For the first weeks of its life all of the nutritional elements the infant needs are supplied by its mother via lactation and nursing.

Lactation involves a host of chemical changes in the rat. During lactation the rat’s pituitary gland produces less adrenal secretions.
The actual stimulation of suckling in turn stimulates the pituitary gland to release growth hormones.
Suckling also stimulates prolactin release by decreasing intra-mammary pressure, thus causing continued milk production (although not indefinitely).

If nursing is stopped, as in the loss of the offspring or weaning of the offspring, the mammary glands will fill up.
This increases intra-mammary pressure which, in turn, causes mammary blood flow to decrease. This triggers the body to decrease the lactation hormones and lactation will end.

Although it is not recommended, it is actually possible for a doe, when presented with continuous foster litters, to sustain lactation for up to a year.

Lactation Hormones

Two major hormones, oxytocin and prolactin, are the most directly related to lactation in the rat.

• Oxytocin fulfills two roles in the pregnant rat. It induces uterine contractions at birth and mediates milk ejection during lactation.
• Prolactin is a hormone that is secreted and regulated by the pituitary gland. Its function is to initiate and sustain lactation in the female rat.

  Prolactin release begins in mid- to late pregnancy in conjunction with the development of the mammary glands. Release continues throughout the maintaining of lactation (galactopoiesis) and declines as the offspring begin to wean.

Colostrum

The initial secretion from the mammary gland is colostrum. The colostrum is lower in volume and higher in nutrients than milk and offers the newborn rat antibodies.

While in the uterus the fetal rat is sheltered from outside pathogens and produces no antibodies. The mother, however, is exposed to antigens and develops maternal immunoglobulin antibodies which are then passed to her offspring via the colostrum. These important maternal antibodies protect the infant rat for the first several weeks of life.

Normal intestinal digestion would render these inactive. In the newborn rat there is limited intestinal absorption for the first 20 days which allows for absorption of IgG.

Milk

Within a few days the colostrum thins and is replaced by milk.
At day 5 the average milk consumption of an individual pup rat is 5 mL with an increase of 0.5 mL per day. By day 18 the increase is 0.6 mL per day.

Milk Composition

• Water:69.3%
• Protein: 11.3%
• Protein/Fat: 0.8%
• Lactose: 2.9%
• Ash: 1.5%
• Total Milk Solids: 31.7%

Water
At 70%, water is the main component of milk. This keeps the neonate rat hydrated and ensures the other components flow smoothly.

Carbohydrates
The main carbohydrate found in milk is lactose, which is a disaccharide. Lactose is unique to milk and is an easily digestible source of glucose.
Glucose provides energy to the neonate. Other forms of carbohydrates are found in the milk at much lower levels. These can include different types of sugars, some of which are thought to be a key to developing healthy gut flora in the intestines of the pup rat.

Milk Fat
Milk contains lipids. At birth the neonate has very little body fat. It converts the lipids in the milk to body fat (adipose), thus providing itself with insulation and protection from environmental temperature drops. It then uses the milk fat as an energy source.

Milk Protein
Caseins (milk proteins) are composed of different amino acids and are responsible for much of the growth and development of the infant rat.

Minerals (Ash)
Calcium and phosphorus are the major minerals found in the milk of the mother rat.
These minerals are required for growth and development of bones and teeth.

Other Components
Milk contains all the major vitamins needed for the neonate as well as enzymes, cellular proteins, and growth factors.

References

1. Iwasaka, T., Umemura, S., Kakimoto, K., Koizumi, H., & Osamura, Y. (2000). Expression of prolactin mRNA in rat mammary gland during pregnancy and lactation. J Histochem Cytochem, 48(3), 389-96. Retrieved December 18, 2008, from the Medline database.
2. Kanno, T., Koyanagi, N., Katoku, Y., Yonekubo, A., Yajima, T., Kuwata, T., Kitagawa, H., & Harada , E. (1997). Simplified preparation of a refined milk formula comparable to rat’s milk: influence of the formula on development of the gut and brain in artificially reared rat pups. J Pediatr Gastroenterol Nutr, 24(3), 242-52. Retrieved December 18, 2008, from the Medline database.
3. Mustafa, A. (n.d.). Dairy Cattle Production (342-450A): An Overview of Milk. Retrieved December 18, 2008, from http://animsci.agrenv.mcgill.ca/courses/450/lectures/2/2.PDF.
4. Nutrition, N. R. C. (US) S. on L. A. (1995). Nutrient Requirements of the Laboratory Rat. In www.ncbi.nlm.nih.gov. National Academies Press (US). Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK231925/