Prenatal administration of methadone in the rat: acoustic startle amplitude and the rest-activity cycle at 30 days of age.

Two doses of methadone were administered by osmotic minipump from Day 8 of gestation through parturition, a dosing technique previously shown to produce physical dependence in the dams. A pair-fed control group received sterile water via minipump and was allowed to eat and drink only the amount consumed by the high-dose group on the same gestation days. A nontreated control group was left undisturbed during pregnancy. All treated and control litters were fostered at birth to untreated dams. The effects of methadone on maternal and offspring toxicity replicated our previous findings. At 29-31 days of age each treated and control animal was tested either for changes in acoustic startle amplitude or the rest-activity cycle. Methadone treated offspring were no different from the controls on either measure. These findings support the hypothesis derived from our earlier research that prenatal exposure to methadone produces a prolonged but transitory opioid abstinence. This is evidenced by increased startle amplitude and a disturbed rest-activity cycle that peaks at approximately 3 weeks of age. We demonstrate that these effects are no longer evident at 4 weeks of age. Together, these findings define a state of hyperexcitability in the young rat that resolves by 1 month of age. This transitory state parallels clinical descriptions of human infants undergoing opiate abstinence.

Prenatal administration of methadone in the rat increases offspring acoustic startle amplitude at age 3 weeks.

Two doses of methadone were administered by osmotic minipump from Day 8 of gestation through parturition, a dosing technique previously shown to produce physical dependence in the dams. A pair-fed control group received saline via minipump and was allowed to eat and drink only the amount consumed by the high dose group on the same gestation days. A nontreated control group was left undisturbed during pregnancy. All treated and control litters were fostered at birth to untreated dams. The effects of methadone on maternal and offspring toxicity replicated our previous findings. At 21-23 days of age, acoustic startle amplitude was measured for each treated and control animal. Because prenatal methadone exposure resulted in reduced body weight at the time of testing, it was necessary to analyze startle amplitude using weight as a covariate. This analysis showed that the methadone treated offspring had a significantly enhanced mean startle amplitude compared with the controls. These findings support the hypothesis derived from our earlier research that prenatal exposure to methadone produces a prolonged state of CNS hyperexcitability similar to clinical descriptions of human infants undergoing opiate abstinence.

Prenatal administration of methadone using the osmotic minipump: effects on maternal and offspring toxicity, growth, and behavior in the rat.

Two doses of methadone were administered by osmotic minipump from day 8 of gestation through parturition. A pair-fed control group received saline via minipump and was allowed to eat and drink only the amount consumed by the high dose group on the same gestation days. A nontreated control group was left undisturbed during pregnancy. All treated and control litters were fostered at birth to untreated dams. Naloxone challenge of the dams after parturition showed that drug treatment produced physical dependence. Methadone treatment reduced maternal weight gain but had no effect on either the frequency of resorptions or birthweight. Both doses of methadone increased perinatal mortality but only the high dose produced a decrement in postnatal growth. To examine the effects of methadone on the rest-activity cycle of the offspring, groups of three littermates from each of the treated and control groups were tested for an 8 h observation period on electronic activity monitors at 22 days of age. No behavioral effects were observed for either control group or the low dose methadone group. The high dose methadone offspring, however, spent less time resting, showed disrupted rhythmicity, and poor state regulation. These findings are discussed in relation to earlier studies using once per day methadone administration as well as clinical descriptions of infants undergoing opiate abstinence.

Prenatal delta-9-tetrahydrocannabinol in the rat: effects on auditory startle in adulthood.

Either 15 or 30 mg/kg of delta-9-tetrahydrocannabinol (delta-9-THC) was administered to pregnant rats by gastric intubation from Day 2 through Day 22 of gestation. Pair-fed and nontreated groups served as controls and all treated and control litters were surrogate fostered at birth to untreated dams. When treated and control male and female offspring were tested for differences in auditory startle at 57-60 days of age, no effects were observed among any of the groups. These findings are compared with other neurobehavioral studies of adult offspring prenatally exposed to cannabis.

Prenatal delta-9-tetrahydrocannabinol in the rat: effects on postweaning growth.

Either 15 or 30 mg/kg of delta-9-tetrahydrocannabinol (delta-9-THC) was administered from Day 2 through Day 22 of gestation. Pair-fed and nontreated groups served as controls and all treated and control litters were fostered at birth to untreated dams. When weighed at 57-60 days of age, pair-fed controls were significantly heavier than the nontreated, whereas the treated animals were intermediate between the controls. These findings are discussed with respect to nutritional studies that have reported postnatal growth enhancement following prenatal maternal undernutrition and the possibility that prenatal delta-9-THC inhibits this effect.

Delta-9-tetrahydrocannabinol during pregnancy in the rat: effects on development of RNA, DNA, and protein in offspring brain.

