Linda Patia Spear

The adolescent brain and age-related behavioral manifestations

To successfully negotiate the developmental transition between youth and adulthood, adolescents must maneuver this often stressful period while acquiring skills necessary for independence. Certain behavioral features, including age-related increases in social behavior and risk-taking/novelty-seeking, are common among adolescents of diverse mammalian species and may aid in this process. Reduced positive incentive values from stimuli may lead adolescents to pursue new appetitive reinforcers through drug use and other risk-taking behaviors, with their relative insensitivity to drugs supporting comparatively greater per occasion use. Pubertal increases in gonadal hormones are a hallmark of adolescence, although there is little evidence for a simple association of these hormones with behavioral change during adolescence. Prominent developmental transformations are seen in prefrontal cortex and limbic brain regions of adolescents across a variety of species, alterations that include an apparent shift in the balance between mesocortical and mesolimbic dopamine systems. Developmental changes in these stressor-sensitive regions, which are critical for attributing incentive salience to drugs and other stimuli, likely contribute to the unique characteristics of adolescence.

Neurobehavioral Changes in Adolescence

Adolescents across a variety of species exhibit age-specific behavioral characteristics that may have evolved to help them attain the necessary skills for independence. These adolescent-related characteristics, such as an increase in risk taking, may be promoted less by the hormonal changes of puberty than by developmental events occurring in brain. Among the prominent brain transformations of adolescence are alterations in the prefrontal cortex, limbic brain areas, and their dopamine input, systems that are sensitive to stressors and form part of the neural circuitry modulating the motivational value of drugs and other reinforcing stimuli. Such developmental transformations of the adolescent brain may predispose adolescents to behave in particular ways and make them particularly likely to initiate use of alcohol and other drugs.

Prenatal cocaine exposure disrupts the development of the serotonergic system

Prenatal cocaine exposure has been found to result in a number of neurobehavioral abnormalities in both clinical and laboratory studies. We have previously shown that cocaine inhibits the growth of developing serotonin neurons in culture. This study examines the effects of cocaine on the developing serotonin system in vivo. Pregnant rats were injected with cocaine (40 mg/kg s.c.) from gestational day 13 to parturition. One group of rats was additionally injected on postnatal days 1-5 with cocaine (10 mg/kg s.c.). [3H]Paroxetine, a selective ligand for the serotonin uptake carrier, was used to quantify serotonin terminal fiber density at one day, one week, and four weeks postnatal. Cocaine exposure was found to significantly decrease [3H]paroxetine-labelled sites and thus the density of serotonin fibers in the cortex and hippocampus at one day and one week postnatal. By four weeks postnatal, no significant effect was observed, indicating that a recovery had occurred. Serotonin immunocytochemistry performed at one month revealed normal fiber distribution in the cortex but a loss of fibers in the CA1 and CA2 hippocampal fields. Postnatal treatment alleviated the effects of prenatal cocaine exposure, resulting in [3H]paroxetine binding levels at one week which were comparable to and, in the cortex, even higher than those of saline controls. We conclude that cocaine delays the maturation of the serotonin system when administered prenatally but may accelerate maturation when administered both pre- and postnatally.

Ontogeny of ethanol elimination and ethanol-induced hypothermia

Ontogeny of ethanol elimination rates and ethanol-induced hypothermia were examined as possible mechanisms contributing to the marked reduction in ethanol sensitivity early in life (Little et al., 1996; Silveri & Spear, 1998) and the notable gender difference in ethanol sleep-time seen in adult animals (Silveri & Spear, 1998). Elimination rates and brain/blood ethanol levels were determined following doses of 1.5 or 4.5 g/kg ethanol in male and female Sprague-Dawley rats at postnatal days (P)16, 26, 36, or 56. Animals were sacrificed at 40, 80, or 160 min post-injection, with ethanol elimination rates estimated from the slope of the regression of blood and brain alcohol levels across the three sampling periods. P16 animals exhibited the slowest rate of ethanol metabolism, while no gender effects were evident at any age. Observed ontogenetic increases in ethanol hypothermia were not systematically related to the ontogeny of ethanol metabolism. Factors other than ontogenetic changes in ethanol metabolism, hypothermia, or the distribution of ethanol between brain and blood must underlie the relative insensitivity to ethanol often reported in young and adolescent organisms, a fruitful area for future studies given the frequent use and misuse of alcohol by human adolescents.

Cocaine-induced behavior in the developing rat.

In this study, the ontogenetic pattern of psychopharmacological responsiveness to the indirect noradrenergic agonist cocaine was compared with that of the α-adrenergic agonist clonidine. Male and female Sprague—Dawley albino rats were given saline or 5, 10, or 25 mg/kg cocaine hydrochloride and were tested using a behavioral time-sampling procedure on Postnatal Days 7, 14, 21, 28, or 35. For comparison, other animals were given 0.5, 1, or 2 mg/kg clonidine and tested using the same procedures on Postnatal Days 7, 14, and 21. During the first 2 weeks of life, administration of either noradrenergic agonist resulted in an increase in locomotor movements and matrix crossings. However, during this time interval, clonidine induced marked wall climbing behavior which was not seen after cocaine administration at any age. Moreover, although the amounts of locomotor movement induced by the two drugs at these ages were of the same magnitude, cocaine administration induced much more of an increase in matrix crossings on Postnatal Day 14 than did clonidine. On Postnatal Day 21, cocaine administration continued to produce an increase in locomotor movements and matrix crossings while clonidine induced cataleptic behavior. Cataleptic behavior was not seen after cocaine administration at any age. The two noradrenergic agonists also had opposite effects on grooming behavior at certain postnatal ages. These results suggest that the ontogenetic patterns of behavior response to these two noradrenergic agonists show not only some similarities but also some notable differences. Possible explanations of these results are discussed. Caution must be exerted in generalizing from the ontogenetic responses to one drug acting upon a neurotransmitter system to the ontogeny of that system in general.

