Neurons in rat suprachiasmatic nucleus exhibit variable ultradian cycles and retain the rhythmic firing patterns entrained by periodic stimuli.

The suprachiasmatic nucleus (SCN) of the hypothalamus serves as the master circadian pacemaker that synchronizes internal rhythms with the environmental light-dark (LD) cycle. Although SCN spike activity is crucial for maintaining circadian rhythms, there is a scarcity of long-term in vivo electrophysiological recording data, especially for ultradian cycles. Using long-term extracellular single-unit recordings in anesthetized rats, we discovered that multiple types of ultradian cycles, which beyond the three previously reported types, can coexist within a single SCN neuron. The period of these ultradian cycles in one single neuron may shift from nearly 2 min to around 10, 30 or around 80 min, or vice versa. These dynamic changes suggest that periodic stimuli may be particularly suitable for probing the electrical characteristics of SCN neurons. We therefore applied periodic light stimulation in anesthetized rats and performed spectral analysis for neuronal activity. We found that SCN neurons can be entrained by short-period LD cycles ranging from seconds to minutes. Notably, the induced oscillatory firing patterns persisted for more than 1.5 h after stimulus cessation, suggesting persistent reverberatory activity within SCN circuitry. The correlation between firing during and after stimulation increased with stimulus duration and cycle number, suggesting that stronger or longer LD exposure may reinforce post-stimulus oscillatory activity. Moreover, light-on cells exhibited more robust post-stimulus oscillations than light-off cells, implying potentially differential roles in photic encoding. These results suggest periodic light changes can induce persistent rhythmic electrical activity within SCN neurons, potentially reflecting a previously unrecognized form of sub-circadian plasticity within the SCN. We propose an "incomplete gear" model as a hypothetical framework for interpreting temporal dynamics in the SCN. In this framework, SCN neurons exhibit variable ultradian cycle periods, which can be conceptualized as a "incomplete gear" with heterogeneous tooth pitches. Periodic light exposure may bias SCN neuronal activity toward shorter or longer ultradian cycles, and the accumulation of these shifts could contribute to changes in the overall periodicity of neuronal activity, potentially contributing to broader temporal reorganization within SCN networks.