Melatonin and Serotonin: Their Synthesis and Effects in Insects

Article information

Chronobiol Med. 2022;4(1):24-28
Publication date (electronic) : 2022 March 31
doi :
Section of Entomology, Department of Zoology, Aligarh Muslim University, Aligarh, India
Corresponding author: Mohd Kaleemullah Farooqi, PhD, Section of Entomology, Department of Zoology, Aligarh Muslim University, Aligarh, U.P. 202002, India. Tel: 91-9045614310, E-mail:
Received 2022 February 28; Revised 2022 March 13; Accepted 2022 March 13.


Melatonin and serotonin are endogenous neurotransmitters and hormones produced by tryptophan metabolism. Melatonin and serotonin are present in the heads, eyes, optic lobes, and brains of various invertebrates. Insects produce serotonin and melatonin via the same pathways that vertebrates do. They are pleiotropic signaling chemicals that regulate a wide range of physiological processes and protect against disease and environmental factors in various species. In insects, they function as neurotransmitters, hormones, and bioregulators, controlling a wide range of endocrines, behavioral, immunological, developmental, and defensive activities, as well as biological cycles.


Melatonin and serotonin are the products of tryptophan metabolism, which are endogenous neurotransmitters and hormones. They are pleiotropic signaling chemicals that regulate a variety of physiological processes as well as counteract disease and environmental stresses in several species. They act as neurotransmitters, hormones, and bioregulators in insects controlling a variety of endocrines, behavioral (including social behavior), immunological, developmental, and protective activities, as well as biological cycles.

Melatonin is an ancient molecule found in alpha proteobacteria and cyanobacteria, indicating that it has been around since the beginning of life. Melatonin (N-acetyl-5-methoxytryptamine), an amino acid derivative, is an evolutionary conservative chemical. Melatonin is found in algae and higher plants [1], as well as nearly every invertebrate group and vertebrates [2].

Bacteria, Eucarya, unicellular and multicellular fungi, plants, and animals, including simple and complex vertebrates and invertebrates, all synthesize this pluripotent molecule with different effects in nature. Melatonin regulates several endocrines, immunological, neurological, metabolic, and defensive activities in addition to the circadian rhythm. Many researchers suggest that melatonin can be utilized as a health supplement because of its multiple biological properties, including antioxidant, anticancer, and antiaging benefits. Melatonin was identified for the first time in invertebrates in the compound eyes of an insect, the locust, Locusta migratoria [3] and it was detected using a radioimmunoassay, which was then confirmed using gas chromatography-mass spectrometry. Furthermore, melatonin is a highly efficient scavenger not only in vertebrates but also in insects [4] and dinoflagellates [5] of metabolic hydroxyl-radicals.

Serotonin, also an ancient molecule with pluripotent and diverse activities, is a result of consecutive metabolic transformations of L-tryptophan that occur in various species, including vertebrates and invertebrates such as worms, insects, fungi, plants, and unicellular organisms.

Several fascinating research studies have revealed serotonin’s ubiquitous and overwhelming activities in vertebrates and invertebrates. Although the number of studies on the role of serotonin in insects is still few, these studies have already demonstrated the critical involvement of serotonin signaling in nearly all key physiological processes in insects. The serotonin transporter (SERT) from the fruit fly Drosophila melanogaster was identified as the first insect amine transporter.


Melatonin and serotonin have been discovered in the heads, eyes, optic lobes, and brains of various invertebrate species. Serotonin and melatonin are produced in insects via the same pathways in vertebrates. Tryptophan-5-hydroxylase-1 (T5H-1) is the rate-limiting enzyme in the biosynthesis of serotonin, which is also involved in the biosynthesis of melatonin. Melatonin is a natural hormone that helps people cope with stressors like cold, heat, UV, and pesticides. The direct link between T5H-1 and melatonin and the underlying mechanism in organisms has yet to be explored.

