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. 2026 Aug 6;55(5):e70161. doi: 10.1111/ahe.70161

Amylin Coexisting With Peptide YY (PYY) and Amylin/PYY Cells Correspond to Subpopulations of Glucagon‐ and Glucagon‐Like Peptide‐1‐Immunoreactive Cells in the Japanese Eel ( Anguilla japonica ) Pancreas

Hirohumi Suzuki 1,✉, Toshiharu Yamamoto 2,3
PMCID: PMC13445147  PMID: 42559671

ABSTRACT

The distribution of amylin‐like substances was investigated by immunohistochemistry in the pancreas of the Japanese eel ( Anguilla japonica ). Amylin‐immunoreactive cells were observed in pancreatic islets. These immunostaining profiles were not observed by the preabsorption of the anti‐amylin serum with synthetic amylin, indicating that the serum specifically recognized eel amylin‐like peptide. Almost all amylin‐immunoreactive cells were immunopositive for peptide YY, suggesting that amylin and peptide YY co‐secreted from the islet cells might participate in the modulation of insulin and glucagon secretion in the pancreatic islets. In addition, weakly glucagon‐immunopositive cells, but not densely glucagon‐immunopositive cells, also corresponded to amylin‐immunoreactive cells. The subpopulation of glucagon‐immunoreactive and glucagon‐like peptide‐1‐immunoreactive cells was also immunopositive for peptide YY. These results suggest that amylin and peptide YY, and also presumably glucagon‐like peptide‐1, are co‐secreted from the amylin/peptide YY cells, and these three peptides modulate food intake through the central nervous system in addition to intrainsular functions.

Keywords: amylin, eel, GLP‐1, pancreas, PYY

1. Introduction

Amylin, or islet amyloid polypeptide, is a 37‐amino acid peptide isolated from the human pancreas (Cooper et al. 1987; Westermark et al. 1986). Pharmacological and physiological studies have demonstrated that amylin modulates pancreatic hormone secretion (Åkesson et al. 2003; Silvestre et al. 1997; Zhu et al. 2011). Interestingly, amylin also affects the central nervous system, leading to anorexigenic feeding (Arnelo et al. 1996; Roth et al. 2006; Rushing et al. 2000). In humans and mice, the coexistence of amylin and insulin on secretory granules in insulin‐producing cells (B cells) has been demonstrated under immunoelectron microscopy (Lukinius et al. 1989; Rindi et al. 1991). However, Tomita (2003a, 2003b) reported that amylin in the human pancreas co‐distributed not only in B cells but also in glucagon‐producing cells (A cells), somatostatin‐producing cells (D cells), and pancreatic polypeptide (PP)‐producing cells (PP cells). PP belongs to the neuropeptide Y family, which includes peptide YY (PYY) (Cerdá‐Reverter and Larhammar 2000). In the avian pancreas, amylin‐immunoreactive cells correspond to PYY‐producing cells (PYY cells) and A cells in Japanese quail ( Coturnix japonica ) (Yanagida et al. 2024). In the reptilian pancreas, amylin‐immunoreactive cells correspond to B cells and PP cells in striped velvet gecko (Homopholis fasciata) (Suzuki and Yamamoto 2018), whereas in the amphibian pancreas, amylin‐immunoreactive cells correspond to X cells (Suzuki and Yamamoto 2014), which are functionally unidentified cells (Etayo et al. 2000) in black‐spotted pond frog ( Pelophylax nigromaculatus ). Since amylin cDNAs have been identified in teleost species (i.e., zebrafish, Danio rerio ; daddy sculpin, Myoxocephalus scorpius ; and Atlantic salmon, Salmo salar ) (Westermark et al. 2002), a wide degree of conservation of amylin in vertebrates has been suggested. In fact, Westermark et al. (2002) reported that amylin‐immunoreactive cells correspond to B cells in the fish Myoxocephalus scorpius and Salmo salar pancreases. Based on these studies, amylin‐coexisting cells vary depending on the species (Table 1), and the reason why amylin is secreted from such varied endocrine cells is unknown. However, this variation may have resulted from adaptation to each vertebrate habitat and evolutionary pressure, which has been documented in prolactin evolutionary changes (Dobolyi et al. 2020).

TABLE 1.

Pancreatic endocrine cell types corresponding to amylin‐immunoreactive cells in various animals.

