The effects of prenatal stress on motivation in the rat pup.

Exposure to prenatal stress (PNS) has been shown to induce a set of psychological and behavioral changes in developing offspring. We used the rodent model to investigate whether PNS produces changes in the ability of the pup to express social motivation. We used a set of behavioral tasks including monitoring ultrasonic vocalizations after isolation, a conditioned place preference, and a novel and familiar odor approach test. Pregnant Long-Evans rats were exposed to an unpredictable, variable stressor twice daily during the third week of gestation. Isolation vocalizations were assessed on postnatal day (PND) 10. Pup affinity for the dam was evaluated on PND 15. Typically, pups display a selective preference for an odor-paired environment only after the odor has been associated with the dam. This previous association produces a positive conditioned stimulus (CS). Normally, pups exposed to a neutral CS (odor paired with cotton balls) do not form this place preference. Results indicate that PNS exposed pups had significantly increased distress vocalizations and an equal preference for the positive and neutral conditioned stimuli. This type of alteration in forming early preferences could be detrimental because of decreases in the specificity of social learning and an impaired responsiveness in social relationships.

Imaging gene expression in live transgenic mice after providing luciferin in drinking water.

Mice expressing the firefly luciferase gene luc under the control of various gene promoters are used to image long-term changes in tumor growth, infection, development, and circadian rhythms. This novel approach enables ongoing regulation of gene expression to be visualized through repeated imaging of luciferase bioluminescence. Typically, luciferin, the luciferase substrate, is injected into mice before they are anaesthetized for imaging. To avoid the effects of handling and stress from injection on expression of the transgene, oral luciferin delivery methods were tested as an alternative to current methods. For unobscured imaging, a transgenic mouse line containing luc controlled by the enhancer and promoter for the major immediate-early gene of human cytomegalovirus (CMV) was crossed with a hairless albino mouse stock (HRS/J), resulting in the Hr-CMV line. Mice given food and water ad libitum readily drank 1-5 mM luciferin in water or apple juice and could be imaged repeatedly on subsequent days without any apparent adverse effects. Oral and injected luciferin produced similar patterns of luminescence in the body areas examined: abdomen, tail vertebrae, gonads, hind leg, foreleg and others, although the tail showed a slightly brighter relative luminescence after oral luciferin. These results show that luciferin is not appreciably degraded in the digestive tract and can be easily administered orally to avoid injection and any concomitant effects on behavior that could alter gene expression.