Neuroendocrine Cells and Neuropeptides
Individual cells of the nervous system and neurosecretory nuclei of the hypothalamus synthesize specific substances known as neuropeptides, with 17 families identified to date. Their mechanism of action is dual:
- Locally: provide signal transmission directly within synapses, acting as neurotransmitters.
- Distally: enter the systemic circulation and affect distant target organs, functioning as true hormones (e.g., ADH, oxytocin, hypothalamic statins and liberins).
Of particular significance are specific groups of neuropeptides with pronounced analgesic properties. Opioid peptides (endorphins) bind to specific receptors, producing powerful analgesic and sedative effects. Neurotensin exhibits actions similar to analgesics, causing a characteristic triad: analgesia, hypothermia (reduced body temperature), and hypotension (decreased blood pressure). In turn, delta-sleep-inducing peptide acts as an endogenous hypnotic, inducing sleep.
Hormonal Regulation of the Digestive System
Many gastrointestinal tract hormones are also found in the central nervous system. The gastric mucosa produces four variants of gastrin, which stimulate gastric motility and secretion while activating the release of glucagon and insulin by the pancreas. Bombesin is also synthesized here, enhancing the secretion of chloride and hydrogen ions (hydrochloric acid) and stimulating pancreatic secretion.
The mucosa of the duodenum produces relatively specific hormones:
- Secretin: inhibits gastrin production while stimulating the release of bile and pancreatic juice.
- Cholecystokinin (pancreozymin): produced in the intestine and the epithelium of pancreatic ducts. It causes contraction of the gallbladder and ducts while stimulating the exocrine pancreas.
Non-specific hormones include histamine and serotonin, which stimulate the secretory and motor activity of the GI tract. Important antagonists include somatostatin (inhibits pancreatic functions) and VIP (vasoactive intestinal peptide — stimulates them). The regulation of GI circadian rhythms is provided by melatonin.
APUD Series Cells
Numerous neuroendocrine cells are grouped into the APUD series strictly based on their biochemical features. The acronym stands for Amine Precursor Uptake and Decarboxylation, reflecting the ability of cells to take up amine precursors (cyclic amino acids: tryptophan, histidine, tyrosine, phenylalanine) and decarboxylate them.
The main distinguishing feature of APUD series cells is their ability to simultaneously produce two types of substances:
- Specific peptide (protein) hormones.
- Biogenic amines (serotonin, catecholamines, histamine).
The synthesis of biogenic amines is not accidental; they perform vital supportive functions: facilitating the exocytosis of the main hormone, increasing capillary permeability for rapid entry of the hormone into the blood, and rendering target cells more sensitive to the humoral signal.
Embryogenesis and Evolution of Scientific Concepts
Endocrine cells have diverse origins involving all germ layers and the mesenchyme:
- Neuroectoderm: hypothalamic neurons, pineal gland, thyroid C-cells, adrenal medulla.
- Foregut epithelium: adenohypophysis, thyroid follicular cells, parathyroid glands.
- Intestinal endoderm: gastroenteropancreatic system.
- Coelomic epithelium (mesoderm): adrenal cortex, ovarian follicular cells.
- Mesenchyme: interstitial cells of the gonads.
The study of these cells led to a revision of the classical rule "one cell — one hormone." With the discovery of the APUD system, the postulate was refined: "one endocrinocyte — one main hormone." However, exceptions are now known. For example, in the anterior pituitary, gonadotrophs simultaneously produce FSH and LH (resulting in 6 hormones across 5 cell types). In the intermediate lobe of the pituitary, a single precursor polypeptide is cleaved to yield multiple hormones: lipotropin, endorphins, and melanocyte-stimulating hormone.