Galanin receptor binding sites in the temporal and occipital cortex are minimally affected in Alzheimer's disease

Synaptic plasticity modulation by circulating peptides and metaplasticity: Involvement in Alzheimer's disease

Synaptic plasticity is a cellular process involved in learning and memory whose alteration in its two main forms (Long Term Depression (LTD) and Long Term Potentiation (LTP)), is observed in most brain pathologies, including neurodegenerative disorders such as Alzheimer's disease (AD). In humans, AD is associated at the cellular level with neuropathological lesions composed of extracellular deposits of β-amyloid (Aβ) protein aggregates and intracellular neurofibrillary tangles, cellular loss, neuroinflammation and a general brain homeostasis dysregulation. Thus, a dramatic synaptic environment perturbation is observed in AD patients, involving changes in brain neuropeptides, cytokines, growth factors or chemokines concentration and diffusion. Studies performed in animal models demonstrate that these circulating peptides strongly affect synaptic functions and in particular synaptic plasticity. Besides this neuromodulatory action of circulating peptides, other synaptic plasticity regulation mechanisms such as metaplasticity are altered in AD animal models. Here, we will review new insights into the study of synaptic plasticity regulatory/modulatory mechanisms which could influence the process of synaptic plasticity in the context of AD with a particular attention to the role of metaplasticity and peptide dependent neuromodulation.

Increased galanin receptor occupancy in Alzheimer's disease

Increased galanin (GAL) may be associated with the cognitive deficits characteristic of Alzheimer's disease (AD). However, both increased and decreased GAL receptor density has been reported in AD brain. Previous studies indicate pre-treatment with guanine nucleotides displaces endogenous GAL from GAL receptors (GALR), providing an indirect measurement of GALR occupancy. In addition, pre-treatment with guanine nucleotides may provide a more accurate measurement of GALR density since it would avoid the masking of GALRs by residual binding of endogenous GAL. Thus, in the present study, we examined the influence of pre-treatment with guanine nucleotides on 125I-GAL binding in multiple regions of normal and AD brain. Our results indicate that GTP pre-treatment enhances GAL binding in specific regions in normal and AD brain. In addition, our results suggest an increase in the number of GALRs occupied by endogenous GAL in the deep layers of the frontal cortex and the lateral hypothalamus of AD subjects compared to normal subjects. The regional differences in GALR density and receptor occupancy between normal and AD subjects may play a role in the cognitive disturbances associated with the disease.

Galanin receptor over-expression within the amygdala in early Alzheimer's disease: an in vitro autoradiographic analysis

Galanin receptor (GALR) expression is increased in various areas of the limbic system in end stage Alzheimer's disease (AD). The amygdaloid complex is a key component of the limbic circuit, is involved in homeostatic and cognitive functions, is impacted in AD and contains the peptide and receptor for galanin. Although GALR expression occurs in the amygdala in end stage AD, it remains to be determined whether a plasticity response occurs early or late in the disease. Therefore, we analyzed the distribution and associated changes in GALR binding in the amygdala during the progression of AD using an in vitro receptor autoradiographic method. Human galanin ([125I]hGAL) receptor binding was performed on brain sections from early and late AD cases, as well as normal age-matched control subjects. In aged controls, densities of [125I]hGAL binding sites were found in the central and the corticomedial nuclei. Relative to controls, possible/early AD cases displayed significantly greater numbers of [125I]hGAL binding sites in the central nucleus and cortico-amygdaloid transition area. In contrast, we found a decrease in the number of binding sites for [125I]hGAL in late as compared to early AD cases. The over-expression of GALRs in subfields of the amygdaloid early in AD suggests that galaninergic systems play a key role in limbic related behavioral changes during the disease process.

Galanin receptor plasticity within the nucleus basalis in early and late Alzheimer's disease: an in vitro autoradiographic analysis

Hypertrophy of fibers containing galanin (GAL), the inhibitory neurotransmitter of acetylcholine, occur on remaining cholinergic nucleus basalis neurons in late stage Alzheimer's disease (AD). The present investigation evaluated whether changes in the number of GAL receptors (GALR) were detectable within the nucleus basalis in the early or late stage of AD when compared to age-matched controls. Postmortem neuropathological specimens were obtained at autopsy from three groups: late AD, early (possible) AD, and normal (age-matched controls) human subjects. Autoradiography of GALR binding was performed on human brain sections from each of the three groups. Analysis of autoradiographic images show no change in the distribution of ([125])hGAL binding sites in early AD cases throughout the nucleus basalis. In contrast, the number of ([125])hGAL binding sites was increased over the anterior nucleus basalis subfield in late stage AD. A region-of-interest densitometric analysis of the anterior nucleus basalis in the late stage AD cases depict an increase in the number of ([125])hGAL binding sites by approximately two-three-fold when compared to normal (age-matched controls). Quantitative measures of ([125])hGAL binding densities were not significantly different in the anterolateral, intermediate or posterior nucleus basalis subsectors of early or late stage AD when compared to age-matched controls. These observations show that the occurrence of overexpression of GALRs coincide with earlier reports showing galaninergic fibers hyperinnervating surviving cholinergic basal forebrain neurons in late stage AD.

