What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Pineal Gland Function: Melatonin and Rhythm

Name: Pineal Gland Function: Melatonin and Rhythm
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

Pineal gland function is the conversion of photic information into a hormonal signal: the gland sits outside the blood-brain barrier as a circumventricular organ, receives indirect light information relayed through the suprachiasmatic nucleus, and releases melatonin almost entirely during darkness. The pineal body functions, in one sentence, as the neuroendocrine translator that turns the daily light-dark cycle into a chemical rhythm the rest of the body can read.

That translation matters across every domestic species. Melatonin sets the phase of the circadian clock, gates reproductive cycling in seasonal breeders, modulates immune and antioxidant defenses, and entrains the sleep-wake cycle. A horse, a ewe, a dog, and a budgerigar all secrete melatonin at night, but they arrive at that rhythm through different anatomy, and those differences explain why some species respond to changing day length and others do not.

What the Pineal Gland Is and Where It Sits

The pineal gland (also called the epiphysis cerebri or pineal body) is a small, pine-cone-shaped neuroendocrine organ attached to the roof of the third ventricle, dorsal to the thalamus and rostral to the cerebellum. It develops as an evagination of the diencephalon, which is why it is described as a circumventricular organ: it lies near the ventricular system and has a fenestrated capillary bed, so it lacks the tight blood-brain barrier that protects most of the brain. That leakiness is functional. Melatonin made inside pinealocytes can pass directly into the circulation without a dedicated transport step [1].

The gland is built around one dominant cell type. Pinealocytes are modified neurons that make up the melatonin-synthesizing population, and they are supported by astrocytes, interstitial cells, vascular endothelium, and connective tissue septa that divide the organ into lobules [2][1]. Single-cell mapping of the primate pineal gland identified pinealocytes as the predominant cell type alongside six glial and vascular lineages, with chromatin regions enriched near melatonin synthesis and phototransduction genes [2].

The Circumventricular Position

Because the pineal sits outside the blood-brain barrier, it is positioned to sense circulating signals and to release hormone directly into the bloodstream. Catecholaminergic nerve fibers enter the human pineal gland and run mainly through the capsule and connective tissue trabeculae, and the density is uneven across sections. In animal glands these fibers carry the classic varicosities of sympathetic terminals, while the human gland examined by immunohistochemistry showed tyrosine hydroxylase-positive fibers without that typical varicose pattern. A small number of neuron-like uni- and bipolar cells were also found inside the lobules, which supports the idea of an intrinsic, intrapineal system that fine-tunes melatonin output through neuron-glial interaction [1].

The Melatonin Synthetic Pathway

Melatonin is not stored in granules like a peptide hormone. It is made on demand from serotonin, and the amount released tracks the amount synthesized. The pathway begins with tryptophan, which is converted stepwise into serotonin and then into melatonin by two enzymes that carry most of the regulatory weight.

The two terminal enzymes are arylalkylamine N-acetyltransferase, abbreviated AANAT, and hydroxyindole-O-methyltransferase, abbreviated HIOMT. AANAT is also called aralkylamine N-acetyltransferase, and HIOMT is also called acetylserotonin O-methyltransferase or ASMT. In the current nomenclature the gene and enzyme are frequently written as ASMT, and both names appear in the primary literature [3][4].

AANAT catalyzes the first committed step of the terminal sequence: the acetylation of serotonin to N-acetylserotonin. HIOMT then methylates N-acetylserotonin to produce melatonin. AANAT is the rate-limiting enzyme, meaning its activity sets the ceiling on how much melatonin the gland can make at any moment. HIOMT is comparatively stable and does not fluctuate nearly as much across the light-dark cycle. This division of labor is the core concept students should carry away: the rhythm of melatonin secretion is, in the short term, the rhythm of AANAT activity [5].

AANAT Is the Rate-Limiting Enzyme

AANAT activity rises steeply after the onset of darkness and collapses with light exposure. In the pineal gland the enzyme is controlled at several levels at once: transcription of the AANAT gene, stability of the AANAT protein, and post-translational modification that tags the protein for degradation. Because of this, a sudden light pulse at night can shut down melatonin synthesis within minutes without waiting for the gene to be switched off [5].

