Role of deep brain photoreceptors in regulation of daily and seasonal responses in birds

Birds exhibit an extraordinary morphological, physiological, and behavioral diversity that allows them to adapt to the diverse environments of our planet. To achieve this, they utilize different sensory structures. One of these structures is located in the deep brain and contains neurons with photopigments (Deep Brain Photoreceptors, DBP) that detect daily and seasonal changes in ambient light (photoperiod), allowing the individual to adjust and synchronize physiological processes with the environment. This DBPs detects and transmits light information to the hypothalamic-pituitary-gonadal axis, regulating the gonadal recrudescence/regression cycle and possibly daily responses in birds.

This work reviews and discusses the state of the art about the presence and functionality of DBPs in a phylogenetic context, with a particular focus on annual reproductive responses and their little-known relationship with daily responses. Exceptions to the seasonal reproductive regulation mechanism, as observed in opportunistic bird species such as the eared dove, where food availability appears to drive the activity of the gonadal oscillator are also discussed. Finally, the possible neural pathways through which DBPs transmit photoperiodic information to the circadian system in birds are proposed.