Neuropharmacology of the snail Helix aspersa and the scorpion Pandinus imperator central neurones

Wright, Nicholas Jeremy Denis (1989) Neuropharmacology of the snail Helix aspersa and the scorpion Pandinus imperator central neurones. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

This thesis is divided into three sections. The first section concerns the responses of identified neurones in the right parietal ganglion of the snail Helix aspersa. Cells F4, 5 and 6 are excited by 5-hydroxy-tryptamine (5-HT) and inhibited by dopamine whereas cells in the F30 area are inhibited by both compounds. Tryptamine and 6-HT are able to activate both types of receptor on the F4, 5 and 6 cells in a dose dependent manner. Their dual action can also be separated by specific antagonists. Ergometrine blocks the dopamine response and reverses tryptamine and 6-HT inhibition into excitation while d-tubocurarine blocks the 5-HT response and reverses tryptamine and 6-HT excitation into inhibition. On F30 cells the compounds appear to act on a dopamine receptor. All are blocked by ergometrine, unaffected by d-tubocurarine and enhanced by low potassium Ringer. The second section utilises the same preparation but is concerned with the chloride-mediated inhibitory response of cell E4 of the visceral ganglion to applied acetylcholine. Four compounds, known to affect chloride movement in a variety of tissues, were tested on this response and compared with the sodium-mediated excitatory response of cells E1 and 2 to applied acetylcholine. Piretanide irreversibly blocked the inhibitory response only and the inhibition increased with time. Furosemide irreversibly blocked both types of response and the inhibition also increased with time. No recovery was seen with either compound. 4-acetamino-4-isothiocyano-2,2'-disulphonic acid stilbene (S.I.T.S.) and ethacrynic acid rapidly and reversibly blocked both types of response. No change in the chloride or sodium equilibrium potential values were noted. Piretanide and furosemide probably act non selectively on the structure of the cell membrane while S.I.T.S. and ethacrynic acid may interact directly with the acetylcholine receptor protein. The third section involves the application of various putative neurotransmitters onto the exposed central nervous system (C.N.S.) of the scorpion Pandinus imperator. Their effects on leg motoneurone output were monitored extracellularly and analysed using the EROS-PC data-logging system. Although the exact nature of the response was obscured as the compounds could act at all or any level of the C.N.S., most did produce definite changes in the recorded spike trains. Gamma-aminobutyric acid (GABA) and acetylcholine were strongly implicated as central transmitters with glycine, glutamate, octopamine, dopamine and 5-HT also having an effect.

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