Biosynthesis of serotonin

Serotonin (5-hydroxytryptamine, 5-HT) is a monoamine, implying that an aliphatic chain with two carbon oxides separates a basic amine group from an aromatic core. The important amino acid tryptophan is the first step in synthesizing 5-HT. Tryptophan hydroxylase (TPH), also known as tryptophan-5-monooxygenase, adds a hydroxyl group to tryptophan to form 5-hydroxytryptophan (5-HTP) (Figure 1) [6]. The non-heme iron and tetrahydrobiopterin-dependent aromatic amino acid hydroxylase family includes TPH, phenylalanine hydroxylase (which catalyses the hydroxylation of the aromatic side-chain of phenylalanine to produce tyrosine), and tyrosine hydroxylase (which converts tyrosine to 3,4-dihydroxyphenylalanine [DOPA]). Cofactors for these enzymes are (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin and O2 [7,8].

Figure 1.

Biosynthesis of serotonin.

5-HTP decarboxylase catalyzes the conversion of 5-HTP to 5-HT in the second phase of 5-HT biosynthesis (Figure 1) [9]. 5-HTP decarboxylase has been proven to be the same enzyme as dopamine decarboxylase, which catalyzes the decarboxylation stage in dopamine biosynthesis and is commonly referred to as aromatic amino acid decarboxylase (AAAD) [10].

Biosynthesis of melatonin

The pathways of melatonin biosynthesis in insects appear to be similar to those seen in vertebrates [2]. The amino acid precursor L-tryptophan is converted to melatonin (N-acetyl-5-methoxytryptamine) by four enzymes: TPH, AAAD, serotonin N-acetyltransferase (NAT), and hydroxyindole-O-methyltransfer (HIOMT) (Figure 2).

Figure 2.

Biosynthesis of melatonin. AcCoA, acetyl coenzyme A; CoA, coenzyme A; SAMe, S-adenosyl-L-methionine; SAH, S-adenosyl-L-homocysteine.

TPH and NAT, two enzymes involved in the multistep conversion of L-tryptophan to N-acetyl-5-ethoxytryptamine, are thought to play a critical regulatory role, with the former limiting the formation of 5-HT (serotonin) and the latter limiting the formation of melatonin [11-14]. AAAD and HIOMT appear to function primarily by mass action, while their actions may be amenable to adaptive control [13]. As an essential amino acid, L-tryptophan cannot be synthesized by vertebrate cells and must be obtained through the diet, implying dietary composition’s importance in the appropriate melatonin synthesis in living animals [12].

5-HTP is formed from tryptophan, decarboxylated to become 5-HT (serotonin). In the vertebrate pineal gland, 5-HT metabolism follows two primary routes: acetylation, which involves NAT activity and results in N-acetylserotonin (NAS), the direct precursor of melatonin, and oxidative deamination, which involves monoamine oxidase (MAO) activity. N-acetylation is the predominant enzymatic route at least in insects [15]. NAT is found in several species, including insects, where it is involved in the inactivation of monoamine neurotransmitters and the formation of catecholamine intermediates required for sclerotization of the insect cuticle. It is unclear what caused the phylogenetic shift from N-acetylation to MAO as an enzyme system for biogenic amine metabolism. Many insects, however, have evolved to avoid the elimination of catecholamine metabolites in aqueous excreta. Water conservation in several insect orders has facilitated their survival in various hostile environments. The subsequent use of monoamines and their N-acetylated metabolites in forming the sclerotized cuticle, which reduces water loss could be a partial explanation [16]. The presence of NAT and NAS in the neural tissues of the fly Drosophila has been known for over two decades.

Furthermore, the presence of the enzyme HIOMT, which is involved in melatonin production from NAS, has recently been suspected in insects. Antibodies produced against the pineal enzyme HIOMT, have been found to bind cockroach cells, indicating an enzymatic feature of the vertebrate pineal gland [17].


Effects of serotonin

In both protostomes and deuterostomes, the biogenic amine serotonin (5-HT) plays a critical role in regulating and influencing many physiological and behavioural processes. Its binding mediates serotonin’s unique actions to membrane receptors and subsequent activation. The great majority of these receptors are members of the G-protein-coupled receptor superfamily. The discovery of 5-HT as a key neuromodulator required a collaborative effort from multiple research teams. In 1940, during the research of constricting factors in the blood that cause hypertension, a serum substance affecting vascular tone was separated and termed “serotonin” [18,19].

