Preweanling Rats Research Articles

Development of intersensory function: Age-related differences in stimulus selection of multimodal compounds in rats as revealed by Pavlovian conditioning.

Preweanling and adult rats were exposed to Pavlovian stimulus pairings with lights, tones, and light-tone compounds as conditional stimuli (CSs). Tone intensity was varied. As was found in previous studies using flavor CSs, odor CSs, or both, in which significant overshadowing or potentiation occurred to an element of a compound, overshadowing was less likely and potentiation more likely for preweanlings than for adults. Several experiments supported the view that these effects reflect an age-related difference in the controlling properties of stimuli: Young rats are more disposed to encode multimodal compounds on the basis of amodal intensity, whereas their elders are more likely to encode modality-specific properties of light and sound. Differential reinforcement in the presence of lights, tones, and light-tone compounds made preweanlings' stimulus selection appear more adultlike and may be a cause of development of intersensory function.

Establishing intermodal equivalence in preweanling and adult rats.

Establishing intermodal equivalence in preweanling and adult rats.

Establishing intermodal equivalence in preweanling and adult rats.

Interactions between olfactory and brightness conditioning were examined in infant (16-day-old) and adult rats. Conditioning to an odor (methyl salicylate [MS] paired with footshock [FS]) was followed by subthreshold conditioning to an unscented black box (paired with FS). Controls were given unpaired presentations in either the 1st or the 2nd phase of this treatment. No age-related differences in conditioning occurred when either phase was presented alone. Significant aversion to the black box was observed after the experimental treatment only in infants, and brightness conditioning potentiated odor conditioning in infants but not adults. In the infants, odor-extinction procedures weakened the aversion to the black box as well as to MS. Olfactory-to-visual transfer required that the same unconditioned stimulus be paired with both conditioned stimuli. These results may reflect an infantile disposition for unitization.

Ethanol-mediated taste aversions and state-dependency in preweanling (16-day-old) rats

Requirements for conditioning of an ethanol-mediated taste aversion in 16-day-old rat pups were examined. Experiment 1 demonstrated that preweanling rats are capable of acquiring, in two trials, an aversion to a 15% sucrose solution when followed by intragastric intubation of a 1.2 g/kg dose of 17% v/v ethanol, but not when followed by a 0.4 g/kg dose. Comparison was with control animals given sucrose followed by an equivalent volume isocaloric Half and Half. When the 0.4 g/kg dose of ethanol preceded sucrose presentation by 30 min (Experiment 2), the aversion was learned, suggesting that the effective delay between the sucrose and the critical consequences of the ethanol had been too long with the former procedure. Expression of the sucrose aversion required, however, the reinstatement of the context of intoxication--state-dependent retention. Finally, the results of Experiment 3B indicated that, in addition to the association between the sucrose and the aversive consequences of alcohol intoxication, the orosensory cues resulting from alcohol's direct elimination, via such processes as respiration and salivation, became associated with the appetitive properties of the sucrose. This was evidenced by a conditioned increase in preference for ethanol odor. Possible age-related differences in the ability to associate stimuli with alcohol's unconditioned consequences, and in state dependency are discussed.

Disruption of noradrenergic, but not serotonergic or opiate, functioning blocks both cardiac and behavioral components of the orienting response in preweanling rats

Previous work in this laboratory established that selective attention, as measured by the behavioral and autonomic expressions of the orienting response (OR), is not disrupted by either dopaminergic or cholinergic receptor blockade. The present experiments extended this pharmacological analysis of the OR. In Experiment 1, preweanling rats were injected with methysergide maleate, a serotonin receptor blocker. Neither the behavioral nor the heart rate (HR) component of the OR was attenuated. In Experiment 2, the opiate receptor blocker naltrexone also failed to inhibit the HR and behavioral expressions of the OR. alpha-1 adrenergic receptor blockade with WB-4101 in Experiment 3 abolished both the HR and behavioral ORs to the pulsating tone. In Experiment 4, clonidine, which inhibits release of norepinephrine by stimulating alpha-2 autoreceptors, attenuated both behavioral and HR ORs to the pulsating tone in a dose-dependent manner. These data, in combination with the prior findings, suggest that norepinephrine is critically involved in the central process underlying the OR in the rat. Dopaminergic, cholinergic, serotonergic, and opiate receptor blockades do not impair selective attention as indexed by HR and behavioral ORs to an auditory stimulus. In contrast, disruption of noradrenergic functioning via either alpha-1 receptor blockade or alpha-2 receptor stimulation disrupts both the HR and behavioral components of the OR. These results indicate that integrity of central noradrenergic functioning is essential for expression of the OR and for stimulus-directed attention.

