Bone remodelling is a complex process, which involves the coupling of bone formation to completed foci of bone resorption, the balance between these 2 processes determines if bone is lost or gained at a particular site. During bone resorption osteoclasts release growth factors sequestered in bone matrix, which are thought to initiate new bone formation. On the other hand, osteoblasts can regulate osteoclast activity through the expression of the counter-acting cytokines, RANKL and OPG. The aim of this project was to determine if factors released during bone resorption impact on the RANKL/OPG system or on osteoclasts directly to regulate bone remodelling. OPG secretion was characterized in a number of osteoblast-like cells and the osteosarcoma cell line MG-63 was chosen as a model for osteoblastic cell behaviour in vitro. EDTA bone extracts prepared from normal human cortical bone powder were used to treat MG-63 cells in vitro. The response to the extract was dependent on the purification procedure used. OPG production was inhibited by partially purified extracts prepared using hydrophobic interaction chromatography, C18 SPE. In comparison extracts prepared using size exclusion centrifugal filters stimulated OPG secretion in confluent MG-63 cells. Therefore bone matrix constituents were able to influence osteoclast activity directly and indirectly through the osteoblastic cells to produce the same response. The simplest mechanism for this co-ordinated response would be the presence of one factor in the extract that is able to influence both osteoblasts and osteoclasts. The identity of the factor responsible for the opposing effects seen in the bone matrix extracts is at the moment unknown. The work presented in this thesis clearly demonstrated that unknown growth factors present in bone matrix influence bone remodelling.

The purpose of this study was to identify the effects of the neurotransmitter, serotonin (SE), on the immune function of peripheral blood mononuclear cells (PBMC) of normal, healthy subjects. This was done as a preliminary investigation to studies on the association of SE with immune changes in autistic subjects. The PBMC isolated from normal male subjects were treated with various concentrations of SE for 48 hrs. Their incubation in SE at a concentration of 10-3 M induced about a 35% decrease in DNA synthesis. However, incubation of the cells in lower concentrations (10-4 to 10-10) of SE produced no significant effect. The ability of natural killer (NK) cells to lyse K562 target cells was also examined after incubation with SE for 48 hrs. The NK activity was almost completely eliminated following incubation in 10-3M of SE, but the activity was not significantly decreased by exposure to lower concentrations of SE. The viability of PBMC was not altered following incubation with SE under identical conditions as those utilized in the NK assay. Preliminary analysis using a fluorescence-activated cell sorter (FACS) of monoclonal antibodies directed against Tll (total T cell), T4 (helper T cell), T8 (suppressor and cytotoxic T cells), B-cell and NK cell markers indicated that the suppressive effect exerted by SE could be attributed to a decrease in the density of these markers or receptors on the cell surface. These findings provide additional evidence for a possible link between neurotransmitters, specifically SE, and immune function.