Researchers have long recognized that learning when to suckle is one of the most important skills a mammalian infant can acquire to ensure its survival. But new research from Binghamton University of the State University of New York is showing that the skill has broader and more compelling implications for the everyday life and growth of infants.
The researchers have found that learning related to suckling is so powerful that newborn rats, which have never been with their mothers and will not normally suckle on an empty surrogate nipple, can be conditioned to do so if the researchers pair a novel scent only once with an infusion of a tiny amount of milk into the mouth. After this pairing, when the newborns again smell the novel scent, they latch onto the empty nipple and suckle four times longer than rat pups that didn't smell the scent in conjunction with an infusion of milk, find Norman Spear and his colleagues, who published their work this month in Behavioral Neuroscience (Vol. 114, No. 3).
Their findings show that there is a powerful learning mechanism behind suckling and, along with providing insight into suckling in infants--including human babies--the work may help illuminate basic learning, memory and reinforcement mechanisms in the brain.
"This work shows among other things that early conditioning is a natural part of infant feeding behavior," says Warren G. Hall, PhD, of Duke University, who studies the link between odor and suckling in human babies "and how powerful these kinds of mechanisms can be and how they operate in the everyday life of infants."
An 'incredibly strong' finding
In all, Spear and his colleagues conducted six conditioning experiments on rat pups younger than five hours old. The general paradigm was simple: The researchers waved a Q-tip scented with lemon over the newborns, followed immediately by five microliters of milk into the pups' mouths. An hour later, the researchers measured how persistently the pups sucked on a surrogate nipple when again exposed to the lemon scent.
The researchers used various controls to verify their findings, including conditions where the newborns were exposed to the milk only or the lemon scent only, or first milk, then lemon odor five minutes later.
Their main finding was that pups who were exposed to the pairing of lemon odor and milk--even just once--suckled on a surrogate nipple 80 percent of the 10-minute testing period while control pups suckled for a mere 20 percent of the time.
"This is an incredibly strong finding," says Spear, who conducted the experiments along with graduate student Sarah Cheslock and researchers Elena Varlinskaya, MD, PhD, and Evgeniy Petrov, MD, PhD.
In fact, pups learn the connection between the odor and milk so well and quickly that they are willing to suckle on a nipple full of saline--a taste that earlier studies find rat pups dislike.
In addition, the researchers found they could wait as long as one minute between the presentation of the lemon scent and the milk infusion and still condition the pups to suckle in response to the lemon scent. This finding is particularly interesting, says Spear, because other research finds that rat pups normally don't tolerate this kind of "trace interval" during conditioning experiments: Unless the two stimuli are paired in time, the pups fail to learn.
"Trace conditioning--having an interval between the stimuli--can be looked at as a way of challenging the memory system," says Spear. "And infants are usually really poor at that. [Our newborns] were tolerating traces up to 60 seconds. While 16-day-old rats have a hard time with even 10-second intervals in other experiments."
There are two possible explanations for this finding, says Spear. Either milk is such an important stimulus for newborn pups that their brains easily hold on to any information about it, or the pups have had so little sensory experience that there are no other experiences for learning to compete with. Spear suspects that although the primacy of sensory experience will be found important, it is critical that milk and its neurochemical consequences are such a powerful stimulus.
But it's not milk per se that seems to be critical to learning, he adds. Instead, it appears to be the sweet taste. Indeed, Spear and his colleagues got the same results if they used sugar and saccharine instead of milk. Neither sugar nor saccharine provides the same nutritional value as milk. In fact, saccharine provides no nutritional value, so it seems to be the sweet taste that drives the learning rather than the caloric consequences, says Spear.
This study adds to a growing body of research showing that infant suckling can be manipulated through conditioning, says Spear. In fact, Duke's Hall and his colleague Sally Coyle, PhD, have been working on a conditioning paradigm to help human infants transition from one type of formula to another, which often has to be done for medical reasons.
In conjunction with the company Appetek, Hall and Coyle have used "flavor dots"--discs of scents such as strawberry--to condition babies to take a bottle containing a new type of formula or a medicine, such as Pedialite, that babies tend to dislike. Similar to what Spear did with his rat newborns, the researchers placed a flavor dot on a bottle providing babies with formula they enjoy. After several pairings, they then put the flavor dot onto a bottle containing new formula or medicine.
"The babies double their intake of the new substance and take it like milk for the first few minutes," says Hall.
This conditioning paradigm is so powerful and robust that Spear hopes it will help illuminate some of the brain's most basic reinforcement and memory mechanisms. Other work in his lab shows that alcohol reinforces suckling just as strongly as milk--another surprising finding because adult rats normally won't drink alcohol unless they have been specially bred for it.
Because brain development in the infant rat is functionally less complex than that of an adult, he believes this may be a good model for understanding the reinforcement mechanisms of alcohol.
"With few neural synapses and neurochemical receptors to investigate, it might be a short-cut for delineating exactly what makes alcohol reinforcing to the brain," he says.
Spear also hopes that the powerful conditioning paradigm can be used to help him understand more about infantile amnesia--the inability for adults to retrieve memories from their infancy and early childhood. He's most interested in testing the idea that infants encode memories differently from adults. Therefore, as adults, the only way to access the memories would be to break that infant code. By studying learning capacity in newborns, Spear hopes to be able to learn more about what they're encoding and how they're encoding it.
This article is part of the Monitor's "Science Watch" series, which reports news from APA's journals.