Central and Peripheral Actions of the Neuropeptide Proctolin on a Postural Neuromuscular Muscle System in the Norway Lobster, Nephrops norvegicus (L.)

Denheen, Maria Theresa (1992) Central and Peripheral Actions of the Neuropeptide Proctolin on a Postural Neuromuscular Muscle System in the Norway Lobster, Nephrops norvegicus (L.). PhD thesis, University of Glasgow.

Abstract

An investigation of the central and peripheral actions of the neuropeptide proctolin in the abdominal superficial flexor muscle (SFM) system of the Norway lobster, Nephrops norvegicus has been undertaken. High performance liquid chromatography (HPLC) and a sensitive bioassay have been used to establish the presence of the peptide in various tissues of Nephrops, including the SFM system. Proctolin-like bioactivity (PLB) in partially purified tissue extracts co-eluted with synthetic proctolin using these techniques. Quantification of bioassay responses to proctolin revealed that higher concentrations of the peptide are present in the lateral bundle of the SFM compared with the medial bundle of the muscle. An analysis of the central effects of proctolin on the spontaneous activities of individual SFM motor neurones and on coupling relationships between different motor neurone pairs has been carried out using an isolated abdominal nerve cord preparation. Proctolin can simultaneously enhance and suppress the firing of individual members of the motor neurone pool; the peptide is effective in upregulating the firing of some motor neurones (f1, f4 and f6) and downregulating the firing of others (f2 and f5). These studies have also shown that proctolin can modulate the connectivity between specific superficial flexor motor neurone pairs by evoking or disrupting premotor connections. Thus, proctolin can reconfigure the postural circuitry by specifically modulating the postural flexor motor output pattern. An analysis of the peripheral actions of proctolin on the two slow fibre phenotypes comprising the SFM has been carried out using both in vitro neuromuscular preparations and isolated single fibres. Proctolin alone has no effect on muscle tension, resting membrane potential or EJP amplitude. However, potentiation of SFM tension is produced by proctolin provided that the muscle is partially depolarised prior to exposure of the peptide. Furthermore, the two slow fibre phenotypes of Nephrops SFM exhibit a differential responsiveness to proctolin, lateral fibres being more responsive to proctolin than medial fibres. These results, suggest that the effect of proctolin is mediated via proctolin-sensitive calcium channels and the differential responsiveness of medial and lateral fibres may reflect differences in the distribution and/or proctolin sensitivity of these channels. Surveys of the pattern of innervation across the SFM have been carried out in an attempt to further investigate the observed correlation between different fibre phenotypes and their pattern of innervation at the level of individual motor neurones. The existence of a 'gross' correlation between fibre phenotypes and pattern of innervation has been confirmed. However, results indicate that the polysynaptic pattern of activity displayed by medial fibres as opposed to the synaptic silence usually observed for lateral fibres is due to the expression of their differential presynaptic properties. Medial fibres show properties of high output synapses and exhibit weak facilitation whereas lateral fibres show properties of low output synapses and facilitate strongly. Further characterisation of the SFM with regard to fibre heterogeneity by investigating a number of neuromuscular parameters including pattern of innervation, facilitation properties, decay constant and EJP amplitude has suggested that the SFM system is more complex than was previously appreciated. A preliminary investigation of the distribution and function of proctolin in the SFM system of the crayfish Pacifastaciis leniusculus has been carried out. Histochemical analysis of the crayfish SFM reveals that it is composed of the same two slow fibre subtypes as Nephrops but that in crayfish these fibre subtypes are distributed in a random fashion throughout the SFM. An examination of the concentration of proctolin in the SFM of crayfish reveals that a similar distribution of the peptide exists to that found in the SFM of Nephrops\ the lateral bundle contains higher concentrations of proctolin than the medial bundle. Given the differences in distribution of the two fibre subtypes within each SFM, these results suggest that proctolinergic innervation of the SFM in both Nephrops and crayfish is position dependent and not correlated with fibre type. Furthermore, fibre type does not appear to be correlated with resting membrane potential or innervation in crayfish SFM. This study has also confirmed that proctolin acts postsynaptically to potentiate neurally induced tension in crayfish SFM.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Douglas Neil
• Keywords: Neurosciences
• Date of Award: 1992
• Depositing User: Enlighten Team
• Unique ID: glathesis:1992-75335
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Nov 2019 20:50
• Last Modified: 19 Nov 2019 20:50
• URI: https://theses.gla.ac.uk/id/eprint/75335