What are they and where are they produced?
Liberins are synthesized by peptidergic neurons of the hypophysiotropic area of the hypothalamus, which includes the ventromedial, dorsomedial, arcuate, supraoptic, and paraventricular nuclei. These cells possess dual properties: as nerve cells they generate electrical impulses, and as endocrine cells they produce neurosecretion.
A total of 7 releasing hormones are known (chemically they are decapeptides):
- Thyrotropin-releasing hormone (TRH)
- Corticotropin-releasing hormone (CRH)
- Growth hormone-releasing hormone (GHRH)
- Melanotropin-releasing hormone (MRH)
- Prolactin-releasing hormone (PRH)
- Luliberin (luteinizing hormone-releasing hormone)
- Folliberin (follicle-stimulating hormone-releasing hormone)
Note: Luliberin has been isolated and synthesized, whereas folliberin has not yet been isolated. It has been established that the same peptide stimulates the pituitary release of both luteinizing (LH) and follicle-stimulating (FSH) hormones. Therefore, a single term has been adopted for gonadotropic releasing hormones — gonadotropin-releasing hormone (GnRH).
Transport mechanism: how do releasing hormones reach the pituitary gland?
There is no direct neuronal connection between the hypothalamus and the anterior pituitary (adenohypophysis). The transfer of releasing hormones occurs via a humoral pathway through a specific vascular network — the hypophyseal portal system.
Stages of transport:
- Synthesized releasing hormones are transmitted along the axons of neurosecretory cells to the median eminence of the hypothalamus.
- There they enter the primary capillary network.
- Carried by the bloodstream through the vessels of the portal system, the neurohormones descend to the pituitary gland.
- They reach the secondary capillary network (sinusoids with fenestrated endothelium), which lies tightly against the glandular cells of the adenohypophysis.
- Releasing hormones act on target cells, stimulating the release of corresponding tropic hormones into the general bloodstream.
How is the production of releasing hormones regulated?
GnRH secretion is genetically programmed and has a pulsatile (circhoral) character. Arcuate nuclei generate GnRH pulses with a frequency of approximately once every 1–3 hours; bursts of enhanced secretion last for several minutes. Depending on the frequency and amplitude of these pulses, the pituitary secretes predominantly either LH or FSH.
Two main mechanisms influence GnRH release:
- Neurotransmitters (messenger substances):
- Stimulate release: norepinephrine, acetylcholine, $\gamma$-aminobutyric acid (GABA).
- Inhibit release: dopamine, serotonin.
- Negative feedback: An increase in the concentration of peripheral gland hormones (e.g., thyroxine, cortisol, estrogens) in the blood acts humorally on the hypothalamus, suppressing the synthesis of the corresponding releasing hormones.
Role in puberty
During the neutral period of childhood (up to 8 years), releasing hormones are produced in the hypothalamus but are secreted only episodically.
In the prepubertal period, GnRH release remains episodic; undulating LH surges occur exclusively during sleep. During this period, FSH and LH are secreted in insignificant amounts as isolated acyclic bursts, while the estradiol level remains very low.
During puberty, maturation of hypothalamic structures and the formation of tight synaptic connections occur. GnRH secretion becomes rhythmic, gonadotropin synthesis increases, the ovaries are activated, and the cyclic secretion of gonadotropic hormones is established.
Class and exam questions
What is the difference between releasing hormones and statins?
Both types of substances are hypothalamic neurohormones, but they exert opposite effects. Releasing hormones stimulate the secretion of tropic hormones in the anterior pituitary, while statins inhibit it.
Is there a separate releasing hormone for FSH?
No, a specific folliberin has not yet been successfully isolated. It has been proven that the release of both LH and FSH is stimulated by the same decapeptide, which was named gonadotropin-releasing hormone (GnRH).
How do releasing hormones reach the pituitary if there is no nerve connection between them?
They are transported humorally via the portal system. Releasing hormones are transmitted along axons to the primary capillary network of the hypothalamus, and then descend with the blood flow into the secondary capillary network of the adenohypophysis.
What else to study on this topic
- Hypothalamo-pituitary system
- Tropic hormones of the adenohypophysis
- Statins (inhibitory hormones)
- Hypophyseal portal system
- Negative feedback mechanism
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