Either 15 or 50 mg/kg of delta-9-tetrahydrocannabinol (THC) was administered from Day 2 through Day 22 of gestation. Pair-fed and nontreated groups served as controls and all treated and control litters were fostered at birth to untreated dams. To determine the effects of THC on offspring brain development, DNA, RNA and protein values were determined at 7, 14, and 21 days of postnatal age. DNA and RNA levels appeared unaffected by THC but brain protein levels of the 50 mg/kg offspring were significantly lower than in the other groups at Day 7 and 14. This suggests that the high THC dose inhibited protein synthesis for at least the first 14 days of life. Subsequently, protein levels of the 50 mg/kg offspring increased rapidly so that there were no differences between any of the groups at 21 days of age. These findings for developing CNS parallel the delayed rate of somatic growth previously reported from our laboratory and suggest a transitory rather than a permanent effect of THC on both somatic and brain growth. We also found that THC produces a significant dose-related increase in the sex-ratio of live male-to-female offspring, a finding we have reported previously.

Sapid savvy in sucklings: the effect of quinine hydrochloride on intraoral negative pressure and intake by 11-13-day-old rat pups.

Three experiments describe the consummatory behavior of 11-13-day-old rat pups during and following experience with a model aversive taste, quinine hydrochloride. Pups were observed while away from the dam and while suckling. Results show that pups actively reject quinine adulterated solutions in both situations. They do so by spitting the solution from the mouth when away from the nipple and by leaving the nipple and/or decreasing their sucking effort when with the dam.

Delta-9-tetrahydrocannabinol during pregnancy in the rat: I. Differential effects on maternal nutrition, embryotoxicity, and growth in the offspring.

Either 15 or 50 mg/kg of delta-9-tetrahydrocannabinol (THC) in sesame oil was administered by gastric intubation to gravid rats during the last two weeks of gestation. A pair-fed control group was administered the vehicle alone and allowed to eat and drink only the amount consumed by the 50 mg/kg group on the same gestation days. A nontreated control group was left undisturbed during pregnancy. All treated and control litters were fostered at birth to untreated dams. Among the dams receiving 50 mg/kg of THC, food and water intake was initially reduced to 75-80% of nontreated controls but then recovered over 3-4 days to approximately a 15-20% reduction until term. Compared with the nontreated dams, both THC dose-level groups and pair-fed control dams gained significantly less body weight from conception to term. Offspring mortality did not differ between the nontreated and pair-fed controls but was significantly higher among both dose-level THC exposed groups. In addition, there was a dose-related increase in the sex-ratio of live male to female offspring as well as significant effects on rate of growth for both sexes. The results are discussed with respect to published animal and clinical studies of cannabinoid exposure during pregnancy.

Delta-9-tetrahydrocannabinol during pregnancy in the rat: II. Effects on ontogeny of locomotor activity and nipple attachment in the offspring.

Either 15 or 50 mg/kg delta-9-tetrahydrocannabinol (THC) in sesame oil was administered by gastric intubation to gravid rats during the last two weeks of gestation. A pair-fed control group was administered the vehicle alone and allowed to eat and drink only the amount consumed by the 50 mg/kg group on the same gestation days. A nontreated control group was left undisturbed during pregnancy. All treated and control litters were fostered at birth to untreated dams. Intact litters from the two THC treated and the two control groups were tested at 3-day intervals from birth to 32 days of age for differences in locomotor activity. In addition, pups were tested for nipple attachment on days 2, 5, 8, 11, and 14 of age. There were no differences in locomotor activity among any of the groups although activity level varied during development. Pups from dams exposed to 50 mg/kg of THC, as well as the pair-fed controls, displayed significantly longer latencies to attach to a nipple. These results suggest that the impaired nipple attachment observed among the high-dose offspring was not a primary effect of THC, but rather was secondary to the significant reduction of food and water intake among the dams. The behavioral findings are discussed with respect to other animal and clinical reports of prenatal cannabinoid exposure.

Olfactory-guided orientation in neonatal rats is enhanced by a conditioned change in behavioral state.

In Experiment 1, 3-6-day-old rats were simultaneously exposed to an unfamiliar odor which they will normally avoid (orange extract) and the odor of maternal saliva, or to either orange alone or saliva alone. One hour later, in a two-odor choice test, those pups which were simultaneously exposed to orange and saliva exhibited an enhanced orientation to the orange odor. In Experiment 2, orange odor was presented while pups were exposed to the odor of saliva, or while they received one of two types of tactile stimulation (stroking or tailpinching). Pups in control groups were first presented with saliva or tactile stimulation and then the orange odor. Pups that had received any of the three simultaneous exposure treatments subsequently exhibited an enhanced orientation toward the orange odor, but not the pups in the control groups. The results suggest that the tendency of neonates to avoid a novel odor can be reversed by pairing that odor with events that elicit significant increases in behavioral activity. The possibility that this phenomenon reflects a classical conditioning process is discussed.

Huddling and independent feeding of neonatal rats can be facilitated by a conditioned change in behavioral state.

Infant rat pups were exposed to a novel odor (orange or cedar) while they received tactile stimulation (stroking of the body) or were presented with odor or stroking alone. The effects of these treatments were assessed on pups' performance in a huddling test (Experiment 1 and 2) and an independent feeding test (Experiment 3). During these tests, only pups that had received the simultaneous presentation of the odor and stroking exhibited an increase in huddling and feeding. The increase in these behaviors was dependent on the presence of the conditioned odor during testing.

