The Hormones and Neurotransmitters Associated with Stress in Various Species of Dolphin

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In order to receive more reliable results, data should be collected from wild, free-ranging dolphins over a long period of time in a humane way, away from any form of human interaction. If this is not possible, research should be conducted on dolphins that have died of natural causes, focusing on their stress levels after death. It has been shown from a variety of different studies that the main limitation of collecting data on dolphins is the effect human interaction has on their stress levels, making it particularly difficult to analyse a control. Captive dolphins are the easiest to collect data from. However, it has been proven that they are constantly under stress through tourist interactions and limited social interactions with other dolphins (Morgan & Tromborg, 2006).

Although wild dolphins are more difficult to capture, monitor and collect data from, they are under less constant stress. However they are recently becoming more and more exposed to stress due to fisheries. This constant stress is so severe that it is beginning to have an adverse impact on reproduction and survival on dolphin species (Curry & Edwards, 1997). In conclusion, it has been proven that stress has an effect on a variety of different hormones and neurotransmitters in many difference species of dolphin. Cortisol is the hormone most affected by stress and plays an important role in the survival of dolphins during stressful situations. However, the other hormones (aldosterone, dopamine, norepinephrine and epinephrine) all work together to enable the body to perform well under stress. Without these hormones during stress, major organs can shut down and dolphins can easily contract a life-threatening disease, resulting in death. This study researched the importance of norepinephrine under a stressful situation, but only under one specific stress. In order to develop a better understanding of the effects of stress on norepinephrine, more research should be conducted on a wider range of dolphin, perhaps by examining whether young calves manage stress in the same way as mature dolphins or if the depth of dive alters the levels of norepinephrine.

When diving, Bottlenose Dolphins (Tursiops aduncus) induce a change in blood circulation to favour flow to the more important parts of the body, the brain and heart. The vasoconstrictors in dolphins stimulate the contraction of muscles in the wall of blood vessels, with noradrenaline being an important hormone in vasoconstriction. Noradrenaline serum levels were monitored while dolphins dived, showing significant increases in noradrenaline levels and a significant decrease in heart rate (Tomoshique, et al., 2017). This showed that in order to perform well under this stressful situation, norepinephrine was very important. Norepinephrine showed to widen the air passages to the lungs and decrease the heart rate of the dolphins, allowing the dolphins to dive effectively. Captive dolphins are exposed to daily stresses, such as handling, tourists and loud noises. When such captive dolphins are examined after death, they show an increase in noradrenaline levels (Stanford, et al., 1984). Norepinephrine is synthesised from dopamine.

When dopamine is transported into vesicles, it is then converted to norepinephrine by the enzyme dopamine β-hydroxylase (DBH). Norepinephrine can then be released from the adrenal medulla into the bloodstream as a hormone (Systems, 2009). Norepinephrine (NE), also known as noradrenaline (NA), is a catecholamine. Norepinephrine has multiple roles as a hormone and a neurotransmitter. Norepinephrine as a stress hormone plays a key role in the fight-or-flight response as well as affecting parts of the brain where attention and responding actions are controlled (Ortenholm, 2004). As a hormone, norepinephrine is also involved in vasoconstriction. Norepinephrine increases or decreases heart rate, widens air passages in the lungs, increases blood pressure and narrows the blood vessels in non-essential organs, enabling the body to perform well in stressful situations (Goldstein, 2010).

Norepinephrine

Developing a capacity to be able to synthesis and store more stress hormones, such as epinephrine, enables dolphins to increase their chances of survival. It enables dolphins to become better equipped for stressful situations, allowing them to perform better while under stress, and making them less likely develop an infection and a long-term disease.

More than 60 Atlantic Bottlenose Dolphins (Tursiops truncates) were examined, after death, to see the effects of stress, both acute and chronic, on the adrenal glands. It showed that the adrenal glands of chronically stressed animals were significantly heavier than those of acutely stressed dolphins and that long-term stress lead to structural differences of the adrenal glands. Some changes showed that over time, a bottlenose dolphin exposed to stressful situations develops more numerous epinephrine-producing cells. This suggests that these differences might have been caused by an individual’s ability to synthesize and store more stress hormones (Clark, et al., 2006). Epinephrine, also known as adrenaline, is a neurotransmitter and a hormone produced and secreted from the adrenal glands. Epinephrine is released in response to stress, in order to prepare the body for the fight-or-flight response, as it increases blood flow to the muscles. A stressful event leads to the activation of nerves connected to the adrenal glands, which in turn trigger the secretion of epinephrine. When stress from a situation begins to subside, the nerve impulses to the adrenal glands start to stop, resulting in epinephrine levels decreasing.

