Investigation of 5-hydroxytryptamine, acetylcholine, octopamine and peptide receptor types using preparations of Helix aspersa pharyngeal retractor muscle (PRM) and heart

The aim of the project was to investigate the 5-hydroxytryptamine (5-HT), acetylcholine (ACh), octopamine, FMRFamide peptide and catch relaxing peptide (CARP) receptor subtypes in molluscan muscle. To achieve this histological and immunocytological investigations were undertaken, and physiological and pharmacological experiments were conducted on isolated nerve-PRM, and heart preparations from H. aspersa. Microscopy showed the PRM to be a smooth muscle with long fibres. The heart had branching fibres but striations were not observed. Immunocytochemistry of the PRM showed a varicose nerve network and immunoreactivity to 5-HT-like and FMRFamide. Like compounds, which appeared to be co-localized. Soluble guanylyl cyclase immunoreactivity was also shown in the PRM, indicating the possibility of a role for nitric oxide in these muscles. 5-HT was a potent agonist when applied on both tissues. At low concentrations it caused an increase in the twitch amplitude of the PRM, with a reduced half relaxation time. At high concentrations the effects upon the time coarse of contraction persisted but the effects on amplitude were inconsistent between preparations. 5-HT caused a concentration-dependent increase in the amplitude of the beat in the heart preparation. 5-HT agonists were applied to the tissues but none were found to be as potent as 5-HT. Equipotent molar ratios (EPMRs) compared to 5-HT were determined and the rank order of potency was established for both preparations. The antagonists ketanserin and ondansetron failed to block the actions of 5-HT on both tissues, and Fluoxethine, a 5-HT uptake blocker, had no significant effect on the PRM. ACh reduced the amplitude of both the PRM twice and the contraction of the heart in a concentration-dependent manner. ACh agonists, nicotine, muscarine and McN-A-343 did not produce consistent effects. Octopamine increased the amplitude of the PRM twitch and the heartbeat in a concentration-dependent manner.