Introduction

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Although GLP-1 is produced by cells in the intestines and was discovered because of its effects on glucose metabolism (Doyle and Egan, 2001), recent studies have shown that GLP-1 can decrease feeding by acting on specific receptors in the brain. When injected intracerebroventricularly, GLP-1 dramatically reduces food and water intake (Gunn et al., 1996; Tang-Christensen et al., 1996; Turton et al., 1996; Wang et al., 1998) and body weight (Donahey et al., 1998; Meeran et al., 1999) in rats. The GLP-1 receptor antagonist exendin (9-39) inhibits the effects of GLP-1 on food and water intake, suggesting that GLP-1 receptors are involved in satiety (Turton et al., 1996). However, the effects of GLP-1 on feeding are not sustained, and mice lacking GLP-1 receptors are lean, eat normally, and do not develop obesity either with aging or after several months of high-fat intake (Scrocchi et al., 1996; Scrocchi and Drucker, 1998). Several studies have demonstrated GLP-1 receptor expression in both the rodent (Goke et al., 1995; Shughrue et al., 1996) and human (Wei and Mojsov, 1995; Satoh et al., 2000) brain. GLP-1 receptors appear to be distributed primarily in the hypothalamus, thalamus, brainstem, lateral septum, subfornical organ, and the area postrema. However, specific binding sites for GLP-1 have also been detected in neurons in the caudate-putamen, cerebral cortex, hippocampus, and cerebellum (Campos et al., 1994; Calvo et al., 1995; Goke et al., 1995). It remains to be established whether GLP-1 is produced by neural cells, but it has been shown that GLP-1 present in the bloodstream can enter the brain (Orskov et al., 1996).

We have recently shown that GLP-1 receptor activation induces neurite outgrowth in PC12 cells and SK-N-SH human neuroblastoma cells by a mechanism involving the second messenger cyclic AMP (Perry et al., 2002). Signals that stimulate cyclic AMP production can protect neurons against death in various paradigms. For example, corticotropin-releasing hormone and urocortin can protect cultured hippocampal neurons against death induced by glutamate and amyloid -peptide (Pedersen et al., 2001, 2002).

In the present study, we have further examined the neurotrophic properties of GLP-1 and its longer-acting analog exendin-4 (Greig et al., 1999; see Perry et al., 2002 for amino acid sequences of GLP-1 and exendin-4) in cultured hippocampal neurons and in a well established rodent model of neurodegeneration. Exendin-4 has been shown to bind to the known GLP-1 receptor in pancreatic  cells (Goke et al., 1993; Thorens et al., 1993) and has several advantages over GLP-1: it has a higher potency than GLP-1, its half-life is approximately 120 min, and it maintains higher plasma levels of insulin over a longer time duration than GLP-1 (Ryan et al., 1998; Greig et al., 1999; Egan et al., 2002). In line with our demonstration that rat hippocampal neurons express functional GLP-1 receptors, we have tested the hypothesis that GLP-1 and exendin-4 can protect against glutamate-induced apoptosis and restore cholinergic marker activity in adult rats following nonselective excitotoxic basal forebrain damage. For several decades, lesion models of Alzheimer's disease in the rat have encompassed one aspect of the human condition, that is, the loss of cholinergic neurons found in the medial septum and nucleus basalis of Meynert (or basal nucleus in rodents). Such models have relied on immunohistochemical correlates of cholinergic phenotype to identify lesion- and treatment-induced changes. Although treatment with growth factors is well documented to protect against lesion-induced cholinergic cell death (Haroutunian et al., 1986; Mandel et al., 1989), recent observations suggest that such neurons may simply down-regulate their phenotype, rather than die, and that treatment with such growth factors may rescue cholinergic neurons (Haas and Frotscher, 1998; Weis et al., 2001). We propose that GLP-1 and exendin-4 possess neurotrophic capabilities and, as such, offer the possibility for restoring the cholinergic phenotype following partial excitotoxic damage within the basal nucleus of the rat. Although originally developed for their insulinotropic properties, GLP-1 and exendin-4 may have potential for reversing or halting neurodegenerative processes observed in central nervous system disorders such as severe epileptic seizures and Alzheimer's disease and in neuropathies associated with type 2 diabetes mellitus.

