Neurotransmitter contribution of neuronal subpopulations affect properties of the circadian clock

In mammals, the suprachiasmatic nucleus (SCN) orchestrates the circadian rhythms even without the daily light–dark cycle (resulting in free running behavior). The SCN neurons can be organized into two heterogeneous subgroups, namely the ventrolateral (VL) subgroup and the dorsomedial (DM) subgroup, which are coupled through different neurotransmitters, i.e., the VL and the DM produce VIP and AVP, respectively. Experiments have found that they also differ in their neural amplitudes. In the present study, we examined the effect of disparities in transmitter signals released by two subgroups on the collective behaviors of the SCN neurons, in particular the free running period and the synchronization degree. The disparity of transmitter signals released by the VL and the DM is represented by the ratio of weight parameters in the mean-field, based on the Poincaré model. We find that the effects depend on the difference of neuronal amplitudes between the two subgroups. If there exists an amplitude difference between the subpopulations, the relationship of the free running period to the ratio is parabolic with a trough. Additionally, the relation between the synchronization degree and the coupling strength is also dependent on the ratio of weight parameters in the mean-field. Interestingly, when the ratio is large, the synchronization degree shows a parabolic-like change with the increase of the coupling strength. Our findings shed light on the rhythmic regulations of the SCN neurons affected by the disparities in the strengths of transmitters released by different subpopulations.