The biological clock works in close interaction with various physiological processes to send commands to the various body organs, and to receive feedback on the body’s needs. According to neuroscientist Ruud Buijs, from the National Autonomous University of Mexico, time is a key factor for regulating temperature, reproduction, metabolism, circulation and the immune system.
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| Neuroscientist Ruud Buijs |
In a conference at the Intercontinental Academia on April 21, Buijs discussed this interaction, illustrating his exposition with numerous examples from studies of animals and humans.
He initially gave a schematic overview of the workings of the hypothalamus, a part of the brain highly connected to primitive parts of that organ and, via the autonomic nerves of the spinal cortex, to other parts of the body, sending them commands from the brain. “In addition to being essential for us to move our hand and other actions, the spinal cortex is also indispensable for the proper functioning of physiological processes. To achieve this type of physiological control we need the hypothalamus.”
The biological clock, which receives information about light and dark directly from the retina, is located in the hypothalamus, near the suprachiasmatic nucleus, the primary center for regulating circadian rhythms.
According to Buijs, it is now possible to remove the biological clock of the brain of a guinea pig and keep it functioning in vitro, maintaining electrical activity in a cycle of approximately 24 hours, autonomously, without having to do anything else.
Accuracy
To illustrate the accuracy of this mechanism, he said that forensic medicine can determine with great precision the time that someone was murdered by analyzing the expression of the biological clock in their organs, especially if the victim is found within 48 hours of death.
The moment when someone comes into the world is likewise determined by the biological clock. He showed a graph showing that the birth of the first child of pregnant residents of Amsterdam peaks at around 8:00 am. (In the Netherlands, babies are generally born with the help of a midwife.) Another chart showed that the peak time was between 4:00 and 5:00 am for the second or subsequent children, something that Buijs attributes to the fact that women become more savvy regarding labor. A third chart, however, shows a peak around noon and refers to births with obstetricians, when “babies are born in the doctor’s time, who induces labor or performs a cesarean section.”
In some cases, the moment of an individual’s death can also be determined by the biological clock, as shown by the fact that the peak of heart attacks occurs in the early part of the morning, with greater incidence on Mondays (perhaps due to the deregulation of schedules over the weekend, ventured Buijs).
Hormones
In Buijs’ view, the biological clock resorts to several mechanisms to enforce rhythms upon the body, using in many cases the hormone corticosterone. “In experiments with mice, corticosterone peak occurs soon after the night period (we know that mice are active at night), while the peak of the hormone melatonin occurs at night, indicating it induces activity in animals.” In humans, melatonin also peaks at night, but unlike what happens to mice, melatonin promotes sleep in humans.
To study this process in an animal antithetical to the mouse, one with diurnal habits, Buijs used the Arvicanthis ansorgei, a wild African rodent. These animals are active in early and late daytime. “We say that the biological clock prepares our body for the onset of the active period. When measuring corticosterone in the animal, it was found that the peak occurs just before active periods, so there are two corticosterone peaks in 24 hours. This means that, somehow, the biological clock adapts to the animal’s life style and adopts two peaks of activity.”
The hypothalamus contains specific areas that control temperature, heart rate and food intake. The biological clock imposes a temporal pattern to them all. “These connections are strong and there is no escaping the biological clock.”
Buijs said that the areas of the hypothalamus linked to food intake exert a type of influence similar to that of the biological clock, and work in harmony with it. He cited as an example the role of temporal and metabolic factors in modulating body temperature. In animals with nocturnal habits, the temperature is higher at night, then lowers and finally rises again, anticipating the active period.
The metabolism influences temperatures, so that, during the daytime period (of repose), they are low. If the biological clock is injured, the rhythm of temperature variation disappears and remains unaffected even by the metabolic factor, confirming the relationship between metabolism and biological clock.
To produce corticosterone, the paraventricular nucleus of the hypothalamus produces a hormone that leads to the production, in another part of the hypothalamus, of the adrenocorticotropic hormone (ACTH), which stimulates the production of corticosterone in the adrenal glands. Therefore, it was to be expected that upon examining the daytime and nighttime levels of corticosterone in an animal, we would find a relationship with the levels of ACTH. But that is not what happens.
To investigate this, Buijs inserted a virus similar to that of rabies in the adrenal glands of mice. Because this virus has the property of being absorbed by nerve terminals, reproducing itself in the body of the cell and migrating to other cells via the nerve terminals, it is possible to follow the chain of command from the brain to the glands.
Thus, it was possible to establish that neurons in the spinal cortex communicate with the adrenals. It was also possible to follow the impulses of the biological clock, proving that it uses not only hormones to send commands to the organs, but also autonomic pathways. This is advantageous, because something introduced into the bloodstream will take a certain amount of time to reach the organs. With a direct connection to the organs, the biological clock prepares them for what is coming in the blood and also for the arrival of hormones.
If the biological clock uses these means to communicate with the body, what means do the organs use to respond to biological clock? “Many scientists still think the biological clock is an autonomous timepiece that requires no feedback. Of course, this is not true. We have evidence that it needs feedback. The biological clock is in constant interaction with the body.”
In mammals and many other animals, this response is regulated by melatonin, which leads the body to bypass the biological clock cycle. Buijs displayed graphs showing the increased production of melatonin in a reindeer in Finland in the autumn, when the duration of night increases from less than one hour at the end of July to more than 11 hours in mid-September. Also in Finland, where the temperatures of certain periods of the year make the night flight of mosquitoes impossible, bats begin making diurnal flights to hunt for food. “Each organism makes manifold efforts to get in balance with the environment, where the length of the day/night cycle will determine the standard daily rhythm and the pace that the animal will adopt.”
