, mood disorders, epilepsy, neurodegenerative conditions, diabetes, and cancer.

Galanin and galanin-like peptide modulate vasopressin and oxytocin release in vitro: the role of galanin receptors

Galanin (Gal) and galanin-like peptide (GALP) may be involved in the mechanisms of the hypothalamo-neurohypophysial system. The aim of the present in vitro study was to compare the influence of Gal and GALP on vasopressin (AVP) and oxytocin (OT) release from isolated rat neurohypophysis (NH) or hypothalamo-neurohypophysial explants (Hth-NH). The effect of Gal/GALP on AVP/OT secretion was also studied in the presence of galantide, the non-selective galanin receptors antagonist. Gal at concentrations of 10(-10 )M and 10(-8 )M distinctly inhibited basal and K(+)-stimulated AVP release from the NH and Hth-NH explants, whereas Gal exerted a similar action on OT release only during basal incubation. Gal added to the incubation medium in the presence of galantide did not exert any action on the secretion of either neurohormone from NH and Hth-NH explants. GALP (10(-10 )M and 10(-9 )M) induced intensified basal AVP release from the NH and Hth-NH complex as well as the release of potassium-evoked AVP from the Hth-NH. The same effect of GALP has been observed in the presence of galantide. GALP added to basal incubation medium was the reason for stimulated OT release from the NH as well as from the Hth-NH explants. However, under potassium-stimulated conditions, OT release from the NH and Hth-NH complexes has been observed to be distinctly impaired. Galantide did not block this inhibitory effect of GALP on OT secretion. It may be concluded that: (i) Gal as well as GALP modulate AVP and OT release at every level of the hypothalamo-neurohypophysial system; (ii) Gal acts in the rat central nervous system as the inhibitory neuromodulator for AVP and OT release via its galanin receptors; (iii) the stimulatory effect of GALP on AVP and OT release is likely to be mediated via an unidentified specific GALP receptor(s).

Chronic pain. Decreased motivation during chronic pain requires long-term depression in the nucleus accumbens

Several symptoms associated with chronic pain, including fatigue and depression, are characterized by reduced motivation to initiate or complete goal-directed tasks. However, it is unknown whether maladaptive modifications in neural circuits that regulate motivation occur during chronic pain. Here, we demonstrate that the decreased motivation elicited in mice by two different models of chronic pain requires a galanin receptor 1-triggered depression of excitatory synaptic transmission in indirect pathway nucleus accumbens medium spiny neurons. These results demonstrate a previously unknown pathological adaption in a key node of motivational neural circuitry that is required for one of the major sequela of chronic pain states and syndromes.

Leptin receptor neurons in the mouse hypothalamus are colocalized with the neuropeptide galanin and mediate anorexigenic leptin action

Leptin acts centrally via leptin receptor (LepRb)-expressing neurons to regulate food intake, energy expenditure, and other physiological functions. LepRb neurons are found throughout the brain, and several distinct populations contribute to energy homeostasis control. However, the function of most LepRb populations remains unknown, and their contribution to regulate energy homeostasis has not been studied. Galanin has been hypothesized to interact with the leptin signaling system, but literature investigating colocalization of LepRb and galanin has been inconsistent, which is likely due to technical difficulties to visualize both. We used reporter mice with green fluorescent protein expression from the galanin locus to recapitulate the colocalization of galanin and leptin-induced p-STAT3 as a marker for LepRb expression. Here, we report the existence of two populations of galanin-expressing LepRb neurons (Gal-LepRb neurons): in the hypothalamus overspanning the perifornical area and adjacent dorsomedial and lateral hypothalamus [collectively named extended perifornical area (exPFA)] and in the brainstem (nucleus of the solitary tract). Surprisingly, despite the known orexigenic galanin action, leptin induces galanin mRNA expression and stimulates LepRb neurons in the exPFA, thus conflicting with the expected anorexigenic leptin action. However, we confirmed that intra-exPFA leptin injections were indeed sufficient to mediate anorexic responses. Interestingly, LepRb and galanin-expressing neurons are distinct from orexin or melanin-concentrating hormone (MCH)-expressing neurons, but exPFA galanin neurons colocalized with the anorexigenic neuropeptides neurotensin and cocaine- and amphetamine-regulated transcript (CART). Based on galanin's known inhibitory function, we speculate that in exPFA Gal-LepRb neurons galanin acts inhibitory rather than orexigenic.