Taste discrimination learning in preweanling rats

Seventeen-day-old rat pups received intraoral infusions of two novel flavors, coffee (0.625% w/v Sanka, decaffeinated) and saccharin (0.5% w/v), of which one (CS+) was paired with a 0.75% body weight, Lp. iiYection of 0.4 M LiCI, and the other (CS-) was presented alone. On the following day, two more infusion tests were conducted to determine intake of each flavor. Intake of the CS+ flavor was markedly suppressed, relative to that of the CS- flavor, although the magni­ tude ofthis effect depended on which flavor was designated CS+ and on the order in which CS+ and CS- were presented on the test day. These results indicate that preweanling rats are capa­ ble of discriminative taste aversion learning. This simple conditioning procedure should provide a useful method for studying the sensory development of the gustatory system and/or the de­ velopmental psychobiology of learning and memory.

Effects of beta-endorphin on DNA synthesis in brain regions of preweanling rats

Recent studies of whole brain in rat pups have shown a marked decrease in DNA synthesis following intracisternal (i.c.) administration of beta-endorphin (BE). This investigation examines DNA synthesis in the cerebral cortex and cerebellum to determine whether the effect shows regional selectivity. Two- to twenty-day-old rats were given a single ic injection of BE, and DNA synthesis was assessed 1 h later. In the cerebral cortex, a region that undergoes major phases of cell multiplication in the immediate pre- and postnatal periods, BE significantly decreased DNA synthesis in 2-day-old rats, and a maximal inhibition was obtained by 4 days of age. In contrast, the cerebellum, a region that grows predominantly after birth, showed less sensitivity to BE during the early postnatal days, and a maximal effect was not attained until 10 days of age. While at 15 days of age the inhibition began to diminish in the cortex, a maximal effect was still seen in the cerebellum. Naloxone prevented the response in both brain regions, indicating the participation of opioid receptors. These results indicate that CNS BE is apparently able to alter DNA synthesis throughout the brain, with the greatest sensitivity occurring in those regions with highest mitotic rates at the time of exposure to BE.

Chronic Administration of a Potent Cholecystokinin Receptor Antagonist, L-364,718, Fails to Inhibit Pancreas Growth in Preweanling Rats

We examined whether endogenous cholecystokinin (CCK) is involved in growth of the preweanling rat pancreas. Twice daily for 14 days, 7-day-old neonatal rats received an oral gavage of either 2.5 mg/kg or 5.0 mg/kg of the potent and specific CCK receptor antagonist L-364,718 (the 2.5 mg/kg dose of antagonist was shown in the present study to abolish totally the pancreas growth-promoting effects of exogenously administered caerulein (CR) in neonatal rats). Control pups received oral gavages of the L-364,718 vehicle alone. The final body weights, pancreas weights, total pancreatic DNA contents, and total pancreatic protein contents did not differ significantly between the 21-day-old control pups and the 21-day-old pups that were pretreated for 14 days with either the low or the high doses of L-364,718. These findings suggest that endogenous CCK is not required for growth of the neonatal rat pancreas.

