Histamine is an autocoid that is found in plenty in the region of wounds and infection, indicating its role in healing.
Even the itch we experience during wound healing also indicates histamine presence.
Histamine is present in many body cells and tissues. It is an essential chemical that controls body physiology.
It acts by two receptors, viz. H1, H2, and H3 are not prominent in health care.
Functions of histamine
In the Whole Body
Histamine is present in mast cells, so during an infection or wound, it is accumulated there.
This histamine acts to attract leukocytes (white blood cells).
It produces constriction in the lungs and intestine and lowers blood pressure.
It is also present without mast cells in other tissue like the brain, skin, and stomach.
Role of Histamine in gastric secretion and peptic ulcer formation
- Histamine stimulates acid secretion in the stomach through its action on H2 receptors.
- Acetylcholine and gastrin act on mast cells in gastric mucosa, resulting in histamine release.
- This histamine acts on H2 receptors on oxyntic cells and stimulates acid release.
- Thus increased histamine secretion causes acidity in the stomach, leading to peptic ulcers over time.
Anti-histamines such as diphenhydramine and hydroxyzine are used to treat peptic ulcers. Anti-histamines work by blocking the histamine receptor.
Histamine in the gastrointestinal tract (GIT)
Histamine influences three major functions of GIT like
- It increases gastric acid secretion through the H2 receptor stimulation and helps in protein digestion.
- Alters mucosal ion secretion.
- Regulates GI motility
Histamine plays a vital role in inflammation.
Due to a bruise or ball injury, it shows accumulation in the region of the blow.
- Redness (rubor),
- Dolor (pain), and
- Heat (calor) in the region.
- H4 receptor-mediated mast cell activation causes the release of several inflammatory mediators via an inflammatory cascade.
- These mediators stimulate the migration of different inflammatory cells into the inflammatory site.
- Histamine also causes itchiness at the wound site due to its accumulation.
In the brain (CNS)
- Histamine acts as a neurotransmitter in the brain.
- It has excitatory effects via H1 and H2 receptors while inhibitory action via H3 receptors.
- Histaminergic neurons are located in the tuberomammillary nucleus of the posterior hypothalamus and project to many parts of the brain.
- Thus exerting its effects on wakefulness, alertness, temperature control, arousal, the release of pituitary hormones, and thirst.
- It influences higher brain functions like sleep-wake regulation, circadian and feeding rhythms, homeostasis, cognitive processes, etc.
- Neuronal histamine is also involved in pain perception. It has antinociceptive properties. Histamine plays a role in the repair of neuronal damage.
- It triggers the release of the adrenaline hormone from the adrenal gland.
- Drugs that work to mitigate histamine include Anti-histamine drugs like cetirizine, promethazine, dimenhydrinate, etc.
- These drugs are used in times of common cold, allergy, vomiting during pregnancy, motion sickness, etc.
- Distorted histamine signaling in the brain affects addictive behaviors and degenerative diseases such as Alzheimer’s, Parkinson’s disease, and multiple sclerosis.
Histamine in narcolepsy
- Narcolepsy is a chronic sleep disorder where the patient displays overwhelming daytime drowsiness and sudden attacks of sleep.
- Though histamine is a wakefulness-promoting substance, there is still a marked increase in the histaminergic system of neurons in the brain of narcoleptic patients.
- This compensatory mechanism compensates for the loss of orexin and hypocretin neurons in the hypothalamus.
- Typically orexin and hypocretin neuropeptides are mainly responsible for wakefulness.
Histamine in migraine
- Excess histamine accumulates within the brain; it acts via H1 receptors to induce a migraine.
- This excess occurs when histamine is not metabolized adequately due to the lack of the enzyme diamine oxidase (DAO).
Histamine on cognition
- Histamine is one of the primary neurotransmitters in the brain, affecting memory and learning.
- It mediates its effects via H1, H2, H3, and H4 receptors, which are G protein-coupled receptors.
- Depending on the receptor it acts, its effect on cognition, learning, and memory may be facilitatory or inhibitory.
Histamine’s Effects on the immune response
- Cells involved in immune response, like macrophages, T and B lymphocytes, endothelial cells, etc., express histamine receptors and secrete histamine.
- Histamine influences the recruitment of the primary effector cells into tissue sites and affects their maturation, activation, polarization, and effector functions.
