Ingestional aversion learning in preweanling rats

Peculiar modulation of taste aversion learning by the time of day in developing rats

The ontogeny of the temporal context modulation of conditioned taste aversion was studied in male Wistar rats using a palatable 1% NaCl solution. A procedure that included two saline preexposures, a single pairing saline-lithium chloride (0.15 M; 1% b.w.) either at the same or a different time of day of preexposures and a one-bottle test at the same time than preexposure was applied. Four age groups (PN32, PN48, PN64, and PN100) covering the complete range from adolescence to the adult period were tested. The results showed no effect of a temporal context shift in PN32. A peculiar enhancement of temporal context-specific saline aversions was exhibited by PN48 and PN64 rats, while the adult typical temporal context specificity of latent inhibition was only evident in PN100 rats. The results are discussed in terms of the peculiar brain functional organization during a protracted adolescence period.

Progressive postnatal increases in Fos immunoreactivity in the forebrain and brain stem of rats after viscerosensory stimulation with lithium chloride

Interoceptive signals have a powerful impact on the motivation and emotional learning of animals during stressful experiences. However, current insights into the organization of interoceptive pathways stem mainly from observation and manipulation of adults, and little is known regarding the functional development of viscerosensory signaling pathways. To address this, we have examined central neural activation patterns in rat pups after treatment with lithium chloride (LiCl), a malaise-inducing agent. Rat pups were injected intraperitoneally with 0.15 M LiCl or 0.15 M NaCl (2% body wt) on postnatal day (P)0, 7, 14, 21, or 28, perfused 60 to 90 min postinjection, and their brains assayed for Fos protein immunolabeling. Compared with saline treatment, LiCl increased Fos only slightly in the area postrema, nucleus of the solitary tract, and lateral parabrachial nucleus on P0. LiCl did not increase Fos above control levels in the central nucleus of the amygdala, bed nucleus of the stria terminalis (BNST), or paraventricular nucleus of the hypothalamus on P0 but did on P7 and later. Maximal Fos responses to LiCl were observed on P14 in all areas except the BNST, in which LiCl-induced Fos activation continued to increase through P28. These results indicate that central LiCl-sensitive interoceptive circuits in rats are not fully functional at birth, and show age-dependent increases in neural Fos responses to viscerosensory stimulation with LiCl.

Late postnatal maturation of excitatory synaptic transmission permits adult-like expression of hippocampal-dependent behaviors

Sensorimotor systems in altricial animals mature incrementally during early postnatal development, with complex cognitive abilities developing late. Of prominence are cognitive processes that depend on an intact hippocampus, such as contextual-configural learning, allocentric and idiocentric navigation, and certain forms of trace conditioning. The mechanisms that regulate the delayed maturation of the hippocampus are not well understood. However, there is support for the idea that these behaviors come "on line" with the final maturation of excitatory synaptic transmission. First, by providing a timeline for the first behavioral expression of various forms of learning and memory, this study illustrates the late maturation of hippocampal-dependent cognitive abilities. Then, functional development of the hippocampus is reviewed to establish the temporal relationship between maturation of excitatory synaptic transmission and the behavioral evidence of adult-like hippocampal processing. These data suggest that, in rats, mechanisms necessary for the expression of adult-like synaptic plasticity become available at around 2 postnatal weeks of age. However, presynaptic plasticity mechanisms, likely necessary for refinement of the hippocampal network, predominate and impede information processing until the third postnatal week.

Olfactory conditioning facilitates diet transition in human infants

We evaluated whether Pavlovian conditioning methods could be used to increase the ingestion of non-preferred solutions by formula-fed human infants. In baseline measures, 5-7 month old infants sucked less frequently and consumed less water than regular formula. During a 3-day olfactory conditioning period, parents placed a small scented disk, the conditioned stimulus, on the rim of their infants' formula bottle at every feeding. Following this training, infants' responses to water were tested when their water bottles had a disk scented with the training odor, a novel odor, or no odor. Infants tested with the training odor sucked more frequently and consumed significantly more water than they had at baseline. Infants tested with no odor or a novel odor consumed water at or below baseline levels. These data demonstrate that olfactory conditioning can be used to enhance ingestion in infants and suggest that such methods may be useful for infants experiencing difficulty when making transitions from one diet to another.

