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.
Fishes rely on the neuromasts of their lateral line system to detect water flow during behaviors such as predator avoidance and prey localization. Although the pattern of neuromast development has been a topic of detailed research, we still do not understand the functional consequences of its organization. Previous work has demonstrated somatotopy in the posterior lateral line, whereby afferent neurons that contact more caudal neuromasts project more dorsally in the hindbrain than those that contact more rostral neuromasts (Gompel N, Dambly-Chaudiere C, Ghysen A. Development 128: 387-393, 2001). We performed patch-clamp recordings of afferent neurons that contact neuromasts in the posterior lateral line of anesthetized, transgenic larval zebrafish (Danio rerio) to show that larger cells are born earlier, have a lower input resistance, a lower spontaneous firing rate, and tend to contact multiple neuromasts located closer to the tail than smaller neurons, which are born later, have a higher input resistance, a higher spontaneous firing rate, and tend to contact single neuromasts. We suggest that early-born neurons are poised to detect large stimuli during the initial stages of development. Later-born neurons are more easily driven to fire and thus likely to be more sensitive to local, weaker flows. Afferent projections onto identified glutamatergic regions in the hindbrain lead us to hypothesize a novel mechanism for lateral line somatotopy. We show that afferent fibers associated with tail neuromasts respond to stronger stimuli and are wired to dorsal hindbrain regions associated with Mauthner-mediated escape responses and fast, avoidance swimming. The ability to process flow stimuli by circumventing higher-order brain centers would ease the task of processing where speed is of critical importance. Our work lays the groundwork to understand how the lateral line translates flow stimuli into appropriate behaviors at the single cell level.
Gamma-aminobutyric acid immunoreactive feedback neurons of the protocerebral tract are a major component of the honeybee mushroom body. They have been shown to be subject to learning-related plasticity and provide putative inhibitory input to Kenyon cells and the pedunculus extrinsic neuron, PE1. We hypothesize, that learning-related modulation in these neurons is mediated by varying the amount of inhibition provided by feedback neurons. We performed Ca2+ imaging recordings of populations of neurons of the protocerebral-calycal tract (PCT) while the bees were conditioned in an appetitive olfactory paradigm and their behavioral responses were quantified using electromyographic recordings from M17, the muscle which controls the proboscis extension response. The results corroborate findings from electrophysiological studies showing that PCT neurons respond to sucrose and odor stimuli. The odor responses are concentration dependent. Odor and sucrose responses are modulated by repeated stimulus presentations. Furthermore, animals that learned to associate an odor with sucrose reward responded to the repeated presentations of the rewarded odor with less depression than they did to an unrewarded and a control odor.
The lateral line system of larval zebrafish is emerging as a model to study a range of topics in neurobiology, from hair cell regeneration to sensory processing. However, despite numerous studies detailing the patterning and development of lateral line neuromasts, little is known about the organization of their connections to afferent neurons and targets in the hindbrain. We found that as fish grow and neuromasts proliferate over the body surface, the number of afferent neurons increases linearly. The number of afferents innervating certain neuromasts increases over time, while it decreases for other neuromasts. The ratio of afferent neurons to neuromasts differs between the anterior and posterior lateral line system, suggesting potential differences in sensitivity threshold or spatial resolution. A single afferent neuron routinely contacts a group of neuromasts, suggesting that different afferent neurons can convey information about receptive fields along the body. When afferent projections are traced into the hindbrain, where a distinct somatotopy has been previously described, we find that this general organization is absent at the Mauthner cell. We speculate that directional input from the lateral line is less important at an early age, whereas the speed of the escape response is paramount, and that directional responses arise later in development. By quantifying morphological connections in the lateral line system, this study provides a detailed foundation to understand how hydrodynamic information is processed and ultimately translated into appropriate motor behaviors.
SUMMARYHoneybees learn to associate an odor with sucrose reward under conditions that allow the monitoring of neural activity by imaging Ca 2+ transients in morphologically identified neurons. Here we report such recordings from mushroom body extrinsic neurons -which belong to a recurrent tract connecting the output of the mushroom body with its input, potentially providing inhibitory feedback -and other extrinsic neurons. The neuronsʼ responses to the learned odor and two novel control odors were measured 24h after learning. We found that calcium responses to the learned odor and an odor that was strongly generalized with it were enhanced compared with responses to a weakly generalized control. Thus, the physiological responses measured in these extrinsic neurons accurately reflect what is observed in behavior. We conclude that the recorded recurrent neurons feed information back to the mushroom body about the features of learned odor stimuli. Other extrinsic neurons may signal information about learned odors to different brain regions.
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