Capsaicin‐induced neuronal degeneration in the brain and retina of preweanling rats

Capsaicin is a neurotoxin known for its ability to cause degeneration of small unmyelinated primary sensory neurons in both spinal and cranial nerves. Although lower motor neurons do not degenerate following capsaicin treatment, the extent to which capsaicin may damage neurons in the brain has not been thoroughly evaluated. This study examines the effects of systemic capsaicin (50-150 mg/kg) on the central nervous system of 10-day-old rats. Rat pups were injected with capsaicin or the injection vehicle and sacrificed 6 hours-10 days later. Brains, spinal cords, and retinas were stained with cupric silver to label degenerating neurons. As previously reported for capsaicin-treated rats, degenerating nerve terminals were present in areas receiving primary afferent input: the spinal cord dorsal horn, spinal trigeminal nucleus, nucleus of the solitary tract, and area postrema. However, degenerating terminals were also present in areas not known to receive primary sensory innervation: the inferior olivary nucleus, sphenoid nucleus, medial and olivary pretectal nuclei, interpeduncular nucleus, interfascicular nucleus, caudal linear, dorsal, median, and paramedian raphe nuclei, supramammillary area, lateral habenula, ventrolateral geniculate nucleus, ventral reuniens nucleus, ventromedial hypothalamic nucleus, lateral hypothalamic and preoptic areas, suprachiasmatic nucleus, septohypothalamic nucleus, bed nucleus of the stria terminalis, lateral septal nucleus, accumbens shell, olfactory bulb, and retina. Some areas where capsaicin caused degeneration in rat pups do not appear to be capsaicin-sensitive in adult rats. Results indicate that (1) capsaicin's neurotoxicity is not limited to primary sensory neurons and (2) developmental factors may alter the capsaicin sensitivity of some neuronal projections within the brain.

Ontogenetic differences in the expresion of odor‐aversion learning in 4‐ and 8‐day‐old rats

Two experiments investigated the ability of preweanling rats, 4 or 8 days of age, to form an odor-LiCl association across various CS-US delays. The results of the odor preference test indicated that 8-day-old subjects acquired an aversion to the CS+ odor when trained with either a 0 or 15 min CS-US delay, while 4-day-old subjects did not exhibit a reduction in preference for the CS+ at any of the CS-US delays tested. The absence of a reduction in preference for the CS+ by 4-day-old rats could not be attributed to their failure to acquire the odor aversion, however; they avoided a texture with which the CS+ odor was paired in a second order conditioning paradigm. The results suggest that outcomes appearing to represent age-related differences in associative learning may in some instances be more appropriately viewed as representing ontogenetic differences in the way in which acquired associations are manifested in behavior.

The ontogeny of opioid receptors mediating opiate-induced feeding in rats

Acute administration of naloxone to preweanling rats does not attenuate independent ingestion of milk until 14 days of age suggesting that the full expression of an endogenous opioid system(s), regulating feeding rats, is not complete prior to this age. The present study was undertaken to examine the functional ontogeny of opioid receptors mediating opiate-induced feeding in rats. Rat pups, satiated with milk, were given intraperitoneal injections of the opiate receptor agonist, morphine, and were allowed free access to milk. Morphine stimulated the intake of milk at 3, 5, 7, 14 and 21 days of age, within 2 hr of injection. A time-course analysis in 7-day-old pups showed greater enhancement of intake between hours 2 and 4, than between hours 0 and 2, for large doses of morphine (0.3 and 1.0 mg kg ) suggesting that morphine-induced behavioral depression, which was observed early in the test session, confounded intake at earlier hours. Administration of the opiate receptor antagonist, naltrexone, produced no effect on intake of its own, but blocked the stimulation of intake by morphine in 5-day-old pups confirming that the effect of morphine on the intake of milk was mediated by opioid receptors. Thus, while a functional endogenous opioid system(s), regulating feeding in rats, is not fully mature until 14 days postpartum, the present results suggest that opioid receptors mediating feeding are functional very early in the postnatal development of the rat.

