Evgeniy S. Petrov

Rapid and robust olfactory conditioning with milk before suckling experience : Promotion of nipple attachment in the newborn rat

An olfactory conditioning paradigm tested the hypothesis that newborn rats are able to learn about events associated with their first experience with milk as early as 3-5 hr after birth. Exposure to lemon odor (conditioned stimulus, [CS]) paired with intraoral milk infusions (unconditioned stimulus, [US]) resulted in strong conditioning: In the presence of the CS, sustained attachment occurred to an empty nipple as if it provided milk, whereas pups in control conditions showed little attachment. A single CS-US pairing was sufficient for strong conditioning, which was evident with a trace interval as long as 60 s. Conditioning was robust enough to promote attachment to a nipple providing saline, which is aversive to the newborn rat, and comparably strong conditioning occurred with sucrose or saccharin as the US. These findings suggest that olfactory conditioning has the potential to modify suckling behavior.

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.

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.

Self-administration of ethanol and saccharin in newborn rats: Effects on suckling plasticity.

Responsiveness to a surrogate nipple providing water, saccharin, 5% ethanol, or 10% ethanol was tested in newborn rats naive to suckling (3-5 hr old) on Postnatal Day (P) 0 and in older neonates with regular suckling experience on P1 or P2. At all ages, pups demonstrated greater nipple attachment for saccharin or 5% ethanol than for water. Intake of saccharin and 5% ethanol was high in newborns, far exceeding that of water. At P1 and P2, pups exhibited a preference for saccharin, but not for 5% ethanol. Preexposure to a nipple providing ethanol or saccharin (but not a nipple alone or fluids alone) increased subsequent responsiveness toward an empty surrogate nipple (no fluid), both 1 hr and 24 hr after preexposure. Although, during preexposure, pups responded most positively to the nipple providing saccharin, the longest time spent on an empty nipple was observed in pups preexposed to 10% ethanol.

Reinforcing properties of ethanol in neonatal rats: involvement of the opioid system.

Toward understanding why infant rats ingest high levels of ethanol without initiation procedures, the authors tested effects of mu and kappa receptor antagonists on ethanol reinforcement in neonatal rats. After an intracisternal injection of CTOP (micro antagonist), nor-Binaltorphimine (kappa antagonist), or saline, newborn (3-hr-old) rats were given conditioning pairings of an odor with intraorally infused ethanol or a surrogate nipple with ethanol administered intraperitoneally (to minimize ethanols gustatory attributes). In each case, these opioid antagonists reduced or eliminated ethanols reinforcement effect. The same effects occurred with saccharin as the reinforcer in olfactory conditioning. The results imply that activation of mu and kappa receptors, apparently acting jointly, is necessary for reinforcement or that antagonists of this activity impair basic conditioning.

Reinforcement From Pharmacological Effects of Ethanol in Newborn Rats

BACKGROUND The preweanling period in the rat is characterized by acceptance of substantial amounts of ethanol and susceptibility to its reinforcing effects. It has been unclear, however, whether the neurobiological basis of ethanol reinforcement properties at this age is in ethanols olfactory, gustatory, or pharmacological effects. METHODS The effectiveness of intraperitoneal (ip) ethanol as a reinforcer for newborn (3-hr-old) rats was tested toward separation of the orosensory and pharmacological sources of ethanol reinforcement. Responsiveness to a test nipple by pups given such pairings was compared with that of pups given unpaired presentations of the nipple and ethanol. RESULTS Reinforcement was assessed in terms of response to a surrogate nipple 1 hr after a single pairing of a similar nipple providing water (conditioned stimulus) and ip injection of ethanol (0.125, 0.25, 0.50, or 0.75 g/kg; unconditioned stimulus). Significant effects of ethanol reinforcement occurred with the lower doses (0.125 and 0.25 g/kg); higher doses of ethanol (0.50 and 0.75 g/kg) had no significant reinforcement effect. A second experiment determined that for conditioning with ip ethanol as the unconditioned stimulus, a conditioned stimulus consisting of only ingesting water or only suckling on an empty nipple also yielded significant reinforcing effects of ethanol, although with less strength than their combination. Both reinforcing doses of ethanol, 0.125 and 0.25 g/kg, yielded detectable concentrations of ethanol in the blood 5 min after injection, which were sustained at a significantly lower level 60 min after administration. CONCLUSIONS These data indicate that aside from possible, and likely weak, hematogenic sources of gustatory and olfactory attributes of ethanol, the basis of ethanols reinforcement effect in neonatal rats is primarily pharmacological. For the pharmacological effects of ethanol to be reinforcing for the neonatal rat, concurrent appetitive activity on a nipple providing a fluid may be necessary for a substantial effect with this paradigm.

