During my doctorate at the Institute for Zoology at the University of Bonn I extracellulary recorded lateral line units in the medial octavolateral nucleus (MON) in the brainstem in goldfish, Carassius auratus. The MON is the first site of sensory processing in the ascending lateral line pathway of fish.Aim of the work was to investigate and characterize the response behavior of these units to different hydrodynamic stimuli to learn more about central processing of lateral line information. It was investigated how MON units respond to a vibrating sphere in terms of different frequencies, locations, and sphere vibration directions. The spatial excitation patterns of Mon units were described and finally the response behavior to water flow in different directions and velocities.

Frequency characteristics

Whereas the frequency response characteristics of primary lateral line afferent fibers have been described at least in some species, nothing is known about the characteristics of brainstem lateral line neurons. I investigated how MON neurons in goldfish, Carassius auratus, respond to sine

wave stimuli of different frequencies.Extracellular recordings were made from single MON neurons  while the lateral line was stimulated with sinusoidal water motions generated by a stationary vibrating sphere.

The findings demonstrate that response behaviors, patterns of discharge and frequency response characteristics of brainstem lateral line neurons are much more diverse than those of primary afferents. This is indicative of the fact that MON neurons integrate information across a large number of afferent fibers that may innervate widely distributed neuromasts.

Spatial excitation patterns

Theoretical data show that information on the position of a vibrating source can be derived from the pressure gradient pattern (equivalent to the velocity pattern) across the array of lateral line receptors (neuromasts). In particular, location and separation of peaks and troughs in the pressure gradient (velocity) patterns change in a predictable way with location and vibration angle of the source.

I studied how this peripheral hydrodynamic information is encoded by neurons in the brainstem medial octavolateralis nucleus (MON), the first site of central integration.Receptive fields of most MON neurons consisted of single excitatory or inhibitory areas that could be fairly narrow or extend across large parts of the body surface, whereas the receptive fields of other neurons consisted of two or even more excitatory or inhibitory areas. In most MON neurons receptive fields changed in some aspect when sphere vibration direction was altered. However, the changes were not consistent and neurons with a clear preference for a distinct direction of sphere vibration were not found.

The data show that only few MON neurons maintain a primary-like representation of the peripheral hydrodynamic information. In most MON neurons, subtle effects of sphere vibration angle on receptive fields occurred, meaning that these neurons maintain the possibility to encode source location and source vibration direction. However, MON neurons apparently do not encode pressure gradient patterns in the same way as primary lateral line afferent fibers.

Responses to bulk water flow

Recordings from afferent fibers indicate that fluctuations within a water flow can be used to determine flow direction and flow velocity if the central lateral line compares the inputs from two or more peripheral receptors that are organized in series on the fish surface. I recorded activity from brainstem neurons in a water flow and determined velocity thresholds, temporal activity patterns, and directional sensitivity. Brainstem neurons integrate inputs from receptors distributed across the body surface. If these neurons determine flow velocity and/or flow direction by comparing peripheral input, they should respond preferentially to particular flow velocities and/or flow directions.

During recording activity of medullary single units the lateral line was stimulated with running water. Flow was applied from anterior to posterior (AP) or from posterior to anterior (PA). Neuronal activity was monitored under two stimulus paradigms: pulsed flow (discrete flow) and ramped flow (velocity continuously increasing).

The majority of the recorded flow-sensitive neurons (78%) responded to both AP and PA flow with either an increase or a decrease in discharge rate. However, a substantial proportion of neurons (22%) were direction-sensitive. Only very few neurons seem to be tuned to a distinct flow velocity or a narrow range of flow velocities. These data presently do not lend strong support to the idea that the velocity of water flow is already determined by neurons at the level of the lateral line brainstem.

In Vivo Whole-Cell Recordings from Brainstem Lateral Line Neurons in Goldfish, Carrasius auratus

In my Diploma Thesis (Electrophysiological and Histological Characterization of Brainstem Lateral Line Neurons in Goldfish, Carassius auratus) the in vivo whole-cell technique was used to record neuronal activity from lateral line single units in the goldfish medial octavolateral nucleus (MON). Neuronal activity was recorded intracellulary with glass micropipettes. Units were filled electrophoretically with neurobiotin and immunohistochemically stained using the DAB procedure. This method offers the possibility to characterize brainstem neurons both electrophysiological and histological.

I investigated whether flow- sensitive and flow-insensitive neurons in the MON of goldfish can be distinguished based on their morphology. Neurons were filled with biocytin and their morphology was reconstructed from histological sections. They were characterized based on morphological criteria.Of the recorded neurons, 67% were flow-sensitive, i.e., they responded to a laminar water flow with a change in discharge rate. 33% were flow-insensitive. A correlation between physiology and morphology was not apparent, i.e., flow-sensitive and flow-insensitive MON-cells could not be distinguished based on their morphology.