Ontogeny of delay versus trace eyeblink conditioning in the rat

The ontogeny of delay versus trace eyeblink conditioning was examined in 19-, 23-, and 30-day-old rat pups. Pairings of a tone conditioned stimulus (CS) and periocular shock unconditioned stimulus (US; 100-ms) were presented in one of three conditioning paradigms: standard delay [380-ms CS, 280-ms interstimulus interval (ISI)], trace (380-ms CS, 500-ms trace interval), or long-delay (980-ms CS, 880-ms ISI). The results of two experiments indicated that standard delay conditioning emerged between 19 and 23 days of age whereas trace and long-delay eyeblink conditioning emerged more slowly from postnatal Days 19 to 30. Because the acquisition profile for long-delay paralleled that of trace and not standard delay, it appears that the relative deficits in the emergence of trace eyeblink conditioning during development reflect difficulty in forming associations over long ISIs rather than the short-term memory demands of the trace conditioning paradigm.

Low body temperature affects associative processes in long-trace conditioned flavor aversion

A series of experiments examined the effect of low body temperature on the associative process in long-trace conditioned flavor aversion. Experiment 1 demonstrated that maintaining a low body temperature between conditioned stimulus (CS) and unconditioned stimulus (US) administration facilitates the associative process and allows a flavor aversion to be conditioned in young rats over an interval that would normally not support conditioning. Experiments 2 and 3 demonstrated that this was due neither to lingering systemic saccharin serving as a CS nor to a cold induced enhancement of US intensity. Experiment 4 demonstrated that inducing hypothermia at various times during a 3-h CS-US interval results in an apparent delay of reinforcement gradient. We propose that a cold induced decrease in metabolic rate slows the internal clock that governs the perception of time and that the CS-US association depends upon perceived contiguity rather than upon an external clock-referenced contiguity.

Ontogenesis of trace conditioning in young rats: dissociation of associative and memory processes

Hooded rats from 15 to 30 days of age were trained with a Pavlovian trace fear conditioning procedure in order to study the development of their capacity to learn associations between events separated in time. For both the auditory and visual systems, the associative processes necessary to learn about temporally contiguous events emerged earlier during ontogenesis than the processes necessary to integrate events separated in time. Furthermore, the ability of the rats to integrate events separated by increasingly long intervals continued to improve as they got older. We suggest that the emergence of the capacity to integrate temporally separate events reflects the maturation of memory processes that retain representations of stimulus events over time, and that these processes continue to mature for a considerable period after the basic associative processes have become functional.

Ontogenesis of learning: V. Variation in associative and nonassociative control of an operant forelimb response in infant rats

To study ontogenetic variation in the mechanisms that control operant responding, infant rats 4-16 days old were trained to lift a forelimb in order to receive an infusion of a sucrose-milk solution (reinforcer). Although the learning processes that contribute to this behavior were to some extent functional in pups 5-6 days old (Experiment V), there were major age-related constraints on the performance of this response. We were unable to observe selective responding with the reinforced forelimb until pups were 9 days old (Experiment I). This was in part because pups tested when 7, but not 9 days old, were highly activated (as measured by responding with both forelimbs) by both the reinforcer (Experiment II) and cues associated with the reinforcer during training (Experiment IV). Implications of these findings are discussed.

A specialization of taste aversion learning during suckling and its weaning-associated transformation

During ontogenesis, altricial mammals advance through different ecological niches which require correspondingly different adaptive strategies. We view learning as an important feature of ontogenetic adaptations and consider the acquisition and expression of taste aversion learning during nursing within this framework. Fifteen-day-old preweanling rat pups fail to acquire conditioned taste aversions while nursing, whereas 20-day-old weanlings readily learn flavor aversions while suckling (Martin & Alberts, 1979). The developmental emergence of taste aversion learning while nursing coincides with reduction in maternal milk supply and the rat pup's transition to a solid food diet. We considered the ontogenetic transformation of learning while nursing as a feeding-related adaptive strategy of the weanling rat and tested two predictions derived from this view: (1) delayed weaning might postpone acquisition or expression of suckling-related taste aversions and (2) conditions that permit weaning (e.g., access to alternate food sources) might induce acquisition or expression of such taste aversions. Both predictions were confirmed.

