Effects of warmth on newborn rats' motor activity and oral responsiveness to an artificial nipple.

Temperature is a powerful regulator of the behavior and physiology of newborn altricial animals. The effects of warmth on newborn rats' oral responsiveness to suckling stimuli and spontaneous motor activity in a thermoneutral environment were investigated. Newborn rat pups' oral grasp responses to an artificial nipple and overall motor activity were recorded for 18 min. Near-term pups were delivered by cesarean section so that their 1st experiences with suckling stimuli could be observed. Experimental pups were warmed for 15 s every 2 min; control pups were not warmed. Warmed pups grasped the nipple fewer times than the not-warmed pups. However, oral grasp durations became longer for the warmed pups but not for the not-warmed pups. Warmth increased pups' motor activity but only while the heat was applied. Warmth in a thermoneutral environment may promote longer nipple attachment during newborns' early feeding experiences.

The first suckling episode in the rat: the role of endogenous activity at mu and kappa opioid receptors.

The present study examined the role of endogenous activity at mu and kappa opioid receptors in attachment to and ingestion of milk from a surrogate nipple in cesarean-delivered newborn rats prior to regular suckling experience. Selective opioid antagonist drugs were injected into the cisterna magna (IC administration) or lateral ventricles (ICV administration). Blockade of endogenous activity at mu opioid receptors by IC administration of the selective antagonist CTOP reduced attachment time and markedly increased disengagements from the nipple. CTOP also increased the intensity of suckling measured as milk intake per min attached to the nipple, when milk was available from the nipple in a free-access regime, and enhanced intake when milk was infused through an intraoral cannula aside from the suckling context. The ICV administration of the selective kappa antagonist nor-BNI considerably increased latency to grasp the surrogate nipple, while time on the nipple and milk intake were decreased. The presented data suggest that populations of mu and kappa receptor-containing neurons, differentiable by the route of antagonist administration, play an important role in initiation and maintenance of suckling behavior in the newborn rat during its first encounter with the nipple and milk. The kappa opioid system is predominantly involved in the initiation of the newborn's behavior directed toward the nipple providing milk. The role of the mu opioid system seems more complicated: it transforms initial oral grasp responses into sustained attachment to the nipple and maintains the intake of milk at a certain physiological level.

Milk delivery schedules and stomach preloading alter patterns of suckling behavior by newborn rats on a surrogate nipple.

Newborn rat pups tested before suckling experience attached to and ingested milk from the surrogate nipple. Time attached to the nipple and amount of milk ingested depended on the schedule of milk infusion through the nipple. More frequent milk infusions resulted in more frequent disengagements from the nipple during the test, less time attached to the nipple, and less body weight gain. The initial patterns of attachment behavior--continuous or intermittent--were reproduced later when rats were tested on the surrogate nipple. Preloading of the stomach with milk effectively altered both attachment and ingestion from the nipple, whereas preloading with the same amount of water had no effect on suckling behavior. The data suggest that newborn rats flexibly adjust their attachment behavior to peculiarities of milk delivery through the surrogate nipple and reproduce the initial attachment pattern when reexposed to the surrogate nipple.

Classical conditioning of responses to an artificial nipple in the rat fetus: mu and kappa opioid systems.

The rat fetus exhibits oral grasp responses when presented with an artificial nipple in utero. Preexposure to milk produces a transient decrease in oral grasp responses. The effect of milk on oral grasping is mediated by endogenous activity in brain opioid systems. Specifically, milk triggers mu activity in rostral brain regions and kappa opioid activity in caudal brain regions to decrease oral grasping of the artificial nipple. Reexposure to the artificial nipple after it has been paired with milk during a classical conditioning trial results in a conditioned reduction in oral grasping that is evident when fetuses are reexposed to the nipple. The classically conditioned decrease in oral grasping is mediated by mu opioid activity in rostral brain regions and kappa opioid activity in caudal brain regions. Endogenous opioid systems, activated by exposure to milk and the nipple, may regulate the processing of sensory information during suckling in the rat.

Developmental psychobiology: a look back on 30 years.

Developmental Psychobiology publishes papers from the disciplines of psychology, biology, neuroscience, and medicine that contribute to an understanding of behavioral development. Research focuses on development in the embryo/fetus, neonate, juvenile, and adult as well as multidisciplinary research that relates behavioral development to anatomy, physiology, biochemistry, genetics, and evolution. The journal represents a broad phylogenetic perspective on behavioral development by including studies on invertebrates, fish, birds, non-human primates, and humans. This paper presents an analysis of empirical articles published since its first issue in 1968. This analysis covers number of authors, sex of first author; evidence of grant support, and participation of investigators from outside the US. Additionally, the analysis includes the topic of research, level of analysis, and subject of experimental study. Over its 32-year history, Developmental Psychobiology has published papers on a wide range of topics representing a broad phylogenetic perspective with a continued focus on behavioral investigation. This analysis revealed trends such as an increase in the number of studies at a physiology/anatomy level, an increase in studies with human subjects, and increases in contributions from investigators outside of the United States as well as women.

