ISSN 0006-2979, Biochemistry (Moscow), 2026, Vol. 91, No. 8, pp. 1396-1405 © Pleiades Publishing, Ltd., 2026.
ISSN 0006-2979, Biochemistry (Moscow), 2026. © Pleiades Publishing, Ltd., 2026.
1396
Neuroprotective Effect of Subconjunctival
Melatonin Injections in 6-Hydroxydopamine-Induced
Neurotoxic Eye Damage
Tatyana A. Pavlenko
1,a
*, Natalya B. Chesnokova
1
, Olga A. Lisovskaya
1
,
Olga V. Beznos
1
, and Andrey V. Grigoryev
1
1
Helmholtz National Medical Research Center of Eye Diseases, Ministry of Health of the Russian Federation,
105062 Moscow, Russia
a
e-mail: tanya1975_@inbox.ru
Received November 1, 2025
Revised April 27, 2026
Accepted April 27, 2026
Abstract— The search for new effective methods of neuroprotection is relevant for the treatment of a wide
range of common eye diseases such as age-related macular degeneration, glaucoma, and diabetic retinopa-
thy. Using a rabbit model of neurotoxic eye damage induced by intravitreal injection of the dopaminergic
neurotoxin 6-hydroxydopamine (6-OHDA), we studied the effect of subconjunctival injections of 0.1% mela-
tonin solution on pathophysiological and biochemical processes in the eye. Administration of 6-OHDA caused
destruction of the retinal pigment epithelium, narrowing of retinal vessels, hemorrhages, decrease in ocular
blood flow, and intraocular pressure, as well as reduced activity of α
2
-macroglobulin and concentrations of
matrix metalloproteinase-9 and angiotensin-converting enzyme in the aqueous humor, vitreous body, and
retina. Subconjunctival melatonin injections over 9 days resulted in the 2-fold on average reduction of
the area of retinal damage, and normalization of the minute volume of ocular blood flow and intraocular
pressure. On day 14, an increase in the α
2
-macroglobulin activity in the aqueous humor (0.66 ± 0.19 vs.
0.49 ± 0.13 nmol/min per 1 mg protein) and vitreous body (0.50 ± 0.15 vs. 0.21 ± 0.07 nmol/min per 1 mg
protein, p < 0.05) was observed, along with a decrease in the matrix metalloproteinase-9 concentration
(118.4 ± 34.6 vs. 524.3 ± 163.2 ng/mg, p < 0.05) and angiotensin-converting enzyme concentration (371.0 ± 52.1
vs. 596.1 ± 171.5 ng/mg, p < 0.05) in the retina. This indicates reduction in the intensity of inflammation
and vascular wall permeability. Melatonin could be considered a promising neuroprotective component in
the complex therapy of retinal diseases.
DOI: 10.1134/S0006297925603880
Keywords: 6-hydroxydopamine, neuroprotectors, neuroinflammation, retina, melatonin, intraocular pressure,
α
2
-macroglobulin, matrix metalloproteinase-9, angiotensin-converting enzyme
* To whom correspondence should be addressed.
INTRODUCTION
The number of patients with neurodegenerative
diseases of both central nervous system and visual
organs is progressively increasing worldwide. The
main causes of visual impairment in common eye
diseases such as age-related macular degeneration,
glaucoma, and diabetic retinopathy are neurodegen-
erative processes in the retina. In Russia, in 2022,
more than 1.25 million patients with glaucoma were
registered, 16,000 people (1.6 per 10,000 population)
with age-related macular degeneration, and the total
incidence of diabetic retinopathy in 2023 ranged from
700,000 to 1.5 million patients [1-3].
Development of new effective methods for early
diagnosis, prevention, and treatment of neurodegen-
eration in ophthalmology is an urgent and socially
significant task. Experimental studies have demon-
strated high potential of melatonin as a neuropro-
tector [4-6]. Melatonin is primarily produced in the
pineal gland, and retina is secondary source of this
hormone. In addition to regulating circadian rhythm,
EFFECT OF MELATONIN ON NEUROTOXIC EYE DAMAGE 1397
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
it suppresses oxidative stress, inflammatory response,
and programmed cell death (apoptosis and autopha-
gy), which are the main pathogenetic mechanisms of
neurodegeneration [7]. This makes promising its use
for neuroprotection, in eye diseases.
