Scott R. Robinson

Kappa opioid mediation of fetal responses to milk.

A series of experiments provided evidence for the existence of a functional opioid system in the fetal rat near term. Application of a tactile probe to the perioral region of the fetus consistently evoked a stereotypic facial wiping response. Administration of low dosages of morphine to the fetus had little effect on nonevoked motor activity but reduced fetal responsiveness to cutaneous stimulation. Milk infused into the mouth of the fetus reduced fetal responsiveness to the tactile probe. Milks effect on cutaneous responsiveness was reversed by injection of the nonspecific opioid antagonist naloxone. The effect of milk on fetal responsiveness to cutaneous stimulation was reversed by the kappa opioid antagonist nor-binaltorphimine, but not by the mu antagonist beta-funaltrexamine. Milk engages the endogenous opioid system of the fetal rat and affects fetal responsiveness by interacting with the kappa receptors of the opioid system.

Prenatal experience with milk: Fetal behavior and endogenous opioid systems

The existence of organized responses to milk in newborn mammals, which lack experience at the nipple, implies the prenatal development of neural and behavioral systems for recognizing, obtaining, and processing milk. Many components of milk-directed behavior have been identified in the fetus. The stretch response expressed by neonatal rats during milk ejection at the nipple can be elicited before birth by infusing milk into the mouth of the fetus. Milk promotes reorganization of fetal motor behavior, facilitates expression of the stretch response, and alters fetal responsiveness to cutaneous stimulation. Pretreatment of fetuses with opioid agonists and antagonists has confirmed involvement of the mu and kappa opioid systems in mediating the effects of milk. Opioids appear to play a dual role in milk-oriented behavior: Initially, opioids suppress behavioral responses of the fetus and neonate to novelty, permitting ingestion of milk, and secondarily, opioid activity can promote learning at the nipple by functioning as a reinforcer. Study of milk-directed behavior in the fetus may promote better understanding of the special needs of preterm human infants.

Rat fetuses respond to chemical stimuli in gas phase

Fetuses exhibit behavioral responses to intraoral infusions of chemical solutions presented in liquid phase, including an increase in overall activity and the production of a species-typical action pattern, facial wiping. A series of experiments were conducted in which chemical compounds with strong olfactory characteristics--citral and cyclohexanone--were presented in gas phase to rat fetuses on day 20 of gestation. Fetuses exhibited similar behavioral responses to stimuli presented in liquid and gas phases, suggesting a common basis for the processing of prenatal chemosensory information. Chemosensory stimuli in gas phase are sufficient to elicit both behavioral activation and a stereotypic action pattern of the rat fetus, patterns that anticipate postnatal olfactory-directed behavior. These results suggest that prenatal chemosensory experience, which normally occurs in an aqueous medium, may be relevant to the development of postnatal behavior that is directed by airborne olfactory cues.

Olfactory bulb transection alters fetal behavior after chemosensory but not tactile stimulation

Rat fetuses respond to an intraoral infusion of lemon extract with an increase in overall activity and facial-wiping behavior. Other studies have suggested a role for olfaction in mediating fetal responses to chemosensory stimuli. In the present study, a micro-knife was used to surgically isolate the main and accessory olfactory bulbs from more caudal structures in the fetal brain. Fetuses that received this transection procedure or a sham treatment showed normal levels of non-evoked motor activity during the period prior to chemosensory infusion. Surgical isolation of the olfactory bulbs had no effect on fetal response to perioral tactile stimulation. Behavioral responses to infusion were diminished but not eliminated in fetuses with olfactory bulb transections. The olfactory bulb, which is functional in spite of its anatomical immaturity, plays a role in the control of fetal behavior.

Milk as the proximal mechanism for behavioral change in the newborn.

Early experience in the context of suckling at the nipple is a crucial determinant of neurobehavioral development in mammals. In particular, milk has been recognized by developmental psychobiologists as an ecologically relevant sensory stimulus and should be viewed by clinicians as more than a source of nutrition in early human development. Gaining access to and processing milk during suckling serves as a primary focus of activity in the newborn. Because the rat fetus lacks prior exposure to milk or other suckling stimuli, it provides an excellent model system for investigating the development of milk related responses. Fetal exposure to milk results in a cascade of behavioral, physiological and neurochemical consequences. Milk‐induced activation of the endogenous opioid system plays an important role in reorganizing fetal motor activity, altering sensory responsiveness and supporting associative learning in the fetus. Subtle changes in contextual stimulation alter the ability of milk to engage different neurochemical systems. These findings suggest that the infants first experiences with milk and other suckling stimuli may have lasting consequences for neurobehavioral development.