Brain glucoprivation and ketoprivation do not promote ingestion in the suckling rat pup

We examined the possibility that brain glucose or ketone availability may control suckling or precocious feeding in the preweanling rat. Brain glucoprivation induced by 5-thio-D-glucose injection into the 4th ventricle did not increase feeding on orally infused milk until 30 days of age, although hyperglycemia was evoked as early as 9 days by the same treatment. Plasma ketone levels varied with suckling status, but pharmacological blockade of hepatic free fatty acid oxidation, which restricts ketone availability (ketoprivation), failed to increase suckling. Because the suckling rat can switch energy substrates under nutritional duress, we combined glucoprivation and ketoprivation. Feeding was suppressed, and suckling was not affected. Finally, we injected ketones into the 3rd brain ventricle and found that they increased feeding. Thus, in contrast to the adult rat, reduced glucose or ketone utilization by the brain does not increase food intake in the preweanling, but increased circulating and brain ketone concentrations may arouse feeding.

Disruption and Recovery of the Orienting Response Following Shock or Context Change in Preweanling Rats

Previous research has shown that placement of preweanling rats in an unfamiliar environment inhibits both heart rate and behavioral orienting responses to an auditory stimulus, and that the orienting response gradually reappears during the following 15 min. Four experiments designed to evaluate two potential explanations of this effect were conducted. If the arousal induced by an unfamiliar environment disrupts sensory information processing, so should the arousal induced by mild electric shock. In two experiments, the orienting response was inhibited in proportion to number of shocks received and rate of recovery was comparable to that observed in the unfamiliar environment. New environments also contain many unfamiliar stimuli which may overload the rats' limited information processing capacity. In two experiments a change in environmental stimulation was shown to inhibit the orienting response. Recovery of the orienting response followed the same time course as that seen following shock or placement in an unfamiliar environment. Although the results of this study clearly demonstrate environmental inhibition of the orienting response, they provide little support for either the "arousal" or the "information overload" hypotheses of orienting response inhibition. The implications of these data for current theoretical conceptualizations of the orienting response are discussed.

Vinyl chloride-induced DNA adducts II: Formating and persistence of 7-(2'oxoethyl)guanine and N2 ,3 ethenoguanine in rat tissue DNA

The formation and persistence of the DNA adducts 7-(2'-oxoethyl)guanine (OEG) and N2,3-ethenoguanine (EG) were investigated in preweanling Sprague-Dawley rats exposed to vinyl chloride (VC). Lactating female CD rats with 10 day old pups were exposed to 600 p.p.m. VC by inhalation for 5 days, 4 h/day. Groups of rats were killed immediately and 3, 7 and 14 days after exposure. The concentrations of OEG and EG were measured in liver, lung, kidney, brain and spleen. HPLC with fluorescence detection was used for OEG detection, and gas chromatography-negative ion chemical ionization mass spectrometry was used for EG detection. In tissues of neonatal rats, the concentrations of both DNA adducts, expressed as pmol/mumols unmodified guanine, were highest in liver (OEG 162 +/- 36, EG 1.81 +/- 0.25), followed by kidney (OEG 29 +/- 1, EG 0.31 +/- 0.02), and lung (OEG 20 +/- 7, EG 0.21 +/- 0.08). No adducts were found in brain or spleen. DNA adducts were detected only in liver (OEG 43 +/- 7, EG 0.47 +/- 0.14) and lung (OEG 20 +/- 5, EG 0.27 +/- 0.03) of the dams. The ratio between EG and OEG was approximately 1:100 in all tissues immediately after exposure. In the liver of the preweanling rats, this ratio increased to 1:14 1 week after exposure, reflecting a greater persistence of EG. A half-life of 62 h was calculated for OEG, and the estimated half-life for EG was greater than 30 days. In view of the slow loss of EG and its high efficiency for causing base-pair mismatch, these results suggest that EG may be an important DNA adduct in VC-induced carcinogenesis.