Galanin and galanin receptors

The development of a strain of galanin knockout mice has provided confirmation of a neuroendocrine role for galanin, as well as supporting results of previous physiological investigations indicating a role for galanin in analgesia and neuropathic pain, and potentially in neuronal growth and regeneration processes. Whether elevation of galanin expression in neurodegenerative disorders such as Alzheimer's disease represents a survival response or exacerbates functional deficit in afflicted individuals remains to be determined. More detailed analysis of the phenotype of the galanin knockout mouse should provide insights into the physiological role of galanin in memory and learning processes, as well as in hypothalamic function and other aspects of neuroendocrine regulation. Biochemical and molecular cloning efforts have demonstrated that the multiplicity of actions of galanin is matched by complexity in the distribution and regulation of galanin and its receptors. A focus on characterisation of galanin receptors has resulted in the molecular cloning of three receptor subtypes to date. The distribution and functional properties of these receptors have not yet been fully elucidated, currently precluding assignment of discrete functions of galanin to any one receptor subtype. It is not currently possible to reconcile available pharmacological data using analogs of galanin and chimeric peptides in functional assay systems with the pharmacological properties of cloned receptor subtypes. This highlights the value of further knockout approaches targeting galanin receptor subtypes, but also raises the possibility of the existence of additional receptor subtypes that have yet to be cloned, or that receptor activity may be modulated by regulatory molecules that remain to be identified. The development of receptor subtype-specific compounds remains a high priority to advance work in this area. The ability to selectively modulate the many different actions of galanin, through a clearer understanding of receptor structure-function relationships and neuronal distribution, promises to provide important insights into the molecular and cellular basis of galanin action in normal physiology, and may provide lead compounds with therapeutic application in the prevention and treatment of a range of disorders.

Centrally administered galanin blocks morphine place preference in the mouse

Galanin is a neuropeptide with appetitive, antinociceptive and neuroendocrine functions. Galanin and galanin binding sites are present in brain areas that mediate reinforcement, such as nucleus accumbens and ventral tegmental area, as well as locus coeruleus, an area known to be involved in development of drug dependence and withdrawal. This localization, coupled with the observation that there is a strong interaction between morphine and galanin in spinal cord, made it of interest to study whether galanin might have effects on morphine reinforcement. Using the place preference paradigm we found that galanin (1 microg i.c.v.) alone does not possess reinforcing or aversive properties but attenuates the preference conditioned by peripheral administration of morphine (5 mg/kg s.c.). Quantitative receptor autoradiography showed that morphine treatment that could condition a place preference decreased galanin binding in the nucleus accumbens and increased galanin binding in the locus coeruleus. In contrast, acute naltrexone administration increased galanin binding in the nucleus accumbens, suggesting that levels of galanin binding are tonically regulated by opioid receptors in that area. Contrary to what is seen in the spinal cord, these results indicate that galanin and morphine have an antagonistic interaction in the brain that results in attenuation of morphine reinforcement by activation of the galaninergic system.

Suggestive evidence for inhibitory effects of galanin on mesolimbic dopaminergic neurotransmission

The objective was to examine effects of galaninrat on forebrain monoamine synthesis and on spontaneous locomotor activity in the rat. The rate of monoamine synthesis was estimated by measuring the accumulation of l-DOPA and 5-HTP, following inhibition of cerebral aromatic l-amino acid decarboxylase by means of NSD-1015 (100 mg kg-1 i.p.), after i.c.v. or intracerebral administration of galanin in adult male Wistar rats. Spontaneous locomotor activity was observed in an automated open-field arena ( approximately 0.5 m2). The i.c.v. administration of galanin (0.5-5.0 nmol bilaterally) produced a dose-dependent, statistically significant, increase in DOPA accumulation throughout the neostriatum, and in the olfactory bulb, indicating an increase in the rate of DA synthesis. No increase was observed in brain areas where noradrenaline is the predominant catecholamine, such as the neocortex or the ventral hippocampus. In addition, there was a tendency for an increase in 5-HTP accumulation in the dorso-lateral neostriatum and in the accumbens. The same i.c.v. administration of galanin produced a dose-dependent, and statistically significant, decrease in spontaneous locomotor activity. The effect on forebrain DA synthesis could also be produced by local bilateral application of galanin (2x1 nmol) into the ventral tegmental area, but not the nucleus accumbens (2x2 nmol). There were no effects on forebrain DOPA or 5-HTP accumulation by the local application of galanin into the locus coeruleus, or into the dorsal raphe nucleus. It is concluded that the neuropeptide galanin modulates forebrain dopaminergic neurotransmission. The effect appears to be mediated at the somato-dendritic level of the meso-neostriatal pathway, and could perhaps be utilized to normalize perturbations ascribed to dysfunction in this neuronal pathway, such as schizophrenia.

