Overall the data demonstrate that preweanlings were generally faster to initially explore and were then subsequently more investigative of and interactive with a wide range of specific stimuli introduced into their environments than adults. In addition to this, preweanling behavioral responses to specific stimuli less impacted by the time of day and stimulus location as suggested by their equal response to stimuli in the least and most active parts of the daily cycle (first and third quartiles, respectively), and the smaller impact of placing stimuli in the center of the environment, a location commonly understood to be anxiety-provoking and hence actively avoided by adults. This result is consistent with our previous work showing that preweanlings can exhibit anxiety-like responses but are normally less anxious than adults (Smith and Morrell, 2007
). The impact of novelty on the interaction with a stimulus depended upon the stimulus that was introduced into the home cage, but it was generally more profound in the adult than in the younger rat such that preweanlings more quickly explored novel stimuli and interacted more with them. That said, the preweanlings do have adult-like responses to certain categories of stimuli suggesting that the maturation of the CNS systems regulating food- and predatory hunting-related behaviors occurs earlier than the regulation of systems underlying exploration of other stimuli (e.g., objects).
It can be concluded that, for the most part, the preweanling has an adult-like capacity to discriminate between closely related stimuli, for example among the pup-like stimuli, but then subsequently interacts with the stimuli in a quantitatively, and sometimes qualitatively, different manner from the mature adult. Together, these data suggest that preweanlings have a greater propensity to gather information about certain categories of specific stimuli in their environment compared with adults and that the experiences generated by this normal, increased interaction with stimuli may crucially impact and shape the developing CNS in this period of life.
We and others have suggested that the well-documented phenomena of increased exploration of the environment by the young rat during the peri-weaning period may be based on having a higher level of motivation for environmental exploration than adults (Gerrish and Alberts, 1997
, Goodwin and Yacko, 2004
; Smith and Morrell, 2007
). Based on our current data, we posit that this enhanced motivation for environmental exploration in the preweanling extends to many specific stimuli within the environment. Alternatively, preweanlings might not have memory abilities as keen as those of adults, may lack the ability to form categorical systems for complex stimuli resulting in increased stimulus investigation because they don’t remember them, or they may have less sensory capacity than adults thereby requiring more interaction to learn about a stimulus. However, these interpretations seem unlikely based on studies examining memory function (Roth and Sullivan, 2001
) and sensory capacity (Bolles and Woods, 1964
; Moorcroft et al., 1971
) in developing rodents. We further posit that the greater responsivity of preweanlings to specific stimuli in their environment may in part facilitate the entrainment or establishment of neuronal networks so that preweanlings acquire the adaptive neural connections and response patterns of adults.
The rapid and extensive responses of preweanlings were particularly notable with regard to the larger objects and those objects with interactive properties. After more rapid contact, preweanlings then spent substantially more time in contact with these objects than the adults. While such activity may be a form of object play (Fagen, 1981
), this specific type of play behavior has not been characterized in developing rodents. In contrast, adults investigated larger and more interactive objects with greater caution. These data extend and are consistent with prior reports showing that adolescent rats interact with objects more than adults (Douglas et al., 2003
; Laviola et al., 2004
; Stansfield et al., 2004
Our tests using three types of objects (small, subject size, and crawl ball) in the novel versus familiar state did not demonstrate any discrimination of novelty, but our testing procedures were different from typical novel object recognition memory test procedures in that those tests are much shorter and present both novel and familiar objects at the same time (Berlyne, 1950
; Besheer and Bevins, 2000
; Ennaceur and Delacour, 1988
). It is possible that for these data, the averaging of exploration of the objects over the full thirty minutes may have obscured any initial response differences, and that the presentation of a single object either in novel or familiar form, as opposed to a test situation where subjects must discriminate between novel and familiar objects, does not facilitate the distinction between object novelty and familiarity.
Furthermore, subjects may have become sufficiently familiar with these specific objects after having been housed with them for 24 hours prior to testing and subsequently found that its interactive properties no longer held their interest thereby resulting in decreased interactions during test. Our tests using the jingle ball and running wheel did however demonstrate marked differences in responses of subjects to their novel versus familiar states demonstrating that both preweanlings and adults can discriminate between novelty and familiarity.