An intrinsic source of behavioral regulation that influences discrete responses to cues important for the initiation of suckling.

The present experiment investigated the relationship between motor activity and oral grasping of an artificial nipple in newborn rats. Pups orally grasped the artificial nipple, and they performed more and longer oral grasps in the latter portion of the nipple presentation. Motor activity was cyclical, and this cyclicity was evident before and during presentation of the artificial nipple. The onset of an oral grasp response was preceded by a period of relatively low motor activity, and the termination of a grasp was followed by relatively high motor activity. The newborn rat pup's intrinsic oscillations in motor activity may regulate the expression of discrete responses to cues important for the initiation of suckling.

How the neonatal rat gets to the nipple. II. Changes during development.

In the course of postnatal development, the motor sequence executed by pups in order to attach to the dam's nipple undergoes extensive changes. During the 1st postnatal week, the pup performs a rotation along the longitudinal axis of its trunk to achieve a supine posture under the mother. The pup then crawls on the maternal ventrum while in the supine posture, searching for, finding, and attaching to a nipple. During the 2nd postnatal week, this sequence is modified and the pup first searches and establishes contact with a nipple before rotating to the supine posture. This sequence of movements is then truncated. By postnatal Day 11, pups may attach to a nipple while in a prone posture. Developmental changes in supination before attaching to the nipple are reminiscent of changes in righting during a similar period of development. These observations support the idea that both righting and postural adjustments involved in attachment to the nipple derive from common motor modules, with righting executed in the direction of gravity and rotation to the nipple executed against the force of gravity. The parallel structure of these behaviors is consistent with a common origin and similar control mechanisms for these distinct motor behaviors that are expressed early in postnatal development.

Sustained attachment to the nipple in the newborn rat depends on experience with the nipple, milk, and the expression of oral grasping.

Newborn rats showed mouthing, licking, and oral grasping when presented with a surrogate nipple. These responses changed after the pup expressed an oral grasp response and experienced milk at the nipple. Newborn pups that ingested milk from the surrogate nipple showed brief oral grasp responses and, when tested 1 hr later, showed sustained attachment to an empty surrogate nipple. Contact with the nipple, oral grasping of the nipple, and experience with milk altered subsequent behavioral responses to the nipple. Classical and instrumental conditioning may play a role in transforming brief oral grasp responses into longer oral grasp responses and sustained attachment to the nipple.

Blood pressure and heart rate in the ovine fetus: ontogenic changes and effects of fetal adrenalectomy.

Ontogenic changes in baseline and 24-h rhythms of fetal arterial blood pressure (FABP) and heart rate (FHR) and their regulation by the fetal adrenal were studied in 18 fetal sheep chronically instrumented at 109-114 days gestation (GA). In the long-term study, FABP and FHR were continuously recorded from 120 days GA to spontaneous term labor (>145 days GA) in five animals. Peak times (PT) and amplitudes (Amp) of cosinor analysis were compared at 120-126, 127-133, and 134-140 days GA. Consistent, significant linear increases in FABP and linear decreases in FHR were observed in all fetuses. Significant 24-h rhythms in FABP and FHR were observed during all the time windows. In the adrenalectomy study, to test the hypothesis that fetal cortisol plays a key role in cardiovascular maturation, fetal adrenals were removed in eight animals (ADX); sham fetal adrenalectomy was performed on five animals (Con). Cortisol (4 microgram/min) was infused intravenously in four ADX fetuses from day 7 postsurgery for 7 days (ADX+F). No significant changes in PT and Amp in FABP and FHR were observed. Plasma cortisol levels remained low in Con and ADX fetuses (<4.9 ng/ml). Cortisol infusion increased fetal plasma cortisol to 22.3 +/- 3.2 ng/ml (mean +/- SE) on day 13 in ADX+F fetuses. FABP increased in control and ADX+F but not ADX fetuses; FHR decreased in control and ADX but rose in ADX+F fetuses. These results suggest that, in chronically instrumented fetal sheep at late gestation, 1) increases in FABP and decreases in FHR are maintained consistently from 120 to 140 days GA, with distinct 24-h rhythms, the PT and Amp of which remain unchanged, and 2) the physiological increase in FABP is dependent on the fetal adrenal; bilateral removal of the fetal adrenals does not prevent the ability of cortisol to produce a sustained increase in FABP.

Thermal, olfactory, and tactile stimuli increase oral grasping of an artificial nipple by the newborn rat.

