The association of octopamine with specific neurones along lobster nerve trunks.

Abstract

Octopamine and its synthetic enzyme, tyramine .beta.-hydroxylase (TBH), were found in high concentrations at 2 points along 2nd thoracic nerve roots in lobsters [Hormarus americanus]. The first was in the proximal section of the 2nd root between the ventral nerve cord and the bifurcation of the root into medial (to flexor muscles) and lateral (to extensors) branches. The 2nd region of high concentration was within a well known crustacean neurosecretory system, the pericardial organ, located close to the ends of the lateral branches of the roots. With several different staining procedures, small clusters of nerve cell bodies were found within the connective tissue sheath in the proximal regions of the 2nd roots. No cell bodies were seen in the pericardial organ regions. Cell bodies were variable in number and position between corresponding roots in the same animal and homologous roots among different animals. The average numbers of cell bodies correlate well with TBH and octopamine content, and with the synthesis of octopamine in these same regions of roots. Small clusters of root cell bodies dissected from preparations had > 500-fold higher activities of TBH than isolated efferent excitatory and inhibitory or afferent sensory axons. Along with octopamine, the preferential synthesis of acetylcholine and serotonin [5-hydroxytryptamine] was seen in proximal segments of roots. Acetylcholine synthesis in these regions could represent transmitter synthesized in the nerve terminals innervating the root cells. The role of serotonin in these regions was not clear, but the amounts of endogenous serotonin were only 1/10 of the amounts of octopamine present. Dopamine was not synthesized from tyrosine in 2nd thoracic roots. If dopa or dopamine were used as precursor compounds, noradrenaline, which was not usually not found in lobsters, could be accumulated in proximal segments of roots. Phenolamines were converted to 2 further metabolites by lobster tissues. The compounds were unidentified and were named fast and slow products on the basis of their migration on electrophoresis at acid pH. Some partial characterization of slow product revealed that it was a mixture of compounds that could convert to mild acid hydrolysis to fast product and parent phenolamine. Nerve cells in the proximal segments of the 2nd thoracic roots seem to contain and synthesize octopamine. Since not all the cells in any single root were analyzed for octopamine or TBH, the possibility that one or more cells contained physiologically interesting substances other than octopamine was not eliminated.