A study on transduction an transmission in catfish ampullary electroreceptor organs

Abstract

Electroreception is the ability of animal species to detect weak electric fields. It is mediated by a sensory system that occurs in some aquatic vertebrates. Electroreception is useful for prey- and predator detection, orientation in space, electrolocation and communication. This thesis deals with the transduction mechanism of catfish ampullary electroreceptor organs. In the first two chapters, patch clamp and FURA-2 experiments are described that are designed to get find out which ion channels are involved in the transduction of the sensory signal. The experiments show that an electrical stimulus induces a calcium-influx into the cells. This stimulus evoked calcium influx can be manipulated by application of calcium channel blockers, as well as sodium and potassium channel blockers. This leads to the conclusion that sodium and potassium channels provide the apical membrane conductance that allows depolarisation of the membrane and activation of the presynaptic calcium channels. In the last two chapters, the issue of spontaneous activity of the afferent nerve fibre is addressed. It is studied whether this spontaneous spike activity arises from continuous neurotransmitter release of the electroreceptor cells, or if it is a property of the nerve itself. The transmission of the receptor cells and the afferent nerve is blocked by application of the neurotoxin tetanus toxin. The results show disappearance of stimulus evoked activity, while the spontaneous activity continues. The conclusion is that the spontaneous activity is generated by the nerve fibre itself. The occurrence of the spontaneous activity is implemented in a mathematical model of the afferent nerve fibre. The model is based on the extensive branching pattern of the afferent fibre. The branching pattern is revealed by an immunocytochemical staining and shows extensive converging. Multiple synapses are present between the single nerve afferent and each of the approximately 20 electroreceptor cells of the organ. The model predicts that the convergence within each organ leads to increased sensitivity.