ISSN 0006-2979, Biochemistry (Moscow), 2026, Vol. 91, No. 8, pp. 1406-1416 © Pleiades Publishing, Ltd., 2026.
1406
Age-Related Alterations
in Retinal Monoamine Neuromodulation
during AMD-Like Retinopathy Development in Rats
Darya V. Telegina
1,a
, Alena O. Kutlimetova
2,b
, Arseniy E. Izyurov
2,c
,
Alexander V. Kulikov
2,d
, and Nataliya G. Kolosova
2,e
*
1
Sirius University of Science and Technology, 354340 Sirius Federal Territory, Russia
2
Institute of Cytology and Genetics Siberian Branch of Russian Academy of Sciences,
630090 Novosibirsk, Russia
a
e-mail: telegina.dv@talantiuspeh.ru
b
e-mail: burnyasheva@bionet.nsc.ru
c
e-mail: IzyurovAE@bionet.nsc.ru
d
e-mail: v_kulikov@bionet.nsc.ru
e
e-mail: kolosova@bionet.nsc.ru
Received March 23, 2026
Revised June 24, 2026
Accepted June 24, 2026
Abstract— Aging is the major risk factor for age-related macular degeneration (AMD), a leading cause of
vision loss in aging populations. Increasing evidence suggests that alterations of neurotransmitter systems
contribute to the pathogenesis of AMD. Although biogenic amines in the retina were first detected over
50 years ago, their age-related dynamics and role in AMD development are still poorly understood. Here,
we compare age-related changes in the concentrations of norepinephrine, serotonin, dopamine, and their
main metabolites in the retinas of senescence-accelerated OXYS rats that develop an AMD-like retinopathy
and Wistar rats. We also assessed activities of monoamine oxidases (MAOs) and tyrosine hydroxylase and
compared changes in these activities with transcriptome data on genes associated with biogenic amine
pathways. The results revealed that in OXYS rats, retinal aging and progression of AMD-like retinopathy
are primarily associated with alterations in the dopaminergic system. Specifically, dopamine and its main
metabolite DOPAC were present in the retinas at higher levels than serotonin and 5-hydroxyindoleacetic
acid. Furthermore, the development of retinopathy in OXYS rats was accompanied by elevated dopamine
and DOPAC levels, decreased MAO activity, and changes in the expression of genes associated with the
dopaminergic synapse signaling.
DOI: 10.1134/S0006297926600870
Keywords: retina, aging, age-related macular degeneration, biogenic amines, OXYS rats
* To whom correspondence should be addressed.
INTRODUCTION
Aging is a predominant risk factor for the devel-
opment of age-related macular degeneration (AMD),
a progressive neurodegenerative retinal disease that
leads to severe central vision impairment. By 2040,
AMD is projected to affect approximately 288 million
older adults worldwide. Besides aging, the develop-
ment of AMD has been associated with various other
factors, including genetic susceptibility and environ-
mental influences [1]. Despite decades of extensive
research, the etiology of AMD remains unknown,
which explains the lack of effective and affordable
treatments. Recent introduction of therapies target-
ing vascular endothelial growth factor (VEGF) has
significantly improved clinical outcomes in patients
with neovascular (“wet”) AMD, a form characterized
by pathological proliferation of new blood vessels in
the retina [2]. However, no treatment currently exists
for patients with “dry” AMD (a progressive loss of
the region of highest visual acuity in the macula) or
geographic atrophy, other than vitamin supplements,
which can only slow the progression of vision loss[3].