5-HT was demonstrated to modulate appetite in several insect species. Neuromodulatory actions of 5-HT were shown to depress feeding in D. melanogaster, while decreased neuronal 5-HT levels increased appetite [20]. 5-HT injection in the hemolymph decreased feeding in another dipteran species, the flesh fly Neobellieria bullata [21]. 5-HT inhibits feeding when injected in the brain of honey bee, and when injected in the gut excited muscle contractions, although general elevation of 5-HT in the bee’s hemolymph did not affect food intake [22]. Moreover, 5-HT increased fluid secretion from salivary glands in Aedes aegypti, the fly Calliphora vicina and the cockroach Periplaneta americana [23-25]. 5-HT acts as a diuretic hormone in Rhodnius prolixus, it is produced soon after the initiation of feeding and stimulates rapid tubule secretion [26,27].

5-HT also has thorough effects on some aspects of learning and memory and is thus a major player in modulating several insect behaviors. By inhibiting the serotonergic system in neurons of D. melanogaster, appetitive olfactory memory performance was considerably reduced [28]. Flies with genetically or pharmacologically reduced 5-HT levels in the brain also had a strongly reduced memory formation in a behavioral test wherein flies were trained to avoid a chamber position associated with high temperature [29]. In honey bees, both memory storage and retrieval were reduced when 5-HT was injected prior to conditioning [30-32]. In the desert locust Schistocerca gregaria, 5-HT and its downstream effector molecules were shown to induce gregarious, swarming behavior [33,34]. Fruit flies with genetically or pharmacologically elevated 5-HT levels showed higher fighting frequencies and more intense fighting than untreated flies [35]. In several dipteran species, manipulating serotonin levels with exogenous serotonin or serotonin agonists has been shown to affect feeding probability, modulate tarsal acceptance thresholds, and change meal size [36-39]. Exogenous serotonin injection decreases carbohydrate feeding in the grey flesh fly, N. bullata, and the queen blows fly, Phormia regina [36,39]. Although many aspects of flies’ feeding physiology have been studied, and many of the nervous pathways involved in feeding regulation have been identified, it is still unclear how and where serotonin influences feeding [40].

Effects of melatonin

Based on a review of the literature, it appears that the presence and synthesis of melatonin in photic and cerebral regions are a common trait of all invertebrates. Melatonin is present in the brain of Musca autumnalis changes regularly, with peak values seen during the dark period of the day-night cycle [41]. NAT activity of roughly 20 nmol/brain/hour in the cockroach brain and/or optic lobe, which is equivalent to that seen in chicken pineal glands at night. NAT does not appear to fluctuate regularly in the cockroach, whereas melatonin concentrations appear to be high at night and low throughout the day [42].

It has now been established that the pineal gland is involved in the transmission of photoperiodic information in vertebrates via the daily pattern of melatonin release. Invertebrates and vertebrates use photoperiodic variations as a time trigger to initiate physiological processes such as reproduction and diapause. Melatonin and its precursors or synthesizing enzymes are now well established in many organs implicated in photoreceptive processes or circadian pacemaking in both vertebrates and invertebrates.

In insects, melatonin synthesis and release follow a circadian rhythm influenced by light-dark cycles in the environment, just like in vertebrates. Insects use photoperiods as a time cue to start postembryonic processes such moulting, eclosion, and diapauses. This means they can tell the difference between long and short days.

Melatonin levels in insect species like M. autumnalis [41], D. melanogaster [43], L. migratoria [3], Gryllus bimaculatus [44], and Ischnura verticalis and Enallagma civile [45], R. prolixus [46], Ischnura graellsii, and Oedipoda caerulescens [47] or Apis mellifera [48] show a circadian rhythm with a scotophase peak typical of vertebrate melatonin production. The brain and optic lobes of the cockroach P. americana were described the same [42].

Even though melatonin is found in insects [3,49] and is thought to have physiological functions [50-53], the short-day mimicking effect was only detected in aphids [51]. Melatonin therapy delayed the commencement of oviposition in Pyrrhocoris apterus [50].

Insects benefit from exogenous melatonin because it activates the antioxidant defense system in response to a toxic insult. The effects of melatonin and its precursor serotonin (5-hydroxytryptamine) on the release of prothoracicotropic hormone (PTTH) from the brain of the studied cockroach species in vitro provided evidence for the first time that melatonin functions as a releaser of this glandotropic neuropeptide in this insect [54].