- Histamine regulates antigen-specific helper T cells(Th1 and Th2 cells).
- Histamine acting via H2 receptor modulates peripheral antigen tolerance induced by T regulatory cells in several pathways.
- The differential expression of H4 receptors on immune cells and their varied intracellular signaling mechanisms lead to the diverse effects of histamine on immune responses.
Histamine in hypersensitivity reactions
- Hypersensitivity reactions are an overreaction of the immune system to an antigen (may be endogenous or exogenous), which in normal conditions would not cause an immune response.
- Histamine is released from mast cells and other mediators in Type 1 hypersensitivity reaction (known as an anaphylactic reaction).
- The mast cells store histamine in the form of granules and release it when they get activated.
- In Type 1, the reaction they are activated when antibodies are released against the antigen via the action of IgE (immunoglobulin E)
In blood vessels
- The vascular walls contain large amounts of histamine located in mast cell and non-mast cell stores.
- Mast cells present in postcapillary venules secrete histamine, which causes protein leakage and edema formation.
- Activation of vascular H1 and H2 receptors causes vasoconstriction and vasodilatation, respectively.
- In humans, the effect of vasodilation predominates.
H2 receptors are located on vascular smooth muscle cells and the vasodilator effects are mediated by cyclic Adenosine monophosphate (cAMP).
H1 receptors are present on endothelial cells, and their stimulation leads to the formation of local vasodilator Nitric Oxide (NO).
This NO, which diffuses from endothelial cells to vascular smooth muscle cells, increases the cGMP concentration, which leads to relaxation of vascular smooth muscle.
Histamine’s effect on venules
- Histamine release from mast cells (in allergic reactions) acts on endothelial cells of venules and highly increases vascular permeability.
- Endothelial cells form a single layer covering the inner surface of the blood vessel.
- The vascular permeability depends on blood flow and the vascular endothelial barrier.
- The endothelial barrier is formed by intercellular adherens junctions consisting of vascular endothelial cadherin (VE-cadherin), catenins, and the cytoskeleton.
- Histamine acts by phosphorylating VE-cadherin, increasing the intracellular calcium concentration in endothelial cells, etc.
Effect of histamine in hypertension
- Histamine has hypertensive effects on the cardiac system.
- It increases blood pressure by acting on peripheral blood vessels.
- In cases of excess histamine accumulation in the Body (histamine intolerance), due to a lack of enzyme diamine oxidase (DAO) (which catalyzes breaking down histamine), the patient experiences high blood pressure.
In wound healing
- Histamine has an accelerating effect on cutaneous wound healing.
- Histamine stimulates angiogenesis, the process of formation of new blood vessels, which increases the rate of healing.
- It also stimulates the exudation of molecules involved in wound healing from blood vessel to site. Histamine also helps in macrophage recruitment.
- Histamine is released from stimulated basophils and mast cells.
- Histamine inhibits chemotaxis, which is the movement of phagocytic cells in the direction of a higher concentration of pathogenic microbe.
- Histamine inhibits the formation of superoxide ions and other free radicals.
- This inhibits the process of phagocytosis.
- Mast cells are present in the heart, mostly around vessels and between myocytes. Histamine is stored in cytoplasmatic granules of mast cells.
- On activation of mast cells, the histamine is released. It produces arrhythmogenic effects. The effects are mediated by H2-receptor.
- It stimulates heartbeat and causes tachyarrhythmias by increasing sinus rate and ventricular automaticity and slowing atrioventricular conduction.
- Histamine also influences sodium and potassium influx, thus affecting depolarization and repolarization.
Effect of histamine on respiratory system and asthma
- Histamine acts on the smooth muscle cells of the respiratory airways and causes contraction, which leads to reflex bronchoconstriction.
- Histamine directly stimulates the H1 receptors present in the smooth wall, or histamine indirectly stimulates afferent vagal fibers.
- Both of these processes lead to bronchoconstriction.
- In allergic asthma, the mast cells get activated. These mast cells secrete histamine.
This histamine causes airway obstruction through 3 processes.
- It stimulates bronchoconstriction,
- Increases secretion in the airways and causes mucosal edema.
- Narrows down the airway passages and causes wheezing.
Thus anti-histamines are often given to treat allergic asthma.
