Article summary with Pavlovian influences over food and drug intake by Woods & Ramsay - 2000

Disruption of homeostasis

Intake of food and drugs have in common that they cause the disruption of homeostasis in our body. There are a number of regulatory systems that are linked to the change of a certain physiological parameter, such as body temperature and the level of glucose in the blood. These systems can then initiate neural responses when a change occurs. Eventually, they can anticipate the disruption of homeostasis and respond to it in advance.

When drugs and food enter our bodies, they act on countless biological processes and thus bring about change. Some of these effects are wanted, others not. Through classical conditioning, people are able to maintain the positive effects of taking drugs and food by minimizing their negative effects. This is a learning principle that is subject to all regulatory systems in our body.z

Biological consequences of food

It used to be thought that the reason people eat is that the level of fuel for energy in critical tissues in the brain is too low. These are the so-called "depletion-repletion" models and there is something to argue with. People do not only eat when they are hungry, so when they still have enough energy. In addition, the balancing processes of eating food take place too slowly. This does not mean that the body itself does not initiate processes if the glucose level is low. That is indeed the case. A low level of glucose causes a number of regulatory responses to take place that have the consequence that a glucose secretion takes place in the blood immediately from the body itself.

However, food and the level of glucose in the body are related to each other. Eating increases the level of glucose in the blood when carbohydrates come from the stomach. To deal with this, the body secretes insulin from the kidneys on which less glucose is released from the liver into the blood. This will restore the balance. It has been discovered that this release of insulin can be brought under stimulus control using classical conditioning.

This means that if a person is confronted with stimuli (CS) that imply that food intake will follow (UCS), the body will release insulin before food is taken (CR) to prevent excessive levels of glucose in the blood. The body gives a kind of anticipated counter-reaction.

In addition to the level of glucose, there are other parameters that are regulated as a result of eating. These are, for example, body temperature and metabolic level for which a counter-reaction is given before the food is taken.

It is important with these findings that people can be flexible with their food intake and that they do not have to do so at set times. Normally someone eats several small meals a day. However, if a person's lifestyle restricts them to taking two meals a day, they must be large to maintain body weight. If no equilibrium restoring processes were to take place in the body, for example, the glucose level would rise considerably. However, a person can only eat large meals if the meal can be predicted.

The influence of drugs on regulatory systems

Even when drugs are taken, there is an interaction with tissues and molecules in the body. The effects of exogenously administered drugs that bind to certain receptors are similar to the effects of endogenous hormones and neurotransmitters. The exception is that the effects of drugs come about in unusual ways or combinations. Drugs can change normal physiology in countless ways and most drugs work in different ways at the same time, creating the well-known drug effect. So just like food, drugs cause certain parameters in the body to change, thereby disrupting homeostasis.

In the case of drugs, these disruptions can also be corrected by learned anticipated responses. Here too the classical conditioning principle seems to be at work. The conditioned stimulus is the environmental cue. The unconditioned stimulus is the drug effect. After repeated offers from the CS-UCS, a conditioned response is elicited by the CS in the absence of the drug. Taking drugs, for example, can cause body temperature to fall (hypothermia) and this is the drug effect.

The unconditioned response (UR) is then the processes that cause the body temperature to rise again. After a few pairings of the CS with the drug effect, the CS alone provokes an increase in body temperature (CR). The net result of the drug intake is then zero and the person has become tolerant.

Research with ethanol has also shown that tolerance for hypothermia due to the ethanol stimulus situation is specific. So a cue is needed that predicts the drug effect that causes tolerance to occur. Taking the drugs on its own is not enough. The condition for tolerance is therefore that there is an association between a regulatory neural reflex (UR) with a certain CS.

In an experiment with rats, it has been shown that they can become tolerant to motor disco coordination based on classical conditioning. The experiment consists of three groups of rats who all learned how to walk on a treadmill (CS) without making mistakes.

The experimental group was then placed in a narrow cylinder which was then rotated so that the rats became dizzy (motor disc coordination = UCS). Every adjustment to the engine control system is then an UR. When the rats were placed on the treadmill, they made many mistakes, but after repeating this procedure, they became tolerant and made fewer errors (CR).

The control group was also placed in a cylinder, but it was not turned. The last group was put in the cylinder that was turned around, but never just before they had to walk on the treadmill.

When the experimental rats proved tolerant, all three groups were spun around in the cylinder. The experimental group then made significantly fewer errors on the treadmill. If instead of spinning the cylinder, ethanol is administered to all rats, the effect appears to be the same. Rats that were tolerant of the motor-coordinating effect of the rotating cylinder were also tolerant of the same effects but then provoked by ethanol. This phenomenon is called cross-tolerance. So it can be that someone is tolerant of a certain drug without ever having taken it.