One of possible strategies for treating and preventing
AMD and for reversing age-related changes in the ret-
ina is the use of agonists and antagonists of mono-
CHANGES IN RETINAL MONOAMINE LEVELS IN RETINOPATHY 1407
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
amines, such as dopamine (DA), serotonin (5-HT), and
noradrenaline (NA), which act as neuromodulators
in the retina [4-6]. Several retrospective studies have
shown that levodopa (L-DOPA), a DA precursor com-
monly used in Parkinson’s disease therapy, reduces
the incidence of AMD and slows down its progression
to geographic atrophy [7-9]. However, interpretation
of these findings is limited, as patients in these stud-
ies received L-DOPA to compensate for DA deficiency
associated with Parkinson’s disease and, therefore,
had initially lower DA levels in the central nervous
system. Onthe contrary, the use of selective serotonin
uptake inhibitors (SSRIs), which are the most frequent-
ly prescribed antidepressants globally, has been asso-
ciated with reduced visual acuity [4] and decreased
thickness of the foveal and perifoveal macular gangli-
on cell complex [10] and, in rare cases, maculopathy
[11]. It is important to note that patients with major
depressive disorder, bipolar disorder, or schizophre-
nia, i.e., diseases characterized by dysregulation of
monoaminergic systems, often exhibit structural and
functional retinal abnormalities [12]. Although bio-
genic amines were first detected in the retina over
50 years ago, age-dependent changes in these systems
in this structure remain unclear, particularly in the
context of neurodegenerative disease progression.
To elucidate the role of monoamines in retinal aging
and AMD development, we used senescence-accelerat-
ed OXYS rats, which develop retinopathy that closely
resembles human AMD in its clinical presentations,
morphology, and ultrastructural features [13]. The
first clinical manifestations of retinopathy can be de-
tected by ophthalmoscopic examination in ~20% of
OXYS rats at the age of 5-6 weeks and are recorded
in all animals at the age of 3-4 months. Pathological
changes progress and reach advanced stages associat-
ed with the loss or significant deterioration of visual
acuity by 14-18 months of age. The type of retinopa-
thy developing in OXYS rats corresponds to the “dry”
form of AMD and is manifested by dystrophic changes
and thinning of the retina, impaired microcirculation
in the choroid, changes in neurotrophic support, ac-
cumulation of lipofuscin and amyloid β, as well as
structural abnormalities of the retina characteristic of
AMD[13]. Themain aim of this study was to compare
levels and metabolism of biogenic amines, as well as
the expression of their receptors in the retina of male
OXYS and Wistar rats at the preclinical (20days) and
early (3-5months) stages, as well as during pathology
progression (16-18 months).
MATERIALS AND METHODS
Animals. The study was performed on OXYS and
Wistar rats aged 20 days, 3 months, and 16 months,
obtained from the Conventional Animal Vivarium
of the Institute of Cytology and Genetics. The ani-
mals were housed under standard laboratory condi-
tions (22 ± 2°C and a 12-h light/dark cycle) in cages
(57×36×20 cm; five animals per cage) with adlibitum
access to standard rodent chow (PK-120-1; Labora-
torsnab, Russia) and water. The rats were euthanized
by CO
2
asphyxiation followed by decapitation. Next,
retinas from both eyes of each rat were carefully
excised (n = 8 per group), transferred into microfuge
tubes, flash-frozen in liquid nitrogen, and stored
at −80°C.
Sample preparation. The retinas were homoge-
nized in 150 μL of 50 mM Tris-HCl, pH 6.0, contain-
ing 1 mM dithiothreitol (DTT) (Sigma-Aldrich, USA).
A 50-μL aliquot of the homogenate was immediately
mixed with 150 μL of 0.6 M HClO
4
and centrifuged
for 15 min at 12,700 rpm (4°C). The resulting su-
pernatant was diluted twofold with water and used
for quantification of biogenic amines by high-per-
formance liquid chromatography (HPLC). The pellet
was resuspended in 1 mL of 0.1 M NaCl and used for
protein quantitation by the Bradford assay (Bio-Rad,
USA) in accordance with the manufacturer’s protocol.
The remaining 100 µL of homogenate was centrifuged
for 15 min at12,700g (4°C). The supernatant was then
transferred into a transparent microfuge tube. The
supernatant and the pellet were stored at −80°C for
subsequent monoamine oxidase (MAO) activity assay.
Monoamine assay by HPLC was performed as
described previously [14]. In brief, concentrations
of NA, DA, 3,4-dihydroxyphenylacetic acid (DOPAC),
serotonin (5-HT), and 5-hydroxyindoleacetic acid
(5-HIAA) were determined in the diluted supernatant
by HPLC on a Luna C18(2) column (length × internal
diameter 10; particle size, 5 μm) with electrochemical
detection at 750 mV with a DECADE II™ electrochem-
ical detector (Netherlands). Calibration curves were
generated using standard mixtures containing 0.5,
1, and 2 ng of each biogenic amine (Sigma-Aldrich).