Transcriptional Control Above AANAT

Enzyme abundance is not the whole story. Homeobox transcription factors are best known for controlling organ development, yet a set of them remains strongly expressed in the adult pineal gland. The cone-rod homeobox gene CRX is one example. In adult rats, delivering short-hairpin RNA through adeno-associated viral vectors to knock down CRX in the mature pineal gland reduced expression of tryptophan hydroxylase 1 and acetylserotonin O-methyltransferase, two melatonin-synthesizing enzymes, mirroring earlier in vitro results from pinealocyte cultures [3]. Transcription factor hubs that integrate circadian and light-responsive signals include CRX, OTX2, LHX4, and RORA [2].

How Photic Input Reaches the Pineal Gland

The pineal gland does not look at the sun directly in mammals. It receives light information secondhand through a defined anatomical route, and the suprachiasmatic nucleus sits at the center of that route.

The pathway runs as follows:

Light strikes the retina and activates intrinsically photosensitive retinal ganglion cells that express melanopsin.
Those ganglion cells project via the retinohypothalamic tract to the suprachiasmatic nucleus (SCN) in the anterior hypothalamus.
The SCN, the master circadian pacemaker, generates a near-24-hour rhythm and projects to the paraventricular nucleus.
Fibers descend to the intermediolateral cell column of the thoracic spinal cord.
Preganglionic sympathetic fibers synapse in the superior cervical ganglion.
Postganglionic sympathetic fibers travel with the nervi conarii to the pineal gland and release norepinephrine onto pinealocytes.
Norepinephrine acting on beta-adrenergic receptors drives the intracellular cascade that raises AANAT activity and melatonin synthesis.

The sign of the signal is inverted along the way. Light arriving at the retina suppresses melatonin production, while darkness removes that suppression and permits synthesis to proceed. The SCN governs melatonin secretion through light-responsive pathways, with light exposure inhibiting and darkness stimulating melatonin production [5].

flowchart TD
    A[Light hits retina] --> B[Melanopsin ganglion cells]
    B --> C[Retinohypothalamic tract]
    C --> D[Suprachiasmatic nucleus]
    D --> E[Paraventricular nucleus]
    E --> F[Thoracic spinal cord]
    F --> G[Superior cervical ganglion]
    G --> H[Norepinephrine on pinealocyte]
    H --> I[AANAT activity rises]
    I --> J[Melatonin released at night]

Circadian Regulation and Feedback

The SCN is the central pacemaker that coordinates sleep-wake cycles, metabolism, and neuroendocrine signaling, and melatonin is its most reliable hormonal output [6]. Core clock genes including CLOCK, BMAL1, PER, and CRY generate the intracellular rhythm that keeps the system cycling even in constant conditions, and melatonin feeds back onto the clock to reinforce phase alignment [5][7].

Melatonin acts through MT1 and MT2 receptors in the SCN and in other brain regions. Signaling through these receptors facilitates sleep onset, circadian rhythm alignment, and sleep maintenance, and melatonin also carries antioxidant, anti-inflammatory, and chronobiotic effects [5]. The chronobiotic property is distinct from the hypnotic one: a properly timed melatonin signal can shift the phase of the clock itself, which is why the hormone is studied as a phase-setting agent rather than only as a sleep-promoting one [5].

Melatonin signaling is not confined to the central nervous system. Melatonin binds MTNR1A on bovine adenohypophyseal cells, activates the cAMP/PKA signaling pathway, promotes FOXO1 protein expression, and FOXO1 binds the GH1 promoter to increase growth hormone synthesis and secretion. Blocking MTNR1A with luzindole or inhibiting downstream steps with 4P-PDOT, 2',5'-dideoxyadenosine, or H-89 interrupts this cascade [8]. This gives melatonin a direct route into somatotropic regulation, separate from its clock-setting role.

Photoreceptive Versus Non-Photoreceptive Pinealocytes

The pineal gland across vertebrates comes in two functional designs, and confusing them is the single most common error in comparative physiology examinations.

Mammalian pinealocytes are not photoreceptive. They are secretory cells that depend entirely on retinal input relayed through the SCN and the sympathetic chain. If you remove the eyes or cut the pathway, the mammalian pineal loses its light response. This is why mammals are described as having a blind pineal that reads light indirectly [5][2].