Our findings on the adult responses to the wheel are consistent with Barnett’s finding from presenting wild rats with an immense novel object, a food bin, which subjects also initially avoided, a reaction he termed neophobia; this avoidance was overcome as the rats had to interact with the bin as their only food source(Barnett, 1958
). Like Barnett’s food bin, the wheel is a stimulus with inherent incentive value for rats. In fact we chose the wheel as the immense object in our protocol because others have demonstrated that wheel running is a robust voluntary activity of adult rodents and that this object has positive incentive salience (i.e., interpreted as a rewarding property) revealed once they are familiar with wheel running activity or its after-effects (Lett et al., 2000
; Werme et al., 2000
). Further, our previous work has shown that p18 preweanlings can engage in wheel running at levels similar to the adult (Smith and Morrell, 2007
) suggesting that wheel running may also have the same positive incentive salience for them.
Newborn pups as stimuli revealed considerable differences between preweanling and adult responses to specific stimuli. While both groups discriminated all three of the stimuli in this category and rapidly approached a live pup, adults actively avoided the pup after a brief period of exploration while the preweanlings spent the majority of the test period with it. Non-maternal adult rats are known to avoid pups for days (Cosnier and Couturier, 1966
; Mayer and Rosenblatt, 1979
; Rosenblatt, 1967
). Our data from both groups on the total pup contact time are consistent with previous reports of pup interaction during the initial phases of maternal sensitization (Bridges et al., 1974
; Kalinichev et al., 2000
; Mayer and Rosenblatt, 1979
; Olazabal et al., 2004
; Olazabal and Morrell, 2005
). These data also provide critical details about how preweanling and adult rats initially approach and investigate newborn pups. The rapid approach by adults to the live and postmortem pup stimulus is consistent with previous reports showing that adult rats readily approach and investigate any conspecific that enters the home territory (Barnett, 1958
; Miczek and de Boer, 2005
Alternatively, given the predatory hunting response of adults to crickets, and since infanticide and pup eating is found among non-maternal rodents (Numan and Insel, 2003
) it might be that rat pups held potential as interesting food in their environment. The level of preweanling interaction with pups during the initial approach and investigation further suggests that, like adults, preweanlings will approach unfamiliar conspecifics in the home environment, but the subsequent interaction with live pups, the delayed postmortem pup interaction, and the absolute lack of infanticide by rat juveniles, this behavioral spectrum is qualitatively different from that of adults.
By also testing responses to a warm but dead pup and a warm pup-sized object, we found that heat was not an important factor for interaction with the pup. These data are consistent with the work of others showing that juveniles and adults spend significantly more time in contact with live pups than rubber pup-sized objects (Gray and Chesley, 1984
), marbles (Sheehan et al., 2000
), or dead pups (Mayer and Rosenblatt, 1979
) and extend their findings by controlling for the thermal properties and characterizing the initial response.
Preweanlings and adults also had different initial interactions with another live stimulus, a cricket. This was a rare case in which preweanlings took longer to approach and contact the stimulus than did adults, but then still followed their general pattern and investigated it more than the adults. It is possible that the preweanling is simply less attentive to small new stimuli placed in their cage, as seen with the small inanimate objects, whereas the adults rapidly attended and responded to an intrusion into their home territory. Responses based on innate fear responses seem unlikely as the young did not freeze and remained very active when exposed to the cricket. However, once the initial investigation was completed, the subsequent response to this stimulus exhibited by some of the preweanlings was surprisingly adult-like as both young and adult subjects engaged in predatory hunting. This finding suggests that the animation provided by crickets may serve to focus the attention of the preweanling to therefore respond with a mature innate hunting response. Even though the preweanlings that engaged in hunting took longer to catch and kill the cricket and spent more time eating it, the overall sequence of behaviors was considerably organized and executed in rapid succession similar to that seen in the adults. Adults in this study engaged in hunting behavior identical to what has been documented (Comoli et al., 2003
; Ivanco et al., 1996
; Kinsley et al., 2006
We posit that preweanlings spent more time eating the cricket than the adults simply because they ate the entire insect. Since preweanlings were presented with nymphs, which are half the size of adult crickets, it is unlikely that the increased consumption time was attributed to relative subject-to-cricket size differences. Others have reported that adult rats will only eat the torso of the cricket and will avoid the head and rear legs, possibly due to the spiny protrusions on the legs (Ivanco et al., 1996
). Our results are consistent with another study showing that young marsupials, specifically adolescent opossums, readily hunt crickets but that their behavior is slightly less developed than the adult (Ivanco et al., 1996
Preweanlings also responded relatively maturely to novel and familiar foods. Both groups ate significantly less novel food than familiar food, investigated the novel food less, and spent less time in contact with the novel food than the familiar one. The pattern of interactions that adults exhibited is consistent with what has been described in the literature as food neophobia (Barnett, 1958
; Chitty and Shorten, 1946
; Weinberg et al., 1978
). Although food neophobia has not been characterized in preweanling rodents, the adult-like patterns of interaction strongly suggest that the behavior of these young animals can be considered neophobic even though they interacted with the novel food slightly more than the adults did. This avoidance of novel foods during postnatal development may serve a protective function.
