The Test Results

FosB Mutant Development and Abnormalities:

FosB mutant homozygous females were born and developed as any normal mouse, but were ten percent smaller than wild type mice. When the fosB homozygous females were mated with fosB homozygous mutant males, the resulting pregnancies were normal and carried to term. However, twenty-four to fourty-eight hours after delivery, the mortality rate of the pups was in excess of fifty percent. The high occurrance of lethality could be attributed to either the mutant mothers, the mutant pups, or both. To isolate the cause of the high mutant pup mortality rate, heterozygous males were mated with homozygous females and vice versa. As a result, it was found that the number of surviving pups in any given pregnancy relied primarily upon the genotype of the mother, and was independent of father or pup genotype. This supports the idea that the survival relies heavily on the nurturing ability of the fosB mothers.

Physical Trait Analysis:

To isolate the defect in the mother that contributed to the death of her pups, a physical trait analysis was performed. The most obvious mechanism leading to pup mortality would be a postnatal lactation defect in the mammary glands. Comparing the mammary glands of the FosB mutant mothers with those of the wild-type mothers, no discernable differences were found. Initially, the mutant mothers failed to lactate postpartum. However, this was due to her failure to nurse the pups and not a physical defect in the mammary glands manifesting from the FosB mutation. In subsequent examinations, the reproductive tracts, hormonal status, and gross anatomy of the pituitary and hypothalmus were investigated. No obvious physical or anatomic abnormalities were found in the FosB mutants that would explain the postnatal pup lethality.

Behavioral Analysis:

After ruling out any physical abnormalities, an explanation concerning some sort of behavioral defect was sought; namely one considering postnatal care of the pups. After birth, it was immediately noted that mutant mothers failed to exhibit normal nurturing behavior: creating a nest, cleaning the pups, retrieving them to the nest, and crouching over them for warmth and nursing. The mutant mothers, rather, sat in a corner and ignored the pups that were scattered around the cage; failing to collect them or huddle over them to keep them warm.

Behavioral Testing:

To identify any behavioral defects, the mutant mothers were subjected to a series of tests.

TEST ONE:

The mothers (normal and mutant) were exposed to their pups for 20 minutes after birth before the cages were disturbed.

The normal mothers spent this time crouched over their pups while the mutant mothers did not warm, nurture, or pay attention to her pups.

TEST TWO:

Each mother was deprived of her pups for one hour and then three pups were returned and placed in three corners of the cage. Wild-type mothers retrieved their first pup in an average of twenty seconds and all three pups within four minutes. Only one mutant mother retrieved all her pups and only about half even retrieved one pup. On average, the retrieval of the pups ocurred 50 times slower than normal in the FosB mutant mothers. It was observed that the mothers did acknowledge the pups, but failed to care for them. However, the behavior of the mutants was improved after multiple pregnancies and observing the behavior of the wild-type mothers. Therefore, the nurturing instinct could be learned over time.

These tests were also performed on mice who were not pregnant. The results remained the same, establishing that the FosB response occurs out of the context of pregnancy. In more tests, it was also shown that the nurturing defect was not sex specific. The effect held for both male and female mutant mice. The fact that it was not sex specific suggests that the defect reflects a function of FosB in the brain circuitry in response to the presence of pups.

TEST THREE:

To further rule out cognitive defects, the mice were subjected to the hidden platform test in the Morris water maze. The mutants performed the same as wild-type mice. Mutant mice did not appear to suffer from any drastic cognitive or intellectual defects in relation to the performance of the wild-type mice.

TEST FOUR:

To eliminate the possibility of sensory defect, the olfactory senses were tested. Mutant mice were essentially indistinguishable in their ability to detect isovaleric acid and pentadecalactone. The hypothalmus was also ruled out as a source of the defect because all other processes regulated by the hypothalmus (adaptation to cold, male aggression, locomotor activity, eating, sexual behavior, etc.) were normal. This suggests that the FosB controls a region of the brain whose function is to control the nurturing response.

TEST FIVE:

The final test tries to explain the expression of FosB in the brain when the ability to nuture is controlled. The fact that there is a specific nuturing defect in the FosB mutant mice relates that FosB is controlled in the brain within a nuturing response region. To test the possibility that the newborn pups trigger the induction of FosB in the brain of the mother, the affect of pup presentation was examined. Sections of the brain were stained with Anti-FosB antibodies before, two, and six hours after the pups were presented to the mutant mothers. The results demonstrated that FosB was clearly visible in the preoptic area of the hypothalmus (POA) after a six hour exposure to the pups. From previous studies, showing that POA is critical for nurturing behavior, and that the disruption of the FosB leads to a nuturing defect, the studies suggest that the FosB may act with the POA neuron in the illustration of nuturing behavior. The analysis of the tests also showed positive results of FosB within several other brain regions including the main and accessory olfactory bulbs and pyriform cortex. Upon presentation of the pups, there was no further indication of FosB in the accessory or main olfactory bulbs. Previous experiments show that olfactory information is critical for nurturing. Therefore, FosB may also affect the integrating of many olfactory inputs that regulate the nuturing response. There were also slight detections of FosB within other regions of the brain, suggesting that FosB may also be affected by subtle environmental changes. The expression of FosB within so many regions of the brain suggests that it may also have other functions in the central nervous system. These results illustrated a great deal about the nature of FosB and its importance in the nurturing behavior of the mother mice. A discussion of these aspects can be found on the following page:

This page was created by Group 3

12/01/96