Neural Mechanism of Photoperiodism

In neural mechanisms underlying photoperiodism, photoperiodic information is received by photoreceptors and is processed in the photoperiodic clock and counter in the brain. The processed signals are then switched to the endocrine organs. Neuroanatomy, microsurgery, and electrophysiology, in combination with RNA interference, have revealed plausible photoperiodic neural circuitries that employ circadian clock cells and neurosecretory cells in the pars intercerebralis and pars lateralis. In the blow fly Protophormia terraenovae and the bean bug Riptortus pedestris, an anterior base of the medulla region containing clock protein PERIOD cells and neuropeptide pigment-dispersing factor cells is a potential site for photoperiodic mechanisms. In P. terraenovae PERIOD nuclear localization suggests that clock cells of s-LNv and DNm have different phase settings to environmental light-dark cycles. By comparing the clock phases of s-LNv and DNm, short and long days are distinguishable. In R. pedestris, pars intercerebralis neurons show a photoperiodic response in their firing activities, depending on the period expression. The neurotransmitter glutamate mediates short-day signals to pars intercerebralis neurons under period expression. In the proposed neural circuitry, cellular responses such as electrical activities, gene expression, and fiber projection patterns to short and long days should be revealed to understand the photoperiodic clock and counter mechanisms in future studies.