There are many different kinds of amino acids, all of them very small molecules, which can link together to form chains of varying lengths. If fewer than 50 amino acids link together the chain is called a peptide. Larger numbers linked together form what are called proteins. Shorter or longer, peptides or proteins, these chains play critical roles in all life, roles that are determined by the unique sequence in which these amino acids are linked together. Each chain folds into a particular shape, which can vary from a helix to a large glob. For one example, a chain with 732 amino acids, which is partly glob like and partly helical, is responsible for making a molecule called lanosterol, which is eventually converted to the hormones that determine if we are male or female.
But don’t judge importance by size. Today I want to tell you about a chain with 36 amino acids, a neuropeptide that is mostly helical and is widely found in the brains and central nervous systems of animals. This kind of peptide responds to the experiences and controls the behavior of many forms of life, from mammals like us, in which it is designated neuropeptide Y, to fruit flies, in which the comparable peptide is neuropeptide F.
There’s a lot of science about these neuropeptides. Let’s look at five studies—the first about fruit flies. Fruit flies are especially handy life forms for scientific study because researchers know how to manipulate their biochemistry. In work carried out at the University of Georgia, researchers altered the genes of the flies to control how much of the neuropeptide was available to the flies. They found that flies with low levels of the neuropeptide had a wooden leg—they could imbibe a great deal of ethanol without much effect. Flies with plenty of the neuropeptide got drunk on very little ethanol and couldn’t take much more than a “sip.” The Georgia results fit what came out of earlier research at the University of Washington: Mice with little of the neuropeptide also had a wooden leg and could keep on drinking while showing little effect. Mice with a surfeit of the neuropeptide, on the other hand, were stumbling after one mouse-sized drink.
Scientists in Sweden turned their attention to rats and how the amount of neuropeptides in a rat’s central nervous systems could be affected by early experiences. Rats that had been separated from their mothers for a few hours a day during the first weeks of life were known to show depressed behavior into adulthood, possibly because they were exposed to high levels of stress in infancy. Remarkably, this stress arising from “maternal deprivation” in the Swedish rats led to decreased levels of the analogous neuropeptide that had been studied in the mice in Washington State and the fruit flies in Georgia. Rats that had full access to motherly love had the usual high levels of the neuropeptide and were not unusually depressed.
These studies demonstrate that a stressful experience can change the lifelong levels of the neuropeptide known to affect an animal’s relationship to ethanol. These results were recently brought together in a study conducted at the University of California in San Francisco, in which male fruit flies were divided into two groups. One group (called rejected-isolated) was put in the presence of female flies who had already mated and who rejected the advances of these horny newcomers. In an interesting behavior, when these same horny flies were put in the presence of what the scientists called “multiple receptive virgin females,” they didn’t go for them. They apparently had learned to expect rejection. The other group of male flies who had not been subjected to rejection (the so-called mated group), when put in the presence of these “multiple receptive virgin females,” took advantage of the situation. As might be expected given the results of the research on the Swedish rats, the rejected-isolated flies had far lower levels of neuropeptide than the flies with access to the receptive virgin females did. And not surprisingly, given the results in Georgia and Washington State, the rejected-isolated flies showed that wooden leg and drank heavily when taken to a source of ethanol.
Apparently, the research on mice, rats and fruit flies has a great deal to do with us. In research at the University of Cincinnati, scientists found that men suffering from combat related post-traumatic stress disorder (PTSD) had lower concentrations of neuropeptide Y in their cerebrospinal fluid than a control group of veterans and civilians who had not experienced combat related stress. I read nothing about alcohol in that study published in 2009, but it wouldn’t be surprising to discover that veterans afflicted with PTSD are able to drink more and with less effect than men not suffering from the disorder.
The authors of the San Francisco fruit fly study pointed out at the end of their article that their work may provide a model for how social experiences affect biological reward systems in other species. That would include us.