In birds, reptiles, amphibians, and fish, pinealocytes retain true photoreceptive capacity. These cells carry photopigments and respond to light directly, and in some species the gland sits under a translucent skull or forms a pineal window that admits light to the organ itself. The avian pineal is therefore both a photoreceptor and a secretory gland, and it can oscillate independently of the eye. Students should not apply the mammalian description of pineal function to a bird or a fish without qualification.

The larval zebrafish illustrates the non-mammalian case well. Ablating the pineal gland through targeted expression of the nitroreductase variant NTR2.0 in pinealocytes, or knocking out the brain-specific homeobox gene bsx required for pineal development, produced complete loss of the gland confirmed by live imaging and loss of arylalkylamine N-acetyltransferase. The pineal gland is required for optimal nighttime immunity to bacterial infection in these larvae, and the immune response to infection is circadian-gated in the intact animal [9].

Comparative Table of Pineal Anatomy and Rhythm

The table below summarizes the species differences most relevant to veterinary study. Melatonin rhythm timing is listed as the phase of peak secretion, not the amplitude, because amplitude varies with age, season, and individual.

Species group	Pineal position and structure	Photoreceptive pinealocytes	Primary melatonin rhythm
Dog and cat	Dorsal to thalamus, stalked, lobulated, outside blood-brain barrier	No, blind pineal dependent on retinal input	High at night, suppressed by light
Horse	Slender stalked gland in the tentorial notch region	No	High at night, drives seasonal reproductive transition
Sheep	Compact gland on the diencephalic roof	No	High at night, photoperiod signal for estrous cycling
Cattle	Compact lobulated gland	No	High at night, melatonin correlates with growth hormone levels
Birds	Superficial or deep, often directly light-sensitive	Yes	High at night, with direct light responsiveness
Reptiles and fish	Dorsal diencephalic, some with pineal window	Yes	High at night, gland can oscillate without the eye
Primates including humans	Small lobulated gland, capsule and trabeculae	No	High at night, suppressed by retinal light input

Melatonin in Horses and Sheep: The Seasonal Breeder Signal

In horses and sheep, circulating melatonin is high at night and low during the day, and that nighttime elevation is the signal the reproductive axis uses to read the season. Because night length changes predictably across the year, the duration of the melatonin peak becomes a calendar. This is the mechanism behind seasonal breeding.

The strategic logic differs between the two species. Sheep are short-day breeders. As day length shortens in autumn, the nocturnal melatonin peak lengthens, and that longer signal drives the gonadotropin-releasing hormone pulse generator toward the estrous cycle. Horses are long-day breeders. As days lengthen in spring, the melatonin peak shortens, and that shorter nightly signal releases the reproductive axis from suppression and permits the spring transition into cyclicity. In both cases the melatonin rhythm is the message, not a byproduct, and this is why manipulating day length or giving artificial light at night can shift breeding in these species.

Melatonin is also produced outside the pineal gland, and in sheep pregnancy changes that extrapineal production. In ewes, AANAT expression increased in the thymus and spleen but fell in the thyroid and endometrium by day 70 of gestation compared with day 16 of the estrous cycle. HIOMT (ASMT) expression rose in the lymph nodes and fell in the spleen, liver, and thyroid across the same interval, and endometrial ASMT showed a stage-specific pattern with no significant difference between early cycle and late gestation [4]. The takeaway is that pineal melatonin sets the endocrine rhythm while local melatonin supports tissue-level antioxidant and immune functions [4][10].

Extrapineal Melatonin and Autocrine Signaling

Melatonin is not made only in the pineal gland. A systematic review of original studies published between 2000 and 2025 found autocrine melatonin signaling reported in the ovary and reproductive system, the gut, skin and hair follicles, the retina, the testes, the liver and metabolic tissues, bone, the cardiovascular and endothelial compartment, and mitochondria, with the highest number of studies addressing immunoregulatory switching [10]. This means melatonin acts as a local tissue messenger at the same time that the pineal provides the systemic rhythm.