Recently, it has been proposed that the synthesis and release of MEL in the head of Spodoptera litura follow a circadian rhythm, and that light inhibits its synthesis [55]. In addition, the existence and distribution of MEL and 5-HT in Orthoptera’s optic lobes have been determined [56].


Funding Statement

Author is very grateful to the Council of Scientific & Industrial Research (CSIR), New Delhi, India for providing financial aid under the Research Associate fellowship (09/112(0676)2020 EMR-I).

The authors have no potential conflicts of interest to disclose.

Availability of Data and Material

Data sharing not applicable to this article as no datasets were generated or analyzed during the study.

Author Contributions

Conceptualization: Mohd Kaleemullah Farooqi. Data curation: Mohd Kaleemullah Farooqi. Formal analysis: Mohd Ali. Funding acquisition: Mohd Kaleemullah Farooqi. Investigation: Mohd Kaleemullah Farooqi. Methodology: Mohd Kaleemullah Farooqi. Project administration: Mohammad Amir, Mohd Kaleemullah Farooqi. Resources: Mohd Kaleemullah Farooqi. Software: Mohammad Amir, Mohd Kaleemullah Farooqi. Supervision: Mohammad Amir. Validation: Mohd Kaleemullah Farooqi. Visualization: Mohd Kaleemullah Farooqi. Writing—original draft: Mohd Kaleemullah Farooqi. Writing—review & editing: Mohd Kaleemullah Farooqi.


Dr. Mohd Kaleemullah Farooqi wishes to extend his gratitude to Prof. Mohammad Amir for his able guidance. Thanks, are also due to Chairman, Department of Zoology, Aligarh Muslim University, for providing facilities.


