75. Hypothalamic releasing and inhibiting factors

Question 1

Hypothalamus

Answer 1

Hypothalamus is the highest cerebral integrator of autonomic (vegetative) functions.
-It synchronizes neural and hormonal regulative activity.

• Hormones produced in the parvocellular area reach the anterior lobe of the pituitary gland (adenohypophysis).
• Hormones produced in the magnocellular area reach the posterior lobe of pituitary gland (neurohypophysis).

Question 2

Two parts of the hypothalamus

Answer 2

HT can be divided into two areas: 
magnocellular area (with large cells) and parvocellular area (with small cells). 

Magnocellular:
Consists of two parts ; supraoptic nucleus and paraventricular nucleus, which are mainly responsible for production of oxytocin and antidiuretic hormone (ADH, or vasopressin).

Parvocellular:
It is formed by ventromedial nucleus, dorsomedial nucleus, and infundibular nucleus.
This area produces those releasing (liberin) or inhibiting (statin) substances, which can reach the anterior lobe of the pituitary gland.

Question 3

Parvocellular area

Answer 3

Parvocellular neurosecretions are transported down the axons, reach portal circulation of pituitary stalk at the area of median eminence and, finally, arrive at the anterior lobe of the pituitary gland. Here they influence the production and the release of several hormones into the vascular system.

Question 4

Magnocellular area

Answer 4

Hormones, produced in the magnocellular region, are transported down inside the axons from the site of production (neurone) to the site of release (posterior pituitary). The posterior lobe of the pituitary gland can be considered as an appendix of the hypothalamus.

Question 5

Axonal transport

Answer 5

The peptide (releasing or inhibiting factor) produced by a certain hypothalamic cell, travels down the axon and it is released to the environment through the terminal apparatus. This is called neurosecretion.

Unlike in synpatic signaling the released substance reaches peripheral circulation for further transport.

Peptides, already inside the capillaries, are transported subsequently down the pituitary stalk (portal vessels) toward the anterior pituitary, where a second capillarization makes it possible for the hypothalamic signal to reach its destination: hormone secreting cells of anterior pituitary gland (adenohypophysis).

Question 6

Proteins of axonal transport

Answer 6

A motor protein, kinesin, is responsible for transport from soma to synapse direction, while to the opposite (returning of residues to the soma) transport is done by dynein.

Question 7

Hypothalamic factors

Answer 7

Parvocellular areas synthesize releasing and inhibitory substances (factors or hormones), which influence tropic-hormone (hormones that have other endocrine glands as their target) production of the anterior pituitary.

-Hypothalamic substances, which influence production and release of pituitary gland, are may times called hypophyseotrop (hypophyseal tropic hormones).

Question 8

Releasing factors - hypothalamus

Answer 8

Hypophysotropic, hormones are produced in the hypothalamus.

TRH: thyrotropin releasing hormone, stimulate thyroid gland hormones.
CRF: corticotropin releasing hormones, adrenocorticotropin stimulating hormone. It facilitates synthesis of ACTH and MSH and opiates, also splitting POMC.
GnRH: gonadotropin releasing hormones: facilitates synthesis of FSH and LH.
GRF: growth hormone releasing factor.
PRF: prolactin releasing factor, responsible for lactation, and ovulation in rat.
MRF: melanocyte stimulating hormone, activator of MSH. Inhibiting factors are DOPAMINE, SOMATOSTATIN, GABA AND VIP.
GIH: growth hormone inhibiting factor.
PIF: Inhibits prolactine release. (dopamine)
VIP: increase synthesis and secretion of prolactine and other anterior pituitary hormones. It acts negatively on synthesis of somatostatin.

Question 9

Inhibiting factors

Answer 9

Parvocellular inhibiting factors (hormones)

• DOPAMINE (important)
• SOMATOSTATIN (important)
• GABA is also known by its general inhibiting effects.
• VIP indirectly acting specific inhibitor
• GIF= growth hormone inhibiting factor, somatotropin inhibiting factor.
• PIF is a systematic name for an inhibiting factor of prolactin release (and production).
• GAP (GnRH Associated Peptid) significantly decrease prolactin production.
• PRL (new)

Question 10

Classical neuroendocrine reflex

Answer 10

-Oxytocin-mechanisms is a neuroendocrine reflex. This was the first mechanism that revealed the close co-operation of neural and endocrine systems.

• The essentials of a neuroendocrine reflex are:
  1) the translation of neural information, originated from the peripheral sensory nerve, to the language of endocrine system with the help of the hypothalamus and 2) that the effect or response is not a neural, but a hormonal response.
• Excitation from udder-nipple afferent sensory fibres reaches the spinal cord by the inguinal nerve. From it, excitation finally arrives at the hypothalamus, where an enhanced oxytocin synthesis is evoked.
• At the same time, oxytocin release increases from the posterior lobe of the pituitary gland. Oxytocin reaches mammary gland by means of blood circulation.