INTRODUCTION:

The isolation of hypothalamic substance that stimulates the release of TSH from the anterior pituitary gland, identified as L-pyroglutamyl-L-histidyl-L-prolineamide (L-pGlu-L-His-L-ProNH2) resulted from research conducted by Guillemin and Schally ¹. The tripeptide identified as TRH was the first hypothalamic release factor identified, shown in figure1 under.

Figure 1: Chemical Structure of Thyrotropin-releasing hormone

The isolation of TRH supported the notions of neuroendocrinology, showing that hormones secreted by neurons act as transducers between two major communication systems of mammals (neuronal and endocrine). TRH is produced predominantly in the PVN and the anterior paraventricular (Pva) nuclei of the hypothalamus, where it functions to regulate TSH and PRL production in the anterior pituitary. Thus, TRH plays a central role in the regulation of the hypothalamic-pituitary-thyroid (HPT) axis. The effects of TRH on both CNS neurons and anterior pituitary secretory cells contain an excitatory component, while TRH receptors are located on two different anterior pituitary cell types, known as thyrotropes (which secrete TSH) and mammotrophs (which secrete PRL).

The receptors on these cell types bind TRH and stimulate the release of TSH and prolactin. In addition, TRH has a significant impact on the activity of many neurobiological systems by providing actions mediated by the CNS that are not dependent on the HPT axis. These CNS mediated actions of TRH have been acknowledged for some time now as having potential to be used in the treatment of a variety of neurological disorders. TRH is produced through post-translational processing yielding an intermediate product of Gln-His-Pro-Gly through two Lys-Arg cleavagese via carboxypeptidase-E. Proline in the Gln-His-Pro-Gly intermediate is then amidated by the peptidyl-glycine a-amidatingmonoxygenase. Finally, GLN cyclization to pGlu is carried out by mammalian glutaminyl cyclase, as demonstrated by Fischer et al. TRH is degraded in serum and most peripheral tissues by thyroliberinase in serum, with a half-life of TRH in humans ranging from approximately 2 to 6 minutes. The pharmacological effects of TRH can be classified into two categories: endocrine and CNS-stimulating actions^2,3.

TRH Pharmacology:

Thyrotropin Releasing Hormone (TRH) is produced by the hypothalamus as a tripeptide comprised of the amino acid sequence pyroglutamyl-histidyl-proline amide. Its primary function in the hypothalamic-pituitary-thyroid (HPT) axis is to be the first hormone to stimulate thyroid function; however, it has also been shown to act on other parts of the CNS and the GI tract, where it has significant neuromodulatory and neurotransmitter activity ⁴.

Neuroendocrinology (The Major Axis):

Thyrotropin-Releasing Hormone (TRH) is critical in regulating the Hypothalamic-Pituitary-Thyroid Axis (HPT) that helps to maintain thyroid hormone homeostasis (figure2). The most commonly studied pharmacological activity of TRH occurs in the anterior pituitary gland. Thyrotropin (TSH) secretion: TRH binds to G-protein coupled receptors found on the anterior pituitary gland; thereby, initiating the synthesis and release of TSH by pituitary thyrotropes which leads to the stimulation of the thyroid gland to secrete T_3 and $T_4$.Prolactin secretion: In humans, TRH can stimulate the release of prolactin secreted from the anterior pituitary gland. Although not a primary physiological regulator (i.e., dopamine is primarily responsible for regulating prolactin secretion), there have been numerous studies showing that administration of TRH will cause a significant increase in prolactin levels when administered at pharmacological doses^5,6.

TRH stimulates the glandular production of Thyroid Stimulating Hormone (TSH) and the release of TSH in all mammals. In addition, the hormone stimulates the release of prolactin, which are both used diagnostically in evaluating hypothalamic and pituitary function. Individuals who are hypothyroid have a greater than normal response to TRH, while hyperthyroid patients have a decreased response to the TRH. TRH is released from neurons in the hypothalamus into the hypothalamo-hypophyseal portal vessels that transport blood between the hypothalamus and the anterior pituitary. Anterior pituitary has many subtypes of hormone-secreting (endocrine) cells. Binding of TRH to its respective receptors on two anterior pituitary cells stimulates the hormone secretion of TSH and prolactin.