Very low toxicity and high bioavailability of
melatonin provide wide possibilities for its applica-
tion. There is a lot of data on the involvement of
melatonin in regulation of physiological processes
in the eye and possible effectiveness of its use for
the treatment of eye diseases accompanied by neu-
rodegenerative processes in the retina – age-related
macular degeneration, diabetic retinopathy, retinopa-
thy of prematurity, glaucoma, etc. [8-11]. Age related
gradual decrease of melatonin production weakens
natural protection of the retina and increases the
risk of neurodegeneration. Replenishing deficiency
with exogenous melatonin may be a promising neu-
roprotective strategy [5, 12]. It has been proven that
it improves mitochondrial function, reduces oxidative
stress and inflammation, suppresses cell apoptosis,
and slows the progression of degenerative changes.
Thus, exogenous melatonin is a multifactorial agent
that can compensate age-related protection decline
and become an important remedy for the prevention
and therapy of age-related eye diseases [13].
The possibilities of topical and systemic mela-
tonin for the treatment of eye diseases have been
studied experimentally and in clinical settings. Top-
ical melatonin was used mainly to study its effect
on intraocular pressure (IOP), that depends on the
functioning of the anterior eye segment structures
[10, 12]. Very few studies have been devoted to the
effect of subconjunctival melatonin injections on neu-
rodegenerative processes in retina. For this purpose,
a model based on oxidative toxicity of glutamate in
combination with L-buthionine-S,R-sulfoximine, which
triggers apoptosis of nerve cells through mechanisms
of cytotoxicity and oxidative stress, was used [14, 15].
Many neurodegenerative diseases of the reti-
na are associated with the deficiency of dopamine,
which is mainly synthesized by amacrine cells of the
retina [16]. However, so far, the dopamine deficiency
model has not been used to study the mechanisms
of melatonin action, particularly, its effect on the re-
nin-angiotensin system activity and protease-inhibitor
balance in the neurodegenerative process in the eye.
We chose a model based on the intravitreal injection
of the dopaminergic neurotoxin 6-hydroxydopamine
(6-OHDA). The mechanism of damage and death of
dopaminergic neurons (amacrine cells in the retina)
under the influence of 6-OHDA involves disruption of
intracellular calcium homeostasis, which leads to cal-
pain activation and mitochondrial damage [17].
The aim of this work was to achieve local dis-
ruption of dopamine production, oxidative stress and
neuroinflammation in the retina using dopaminergic
neurotoxin, in order to reproduce the main patho-
genetic mechanisms of the development of neuro-
degenerative process in the eye and to estimate the
possibility of topical use of melatonin to stop neu-
roinflammation.
MATERIALS AND METHODS
The study was performed on 30 male chinchilla
rabbits 2.0 kg weight. The animals were purchased
from FGUP OPH “Manikhino” and kept in the ex-
perimental centre of the Helmholtz National Medi-
cal Research Center of Eye Diseases at air tempera-
ture of 20-22°C, humidity of at least 45%, and 12-h
light cycle with free access to standard rabbit feed
and water.
All experiments with animals were carried out in
accordance with the recommendations of the “State-
ment for the Use of Animals in Ophthalmic and
Visual Research” of the Association for Research in
Vision and Ophthalmology (ARVO). The protocol of
this study was approved by the Ethics Committee of
the Helmholtz National Medical Research Center of
Eye Diseases of the Ministry of Health of Russia (Pro-
tocol no. 63/1, dated December 1, 2024).
