Budick, S.B.*; O’Malley, D.M.: Minimal Behavioral Deficits Are Observed After Laser-ablation of the nMLF in Larval Zebrafish

Larval zebrafish exhibit a range of locomotive behaviors including swimming, turning and escaping. These behaviors are controlled by a relatively small group of brainstem neurons including cells of the nucleus MLF (nMLF) plus reticulospinal, vestibulospinal and T-reticular neurons. Little is known of the function of the nMLF cells, though they constitute perhaps 15% of the total descending motor control system in larval zebrafish. The nMLF cells are suggested to be involved in the initiation of swimming (Bernau et al. 1991; Uematsu and Todo, 1997) and might constitute part of the midbrain locomotor region. Because laser-ablation is a useful technique for revealing the functional role of individually identified hindbrain neurons (Liu and Fetcho, 1999), we attempted to laser-ablate the nMLF both unilaterally and bilaterally (a minimum of 7 cells/hemisphere), using a 15mW Kr-Ar laser. Larval behavior was then analyzed after a recovery period of at least 24 hours. Spontaneous locomotion, as measured by the number of swim bouts and the total distance swam over 30 minutes, was not effected by ablation. Furthermore, kinematic analysis of high-speed imaging data revealed no major deficits in either the escape-response or swimming. The data do suggest, however that the nMLF may be important for generating high angular velocity, large amplitude escape responses. Occasionally, multiple, spontaneous C-bends occurred in rapid succession. A striking feature of these C-bends was that they resulted in little forward propulsion. Such abnormal C-bends were not observed in control fish. These results suggest that nMLF cells are not necessary for the initiation/control of swimming, but may contribute to the coordination or control of the escape response. Supported by NIH Grant NS 37789.