Taste aversion learning in preweanling rats exposed to alcohol prenatally

The effects of prenatal alcohol exposure on the development of a conditioned taste aversion were examined in preweanling rat pups. Mothers of these pups were fed isocaloric liquid diets containing either 35 or 0% ethanol-derived calories (EDC) from gestation days 6 through 20. A pair-feeding procedure was employed, and an ad lib lab chow control group was also included. At 5, 10, or 15 days of age, pups were infused with a saccharin solution through a cannula implanted in the oral cavity. Half of the pups in each group were then injected with lithium chloride (LiCl), which served as the poisoning agent, and the other half with sodium chloride (NaCl) as a control. Animals were subsequently tested for a conditioned aversion to the saccharin solution. At 15 days of age, all of the pups in the LiCl-poisoned group demonstrated a conditioned taste aversion to the saccharin solution, but the degree of this aversion was less in alcohol-exposed offspring. At 10 days of age, a taste aversion was learned, although it was not as strong as that shown by 15-day-old pups, and it appeared to be learned equally well by all of the prenatal treatment groups. At 5 days of age, there was marginal support for taste aversion learning. Again, it did not interact with prenatal treatment. The ontogenic differences in taste aversion learning exhibited by alcohol-exposed offspring relative to controls are discussed in terms of altered hippocampal development.

Ontogenesis of learning: I. Variation in the rat's reflexive and learned responses to gustatory stimulation

These experiments indicate that by Day 15 after birth, the processes that mediate a number of taste-controlled behaviors in the rat are functional. These include the sensory processes necessary to detect and respond reflexively to sucrose, the event-learning processes that reduce the rat's neophobic reaction to sucrose, and the integrative-learning processes that enable it to learn an aversion to sucrose when paired with lithium toxicosis, even when these events are separated by 1 hr. These capacities, however, did not emerge simultaneously. Those necessary to detect and respond reflexively to sucrose emerged prior to those that contribute to the learned control of taste-guided behaviors. It is argued that these age-related dissociations in behavioral capacities reflect a caudal-to-rostral maturational sequence of components of the ascending gustatory system that are thought to underlie these capacities.

The effect of CS-preexposure on conditioned taste aversion in young and adult rats

Young and adult rats were given nonreinforced exposures to the flavor-CS prior to a taste aversion conditioning session. CS-preexposure prevented the conditioning of a taste aversion in young rats but only attenuated conditioning in adults. These results suggest that an enhanced CS-preexposure effect may account for the previously reported weaker conditioned taste aversion observed in young rats, as compared to adults, when there is a protracted interstimulus interval.

Experimental methods in behavioral teratology

The possibility that the exposure of the embryo to certain chemical substances can lead to behavioral disturbances is known from human epidemiological studies, e.g., in chronic poisoning with mercury and ethanol. Therefore, efforts are made to develop toxicological techniques with which new behavioral teratogens can be recognized. The review describes the most important experimental methods which are presently explored, and which are based on a rich body of knowledge accumulated by experimental psychologists. Most of the tests were developed with small animals, mostly with rats. They range from a rather straightforward determination of various reflexes to complex behavioral situations involving mechanical devices, operant conditioning techniques and procedures evaluating social behavior. In applying these methods in routine toxicology, it is important to remember, that many behavioral effects determined in newborn and adult animals are subtle. Moreover, they are influenced by a large variety of environmental factors affecting the health and the behavior of the mothers and of the offspring in the early and later phases of development. Therefore, the experiments must be conducted under highly standardized conditions and must be controlled rigorously. It is concluded that the best experimental strategy for the evaluation of potential behavioral teratogens is not yet established. Therefore, it would be premature to decide on a fixed protocol to be included in routine animal safety experiments for drugs and other chemical substances.

Conditioned taste aversion as a function of age in mature male rats

Experimental groups of young mature (90-120 days), mature (365-395 days), and aged (730-760 days) Fischer-344 rats were allowed to drink a saccharin solution followed by lithium chloride toxicosis initiated at one of three intervals, either 15, 60, or 240 minutes. Control groups were given saline placebos according to the same schedule. In a preference test conducted 48 hours after conditioning, there was little evidence of age differences in the acquisition of a saccharin aversion. Age differences were noted in the extinction of the aversion which was tested by monitoring preference over a period of 32 days of continuous access to saccharin and water. Older animals tended to show greater resistance to extinction.

Developmental changes in amphetamine-induced taste aversions

In this study a conditioned taste aversion paradigm was employed to examine the ontogenetic trend in psychopharmacological responsiveness to amphetamine among infant (18 days of age), periadolescent (35 days of age), and young adult (52 days of age) rats. The ability of amphetamine to alter taste preference increased with dosage level and this effect interacted with age. Infant rats demonstrated greater sensitivity to the taste aversion inducing properties of amphetamine than either periadolescent or young adult animals. In contrast, periadolescent animals demonstrated a marked resistance to amphetamine's taste aversion inducing properties when compared with either infant or young adult animals. This developmental pattern in amphetamine drug responsiveness seen utilizing the taste aversion paradigm parallels the previously examined ontogenetic trend in amphetamine response using locomotor activity as a response measure.