The animals were randomly divided into 3
groups of 10 rabbits each. In two groups, a neurode-
generative process in the retina was modeled via in-
travitreal injection of a 6-hydroxydopamine solution
(Sigma-Aldrich, USA) in a sterile NaCl 0.9% contain-
ing 0.5% ascorbic acid (m/V) according to the pro-
tocol developed at the Helmholtz National Medical
Research Center of Eye Diseases [18]. Ascorbic acid
in this case acts as a stabilizer of 6-OHDA, prevent-
ing its premature oxidation, according to the original
method [19]. For treatment, subconjunctival injections
of 0.1% melatonin solution (m/V) (Sigma-Aldrich) in
a sterile NaCl 0.9% containing 5% dimethyl sulfoxide
(V/V) were used (Table 1). Time points for examina-
tions were chosen based on the data obtained during
the development of the model [18].
The first (control) group of animals was intravit-
really injected once with 0.1 ml of sterile NaCl 0.9%
containing 0.5% ascorbic acid (m/V) in both eyes.
The other animals (groups 2 and 3) received intra-
vitreal injection of 0.25 mg of 6-OHDA in 0.1 ml of
sterile NaCl 0.9% containing 0.5% ascorbic acid (m/V)
in both eyes. Group 2 received subconjunctival in-
jections of 0.05 ml of NaCl 0.9% as placebo once per
day in both eyes. Animals in group 3 were treated
with 0.05 ml of 0.1% melatonin solution (m/V) in NaCl
0.9% containing 5% dimethyl sulfoxide (V/V) subcon-
junctivally in the same mode. Treatment was carried
out for 9 days.
PAVLENKO et al.1398
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
Table 1. Experimental design
Groups Group 1 (Control) Group 2 (Model) Group 3 (Model)
1) Start of experiment – intravitreal injection
Composition
of intravitreal
injections
0.1 ml: NaCl 0.9% (m/V),
ascorbic acid 0.5% (m/V)
0.1 ml: 6-OHDA 0.25 mg,
NaCl 0.9% (m/V),
ascorbic acid 0.5% (m/V)
0.1 ml: 6-OHDA 0.25 mg,
NaCl 0.9% (m/V),
ascorbic acid 0.5% (m/V)
2) Treatment duration: 9 days, starting from the day of 6-OHDA injection
Composition
of subconjunctival
injections
0.05 ml: NaCl 0.9% (m/V),
DMSO 5% (V/V)
0.05 ml: NaCl 0.9% (m/V),
DMSO 5% (V/V)
0.05 ml: melatonin 0.1% (m/V),
NaCl 0.9% (m/V),
DMSO 5% (V/V)
3) IOP measurement daily, plethysmography on days 7 and 14, fundus examination on day 14
4) Material collection and examination on days 7 and 14: 10 eyes from each group, including control
Biochemical studies total protein
concentration
α
2
-macroglobulin activity matrix metalloproteinase-9
concentration
Material for
research
aqueous humor,
vitreous body, retina
aqueous humor, vitreous
body, retina
aqueous humor, retina
In all animals, IOP was measured daily using an
automatic tonometer for animals, TonoVet (Tiolat, Fin-
land). Retina was examined using indirect ophthal-
moscopy with an aspherical lens +78 diopters against
a dilated pupil. Intensity of ocular blood flow was
estimated before the introduction of neurotoxin and
on days 7 and 14 after it by measuring the minute
blood flow using an Ophthalmoplethysmograph OP-A
(SKTB Optimed, Russia).
On day 14, fundus was examined via indirect
ophthalmoscopy with an aspherical lens +78 diopters
against a dilated pupil. The area of retinal pigment
epithelium disorganization was visually assessed in
points: 1 point corresponded to 1 quadrant of the
visible area of the retina. Neurovascular disorders in
the retina were also estimated: presence of edema,
indicating pathologically increased vascular permea-
bility, areas of ischemia, and hemorrhages.
Aqueous humor was collected via paracente-
sis with an insulin syringe under topical anesthe-
sia with Inocaine (oxybuprocaine 0.4%). Next, the
animals were euthanized, and eyes enucleated. The
eyeball was dissected along the ora serrata, vitreous
body and retina were collected. The vitreous body
was centrifuged for 10 min at 5000 rpm (1846g) with
cooling, and supernatant collected for examination.
The retinas were homogenized in ice-cold NaCl 0.9%
(0.3 ml per 1 retina) using an ultrasonic homogeniz-
er UP50H (Hielscher, Germany), centrifuged under the
same conditions, and supernatant collected for exam-
ination. The same procedures were repeated with the
remaining animals on day 14.
