Genetic background, nature of event, and time of exposure to event direct the phenotypic expression of a particular genotype

Individual differences in drug dependence in rats: the role of genetic factors and life events

Drug dependence and addiction is a chronic mental illness that has far reaching consequences for society in terms of economic loss, health costs and judicial problems. A crucial question in drug addiction, is what factors are involved in its aetiology, and especially what mediates the shit from use to abuse. As in most other mental illnesses, addiction can best be described using the so-called three hit model, which states that a disease results from an interaction between genetic factors, early lie events and late environmental factors. However, the precise nature of these factors still remains to be elucidated. This present review discusses the results from an animal model in which these three different hit are currently being investigated. The apomorphine susceptible (APO-SUS) and apomorphine unsusceptible (APO-UNSUS) rats, originally selected on the basis of their behavioural response to the dopaminergic agonist apomorphine, were recently found to be genetically different in the number of gene copies of a component of the gamma-secretase complex called Aph-1b. Whereas APO-UNSUS rats have three copies of the gene, APO-SUS rats have either 1 or 2 copies. In addition we have shown that these rats show differences in cocaine and alcohol self-administration, and that both early life events and late environmental factors can alter this self-administration behaviour. Thus, the data so far support the hypothesis that the APO-SUS and APO-UNSUS rats offer an interesting animal model for drug dependence in which genes and environment interact. We finally propose a theoretical model which can explain this gene-environment interaction.

Stress and the brain: from adaptation to disease

In response to stress, the brain activates several neuropeptide-secreting systems. This eventually leads to the release of adrenal corticosteroid hormones, which subsequently feed back on the brain and bind to two types of nuclear receptor that act as transcriptional regulators. By targeting many genes, corticosteroids function in a binary fashion, and serve as a master switch in the control of neuronal and network responses that underlie behavioural adaptation. In genetically predisposed individuals, an imbalance in this binary control mechanism can introduce a bias towards stress-related brain disease after adverse experiences. New candidate susceptibility genes that serve as markers for the prediction of vulnerable phenotypes are now being identified.

Gene dosage effect on gamma-secretase component Aph-1b in a rat model for neurodevelopmental disorders

A combination of genetic factors and early life events is thought to determine the vulnerability of an individual to develop a complex neurodevelopmental disorder like schizophrenia. Pharmacogenetically selected, apomorphine-susceptible Wistar rats (APO-SUS) display a number of behavioral and pathophysiological features reminiscent of such disorders. Here, we report microarray analyses revealing in APO-SUS rats, relative to their counterpart APO-UNSUS rats, a reduced expression of Aph-1b, a component of the gamma-secretase enzyme complex that is involved in multiple (neuro)developmental signaling pathways. The reduced expression is due to a duplicon-based genomic rearrangement event resulting in an Aph-1b dosage imbalance. The expression levels of the other gamma-secretase components were not affected. However, gamma-secretase cleavage activity was significantly changed, and the APO-SUS/-UNSUS Aph-1b genotypes segregated with a number of behavioral phenotypes. Thus, a subtle imbalance in the expression of a single, developmentally important protein may be sufficient to cause a complex phenotype.