In rats the reward state of the female is unique immediately postpartum; she chooses pups over cocaine. We seek to determine if the neuroendocrine basis of this unique reward state has a biological grounding in which motivation for drug reward is less, and therefore of particular interest for the human condition. We also seek to understand the neural substrate on which the neuroendocrine factors act to alter the reward state of the early postpartum female rat. We use a place preference paradigm that trains the rats to associate particular environmental cues with two unconditioned stimuli, pups and cocaine, and then tests their preference. We will expand this paradigm by systematically testing two variables, the strength of the drug stimulus, and the age of the pup stimulus (Aim 1). We hypothesize that the neuroendocrine status of the female is the basis for the genesis of the unique reward state of the early postpartum period.
In Aim 2, we will test this by applying well-established models of maternal behavior that vary the basis of the behavior. We will (1) use pup-induced maternal behavior in virgin female rats to determine whether maternal status alone (the behavioral state) is sufficient to generate the specialized reward state. Then we will (2) add estrogen, (3) determine whether the hormones of pregnancy alone, in the absence of the hormones and experience of parturition and the postpartum period (maternal pregnant rat hysterectomized-ovariectomized on day 20 of pregnancy), are sufficient, and (4) determine when and if the specialized reward state occurs in pregnancy.
In Aim 3, we will use this unique behavioral model to examine the neural substrate of these reward processes. The neural substrate that is engaged during this specialized reward state of the female will be identified using immunocytochemistry for three dependent variables, c-Fos to determine general engagement/activation of neurons, Homer 1a to identify glutamatergic-related activation, and CART peptide to determine pharmacologic aspects of the activation.
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