Exposure to cocaine alters dynorphin-mediated regulation of excitatory synaptic transmission in nucleus accumbens neurons

BACKGROUND

Dysregulation of excitatory synaptic input to nucleus accumbens (NAc) medium spiny neurons (MSNs) underlies a key pathophysiology of drug addiction and addiction-associated emotional and motivational alterations. Dynorphin peptides, which exhibit higher affinity to κ type opioid receptors, are upregulated within the NAc upon exposure to cocaine administration, and the increased dynorphin-signaling in the NAc has been critically implicated in negative mood observed in cocaine- or stress-exposed animals. Despite such apparent behavioral significance of the NAc dynorphins, the understanding of how dynorphins regulate excitatory synaptic transmission in the NAc remains incomplete.

METHODS

We used electrophysiological recording in brain slices to examine the effects of dynorphins on excitatory synaptic transmission in the NAc.

RESULTS

We focused on two key dynorphins, dynorphin A and B. Our current results show that dynorphin A and B differentially regulated excitatory postsynaptic currents (EPSCs) in NAc MSNs. Whereas perfusions of both dynorphin A and B to NAc slices decreased EPSCs in MSNs, the effect of dynorphin A but not dynorphin B was completely reversed by the κ receptor-selective antagonist nor-binaltorphimine. These results implicate κ receptor-independent mechanisms in dynorphin B-mediated synaptic effects in the NAc. Furthermore, repeated exposure to cocaine (15 mg/kg/day via intraperitoneal injection for 5 days, with 1, 2, or 14 days withdrawal) completely abolished dynorphin A-mediated modulation of EPSCs in NAc MSNs, whereas the effect of dynorphin B remained largely unchanged.

CONCLUSIONS

Given the quantitatively higher abundance of dynorphin B in the NAc, our present results suggest that the dynorphin B-mediated, κ receptor-independent pathways predominate in the overall effect of dynorphins in cocaine-pretreated animals and potentially in cocaine-induced alterations in mood.

Galanin-like peptide: a key player in the homeostatic regulation of feeding and energy metabolism?

The hypothalamus has a critical role in the regulation of feeding behavior, energy metabolism and reproduction. Galanin-like peptide (GALP), a novel 60 amino-acid peptide with a nonamidated C-terminus, was first discovered in porcine hypothalamus. GALP is mainly produced in the hypothalamic arcuate nucleus and is involved in the regulation of feeding behavior and energy metabolism, with GALP-containing neurons forming networks with several feeding-regulating peptide-containing neurons. The effects of GALP on food intake and body weight are complex. In rats, the central effect of GALP is to first stimulate and then reduce food intake, whereas in mice, GALP has an anorectic function. Furthermore, GALP regulates plasma luteinizing hormone levels through activation of gonadotropin-releasing hormone-producing neurons, suggesting that it is also involved in the reproductive system. This review summarizes the research on these topics and discusses current evidence regarding the function of GALP, particularly in relation to feeding and energy metabolism. We also discuss the effects of GALP activity on food intake, body weight and locomotor activity after intranasal infusion, a clinically viable mode of delivery.

Regulation of energy homeostasis by bombesin receptor subtype-3: selective receptor agonists for the treatment of obesity

Bombesin receptor subtype 3 (BRS-3) is a G protein coupled receptor whose natural ligand is unknown. We developed potent, selective agonist (Bag-1, Bag-2) and antagonist (Bantag-1) ligands to explore BRS-3 function. BRS-3-binding sites were identified in the hypothalamus, caudal brainstem, and several midbrain nuclei that harbor monoaminergic cell bodies. Antagonist administration increased food intake and body weight, whereas agonists increased metabolic rate and reduced food intake and body weight. Prolonged high levels of receptor occupancy increased weight loss, suggesting a lack of tachyphylaxis. BRS-3 agonist effectiveness was absent in Brs3(-/Y) (BRS-3 null) mice but was maintained in Npy(-/-)Agrp(-/-), Mc4r(-/-), Cnr1(-/-), and Lepr(db/db) mice. In addition, Brs3(-/Y) mice lost weight upon treatment with either a MC4R agonist or a CB1R inverse agonist. These results demonstrate that BRS-3 has a role in energy homeostasis that complements several well-known pathways and that BRS-3 agonists represent a potential approach to the treatment of obesity.