According to Buijs, different areas of the brain produce the same neurotransmitter. The biological clock is one of the areas that produce vasopressin, an antidiuretic hormone with vasoconstrictive effects that also acts as a neurotransmitter in the brain. The biological clock produces vasopressin for an area that is also influenced by gonadal hormones.
Buijs showed images of two areas in the brain of a mouse with vasoconstrictive innervation, one that is influenced by gonadal hormones and another sensitive to the biological clock. When the mouse is neutered, vasoconstriction in the first area disappears, but remains in the area that is sensitive to the biological clock. This would indicate the possibility of an eventual loss of vasoconstrictive innervation for physiological or functional reasons. According to Buijs, this possibly occurs through the reduction of gonadal hormones, e.g., during preparation for winter, when the animal hibernates.
The size of the testicles and the testosterone levels of the European hamster (an animal that hibernates) are much greater in summer than in winter. Both the size and the level decrease abruptly between late July and late August, apparently preparing the animal to survive the coming winter. “The hamster goes into hibernation for four or five days, wakes up for 24 hours and eats, drinks and urinates a little, then goes back into hibernation, in a very well-organized process in temporal terms. If testosterone is given to the animal during this period, it will not hibernate, will attempt live in open spaces and will die.”
Images of certain area of the hamster’s brain (the same one observed in the mouse of the previous example) are completely different in the summer and in winter, indicating how the animal’s rhythm influences the central nervous system. The decrease of gonadal hormones prepares the animal for winter. The loss of vasoconstriction in the septum allows it to adapt its physiology and plummets its temperature to 5°C.
Type 2 diabetes and obesity
Two other examples of disordered physiological processes possibly caused by a desynchronization between the biological clock and the physiological mechanisms themselves are, according to Buijs, the onset of type 2 diabetes and the development of obesity.
In the case of type 2 diabetes, this might have to do with the fact that the brain needs more glucose for the active period of the individual’s daily cycle. The amount of sugar (glucose) consumed by the brain in 24 hours is 100 g and the quantity available for the rest of the body is 5 g. “The selfish brain competes with the rest of the body for energy. ‘Compete,’ however, is not a good verb, because the brain is the boss and orders that the sugar be given to it.”
An experiment was carried out with people suffering from type 2 diabetes and had twice the glucose levels of people without the disease. Although the level of glucose is already quite high in patients, it begins to rise even further around 5:00 am, preparing the body for the active period of the day.
The explanation for this, according to Buijs, is that the brain becomes more active and requires more energy in the beginning of the period of greater activity. To meet the demands of the brain, the biological clock prepares the body to make more glucose available.
When the body provides more glucose, it peak in the bloodstream quite rapidly, but then the level drops in a short time. By observing this phenomenon throughout the day, it was found that peaks in blood glucose levels decrease until the onset of the active period.
Interestingly, if you compare both phenomena, you’ll see that the glucose peak in the blood corresponds to lowest glucose peak in the muscles. “This means that the biological clock is doing two things at once: on the one hand, it is stimulating the production of glucose; on the other, it is making the brain absorb more glucose. A perfect preparation for the active period of the day.”
What is the role of the biological clock in obesity? Buijs said one of the correlations has to do with the period of sleep: the shorter the period, the greater the chances of developing obesity. But there’s another correlation, related to factors that promote the growth of fatty tissue.
Actually, the autonomic nervous system involves two systems (the sympathetic and the parasympathetic), by means of which the brain sends commands for the fatty tissues to grow. By injecting a virus similar to that of rabies in retroperitoneal fat (the back of the abdominal cavity) of mice, it is possible to identify the area of the brain that controls the parasympathetic system – which is, in general, the system for repose. “When cutting the innervation of the system, the uptake of glucose decreases, indicating that a command from the brain is required for the fatty tissue to absorb it.”
By analyzing the abdominal fat of two 14-year old boys, one non-diabetic and the other diabetic, the clinical finding was that the accumulation of fatty tissue in the gastrointestinal compartment is associated with the disease. According to Buijs, understanding that the parasympathetic system is important for the accumulation of fat suggests that the system’s commands for the gastrointestinal compartment may be stronger than the commands for the subcutaneous area. This might mean that both regions need other body signals; otherwise, the brain will not be able to distinguish between the compartments.
To resolve this doubt, markers were injected in abdominal tissue of mice and it was found that in the autonomic center (which commands the fatty tissues) there is a pair of different nerves to command the fat of each compartment. The differentiation of nerves may be followed up to the hypothalamus, where the differentiation can actually be seen in one of the structures that receive information from the biological clock. Thus, we find that the biological clock has nerves that communicate only with some part of the body not with another.
The conclusion of these experiments is that different controls for different tissues is what enables the centralized control of fat distribution. According to Buijs, this can be seen in the fact that individuals who accumulate abdominal fat suffer an imbalance in the body’s fat compartments, suggesting that some cases of diabetes and hypertension may involve this type of imbalance in the commands of the autonomic nervous system – not only in commands stemming from the hypothalamus, but in those from the biological clock itself.
Buijs’ working hypothesis for future research is that disarray in the reciprocal relationship between the biological clock and the organs – at any level and at any stage of life – can result in illness. “The disease can be induced, for instance, by ingesting food at the wrong moments during the 24 hour cycle.”
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