Light- and dark-dependent orexinergic neuronal signals promote neurodegenerative phenomena accounting for distinct behavioral responses in the teleost Thalassoma pavo

The neuropeptides hypocretins/orexins (ORX) areknown to control state-dependent activities such assleep–wakefulness, energy homeostasis, thermoregulation, and maternal behaviors. To date, interests regarding environmental-related ORX-ergic neuronal functions have dealt with mammals; only recently is attention beginning to be directed toward aquatic vertebrates. Here, photoperiod-dependent effects of ORX-A on behavioral, neurodegenerative and transcriptional activities were evaluated in some forebrain areas of a teleost Labridae (ornate wrasse, Thalassoma pavo). The ornate wrasse, when treated intraperitoneally with a high physiological dose (100 ng/g) of ORX-A and exposed to a natural photoperiod(12L:12D), exhibited very high (P < 0.001) locomotionand feeding behaviors. ORX-A in the presence of aconstant light photoperiod accounted for numericallyeven greater (>500%) feeding frequencies. Conversely,constant dark conditions very strongly reduced feeding habits and moderately (P < 0.05) increased resting states. In this case, the same ORX-A and photoperiodic conditions responsible for altered behaviors also induced neurodegenerative processes in the different hypothalamic, mesencephalic, and telencephalicneuronal fields. Interestingly, this ORX-A treatment seemed to be correlated to greater up-regulatorypatterns of ORX receptor mRNA in these samebrain areas, above all under constant light conditionsrather than natural photoperiod. On the other hand,telencephalic sites provided a very active expressioncapacity during the dark phase. Overall, these resultssuggest for the first time that at least in the ornatewrasse, light- and dark-dependent ORX-ergic neuronalactivities are able to cause short- and long-termabnormal motor behaviors, likely through neurodegenerative and transcriptional events in a brain regional manner.