Functional and ultrastructural analysis of reafferent mechanosensation in larval zebrafish

The ability to differentiate between external sensory inputs and reafferent inputs that arise from one’s own locomotion is imperative to navigate environments. This was explored in a study done in a collaboration with Iris Odstrcil and EngertLab.

Suppression of these reafferent signals has been observed in many model organisms, including zebrafish. However, it is still unknown precisely how efferent and afferent pathways contribute to processing lateral line and suppressing reafferent input.

Two efferent pathways that send signals to the lateral line are the cholinergic octavolateral efferent nucleus (OEN) and the dopaminergic efferent to the lateral line (DELL), which originate in the hindbrain and ventral hypothalamus respectively.

Labeling identifies clusters of cells belonging to the DELL and OEN pathways (left). Schematic showing the relative positions of the DELL and OEN (right). The DELL is located rostral to the OEN in the brain.

Using 2-photon functional imaging, we observed that sensory neurons in the posterior lateral line respond to water flow (exafferent stimuli), but don’t respond to their own locomotion (reafferent stimuli) in the absence of a water flow stimulus.

The average single-cell responses in the presence of (C) and the absence of (F) a water flow stimulus. The population-level neural responses are additionally shown in both conditions with (D) and without flow (G).

To further parse through the DELL and OEN populations, imaging and behavior tracking found that DELL neurons were activated in response to locomotion and stimuli that did not invoke a motor response. OEN neurons were only activated in response to locomotion.

The top and bottoms panels show the neural responses to different stimuli in the DELL and OEN, respectively. The fluorescence activity of 4 neurons in the DELL (A) and the OEN (G) and the tail angle based on swim behavior in the same time course. The average fluorescence responses (top) and swim probabilities (bottom) are shown in response to swim, visual motion, tap, flow, and heat stimuli in the DELL (B-F) and OEN (H-L) neurons.

These experiments show not only that the posterior lateral line can distinguish between stimulation from exafferent and reafferent stimuli, but also that OEN neurons may contribute to reafferent inhibition.

Using electron microscopy, we additionally analyzed the fine anatomy of neuromasts to fully understand the innervation of these lateral line organs.

OEN neurons appear to broadly innervate hair cells while DELL neurons may interfere with other axons of OEN/DELL origin and indirectly target hair cells. We expect that the OEN directly affects hair cell sensitivity, while the DELL has a more modulatory effect.

*OEN (red) neurons make closer contact with hair cells than DELL (orange) neurons.*

Ablation of OEN cells resulted in heightened responses in the posterior lateral line where they did not exist in the non ablated condition. On the other hand, ablation of DELL cells only resulted in minor increase in variability within cell populations and fish.

The top and bottoms panels show the neural responses pre- and post-ablation in the DELL and OEN, respectively. Schematic showing the ablations performed in the DELL (A) and OEN (D) pathway. Average fluorescence response and swim probability in the contralateral and ipsilateral DELL (B) and OEN (A) pre- and post-ablation. The difference in the fluorescence response between swim and no swim conditions in both hemispheres of the DELL (C) and OEN (F) pre- and post-ablation.

A similar effect as observed in OEN ablation was noticed in a cholinergic a9 receptor subunit knockout model, confirming that the cholinergic efferent pathway is required for reafferent suppression in the lateral line.

In summary, we modeled the anatomy and connectivity of OEN/DELL neurons and neuromasts and identified their role in modulating exafferent and reafferent stimuli. Check out the full study and share it on Twitter!

Functional and ultrastructural analysis of reafferent mechanosensation in larval zebrafish