Adrenaline

Adrenaline, also known as epinephrine and alternatively spelled adrenalin, is a hormone and medication which is involved in regulating visceral functions. It appears as a white microcrystalline granule. Adrenaline is normally produced by the adrenal glands and by a small number of neurons in the medulla oblongata. It plays an essential role in the fight-or-flight response by increasing blood flow to muscles, heart output by acting on the SA node, pupil dilation response, and blood sugar level. It does this by binding to alpha and beta receptors. It is found in many animals, including humans, and some single-celled organisms. It has also been isolated from the plant Scoparia dulcis found in Northern Vietnam.

Medical uses

As a medication, it is used to treat several conditions, including allergic reaction anaphylaxis, cardiac arrest, and superficial bleeding. Inhaled adrenaline may be used to improve the symptoms of croup. It may also be used for asthma when other treatments are not effective. It is given intravenously, by injection into a muscle, by inhalation, or by injection just under the skin. Common side effects include shakiness, anxiety, and sweating. A fast heart rate and high blood pressure may occur. Occasionally it may result in an abnormal heart rhythm. While the safety of its use during pregnancy and breastfeeding is unclear, the benefits to the mother must be taken into account.
A case has been made for the use of adrenaline infusion in place of the widely accepted treatment of inotropes for preterm infants with clinical cardiovascular compromise. Although sufficient data strongly recommends adrenaline infusions as a viable treatment, more trials are needed to conclusively determine that these infusions will successfully reduce morbidity and mortality rates among preterm, cardiovascularly compromised infants.
Epinephrine can also be used to treat open-angle glaucoma, as it increases the outflow of aqueous humor from the eye, which lowers intraocular pressure.

Physiological effects

The adrenal medulla is a major contributor to total circulating catecholamines, though it contributes over 90% of circulating adrenaline. Little adrenaline is found in other tissues, mostly in scattered chromaffin cells and in a small number of neurons that use adrenaline as a neurotransmitter. Following adrenalectomy, adrenaline disappears below the detection limit in the bloodstream.
Pharmacological doses of adrenaline stimulate α₁, α₂, β₁, β₂, and β₃ adrenoceptors of the sympathetic nervous system. Sympathetic nerve receptors are classified as adrenergic, based on their responsiveness to adrenaline. The term "adrenergic" is often misinterpreted in that the main sympathetic neurotransmitter is noradrenaline, rather than adrenaline, as discovered by Ulf von Euler in 1946. Adrenaline has a β₂ adrenoceptor-mediated effect on metabolism and the airway, with no direct neural connection from the sympathetic ganglia to the airway.
Walter Bradford Cannon originally proposed the concept of the adrenal medulla and the sympathetic nervous system being involved in the flight, fight, and fright response. But the adrenal medulla, in contrast to the adrenal cortex, is not required for survival. In adrenalectomized patients, hemodynamic and metabolic responses to stimuli such as hypoglycemia and exercise remain normal.

Exercise

One physiological stimulus to adrenaline secretion is exercise. This was first demonstrated by measuring the dilation of a pupil of a cat on a treadmill, later confirmed using a biological assay of urine samples. Biochemical methods for measuring catecholamines in plasma were published from 1950 onwards. Although much valuable work has been published using fluorimetric assays to measure total catecholamine concentrations, the method is too non-specific and insensitive to accurately determine the very small quantities of adrenaline in plasma. The development of extraction methods and enzyme–isotope derivate radio-enzymatic assays transformed the analysis down to a sensitivity of 1 pg for adrenaline. Early REA plasma assays indicated that adrenaline and total catecholamines rise late in exercise, mostly when anaerobic metabolism commences.
During exercise, the adrenaline blood concentration rises partially from the increased secretion of the adrenal medulla and partly from the decreased metabolism of adrenaline due to reduced blood flow to the liver. Infusion of adrenaline to reproduce exercise circulating concentrations of adrenaline in subjects at rest has little hemodynamic effect other than a slight β₂-mediated fall in diastolic blood pressure. Infusion of adrenaline well within the physiological range suppresses human airway hyper-reactivity sufficiently to antagonize the constrictor effects of inhaled histamine.
A link between the sympathetic nervous system and the lungs was shown in 1887 when Grossman showed that stimulation of cardiac accelerator nerves reversed muscarine-induced airway constriction. In experiments in the dog, where the sympathetic chain was cut at the level of the diaphragm, Jackson showed that there was no direct sympathetic innervation to the lung, but bronchoconstriction was reversed by the release of adrenaline from the adrenal medulla. An increased incidence of asthma has not been reported for adrenalectomized patients; those with a predisposition to asthma will have some protection from airway hyper-reactivity from their corticosteroid replacement therapy. Exercise induces progressive airway dilation in normal subjects that correlates with workload and is not prevented by beta-blockade. The progressive airway dilation with increasing exercise is mediated by a progressive reduction in resting vagal tone. Beta blockade with propranolol causes a rebound in airway resistance after exercise in normal subjects over the same time course as the bronchoconstriction seen with exercise-induced asthma. The reduction in airway resistance during exercise reduces the work of breathing.

