February 2, 2023
Keio University
Madoka Utsumi, a first-year master's student at the Graduate School of Science and Technology, Keio University, along with Professor Kotaro Oka and Associate Professor Koji Hotta from the university's Faculty of Science and Technology, have revealed that a pair of motor neurons responsible for the rhythmic spontaneous movements in the early development of the sea squirt *Ciona intestinalis* Type A exhibit a seven-stage change in neural activity as development progresses.
Rhythmic motor patterns such as walking and swimming are generated by the central pattern generator (CPG) located in the spinal cord. However, the dynamics of the individual neurons that control this circuit and the formation mechanism of the CPG circuit responsible for rhythmic neural activity remain unclear.
In this study, we investigated this mystery using the sea squirt *Ciona intestinalis* Type A, the closest living relatives of vertebrates, for which the cell lineage and complete neural connectivity information in the larval stage are known. The swimming larvae of *Ciona intestinalis* Type A can swim by swinging their tails through alternating contractions of the tail muscles on the left and right sides. Our previous research has shown that the rhythm of the Early Tail Flick (ETF), an initial motor behavior during the late tailbud stage of the ascidian, is generated by a pair of motor neurons, MN2, located in the motor ganglion (Akahoshi et al ., 2021). However, that study only observed the neural responses of MN2 up to the pre-hatching stage (St. 24), and it remained unclear whether MN2 was involved in generating the rhythm for the side-to-side tail swinging during and after the swimming larval stage.
In the present study, we succeeded in long-term measurement of the calcium responses of the left and right MN2 neurons. Based on criteria such as their firing intervals and synchronous/asynchronous activity, we showed that the firing patterns of MN2 can be divided into seven phases. By Phase III, the MN2 rhythm converges to a cycle of several tens of seconds; from Phase V, the rhythms of the left and right MN2 begin to synchronize; in Phase VI, the period lengthens; and finally, even in Phase VII, when the tail regresses due to metamorphosis, movement ceases, and neural activity is no longer required, the MN2 neurons continued to fire. This suggests that the MN2 neurons continuously control the tail movements throughout the life of the swimming larva, from the time the ascidian's tail begins to move, through the swimming phase, until the tail disappears. The results of this study are expected to be a significant discovery that will contribute to elucidating the formation process of CPG circuits.
The research results were published online in the Swiss scientific journal Frontiers in Cell and Developmental Biology on January 13, 2023.
The full press release is available below.