Figure 2: Hypothalamic–pituitary–thyroid (HPT) axis showing negative feedback loop for releasing TRH

Central Nervous System (CNS) Activities:

Extrahypothalamic TRH pathways are independent from the thyroid system. TRH is believed to be a modulator of several known neurotransmitter systems (e.g., Dopa-mine, Serotonin, Acetylcholine, Opiates). For example, TRH has an important role as a neuro-modulator in the regulation of drugs that affect the listed neurotransmitters and others. TRH is possibly utilized as a neurotransmitter via particular receptors found throughout the central nervous system and is metabolized rapidly via unique catabolic processes. TRH's neuroactivity in the central nervous system has generated interest in developing novel therapies to treat many different neurological disorders including central nervous system injuries, seizures, depressive disorders, cognitive deficiencies, and spinocerebellar degeneration ^7,8.

TRH exerts its stimulating effects in the brain and does so independently from the thyroid axis and often produces analeptic (stimulatory) effects including:

· Arousal/Vigilance: TRH has been shown to increase locomotion and prevent the sedative effects of many CNS depressants including alcohol, barbiturates, and anesthetics and is therefore be commonly assessed for its potential ability to induce wakefulness.

· Antidepressant Effects: TRH may enhance the effectiveness of the monoamine neurotransmitters dopamine and norepinephrine, thereby causing a temporary elevation in mood.

· Neuroprotection: TRH has been found to provide neuroprotection from spinal cord injuries and head trauma by enhancing blood flow to the brain and decreasing lipid peroxidation.

TRH and the Autonomic Nervous System Effects:

The autonomic nervous system is also affected when TRH is administered. Many changes to physiology occur with the administration of TRH:

· Blood Pressure and Heart Rate: TRH stimulates sympathetic outflow, which leads to an increase in arterial blood pressure and heart rate after central administration of TRH.

· Thermoregulation: TRH plays an important role in maintaining body temperature. TRH induces shivering and non-shivering thermogenesis, ultimately increasing the amount of heat produced by the body ⁹.

· Gastrointestinal Effects: TRH has receptors in the brainstem nuclei that regulate actions of the digestive system. a) Increased Gastric Secretion: TRH stimulates the vagal nerve, which increases the secretion of gastric acid and passive pepsinogen. b) Motility: It enhances GI motility and transit time. Because of these effects, TRH is sometimes linked to the "brain-gut" axis communication ¹⁰.

These different roles has been summarised in figure 3 under.

TRH role in controlling seizures:

It has been shown by experimental evidence that TRH plays a role in controlling seizures. The induction of seizures has shown to lead way to an increase in TRH levels and to a decrease in TRH receptor levels located in the limbic system regions of the brain when a variety of seizure inducing agents (e.g., Pentyle-netetrazol, kainic acid) are used to induce generalized seizures. These findings indicate that TRH is involved in modulating seizures. Pharmacological studies with TRH and its derivatives have demonstrated reduced seizure activity from kindling or PTZ or glutamate seized (effects observed with male and female rats). TRH has been found to be beneficial to patients suffering from intractable epilepsy, including infantile spasms, Lennox-gastaut syndrome (LGS), myoclonic seizures, generalized seizures and refractory partial seizures. Additionally, TRH acts to produce a lasting increase in TRH levels in the case of generalized electroconvulsive induced seizures. TRH has also been found in multiple animal models to not only increase the anticonvulsant effect of phenobarbital but also produce its anticonvulsant effect documented in multiple animal models of seizures. Although there is a wealth of preclinical evidence suggesting that TRH may act as either an anticonvulsant or as an adjunct to known anticonvulsant therapies, only limited clinical data exist to support its use as a treatment for refractory epilepsy ¹¹.

TRH role in controlling Parkinson’s Disease:

It is still not fully understood what causes PD, but there is an ample amount of biochemical research demonstrating the role of processes such as oxidative stress and inhibition of mitochondrial function in PD, as well as models of PD, which have been created using neurotoxic substances, including but not limited to; 6-hydroxydopamine (6-OHDA), 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or dopamine (DA). Both MPTP and 6-OHDA have been shown to induce the loss of striatal levels of DA, the loss of immunoreactive (TH-ir) fibers within the striatum, the loss of TH-ir neurons in the substantia nigra, and aggregation of non-fibrillar synuclein. Research has also shown that the 6-OHDA model of neurodegeneration involves damage via the production of hydrogen peroxide and hydroxyl radicals in the presence of iron. TRH has neuroprotective effects upon nigrostriatal DA neurons, and therefore it may represent a new avenue of treatment for PD due to its potential to promote DA release ¹².

