Figure 7. Using the model to simulate the effect of hypertonic saline injection.

An injection of hypertonic saline causes a rapid increase in osmotic pressure. The rapid increase in input causes some initially slow firing vasopressin cells to shift immediately to fast continuous firing before settling in a phasic pattern after a long delay (∼10 min) [26]. We hypothesise that this is due to insufficient availability of dendritic dynorphin, which takes time to upregulate. We tested this using an extension to the basic model and were able to reproduce the effect observed in vivo. Osmotic pressure was initially set at 295 and increased to 315 by injection at 5 min. We suggest that releasable dynorphin store upregulation is dependent on a slow activity-dependent vesicle transport mechanism, T. The extended dynorphin mechanism uses parameters k_T = 0.00001, λ_T = 500000, k_{D store} = 0.02, λ_{D store} = 1000000, D _spike = 0.1, D _storecap = 10, and τ_O = 200000.