Neurones are classified into types partly according to the transmitter they release at their synaptic endings on other nerve cells. Some nerve cells do something different - they secrete hormones at their nerve endings, i.e. a hormone is released into the blood stream and circulates through the heart to reach the target organ. Supraoptic neurones have axons that pass into the posterior pitutiary (neurohypophysis), and release vasopressin (antidiuretic hormone) into the general circulation, and this hormone acts on the kidney in the control of water balance.
Other hypothalamic neurones release hormones (called releasing hormones or releasing factors) into capillaries in the hypothalamus. These capillaries are one part of a portal system: they join togther to form the pituitary portal vein, which in turn opens up into a second set of capillaries within the anterior pituitary (adenohypophysisis). This specialised system of blood vessels, the hypothalamo-hypophyseal portal system (the pituitary portal system), carries releasing hormones from the hypothalamic capillaries to the adenohypophysial capillariesof the anterior lobe of the pituitary gland, where the releasing factors regulate the secretion of anterior pituitary hormones. Neurosecretion is the process whereby neurones release hormones (releasing hormones) into the vascular system. In the neurohypophysis the release is directly into the systemic circulation, but in the hypothalamus the hormones are released into the pituitary portal system.
Posterior lobe of the Pituitary (Neurohypophysis)
The posterior lobe of the pituitary gland (neurohypophysis) contains the terminals of axons whose cell bodies are within the hypothalamus. Activity in these neurons causes the release of two hormones into blood flowing through the posterior pituitary. These hormones are Vasopressin (Antidiuretic hormone, ADH) and Oxytocin.
Origin of Vasopressin and Oxytocin
The release of vasopressin and oxytocin from the neurohypophysis occurs when action potentials pass down the axons of neurons that originate in the hypothalamus and whose terminals contain the hormones. These hormones are manufactured in the nerve cell bodies in the supraoptic and paraventricular nuclei of the hypothalamus and are transported inside their axons to their axonal terminals, where they are released into the blood stream.
The physiological function of Vasopressin (also known as Anti-Diuretic Hormone ADH) is to control water balance and its main site of action is on the kidney. Its secretion is controlled by the hypothalamus where sensory neurones monitor the osmolality of the blood, and these regulate the release of ADH. This is an important mechanism in fluid homeostasis.
Oxytocin is synthesised in the Paraventricular Nucleus of the hypothalmus, a large nucleus that can be subdivided. One group of paraventricular neurones send their axons into the neurohypophysis, where oxytocin is secreted directly into the neurohypophyseal capillaries (which are not part of the pituitary portal system). The release of oxytocin into the blood stream during labour and during the 'let-down' of milk in the breast during suckling is due to this pathway.
Another one part of the paraventricular nucleus sends axons to the autonomic areas of the spinal cord, where oxytocin acts as a neurotransmitter rather than as a hormone.
Image source: encyclopedia britannica
Pituitary Portal System
In most parts of the body, blood passes from the arterial system to the venous system via a single network of capillaries. But in the hypothalamus and pituitary there are two networks of capillaries in series, jointed by a portal vein. Blood flows into the hypothalamus and receives releasing factors from neuroendocrine cells; this blood flows down the pituitary portal vein into the second set of capillaries in the adenohypophysis, where the releasing factors influence the secretion of hormones.
The Anterior lobe of the Pituitary Gland (Adenohypophysis)
The anterior lobe of the pituitary receives blood that has already passed through the hypothalamus via the pituitary portal system. In the hypothalamus neurosecretory neurons secrete releasing hormones that act on endocrine cells in the anterior pituitary, only a short distance away. These cells, in turn, release hormones that enter the general circulation to act on distant endocrine glands, such as the thyroid, adrenal cortex and the gonads.
The anterior pituitary gland has different types of cells, each producing hormones released into the systemic circulation that regulate the activity of distant endocrine organs in the body. The anterior pituitary hormones regulate the activity of the ovaries and testes, thyroid and adrenal glands.
Other anterior pituitary hormoes are concerned with growth and glucose homeostasis (growth hormone, somatotropin), and the production of milk (prolactin).
ANTERIOR PITUITARY HORMONES
Thyroid Stimulating Hormone (TSH)
Thyroid Stimulating Hormone (TSH) is released from the anterior pituitary and stimulates the thyroid gland to release thyroxine (T4) and tri-iodothyronine (T3). These hormones are of importance in regulating metabolism, and in the development of the skeletal and nervous systems. The release of TSH by the anterior pituitary is controlled by TSH releasing factor (TSH-RF) by the hypothalamus.
Adrenocortical trophic hormone (ACTH, Corticotrophin)
Adrenocortical trophic hormone (ACTH) is released from the anterior pituitary and regulates the physiological release of glucocorticoids from the adrenal cortex.
