Release of octopamine by Leydig cells in the central nervous system of the leech Macrobdella decora, and its possible neurohormonal role.

1. The release of octopamine by the central nervous system of the leech Macrobdella decora was examined. Isolated ganglia or chains of ganglia were incubated in salines of varying composition, and the release of octopamine into the perfusate was measured using a radioenzymatic method. In some experiments this release was correlated with the electrical activity of the octopamine-containing Leydig cells, as measured via a microelectrode in the cell body. 2. Chains of ganglia incubated in normal saline released 0.04 pmol octopamine/ganglion/3 min incubation period. This amount was not significantly increased by either the monoamine oxidase inhibitor iproniazid phosphate (0.1 mM) or the uptake inhibitor desipramine (10 microM) alone, but was by both together. Nominally calcium-free saline containing 20 mM Mg++ significantly decreased octopamine release. 3. High K+ saline increased octopamine release significantly in both standard saline and one containing the blocking agents. This increase was sevenfold in saline containing iproniazid phosphate and desipramine, and was significantly greater than that obtained in saline without the blockers. This provides further evidence for the role of octopamine as a neuroactive substance in the leech by indicating the existence of possible mechanisms for its uptake and/or inactivation. 4. Octopamine release was positively correlated with the firing frequency of Leydig cells. No release was detectable when the cells were prevented from firing by the injection of hyperpolarizing current. Release was frequency-dependent when the cells fired at frequencies of 0.1-1.0 spikes/s. Elevating the external calcium concentration from 1.8 to 5.4 mM significantly increased octopamine release at all frequencies tested, except for 0 spikes/s, at which release remained below detectabilty.(ABSTRACT TRUNCATED AT 250 WORDS)