The following parameters were chosen for char-
acterizing severity of inflammation and microcircula-
tory disorders in retina: concentration of total protein
in the vitreous body (indicator of vascular permea-
bility), activity of α
2
-macroglobulin (an acute-phase
inflammatory protein and a broad-spectrum protease
inhibitor), concentration of angiotensin-converting en-
zyme (ACE) and matrix metalloproteinase-9 (MMP-9)
in aqueous humor and retina homogenate.
Concentrations of ACE and matrix MMP-9 were
determined via ELISA using diagnostic kits: ELISA kit
for ACE and ELISA kit for MMP-9 (Cloud-Clone Corp.,
USA). Activity of α
2
-macroglobulin was determined by
measuring of the ability of trypsin to cleave the spe-
cific substrate N-benzoyl-DL-arginine-p-nitroanilide in
the presence of a trypsin inhibitor from soybean [20]
(all reagents from Sigma-Aldrich). To eliminate inac-
curacies associated with heterogeneity of the struc-
ture of the retina and vitreous body and errors in
the collection of biological material, the obtained data
were expressed per 1 mg of total protein content,
determined by the Lowry method [21]. Absorbance
measurements at 405 nm were carried out with a
multifunctional spectrophotometer for microplates
Synergy MX (BioTek, USA).
Statistical processing of the results was carried
out using the Excel and Statistica 10.0 software pack-
ages. Significance of differences between the groups
with a significance level of at least 95% was evalu-
ated using the non-parametric Mann–Whitney U test.
The results are presented as an arithmetic mean and
standard error of the arithmetic mean (M ± SEM).
EFFECT OF MELATONIN ON NEUROTOXIC EYE DAMAGE 1399
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
RESULTS
According to the fundus examination on the day
14 after intravitreal injection of 6-OHDA, the area of
retinal damage was on average 2 points, i.e., 1/2 of
retinal surface. In all eyes neurovascular disorders
were noted: multiple hemorrhages of the retinal ves-
sels, constriction and tortuosity of retinal vessels,
scarcity of choroidal vessels, and retinal edema. In the
group receiving subconjunctival melatonin injections,
the area of damage was 2 times less and averaged
1 point (1/4 of the retinal area), while neurovascular
disorders in the form of single hemorrhages were de-
tected in only half of the eyes. In the control group,
after intravitreal injection of the control solution, no
pathological changes on the fundus were detected.
Administration of 6-OHDA caused the decrease
in intraocular blood flow due to the blood vessel
constriction. This corresponds with our previously
obtained data that 6-OHDA causes increase of the
vasoconstrictor endothelin-1 concentration in reti-
na [18]. After the injection of NaCl 0.9% (placebo), on
the day 7, increase in the minute volume of ocular
blood flow was observed, which is apparently due to
the local irritating effect (Fig. 1). Melatonin injections
caused the normalization of intraocular blood flow,
thus indicating that melatonin reduced the vascular
response to the neurotoxin.
After intravitreal injection of NaCl 0.9% IOP re-
mained elevated for 7 days due to the introduction of
extra volume into the eye (Fig. 2). Due to the effect
of 6-OHDA, IOP decreased over 7 days, followed by
the dramatic increase on the day 10. Melatonin did
not affect the IOP reduction after neurotoxin admin-
istration during the first 7 days, but in the course of
treatment, IOP gradually returned to the normal val-
ues, without increase on day 10.
Total protein concentration in the aqueous hu-
mor on the day 7 was the same in all groups, but
on the day 14, protein concentration in animals that
received 6-OHDA was 2 times higher than in controls
(Table 2). The same was found in the vitreous body
on the day 7 and on the day 14.