Extrapineal melatonin uses the same terminal enzymes. AANAT and HIOMT are expressed in thymus, spleen, liver, lymph nodes, thyroid, duodenum, and endometrium in sheep, and their expression shifts with pregnancy stage [4]. In one study on skin pigment cells, isolated melanophores from Xenopus laevis responded strongly to melatonin but only slightly to direct light, while whole tadpole pigmentation showed robust circadian regulation that was mainly hormone-driven. Melatonin treatment inhibited melanin synthesis and shifted core circadian gene expression toward a dark-phase pattern [11]. That result is a useful reminder that in pigment cells, as in many tissues, the hormone signal carries more circadian weight than local light sensing.

How Melatonin Signaling Is Observed and Tested

Veterinarians and researchers assess the melatonin system in several ways, and each method has a different intended use.

Salivary melatonin is measured because the hormone crosses into saliva and tracks the free fraction in blood. In patients with non-hydrocephalic symptomatic pineal cysts, salivary melatonin was sampled at half-hour intervals between 08:30 p.m. and 01:00 a.m. to describe the early-night rise and relate it to sleep quality and headache diaries [12]. This design is useful for describing a rhythm in an individual, while a single random sample is far less informative.

Breast milk melatonin can be measured by competitive ELISA and used to track maternal circadian output. A randomized controlled trial in premature infants that provided kangaroo care twice daily for three days found no statistically significant change in maternal breast milk cortisol or melatonin levels, though the intervention did significantly affect the infants' Bispectral Index values, heart rate, respiratory rate, oxygen saturation, and body temperature [13].

Pineal morphology can be assessed by high-resolution T1- and T2-weighted magnetic resonance imaging, which quantifies total pineal volume, parenchymal volume, and cyst prevalence. In a cross-sectional study of 110 healthcare workers, total and parenchymal pineal volumes did not differ between 32 night workers and 78 day workers, but pineal cyst prevalence was higher among night workers. Within the night-work group, greater pineal parenchymal volume correlated positively with sleep-maintenance disturbance, and pineal volume increased during early night-work exposure before stabilizing with longer exposure [14].

At the experimental level, single-cell and spatial multiomic methods now map the regulatory architecture of the gland directly. Integrating single-nucleus RNA sequencing, single-nucleus ATAC sequencing, and spatial transcriptomics in the macaque pineal gland resolved a dual-layer regulatory architecture: melatonin synthesis programs are robustly organized across cells, while circadian clock regulators show a sparse spatial pattern [2].

Cell-culture and organoid systems make the human gland accessible. Human pineal gland organoids recapitulate pinealocyte maturation, noradrenergic responsiveness, and melatonin synthesis, providing a platform for studying circadian dysfunction and sleep disturbances associated with disease [15].

Clinical Relevance, Limitations and Common Mistakes

Melatonin rhythm disruption appears across a wide range of conditions, and the direction of causality is often uncertain. Circadian rhythm sleep disorder is associated with a significantly higher risk of open-angle glaucoma at one, three, and five years in a large retrospective cohort, and the risk remained elevated among patients not using melatonin, while melatonin users did not show a statistically significant difference from controls [16]. Neurodegenerative and pediatric reviews describe the same association from the opposite direction: decreased melatonin secretion, altered SCN signaling, and sleep disturbance are features of Alzheimer disease, and clock gene mutations in BMAL1, PER, CRY, and CLOCK accompany the pathology [6][7]. In pediatric neurological conditions, immature circadian control interacts with oxidative stress and inflammation, and melatonin is repeatedly identified as a circadian-regulated mediator in hypoxic-ischemic encephalopathy, autism spectrum disorder, metabolic encephalopathies, and sepsis [17].

Metabolic and musculoskeletal tissues are also clock-sensitive. Core circadian clock proteins are downregulated in degenerated intervertebral disc tissue across species, and cyclic melatonin delivery upregulated BMAL1 and CLOCK, restored circadian rhythm, and enhanced extracellular matrix synthesis in a rat model [18]. In high-fat-diet obesity models, restoring the physiological rhythm of melatonin and body temperature realigned hepatic gene oscillation patterns, and the effect ran through the sphingolipid pathway via S1PR4/TRAF2 signaling [19].