Melanocyte Stimulating Hormone (MSH) stimulates melanocytes in the skin to produce a colored pigment, melanin. This hormone is a byproduct of the production of ACTH: MSH, ACTH and endorphins are all produced by the breakdown of pro-opiomelanocortin (POMC), a molecule encoded by a gene that is expressed in some anterior pituitary cells.
Follicle Stimulating Hormone and Luteinising hormone are responsible for the release of male and female hormones from the testis and ovary. In females the rhythmic release of pituitary gonadal hormones controls ovulation and the release of oestrogen and progesterone from ovarian follicles; in turn these sex hormones are responsible for the menstrual cycle and female secondary sexual characteristics. In males, FSH causes the release of testosterone from the Leydig cells of the testis; testosterone is important in spermatogenesis and the development of male secondary sexual features.
Prolactin is a peptide hormone released by cells of the anterior pituitary and stimulates mammary glands to produce milk. The secretion of prolactin is controlled by cells in the hypothalamus that release dopamine, a releasing factor that circulates in the pituitary portal system. Unlike most releasing factors, dopamine, an amine, has an inhibitory effect on the release of prolactin.
Growth Hormone (Somatotropin)
Growth Hormone is a peptide hormone released by cells of the anterior pituitary and acts on many tissues, particularly children's bones. It also has important effects on glucose, fat and protein metabolism throughout life. It is released from the anterior pituitary during sleep.
HYPOTHALAMIC RELEASING HORMONES (FACTORS)
Thyrotrophin Releasing Hormone (TRH, TRF)
TRH is a small peptide produced in the hypothalamus and secreted into the hypothalamic capillaries to be carried through the pituitary portal system to TSH releasing cells in the anterior pituitary. Its action is excitatory and causes the release of Thyrotrophin (Thyroid Stimulating Hormone)
Corticotrophin Releasing Hormone (CRH, CRF)
CRH is a peptide secreted into the hypothalamic capillaries and carried to the anterior pituitary where it stimulates the release of ACTH.
Growth Hormone Releasing Hormone (GHRH) is a peptide released by neurones in the arcuate nucleus, and carried by the pituitary portal system to the anterior pituitary, where it stimulates growth hormone (GH) secretion. Growth Hormone is released in a pulsatile manner due to the pattern of release of GHRH; it is released during sleep.
Growth hormone secretion is also influenced by Somatostatin, a peptide also known as Growth Hormone Inhibitory Releasing Factor (GHIRF), produced by the hypothalamus.
Gonadotropin-releasing hormone (GnRH), also known as follicle-stimulating hormone–releasing hormone (FSH-RH) and luteinizing hormone–releasing hormone (LHRH) are hypothalamic factors that control the release of FSH and LH. GnRH is secreted in pulses which occur regularly in the male, with a diurnal cycle, but in females, the frequency and size of the pulses varies during the menstrual cycle.
Prolactin Inhibitory Releasing Factor
Prolactin release form the anterior pituitary gland is controlled by the circulation of an inhibitory substance produced by the hypothalamus through the pituitary portal system. This releasing factor is known to be dopamine - an amine, unlike the other releasing factors.
Hormones vs Neurotransmitters
Oxytocin is synthesised in the Paraventricular nucleus of the hypothalmus, and one part of this nucleus sends axons to the autonomic areas of the spinal cord, where oxytocin acts as a neurotransmitter rather than a hormone. Note the importance of the light-dark cycle in regulating the activity of the suprachiasmatic nucleus, which controls the sleep/waking cycle as well as other physiological functions that have diurnal rhythms.
Many peptides are produced by neurones, including central neurones and the enteric neurones. The term 'brain-gut axis' refers to the fact that many peptides found in the gastro-intestinal tract are also found in the CNS, and some of these can act as neurotransmitters or hormones.
Substance P is found in the gut, is present in primary afferents concerned with nociception, and in many parts of the CNS.
Cholecystokinin (CCK) is an intestinal hormone, but also a CNS neurotransmitter. Other neuropeptides include Neuropeptide Y (NPY) and the Enkephalins (ENK).
In the hypothalamus, ENK are made as a product of the synthesis of AdrenoCorticoTrophic Hormone (ACTH) and Melanocyte Stimulating Hormone from a peptide called Pro-Opiomelanocortin (POMC). All of these products derive from POMC, but have different actions. Endorphins can be released into the circulation, but smaller related ENK peptides act as neurotranmitters.
Image source: commons wikipedia
The diagram show the products of Pro-Opiomelanocortin (POMC), which included Adrenocorticotrophic Hormone (ACTH), Enkephalins and Melanocyte stimulating hormone (MSH)