Visualization of the nitrergic system and modulation of synaptic transmission by NO in the pulmonate snail, Helix pomatia

Nitric oxide (NO) modulates synaptic transmission in vertebrates and invertebrates. The putative neurotransmitter NO is generated by the enzyme nitric oxide synthase (NOS). This enzyme metabolises the amino acid arginine to NO and citrulline. NO activates a guanyly cyclase and induces the intracellular accumulation of cGMP. The proposed study is concerned with: Localization of putative NO producing cells and the cytochemical localization of arginine, citrulline, and cGMP in the nervous system of the pulmonate snail, Helix pomatia. The elucidation of the microanatomical distribution of molecules associated NO generation indicate which neural pathway employ NO as a transmitter and which cells co-operate to generate and terminate a NO signal. Modulation of synaptic transmission by NO in the snail nervous system. These experiments should establish the significance of NO on electrical excitability of neurons. In particular, the effect of NO on voltage gated ion channels and transmitter modulated ion conductances is tested. The significance of the results is discussed in context with animal behaviour, in particular with chemosensory processing and feeding behaviour.

Cognitive processes like learning and memory, emotions, conscious movements or control of autonomous processes originate in the continuous communication among neurons. Communication is due to the release of chemical messengers, which are sensed by other cells. Messengers either induce a transient change of neuronal activity or lay the foundation of a permanent memory trace. Nitric oxide, a gaseous messenger molecule within and outside the nervous system, has attracted the attention of numerous researchers because of its multitude of functions. Analysis of the relatively simple nervous system of gastropods has significantly contributed to our current interpretation of the cellular and molecular basis of behaviour and memory. In the present project, we addressed following questions: (1) Do snails produce NO? (2) How is the production of NO regulated? (3) How does NO modulate neuronal excitation? (4) Does NO interact with iron metabolismus? Our investigations showed that the pulmonate snail Helix pomatia contains a gene for a nitric oxide synthesizing enzyme (NOS). Using molecular biological techniques, we analyzed the primary structure of the snail NOS and compared its structure with the human NOS. Our results showed that snail und human NOS share numerous similarities indicating similarities in the regulation of the enzyme. Histochemical studies revealed the distribution of enzyme, its substrate L - arginine and its product L - citrulline in different cells. This indicates that NO production depends on the supply of the substrate from other cells, which could prevent overproduction of NO. Using electrophysiological experiments, we showed that NO increases electrical activity by suppression of a potassium conductivity and increase of calcium conductivity. Furthermore, nitric oxide regulates iron metabolism via modulation of ion regulating proteins.