Biogenic amines and their respective receptors in visceral muscles of the locust, Locusta migratoria

The biogenic amine serotonin is an important neuroactive chemical believed to play both central and peripheral roles in many insect species. The distribution and association of serotonin with the alimentary canal of the locust were investigated by immunchistochemistry, HPLC and physiological assays. Serotonin-like immunoreactive processes were present throughout the alimentary canal including the foregut, midgut, gastric caeca and hindgut. Serotonin-like immunoreactive processes were also found to be entering the ventricular ganglion, arborizing and exiting the ganglion and innervating various regions of the alimentary canal. HPLC analysis coupled to electrochemical detection confirmed the presence of serotonin and indicated that the highest content of serotonin was in the midgut followed by the foregut, hindgut, and then gastric caeca. Serotonin reduced the tonus and inhibited proctolin-induced contractions of the midgut circular muscles in a dose-dependent manner. Pharmacological studies using a selection of serotonergic compounds revealed that alpha-methylserotonin and mianserin were the most potent agonist and antagonist respectively. These studies indicate that the action of serotonin on the midgut is presumably mediated via a 5-HT2-like receptor subtype.; The activities of all biogenic amines are mediated by a group of transmembrane proteins, G protein-coupled receptors (GPCRs). In order to isolate GPCRs from the locust visceral muscles, two cDNA libraries were constructed based on the midgut and oviduct tissues. Screening of these libraries resulted in the isolation of several partial receptor cDNAs. Further characterization of one of the cDNAs resulted in the isolation of a putative octopamine/tyramine receptor with sequence similarities to biogenic amine receptors particularly to a previously cloned tyramine receptor from the locust. Comparison of the genomic and cDNA sequences indicated that the gene is intronless. Expression analyses using RT-PCR and Northern blot indicated the presence of receptor transcripts throughout the brain, ventral nerve cord, midgut and oviduct. In situ hybridization resulted in the expression of the receptor mRNA in a small number of cells likely to be midgut endocrine-like cells and in a number of spindle-shape cells located on the common oviduct. Molecular biology and physiological studies on the locust oviduct raise the possibility that this receptor is likely to be a tyramine receptor subtype.