1. Balzer I, Hardeland R. Melatonin in algae and higher plants‐possible new roles as a phytohormone and antioxidant. Bot Acta 1996;109:180–183.
2. Vivien-Roels B, Pévet P. Melatonin: presence and formation in invertebrates. Experientia 1993;49:642–647.
3. Vivien-Roels B, Pevet P, Beck O, Fevre-Montange M. Identification of melatonin in the compound eyes of an insect, the locust (Locusta migratoria), by radioimmunoassay and gas chromatography-mass spectrometry. Neurosci Lett 1984;49:153–157.
4. Anisimov VN, Mylnikov SV, Oparina TI, Khavinson VK. Effect of melatonin and pineal peptide preparation epithalamin on life span and free radical oxidation in Drosophila melanogaster. Mech Ageing Dev 1997;97:81–91.
5. Antolín I, Obst B, Burkhardt S, Hardeland R. Antioxidative protection in a high-melatonin organism: the dinoflagellate Gonyaulax polyedra is rescued from lethal oxidative stress by strongly elevated, but physiologically possible concentrations of melatonin. J Pineal Res 1997;23:182–190.
6. Roberts KM, Fitzpatrick PF. Mechanisms of tryptophan and tyrosine hydroxylase. IUBMB Life 2013;65:350–357.
7. Fitzpatrick PF. Tetrahydropterin-dependent amino acid hydroxylases. Annu Rev Biochem 1999;68:355–381.
8. Hufton SE, Jennings IG, Cotton RG. Structure and function of the aromatic amino acid hydroxylases. Biochem J 1995;311:353–366.
9. Kim-Ha J, Smith JL, Macdonald PM. oskar mRNA is localized to the posterior pole of the Drosophila oocyte. Cell 1991;66:23–35.
10. Lovenberg W, Weissbach H, Udenfriend S. Aromatic L-amino acid decarboxylase. J Biol Chem 1962;237:89–93.
11. Reiter RJ. Pineal melatonin: cell biology of its synthesis and of its physiological interactions. Endocr Rev 1991;12:151–180.
12. Arendt J. Melatonin and the mammalian pineal gland. London: Chapman & Hall; 1995.
13. Klein DC, Weller JL. Indole metabolism in the pineal gland: a circadian rhythm in N-acetyltransferase. Science 1970;169:1093–1095.
14. Thomas KB, Iuvone PM. Circadian rhythm of tryptophan hydroxylase activity in chicken retina. Cell Mol Neurobiol 1991;11:511–527.
15. Evans PH, Fox PM. Enzymatic N-acetylation of indolealkylamines by brain homogenates of the honeybee, Apis mellifera. J Insect Physiol 1975;21:343–353.
16. Smith TJ. Phylogenetic distribution and function of arylalkylamine N-acetyltransferase. Bioessays 1990;12:30–33.
17. Takeda M, Endo Y, Saito H, Nishimura M, Nishitsutsuji-Uwo J. Neuropeptide and monoamine immunoreactivity of the circadian pacemaker in Periplaneta. Biomed Res 1985;6:395–406.
18. Rapport MM, Green AA, Page IH. Serum vasoconstrictor, serotonin; isolation and characterization. J Biol Chem 1948;176:1243–1251.
19. Rapport MM, Green AA, Page IH. Crystalline serotonin. Science 1948;108:329–330.
20. Neckameyer WS. A trophic role for serotonin in the development of a simple feeding circuit. Dev Neurosci 2010;32:217–237.
21. Dacks AM, Nickel T, Mitchell BK. An examination of serotonin and feeding in the flesh fly Neobellieria bullata (Sarcophagidae: Diptera). J Insect Behav 2003;16:1–21.
22. French AS, Simcock KL, Rolke D, Gartside SE, Blenau W, Wright GA. The role of serotonin in feeding and gut contractions in the honeybee. J Insect Physiol 2014;61:8–15.
23. Novak MG, Ribeiro JM, Hildebrand JG. 5-hydroxytryptamine in the salivary glands of adult female Aedes aegypti and its role in regulation of salivation. J Exp Biol 1995;198:167–174.
24. Novak MG, Rowley WA. Serotonin depletion affects blood-feeding but not host-seeking ability in Aedes triseriatus (Diptera: Culicidae). J Med Entomol 1994;31:600–606.
25. Berridge MJ, Patel NG. Insect salivary glands: stimulation of fluid secretion by 5-hydroxytryptamine and adenosine-3’,5’-monophosphate. Science 1968;162:462–463.
26. Donini A, O’Donnell MJ, Orchard I. Differential actions of diuretic factors on the Malpighian tubules of Rhodnius prolixus. J Exp Biol 2008;211:42–48.
27. Maddrell SH, Herman WS, Mooney RL, Overton JA. 5-Hydroxytryptamine: a second diuretic hormone in Rhodnius prolixus. J Exp Biol 1991;156:557–566.
28. Sitaraman D, LaFerriere H, Birman S, Zars T. Serotonin is critical for rewarded olfactory short-term memory in Drosophila. J Neurogenet 2012;26:238–244.
29. Sitaraman D, Zars M, Laferriere H, Chen YC, Sable-Smith A, Kitamoto T, et al. Serotonin is necessary for place memory in Drosophila. Proc Natl Acad Sci U S A 2008;105:5579–5584.
30. Bicker G, Menzel R. Chemical codes for the control of behaviour in arthropods. Nature 1989;337:33–39.
31. Menzel R, Heyne A, Kinzel C, Gerber B, Fiala A. Pharmacological dissociation between the reinforcing, sensitizing, and response-releasing functions of reward in honeybee classical conditioning. Behav Neurosci 1999;113:744–754.
