Seasonal And Diel Rhythms Regulate Multistability In A Teleost Vocal Pattern Generator
The central pattern generator controlling vocalizations in songbirds has been investigated for more than 30 years and produced a wealth of information about the morphological and physiological underpinnings of seasonal change in song production and stereotypy. Now departing the aerial lifestyle for an aquatic one, we find a teleost fish that presents not only an annual reproductive rhythm in vocal motor circuit function, but a daily and activity-dependent one as well. The plainfin midshipman, Porichthys notatus, spends winters in the deep, offshore waters of the Pacific coast from Baja to Alaska from where they migrate into the tidal zone in spring and summer to spawn. At night, the parental and highly vocal, type I male uses an acoustic beacon, the advertisement hum, to attract the females to his rocky excavation to lay her eggs, a nest also defended with several agonistic calls. These calls can be studied in a neurophysiological, or "fictive call" preparation, in which the vocal circuit is activated by microelectrical stimulation and the rhythmic output easily monitored by an extracellular electrode on the ventral root nerve that innervates the vocal muscle of the swim bladder. Since this rhythmic motor volley, "or fictive vocalization" directly predicts the temporal properties of the natural calls, it serves as a valid measure of natural plasticity in a dedicated motor circuit. The following studies present for the first time the full repertoire of midshipman fictive calls and how seasonal and diel physiological changes in vocal circuit function determine its variable output. Furthermore, the activity-dependence of these rhythmic fictive calls and their patterning by spatially dynamic levels of GABAergic inhibition may reveal a functional partitioning in the circuit, such that rhythm and duration are controlled at one level, and the frequency shift that distinguishes a broadband grunt from a multiharmonic hum occurs at another.
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