Taxa Species Endocrine cell types References
Mammalia

Homo sapiens

Mus musculus

B cells

A, B, D, PP cells

B cells

Lukinius et al. (1989), Rindi et al. (1991)

Tomita (2003a, 2003b)

Rindi et al. (1991)
Aves Coturnix japonica A, PYY cells Yanagida et al. (2024)
Reptilia Homopholis fasciata B, PP cells Suzuki and Yamamoto (2018)
Amphibia Pelophylax nigromaculatus X cells Suzuki and Yamamoto (2014)
Teleost

Myoxocephalus scorpius

Salmo salar

B cells

B cells

Westermark et al. (2002)

Westermark et al. (2002)
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Eels (family Anguillidae and order Anguilliformes) belong to one of the basal teleost groups and are a valuable anguilliform fishery. However, amylin distribution has not been investigated in the anguilliform fish pancreas. Amylin and PYY are both effectively related to feeding behaviour (Karra et al. 2009; Lutz 2012; Roth et al. 2007). In fish aquaculture, feeding control is essential, so knowledge of this bioactive peptide colonization is important. In this study, we examined amylin distribution in the pancreas of Japanese eel ( Anguilla japonica ) using immunohistochemical techniques and elucidated its association with other pancreatic hormones.

2. Materials and Methods

2.1. Animals

In June 2025, five male Japanese eels ( A. japonica ), all approximately 45 cm in length, were obtained from a local dealer in Wakayama Prefecture, Japan. Before experimentation, the eels were kept for 3 days at 24°C in freshwater tanks. Experiments were performed under the authority of the Animal Care and Use Committee of the University of Teacher Education Fukuoka.

2.2. Fixation and Sectioning

The five eels were anaesthetised deeply by immersion in 0.05% ethyl 3‐aminobenzoate methanesulfonate (MS‐222, E10521; Sigma‐Aldrich, St. Louis, MO, USA). Their hearts were dissected and perfused transcardially with 0.81% NaCl and then fixative (4% formaldehyde and 0.2% picric acid in 0.1 M sodium phosphate buffer [PB; pH 6.9]). The pancreas and intestines were removed immediately and further fixed for 2 days in the same fixative at 4°C. After fixation, the samples were stored in PB. The samples were submerged in a solution containing 10% sucrose and 50% O.C.T. compound (Sakura Finetek Japan Co. Ltd., Tokyo, Japan) for 1 day. The samples were then cut with a cryostat (HM525; MICROM, Walldorf, Germany) into 6‐μm thick cross sections, and the sections were thaw‐mounted on MAS‐coated glass slides (MAS‐04; Matsunami Glass Ind., Kishiwada, Japan).

2.3. Immunohistochemistry

We performed immunostaining as described previously by Suzuki et al. (2022). Briefly, the sections were washed overnight in 0.1 M PB (pH 7.4) containing 0.9% saline (PBS) and incubated with the primary antibody, that is, rabbit anti‐rat amylin serum (Table 1), diluted in PBS containing 1% bovine serum albumin and 0.3% Triton X‐100 (PBS‐BSAT), for 24 h at 4°C. After washing in PBS, the sections were incubated with the secondary antibody, that is, biotinylated goat anti‐rabbit IgG antibody (BA‐1000; Vector Laboratories, Burlingame, CA, USA) diluted to 1:100 in PBS‐BSAT, for 1 h at room temperature. Next, we washed the sections again in PBS and incubated them with avidin‐biotin‐horseradish peroxidase complex (ABC) (PK‐6100; Vector Laboratories; diluted to 1:200 in PBS‐BSAT) for 30 min at room temperature. After final washing in PBS, the sections were reacted with 0.02% 3,3′‐diaminobenzidine tetrahydrochloride (DAB) and 0.005% hydrogen peroxide in 0.05 M Tris–HCl buffer solution (pH 7.4). Then, the sections were counterstained with thionine and covered with a Malinol mounting medium (No. 2009‐3; Muto Pure Chemicals, Tokyo, Japan) and a coverslip. We prepared antibody controls for the anti‐amylin serum by omitting the antibody from the first incubation solution or by using the serum preabsorbed with synthetic rat amylin (10 μg/mL, 4220‐v; Peptide Institute, Osaka, Japan). Moreover, we tested the cross‐reactivity of anti‐amylin serum with PYY by serum preabsorption with synthetic human PYY (10 μg/mL, No. 7650; Peninsula Laboratories, San Carlos, CA, USA).