Experimental study of the first suckling episode: Rat pups ingest fluids through a surrogate nipple.

The present study reported a new technique in which pups ingested fluids from a surrogate nipple. Cesarean-delivered pups tested before suckling experience showed oral grasp responses and ingested milk from the surrogate nipple. Pups ingested equal amounts of distilled water and milk and rejected saline. After ingesting milk from the surrogate nipple, pups remained attached to an empty surrogate nipple, while pups exposed to distilled water or saline did not show sustained attachment. Brief experience with milk from an oral cannula or from a surrogate nipple elicited sustained attachment to an empty nipple. Pups ingesting milk from a surrogate nipple showed increased intake of water and saline from the nipple when tested subsequently. The surrogate nipple provides a new technique for experimental study of early suckling behavior. The results suggest that initial experiences with milk may reinforce components of early suckling behavior.

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.

Newborn rats' first suckling experience: Taste differentiation and suckling plasticity

The present study examined responsiveness of newborn rats to a surrogate nipple providing fluids with basic tastes (sweet, sour, bitter and salty) and assessed the effects of this first gustatory experience on subsequent responding to the nipple itself (empty nipple) or the nipple providing water. Responsiveness (attachment to and ingestion from a surrogate nipple providing saccharin, saline, quinine or ammonium chloride) was compared with that toward a nipple providing water. Compared to water, saline and quinine significantly reduced attachment to and ingestion from the nipple, while saccharin and milk significantly increased attachment behavior. Ammonium chloride increased attachment but not ingestive behavior. Suckling experience with saline, quinine and ammonium chloride attenuated both attachment and ingestive behaviors when subjects were tested 1 h later with an empty surrogate nipple or a nipple providing water. Experience with saccharin and milk (but not water) increased both measures. The data suggest that in newborn rats, as early as a few hours after birth, mechanisms of gustatory detection have control over suckling behavior. Initial experience with the tastants available from the nipple in the first suckling episode may alter further responsiveness to the nipple itself, mediated perhaps by mechanisms of appetitive and aversive conditioning.

Dynorphin A (1-13) and responsiveness of the newborn rat to a surrogate nipple: immediate behavioral consequences and reinforcement effects in conditioning.

The role of endogenous opioid system in learning (Pavlovian conditioning) and reinforcement was studied in newborn rats by pairing central injection of dynorphin A (1-13), an endogenous ligand with high affinity for kappa opioid receptors, with a surrogate nipple. For conditioning, the nipple served as the conditioned stimulus and dynorphin as the unconditioned stimulus. Dynorphin was found to elicit an immediate unconditioned, dose-dependent increase in the neonatal responsiveness to a surrogate nipple providing water, an effect apparently mediated through the kappa opioid receptors. This immediate effect of dynorphin was evident in the context of suckling behavior but not in the context of adult-like drinking when water was delivered through an intra-oral cannula. One hour after injection, the unconditioned stimulatory effect of dynorphin had dissipated and no longer affected responsiveness to the nipple. However, pairing of centrally injected dynorphin and suckling on a nipple delivering water substantially enhanced responsiveness to a nipple 1h later. This suggested conditioning, and hence reinforcement, by the centrally injected dynorphin. The conditioned sustained nipple attachment was mediated through both kappa and micro opioid receptors. These experiments confirm a central role for the brain opioid systems in reinforcement of neonatal behavior on the nipple.