Epinephrine

Dopamine helps to maintain a good mood and prevent pain. Prolonged stress depletes dopamine levels in the body which instigates the release of stress hormones, in this case mainly cortisol. This suggests why the dopamine levels within the dolphins of this study decreased so significantly when being caught for so long. Free-ranging dolphins were examined based on their stress response by measuring their physiologic stress response to capture and release. Over 150 bottlenose dolphins (Tursiops truncates) were collected from two study sites (Charleston and the Indian River Lagoon) over 4 years. The stress hormones were measured in the blood, which included dopamine, aldosterone, cortisol, epinephrine and norepinephrine. Dopamine concentrations decreased significantly as the time being captured increased (Fair, et al., 2011).

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However, cortisol levels seemed to increased significantly from the initial capture and continued to do so while being caught. Studies have proved that when dolphins are exposed to stress, dopamine levels rise. (Brock, 1999). However, if the dopamine levels do not rise while under constant stress, over time this can alter the behaviour of a dolphin. Behaviours can change so drastically that a dolphin would lose their appetite, become increasingly agitated and increasingly aggressive. This ultimately can lead to their death, suggesting that a dolphin’s survival and reproductive success is ensured due to its ability to withstand stress (Brock, 1999). Dopamine (3,4-dihydroxyphenethylamine) is a catecholamine produced in the dopaminergic neurons and in the hypothalamus. It has a role in the body as a neurotransmitter that initiates adrenalin during the activation of a stress response. Dopamine acts on G protein-coupled dopamine receptors (GPCRs) (Kebabian & Calne, 1979).

Dopamine

There are numerous limitations to this study. For example, only two dolphins were examined. For more reliable results, the same study should be repeated on more individuals and possibly on different species of dolphin. Researchers should also take into account that the individuals in this study were captive dolphins, and that this could affect the dolphins’ levels of stress. A study conducted on two adult Bottlenose Dolphins (Tursiops truncates) exposed to stress showed the effects that stress has on the hormone, aldosterone. These two dolphins were housed individually in captivity, over a 10-day period, and were exposed to decreasing water temperatures. Voluntary blood drawbacks were taken from each dolphin every 2-3 days, and the serum was analysed via radioimmunoassay. As the water temperature decreased, the dolphins showed an increase in serum aldosterone, with aldosterone levels being twice as high at the coldest water temperature (Houser, et al., 2011). When in a stressful situation, increased aldosterone levels activate mineralcorticoid receptors which in turn increases oxidative stress and inflammation leading to cardiovascular and renovascular disease. Aldosterone then targets the kidneys (particularly the kidney tubules), increasing the absorption of sodium and water which increase blood volume and pressure. This ensures an adequate delivery of oxygen and nutrients to the body while under stress.

Numerous studies have begun to support the evidence that exposure to stress (particularly chronic stress) is now a risk factor for cardiovascular disease (CVD) in many mammals. Aldosterone is released in response to a stressful situation, activating the hypothalamic-pituitary-adrenal (HPA) axis. Studies have shown that stress, both inflammation and oxidative, plays a key role in the development of aldosterone-induced cardiovascular problems (Yoshimoto & Hirata, 2007). Aldosterone is also responsible for regulating sodium (Bollag, 2014), therefore has an important role in the regulation of blood volume and blood pressure (Funder, 2004). Having too low aldosterone levels in the body can lead to hypotension (low blood pressure) and circulatory shock, whereas excessive aldosterone levels in the body can cause hypertension (long-term high blood pressure) which can lead to congestive heart failure (Bollag, 2014). Aldosterone is a mineralcorticoid, steroid hormone produced in the adrenal glands. When stress arises, this causes a release of corticotropin releasing hormone (CRH) from the hypothalamus of the brain. As these hormone levels rise, this stimulates the secretion of the adrenocorticotropin (ACTH) hormone from the anterior pituitary. These hormones act on the adrenal gland, which produce aldosterone (Chan, 2010). Aldosterone This study only focuses on two hormones associated with stress, cortisol and aldosterone. In order to gain more understanding of the effects of stress on hormones, further research should be conducted to see the effects of oil toxicity on the catecholamines associated with it. Figure 1 shows that when the dolphins ingested the oil, it lead to hypoadrenocorticism, decreasing the hormones associated with stress in the body, such as cortisol and aldosterone. This lead to the deaths of the dolphins affected by the oil spill. Similar effects occurred when the dolphins inhaled the oil. Lung injury, hepatic injury and maternal transfer all occurred which lead to chronic disease and death.

