A novel in vivo approach to study circadian rhythmicity of glucocorticoids using adeno-associated virus

Abstract

The hypothalamic-pituitary-adrenal (HPA) axis regulates glucocorticoid release in a circadian rhythm as well as in response to stress. The adrenal gland is made up of two distinct tissues: the cortex, which produces glucocorticoids, and the medulla. The cortex responds to hormonal cues while the medulla receives input from the splanchnic nerve. Interestingly, cutting the splanchnic nerve has been shown to affect cortical function, indicating a possible interaction between the two tissues. However, the precise mechanism of this interaction remains unknown. An experimental approach that genetically manipulates adrenal activity could potentially delineate the functional relationship between medulla and cortex. One option is to infect adrenal tissue with recombinant adeno-associated virus (AAV) capable of silencing a gene of interest in a specific tissue type. This is a novel approach in the adrenal gland, so we must first develop a protocol that will optimize methodological variables such as AAV serotype, surgical procedure, injection volume, and infection time-course. We tested various combinations of these variables and used GFP-tagged AAV to assess the degree of infection in rats. Thus far, we have determined that the serotype AAV8 yields the greatest degree of infection, and AAV2 and AAV5 seem to selectively infect the cortex. In addition, the concentration of virus injected is more important than the volume, and the injection method can have a large impact. Stress tests revealed that injection of AAV-GFP does not appear to alter adrenal function. AAVs thus appear to be promising tools in neuroendocrine studies, as they provide a means for genetically modifying the adrenal gland. Future experiments will use this new technique to analyze the functional relationship between adrenal cortex and medulla.