Read the first of three excerpts from the chapter “Why do people take drugs?” from David Nutt’s bestselling book “Drugs Without the Hot Air“.

Why do people take drugs?

“Drugs are the product of a complex evolutionary game. As fungi and plants evolved, some developed chemicals in their leaves or seeds that deterred the insects and other animals that fed on them, helping the plants to survive and reproduce. These chemicals mimicked the natural substances in insects’ brains which told them how to behave, confusing the insects, or overloading their nervous systems and poisoning them.

Insects and the larger animals that followed them evolved in turn, adapting to these changes and sometimes developing a liking for the plant chemicals. Many animals in the wild seek out drugs: goats eat coffee beans, and pigs and elephants gorge on the alcohol in rotting fruit. In laboratory settings, small mammals such as mice and rats have remarkably similar reactions to humans, and become addicted to the same sort of drugs as we do. Most of the drugs we use today are either made directly from plants, or are synthetic derivatives of these plant chemicals. To understand how they work, we need to understand some of the basic mechanisms of the brain.

Chemicals in the human brain

The chemicals that send messages between nerve cells (neurons) in our brains are called neurotransmitters; they respond to our environment and tell us how to behave. When we’re hungry our bodies tell us to eat, and when we’re full our bodies tell us to stop, just as when we’re safe we need to be able to relax, and when we’re in danger we need to be alert.

A neuron releases ^†neurotransmitters into the synapse (gap) between it and a neighbouring neuron. The neurotransmitters move across the gap to the other neuron, where they activate receptors specifically designed to recognize the particular chemical (Figure 4.1), and so create feelings – for example, of hunger or fear. These neurotrans- mitters are then reabsorbed at reuptakesites(Figure 4.2) when the signal isn’t needed anymore – for example, when a predator has gone. (At the reuptake site, special transporter proteins in the cell wall allow the neurotransmitter molecules, which are large, to pass into the interior of the neuron. We’ll see later than some drugs work by blocking the transporters and preventing the reabsorption of the neurotrans- mitter.)

A typical day without drugs

The brain is extremely complex, and there’s still a lot we don’t know, although in the last two decades neuroimaging techniques have vastly improved our understanding of how neurotransmitters work. The most important chemicals, and a brief summary of what they do, are listed in Table 4.1 on page 65. As we’ll see shortly, drugs target receptors designed to respond to these natural chemicals; the better we understand natural chemicals, the better we’ll understand the effects of the drugs that mimic them.

To illustrate how these chemicals work, let’s meet Ben, a cleanliving man who doesn’t like to take any drugs at all – not even coffee. As he wakes up and gets out of bed, glutamate is released, kick-starting his body’s transition into being awake. He drives into work, getting stuck in traffic; it’s really important he’s on time today, and his brain is flooded with noradrenaline as he becomes angry and stressed at the thought of being late. When he gets to work, it turns out his boss is also late, so Ben isn’t in trouble after all, and a rise in serotonin levels makes him feel better. As lunchtime approaches, there’s a dip in his cholecystokinin which makes him feel hungry, so he goes to the canteen and his cholecystokinin level rises again as he eats.

After lunch Ben gives an important presentation, which his boss is really pleased with, and his being congratulated causes the release of the reward chemicals endorphins and dopamine. On the way home he has an argument on the phone with his wife, and his serotonin drops making him feel miserable, but after going for a run his endorphin levels go up and he feels a lot happier. While making dinner to apologize, he cuts his finger and endocannabinoids and endorphins help numb the pain. As night falls, adenosine builds up in the brain, glutamate levels fall, and GABA levels rise, making Ben feel tired and ready for sleep.”

Read the second part here.