Methodological considerations in neurobehavioral teratology

Neurobehavioral teratology is a rapidly expanding field benefitting from recent advances in neurobiology and behavior and from the increasing availability of compounds with specific pharmacological actions. There is evidence that data derived from animal studies are clinically pertinent and hence animal studies are useful in extending clinical findings, in anticipating consequences of early drug exposure and, by determining the underlying neural mechanisms, in developing therapeutic approaches. However, the usefulness of animal studies crucially depends on the reliability and sensitivity of the methods used. We highlight the importance of appropriate selection of the route, dose, frequency, duration and timing of drug administration. We also emphasize the importance of not confounding treatment with litter effects and suggest that either the litter be used as the unit of analysis, or that each litter contribute only one pup to each test condition. We discuss the question of the time of testing and of testing, not only under baseline conditions, but also in conditions in which the offspring are exposed to stressful, pharmacological or cognitive challenges. We hope that future studies will benefit from these considerations and avoid the methodological weaknesses that beset some of the early studies in this field.

Cocaine and development: A retrospective perspective

Neurobehavioral alterations evident in offspring of Sprague-Dawley rat dams exposed to 40 mg/kg/day cocaine subcutaneously from gestational days 8-20 are reviewed. Consequences for offspring are often age dependent: for instance, reliable deficits in classical conditioning are evident during the early postnatal period, whereas cognitive effects are less pervasive in adulthood, although apparent in tasks such as reversal training. Gender of offspring is another variable of importance, particularly when testing animals in adulthood, with adult male offspring being more likely than their female counterparts to exhibit alterations following the prenatal exposure regimen. Characteristics of the test situation likewise influence detection of outcome effects, with effects particularly likely to emerge under stressful testing conditions or other challenges to the organism. Under these circumstances, alterations in responsiveness to stressors also sometimes emerged in offspring of pair-fed (PF) dams (whose food intake was restricted to match that of cocaine-exposed [COC] dams); these findings perhaps should not be surprising given that pair feeding is a stressor and prenatal stress is known to alter later stressor responsiveness. Although several approaches to equate food intake or avoid pair feeding have yielded disappointing findings, one promising approach is to initiate cocaine administration prior to mating followed by exposure throughout gestation. Premating exposure to cocaine was sufficient to eliminate anorexic effects of drug delivery during pregnancy, although it remains to be seen how similar the pattern of neurobehavioral alterations that emerge with this extended exposure regimen will be to effects seen following more restricted gestational exposure.

Neurobehavioral assessment during the early postnatal period

Few laboratories investigating the neurobehavioral consequences of developmental toxicants assess offspring early in ontogeny other than examining physical maturation, reflex development and perhaps locomotor activity, measures which tap only a limited portion of the neurobehavioral capacities of young organisms. The importance of including a wider range of neurobehavioral assessments during the early postnatal period in developmental toxicology test batteries is discussed. Special considerations for the design of testing early in life are enumerated, and examples are given of suckling, cognitive and psychopharmacological tests that have been shown to be sensitive indicators early in life of the effects of gestational drug exposure.

Ontogenesis of morphine-induced behavior in the rat

Ten, seventeen and twenty-four day old rats were observed using a behavioral-time sampling procedure following injection of saline, 0.1, 0.5, 1.0 or 5.0 mg/kg/5cc morphine sulphate. At Day 10, the predominant response to morphine was a depression of locomotor activity. At the 5 mg/kg dose, catalepsy was also seen. In animals of this age, 0.1 mg/kg morphine, which was not sufficient to depress activity, also had no effect on stereotyped gnawing/mouthing behavior, nor did it produce any increase in locomotor activity. A morphine-induced increase in locomotion was first seen on Day 17 (after 0.5 mg/kg morphine). As with Day 10 animals, Day 17 animals given 5 mg/kg morphine showed a depression of locomotion and catalepsy. Stereotypic gnawing/mouthing behavior was first seen in Day 24 animals (after 5 mg/kg morphine), although no dose of morphine produced significant differences in activity at this age. Possible mechanisms resulting in these marked alterations in behavioral response patterns to morphine during this two week period of ontogeny are discussed.

Effects of prenatal cocaine on behavioral adaptation to chronic stress in adult rats

Prenatal exposure to cocaine in rats has previously been shown to alter the behavioral and hormonal responses to acute stressors, although no work has yet examined stress adaptation in these animals in adulthood, a possibility examined in this experiment. Male and female offspring of Sprague-Dawley rat dams given 40 mg/kg/3 ml subcutaneously daily from gestational days 8-20 (C40), saline injected and pair-fed dams (PF), and non-treated dams (NT) were tested in adulthood (90-120 days). Offspring were given a 5-min open field test 24 h following the last of 1 (Acute), 9 (Chronic) or 0 (control) daily 15-min intermittent footshock sessions. Substantially more behavioral adaptation was evident in NT offspring than in C40 and PF animals. The attenuated stress adaptation seen in C40 offspring extends prior work showing altered stress responsiveness in these animals, although the PF data caution against the conclusion that this lack of stress adaptation necessarily reflects gestational exposure to cocaine per se.