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)
- Luteinizing hormone-releasing hormone (LHRH), also known as luliberin
- Follicle-stimulating hormone-releasing hormone (FSHRH), also known as folliberin
Note: Luliberin has been isolated and synthesized, whereas folliberin has not yet been successfully 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 is adopted for gonadotropic releasing hormones — gonadotropin-releasing hormone (GnRH).
Transport Mechanism: How do Releasing Hormones Reach the Pituitary?
There is no direct neuronal connection between the hypothalamus and the anterior pituitary (adenohypophysis). The transfer of liberins is carried out via a humoral pathway through a specialized vascular network — the hypophyseal portal system.
Stages of transport:
- Synthesized liberins are transported via axons of neurosecretory cells to the median eminence of the hypothalamus.
- There they enter the primary capillary network.
- Carried by blood flow through the portal vessels, the neurohormones descend to the pituitary.
- They reach the secondary capillary network (sinusoids with fenestrated endothelium), which lies tightly against the glandular cells of the adenohypophysis.
- Liberins act on target cells, stimulating the release of corresponding tropic hormones into the general bloodstream.
How is the Production of Releasing Hormones Regulated?
The secretion of GnRH is genetically programmed and has a pulsatile (circhoral) pattern. 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 predominantly secretes either LH or FSH.
The release of GnRH is influenced by two main mechanisms:
- Neurotransmitters (chemical messengers):
- 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 secreted only episodically.
In the prepubertal period, GnRH release remains episodic; wave-like surges of LH occur exclusively during sleep. During this period, FSH and LH are secreted in insignificant amounts as isolated acyclic bursts, and estradiol levels remain very low.
In the pubertal period, maturation of hypothalamic structures occurs along with the formation of tight synaptic connections. GnRH secretion becomes rhythmic, gonadotropin synthesis increases, the ovaries are activated, and the cyclic secretion of gonadotropic hormones is established.