Immunotoxicity of tributyltin oxide in rats exposed as adults or pre-weanlings

A comparison was made between adult and pre-weanling rats of the immunotoxic effects of subacute dosing with bis(tri- n-butyltin) oxide (TBTO). Adult (9 weeks old) male Fischer rats were dosed by oral gavage with TBTO for 10 consecutive days at 1.25–10 mg/kg per dose or 3 times/week for a total of 10 doses at 5–20 mg/kg per dose. Adults rats similarly dosed by oral gavage with 6 mg/kg per dose cyclophosphamide (CY) served as positive controls. Pre-weanling rats (3–24 days old) were dosed 3 times/week for a total of 10 doses at 2.5, 5 or 10 mg/kg per dose. At various times after dosing rats were evaluated for alterations in body and lymphoid organ weights, mitogen and mixed lymphocyte reaction (MLR) lymphoproliferative (LP) responses, natural killer (NK) cell activity, cytotoxic T lymphocyte (CTL) responses and primary antibody plaque-forming cell (PFC) responses. In adult rats given 10 daily doses of TBTO, thymic involution was observed at a dosage of 2.5 mg/ kg and mitogen responses to Con A and PHA were suppressed at 5 mg/kg. The PFC response was enhanced in adult rats dosed daily at 2.5 mg/kg. A dosage of 5 mg/kg given intermittently (3 times/week) to adults or pre-weanlings resulted in thymic involution. Reductions in mitogen responses were observed in adults dosed intermittently at 10 and 20 mg/kg and in pre-weanlings at 5 and 10 mg/kg. The MLR response was suppressed in adult rats dosed intermittently at 20 mg/kg and in pre-weanling rats at 10 mg/kg. NK cell activity was suppressed only in pups dosed intermittently at 10 mg/ kg. CTL responses were not affected in either age group. Within 3 weeks following the last exposure of adult rats to TBTO all parameters returned to normal. On the other hand, LP responses to mitogens were suppressed in 10-week-old rats that were dosed with 10 mg/kg TBTO as pre-weanlings. However, this exposure regimen resulted in reductions in body weight that persisted for up to 13 weeks of age, which suggests that TBTO may be a developmental toxicant. These data indicate that while exposure of young rats to TBTO resulted in immune alterations at doses lower than those required to suppress responses in adults, the observed effects may also be influenced by the developmental toxicity of this compound.

The effects of prenatal exposure to delta-9-tetrahydrocannabinol on the rest-activity cycle of the preweanling rat

Either 15 or 50 mg/kg of delta-9-tetrahydrocannabinol (delta-9-THC) was administered from Day 2 through Day 22 of gestation. Pair-fed (0 mg/kg) and nontreated groups served as controls and all treated and control litters were fostered at birth to untreated dams. To examine whether delta-9-THC produces effects on the rest-activity cycle of the offspring, groups of 3 littermates from each of the treated and control groups were tested for an 8 hr observation period on electronic activity monitors at 17, 22 and 30 days of age. Whereas all treated and control groups showed age-dependent changes in activity, neither activity level nor the rest-activity pattern were affected by delta-9-THC. These findings are discussed in relation to neurobehavioral studies of activity changes following prenatal exposure to cannabis.

Dopamine D‐2 receptor mediation of response suppression learning of young rats

In three experiments, the effects of augmenting or blocking dopamine (DA) D-2 receptor activity on the ontogeny of response suppression learning of preweanling rat pups were determined. In the initial experiment, rat pups were trained to traverse a straight alley for nipple attachment to an anesthetized dam. When footshock (0.2 mA, 0.5 sec) was made contingent on responding, younger (11- and 13-day-olds) rat pups were deficient to older (17- and 19-day-olds) pups at withholding punished responding. In the subsequent experiments, response suppression learning was assessed after injecting 11- and 17-day-old rat pups with the specific DA D-2 agonist, LY 171555 (0.005-, 0.01-, and 0.1-mg/kg, i.p.), or the specific DA D-2 antagonist, sulpiride (5.0-, 15.0-, and 50.0-mg/kg, i.p.). LY 171555 enhanced the punished responding of both the 11- and 17-day-old rat pups; whereas, sulpiride increased the punished responding of the 17-, but not the 11-day-olds. In four additional experiments, the effects of LY 171555 and sulpiride on the locomotor activity, nociception, and reinforcement processes of 17-day-old rat pups was assessed. Rat pups given LY 171555 (0.01 mg/kg, i.p.) exhibited enhanced locomotor activity and a trend towards hyperanalgesia using a hot plate task. Sulpiride (15.0 mg/kg, i.p.) completely antagonized LY 171555's activity enhancing effects and had hyperalgesic properties. In two experiments, sulpiride did not affect the nonpunished appetitive responding of the 17-day-olds; whereas, haloperidol-treated pups responded on fewer reinforced trials than did saline-treated pups. Therefore, these results indicate that the response suppression learning of 17-day-old rat pups is mediated, at least partially, by a DAD-2 receptor system, and that D-2 receptors are also involved in the locomotor activity and nociceptive responses of young rat pups.

