We like to think we are in charge of our own behaviour that our thoughts are under our conscious control and that our actions are mostly reasonable. But our behaviour is also in the sway of an ancient system of mind control: hormones. These protein messengers are best known for their fundamental duty as regulators think of insulin and blood sugar, for example but they also bathe the brain in chemical information that tells us about the world around us and the people in it. Can a surge in a particular hormone make us feel and act like a totally different person? And if so, are we right to blame our out-of-control moments on some kind of biochemical signalling? Here, we look at some of the big notions about how hormones mess with your head and sift fact from fiction.
Oxytocin equals love
Of all the hormones, oxytocin undoubtedly has the best PR. Widely known as the love hormone and the cuddle chemical, it has a reputation for spreading goodwill among humankind. It has even been touted as a potential treatment for autism, anxiety, depression and chronic pain. Released during childbirth, breastfeeding and orgasm, oxytocin induces maternal and mate-bonding behaviour in many animals, including ourselves. In 2005, the possibility of hacking this system first reared its head when researchers found that people given an oxytocin nasal spray were more likely to trust others around them. Subsequent studies have found that sniffing oxytocin increases generosity, cooperation and empathy. Now the sprays are sold online, promising to improve your sex life, reduce anxiety and create feelings of trust. But not everyone believes the hype. Reviewing the evidence in a paper published last year, Mike Ludwig at the University of Edinburgh, UK, pointed out that no one has replicated the 2005 trust experiment, and that even the original researchers are backing away from its conclusions. Nor has it been proven that oxytocin can cross the blood-brain barrier. Studies of cerebrospinal fluid taken from people who had sniffed the hormone just beforehand suggest that it might, but it is too early to say for sure, Ludwig says. Even if oxytocin does indeed enter the brain, its effects appear to depend on context. Studies in mice suggest it alters brain circuitry so as to focus attention on socially relevant cues. Transplant such an effect to humans complex social lives and it could be a double-edged sword, promoting group bonding but perhaps also increasing hostility to outsiders. Other studies suggest that large doses of oxytocin may increase anxiety by making people oversensitive to what others say about them. All things considered, it might be best to get our warm and fuzzy feelings from real cuddles for the time being.
Periods make you see red
The comedian Roseanne Barr once said, Women complain about PMS, but I think of it as the only time of the month when I can be myself. When premenstrual syndrome causes a woman to morph into an angry, tearful or irritable version of herself, its pretty clear that hormones are to blame. Which ones and what they are doing to the brain is less clear. Part of the problem is that the menstrual cycle involves four hormones, each peaking at different times. Subtle differences in their fluctuations and in symptoms from one woman to another make it far from simple to pin down the effects of any particular hormone. The main players in the cycle are oestrogen (specifically a form of it called oestradiol) and progesterone, both of which affect parts of the brain. High progesterone levels, for instance, have been linked to increased activity in the amygdala, a brain area involved in threat detection, which may explain why nerves can feel so raw in the week before a period.
Highs and lows
Mood swings could also be the result of a sudden drop in oestrogen, high levels of which confer emotional resilience. One intriguing possibility is that its not so much the drop-off that causes the bad mood as the unusual high that precedes it. Oestradiol seems to be responsible for a phase of good mood and well-being shortly before ovulation, when levels peak, says Belinda Pletzer at the University of Salzburg, Austria. This lack of a peak has been discussed as one reason for negative mood changes in women on the pill, who dont ovulate, Pletzer says. A slower decline in oestrogen before menopause also protects women from mood alterations, says Julia Sacher, a neuroscientist at the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany. Why some women seem more hormonal than others may have less to do with different levels of hormones than with an individuals sensitivity to them. A study published early this year linked a persons susceptibility to PMDD, a more severe form of PMS, with overexpression of a group of genes that regulate the cells response to hormones.
Hormones make you hangry
Ever felt ready to fight for the last biscuit? Hanger feeling angry due to hunger is incredibly common, and ghrelin, the so-called hunger hormone, is implicated. Released when the stomach is empty, it triggers a rise in levels of neuropeptide Y (NPY), a neurotransmitter involved in making us want to eat and in regulating anger and aggression. People with intermittent explosive disorder, characterised by impulsive aggression, have above-average levels of NPY. Whats more, the more NPY in circulation, the greater the fall in the level of another neurotransmitter, serotonin. Low serotonin has been linked with reduced communication between the amygdala the brains threat detector and the prefrontal cortex, which regulates emotional responses. In such a situation, it might be harder to rein in emotions when we are stressed. But hormone and neurotransmitter levels alone dont dictate whether you are likely to lose it when peckish. The amount of connectivity between prefrontal cortex and amygdala varies between individuals, suggesting that some people may be more predisposed to hanger. Those who are can take comfort from the idea that it could well be an adaptive trait. From an animal perspective, being angry and possibly more aggressive when youre hungry can certainly increase the chances of survival, says Luca Passamonti, a neuroscientist at the University of Cambridge.