	Materials and Methods

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Cell Culture. Hippocampal neuronal cultures were prepared from 18-day-old embryonic Sprague-Dawley rats using methods similar to those described previously (Mattson et al., 1995). Briefly, cells were dissociated by mild trypsinization and trituration and plated in minimal essential medium containing 10% fetal bovine serum and 1% antibiotic solution (10⁴ U/ml penicillin G, 10 mg/ml streptomycin, and 25 µg/ml amphotericin B; Sigma-Aldrich, St. Louis, MO). Hippocampal neurons were plated at a density of 100,000 cells/ml on 25-mm-diameter poly-D-lysine-coated, glass coverslips. Three hours after plating, the medium was replaced with serum-free Neurobasal medium containing 1% B-27 supplement (Invitrogen, Carlsbad, CA). Immunofluorescence staining for mitogen-activated protein-2 (neurons) and glial fibrillary acidic protein (GFAP; astrocytes) showed that more than 98% of the cells were neurons, and the remainder were predominantly astrocytes. Cultures were used within 7 to 10 days of plating.

Competitive Binding of GLP-1 to GLP-1 Receptor in Hippocampal Neurons. Binding studies were performed as described by Montrose-Rafizadeh et al. (1997b). Duplicate hippocampal neuronal cultures were washed in 0.5 ml of binding buffer and subsequently incubated in 0.5 ml of buffer containing 2% bovine serum albumin, 17 mg/l diprotin A (Bachem California, Torrance, CA), 10 mM glucose, 0.001 to 1000 nM GLP-1, and 30,000 cpm of ¹²⁵I-GLP-1 (Amersham Biosciences UK, Ltd., Little Chalfont, Buckinghamshire, UK), overnight at 4°C. At the end of the incubation, the supernatant was discarded, and the cells were washed three times in ice-cold phosphate-buffered saline (PBS) and incubated at room temperature with 0.5 ml of 0.5 M NaOH and 0.1% SDS for 10 min. Radioactivity in cell lysates was measured in an Apec-Series gamma counter (ICN Biomedicals, Inc., Costa Mesa, CA). Specific binding was determined as the total binding minus the radioactivity associated with cells incubated in the presence of a large excess of unlabeled GLP-1 (1 µM). The GLP-1 concentration associated with 50% binding, EC₅₀, was determined by logit plot analysis.

Intracellular cAMP Determination. To demonstrate the presence of functional GLP-1 receptors, cyclic AMP was measured according to the method of Montrose-Rafizadeh et al. (1997a). Triplicate hippocampal neuronal cell cultures were treated with 10 nM GLP-1 and harvested at 5-min intervals after the onset of drug treatment for a total period of 30 min. Cells harvested at the start of drug treatment (0 min) were used for baseline levels of cAMP.

Cell Survival. The fluorescent DNA binding dye Hoescht 33342 was used to measure apoptotic cell death. Neurons were incubated in Locke's buffer with GLP-1 (10 nM) or exendin 4 (0.3 µM) alone, and in combination with glutamate (10 µM) for 16 h. The concentration of GLP-1 used was based on the EC₅₀ value derived from the binding experiment, which was demonstrated to stimulate the release of cAMP, and which induced differentiation without causing cell death in our previous neuronal cell studies (Perry et al., 2002). Exendin-4 is well documented to bind at the GLP-1 receptor (Goke et al., 1993, 1995), and we have based our dose on that used previously in cultured neuronal cells (Perry et al., 2002). Cells were fixed in a solution of 4% paraformaldehyde in PBS, and membranes were permeabilized with 0.2% Triton X-100. Following incubation with Hoechst 33342 (1 µM) for 30 min, nuclei were visualized under epifluorescence illumination (340-nm excitation, 510-nm barrier filter) using a 40× oil immersion objective. Approximately 200 cells were counted in at least three separate dishes for each treatment condition, and experiments were repeated at least twice. Cells were considered apoptotic if nuclear DNA was fragmented or condensed, whereas cells with nuclear DNA of a more diffuse and uniform distribution were considered viable. At the time of counting, the investigator was unaware of the identity of the treatment groups. The percentage of cells with condensed or fragmented nuclei was determined in each culture.