To determine the amylin‐immunoreactive cell types, we carried out double‐immunofluorescent staining, combining the anti‐amylin serum with antibodies against four pancreatic hormones (i.e., PYY, glucagon, insulin, and somatostatin) as primary antibodies (Table 2). To detect the immunoreactivities of amylin and the hormones, we applied fluorescein‐, rhodamine‐, or Alexa Fluor 555‐conjugated secondary antibodies (Table 3). These primary and secondary antibodies were diluted in PBS‐BSAT. In the cases of the amylin and PYY, and amylin and insulin combinations, we used fluorescein‐conjugated donkey anti‐rabbit IgG to visualize amylin immunoreactivity and Alexa Fluor 555‐conjugated goat anti‐guinea pig IgG to visualize PYY and insulin. On the other hand, in the case of the amylin and glucagon combination, we detected amylin immunoreactivity and glucagon immunoreactivity using fluorescein‐conjugated donkey anti‐rabbit IgG and rhodamine‐conjugated donkey anti‐goat IgG, respectively. In contrast, in the case of the amylin and somatostatin combination, we detected amylin immunoreactivity and somatostatin immunoreactivity using fluorescein‐conjugated donkey anti‐rabbit IgG and rhodamine‐conjugated donkey anti‐rat IgG, respectively.

TABLE 2.

Primary antibodies used in the study.

Antigen Host Source and code no. Optimal dilution
Amylin (rat) Rabbit Peptide Institute, Osaka, Japan; 14220‐v 1:500
PYY (human) Guine Pig PROGEN Biotechnik, Heidelberg, Germany; No. 16066 1:3000
Glucagon (Pig) Goat AbD Serotec, Oxford, UK; No. 4660‐0610 1:1000
Insulin (human) Guine Pig Yanaihara Insutitute, Fujinomiya, Japan; Y370 1:5000
Somatostatin (human) Rat Protos Biotech Corporation, New York, NY, USA; NP‐105 SSTrat 1:1000
GLP‐1 (human) Rabbit Yanaihara Insutitute; Y320 1:10,000
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TABLE 3.

Secondary antibodies that were used in the double‐immunofluorescent staining process.

Antigen Host Labeling Source and code no. Optimal dilution
Rabbit IgG Donkey Fluorescein Millipore, Temecula, CA, USA; AP182F 1:100
Rabbit IgG Donkey Rhodamine Millipore; AP182R 1:100
Goat IgG Donkey Rhodamine Millipore; AP180R 1:100
Guinea pig IgG Goat Alexa Fluor 555 Invitrogen Corp., Carlsbad, CA, USA; A21435 1:100
Guinea pig IgG Donkey Fluorescein Millipore; AP193F 1:100
Rat IgG Donkey Rhodamine Millipore; AP189R 1:100
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To clarify the association between PYY and glucagon, we also performed double‐immunofluorescent staining to examine the coexistence of PYY and glucagon, as well as PYY and glucagon‐like peptide‐1 (GLP‐1), a bioactive peptide derived from a glucagon precursor (Holst et al. 1994). For primary antibodies, guinea pig anti‐PYY serum, goat anti‐glucagon antibody, and rabbit anti‐GLP‐1 serum were used (Table 2). Immunoreactivities of PYY, glucagon, and GLP‐1 were detected by fluorescein‐conjugated donkey anti‐guinea pig IgG, rhodamine‐conjugated donkey anti‐goat IgG, and rhodamine‐conjugated donkey anti‐rabbit IgG, respectively (Table 3).

3. Results

Amylin‐immunoreactive cells were observed in pancreatic islets characterized by ellipsoidal large‐type (longitudinal axis of 500–640 μm) and spherical small‐type (80–130 μm in diameter) shapes (Figure 1a), of which islet shapes were similar to the previous description of the European eel (L'Hermite et al. 1985). These islets containing amylin‐immunoreactive cells were observed in the 3‐cm cranial region of the pancreas. Amylin‐immunoreactive cells were ovoid or ellipsoid in shape (longitudinal axis of 10–20 μm) with or without thin cytoplasmic processes. These amylin‐immunoreactive staining structures were not observed after the preabsorption of the anti‐amylin serum with synthetic amylin (Figure 1b). Almost all the amylin‐immunoreactive cells were immunopositive for PYY (Figure 2a–c). In this study, glucagon‐immunoreactive cells appeared as two types: densely immunopositive cells and weakly immunopositive cells (Figure 2e). Amylin‐immunoreactive cells corresponded to weakly glucagon‐immunoreactive cells, but not to densely glucagon‐immunoreactive cells (Figure 2d–f). Amylin‐immunoreactive cells did not correspond to either insulin‐immunoreactive cells (Figure 2g–i) or somatostatin‐immunoreactive cells (Figure 2j–l). Preabsorption of the anti‐amylin serum with PYY had no effect on amylin‐immunoreactive profiles (not shown).