Emotional responses

Every emotional response has a behavioral, an autonomic, and a hormonal component. The hormonal component includes the release of adrenaline, an adrenomedullary response to stress controlled by the sympathetic nervous system. The major emotion studied in relation to adrenaline is fear. In an experiment, subjects who were injected with adrenaline expressed more negative and fewer positive facial expressions to fear films compared to a control group. These subjects also reported a more intense fear from the films and greater mean intensity of negative memories than control subjects. The findings from this study demonstrate that there are learned associations between negative feelings and levels of adrenaline. Overall, the greater amount of adrenaline is positively correlated with an aroused state of negative emotions. These findings can be an effect in part that adrenaline elicits physiological sympathetic responses, including an increased heart rate and knee shaking, which can be attributed to the feeling of fear regardless of the actual level of fear elicited from the video. Although studies have found a definite relation between adrenaline and fear, other emotions have not had such results. In the same study, subjects did not express a greater amusement to an amusement film nor greater anger to an anger film. Similar findings were also supported in a study that involved rodent subjects that either were able or unable to produce adrenaline. Findings support the idea that adrenaline has a role in facilitating the encoding of emotionally arousing events, contributing to higher levels of arousal due to fear.

Memory

It has been found that adrenergic hormones, such as adrenaline, can produce retrograde enhancement of long-term memory in humans. The release of adrenaline due to emotionally stressful events, which is endogenous adrenaline, can modulate memory consolidation of the events, ensuring memory strength that is proportional to memory importance. Post-learning adrenaline activity also interacts with the degree of arousal associated with the initial coding. There is evidence that suggests adrenaline does have a role in long-term stress adaptation and emotional memory encoding specifically. Adrenaline may also play a role in elevating arousal and fear memory under particular pathological conditions, including post-traumatic stress disorder. Overall, "Extensive evidence indicates that epinephrine modulates memory consolidation for emotionally arousing tasks in animals and human subjects." Studies have also found that recognition memory involving adrenaline depends on a mechanism that depends on β adrenoceptors. Adrenaline does not readily cross the blood-brain barrier, so its effects on memory consolidation are at least partly initiated by β adrenoceptors in the periphery. Studies have found that sotalol, a β adrenoceptor antagonist that also does not readily enter the brain, blocks the enhancing effects of peripherally administered adrenaline on memory. These findings suggest that β adrenoceptors are necessary for adrenaline to have an impact on memory consolidation.

Pathology

Increased adrenaline secretion is observed in pheochromocytoma, hypoglycemia, myocardial infarction, and to a lesser degree, in essential tremor. A general increase in sympathetic neural activity is usually accompanied by increased adrenaline secretion, but there is selectivity during hypoxia and hypoglycemia, when the ratio of adrenaline to noradrenaline is considerably increased. Therefore, there must be some autonomy of the adrenal medulla from the rest of the sympathetic system.
Myocardial infarction is associated with high levels of circulating adrenaline and noradrenaline, particularly in cardiogenic shock.
Benign familial tremor is responsive to peripheral β adrenergic blockers, and β₂-stimulation is known to cause tremor. Patients with BFT were found to have increased plasma adrenaline but not noradrenaline.
Low or absent concentrations of adrenaline can be seen in autonomic neuropathy or following adrenalectomy. Failure of the adrenal cortex, as with Addison's disease, can suppress adrenaline secretion as the activity of the synthesizing enzyme, phenylethanolamine-N-methyltransferase, depends on the high concentration of cortisol that drains from the cortex to the medulla.