Physiological responses of mechanosensory systems in the head of larval zebrafish (Danio rerio)

The lateral line system of zebrafish consists of the anterior lateral line, with neuromasts distributed on the head, and the posterior lateral line, with neuromasts distributed on the trunk. The sensory afferent neurons are contained in the anterior and posterior lateral line ganglia, respectively. So far, the vast majority of physiological and developmental studies have focused on the posterior lateral line. However, studies that focus on the anterior lateral line, especially on its physiology, are very rare. The anterior lateral line involves different neuromast patterning processes, specific distribution of synapses, and a unique role in behavior. Here, we report our observations regarding the development of the lateral line and analyze the physiological responses of the anterior lateral line to mechanical and water jet stimuli. Sensing in the fish head may be crucial to avoid obstacles, catch prey, and orient in water current, especially in the absence of visual cues. Alongside the lateral line, the trigeminal system, with its fine nerve endings innervating the skin, could contribute to perceiving mechanosensory stimulation. Therefore, we compare the physiological responses of the lateral line afferent neurons to responses of trigeminal neurons and responsiveness of auditory neurons. We show that anterior lateral line neurons are tuned to the velocity of mechanosensory ramp stimulation, while trigeminal neurons either only respond to mechanical step stimuli or fast ramp and step stimuli. Auditory neurons did not respond to mechanical or water jet stimuli. These results may prove to be essential in designing underwater robots and artificial lateral lines, with respect to the spectra of stimuli that the different mechanosensory systems in the larval head are tuned to, and underline the importance and functionality of the anterior lateral line system in the larval fish head

Zebrafish Larvae Exhibit Rheotaxis and Can Escape a Continuous Suction Source Using Their Lateral Line

Zebrafish larvae show a robust behavior called rheotaxis, whereby they use their lateral line system to orient upstream in the presence of a steady current. At 5 days post fertilization, rheotactic larvae can detect and initiate a swimming burst away from a continuous point-source of suction. Burst distance and velocity increase when fish initiate bursts closer to the suction source where flow velocity is higher. We suggest that either the magnitude of the burst reflects the initial flow stimulus, or fish may continually sense flow during the burst to determine where to stop. By removing specific neuromasts of the posterior lateral line along the body, we show how the location and number of flow sensors play a role in detecting a continuous suction source. We show that the burst response critically depends on the presence of neuromasts on the tail. Flow information relayed by neuromasts appears to be involved in the selection of appropriate behavioral responses. We hypothesize that caudally located neuromasts may be preferentially connected to fast swimming spinal motor networks while rostrally located neuromasts are connected to slow swimming motor networks at an early age

The effects of selective neuromast ablation on the ability of larvae to avoid a suction source.

<p>A. DASPEI-labeled neuromasts in 5 day post fertilization larvae with sections of the posterior lateral line (PLL) ablated with neomycin. Intact neuromasts labeled with DASPEI are highlighted with white arrowheads, while white boxes indicate regions where neomycin was applied. Note that due to the transparency of the larvae, at times labeled neuromasts from the opposite side of the body are seen. Five different treatments were tested, from top to bottom: (1) larvae with rostral neuromasts of the PLL ablated, (2) middle neuromasts of the PLL ablated, (3) caudal neuromasts of the PLL ablated, (4) complete PLL ablated and (5) sham treated control group. B. Percent larvae that escape (black bars) and are captured by (white bars) the suction source. There is a significant difference between the control (N = 78) and the complete PLL ablated group (N = 36) as well as between the control and the caudal neuromasts ablated group (N = 32). We also found a significant difference between the complete PLL ablated group and middle neuromasts ablated group (N = 27). No significant effects where found for the rostral neuromasts ablated group (N = 18). All groups were tested using a Fisher's exact test (***p<0.001, **p<0.01, *p<0.05). C. Time series showing the position of a larva with caudal neuromasts ablated (white circles) captured by the suction source (located at the origin of the coordinate system), and a larva with middle neuromasts ablated (black circles) bursting away from the suction source. Start and endpoint of each path are indicated.</p

Characteristics of bursting behavior in the suction chamber for untreated larvae.

<p>A. Percentage of fish initiating a burst at a given radial distance from the suction source, where the origin of the suction source is at 0 cm. Most larvae initiate a swimming burst between 0.5–1.0 cm (1–2 body lengths) away from the suction source, while far fewer larvae initiate a burst closer or further away (N = 98). B. Relationship between burst distance and location of burst initiation. The closer to the suction source a burst is initiated the farther the distance traveled during the burst (N = 98, R² = 0.09, p = 0.02). C. Velocity of passively drifting euthanized larvae as a function of distance from the suction source. The average drifting velocity of ten bodies was smoothed using a cubic spine (gray shaded area represents the standard error of the mean, N = 10). D. Average burst velocity as a function of the location of burst initiation. The closer to the suction source the burst is initiated, the faster the burst velocity (N = 98, R² = 0.06, p = 0.01).</p

Larvae burst away from a continuous suction source at 5 days post fertilization.

<p>A. Schematic of the experimental suction chamber which is immersed in a large volume bath and connected with tubing to a retention flask and vacuum motor. B. Comparison of two suction avoidance behaviors. Position of a larva (black line) drifting towards and then bursting away from the suction source (origin at 0 cm), and another larva continuously holding station instead of bursting (gray line). C. Time series of a rheotactic larva (gray circles) escaping the suction source (arrow) with a quick burst of swimming. Frames taken every 350 ms.</p

Projections of the Diencephalospinal Dopaminergic System to Peripheral Sense Organs in Larval Zebrafish (Danio rerio)

Dopaminergic neurons of the descending diencephalospinal system are located in the posterior tuberculum (PT) in zebrafish (Danio rerio), and correspond in mammals to the A11 group in hypothalamus and thalamus. In the larval zebrafish, they are likely the only source of central dopaminergic projections to the periphery. Here, we characterized posterior tubercular dopaminergic fibers projecting to peripheral sense organs, with a focus on the lateral line neuromasts. We labeled and identified catecholaminergic neurons and their projections by combining two immunofluorescence techniques, (i) using an antibody against Tyrosine hydroxylase, and (ii) using an antibody against GFP in transgenic zebrafish expressing in catecholaminergic neurons either membrane-anchored GFP to track fibers, or a Synaptophysin-GFP fusion to visualize putative synapses. We applied the CLARITY method to 6 days old whole zebrafish larvae to stain and analyze dopaminergic projections by confocal microscopy. We found that all lateral line neuromasts receive direct innervation by posterior tubercular dopaminergic neurons, and tracked these projections in detail. In addition, we found dopaminergic fibers projecting to the anterior and posterior lateral line ganglia, and extensive central dopaminergic arborizations around the terminal projection field of the lateral line afferent neurons in the hindbrain medial octavolateralis nucleus (MON). Therefore, dopaminergic innervation may affect lateral line sense information at different processing stages. Additional dopaminergic fibers innervate the trigeminal ganglion, and we observed fine catecholaminergic fibers in the skin with arborization patterns similar to free sensory nerve endings. We also detected potentially dopaminergic fibers innervating inner ear sensory epithelia. Therefore, the diencephalospinal A11-type dopaminergic system may broadly modulate peripheral senses. We also briefly report peripheral sympathetic catecholaminergic projections labeled in our experiments, and their innervation of the developing intestine, swim bladder and abdominal organs