Neurons reactive to antibodies against serotonin in the stomatogastric nervous system and in the alimentary canal of locust and crickets (orthoptera, insecta)

Immunoreactive neurons in the stomatogastric nervous system and in the alimentary tract of the locust Schistocerca gregaria and the crickets Gryllus bimaculatus and Acheta domesticus have been examined using antibodies against serotonin (5-hydroxytryptamine; 5-HT). For comparative anatomical analysis cobalt chloride infusion was applied. The innervation of the visceral muscles was studied electron microscopically. In all three species the majority of the 5-HT-immunoreactive cell bodies of the stomatogastric nervous system occur in the frontal ganglion in which 30-40% of the total number of cell bodies react with anti-5-HT. In the occipital ganglion only two to four cell bodies (1-2%) are 5-HT-immunoreactive. Single immunoreactive cell bodies were observed in the ventricular ganglia in only a few preparations. The 5-HT-immunoreactive neurons in the frontal ganglion are pseudounipolar or multipolar. The main process of the 5-HT-immunoreactive neurons of the frontal ganglion descend along the entire stomatogastric nervous system. Some arborizations of these processes ascend into the brain and others supply the neuropil of all stomatogastric ganglia. Side branches leave the stomatogastric nervous system and form a plexus along the surface of the entire intestinal tract from where 5-HT-immunoreactive fibers supply: all muscle layers of the muscularis; the external dilator muscles of the foregut and probably some somatic muscles, neurohaemal organs and Malpighian tubules (excretory system). Serotonin-immunoreactive fibers further proceed into salivary gland and the retrocerebral complex (corpora cardiaca and corpora allata). The retrocerebral glandular complex and the hindgut receive additional immunoreactive neurons from the central nervous system. Electron microscopic analysis of nerves innervating the muscle layers of the alimentary tract revealed one type of 5-HT-immunoreactive and one or two types of non-5-HT-immunoreactive fibers. All fiber types contact the sarcolemma of muscle fibers without any obvious synaptic membrane specializations. The 5-HT-immunoreactive fibers are in some regions in close contact with the haemolymph. These regions can be regarded as sites of neurohormonal release. The distribution of serotonin-immunoreactive neurons suggests that 5-HT acts as a neurotransmitter and/or modulator on intestinal muscles and some somatic muscles and glandular cells, and as a neurohormone released from neurohaemal sites into the body fluid.