Histamine: Synthesis, Release And Functions

Histamine is an idazoic amine involved in the local responses of the immune system. This molecule also regulates normal functions in the stomach and acts as a neurotransmitter in the central nervous system (CNS). Outside the CNS, it acts as a mediator of numerous physiological processes.

Since the 1950s, scientists have known that this molecule exists in the brain, but until recently they did not know what it did. We’ll explain its role in the brain later in this article.

In the body, histamine is mainly stored in the mast cells of the connective tissue and in the basophil cells of the blood. In fact, these cells are extremely secretive, forming a system that responds to a wide variety of endogenous and exogenous stimuli through multiple cellular mechanisms.

Under normal conditions, histamine from mast cells is stored in secretory granules containing a matrix of heparin and various proteins. Together with the various hydrolases, it is mostly connected to the matrix by ionic bonds, but a small amount can be loose.

Release Mechanisms

In order for histamine to be released, it must cross the granular and cell membranes. The release may be cytotoxic, upon rupture of both membranes, or exocytotic, by fusion of the membranes without cellular degradation.

Your body releases histamine during physiological processes such as the secretion of gastric juice. However, it is much better known for its participation in pathological processes, where it is released in a more or less explosive way, such as in inflammatory reactions and in immediate hypersensitivity reactions.

In these cases , histamine is often one of several mediators that the body releases at the same time. There are multiple physical and chemical agents that trigger the release of histamine. The physical includes:

• Warmth
• Radiation
• Cold
• Trauma

As for the chemical agents, there is an extraordinary number and a wide variety of them. They have to find receptor molecules in the membrane with which they interact.

Depending on the type of interaction, a series of different steps will be activated that ultimately increase the intracellular calcium concentration. Among these stimuli we can find the following:

• Antigens
• Cytotoxic agents
• Enzymes

General reaction of histamine in the human body

When injected into the general circulation, histamine can cause the skin to turn red and can cause tachycardia, throbbing headache and hypotension. These effects are directly dose-dependent. This means that if you increase the dose of histamine administered, it also increases the effects.

Locally, it produces edema, pruritus (itching), urticaria (hives), and bronchoconstriction. In addition, histamine, along with other mediators, contributes to anaphylactic shock.

On the other hand, it plays an obvious role in the immune response, in inflammation, secretion of gastric juices and regulation of blood circulation, among other functions that we will see below.

Functions of histamine in the body

There are many roles that histamine plays in our body. Below we will describe 5 of them.

Allergic Processes

Histamine is a modulator of both humoral and cellular immune responses, as well as an important mediator of hypersensitivity reactions. When applied in large doses, or released during anaphylaxis, it causes an extreme drop in blood pressure.

Neurotransmission

Histamine can act as a neuromodulator, regulating responses to other neurotransmitters. It interacts like acetylcholine, opiates, GABA, and so on. Furthermore, it increases the excitability of CNS neurons, regulates hypothalamic functions, sleep-wake relationship, appetite and vegetative functions.

Cardiovascular regulation by histamine

It acts as a vasodilator by interacting with histamine receptors. Histamine release leads to an increase in capillary permeability due to effects on small blood vessels from the outflow of plasma proteins and fluids from the cells.

Effect on extravascular smooth muscle

Histamine causes smooth muscle contraction. However, the response varies widely, even in the same person. Small doses of the molecule also cause bronchoconstriction in people with bronchial asthma, as we mentioned earlier.

Secretion of gastric juice

This molecule also promotes the secretion of juices in the stomach and triggers the secretion of acid from parietal cells. In addition, it significantly increases the production of pepsinogen and intrinsic factor. Therefore, it is the main mediator of acid secretion from the stomach, by stimulating H2 receptors.