Animals and Surgical Procedures. The study was undertaken using a total of 35 adult male Fischer-344 rats weighing approximately 300 g each. The animals were housed under controlled light/dark and temperature conditions with food and water available ad libitum. Rats were anesthetized with ketamine (90 mg/kg) and acepromazine (0.91 mg/kg). Stereotaxic surgery was carried out as described (Perry et al., 2001). Ibotenic acid dissolved in 0.1 M PBS was infused unilaterally into the left lateral branch of the forebrain bundle, which was referred to as the basal nucleus by Paxinos and Watson (1998), at 10 µg/µl (0.5 µl, two sites). Prior pilot examination of the efficacy of this particular batch of toxin demonstrated that this dose produced a 60% loss of choline acetyltransferase (ChAT)-positive immunoreactivity in the basal forebrain, with a comparable loss of projections to the cortex. Although ibotenic acid is well documented to produce nonselective damage, in this particular series of experiments we elected to use cholinergic neurons as a marker to quantify the degree of degeneration. The rationale for the experimental parameters included the following. First, by sparing a proportion of cell bodies, we ensured that the infused peptide would have a residual population on which to exert possible trophic effects. Second, unilateral rather than bilateral damage was induced such that contralateral comparisons could be made within individual animals rather than simply between groups. A second series of animals receiving infusions of vehicle were used as controls. Each infusion was performed over 2.5 min, and an additional 2.5 min were allowed for diffusion before the cannula was retracted. After 2 weeks, animals were reanesthetized and stereotaxically implanted with an intracerebroventricular cannula into the right lateral ventricle (anterior-posterior 0.8 mm, lateral to bregma +1.4 mm, ventral to dura 4.0 mm).

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Immunohistochemistry. Adjacent coronal brain sections were taken at 40-µm thickness, through the lesion area, and processed free-floating for ChAT using the polyclonal goat anti-ChAT antibody at 1:100 dilution (Chemicon International Inc., Temecula, CA), and for GFAP using the polyclonal rabbit anti-GFAP antibody at 1:750 dilution (Chemicon). Visualization of positive immunoreactivity was carried out using an avidin-biotin/horseradish peroxidase protocol. In addition, one series of sections was stained for acetylcholinesterase activity, as a histochemical marker for cholinergic neurons of the basal forebrain using a modified method by Geula and Mesulam (1989). An additional series of sections was mounted onto gelatin-coated slides and stained with cresyl violet to visualize cell bodies.

Quantification and Statistical Analyses. In the cell culture studies, the percentage of cells undergoing apoptosis as a result of each treatment condition were subjected to ANOVA using StatView statistical software (StatView, Cary, NC). Following significant main effects, a posteriori comparisons of treatment versus control were made using Tukey's honestly significant difference (HSD) test, using the pooled ANOVA error term and degrees of freedom.

	Results

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GLP-1 and Exendin-4 Protect Cultured Hippocampal Neurons against Cell Death Induced by Glutamate. Binding of ¹²⁵I-GLP-1 to cultured hippocampal neurons was displaced, concentration dependently, by unlabeled GLP-1 (Fig. 1A). The concentration of GLP-1 required to displace 50% bound ¹²⁵I-GLP-1 was determined by logit plot analysis and required a concentration of 14 nM GLP-1 (r = 0.999) in cultured hippocampal neurons.

View larger version (21K): [in this window] [in a new window]   Fig. 1.   Displacement of 125I-GLP-1 binding with cold GLP-1 (A), GLP-1-stimulated release of cAMP (B), and protection against glutamate-induced apoptosis (C) in cultured hippocampal neurons. 125I-GLP-1 binding to intact cultured hippocampal neurons was competed with various concentrations of GLP-1. The data are normalized to maximum values obtained in the presence of 1 µM GLP-1. Each data point represents the mean of two experimental values and is presented as the percentage of maximum binding in the absence of cold peptide. cAMP levels were assayed over 30 min of incubation with 10 nM GLP-1 (B). Vertical error bars represent ± standard error of the mean of three individual experimental values. Treatment with 10 nM GLP-1 or 0.3 µM exendin-4 completely protected against the apoptotic effects of 10 µM glutamate (C). Cultures were treated overnight, fixed with 4% paraformaldehyde, and stained with Hoechst 33342. The number of apoptotic nuclei was counted, and the values are presented as the pooled mean of six individual dishes per treatment condition. Vertical error bars represent ± standard error of the difference between the means. Significant difference from control: , p < 0.01, and , p < 0.001.