Facilitation and impairment of conditioning in the preweanling rat after prior exposure to the conditioned stimulus

Olfactory aversion conditioning of preweanling rats, 10 or 18 days postnatal, was tested after some had been given nonreinforced experience with the to-be-conditioned odor stimulus. In three experiments, it was established that the amount of prior exposure to the CS determined the effectiveness of conditioning. For both ages, odor-shock conditioning was more likely impaired with longer durations of preexposure. This effect was more apparent in the older animals. Low to moderate degrees of prior exposure to the CS under some conditions facilitated, rather than impaired, olfactory conditioning in the 10-day-old rat. This result is in agreement with one previous study in which facilitation in learning was reported for this age after short-term preexposure to the CS. The present study adds to previous data on differential effects of CS preexposure on conditioning. Although the conditions under which facilitation rather than impairment occurs are not yet clear, age-related differences in the effects of CS preexposure were apparent in the present experiments.

Lipogenic potential in liver of the preweanling rat: influence of dietary cholesterol.

We examined the effect of a lower dietary cholesterol load on hepatic lipogenic capacity and plasma cholesterol concentrations during the normal suckling period in artificially reared preweanling rats. The artificially reared rats were fed a milk formula that contained low or normal concentrations of cholesterol during the period from the 5th to 17th day after birth. The activities of HMG-CoA synthase and HMG-CoA reductase in livers of 17-day-old rat pups reared on the low-cholesterol diet were enhanced three- to five-fold over those observed in the age-matched rats in the normal cholesterol and mother-reared control groups. The concentration of cholesterol in plasma of rats reared on the low-cholesterol milk was about 20% lower than that for mother-reared controls. In contrast, rats reared on milk with normal cholesterol content exhibited plasma cholesterol levels about 25 and 50% higher than the mother-reared and low cholesterol groups, respectively. The long-term metabolic consequences of rearing rats on milk formulations without adequate cholesterol remains to be determined.

Effects of 8-OH-DPAT on ultrasonic vocalization and audiogenic immobility reaction in pre-weanling rats

Emotional reactivity in preweanling rats was assessed by observations of two reactions characteristic for two different age periods, respectively. One reaction, ultrasonic vocalization in rat pups separated from mother and littermates, was observed during early postnatal age (10 days). The other reaction, rigid immobility elicited by a sudden sound, was observed at the age of weaning (20 days). Previous studies from this laboratory indicated that the serotonergic system was involved in the control of both of these reactions. In the present study this notion was further tested by investigating the effects on these reactions of the congener of ergot 8-hydroxy-2-(di- n-propylamino)-tetralin (8-OH-DPAT), characterized as a centrally-acting, potent and selective agonist of 5-hydroxytryptamine (5-HT). Rat pups, 10 days of age, were treated subcutaneously with 7.5, 15 and 30 μg/kg of 8-OH-DPAT and subsequently tested for ultrasonic vocalization. Animals, 20 days old, were treated with 15, 30 and 60 μg/kg of 8-OH-DPAT before testing of the immobility reaction. The results showed a dose-dependent decrease of the amount of ultrasonic vocalization and of the duration of the immobility reaction, indicating an anxiolytic-like action of 8-OH-DPAT in both behavioural patterns. A possible explanation for the antagonistic effect of 8-OH-DPAT is that this drug exerts a preferential agonistic effect on presynaptic (auto-) 5-HT receptors.