Novel and highly distributed classes of hindbrain neuronal activity contribute to sensory processing and motor control in the Xenopus laevis tadpole

Abstract

Locomotion is a key feature of healthy animals, which depends on their ability to move -or not to move- for their survival. The hatchling Xenopus laevis tadpole responds to trunk skin stimulation by swimming away, and its developing nervous system is simple enough to make it an ideal model organism to study the control of locomotion. This vertebrate embryo relies on excitatory cells in the skin to detect the sensory stimulus, which is quickly sent to the brain via ascending sensory pathway neurons. When the stimulation is strong enough, descending reticulospinal neurons are activated in the hindbrain and the spinal cord, after a long and variable delay. The activation of reticulospinal neurons indicates the initiation of swimming and sustains the rhythmic firing of CPG (central pattern generator) neurons, among which are motor neurons. The tadpole is then able to rhythmically contract trunk muscles, allowing the undulatory movement of swimming. However, how the tadpole developing brain exerts descending control over reticulospinal neurons, and thus over the spinal CPG centers, is not fully understood yet. In this work, we recorded extracellular activity in the hindbrain of the tadpole to identify firing units that are involved in the long and variable delay to swim initiation following trunk skin stimulation. We isolated firing units that mediate distinct motor output. We subsequently grouped them in populations based on their firing patterns in response to skin stimulation and motor output. We propose a novel neural circuitry for sensory processing and descending motor control exerted by the hindbrain of the hatchling tadpole, which could account for the long and variable delay to reticulospinal neuron activation, and thus swim initiation.

Early midbrain sensorimotor pathway is involved in the timely initiation and direction of swimming in the hatchling Xenopus laevis tadpole

“…Excitability due to sensory stimulation on one side of the skin is transmitted to both sides of the hindbrain and can initiate swimming on either side of the tadpole’s body (Buhl et al, 2012;Buhl et al, 2015;Roberts et al, 2019;Ferrario et al, 2021;Messa and Koutsikou, 2021). In this study, following left trunk skin stimulation, control tadpoles (n=10) initiated swimming more often on the unstimulated side (contralateral to the stimulus; Figure 3B) at percentages of 41% ipsilateral (13/32 trials) and 59% contralateral (19/32 trials).…”

“…Excitability due to sensory stimulation on one side of the skin is transmitted to both sides of the hindbrain and can initiate swimming on either side of the tadpole's body (Buhl et al, 2012;Buhl et al, 2015;Roberts et al, 2019;Ferrario et al, 2021;Messa and Koutsikou, 2021).…”

“…A third glass suction electrode was attached to the trunk skin on the right side of the body, at the level of the anus, to deliver electrical stimuli to the trunk skin. A schematic view of the electrode positions can be seen in Figures 2A, B (Koutsikou et al, 2018;Messa and Koutsikou, 2021). Electrical stimulation was delivered via a custom-made TTL pulse generator automatically driven through software (Signal 7, CED, Cambridge, UK).…”

An early midbrain sensorimotor pathway is involved in the timely initiation and direction of swimming in the hatchling Xenopus laevis tadpole

“…Two borosilicate glass suction electrodes (diameter ∼50 µm) were attached to both sides of the tadpole's body approximately at the level of the 4th cleft to record ventral A third glass suction electrode was attached to the trunk skin on the right side of the body, at the level of the anus, to deliver electrical stimuli to the trunk skin. A schematic view of the electrode positions can be seen in Figure 2A (Koutsikou et al, 2018;Messa and Koutsikou, 2021). Electrical stimulation was delivered via a custom-made TTL pulse generator automatically driven through software (Signal 7, CED, Cambridge, UK).…”

“…Tadpoles with the Contra MHB + ML lesion (n = 9 tadpoles) also showed significantly longer latencies compared to control animals (n = 10 control tadpoles; p = 0.0011, Kruskal-Wallis/Dunn's test; control vs Contra MHB + ML median: 126.2, IQR: 104.8-198.8 ms; Figure 3A). Excitability due to sensory stimulation on one side of the skin is transmitted to both sides of the hindbrain and can initiate swimming on either side of the tadpole's body (Buhl et al, 2012(Buhl et al, , 2015Roberts et al, 2019;Ferrario et al, 2021;Messa and Koutsikou, 2021). In this study, following left trunk skin stimulation, control tadpoles (n = 10) initiated swimming more often on the unstimulated side (contralateral to the stimulus; Figure 3B) at percentages of 41% ipsilateral (13/32 trials) and 59% contralateral (19/32 trials).…”