Chronic Treatment with GLP-1 or Exendin-4 Attenuates the Ibotenic Acid-Induced Cholinergic Marker Deficit in Adult Rats. Choline acetyltransferase immunoreactivity was used as a marker for cholinergic neurons throughout the basal forebrain. The ChAT antibody stained numerous, large, multipolar neurons, with a similar size and distribution to the acetylcholinesterase-positive cells. The immunocytochemical staining had low background and provided a clear picture of the cell morphology (Fig. 2). Injection of ibotenic acid with subsequent infusion of vehicle resulted in a substantial (43%) loss of ChAT-immunoreactive neurons (Fig. 3, bar 4) over an approximately 1-mm radius from the injection site in the left basal nucleus, which was within the realm of that predicted from the pilot study. The sham-operated control group receiving vehicle infusion showed an increase in the percentage of ChAT-positive cell bodies in the left basal nucleus relative to the right basal nucleus (Fig. 3, bar 1). Clearly, this was unexpected and will be addressed later in this section. Ibotenate-lesioned animals that received GLP-1 or exendin-4 infusions had a decreased loss of ChAT-immunoreactive cell bodies in the left basal nucleus relative to those lesioned animals that received vehicle infusion. More specifically, infusion of exendin-4 produced a decrease in the loss of ChAT-immunoreactive cell bodies in the left basal nucleus from 43%, as was apparent following vehicle infusion, to just 24% below that of the right basal nucleus (Fig. 3, bar 5). Furthermore, GLP-1 infusion resulted in more striking reversal effects, decreasing the loss of ChAT-positive immunoreactive cell bodies in the left basal nucleus to just 6% below that of the right basal nucleus (Fig. 3, bar 6). Standard ANOVA demonstrated an overall significant effect of treatment condition (F = 21.363, df = 5,28; p < 0.001). Multiple comparisons of peptide versus vehicle treatment (Tc = 14.14 and 19.71) revealed significant improvements in ChAT immunoreactivity in the left basal nucleus following infusion of exendin-4 (p < 0.05) and GLP-1 (p < 0.01) after an ibotenic acid lesion. Although infusion of GLP-1 following an ibotenic acid lesion decreased the ChAT-positive cell loss in the left basal nucleus to produce near-equal values with the right side, the overall percentage of difference was still significantly lower than that of the sham vehicle group (p < 0.001). This is likely due to the perceptible increase in ChAT immunoreactivity in the sham group receiving vehicle infusion (Fig. 3, bar 1).

View larger version (178K): [in this window] [in a new window]   Fig. 2.   Choline acetyltransferase and glial fibrillary acidic protein immunoreactivity in the basalis nucleus. ChAT-positive immunoreactivity, ipsilateral (A and C) and contralateral (B and D), to a partial ibotenic acid lesion is shown. Panels A and B, and C and D depict the left and right basal nucleus from individual animals that received vehicle infusion and GLP-1 infusion, respectively. ChAT-positive immunoreactivity in the ipsilateral basal nucleus in an animal that received vehicle infusion (A) was substantially lower than that in the ipsilateral basal nucleus in an animal that received GLP-1 infusion (C). GFAP immunoreactivity, a marker for reactive astrocytes produced in response to injury, demonstrated areas of positive immunoreactivity surrounding the site of cannula implantation and lining of the lateral ventricle in the vicinity of the site of infusion. Interestingly, infusion of GLP-1 produced a more elevated glial reaction in the basal nucleus on the infusion side (F) than that apparent as a result of the lesion (E) or after vehicle infusion.

View larger version (21K): [in this window] [in a new window]   Fig. 3.   Percentage of difference in the Abercrombie-corrected number of ChAT-immunoreactive cell bodies in the ipsilateral basal nucleus (lesion side) relative to the intact contralateral basal nucleus in sham and ibotenic acid animals receiving i.c.v. infusion of vehicle (aCSF), exendin-4, or GLP-1. Vertical error bars represent the standard error of the difference between the means. Significant difference from ibotenic acid vehicle group: , p < 0.05 and , p < 0.01.