Table 2. Total protein concentration in the liquids and
tissues of rabbit eyes with experimental model of ret-
inal neurodegeneration (mg/ml)
Group
Control
group
Without
treatment
0.1%
Melatonin
injections
Aqueous humor
7 days 3.06 ± 0.19 4.07 ± 0.76 3.84 ± 0.64
14 days 1.86 ± 0.33 3.57 ± 0.83* 3.53 ± 0.64*
Vitreous body
7 days 3.04 ± 0.73 6.94 ± 1.38* 4.84 ± 0.45#
14 days 1.42 ± 0.19 3.66 ± 0.66* 3.17 ± 0.26*
Retina
7 days 4.66 ± 0.87 6.73 ± 1.23* 5.09 ± 0.28#
14 days 5.11 ± 0.10 6.22 ± 0.39* 5.59 ± 0.39
Note. * p < 0.05– compared to the control group. # p < 0.05–
compared to the group without treatment.
Fig. 1. Changes in the minute blood flow in the eye of rab-
bits after a single intravitreal injection of 6-OHDA without
treatment and after the course of daily subconjunctival injec-
tions of 0.1% melatonin: C– control group, NT– non-treated
group, Mt – group receiving 0,1% melatonin. * Significant
difference from the control group, p < 0.05. # Significant dif-
ference from the placebo group, p < 0.05.
Fig. 2. Dynamics of IOP in rabbits after single intravitreal in-
jection of 6-OHDA without treatment and after the course of
daily subconjunctival injections of 0.1% melatonin solution:
1 – control group, 2 – group without treatment, 3 – group
receiving melatonin treatment. * Significant difference from
the control group, p < 0.05. # Significant difference from the
placebo group, p < 0.05.
PAVLENKO et al.1400
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
a b
c
Fig. 3. Activity of α
2
-macroglobulin in the aqueous humor (a), vitreous body (b), and retina (c) of rabbits on day 14 af-
ter single intravitreal injection of 6-OHDA without treatment and after the course of daily subconjunctival injections of
0.1% melatonin: C – control group, NT – non-treated group, Mt – group receiving 0.1% melatonin. * Significant difference
from the control group, p < 0.01. # Significant difference from the placebo group (injections of saline), p < 0.05.
a b
Fig. 4. MMP-9 concentration in aqueous humor (a) and retina (b) of rabbits on day 14 after single intravitreal injection of
6-OHDA without treatment and after the course of daily subconjunctival injections of 0.1% melatonin solution: C – control
group, NT – non-treated group, Mt – group receiving melatonin treatment. * Significant difference from the control group,
p < 0.05. # Significant difference from the placebo group (injections of saline), p < 0.05.
In the non-treated group, activity of α
2
-macro-
globulin in the retina and vitreous body significantly
decreased on the day 14 (p < 0.01 compared to the
control group) (Fig. 3). After the course of subcon-
junctival melatonin injections, on the day 14, activity
of α
2
-macroglobulin in the aqueous humor was high-
er than in the non-treated group (p < 0.05).
After the neurotoxin administration, concentra-
tion of ACE and MMP-9 in retina increased signifi-
cantly (Figs. 4 and 5). Melatonin injections caused a
EFFECT OF MELATONIN ON NEUROTOXIC EYE DAMAGE 1401
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
a b
Fig. 5. Changes in ACE concentration in the aqueous humor (a) and retina (b) of rabbits on day 14 after a single intravitreal
injection of 6-OHDA without treatment and after the course of daily subconjunctival injections of 0.1% melatonin solution:
C – control group, NT – non-treated group, Mt – group receiving 0.1% melatonin. * Significant difference from the control
group, p < 0.05. # Significant difference from the placebo group (injections of saline), p < 0.05.
significant decrease in the concentration of ACE and
MMP-9 in the retina, indicating its anti-inflammatory
and anti-ischemic effects.
DISCUSSION
The revealed disorders of protease-inhibitor bal-
ance in tissues and liquids of the eye (decrease in
α
2
-macroglobulin activity in the vitreous body and
increase in MMP-9 and ACE contents in the aqueous
humor and retina, as well as increase of total protein
concentration in the liquids and retina) in combina-
tion with the signs of ischemia indicate the develop-
ment of neuroinflammation, which leads to neuro-
vascular disorders in the course of retinal damage
caused by 6-OHDA [22-24].
