Histamine mediated neuropeptide gene expression in bovine chromaffin cells

Abstract

The adrenal medulla regulates the acute stress response and is itself controlled through a variety of signals, including neuronal, endocrine and paracrine factors. Additionally, recent literature suggests that adrenal medullary function may also be influenced by signals from the immune system, such as interleukin-1, interleukin-6 and tumor necrosis factor-α. Secretions from the adrenal medulla may in return influence the immune response thereby suggesting a bi-directional relationship between the immune system and the stress response. Recent microarray data from our laboratory have indicated that isolated bovine adrenal medullary chromaffin cells are also responsive to histamine, another immune-derived signal. Exposure to histamine results in marked changes to their neuropeptide gene expression. The aims of this thesis were to investigate this histamine-mediated neuropeptide gene expression by using real time PCR to validate the microarray results, to examine the signalling mechanisms involved and to explore any interactions between histamine and other known regulators of adrenal medulla. Cultured bovine chromaffin cells were incubated in absence or presence of histamine for 6-48h. The mRNA was extracted and analysed using qRT-PCR for the neuropeptides which were vasoactive intestinal peptide (VIP) and galanin. Additional experiments were performed using protein kinase inhibitors to examine the signalling pathways involved. Immunocytochemistry was also carried out in an attempt to localise the histamine-induced changes VIP protein expression. The results confirm histamine induces an increase in both VIP and galanin mRNA, the latter being mediated by protein kinase C. The pathway responsible for stimulating VIP mRNA was not positively identified but does not involve protein kinase A or C. Synergistic interactions were seen between histamine and other adrenal medullary activators (PACAP and IL-6) for VIP but not galanin mRNA. Immunocytochemistry however did not show any localisation of VIP, which was probably due to the VIP antibody being very non-specific. Thus whether the changes in neuropeptide gene levels result in alterations in protein expression remains unresolved. Given that VIP and galanin are both implicated in anti-inflammatory roles these results are consistent with the concept of a bi-directional relationship between stress response and the immune system.