Octopamine neurons in lobsters: location, morphology, release of octopamine and possible physiological role.

Octopamine cells are found along second thoracic roots, where they serve as neurosecretory neurons capable of releasing octopamine at two distinct points: one into the hemolymph immediately before it enters the gills; one into the hemolymph immediately after it leaves the gills. The octopamine cells receive a cholinergic synaptic input. We presume that this input is from processes of peripheral sensory cells bringing information to the CNS. Octopamine can increase the strength of contraction of exoskeletal muscles and, at higher concentrations, can induce contractures in these muscles. These effects can be interpreted as a resetting of the level of ionized calcium within muscle fibers (the contracture) to a higher value or a possible enhanced entry of calcium ions during nerve stimulation (increased strength of contraction). The observed effects are of a prolonged duration, outlasting the time of application of octopamine by some 20-40 minutes. We do not know if this effect on muscle tension production is the normal physiological role of octopamine. Other possible roles will be explored in the future. The pathway involving the octopamine neurons in lobsters may provide a model neurohumoral system that can be studied and understood in detail from the level of sensory input to the level of behavioral output.