Studies on neuronal 5-HT in a gastropod mollusc, helix pomatia (L.)

Abstract

This thesis describes information obtained on the structure of 5-HT-containing neurons, on the mechanisms of transport of 5-HT and its precursors into and within neurons, on the nature of the blood supply to the CNS, and on the function of 5-HT-containing neurons within the CNS of Hellix pomatia. In particular data is obtained for the giant serotonin-containing neuron (GSG) in each cerebral ganglion. Dense-cored vesicles of mean diameter 100 nm are present in the perikarya and axon branches of the GSCs. Vesicles of similar appearance are present in the presumed presynaptic endings of the GSCs. Evidence is presented which suggests that such vesicles sequester 5-HT, The fine structure of presumed presynaptic endings making synaptic connections with the GSCs is described. Following exposure to tritiated 5-HT, electron microscope autoradiography showed that silver grains, often in very high concentrations, were located over certain fine axon branches thought to be nerve endings. These processes contained small dense-cored vesicles, which were morpliologically similar to those thought to sequester 5-HT to the perikarya of the GSCs. It is suggested that re-uptake into nerve endings is a mechanism of inactivation of 5-HT in the Following exposure to tritiated 5-HTP, silver grains were observed over the perkarya of the GSCs and other known 5-HT-contalning neurons. There was no indication that 5-HTP was taken up by nerve endings or by non-nervous structures. The accumulation of tritiated tryptophan was less specific; all the neuron perikarya took up this substance. The CMS of H. pomatla is supplied by branches of the anterior aorta. Capillaries from these branches open into a, blood space which is adjacent to, and continuous over the surface of the nervous tissue. Blood passes from this space through the epineural sheath into the body cavity sinuses. Three tissue layers separate the blood spaces from the nervous tissue. These are (i) a luminal endothelium, (ii) a connective tissue layer, and (iii) glial cells. The luminal endothelium and connective tissue are freely permeable to uncharged particles of 10 nm or less. Eloctrophysiological analysis showed that each GSC sends axon branches to muscles in the lips of the animal. Selective stimulation of the GSCs resulted in an increase of electrical activity recorded from these muscles, but no change in their length. This effect was mimicked by 5-HT applied to the muscles. It is suggested that the GSC has a facilitatory effect on the lip muscle potentials.