FIGURE 1.

FIGURE 1

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Micrographs of serial sections of the A. japonica pancreas showing amylin immunoreactivity (a) and effects of preabsorption of the anti‐amylin serum with synthetic amylin (b). The black star indicates a large pancreatic islet. Arrowheads indicate a small pancreatic islet.

FIGURE 2.

FIGURE 2

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Fluorescent micrographs of double‐immunostained sections of the A. japonica pancreatic islets indicating amylin (a, d, g, and j), with peptide YY (PYY) (b), glucagon (e), insulin (h), and somatostatin (k) immunoreactivities. Panels (c), (f), (i), and (l) present the merged images of (a and b), (d and e), (g and h), and (j and k), respectively. Arrows in (a–c) and (d–f) indicate amylin‐immunoreactive cells that are also immunopositive for PYY and glucagon, respectively. Arrows in (g, i, j, and l) indicate amylin‐immunoreactive cells that are immunonegative for insulin or somatostatin. Arrowheads in (e, f, h, i, k, and l) indicate amylin‐immunonegative cells that are immunopositive for glucagon, insulin, or somatostatin.

PYY‐immunoreactive cells corresponded to the subpopulation of glucagon‐immunoreactive cells (Figure 3a–c). PYY‐immunoreactive cells also corresponded to the subpopulation of GLP‐1‐immunoreactive cells (Figure 3d–f).

FIGURE 3.

FIGURE 3

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Fluorescent micrographs of double‐immunostained sections of the A. japonia pancreatic islets indicating peptide YY (PYY) (a and d) to compare with glucagon (b) or glucagon‐like peptide‐1 (GLP‐1) (e) immunoreactivities. Panels (c) and (f) present the merged images of (a and b) and (d and e), respectively. Arrows in (a–c) and (d–f) indicate PYY‐immunoreactive cells that are also immunopositive for glucagon and GLP‐1, respectively. Arrowheads in (b and c) and (e and f) indicate PYY‐immunonegative cells that are immunopositive for glucagon and GLP‐1, respectively.

4. Discussion

Amylin‐immunoreactive staining profiles were abolished by the preabsorption of the anti‐amylin serum with synthetic amylin. In addition, the staining profiles were not affected by the preabsorption of the serum with synthetic PYY. These immunohistochemical results indicate that the amylin antiserum definitely reacted to the amylin‐like peptide, but not to PYY. Moreover, amylin immunoreactive cells corresponded to only the subpopulation of glucagon‐immunoreactive cells, suggesting that the anti‐amylin serum does not have cross‐reactivity with glucagon.

In this study, we demonstrated that amylin‐immunoreactive cells correspond to PYY‐immunoreactive cells in the pancreatic islets of the eel pancreas. A previous study conducted on A. japonica demonstrated the presence of PYY‐immunoreactive cells in pancreatic islet (Ding et al. 1997). PYY, which consists of 36 amino acid residues, was first isolated from porcine intestines (Tatemoto and Mutt 1980). In the rat and mouse pancreas, pharmacological experiments have demonstrated that amylin modulates insulin (Åkesson et al. 2003; Silvestre et al. 1997; Zhu et al. 2011) and glucagon secretion (Åkesson et al. 2003). In addition, PYY modulates insulin and glucagon secretion (Bertrand et al. 1992; Böttcher et al. 1989). Thus, amylin and PYY co‐secreted from PYY‐producing cells in the eel pancreas might participate in the modulation of insulin and glucagon secretion in pancreatic islets.

In addition to the modulatory function of hormone secretion, amylin possesses an anorexigenic function in the mammalian brain (Li et al. 2019; Lutz 2012; Lutz et al. 2018). In the rat brain, a specific amylin‐binding site and a PYY‐binding site have been revealed by autoradiography in various brain regions, including the hypothalamus (Ohkubo et al. 1990; Sexton et al. 1994), suggesting that amylin and PYY are responsible for appetite (Rossi and Stuber 2018). In the mouse brain, permeability of amylin and PYY through the blood–brain barrier has been demonstrated (Banks and Kastin 1998; Banks et al. 1995; Nonaka et al. 2003). In rats, a significant reduction in food intake followed peripheral administration of amylin (Roth et al. 2006), such as subcutaneous infusion (Arnelo et al. 1996) and intraperitoneal injection of amylin (Rushing et al. 2000). Intraperitoneal injection of PYY also reduced food intake in rats (Batterham et al. 2002) and mice (Challis et al. 2003). Interestingly, in rats, the simultaneous intraperitoneal injection of both amylin and PYY induces a reduction in food intake and body weight more effectively than either amylin or PYY alone (Roth et al. 2007). In fish, a significant reduction in food intake was reported after intraperitoneal injection of amylin in Siberian sturgeon ( Acipenser baerii ) (Wang et al. 2022) and intraperitoneal injection of PYY in goldfish ( Carassius auratus ) (Gonzalez and Unniappan 2010). Thus, amylin and PYY co‐secreted from PYY‐producing cells in the eel pancreatic islets may modulate food intake in addition to intrainsular functions.