Blood pressure responses to milk ejection in the young rat.

When young rats receive milk ejections from their mothers they exhibit abrupt 30-50% increases in blood pressure. In our first experiment, we recorded, via carotid cannulae, blood pressure and heart rate during natural nursing bouts. Blood pressure changes coincident with behaviorally defined milk ejections were twice as large as the changes associated with other behaviors observed. In addition, the increases in blood pressure during milk ejection were significantly greater when pups were separated from their mothers for 18-20 hours prior to testing. A second experiment showed that these responses were independent of the mother's overt behavior because virtually identical results were obtained when pups received milk from anesthetized dams stimulated to release milk with oxytocin. These results are discussed with regard to possible immediate functions of the response, and long term effects of repeated cardiovascular activation in early development.

A method for recording and analyzing intra-oral negative pressure in suckling rat pups.

A method for recording and analyzing intra-oral negative pressure in suckling rat pups is described. The technique is capable of detecting changes in sucking frequency, rate and amplitude. Three separate patterns of sucking are defined on the basis of these measures.

Periadolescence: age-dependent behavior and psychopharmacological responsivity in rats.

The behavior and psychopharmacological sensitivity of periadolescent rats are examined in this review. Periadolescent rats are hyperactive and engage in more conspecific play behavior than younger or older rats. When compared with other-aged rats, periadolescents exhibit enhanced performance in simple active-avoidance learning tasks, but perform poorly in more complex appetitive and avoidance learning tasks in which increases in locomotor activity do not improve performance, perhaps as a result of age-specific alterations in selective attention or stimulus processing. Such behavioral "anomalies" of periadolescent animals observed in traditional laboratory situations may be in some way adaptive when considered in the context of the animals' natural habitat. In terms of psychopharmacological responsiveness, periadolescent rats, when compared with younger or older animals, are less sensitive to catecholaminergic agonists but are more responsive to the catecholaminergic antagonist haloperidol. This pattern of psychopharmacological sensitivity suggests that the catecholaminergic systems may be temporarily hyposensitive during the periadolescent period. Evidence is presented that a negative feedback system in the form of dopamine autoreceptors may become functionally mature in mesolimbic brain regions during the periadolescent period. The possibility is presented that maturation of these self-inhibitory autoreceptors might result in a temporary decrease in the efficacy of mesolimbic dopamine projections, perhaps contributing to the psychopharmacological and behavioral characteristics of periadolescent animals. In support of this suggestion, evidence is reviewed indicating that the behavior of adult animals with lesions of the ventral tegmental area, a region containing cell bodies from which these mesolimbic dopaminergic projections originate, resembles that of periadolescent rats.

The role of intraoral and gastrointestinal cues in the control of sucking and milk consumption in rat pups.

Nutritive deprivation, suckling deprivation, gastronintestinal fill, and milk availability contribute to the control of sucking (as measured by jaw-muscle electromyograph) and ingestion of milk (provided via a tongue cannula) in 11-13-day-old rat pups. Depriving pups of the opportunity to suckle reliably increases subsequent sucking and milk intake. Intraoral delivery of milk also increases sucking, regardless of whether or not pups are suckling-deprived. Gastrointestinal preloads have no effect on sucking if pups are not receiving milk, but reliably block the increase in sucking which accompanies milk delivery. Finally, milk delivered to the pup's mouth prior to a suckling opportunity can either enhance or attenuate subsequent sucking depending on whether pups are allowed to consume milk while suckling. In all cases, a particular mode of sucking ("rhythmic" sucking) is most affected by experimental manipulation, and appears to be an important component of the pup's ingestive behavior.

Short- and long-term effects of various milk-delivery contingencies on sucking and nipple attachment in rat pups.

Varied milk delivery contingencies differentially affect sucking in rat pups as measured by jaw-muscle electromyographic activity. In Experiment I we found that 11-13-day-old pups sucked more frequently when receiving tiny intermittent pulses of milk than when receiving no milk, and continued to suck at a high rate for at least 30 min after cessation of milk delivery. In subsequent experiments we found that pups 13-18 days of age engaged in more frequent sucking if receiving milk continuously rather than intermittently, and that this increased rate of sucking persisted for at least 24 hr after cessation of milk delivery. Pups given experience with continuous milk delivery from 14 to 18 days of age also spent significantly more time attached to the nipple at 19 days of age than did pups in control groups. It was concluded that experience with different milk delivery schedules can affect subsequent sucking and nipple attachment behavior.

Suckling infant rats learn a preference for a novel olfactory stimulus paired with milk delivery.

When presented a novel olfactory stimulus while suckling a passive dam, 11- to 14-day-old rat pups acquire a conditioned preference for that stimulus. The magnitude of the conditioned preference is greater if the pups received milk while suckling than if they did not. The results indicate that infants are capable of learning while suckling and that milk delivery plays a role in this associative process.