TRH in thermoregulation:

TRH is important in coordinating several responses to cold. When injected into the central nervous system, TRH increases body temperature and opposes the hypothermic effects of a number of different agents, including barbiturates and the following: ethanol, chlorpromazine, bombesin, neurotensin, and endorphin. TRH antagonizes morphine-induced hypothermia when given systemically, although it does not affect analgesia to any great extent ¹³.

TRH and drugs of abuse:

Because TRH has an ergotropic effect and is a locomotor stimulant, there is reason to believe that there is an interaction between TRH and drugs of abuse. In one study with rats, administration of TRH systemically was found to mimic cocaine by inducing locomotor activation via the release of DA and serotonin in the nucleus accumbens and striatum. In addition, TRH has been shown to counteract many of the depressant effects of morphine — these effects include sedation and hypothermia and catalepsy.

Stereochemistry of TRH:

Empirical energy calculations were used to explore the potential conformational space of the TRH molecule. With TRH being a small molecular entity, it has a relatively large amount of conformational space available to compose the conformational data. The TRH molecule has eight bonds around which the molecule may be rotated (by manipulating the dihedral angles ψ1, w₁, φ2, ψ2, χ1, χ2, w₂ and ψ3), but because φ1 and φ3 must remain fixed in their geometric configuration by virtue of the pGlu and proline rings, only six dihedral angle values will create a potential conformation. This is because θ1 and θ2 may be restricted to a small amount of available conformational space. All possible conformations of the TRH molecule can be generated using the best values from the six dihedral angles, ψ1, φ2, ψ2, χ1, χ2, and ψ3 (figure4). The dihedral angle φ2 and ψ2 have the most impact on molecular structure because these dihedral angles correspond to the torsional angles φ and ψ of the a-carbon of the central histidine and produce the most substantial variation in structure; thus, all six dihedral angles will be examined in detail considering, among other things, the presence of hydrogen bonds between the amide proton of proline and the carbonyl oxygen of pGlu (n-N and histidine a-NH) and hydrogen bonds at the n-N to a-N hydrogen ^14,15.

Figure 4: Different dihedral angles in TRH

Bladgon et al. proposed a “hair pin turn” conformation based upon minima calculated for potential energy indicating that this form of the molecule is less than the “stretched form” proposed by Burgess et al. The latter author showed how important each amino acid within a TRH molecule is in determining the shape of that molecule. The large and inflexible nature of the pGlu at the N-terminal of the molecule allow it to remain relatively unconfined yet still cause significant extension of the peptide chain due to the extended angles φ2 and ψ2. Thus the role of the His side chain in relation to the C-terminal amino acid provides an indication of another major role that residue plays in the mechanism of molecular recognition between TRH and its receptor site. Additionally, the results of energy calculations for N-methylhistidine TRH derivatives coupled with biological activity studies imply that conformational differences in the histidyl amino acid residue directly affect binding affinity for TRH at its respective receptor site ¹⁶. Finally, any alteration to the C-terminal amide function of TRH will affect the ability of TRH to bind to its receptor; therefore, it is reasonable to assume that the C-terminal amide group is an important component with regards to successful binding between TRH and its receptor.

The Nuclear Magnetic Resonance (NMR) studies of the temperature dependence of the chemical shifts of amide hydrogens were conducted on triacidic (TRH), (Nt (1)-methyl-His)TRH and (2-thienyl-Ala2)TRH, as well as the original compound (TRH) and were determined to all have similar-respective chemical shifts, thereby indicating that none of these protons are involved in hydrogen bonding. The presence of a partially present conformational type in the TRH chemical structure through the presence of the cis-isomeric form of proline was established through 13C-NMR demonstrated by the doubling of the 13C-pyrrolidine chemical shift with respect to its relative percentage representation of 15 to 20%. There appear to be steric or conformational preferences with respect to the His-Pro bond in the structure of TRH. NMR relaxation time values of the b, g and Dpyrrolidine carbons in TRH are greater than that of a-carbon so there is a suggestion of rapid interconversion of the endo and exo forms of the carbon ring structures; however, the evaluation of the vicinal coupling constants from a sample of [85% 13C enriched proline]TRH indicates the overwhelming presence of the CD-endo puckered conformational isomeric form. ¹⁷.