In the eel pancreas, weakly glucagon‐immunopositive cells, but not densely glucagon‐immunopositive cells, correspond to amylin‐immunoreactive cells. One possible explanation for this phenomenon might be that densely and weakly immunoreactive cells mainly produce glucagon and GLP‐1, respectively. As the glucagon precursor, proglucagon, contains amino acid residues of GLP‐1 and glucagon itself (Holst et al. 1994), the anti‐glucagon antibody can recognize proglucagon in the eel pancreatic islets. Amylin, PYY, and GLP‐1 may be co‐localized in the eel pancreatic islets, similar to quail pancreatic extra‐islet cells (Yanagida et al. 2024). GLP‐1 expression has been detected in the mammalian pancreas (Kreymann et al. 1991; Song et al. 2019). In human pancreatic islets, GLP‐1 immunoreactivity was detected on the secretory granules in the glucagon‐producing cells under immunoelectron microscopy (Mojsov et al. 1986). In rodents, GLP‐1 strongly stimulates insulin secretion induced by glucose (Ahrén and Pacini 1999; Göke et al. 1993). In eels, GLP‐1 co‐secreted with amylin and PYY may participate in the modulation of insulin secretion in pancreatic islets. In addition, GLP‐1 is an important hormone associated with appetite and weight maintenance, and peripheral administration of GLP‐1 affects the central regulation of feeding (Dailey and Moran 2013; Shah and Vella 2014). Among fish, intraperitoneal injection of GLP‐1 significantly reduces food intake in rainbow trout ( Oncorhynchus mykiss ) (Chivite et al. 2021) and coho salmon ( Oncorhynchus kisutch ) (White et al. 2016). Therefore, GLP‐1 secreted from the pancreatic islet may also modulate food intake, together with amylin and PYY, in eels. Hormonal treatments have practical applications in fish aquaculture. For example, gonadotropic hormones are utilized for maturation of sperm and eggs (Miyanishi and Nagano 2022; Mylonas et al. 2017; Suzuki et al. 2024). In farmed fish, an increase in food intake successfully occurs after intraperitoneal injection of growth hormone in rainbow trout (Johnsson and Björnsson 1994) and neuropeptide Y in olive flounder ( Paralichthys olivaceus ) (Li et al. 2017). Thus, amylin, PYY, and GLP‐1 may control feeding behaviour in fish aquaculture.

Based on limited data, amylin‐coexisting patterns are roughly classified into two groups: (1) coexisting with B cells like in humans, mice, a reptile (Homopholis fasciata), and two species of teleosts ( Myoxocephalus scorpius and Salmo salar ); and (2) coexisting with A cells like in a bird ( Coturnix japonica ) and in the eel ( Anguilla japonica ) as presented in this report. Teleosts have two types of coexisting patterns, which suggests that both amylin‐coexisting patterns may be relatively ancient: one pattern is conserved in mammals and reptiles, while another pattern is conserved in birds. Although available data to elucidate amylin evolution are scarce, comparative studies may provide new insights and connect with new phases of investigation.

In conclusion, we demonstrated that amylin‐immunoreactive cells corresponded to PYY cells and a subpopulation of A cells in the eel pancreas. Amylin may modulate pancreatic hormone secretion by autocrine and/or paracrine fashions and may affect food intake through the brain in concert with PYY and GLP‐1. Although the present study lacked physiological data, in the future, physiological and pharmacological studies of amylin and its receptor ligands may elucidate concrete amylin functions in the eel pancreas.

Funding

This study was supported by Yoshiki Moriguch Grant.

Conflicts of Interest

The authors declare no conflicts of interest.

Acknowledgements

This study was supported in part by a Yoshiki Moriguch Grant to H.S. from the University of Teacher Education Fukuoka.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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