Effects of galanin on monoaminergic systems and HPA axis: Potential mechanisms underlying the effects of galanin on addiction- and stress-related behaviors

Like a number of neuropeptides, galanin can alter neural activity in brain areas that are important for both stress-related behaviors and responses to drugs of abuse. Accordingly, drugs that target galanin receptors can alter behavioral responses to drugs of abuse and can modulate stress-related behaviors. Stress and drug-related behaviors are interrelated: stress can promote drug-seeking, and drug exposure and withdrawal can increase activity in brain circuits involved in the stress response. We review here what is known about the ability of galanin and galanin receptors to alter neuronal activity, and we discuss potential mechanisms that may underlie the effects of galanin on behaviors involved in responses to stress and addictive drugs. Understanding the mechanisms underlying galanin's effects on neuronal function in brain regions related to stress and addiction may be useful in developing novel therapeutics for the treatment of stress- and addiction-related disorders.

Galanin-like peptide: a role in the homeostatic regulation of energy balance?

Galanin-like peptide (GALP) is a neuropeptide that has been proposed to play a role in the regulation of food intake behaviour and body weight. However, the actions of GALP on energy balance are complex. In rats, it appears to impel both appetite stimulating and suppressing effects, whereas in mice, the only effect is a reduction in food intake. Thus, it is currently unclear whether GALP is important in the homeostatic regulation of energy balance, or if it produces effects on appetite and body weight by non-specific actions. This review discusses current evidence of the role of GALP with respect to energy balance, and the mechanisms involved in its regulation. We describe recent evidence that suggests that GALP may elicit differential effects in different rodent species. Furthermore, we provide an insight into a potential novel role for GALP in inflammation, and discuss how this may relate to the non-homeostatic regulation of energy balance.

Alarin is a vasoactive peptide

Galanin-like peptide (GALP) is a hypothalamic neuropeptide belonging to the galanin family of peptides. The GALP gene is characterized by extensive differential splicing in a variety of murine tissues. One splice variant excludes exon 3 and results in a frame shift leading to a novel peptide sequence and a stop codon after 49 aa. In this peptide, which we termed alarin, the signal sequence of the GALP precursor peptide and the first 5 aa of the mature GALP are followed by 20 aa without homology to any other murine protein. Alarin mRNA was detected in murine brain, thymus, and skin. In accordance with its vascular localization, the peptide exhibited potent and dose-dependent vasoconstrictor and anti-edema activity in the cutaneous microvasculature, as was also observed with other members of the galanin peptide family. However, in contrast to galanin peptides in general, the physiological effects of alarin do not appear to be mediated via the known galanin receptors. Alarin adds another facet to the surprisingly high-functional redundancy of the galanin family of peptides.

Immunolesions of glucoresponsive projections to the arcuate nucleus alter glucoprivic-induced alterations in food intake, luteinizing hormone secretion, and GALP mRNA, but not sex behavior in adult male rats

Metabolic signals such as insulin, leptin and glucose are known to alter hypothalamic function. Although insulin and leptin are known to directly alter hypothalamic areas that regulate reproduction, the mechanisms by which glucose alters reproductive function are not as clear. Catecholaminergic neurons in the A1/C1 region in the hindbrain are glucose-responsive and project to the arcuate nucleus. To determine if this pathway is involved in the regulation of sex behavior and luteinizing hormone (LH) secretion, this catecholaminergic pathway was lesioned with injections of saporin conjugated with anti-dopamine-beta-hydroxylase (DSAP) or unconjugated saporin (SAP) in adult male rats. Rats were given glucoprivic challenges and feeding and sex behavior was observed. As was expected, the DSAP-treated rats showed decreased feeding during glucoprivation (250 mg/kg 2-deoxy-D-glucose, 2DG) compared to SAP controls. Glucoprivation caused a significant reduction in sex behavior in both SAP and DSAP animals equally, compared to saline treatments (p < 0.05). At the end of the experiment, animals were given a final challenge with 2DG or saline, euthanized by decapitation and trunk blood was assayed for plasma LH levels. In situ hybridization analysis revealed that 2DG treatment caused a significant reduction in GALP mRNA in SAP controls compared to saline treatment. This reduction in GALP mRNA was prevented with DSAP treatment. In SAP animals, 2DG elicited a significant decrease in plasma LH levels (p < 0.05); this reduction in plasma LH was absent in the DSAP-treated male rats. These data indicate that the A1/C1 efferents to the ventromedial hypothalamus are involved in the glucostatic regulation of GALP mRNA, feeding behavior and LH secretion, but not sex behavior in the adult male rat.