Progressions in TRH derivatives:

Factors which limit the ability of TRH to serve as a drug include poor solubility in fats (lipids), a requirement of supratherapeutic doses to achieve central nervous system (CNS) effects and a short half-life due to the action of proteolytic enzymes. In order to provide TRH with desirable characteristics when compared to these limitations TRH has undergone numerous modifications to provide it with stability against pyroglutamate amino peptidases (PAPs) and/or prolyl endopeptidase (PEP). This modification also aims to separate TRH's CNS effects from its endocrine effects, allowing for selective highly potent derivatives of TRH. In order to create more stable, potent, and derivatives to the parent peptide, TRH derivatives have been synthesized through modification of the three amino acid residues ¹⁸.

A number of promising TRH derivatives have been reported in which the pGlu residue at the N-terminus has been replaced with an amino acid that does not occur in proteins. TA-0910, also known as Taltirelin, has produced significant CNS effects (analeptic activity, prevention of reserpine-induced hypothermia, and increases in spontaneous motor activity) at doses that are approximately 100 times lower than for the parent peptide. CG-3703, also known as Montirelin, has produced potent antiepileptic activity (at doses that are approximately 10 times lower than TRH when administered intravenously) and has a longer duration of action compared with TRH when evaluated in spontaneous epileptic rats. Azetirelin, also known as YM-14673, exhibits 10- to 100-fold greater potency than TRH and produces analeptic activity that has 8- to 36-fold longer duration compared to the parent peptide, as well as preventing reserpine-induced hypothermia, in mice. However, azetirelin has low oral bioavailability because of poor membrane permeability and a lack of lipophilicity. All these analogues are shown in figure 5.

Figure 5: pGlu residue modified TRH derivatives

Few TRH derivatives were known to have a ProNH2 residue modification. The biologically stable RX-77368 (figure 6) was found to have increased neuro-pharmaceutical potency; however, testing it for motor neuron disease did not yield the desired outcome. It was limited to use in stimulating gastric hypermotility for treating peptic ulcer disease.

Figure 6: ProNH2 residue modified derivative

The dithionated derivative i.e. with modified pGlu and ProNH2 residues (figure 7), caused TSH secretion to drop by 50% and had selectivity for receptor type in the pituitary compared to TRH. MK-771 exhibited 35 times greater CNS activity than TRH while having equipotent TSH releasing activity compared to TRH; however, MK-771 had rapid degradation from rat brain homogenate by prolyl oligopeptidase and poor intestinal transport, which led to its abandonment ¹⁹.

Figure 7: Newer Dithionated derivatives

TRH is an excellent test model for the predictive ability of computer methods to determine receptor-bound peptide configurations given its six rotatable bonds. A molecular modelling procedure has also been used to find conformations found in all active TRH derivatives.

CONCLUSION:

The main focus of research into TRH's clinical applications has been not on its endocrine functions but rather on all of the stimulatory effects it has exhibited in the CNS; however, the pathways by which TRH produces those effects are still poorly understood. The action of TRH and TRH derivatives to counterbalance some of the CNS depressant actions of various drugs such as pentobarbital etc. has been noted. TRH (and certain TRH derivatives) has been studied for potential therapeutic benefit in an array of infarct/ischemic states, cerebrovascular disease, and spinal cord injuries. For example, taltirelin (commercially known as Ceredist) has been approved in Japan as a treatment for spinocerebellar degeneration, while montirelin is pending approval for use in advancing recovery from post-concussive states due to head injury via restoration of consciousness. Posatirelin has progressed into Phase III studies as a cognitive enhancer. Other promising CNS active TRH derivatives such as JTP-2942 and 2-ARA-53a6 are in various stages of the preclinical/clinical development process in order to evaluate their therapeutic potential/benefit, as related to the treatment of a variety of serious CNS disorders. Although there is great promise with respect to the clinical potential of TRH analogues for the treatment of a broad-spectrum of CNS disorders, the full extent of that promise is currently being investigated through basic research and clinical trials.