Introduction of the dopaminergic neurotox-
in 6-OHDA into the eye causes oxidative stress and
mitochondrial dysfunction in the dopaminergic neu-
rons – the main causes of neurovascular disorders in
neurodegenerative processes, leading to the death of
nerve cells by apoptosis [25, 26].
Changes in intraocular pressure, as well as pu-
pillary reaction and ocular blood flow under the
influence of the neurotoxin, are associated with its
effect on the sympathetic innervation of the ante-
rior eye segment structures, which are responsible
for the production and outflow of aqueous humor
and the tone of blood vessels via dopamine recep-
tors.
Decrease in the IOP followed by the signifi-
cant increase revealed in the non-treated group
was also observed in our previous experiments.
The mechanism of this requires further study; in
particular, it may be associated with the two-phase
change in dopamine content in the retina after ad-
ministration of 6-OHDA, which decreases during the
first 7 days and then sharply increases on the day 10,
probably due to a compensatory reaction [18].
After the course of subconjunctival administra-
tion of melatonin, we revealed a significantly lower
severity of pathological changes in the retinal pig-
ment epithelium and retinal vessels. The IOP level
recovered gradually, without increase on the day
10, unlike in the group without treatment. The ef-
fect of melatonin may be due to its ability to sup-
press mitochondrial dysfunction and oxidative stress
in the non-pigmented ciliary epithelium involved in
the production of aqueous humor and maintenance
of IOP [27].
At the same time decreased protein content in
the eye liquids indicating vascular permeability re-
duction was observed. Activity of α
2
-macroglobulin,
on the contrary, increased. This could be considered
as a favorable sign, because in another study in the
same animal model investigating the action of indo-
methacin and thioctic acid, higher level of α
2
-macro-
globulin activity after treatment corresponded to the
significantly smaller area of retinal damage [28].
Anti-inflammatory and neuroprotective effects
of melatonin could be implemented via several path-
ways. First, melatonin is a powerful antioxidant [29],
and suppression of oxidative stress leads to the de-
crease in production of many pro-inflammatory fac-
tors. In addition, melatonin is able to regulate expres-
sion of their genes [30, 31].
It is known that by suppressing the synthesis
of MMP-9 in endothelial cells, melatonin contrib-
utes to stabilization of the blood-retinal barrier and
PAVLENKO et al.1402
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
decrease in the vascular permeability [32]. This is
proved by the revealed decrease in the protein per-
meation into the aqueous humor and vitreous body.
Melatonin suppresses expression of the MMP-9 and
MMP-2 by blocking signaling pathways mediated by
transcription factor NF-κB, phosphoinositide-3-kinase
and protein kinases B (PI3K/Akt pathway), and simul-
taneously activate expression of tissue inhibitor of
metalloproteinases TIMP-1 and reduce the content of
pro-inflammatory cytokines (tumor necrosis factor-α,
interleukin-1β, interleukin-6), that also leads to the
decrease in the production of MMP-9 [29-31].
The renin-angiotensin system plays an import-
ant role in the development of retinal pathology of
various etiology. Its components are involved in the
development of inflammation, ischemia, neovascular-
ization, and also cause ocular hypertension [33, 34].
Itis known that melatonin is able to regulate activity
of this system [35], although the direct effect of mela-
tonin on the content of ACE in the tissues and liquids
of the eye has not yet been revealed.
It has been experimentally proved that, in car-
diovascular system, melatonin and angiotensin have
the opposite effects, apparently due to the antiox-
idant and sympatholytic effects of melatonin [36].
Activation of the renin-angiotensin system has a
close bidirectional relationship with oxidative stress.
Its severity positively correlates with the expression
of ACE, angiotensin 2, angiotensin 1 and angiotensin 2
receptors [37]. Angiotensin 2 itself activates produc-
tion of superoxide anion and hydrogen peroxide [38].