Galanin receptors in the hippocampus and entorhinal cortex of aged Fischer 344 male rats

Galanin (GAL) has been proposed to be an inhibitory modulator of cholinergic memory pathways because it acts within the hippocampus to inhibit the release and antagonize the postsynaptic actions of acetylcholine. Here we have used: 1) slice binding and quantitative autoradiography to assess the density and occupancy of GAL receptors; and 2) in situ hybridization histochemistry to assess expression of the GALR1 receptor subtype in the ventral hippocampus of 3-month-old and 21-month-old Fischer 344 male rats. We detected a small but significant (p < or = 0.0003) age-related reduction in 125I-GAL binding-site density in the ventral hippocampus and entorhinal cortex under standard binding conditions. Post-hoc analysis indicated that this reduction with age persisted in the CA1 radiatum and entorhinal cortex following GTP-induced desaturation to unmask pre-existent GAL receptors occupied by endogenous ligand. It was not associated with a significant change in peak GALR1 gene expression in the hippocampus. Because a portion of GAL receptors in this region have been postulated to function as presynaptic auto-receptors on cholinergic fiber terminals, the reduction in GAL binding sites with age may be a consequence of age-related alterations in GAL receptor expression by basal forebrain cholinergic neurons which project to the ventral hippocampus.

Characterization and localization of galanin receptors in human entorhinal cortex

The neuropeptide galanin (GAL) has a widespread distribution throughout the human cortex. The entorhinal cortex (ENT) plays a crucial role in the transfer of cortico-cortical information related to memory and displays severe degeneration in Alzheimer's disease (AD). However, very little is known about the pharmacology of the GAL receptor (GALR) in normal human ENT. Therefore, we pharmacologically visualized their distribution and characterized GALRs using in vitro receptor autoradiography and radioligand binding assays. Autoradiograms revealed intense GALR labeling, mainly in the substantia innominata, hypothalamus, the bed nucleus of the stria terminalis and within layers 2 and 4 of the ENT. Kinetic experiments showed that saturation of GALR sites by [125I]GAL (human) (hGAL) occurred within 2 h and that this binding readily reversed in the presence of a GTP analog, but not in the presence of excess unlabeled hGAL. Analysis of [125I]hGAL binding data from saturation experiments gave KD values of 98.6+/-21.6 pM, Bmax values of 52.9+/-32.4 fmol/mg protein and identified a high and low affinity state of the GALR. The presence of 5'-guanylylimidodiphosphate (GppNHp) or NaCl reduced the agonist labeling of hGALR in ENT membranes.

A confocal microscopic analysis of galaninergic hyperinnervation of cholinergic basal forebrain neurons in Alzheimer's disease

The galanin (GAL) containing peptide fiber system innervates the basal forebrain and has been shown to hyperinnervate remaining cholinergic neurons in Alzheimer's disease (AD). GAL modulates the release of acetylcholine and, therefore, may depress this neurotransmitter in surviving cholinergic basal forebrain (CBF) neurons in AD. The aim of this study was to identify putative synaptic contacts between GAL immunoreactive processes and CBF neurons and evaluate whether these processes hypertrophy in AD patients. We observed by confocal laser microscopy a hyperinnervation of GAL-containing fibers in both AD and Parkinson's disease patients with concurrent AD (PD/AD). Galaninergic fibers were often seen in direct apposition to remaining CBF neurons and enwrapped cholinergic cell soma and dendrites. Our results demonstrate that GAL-containing fibers are in direct apposition to CBF neurons in normal-aged humans and that this phenotype is enhanced in AD and PD/AD, suggesting that direct synaptic contacts occur between GAL-containing fibers and CBF neurons. Because GAL can modulate acetylcholine release from cholinergic neurons, hyperinnervation of GAL fibers in AD and PD/AD patients may further decrease release of acetylcholine from remaining CBF neurons. We propose that therapies based solely on acetylcholinesterase inhibitors may be insufficient to effectively increase cortical levels of acetylcholine.

Galanin receptors: involvement in feeding, pain, depression and Alzheimer's disease

Galanin, a neuroendocrine peptide with a multitude of functions, binds to and acts on specific G-protein coupled receptors. Only one galanin receptor subtype, GalRI, has been cloned so far, although pharmacological evidence suggests the presence of more than one galanin receptor subtype. These receptors mediate via different Gi/Go-proteins the inhibition of adenylyl cyclase, opening of K+-channels and closure of Ca2+-channels. Galanin inhibits secretion of insulin, acetylcholine, serotonin and noradrenaline, while it stimulates prolactin and growth hormone release. Determination of structural components of galanin receptors required for binding of the peptide ligand as carried out recently will facilitate the screening and design of molecules specifically acting on galaninergic systems with therapeutic potential in Alzheimer's disease, feeding disorders, pain and depression.