Pup exposure differentially enhances foraging ability in primiparous and nulliparous rats

Abstract

The role of maternal experience (i.e., pregnancy and pup exposure) on rats' performance in a foraging task was assessed. Primiparous (P) and nulliparous (N) animals were either exposed to pups for 21 days (+) or received no pup exposure (−). Following habituation trials, all animals were tested in spatial and cued versions of the dry land maze (DLM) for three days (three trials per day). In the spatial DLM, the presence of pups decreased latencies in both N and P groups in Trial 5 and P+ rats exhibited shorter latencies to baited food wells than P− animals on Trial 6. In the subsequent probe trial, P+ animals spent significantly more time in proximity to the previously baited well than P− rats. Pups enhanced performance of both P+ and N+ groups in trial 6 of the cued test. Thus, in the spatial task, the individual components of the maternal experience (e.g., pregnancy, parturition, lactation, and pup exposure) converge to produce behavioral modifications in the DLM spatial and probe tasks that enable the female to care for her offspring, in this case, by enhancing foraging behavior. Further, in one trial of each version of the task, pup exposure enhanced performance in N animals suggesting that, in isolation, pup exposure may be a more important influence on ancillary maternal behavior than the pregnancy itself.

Introduction

The female rodent brain undergoes significant changes in response to the varying levels of gonadal hormones accompanying the different stages of the estrus cycle. Initially, these cytoarchitectural modifications were observed in the medial preoptic area (mPOA), a brain region traditionally viewed as an area susceptible to gonadal hormone-induced alterations [1], [2]. Subsequent investigations of the female brain, however, confirmed a relationship between the hippocampus, a brain area that is not typically viewed as playing an integral role in reproductive functioning, and fluctuating levels of gonadal steroids. Specifically, ovariectomized rats experience a reduction in the density of dendritic spines found in CA1 pyramidal neurons, an effect reversed by estrogen replacement [3]. This plasticity in the CA1 spines was shown to be cyclical in nature, evidenced by a 30% decrease in dendritic spine density as the animal progressed from proestrus to estrus (i.e., 24 h period) [4]. Lower levels of estrogen are also associated with decreased CA1 synapse density [5] and decreased multiple synapatic contacts in CA1 presynaptic boutons [6]. Functional changes such as altered electrophysiological recordings in CA1 and CA3 neurons [also accompanied by increased hippocampal brain derived neurotrophic factor (BDNF) immunoreactivity on the day of estrus in this study] [7], an estrogen dependent increase in sensitivity to NMDA receptor input [8], enhancement of LTP in the hippocampus [9], and higher phosphorylated proteins in CA1 dendrites [10] have also been reported. Finally, more new BrdU-labeled cells (likely neurons) have been found in the hippocampus of female rodents in proestrus than in diestrus [11].

Following the establishment of estrogen-induced neuroanatomical and neurophysiological changes, researchers continued their investigations to discern the behavioral relevance of these effects. Because of the hippocampus's confirmed role in learning/memory systems, rats in the various stages of the estrus cycle have been assessed in several types of learning tasks. Although the aforementioned neuroanatomical changes are dramatic, the behavioral data are less clear. Increased arm choice accuracy in the radial arm maze has been observed in rats receiving estradiol via silastic capsules for either 24 h or 30 days prior to maze training, compared to controls [12]. Further, chronic administration of estrogen enhanced performance in win-shift trials and in the radial arm maze following 21 days of hormone administration [13]. Conversely, when researchers have investigated learning abilities within the confines of the natural fluctuations of the gonadal hormones over the estrus cycle, different results have been reported. In one study, no learning impairment was observed in estrus animals (compared to proestrus) when tested in a Morris water maze [14]. In another study, estrus animals performed better than proestrus animals in the Morris water maze, whereas they performed more poorly than proestrus animals in a cue task adapted to the water maze [15]. Finally, water maze performance of meadow voles in male-induced constant behavioral estrus fluctuated in response to estradiol levels; specifically, high estradiol females exhibited longer latencies to the platform [16]. In sum, the only apparent consistency in these behavioral studies has been the inconsistencies associated with the findings.

It is likely that learning performance in rodents at various stages of the estrus cycle is sensitive to many task-related stimuli; for example, proestrus rats perform better in a water maze in warm water whereas estrus rats perform better in cold water [17]. Further, even within the same task, rats in different stages of estrus exhibit different learning strategies, with estrus rats showing a “place” strategy and proestrus rats preferring a “response strategy” [18]. Other experimental variations such as exogenous vs. endogenous sources of estrogen, the use of different species, different tasks (e.g., appetitive vs. escape), and varying durations of estrogen exposure contribute to these diverse findings.

It is also possible that the lack of conclusive findings in the behavioral studies previously described is due to the factor of timing of behavioral tests. When one considers the significant events that typically follow proestrus and estrus in the rat's natural environment (i.e., mate location and selection, copulation, pregnancy and parturition), it is possible that the increase in dendritic spines observed during proestrus may serve to foster the development of the hippocampus so that the female is optimally prepared to meet the demands of her pups [19]. Additionally, the surprising relationship between the hippocampus and the fluctuating levels of gonadal hormones becomes more relevant as the ancillary behaviors necessary for successful maternal behavior are considered (e.g., more efficient foraging, multi-tasking associated with the multiple demands of many pups, enhanced defense from environmental threats). Accordingly, it has been suggested that the hippocampus may alter its optimized computational functions in the various estrus cycles as the demands shift to assure both individual and species survival [20]. Indeed, mate selection, copulation, pregnancy, parturition, and lactation represent a continuum of adaptive neurobiological changes necessary for the proper development and maintenance of offspring [21].

If consideration of natural gonadal hormone fluctuations in the larger context of reproductive experience is important in discerning the accurate nature of hormonal influences in the female, \ consistent effects will likely be revealed when animals are tested following pregnancy, a time consisting of chronic and natural elevations of estrogen (and progesterone). Three decades ago, researchers reported that pregnancy resulted in greater cortical thickness, an effect also found in animals housed in enriched environments [22]. Although these animals were examined the day following parturition, an extended time with the novel sounds, smells, sights and tactile stimulation provided by the pups may have served as further enrichment for the maternal animals. Confirming this notion, it was demonstrated that multiparous and primiparous animals exposed to their pups from 14–21 days performed better in two variations of a spatial foraging task [23]. Additional research has shown enhanced social learning in postpartum female rats [24] and, focusing on alterations during pregnancy, enhanced spatial working memory has been observed during the first and second trimesters of the pregnancy, but not in the third trimester [25]. The number of pregnancies may also influence postpartum learning; research with sheep, for example, suggests that biparous ewes identify their lambs using visual and auditory information after 6 h of mother–young contact whereas primiparous ewes using visual and auditory cues required 24 h to demonstrate a preference for their own offspring [26]. Thus, neurobiological findings in maternal mammals are more consistent than research focusing on the aforementioned short-term hormonal fluctuations. The role of the putative enriched environment (consisting of novel sensory stimuli) provided by the offspring, as well as any learning enhancements provided by the pregnancy itself, may be significant variables involved in learning phenomena.

The present study examined the specific influences of two maternal-related variables (i.e., pregnancy and/or pup exposure) on the female rats' performance in two versions of a foraging task. Primiparous and nulliparous rats, either exposed to pups or no pup stimuli, were used in this study. We hypothesized that the animals naturally allowed to experience both pregnancy and pup exposure would exhibit advantages over animals with only partial involvement with maternal experience.