Characterization of 5-hydroxytryptamine receptors in the snail, 'Helix aspersa'
MetadataShow full item record
The aim of this investigation was to characterize those 5-HT receptors present in three different tissues of the common garden snail. Helix aspersa, into one or more of the categories already described for vertebrate 5-HT receptors. Specific 5-HT receptor agonists and antagonists which had been developed and used to help characterize, and subsequently classify, the various types of 5-HT receptor in vertebrates, were utilized in this study. The three preparations from Helix included: i) identified neurones in the visceral ganglion ii) the heart and iii) the pharyngeal retractor muscle (PRM). The action of 5-HT on identified neurones in the visceral ganglion was studied using the electrophysiological techniques of both voltage- and current- damp. Under voltage-clamp conditions the response of the identified neurones to iontophoretic application of 5-HT was seen to be an inward current of approximately 3-10 nA. Under current-clamp conditions the response to 5-HT was an excitatory depolarization leading to the firing of action potentials of approximately 3-15 mV. Both responses showed rapid desensitization to repetitive applications of 5-HT and were blocked by tubocurarine. No specific 5-HT receptor antagonist to this 5-HT response in Helix neurones was found. The action of 5-HT was mimicked by 5-CT and a- Me-5-HT both of which showed similar-sized responses to 5-HT, whereas sumatriptan gave smaller responses than those of 5-HT. 5-HT had a positive inotropic effect on the heart. The excitatory action of 5-HT on the heart was studied using an organ bath methodology with application of the 5-HT receptor agonists and antagonists at suitable concentrations. No specific 5-HT receptor antagonist was found for the cardioexcitatory effect of 5-HT. The full rank order of potency for the 5-HT receptor agonists tested was 5-HT > methylergometrine = ergotamine = 5-CT > -Me-5-HT = sumatriptan > methysergide = 2-Me-5-HT = tryptamine 8-OH-DPAT. 5-HT caused relaxation in the PRM and was found to inhibit, in a dose- dependent manner, acetylcholine (ACh)-induced contraction in the muscle. This inhibition of ACh-induced contraction by 5-HT in the PRM was studied using an organ bath methodology with application of 5-HT receptor agonists and antagonists at suitable concentrations. No specific 5-HT receptor antagonist for the inhibition of ACh-induced contraction was found. The rank order of 5-HT receptor agonist potency was 5-HT > 5-CT > sumatriptan = ergotannine = methysergide >> -Me-5-HT = 2-Me-5-HT. The effect of 5-HT on cyclic adenosine 3',5'-monophosphate (cAMP) levels within Helix heart and pharyngeal retractor muscle (PRM) tissue were monitored in this investigation. 5-HT caused a dose-dependent increase in cAMP both in Helix heart and PRM tissue. The 5-HT receptors within Helix are not readily characterized into any of the categories of 5-HT receptor that have been already classified in vertebrates: Helix 5-HT receptors appear to be unique in the fact that they are unclassifiable in terms of the vertebrate 5-HT receptor classification. The evidence presented in this investigation is discussed in terms of the molecular biology of receptors: this includes the hypothesis that the 5-HT receptors particularly in Helix heart and PRM tissue could be related to a family of G-protein-coupled receptors whereas the neuronal 5-HT receptors in Helix are more likely to be integral to an ion channel.
Thesis, PhD Doctor of Philosophy
Items in the St Andrews Research Repository are protected by copyright, with all rights reserved, unless otherwise indicated.