32. Mercer AR, Menzel R. The effects of biogenic amines on conditioned and unconditioned responses to olfactory stimuli in the honeybeeApis mellifera. J Comp Physiol 1982;145:363–368.
33. Anstey ML, Rogers SM, Ott SR, Burrows M, Simpson SJ. Serotonin mediates behavioral gregarization underlying swarm formation in desert locusts. Science 2009;323:627–630.
34. Ott SR, Verlinden H, Rogers SM, Brighton CH, Quah PS, Vleugels RK, et al. Critical role for protein kinase A in the acquisition of gregarious behavior in the desert locust. Proc Natl Acad Sci U S A 2012;109:E381–E387.
35. Dierick HA, Greenspan RJ. Serotonin and neuropeptide F have opposite modulatory effects on fly aggression. Nat Genet 2007;39:678–682.
36. Long TF, Murdock LL. Stimulation of blowfly feeding behavior by octopaminergic drugs. Proc Natl Acad Sci U S A 1983;80:4159–4163.
37. Brookhart GL, Edgecomb RS, Murdock LL. Amphetamine and reserpine deplete brain biogenic amines and alter blow fly feeding behavior. J Neurochem 1987;48:1307–1315.
38. Novak MG, Rowley WA. Serotonin depletion affects blood-feeding but not host-seeking ability in Aedes triseriatus (Diptera: Culicidae). J Med Entomol 1994;31:600–606.
39. Dacks AM, Nickel T, Mitchell BK. An examination of serotonin and feeding in the flesh fly Neobellieria bullata (Sarcophagidae: Diptera). J Insect Behav 2003;16:1–21.
40. Haselton AT, Downer KE, Zylstra J, Stoffolano JG. Serotonin inhibits protein feeding in the blow fly, Phormia regina (Meigen). J Insect Behav 2009;22:452–463.
41. Wetterberg L, Hayes DK, Halberg F. Circadian rhythm of melatonin in the brain of the face fly, Musca autumnalis De Geer. Chronobiologia 1987;14:377–381.
42. Binkley SA. The clockwork sparrow: time, clocks, and calendars in biological organisms. Englewood Cliffs: Prentice Hall; 1990.
43. Finocchiaro L, Callebert J, Launay JM, Jallon JM. Melatonin biosynthesis in Drosophila: its nature and its effects. J Neurochem 1988;50:382–387.
44. Itoh MT, Hattori A, Nomura T, Sumi Y, Suzuki T. Melatonin and arylalkylamine N-acetyltransferase activity in the silkworm, Bombyx mori. Mol Cell Endocrinol 1995;115:59–64.
45. Tilden AR, Anderson WJ, Hutchison VH. Melatonin in two species of damselfly, Ischnura verticalis and Enallagma civile. J Insect Physiol 1994;40:775–780.
46. Gorbet DJ, Steel CG. A miniature radioimmunoassay for melatonin for use with small samples from invertebrates. Gen Comp Endocrinol 2003;134:193–197.
47. Vieira R, Míguez JM, Aldegunde M. GABA modulates day-night variation in melatonin levels in the cerebral ganglia of the damselfly Ischnura graellsii and the grasshopper Oedipoda caerulescens. Neurosci Lett 2005;376:111–115.
48. Yang L, Qin Y, Li X, Song D, Qi M. Brain melatonin content and polyethism in adult workers of Apis mellifera and Apis cerana (Hym., Apidae). J Appl Entomol 2007;131:734–739.
49. Itoh MT, Hattori A, Sumi Y, Suzuki T. Identification of melatonin in different organs of the cricket, Gryllus bimaculatus. Zool Sci 1994;11:577–581.
50. Hodkova M. Indication of the role of melatonin in the regulation of reproduction in Pyrrhocoris apterus (Heteroptera). Acta Entomol Bohemos 1989;86:81–85.
51. Hardie J, Gao N. Melatonin and the pea aphid, Acyrthosiphon pisum. J Insect Physiol 1997;43:615–620.
52. Richter K, Peschke E, Peschke D. A neuroendocrine releasing effect of melatonin in the brain of an insect, Periplaneta americana (L.). J Pineal Res 2000;28:129–135.
53. Yamano H, Watari W, Arai T, Takeda M. Melatonin in drinking water influences a circadian rhythm of locomotor activity in the house cricket, Acheta domesticus. J Insect Physiol 2001;47:943–949.
54. Richter K, Peschke E, Peschke D. Effect of melatonin on the release of prothoracicotropic hormone from the brain of Periplaneta americana (Blattodea: Blattidae). Eur J Entomol 1999;96:341–345.
55. Subala SPRR, Shivakumar MS. Changes in light and dark periods affect the arylalkylamine N-acetyl transferase, melatonin activities and redox status in the head and hemolymph of nocturnal insect Spodoptera litura. Biol Rhythm Res 2017;49:13–28.
56. Freelancea BC, Leone L, Anderson CR, Jones TM. A method for paraffin sectioning and identification of indoleamines in the brain of insects with a sclerotized cuticle. J Histotechnol 2017;40:66–72.

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Figure 1.

Biosynthesis of serotonin.

Figure 2.

Biosynthesis of melatonin. AcCoA, acetyl coenzyme A; CoA, coenzyme A; SAMe, S-adenosyl-L-methionine; SAH, S-adenosyl-L-homocysteine.