Fetal behavior and the dopamine system: Activity effects of D1 and D2 receptor manipulations

Binding studies have indicated that D1 and D2 dopamine receptor subtypes are present in rats before birth, but it is not known whether these receptors are functional during the prenatal period. In the present study, day-21 rat fetuses were prepared for direct observation after pharmacological manipulation of D1 and/or D2 receptors. The D1 agonist SK&F38393 induced a marked increase in fetal activity (i.e., forelimb, rearlimb, and head movements) while the D2 agonist quinpirole produced a slight suppression in activity. Coadministration of both agonists resulted in low levels of fetal activity, suggesting an interaction between D1 and D2 receptors. Administration of the D2 antagonist sulpiride resulted in little change in fetal behavior but was effective in blocking the behavioral activation induced by the D1 agonist. The D1 antagonist SCH23390, administered alone or in combination with the D2 antagonist, produced a modest increase in fetal activity that included mouthing and facial wiping behavior. These data provide evidence that the dopamine system is functional and capable of mediating behavioral effects in the near-term rat fetus. Further, manipulation of dopamine receptors results in a different pattern of behavioral effects than has been reported in older animals. The observation that fetal behavior is influenced by these pharmacological challenges suggests that drugs of abuse known to affect the dopamine system, such as cocaine, may cause profound changes in fetal behavior in utero that could consequently lead to alterations in behavioral and CNS development.

Heart rate response of the rat fetus and neonate to a chemosensory stimulus

Resting heart rate (HR) and HR responses of fetal and neonatal rats are described before and after intraoral infusion of isotonic saline or lemon solution. Stable measurements of resting HR were obtained for fetuses over the last three days of gestation (E19, E20, E21) and pups on the day of birth (P0) and four subsequent postnatal ages (P1, P3, P5, P7). Resting HR decreased significantly on P0 relative to the three prenatal ages and exhibited a linear increase thereafter. Variability in resting HR was pronounced on E21, decreased sharply after birth, and gradually increased through P7. Developmental changes in the HR response of fetuses and pups were evident following infusion of lemon. Fetal HR responses to lemon were characterized by bradycardia, which increased in magnitude through P1, diminished after P1, and eventually changed to tachycardia by P7. Both resting HR and HR responses to chemosensory stimulation point to the immediate perinatal period as a time of quantitative and qualitative change during sensory development.

Conditioned opioid activity in the rat fetus.

Classical conditioning in the rat fetus (Embryonic Day 20) was investigated in 4 experiments. Reexposure to a conditioned stimulus (CS; sucrose), after 3 pairings with an unconditioned stimulus (US; milk), reduced fetal facial wiping in a bioassay of perioral cutaneous responsiveness. Reduced responsiveness was evident only in subjects that received paired presentations of the CS and US and cannot be attributed to habituation, sensitization to the CS, or protracted effects of US exposure during conditioning trials. Fetuses attended to the chemosensory, not the tactile, qualities of the sucrose infusion during CS reexposure. Changes in fetal responsiveness resulted from conditioned activity in the endogenous opioid system, specifically at mu opioid receptors. These data confirm that the rat fetus is capable of exhibiting a conditioned opioid response in utero.

Habituation in the rat fetus

Rat fetuses exhibit motor and cardiac responses to chemosensory stimulation on Days 20 and 21 of gestation. The first experiment demonstrated that fetuses exhibit an increase in overall motor activity and decrease in heartrate in response to an initial intraoral infusion of a lemon solution. After a series of nine exposures, however, fetuses no longer exhibit motor or cardiac responses to lemon infusion, suggesting the existence of a habituation-like process. Responsiveness recovers spontaneously following a 3-to 9-min period without stimulation. In a second experiment, a dishabituation treatment was administered to distinguish habituation, which is a centrally mediated decrement in response, from effector fatigue, sensory adaptation, and other peripheral mechanisms that can result in reduced responsiveness. A single infusion of mint following a series of nine lemon exposures was effective in reinstating fetal motor responses to lemon on both Days 20 and 21, but reinstated cardiac responses only on Day 21. Rat fetuses habituate to repeated chemosensory stimulation, suggesting the utility of the habituation paradigm in measuring CNS development during the perinatal period.

Behavioral effects of milk in the rat fetus

Intraoral infusion of milk to the rat fetus promoted changes in behavior (mouth and rearlimb movements), reduced responsiveness to perioral cutaneous stimulation, and resulted in expression of a fetal stretch response. Milk also altered the temporal organization of fetal movements over periods up to 30 min. The orosensory characteristics of milk, in the absence of ingestion, was sufficient to evoke these behavioral effects. Reduced responsiveness to a perioral stimulus had a rapid onset (< 30 s) and brief duration (< 5 min). The ability to disrupt changes in motor organization and the stretch response with an intervening stimulus also exhibited a time course after milk infusion. The findings that tactile stimulation and biomechanical manipulations of fetal movement promote rearlimb activity and facilitate expression of the stretch are consistent with a dynamic perspective of early motor development in the fetus.