It is unlikely that our behavioral results, specifically that both preweanlings and adults ate more familiar food than novel food, were affected by extreme hunger or by food palatability. Our previous work (Smith and Morrell, 2003
) and that of others (Countryman et al., 2005
) showed that food deprivation is necessary to invoke the approach and consumption of even a familiar food. We deliberately avoided prolonged food deprivation because this causes severe hunger that overrides the normally avoidant responses of the animals. In response to extreme levels of food deprivation, rats will readily eat novel food (Burns et al., 1996
; Smith and Morrell, 2003
). Subjects in this experiment sampled only the novel food and were given a sufficient amount of both foods during the test to ensure that consumption of the novel food was not due to a need to relieve hunger simply because all of the familiar food had already been eaten. We presented subjects with two equally palatable and nutritious foods because previous studies have shown that foods with higher fat (Hansson, 1973
) and/or sugar content (Bhardwaj and Khan, 1978
; Khan, 1974
) are more palatable to rats and other rodents. Further, our previous work has shown that preweanlings and adults respond similarly by initially avoiding a food containing a high sugar content that is highly palatable to rodents (i.e., Fruit Loops) following modest food deprivation (Smith and Morrell, 2003
). Thus the limited sampling of the novel food could not be attributed to decreased palatability or nutritional content.
Taken together, our data show that preweanlings respond to most specific stimuli more quickly and with more interaction than adults, while a more adult-like behavioral pattern is observed when preweanlings are presented with a few select stimuli such as food and in the prey-catching phase of cricket interaction. Certainly, the greatest differences in stimulus-specific exploration between preweanlings and adults were revealed with the pup-like category of stimuli. These data further suggest that stimulus-directed exploration may have different developmental trajectories that are stimulus-specific. For example in the case with the newborn pup stimulus previous reports have shown that pup avoidance emerges by p23 and reaches an adult level before the onset of puberty during early adolescence (p30) (Mayer and Rosenblatt, 1979
). Other stimuli that elicited considerable differences in responses between young rats and adults were large interactive objects, particularly an object the same size as the subject, the crawl ball, and an immense object, the running wheel. Stimulus novelty and familiarity with the immense object further revealed age-related behavioral differences.
For the most part, these data demonstrate that young rats explore specific stimuli in the environment in a fundamentally different way than adults during what are normally the days of their first independent forays from the maternally-protected nest. Overall, the preweanlings were less inhibited in their interaction with stimuli. These experiences may place the developing rodent at greater risk for danger but they also significantly shape the developing CNS such that preweanlings acquire the adaptive neuronal connections and response patterns of adults. It is also the case that the greater risk may be mitigated by maternal presence, as the young often accompany the female in her foraging. The greater sensitivity of preweanlings to some specific stimuli in their environment is important to consider when using stimulus-specific exploration as a tool to assess normal and pathological behaviors in preclinical models of mental health disorders during postnatal development, such as autism and obsessive-compulsive disorder, of which a fundamental characteristic is dysfunctional stimulus-specific interaction.
The fact that most stimulus interactions are quantitatively more robust in the preweanling suggests that CNS systems regulating these relatively refined interactions with the environment may be crucially different at p18 versus p60. Some of these interactions are so extensive that what is in fact a normal range of interactions in the preweanling might be misunderstood as perseverative if considered solely by standards of adult interactions with specific stimuli. For the human condition, our data extends the concept that interventions designed for the adult CNS could be inappropriate for the developing CNS of the young diagnosed with disorders characterized by inappropriate stimulus interactions. In future studies, these data will provide essential points of comparison for determining what constitutes aberrant behavior in transgenic or knockout models of physical and mental diseases such that more effective pharmacologic and behavioral interventions can be designed for humans.