Cortisol is bad for you
Cortisol is widely thought of as the bad guy, a stress hormone linked to chronic health conditions, and therefore many think its level in the body should be lowered at all costs. Supplements are sold online that claim to help rid the body of the stuff. This might not be such a good idea, however. Cortisols job is to trigger the release of glucose into the bloodstream. This happens first thing in the morning to provide a much-needed energy boost to get us out of bed. And in times of stress, it gives us the energy to respond to a mental or physical challenge. In the right context, then, cortisol is definitely not bad for you. People with Addisons disease, who produce too little of it or none at all, experience debilitating symptoms including fatigue, and require daily treatment to top up the hormone. On the other hand, too much cortisol in the long term affects the brain in a number of ways. It can impair the generation of new cells in the hippocampus, a region involved in memory, and is also implicated in depression. Its not the absolute level of cortisol that matters, so much as the pattern of cortisol reactivity and recovery, says stress researcher Matthew Stults-Kolehmainen at Columbia University in New York. Fortunately, there is a simple way to achieve a beneficial pattern. Regular exercisers experience useful cortisol surges, says Stults-Kolehmainen, and levels drop quickly after the stress of exercise has passed.
Testosterone makes men angry and bald
Raging testosterone has been blamed for everything from wars to hooliganism to the banking crisis. Yet its reputation for putting men on the attack does not stand up to scrutiny. A study published last year, for example, showed that although high testosterone levels are linked to status-seeking behaviour, the form that behaviour takes depends on social norms. While men given an injection of testosterone were more likely to punish someone who treated them unfairly in a game, they were also more likely to reciprocate if their opponent was generous. As for baldness, contrary to popular belief, it isnt a sign of high testosterone levels. Hair loss is down to an enzyme called 5-alpha-reductase, which converts testosterone to dihydrotestosterone, a compound that causes hair follicles to shrink and die. Just a small amount of testosterone is enough to make a destructive dose of dihydrotestosterone, and genetics determines both the amount of the enzyme a man produces and how sensitive his follicles are to its product. The idea that falling testosterone causes a male menopause is also largely a myth. Testosterone falls by an average of 1 per cent per year after the age of 30, but only 2 per cent of men experience full-blown symptoms, including loss of libido, a drop in physical fitness and fatigue. In most cases, the cause is not the age-related drop in testosterone, but being overweight. Abdominal fat converts testosterone to oestrogen and its likely that this causes the symptoms, says Herman Leliefeld, a urologist based in the Netherlands.
Mums brains turn to mush
Parenthood changes everything, and at least some of that comes down to the way that a flood of hormones prepares the mothers brain for the challenges ahead. Some of these changes happen in pregnancy, but far from turning the brain to mush, most are beneficial. One is an increase in the relative amounts of grey matter in brain regions involved in social cognition, priming the brain for empathy and reasoning. Women in the third trimester also have a reduced stress response, which protects the baby from high cortisol levels that can trigger early birth. It also means that soon-to-be mothers may be less likely to feel stressed than usual. The changes dont end when the baby arrives. In experiments, rat mothers have sharper foraging skills and reaction times, taking just 50 seconds on average to find food hidden in their cage versus 270 seconds for rats that havent had pups. And human brain scans have shown that in the weeks and months following birth, areas of the mothers brain involved in reward processing, reasoning, empathy and regulating emotion all bulk up. The researchers linked the changes to an increase in oestrogen, oxytocin and another hormone, prolactin, although how they bring these changes about isnt yet clear. Laura Glynn, who researches maternal brain changes at Chapman University, California, says that its probably a combination of hormones making the brain more malleable, and sensory stimulation by the baby. It could be that, as in the case of oxytocin, the hormones make mothers brains more sensitive to the world around them. The effects can be long-lasting. Having children alters the mothers hormone levels for decades, says Liisa Galea at the University of British Columbia, Canada, so its not surprising that we see long-lasting effects on behaviour. Fathers, too, show a rise in oxytocin and also in prolactin, which lowers testosterone. In one study, fathers had lower testosterone levels that other men of the same age. Those who spent 3 hours a day or more with their kids had the lowest testosterone levels of all. Perhaps the hormone changes make men more attentive fathers.
Changing gender changes your brain
Hormones affect the brain and behaviour, so it stands to reason that gender transition, which begins with hormone therapy, would affect both. Its true that there are anecdotal reports of increased aggression in people who have transitioned from female to male and thus have more testosterone than before, and of oestrogen therapy making people feel more emotionally connected and slower to anger. But attributing such changes directly to hormones is tricky. It is difficult to disentangle potential direct effects of testosterone reduction from indirect effects of desired physical changes, says Marco Colizzi at Kings College Londons Institute of Psychiatry, who studies links between hormones and mental health. Nevertheless, there is some evidence that testosterone therapy given to people transitioning from female to male raises levels of a protein called SERT towards what we see in people designated male at birth. SERT transports the neurotransmitter serotonin into nerve cells, and plays an important role in mood and anxiety disorders the antidepressant fluoxetine (Prozac) for example, blocks its activity. Such disorders are more common in women, who have a naturally lower levels of SERT. This suggests that hormonal treatments might reduce mental health risks. However, one of the researchers involved in the work, Georg Kranz at the Medical University of Vienna, Austria, says there are limits to hormone therapys effect on the brain simply because so much of the brains wiring is laid down during development. Brain scans have hinted at ways to identify what a transgender brain looks like, which suggests there is much still to learn about how genes, development and hormones work together to build a brain.
New Scientist, 9 August 2017 ; http://www.newscientist.com/