Peak areas were estimated using LabSolution LG/GC
software, version 5.54 (Shimadzu Corporation, Japan)
and quantified against the corresponding standard
curves. Monoamine concentrations were normalized
to the total protein content determined by the Brad-
ford assay.
Tyrosine hydroxylase (TH) activity assay. TH
activity was measured as previously described [15].
A 15-μL aliquot of the supernatant was incubated for
15 min at 37°C in the presence of 0.3 mM L-tyrosine
(Sigma-Aldrich), 0.3 mM 6,7-dimethyl-5,6,7,8-tetrahy-
dropteridine (cofactor) (Sigma-Aldrich), decarboxylase
inhibitor m-hydroxybenzylhydrazine (Sigma-Aldrich),
5 μM catalase (Sigma-Aldrich), and 1 mM DTT in
a final volume of 25 μL. The reaction was termi-
nated by adding 75 μL of 0.6 M HClO
4
, followed
TELEGINA et al.1408
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
by centrifugation at 14,000g for 15 min. The superna-
tant was diluted twofold with water, and the L-DOPA
concentration was determined in the diluted super-
natant by HPLC as described above using standard
solutions of L-DOPA (25, 50, and 100 pmol; Sigma,
USA). Additional 10 μL of the supernatant was com-
bined with 90 μL of 0.1 M NaOH for protein quanti-
tation by the Bradford method. The TH activity was
expressed in pmol of L-DOPA formed per minute per
mg of protein.
MAO activity assay. The MAO activity was as-
sessed using 5-HT as a substrate as described in
[16, 17], with modifications, and was defined as the
amount of 5-hydroxyindoleacetic aldehyde synthe-
sized per minute per mg of protein. In brief, the
pellet was resuspended in 100 μL of 50 mM Tris-HCl,
pH 7.6, using a motor-driven grinder (Z359971, Sigma-
Aldrich) and centrifuged for 15 min at 435g (4°C).
A 10-μL aliquot of the turbid supernatant was in-
cubated for 10 min at 37°C with 0.1 mM 5-HT in a
final volume of 25 μL. The reaction was terminated
by adding 75 μL of 0.6 M HClO
4
, followed by a 15-
min centrifugation at 14,000g The supernatant was
diluted twofold with water, and the concentration
of 5-hydroxyindoleacetic aldehyde was determined
by HPLC using 5-hydroxyindoleacetic aldehyde stan-
dards (500, 1000, and 2000 pmol; Cymit Quimica S.L.,
Spain). Additional 10 μL of the turbid supernatant
was combined with 90 μL of 0.1 M NaOH for protein
quantitation by the Bradford method. MAO activity
was expressed in pmol of 5-hydroxyindoleacetic alde-
hyde formed per minute per mg of protein.
Transcriptome data analysis. Retinal gene ex-
pression profiles in OXYS and Wistar rats at 20 days,
3 months, and 18 months (n = 3 in each group) have
been previously characterized via high-throughput
transcriptome sequencing (RNA-seq) on the Illumina
platform (see [18] and [19] for detail). Differences in
the expression levels were considered significant at
p-value < 0.01. Differentially expressed genes were
then compared with a list of genes associated with
the dopaminergic synapse (rno04728), serotoniner-
gic synapse (rno04726), and tryptophan and tyrosine
metabolism (rno00380 and rno00350, respectively) in
the KEGG pathway database (https://www.genome.jp/
kegg/).
Statistical analysis was performed using the
STATISTICA 10.0 software package (StatSoft, USA). The
normality of data distribution was assessed by the
Shapiro–Wilk test, and the homogeneity of varianc-
es was evaluated by the Levene’s test. Outliers were
identified by the Dixon’s Q test and excluded from
subsequent analysis. Comparisons between groups
were performed using factorial analysis of vari-
ance (ANOVA) and the nonparametric Kruskal–Wal-
lis test with a post hoc comparison of group means.
It should be noted that both parametric and non-
parametric statistical methods produced comparable
results. The genotype (strain) and age were consid-
ered independent factors. Results are presented as
mean ± standard deviation (SD). Differences were
considered statistically significant at p < 0.05.