	Discussion

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The present results indicate that GLP-1 and exendin-4 may have novel therapeutic potential for the treatment of neurodegenerative disorders. We have demonstrated that GLP-1 and exendin-4 provide protection against cell death induced by glutamate neurotoxicity, which is mediated by calcium influx through N-methyl-D-aspartate receptor channels in cultured hippocampal neurons (Mattson et al., 1995). Moreover, using a well established model of neurodegeneration in the rat, GLP-1 and exendin-4 demonstrated significant reversal of the cholinergic marker deficit in the basal forebrain induced by ibotenic acid.

GLP-1 and exendin-4 have previously been shown to possess neurite outgrowth-promoting properties in rat pheochromocytoma cells and in a human neuroblastoma cell line (Perry et al., 2002). We demonstrated that primary hippocampal neurons exhibit a robust increase in cAMP levels when exposed to a single dose of GLP-1, indicating the presence of functional GLP-1 receptors in those neurons. Similar to our demonstration in PC12 and SK-N-SH cells, cAMP is rapidly released after activation with a single dose of GLP-1, reaching maximal levels within 15 min. Cyclic AMP levels decrease back to baseline within 30 min, regardless of the continuous stimulation of the GLP-1 receptor. This "pulsatile" synthesis can be restored following a suitable "recovery phase", which is a classic response to GLP-1 in  cells of the pancreas.

When cultured hippocampal neurons were subjected to excitotoxic insult, GLP-1 and exendin-4 were able to provide complete protection from the glutamate-induced cell death, as has been shown by other neurotrophic factors (Cheng et al., 1994; Mattson et al., 1995). These results suggest that such peptides may play a significant role in protecting hippocampal neurons against excitotoxic damage and potentially other types of brain injury. The dose range of the peptides used in these studies was focused around the EC₅₀ value for GLP-1 derived from the binding study and the doses we have previously used to examine trophic properties in neuronal-like cells as well as those currently used in ongoing experiments related to pancreatic  cell function. Indeed, all doses of peptides used for the in vivo studies reflect those in current clinical investigations of the efficacy of GLP-1 and exendin-4 in patients with type 2 diabetes mellitus (Perfetti et al., 2000; Doyle and Egan, 2001; Larsen et al., 2001), together with the relative potencies of the peptides for the GLP-1 receptor.

Following translation of the in vitro effects to a well established model of neurodegeneration in the rat, we have shown complete amelioration of the ibotenic acid-induced cholinergic marker deficit following infusion of GLP-1. Using ChAT-positive immunoreactivity as the marker for cholinergic cell bodies within the basal forebrain, treatment with exendin-4 significantly decreased the loss of ChAT-positive cell bodies in the lesioned area compared with the vehicle-treated lesion group. The GLP-1-mediated effect was more pronounced than the exendin-4 effect, affording essentially complete recovery of the lesion deficit.

Despite the GLP-1-mediated restoration of ChAT-positive neurons in the lesioned basal forebrain, comparison with the sham-operated vehicle-treated group showed a significant difference. This appears to be due to the apparent increase in ChAT-positive immunoreactivity in the sham vehicle group (Fig. 3, in which the number of ChAT-positive neurons in the left basal nucleus is presented as a percentage of the number in the right basal nucleus). Analysis of separate left and right ChAT-immunoreactive cell body counts in the sham vehicle group suggested that there was a deficit/down-regulation of ChAT immunoreactivity in the right basal nucleus. Clearly, as the control group, the expectation would be to find equal numbers of cell bodies on the left and right sides. This suggests that disturbance of tissue integrity (as a result of cannula implantation and/or treatment delivery), however mild or nonspecific, may produce a disruption of certain markers within particular cell populations. In support of this, Torres and colleagues (1994) have demonstrated that injections of saline into the septum or diagonal band induce a loss of parvalbumin immunoreactivity in the vicinity of the injection, which was as extensive as the loss following a lesion. The lack of significant effects between groups when right basal nucleus ChAT-positive counts were analyzed separately demonstrates that all groups were affected equally and does not detract from the clarity of the GLP-1- and exendin-4-mediated amelioration of cholinergic neuron atrophy. In this series of experiments, the unilateral lesion deficit with contralateral i.c.v. cannula implantation allowed each animal to act as its own control (lesion side versus intact side). However, with the overall increase in the percentage of ChAT-positive cell bodies in the sham aCSF group (Fig. 3), it was necessary to analyze left and right basal nucleus cell body counts separately and confirm that this was not a spurious trophic phenomenon of aCSF infusion (such effects were not apparent in the sham GLP-1 or sham exendin-4 groups) but more likely due to disruption of tissue integrity common across all groups, as was the case. Future studies will address this issue with bilateral rather than unilateral lesion and cannula implantation parameters such that more reliable "between"-groups comparisons can be made. Regardless, this should not detract from the clarity of our peptide data. We have demonstrated complete recovery of cholinergic marker activity following an excitotoxic nonselective ibotenic acid lesion, which we can attribute to the trophic influence in the basal nucleus of GLP-1 treatment.

