Rattus Biologicus:
Oxytocin - A Hormone of Affiliation

It is well known that rats are social animals. Rats express several types of affiliative behaviors, including those associated with reproduction, grooming and parenting. The word "affiliation" is derived from the Latin affiliare (to adopt), and includes different forms of social behavior which involve bringing conspecifics into close proximity for the formation of a social bond. Different hormonal milieus are responsible for the onset and maintenance of these actions, but all of them are known to be controlled (at least in part) by the hormone oxytocin.

Oxytocin is familiar to most of us as a hormone of pregnancy. Oxytocin levels are elevated in the bloodstream of mammals at the time of birth. During labor in humans, it is often administered in the form of Pitocin in order to facilitate the expulsion of the fetus. This is by far its most familiar effect in all mammals -- to increase the contractility of the uterus. The uterus is a smooth muscle, and oxytocin causes contraction of other smooth muscles in the mammary glands, in order to elicit the milk let-down reflex in response to suckling.

Have you ever wondered at the transformation of the female rat into a loving and caring mother after giving birth? The very same hormone that facilitates labor in the mother rat -- oxytocin -- is also responsible for the parenting behavior that follows parturition. In addition to acting at peripheral locations (i.e., uterus, mammary glands), oxytocin acts in the brain to mediate behaviors that ensure reproductive success.

Oxytocin is a peptide hormone that is actually produced in two discrete groups of neurons in the brain of all mammals. One group of oxytocin-producing neurons projects to the posterior pituitary, which is an endocrine gland located at the base of the brain. From the pituitary, oxytocin is released into the bloodstream, whereby it exerts the well-known peripheral effects like uterine contraction and milk let-down. The other group of oxytocin-containing neurons projects directly to specific brain areas that are known to mediate maternal behaviors. By acting locally as a chemical messenger in these brain areas, oxytocin acts as a regulator or controller of maternal behaviors.

Maternal behavior, i.e. caretaking, serves to protect, warm, feed and clean newborn pups until they are able to perform these tasks themselves. This behavior is sharply defined and begins immediately after parturition in the rat. In response to her young, a female nurses by crouching over her pups and making her nipples available. She will lick her pups to elicit elimination, and retrieve strayed pups by carrying them in her mouth back to the nest. When presented with nest material, maternally responsive females respond by building a nest. Finally, a mother rat will attack and drive away intruders when caring for her young. In contrast, a nonpregnant female will immediately withdraw when presented with pups, or even cannibalize them. Moreover, females would rather pick up food pellets vs. pups prior to parturition, but show a clear preference for the pups after giving birth. However, if oxytocin is administered to a virgin rat, she will begin to exhibit maternal behaviors towards young rats.

It is likely that a rise in oxytocin and other hormones of pregnancy during gestation and parturition are responsible for the "switching on" of maternal behaviors. During pregnancy, the ovary produces elevated levels of estrogen. It is believed that estrogen serves to "prime" the brain for oxytocin binding, actually making oxytocin receptors (=binding sites) more plentiful during this time. So, even though oxytocin is always present within the brain, the increase in oxytocin receptor binding just prior to and at parturition is what triggers the onset of maternal behaviors in the rat.

Oxytocin may also facilitate bonding of young to their mother by promoting pup attachment to the nipple. Oxytocin is very concentrated in the milk of lactating rats. Washing the belly of a rat mother removes an important olfactory cue for pup attachment. Topical oxytocin administration on the mother has been reported to re-instate nipple attachment within minutes, even in the absence of milk ejection. It is known that more receptors for oxytocin are present in the fetal rat brain compared to the adult rat brain. Perhaps oxytocin from the mother is triggering affiliative behavior in pups by binding to olfactory areas of the brain, thus setting off a signaling cascade to stimulate feeding.

Oxytocin is associated with reproduction not only in female rats, but also in males. It has been shown that oxytocin administration improves the copulatory performance in rats by shortening the latency between mounting of the female and ejaculation of sperm. In addition, chronically administered oxytocin increases nonsexual social interactions in both male and female rats. In rats that are administered oxytocin over long periods of time, the duration of physical contact with members of the opposite sex is significantly increased Moreover, grooming and genital sniffing of females by male rats increases. It appears that oxytocin may play a role in making our pet rats more friendly!

So, the next time you are lucky enough to observe your rat go through the activities of motherhood, think about the incredible interplay of hormones that enable her to successfully raise her young. When it comes to survival of a species, we humans do not differ much from rats in terms of hormonal control of reproduction and parental care. Perhaps oxytocin may even play role in the bonding of us rat owners with our long-tailed friends!