Caesarean-delivered rat pups tested before any suckling experience show oral grasp responses after stimulation with an artificial nipple. Manipulating the sensory stimuli present at the time of testing alters behavioral responses to the nipple. Specifically, when the nipple is warm, when pups are tested in the presence of amniotic fluid or milk odor, or when pups are tested in the presence of a conspecific, oral grasping of the artificial nipple is increased. Pups respond to the nipple with a shorter latency, show more oral grasp responses, and the individual grasp responses are longer in duration. The experiments suggest that the newborn rat pup exhibits a basic set of behaviors in response to the nipple early in development and that sensory stimuli normally present during the expression of suckling increase oral appetitive behaviors evoked by the nipple.

Variation in motor activity on different time scales and responsiveness to oral stimulation in the rat fetus.

The near-term rat fetus exhibits brief oral grasp responses to discrete presentations of an artificial nipple. In the present experiment, an artificial nipple was presented to individual fetal subjects 10 times. Five of the presentations were timed to occur when spontaneous fetal motor activity was low and five while activity was high, as determined by the baseline activity for the individual fetus. The likelihood of responding to the artificial nipple was increased when the fetus was relatively inactive at the moment of stimulus presentation. Furthermore, stimulus presentations that resulted in oral grasping of the artificial nipple were associated with greater point-to-point variability (2-s intervals) in motor activity during the 30-s period preceding the presentation of the artificial nipple. This pattern of results indicates that the recent history of general motor activity as well as the level of activity at the instant of stimulus presentation may contribute to variation in responding to biologically relevant stimuli early in development.

Endogenous opioids and the first suckling episode in the rat.

Endogenous activity at opioid receptors affects the appetitive behavior of Caesarean-delivered rat pups during presentation of a surrogate nipple that provides milk. Blockade of opioid receptors by peripheral injection of naloxone has no effect on responses evoked by the surrogate nipple. Similarly, blockade of caudal brain opioid receptors by injection of naloxone into the cisterna magna has no effect on the pup's behavior in response to the surrogate nipple. However, blockade of rostral opioid receptors by injection of naloxone into the cerebral ventricles increases the latency to the first oral grasp response, decreases total time on the nipple, and virtually eliminates ingestion of milk from the surrogate nipple (Experiment 1). Blockade of endogenous opioid activity does not affect responses to a nipple that provides distilled water (Experiment 2) or to an empty surrogate nipple (Experiment 3). These data indicate that during the initial suckling episode endogenous opioids in rostral brain regions affect the pup's behavioral responses to the nipple. The results are consistent with the hypothesis that milk engages opioid systems during the first suckling and that endogenous opioids play a role in early suckling.

The development of nuchal atonia associated with active (REM) sleep in fetal sheep: presence of recurrent fractal organization.

The behavioral state of active or rapid eye movement sleep (REMS) is dominant during fetal life and may play an important role in brain development. One marker of this state in fetal sheep is neck nuchal muscle atonia (NA). We observed burst within burst NA patterns suggestive of recurrent fractal organization in continuous 13 day in utero recordings of NA during the third trimester. Consistent with fractal renewal processes, the cumulative mean and standard deviation (SD) diverged over this time and the tail of NA distributions fit a stable Lévy law with exponents that remained invariant over the periods of development examined. The Hurst exponent, a measure of self-affine fractals, indicated that long-range correlations among NA intervals were present throughout development. A conserved complex fractal structure is apparent in NA which may help elucidate ambiguities in defining fetal states as well as some unique properties of fetal REMS.

How the neonatal rat gets to the nipple: common motor modules and their involvement in the expression of early motor behavior.

One-day-old rat pups adopt a supine posture before attaching to the mother's nipple. Body rotations performed to reach the nipple occur in a typical kinematic structure. First, the pup rotates along the longitudinal axis of the trunk and lies on its side. Next, the pup arches the trunk to achieve a U-shaped posture and then rapidly relaxes the trunk. A second cephalocaudal rotation follows at the peak of trunk relaxation as the pup achieves a supine posture. After reaching a supine posture, the pup crawls to a nipple by performing "stepping" movements on the mother's ventral surface. The kinematic structure of these movements is reminiscent of the structure of righting as seen in the newborn rat. Both righting and achieving a supine posture under the mother involve the expression of common motor modules. During righting the modules are executed in the direction of gravity, and when achieving a supine posture the modules are executed against the force of gravity. Simple motor behaviors expressed by the rat pup during early postnatal development may have common origins and common control mechanisms.

Endogenous mu opioid systems and suckling in the neonatal rat.