Taken together, the ability of GLP-1 to protect against glutamate-induced toxicity in cultured cells and to restore cholinergic marker function following excitotoxic-induced damage within the basal forebrain demonstrates that the beneficial effects of these peptides are likely not specific for excitotoxicity or indeed cholinergic neuronal death but may represent a trophic property for all neuronal GLP-1 receptor-expressing cells. Further studies are ongoing to establish whether other neuronal populations are affected by GLP-1.

The mechanisms by which GLP-1 and its naturally occurring analog, exendin-4, are able to exert beneficial effects within the denervated basal forebrain remain unclear. Previous studies have documented the presence and cellular localization of GLP-1 binding sites in the rodent (Kanse et al., 1988; Goke et al., 1995; Shughrue et al., 1996) and human (Satoh et al., 2000) brain. However, such studies have been severely hampered by the lack of a specific antibody against the GLP-1 receptor. As an alternative to direct immunocytochemical labeling, a number of experimental approaches have been used, which involve indirect techniques for the identification of GLP-1 receptors in the brain. First, Northern blot analysis of polymerase chain reaction-amplified GLP-1 receptor mRNA transcripts demonstrated high levels of the GLP-1 receptor mRNA in brainstem and cerebellum, whereas cortex and hypothalamus expressed lower levels (Campos et al., 1994). Second, binding studies using ¹²⁵I-GLP-1 (Goke et al., 1995), further demonstrated the presence of the GLP-1 receptor in the subfornical organ, arcuate nucleus of the hypothalamus, interpeduncular nucleus, parategmental nucleus, inferior olive, and the nucleus of the solitary tract. Third, following cloning of the GLP-1 receptor (Thorens et al., 1993), in situ hybridization studies demonstrated mRNA expression in the basal portion of the frontal cortex, substantia innominata, nucleus accumbens, nucleus basalis magnocellularis, ventral pallidum, lateral septum (and, to a lesser degree, the medial septum), diagonal band of Broca, medial/central and basolateral nuclei of the amygdala, and the CA2 to CA3 region of the hippocampus (Merchenthaler et al., 1999) and, more recently, on glial cells following mechanical brain injury (Chowen et al., 1999). Good correlations have been found between the in situ studies and previous immunocytochemical observations on the location of GLP-1-immunoreactive cells (Jin et al., 1988). Although the highest density of GLP-1 receptors seems to be in circumventricular areas, where generally large numbers of peptide receptors are located, there are moderate to low densities elsewhere in the brain. In particular, Merchenthaler et al. (1999) specify GLP-1 receptor mRNA-expressing cells in basal forebrain areas and the CA2 to CA3 region of the hippocampus.

In summary, from our demonstration that GLP-1 binds and stimulates cAMP production in primary hippocampal neurons, protects against apoptotic cell death in the same hippocampal cells, and restores cholinergic marker function following excitotoxic damage in the basal forebrain, it seems highly probable that GLP-1 receptor expression has a cholinergic contingent. Although we have demonstrated the functional presence of GLP-1 receptors on cultured hippocampal neurons (intracellular cAMP increased after GLP-1 stimulation), such cells are embryonic, and we cannot conclude that they perfectly represent the phenotype of cholinergic cell bodies in the basal nucleus of adult rats. Without a selective antibody to the GLP-1 receptor (the currently available polyclonal antibody produced extensive nonspecific staining in these studies and was considered not selective for the GLP-1 receptor), it remains difficult to establish the precise location of the GLP-1 receptor or the mechanism by which GLP-1 and exendin-4 exert their beneficial effects. Nevertheless, our findings suggest that these peptides may offer the possibility for rescue of damaged neurons in either the central or peripheral nervous systems associated with neurodegeneration. Our ongoing studies are focused on elucidating the functional effects and mechanisms of action of GLP-1 and related analogs within these systems.