Comparison of the maturation of the adrenergic and serotonergic neurotransmitter systems in the brain: implications for differential drug effects on juveniles and adults

Age of onset of mental disorders: a review of recent literature

PURPOSE OF REVIEW

The aim of this article is to review recent epidemiological research on age-of-onset of mental disorders, focusing on the WHO World Mental Health surveys.

RECENT FINDINGS

Median and inter-quartile range (IQR; 25th-75th percentiles) of age-of-onset is much earlier for phobias (7-14, IQR 4-20) and impulse-control disorders (7-15; IQR 4-35) than other anxiety disorders (25-53, IQR 15-75), mood disorders (25-45, IQR 17-65), and substance disorders (18-29, IQR 16-43). Although less data exist for nonaffective psychosis, available evidence suggests that median age-of-onset is in the range late teens through early 20s. Roughly half of all lifetime mental disorders in most studies start by the mid-teens and three quarters by the mid-20s. Later onsets are mostly secondary conditions. Severe disorders are typically preceded by less severe disorders that are seldom brought to clinical attention.

SUMMARY

First onset of mental disorders usually occur in childhood or adolescence, although treatment typically does not occur until a number of years later. Although interventions with early incipient disorders might help reduce severity-persistence of primary disorders and prevent secondary disorders, additional research is needed on appropriate treatments for early incipient cases and on long-term evaluation of the effects of early intervention on secondary prevention.

Childhood and adolescent depression: why do children and adults respond differently to antidepressant drugs?

Childhood and adolescent depression is an increasingly problematic diagnosis for young people due to a lack of effective treatments for this age group. The symptoms of adult depression can be treated effectively with multiple classes of antidepressant drugs which have been developed over the years using animal and human studies. But many of the antidepressants used to treat adult depression cannot be used for pediatric depression because of a lack of efficacy and/or side effects. The reason that children and adolescents respond differently to antidepressant treatment than adults is poorly understood. In order to better understand the etiology of pediatric depression and treatments that are effective for this age group, the differences between adults, children and adolescents needed to be elucidated. Much of the understanding of adult depression has come from studies using adult animals, therefore studies using juvenile animals would likely help us to better understand childhood and adolescent depression. Recent studies have shown both neurochemical and behavioral differences between adult and juvenile animals after antidepressant treatment. Juvenile animals have differences compared to adult animals in the maturation of the serotonergic and noradrenergic systems, and in dose of antidepressant drug needed to achieve similar brain levels. Differences after administration of antidepressant drug have also been reported for adrenergic receptor regulation, a physiologic hypothermic response, as well as behavioral differences in two animal models of depression. The differences between adults and juveniles not only in the human response to antidepressants but also with animals studies warrant a specific distinction between the study of pediatric and adult depression and the manner in which new treatments are pursued.

Differential Effects of the Tricyclic Antidepressant Desipramine on the Density of Adrenergic Receptors in Juvenile and Adult Rats

Although the tricyclic antidepressants, such as desipramine (DMI), are among the most efficacious treatments for adult depression, they are not effective in treating childhood and adolescent depression. Because the adrenergic nervous system is not fully developed until late adolescence, we hypothesized that the mechanisms regulating receptor density may not yet be mature in young mammals. To test this hypothesis, the effects of DMI treatment on cortical alpha-1-, alpha-2-, and beta-adrenergic receptors were compared in juvenile and adult rats. DMI was delivered either by 4 days of twice daily injections to postnatal day 9 to 13 (4 and 7 mg/kg/day) and adult (20 mg/kg/day) rats, or by 2 weeks of continual drug infusion (osmotic minipumps) to postnatal day 21-35 (15 mg/kg/day) and adult (10 mg/kg/day) rats. These delivery paradigms gave juvenile brain concentrations of DMI similar to those in adult rats. The beta-adrenergic receptor was down-regulated with both treatment paradigms in both juvenile and adult rats. By contrast, in the postnatal day 9 to 13 rats, there was a dose-dependent up-regulation of the alpha-1 in the cortex and alpha-2-adrenergic receptor in the prefrontal cortex, whereas there was no change in density in adult rats. These differences in the alpha-adrenergic receptor regulation after DMI treatment suggest that the lack of efficacy of tricyclic antidepressants in treating childhood depression may be related to immature regulatory mechanisms for these receptors.