A team of researchers at the University of Edinburgh have found that cells in the hypothalamus are activated in response to food and in response to the expectation of being fed:
We trained rats to a regime of scheduled feeding, in which food was available for only 2 hr each day. After 10 days, rats were euthanized at defined times relative to food availability, and their brains were analyzed to map Fos expression in neuronal populations to test the hypothesis that some populations are activated by hunger whereas others are activated by satiety signals. Fos expression accompanied feeding in several hypothalamic and brainstem nuclei. Food ingestion was critical for Fos expression in noradrenergic and non-noradrenergic cells in the nucleus tractus solitarii and area postrema and in the supraoptic nucleus, as well as in melanocortin-containing cells of the arcuate nucleus. However, anticipation of food alone activated other neurons in the arcuate nucleus and in the lateral and ventromedial hypothalamus, including orexin neurons. Thus orexigenic populations are strongly and rapidly activated at the onset of food presentation, followed rapidly by activity in anorexigenic populations when food is ingested.
In the study, carried out by Gareth Leng and his colleagues, c-Fos expression was used as a marker for neuronal activity. c-Fos is an immediate-early gene expressed in neurons upon activation. This is apparently the first study to investigate the changes in brain activity that occur during the course of a meal. It also provides a neurobiological basis for a set of classic 19th Century psychology experiments.
Ivan Pavlov (1849-1936) was a physiologist and experimental psychologist who discovered that the learned association between two events can be used to elicit certain responses to specific stimuli. The method for manipulating associative learning developed by Pavlov has come to be known as classical (or Pavlovian) conditioning.
For his experiments, Pavlov developed a technique for collecting and measuring the salivary secretions of dogs (left). This consisted of an apparatus which included a tube, or fistula, that was inserted into the ducts of the salivary glands, so that whenever saliva was produced, it was collected in a measuring cylinder.
Dogs salivate when presented with food. This is a natural physiological response which prepares the digestive system for the food. Thus, prior to any associativelearning, the food is referred to as an unconditioned stimulus, and salivation is the unconditioned response.
In his experiments, Pavlov rang a bell whenever he presented the dog with food. With repeated pairings of the bell and the presentation of food, the dog would start to salivate whenever it heard the bell ringing, even in the absence of food. Because of the learnt association between the ringing of the bell and the presentation of food, it had learnt to expect to be presented with food whenever it heard the bell ringing. The ringing bell is what is called the conditioned stimulus, and the salivation that occurs as a result is the conditioned response.
Classical conditioning is used today by psychiatrists as a form of behaviour modification. Aversion therapy, for example, is a technique used to try and eliminate undesirable behaviour in patients. This is done by associating that behaviour with something unpleasant or discomforting. For example, alcoholics are sometimes treated with emetic drugs, which make them vomit whenever they consume alcohol, in the hope that the learnt association between drinking alcohol and vomiting will eliminate the alcoholism.