RESULTS
Changes in monoamine concentrations in the
rat retina during aging and in the development of
AMD-like retinopathy. First, we assessed biogenic
amine levels in the retina of Wistar and OXYS rats
of different ages (20 days, 5 months, and 16 months)
by HPLC. We reliably detected NA, DA, DOPAC,
5-HT, and 5-HIAA in the retina, whereas the concen-
tration of homovanillic acid was below the detection
level.
According to two-way ANOVA, retinal concentra-
tions of NA (F
2,48
= 5.88, p < 0.01), DA (F
2,47
= 106.551,
p < 0.001), DOPAC (F
2,48
= 42.32, p < 0.001), and
5-HIAA (F
2,45
= 11.77, p < 0.001) depended on age.
In addition, the levels of DA (F
1,47
= 19.48, p < 0.001),
DOPAC (F
1,48
= 11.41, p < 0.01), and 5-HT (F
1,47
= 5.19,
p < 0.05) depended on the rat genotype (Fig. 1).
Despite these overall effects, pairwise compari-
sons of group mean revealed no changes in the NA
and 5-HT levels. The concentration of 5-HIAA was
found to be significantly increased in the retinas of
both Wistar and OXYS rats at the age of 16 months
(p < 0.05). To evaluate 5-HT turnover, we calculated
the 5-HIAA : 5-HT ratio. Two-way ANOVA revealed
significant effects of both age (F
1,46
= 4.14, p < 0.05)
and genotype (F
1,46
= 8.59, p < 0.01) on this ratio.
The 5-HIAA : 5-HT ratio was higher in 20-day-old
OXYS rats compared with age-matched Wistar rats
(p < 0.05) and decreased in OXYS rats by 4 months of
age (p < 0.05).
DA levels were elevated at the age of 5 months
(p < 0.05), whereas DOPAC concentration progressive-
ly increased up to 16 months of age in the retina of
both strains (p < 0.05). Moreover, the levels of both
DA and DOPAC were higherin the retina of OXYS rats
at the age of 5 and 16 months compared with age-
matched Wistar rats (p < 0.05). The index of DA turn-
over (estimated as the DOPAC : DA ratio) depended
on age (F
1,47
= 8.45, p < 0.001) but not on genotype.
This ratio decreased at the age of 4months in Wistar
rats (p < 0.05), while no significant difference in the
DOPAC : DA ratio was observed between the strains.
TH and MAO activities. Next, we analyzed the
activity of TH, which catalyzes L-tyrosine hydroxyl-
ation to L-DOPA, and MAOs, which catalyze the oxi-
dative deamination of monoamines such as DA, sero-
tonin, and epinephrine, in the rat retina. MAO activity
CHANGES IN RETINAL MONOAMINE LEVELS IN RETINOPATHY 1409
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
Fig. 1. Concentrations of biogenic amines and activities of TH and MAO in the retina of Wistar and OXYS rats at different
ages. Data are presented as mean ± SD (n = 7-8); * p < 0.05 vs. strain; # p < 0.05 vs. previous age.
showed a significant age dependence (F
2,48
= 32.11,
p < 0.0001) but was not influenced by the animal
genotype. In Wistar rats, MAO activity increased pro-
gressively with age (significant differences were de-
tected between 20 days and 5 months and between 5
and 16 months, p < 0.05). In OXYS rats, MAO activity
also increased significantly from 20 days to 5months
(p < 0.05) but remained stable thereafter. Significant
differences between the rat strains were found only
at the age of 16 months, when MAO activity was
approximately 1.3-fold lower in OXYS rats than in
Wistar rats (p < 0.05). In contrast, TH activity was
unaffected by either age or genotype, with no signif-
icant differences detected across the groups (Fig. 1).
TELEGINA et al.1410
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
Fig. 2. a) Differences in the expression of monoamine-associated genes between Wistar and OXYS rats at the age of 20days,
3 months, and 18 months. b) GO chord plot showing a relationship between differentially expressed genes and associated
Gene Ontology(GO) terms. c) Venn diagrams illustrating the overlap of differentially expressed monoamine-associated genes
at the age of 20 days, 3 months, and 18 months. Colors indicate upregulated (red) or downregulated (blue) genes.