There are two kinds of G-Protein Coupled Receptors that mediate TRH signaling. A lot more information still needs to be determined to clarify the molecular mechanisms involved in distinguishing the TRH hormonal actions from the CNS effects. Identifying the factors involved in differentiating between TRH-R1 and TRH-R2 could also provide further insight into the mechanism of action of TRH within the hormonal and CNS system. The focus of recent studies has been on the development of more selective TRH analogs that demonstrate improved CNS bioavailability. Results obtained by modifying either the pGlu or His residues separately on TRH produced some of the most compelling findings to date. Generally, TRH analogs modified to omit the pGlu residue are more effective CNS analogs, with many retaining their hormonal activity, although to a lesser degree. TRH analogs designed to omit the His residue display some of the most TRH-R2 specific analogs and produced substantial TRH receptor activation with reduced binding affinity. TRH analogs designed via simultaneous modification of both the pGlu and His residues on TRH provide analogs that have both a high affinity for binding but at the same time have a high specificity for TRH-R2.It is important to create new selective TRH-R2 analogs, they enable us develop new methods to study how TRH works in both an endocrine (hormone) and a central nervous system (CNS) context through its receptor. Additionally, recent studies regarding the role of TRH in the regulation of “homeostasis” within the central nervous system open up more opportunities to develop highly selective and effective TRH analogs that have CNS activity.

REFERENCES:

1. Vargas Y, et al. Thyrotropin-releasing hormone and food intake in mammals. Front Endocrinol. 2024; 15: 11205479.

2. Li X, et al. Emerging roles, mechanisms, and therapeutic potential of TRH. Brain Sci. 2026; 16(2): 229.

3. Zheng Y, et al. Functional evolution of thyrotropin-releasing hormone signaling systems. Front Endocrinol. 2025; 16:11890989.

4. Joseph-Bravo P, et al. TRH: the first hypophysiotropic hormone revisited. J Endocrinol. 2015; 226(2): T85–T92.

5. Rehman G, et al. Thyroid hormone receptor beta and endocrine regulation. Biomed Pharmacother. 2023; 162: 114678.

6. Kobayashi N, et al. Synthesis and evaluation of TRH mimetics with CNS activity. Bioorg Med Chem. 2021; 29: 115876.

7. Alvarez E, et al. new efforts to demonstrate TRH as a therapeutic agent. Int J Mol Sci. 2023; 24(13): 10456.

8. McMackin R, et al. JAK4D: a novel TRH analog for neurodegenerative disorders. Neuropharmacology. 2026; 210: 109034.

9. Kelly J, et al. TRH-based compounds and neurodegenerative models. Neuropharmacology. 2014; 89: 389–399.

10. Vale W. Pharmacology of thyrotropin-releasing factor. In: Handbook of Experimental Pharmacology. Springer; 2010. p. 45–67.

11. Charli JL, et al. TRH-degrading ectoenzyme as therapeutic target. Int J Mol Sci. 2020; 21(10): 3552.

12. Guillemin R, Schally AV. Structure and synthesis of TRH. Science. 1969; 165(3892): 175–177.

13. Gary KA, et al. TRH hypothesis of homeostatic regulation. J Pharmacol Exp Ther. 2003; 305(2): 410–416.

14. Yarbrough GG, et al. TRH in the neuroaxis: therapeutic implications. Med Hypotheses. 2007; 69(6): 1249–1256.

15. Parker CR, et al. TRH binding sites in human brain. Peptides. 1984; 5(4): 629–635.

16. Itadani H, et al. Cloning of TRH receptor subtype TRHR2. BiochemBiophys Res Commun. 1998; 250(1): 68–71.

17. Griffiths EC. Clinical applications of TRH. Clin Endocrinol. 1987; 26(2): 123–135.

18. Boschi G, et al. Antinociceptive properties of TRH. Eur J Pharmacol. 1983; 94(3–4): 317–322.

19. BenchChem. Chemistry of TRH and its analogs: a guide for drug development. 2020.

Received on 18.04.2026 Revised on 15.05.2026

Accepted on 08.06.2026 Published on 04.07.2026

Available online from July 30, 2026

Asian J. Research Chem.2026; 19(4):357-362.

DOI: 10.52711/0974-4150.2026.00054

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