Histamine and platelet aggregation
- When histamine is added to a system, the platelets display increased levels of cAMP. Platelet aggregation is inhibited by histamine.
- When platelets are stimulated and begin to aggregate, there is also a histamine release.
- This release precedes aggregation.
Histamine in allergic rhinitis
- Histamine is a major mediator in the development of allergic rhinitis.
- It may be secreted by mast cells or basophils.
- Nasal administration of histamine causes sneezing, itching, rhinorrhea, and nasal congestion.
- Histamine acts via H1 receptors.
- Anti-histamines are used to treat rhinitis and reduce sneezing.
Histamine on sleep-wakefulness
- The posterior hypothalamus contains a well-developed system of histaminergic neurons that project to most major areas in the brain.
- They act via H₁ and/or H₃ receptors and have a major role in regulating sleep-wakefulness.
- Histamine or H₁ receptor agonists cause wakefulness, while administration of H₁ receptor antagonists induces sleep.
- Histamine release in the hypothalamus is highest during the awake state.
- The histaminergic neurons show maximum activity during the state of high vigilance.
- Their activity reduces during rapid eye movement (REM) and non-REM sleep.
Histamine in depression
- The state of wakefulness, the sleep-wake cycle, learning and memory, appetite, emotions, etc., are controlled by the histaminergic system of neurons present in the CNS.
- The histamine receptor present in the brain is H1.
- Patients who are depressed show decreased binding of histamine to the H1 receptor.
- The more the severity of the depressive symptoms, the more is the decrease in binding affinity to H1 receptors.
Histamine in ocular allergy
- Ocular allergy is the inflammatory reaction of the eye surface in response to allergens present in the environment.
- When mast cells in the eye are exposed to allergens, they release histamine, which mediates the inflammatory processes and causes the eye to become red, watery, and itchy.
Histamine’s role in vomiting
- During motion sickness, histamine is the neurotransmitter transmitting signals from the inner ear to the brain. This induces vomiting.
- In food allergies, histamine is released from mast cells in the gastrointestinal tract when the mast cells are activated on exposure to an antigen.
- Histamine influences the chemoreceptor trigger zone in the medulla oblongata, releasing chemicals like serotonin and dopamine.
- These chemicals act on the vomiting center and induce emesis/vomiting.
Histamine and vertigo
- Vertigo is when a person suddenly starts feeling off-balance and feels a spinning sensation.
- Histamine is used in the treatment of vertigo due to its vasodilating properties.
- It is also used to treat inner ear disturbances.
- Histamine regulates the vestibular function and cerebral circulation.
- Thus it is often used to treat vertigo.
Histamine and burns
- In burn injury, the mast cells are activated, and they release histamine.
- This histamine cause dilation of arteries and constriction of venules.
- This leads to more perfusion. It also increases vessel permeability in the capillaries.
- Thus edema forms in burn injuries.
Histamine and eczema
- Eczema is a condition where the skin becomes inflamed, rough, itchy, and cracked.
- Histamine is released from basophil leucocytes of such patients. Histamine aggravates eczema by causing an inflammatory reaction.
- Histamine, in this case, acts via the H1 receptor.
Histamine and pain
- Histamine influences the processing of nociceptive information, acting in an antinociceptive manner in the CNS while it acts in a nociceptive manner in the PNS (peripheral nervous system).
- Histamine is released when tissue damage or injury causes pain hypersensitivity, like neuropathic pain after nerve damage where histamine is released from mast cells.
- The release of histamine causes the recruitment of macrophages and neutrophils.
- H3 receptor and H4 receptor are involved in the modulation of neuropathic pain.
- When applied to the skin, histamine can also induce an itching sensation and pruritis.
Histamine in anaphylaxis
- Anaphylaxis is a highly severe allergic reaction where the immune system(mast cells and basophils) releases histamine, basophils, mast cells, and other chemical substances in response to allergens.
- Histamine exerts its effects through the activation of histamine 1 (H1) and histamine 2 (H2) receptors and causes the symptoms of anaphylaxis to occur.
- Vasodilation, hypotension, and flushing are caused due to histamine action on both H1 receptors and H1 receptors.
- Action on H1 receptors alone causes tachycardia, vascular permeability, coronary artery vasoconstriction, bronchospasm, pruritus, and rhinorrhea.