Emergent Synchronous Bursting of Oxytocin Neuronal Network
Figure 9
Effect of Suckling on Electrical Activity of Model Cells A and B.
In the model, suckling results in activity-dependent retrograde inhibition of the synaptic inputs. Accordingly, as synaptic input level increases, electrical activity increases faster in the absence of suckling than in its presence. (A) The blue squares show the mean firing rate of model cells with normal network interaction (i.e. with suckling input), as a function of the mean synaptic input rate. The red diamonds show the behaviour in the same conditions but without suckling. The inhibitory input is fixed at 80 Hz. Thus in the model, suckling reduces the background activity of the fastest firing cells. (B) As in (A), but plotting the coefficient of variation (CV) of the interspike intervals (SD (interval)/mean (interval)). This standard measure of the irregularity of firing shows that the model cells fire more regularly as the mean level of synaptic input increases. The effect of suckling is to increase the irregularity of firing of neurons, mainly by reducing the firing rate of the fastest cells. (C) In the model, during suckling, the frequency of milk-ejection bursts is related to the average level of synaptic input in a biphasic manner. At very low and at very high levels of input, bursting will not occur. The frequency of bursts was obtained by simulating the model at varying excitatory input rates and, also shows the effect of altering the frequency threshold for dendritic release: the higher the threshold, the fewer bursts will occur. (D) The frequency of bursting depends on how homogeneous the background firing rate of oxytocin cells is. Here, we looked at the effect of a spatially inhomogeneous input on bursting frequency (mean of five trials of 50 min). Homogeneity is measured as the ratio of the SD of the synaptic input rate over the mean. The bars are SD. (E)The effect of endocannabinoids in the model is to increase the range of synaptic input rates compatible with bursting, and to make the mean rate at which bursts occur relatively independent of synaptic input rate within this range. Here, this is illustrated by looking at the consequences of removing the effect of endocannabinoids (no EC, by = 0). This is true for all the threshold values considered, the α setting panel shows the comparison for the control case (fth = 20 Hz).