Localized injections of the mu antagonist CTOP into intracisternal (i.c.) or intracerebroventricular (i.c.v.) sites altered the behavior of 1-day-old rat pups during continuous exposure to an artificial nipple. Blockade of mu opioid receptors by i.c. injection decreased oral responsiveness to the nipple, while blockade of receptors by i.c.v. injection of CTOP increased oral responsiveness. The injection of CTOP into the i.c. site produced a transient reduction in body weight gain in pups suckling from their mother, while injection into the i.c.v. site had no effect. When cesarean-delivered pups were tested prior to suckling, injection of CTOP into the i.c. site increased latency of the first nipple attachment and decreased total time attached to a surrogate nipple providing milk. Injection of CTOP into the i.c.v. site decreased latency to the first nipple attachment. The results indicate that there is a caudal population of opioid receptors that is involved in the initiation of suckling behavior and a rostral population that plays a role in decreasing responsiveness at the nipple.

The newborn rat ingests fluids through a surrogate nipple: a new technique for the study of early suckling behavior.

In this report we describe an apparatus and procedure that permits a newborn rat pup to ingest test fluids including milk through a surrogate nipple. The surrogate nipple represents a new testing situation for the experimental study of sensory and neurochemical controls of suckling behavior immediately after birth.

Variability in general activity and the expression of complex behavior in the fetal rat (Rattus norvegicus).

Rat fetuses exhibit intrinsic fluctuations in general motor activity and respond to an artificial nipple (AN) with mouthing and oral grasping behavior. The present study examined the relation between the organization of general activity and the expression of these specific responses to an AN on Embryonic Day 21. In Experiment 1, continuous exposure to the AN resulted in nonspecific behavioral activation characterized by an increase in amplitude and high-frequency variability. In Experiment 2, increased amplitude and variability in general activity preceding discrete presentations of the AN resulted in more mouthing and oral grasping responses to the AN. These results suggest that presentation of the AN triggers behavioral reorganization in which the level and variability of overall activity may facilitate expression of well-defined action patterns.

Endogenous mu opioid systems and perioral responsiveness in the rat fetus.

Pharmacological manipulation of mu opioid receptors located in rostral and caudal parts of the brain produces distinctive changes in perioral responsiveness to nipple-like tactile stimulation in the E20 rat fetus. Blockade of caudal mu opioid receptors by intracistema magna (I.C.) injection of the selective mu antagonist drug CTOP reduces appetitive responses directed toward the artificial nipple. In contrast, blockade of mu opioid receptors in the rostral part of the brain by intracerebroventricular (I.C.V.) administration of CTOP increases fetal responsiveness to perioral cutaneous stimulation including oral capture and grasping of the artificial nipple. This pattern of the results suggests that there are at least two functionally different neuronal populations of mu opioid receptor-containing neurons that are involved in the regulation of the perioral responsiveness in the E20 rat fetus. The caudal part of this mu opioid system increases perioral responsiveness while the rostral part of the system decreases responsiveness to nipple-like perioral stimulation. These findings suggest the possibility that mu opioid systems may play a functional role in regulating neonatal behavior at the nipple.

Oral grasping of a surrogate nipple by the newborn rat.

Newborn rat pups exhibit oral appetitive behaviors when presented with an artificial nipple. These behaviors include mouthing and licking movements and expression of stereotyped oral grasp response. Caesarean-delivered pups show increased responding to the nipple over the first 5 h after birth that is independent of experience with the nipple. Mimicking maternal licking by stimulating the anogenital region of the newborn rat with a soft paintbrush increases response to the nipple. Pups tested after 24 hr of normal experience respond to the artificial nipple when tested immediately after separation from the mother. However, oral grasping of the nipple is more frequent in 1-day-old pups tested 3 or 5 hr after separation from the mother. Study of behavioral responses to the artificial nipple promises to provide information about sensory and neurochemical controls of the initial suckling episode.

Classical conditioning in the fetal rat with a long delay between presentation of CS and US.

A single paired presentation of the artificial nipple and milk results in classical conditioning of changes in perioral responsiveness in the E20 rat fetus. This classical conditioning is evidenced by a reduction in responding to perioral tactile stimulation. The results of Experiment 1 confirmed the specificity of milk as an unconditioned stimulus to support classical conditioning. Experiment 2 demonstrated that single-trial classical conditioning with the artificial nipple CS and milk US was possible with a delay of 30 s between nipple and milk presentations. Further, measurements of fetal motor behavior during the delay between CS and US presentations indicated that a single 15-s presentation of the artificial nipple increased movements of the mouth for 30 s after removal of the artificial nipple. Experiment 3 demonstrated that three exposures to the artificial nipple prolonged the expression of mouthing for up to 120 s and made possible single trial classical conditioning with a delay of 120 s between CS and US presentations. The capacity of the fetal CNS to maintain a "behavioral trace" for an ecologically important stimulus, such as the nipple, could have adaptive significance in the early development of motivated behavior.