Showing the health effects (yellow), and what these lead to (blue) (Schwacket, et al., 2013). Based on a study investigated after the Deepwater Horizon Oil Spill in 2010, it showed that dolphins, particularly Common Bottlenose Dolphins (Tursiops truncates), suffered from hypoadrenocorticism (Schwacket, et al., 2013). Hypoadrenocorticism, also known as Addison’s disease, is caused by having too little cortisol in the body. This was consistent with adrenal toxicity caused by the oil in the water. This disease caused lower glucose levels in the blood and increased inflammation, resulting in a poor body condition, increasing disease susceptibility and lowering the chances of survival (Schwacket, et al., 2013). Cortisol also plays an important role in pregnant mammals. Cortisol induces a variety of enzymes before birth. During birth, losing the placenta deprives the foetus of a source oxygen and natural sugars, known as glucose, as well as heat. In preparation for this, in the late stages of pregnancy, organs undergo maturational changes. The changes are regulated by cortisol and include: the lungs maturing structurally and functionally, and glycogen beginning to accumulate in the liver and thermogenesis (Liggins, 1994).

When cortisol levels in the body get too high due to continuous stresses, many species of dolphin can suffer from high blood pressure, high blood sugar, depression (particularly in captive dolphins) and even death (Marino & Lilienfeld, 2007). Having too high of a cortisol level can also lead to the temporary shutdown of digestion and reproduction (Ugaz, et al., 2012). Cortisol levels in the body should always be maintained. However, when cortisol levels decrease significantly, the hypothalamus (area in the brain) releases the corticotrophin-releasing hormone. This causes the pituitary gland to start secreting adrenocorticotropic hormone into the bloodstream. As the adrenal gland begins to detect the higher levels of adrenocorticotropic hormone within the bloodstream, the adrenal gland begins the secretion of cortisol (Fair, et al., 2011). Cortisol is one of the glucocorticoids, and is a steroid hormone transported by the blood and controlled by the hypothalamus, pituitary and adrenal glands. Nearly every cell in the body have receptors for cortisol. The hormone, cortisol has many different effects within the body including: regulating blood sugar levels, acting as an anti-inflammatory, regulating memory formation, controlling salt and water balance, influencing blood pressure and helping the development of a foetus. (Dickerson & Kemeny, 2004)

Cortisol

When a stressful situation arises, mammalian brains initiate a stress response. Stress activates the HPA-axis (hypothalamic-pituitary-adrenal axis) which includes the hypothalamus, pituitary and adrenal gland. The adrenal gland releases the catecholamines, norepinephrine and epinephrine as well as steroid hormones, cortisol and aldosterone. Captive dolphins frequently experience more stress than free-ranging dolphins. This stress arises through noises emitted from the tanks cleaning systems, people crowding around the pools, malnourishment, performing, and people swimming with them. Wild, free-ranging dolphins also suffer from stress. Acute stress can be caused by net entanglement, boat strike or acute infection. Chronic stress can be caused by mothers being separated from their infant or through long-term disease (Clark, et al., 2006). Stress is a result from demanding circumstances that increase an organism’s susceptibility to disease. Stress is known as a state of mental or emotional strain. The way in which an individual deals with stress through a physiological stress response, is often the only way in which they survive stressful situations. (Clark, et al., 2006) What is Stress? There are numerous hormones and neurotransmitters in mammals that are involved with stress, such as steroid hormones and catecholamines. The steroid hormones involved in stressful situations are cortisol and aldosterone released from the adrenal glands. The catecholamines involved in stress are dopamine (which is a neurotransmitter), norepinephrine (also known as noradrenaline, which is both a hormone and neurotransmitter) and epinephrine (adrenaline). Stress can be seen in dolphins in numerous ways, mainly through vocalisation and their behaviour. Whistles increase in both volume and frequency when under stress.

Changes in their behaviour can be seen through pacing, abstaining from eating, becoming aggressive and becoming ill. Stress is measured in dolphins through hormone levels within their blood and blubber. Dolphins are thought to be one of the most intelligent mammals in the world as they form complex bonds, show emotion and can communicate (using whistle-type noises) with individuals of not only the same species, but other species (Lusseau, 2003). However, as they are such complex mammals, they are prone to stress, particularly in captivity, which can have such a negative effect, it can lead to death. Dolphins are marine mammals that live in groups known as pods, of up to a dozen individuals (Bazua-Duran & Au, 2004). They are highly sociable mammals that establish close links with other individuals in their pod and seem to show empathic, cooperative and altruistic behaviours (Wiszniewski, et al., 2008). Dolphins rely on acoustic signals. The acoustic signals determine and negotiate their environment, both physical and social (Smolker, et al., 1992). They also rely on echolocation to manoeuvre.

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