The main pathways of melatonin effect upon
the renin-angiotensin system are neutralization of
the pro-oxidant effect of angiotensin 2 and increase
in the concentration of ACE2 – an enzyme catalyz-
ing conversion of angiotensin 1 to angiotensin 1-9
and of angiotensin 2 to angiotensin 1-7 which leads
to the decrease in the angiotensin 2 level. This is
achieved by suppression of oxidative stress due to
the antioxidant action of melatonin itself and mel-
atonin-induced activation of the expression of anti-
oxidant enzymes superoxide dismutase, catalase, and
glutathione peroxidase [36, 38, 39]. In addition, mel-
atonin triggers and enhances other signaling path-
ways, mediated by inhibiting caspases and activating
SIRT1 (a deacetylase that suppresses the acetylation
of ACE2), and also blocks the signaling pathway me-
diated by the extracellular signal-regulated kinase
and the universal transcription factor NF-κB, which
leads to inhibition of pro-inflammatory mediators
formation [40, 41]. Ultimately, all these processes
lead to the increase in concentration of ACE2 and
angiotensin 1-7, decrease in the level of ACE and an-
giotensin 2, which, in turn, helps to reduce severity
of inflammation, oxidative stress, and endothelial
dysfunction.
CONCLUSION
The presented data indicate that melatonin,
when administered subconjunctivally, suppresses de-
velopment of neuroinflammation and promotes res-
toration of pathophysiological and biochemical pro-
cesses in the eye tissues disrupted by dopaminergic
deficiency. The ability of melatonin to exert a neu-
roprotective effect with simultaneous IOP reduction
makes it a promising drug for the treatment of glau-
coma. It could also be used for age-related macular
degeneration treatment, especially since it is known
that such patients have lower content of melatonin in
blood and tear fluid than individuals of the same age,
but without macular degeneration [42].
Despite the fact that several melatonin-based
ophthalmologic drugs have been developed and
patented worldwide, and preclinical studies have
demonstrated their effectiveness, their clinical use is
still far away [43]. Recently, the use of melatonin in
the form of eye drops for the treatment of neurode-
generative eye diseases has been actively discussed.
Undoubtedly, advantage of using eye drops is non-in-
vasiveness of the treatment. However, it is difficult
to affect acute neurovascular disorders in the reti-
na, such as the one reproduced in our model, with
eye-drops, since the drug needs to penetrate to the
posterior eye segment through many tissue barriers.
Unlike eye-drops, subconjunctival injections facilitate
penetration of a drug with significantly fewer obsta-
cles. A drug administered via subconjunctival injec-
tions immediately enters the eye blood vessels and
retina, achieving high concentration of the drug in it
with significantly fewer side effects.
The model of neurotoxic retinal damage we pro-
posed could be possibly used for further study of
the effectiveness of melatonin as a neuroprotector
not only in the eye diseases but also in neurodegen-
erative processes in the central nervous system and
would contribute to its introduction into the clinical
practice.
Abbreviations
6-OHDA 6-hydroxydopamine
ACE angiotensin-converting enzyme
DMSO dimethyl sulfoxide
IOP intraocular pressure
MMP matrix metalloprotease
Contributions
T. A. Pavlenko – concept and supervision of the
work; T. A. Pavlenko, O. V. Beznos, O. A. Lisovska-
ya, and A. V. Grigoryev – conducting experiments;
O. V. Beznos – data processing; T. A. Pavlenko and
N. B. Chesnokova – writing and editing the text of
the article.
EFFECT OF MELATONIN ON NEUROTOXIC EYE DAMAGE 1403
BIOCHEMISTRY (MOSCOW) Vol. 91 No. 8 2026
Funding
This work was financially supported by the Ministry
of Health of the Russian Federation (reg. no. NIOKTR
124020100001-1).
Ethics approval and consent to participate
All experiments with animals were carried out in accor-
dance with the rules established by GOST 33215-2014
“Guidelines for the Care and Use of Laboratory Animals”
and the recommendations of the “Statement for the Use
of Animals in Ophthalmic and Visual Research” of the
Association for Research in Vision and Ophthalmology
(ARVO). The protocol of this study was approved by the
Ethics Committee of the Helmholtz National Medical Re-
search Center of Eye Diseases of the Ministry of Health
of Russia (Protocol no. 63/1, dated December1, 2022).
Conflict of interest
The authors of this work declare that they have no
conflicts of interest.
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