Melatonin also has direct effects on growth and reproduction. In bovine adenohypophyseal cells, melatonin binds MTNR1A and promotes growth hormone synthesis and secretion through the cAMP/PKA/CREB/FOXO1 pathway, and bovine serum melatonin levels correlate with growth hormone levels [8]. In reproduction, melatonin links circadian signaling to mitochondrial physiology and redox homeostasis, regulating Nrf2 signaling, mitochondrial permeability transition, electron transport chain efficiency, and antioxidant defenses [20].

Misconceptions Students Most Often Hold

The first misconception is that the pineal gland receives light directly in all animals. In mammals it does not. The retina and the SCN sit in the pathway, and the sympathetic chain delivers the final signal.

The second is that melatonin is stored and released like insulin. Melatonin is synthesized on demand, and AANAT is destroyed or inhibited quickly when light returns.

The third is that HIOMT drives the rhythm. AANAT is the rate-limiting enzyme. HIOMT is comparatively stable and does not explain the daily oscillation on its own [5].

The fourth is that melatonin only affects sleep. It acts on MT1 and MT2 receptors in the SCN and other brain regions, directly modulates adenohypophyseal hormone output, supports immune and antioxidant function, and is produced in extrapineal tissues where it acts locally [5][8][10].

The fifth is that any animal with a pineal gland responds to day length the way a horse does. Seasonal breeding depends on the interpretation of the melatonin signal by the hypothalamic-pituitary-gonadal axis, and the direction of that interpretation differs between short-day and long-day breeders.

The sixth is that a single blood or saliva melatonin value describes the system. The rhythm is the variable that matters, and that is why sampling protocols use multiple time points across the night [12].

This article is educational and is not a substitute for veterinary diagnosis or treatment. Individual patients require examination and clinical judgment by a licensed veterinarian.

Quick Review

The pineal gland is a circumventricular organ on the roof of the third ventricle, outside the blood-brain barrier, so it can release melatonin directly into the blood [1].
Melatonin is made from serotonin by AANAT and HIOMT, and AANAT is the rate-limiting enzyme [5].
Light information reaches the mammalian pineal indirectly: retina to SCN to paraventricular nucleus to thoracic cord to superior cervical ganglion to pinealocyte [5].
Norepinephrine from sympathetic terminals raises AANAT activity in darkness, so melatonin is high at night and suppressed by light [5].
Mammalian pinealocytes are secretory and blind to light, while avian, reptilian, and fish pinealocytes are directly photoreceptive.
In horses and sheep, nocturnal melatonin is the photoperiodic signal that drives seasonal breeding, with sheep as short-day breeders and horses as long-day breeders.
Melatonin acts through MT1 and MT2 receptors in the SCN and elsewhere, and it is also produced in many extrapineal tissues where it acts locally [5][10].

Frequently Asked Questions

What does the pineal gland do in animals?

It converts the daily light-dark cycle into a melatonin rhythm that times circadian, reproductive, immune, and metabolic functions. The pineal gland produces the hormone melatonin, the rate of synthesis of which is regulated by afferent catecholaminergic fibers [1].

Why is melatonin called the darkness hormone?

Because its secretion rises when light is absent and falls when light is present. Light exposure inhibits melatonin production and darkness stimulates it, relayed through the suprachiasmatic nucleus [5].

Which enzyme controls how much melatonin is made?

AANAT, arylalkylamine N-acetyltransferase, is the rate-limiting enzyme. It acetylates serotonin to N-acetylserotonin, after which HIOMT methylates that intermediate to yield melatonin [5].

Do birds and mammals have the same pineal structure?

No. Mammalian pinealocytes are secretory and depend on retinal input through the SCN, while bird, reptile, and fish pinealocytes are photoreceptive and can sense light directly [9].

Why do horses and sheep use melatonin to time breeding?

They are seasonal breeders, and the changing length of the nighttime melatonin peak across the year provides a reliable photoperiodic calendar. Melatonin is high at night in both species, and the reproductive axis reads that duration rather than the absolute concentration.

Can an animal be normal with a structurally abnormal pineal gland?

Yes. Pineal cyst prevalence was higher among night workers in one imaging study, while total and parenchymal pineal volumes did not differ between night workers and day workers [14]. Structure and function do not always move together, which is why rhythm data and imaging are interpreted side by side.