Aminergic neuron systems of lobsters: morphology and electrophysiology of octopamine-containing neurosecretory cells

In the American lobster (Homarus americanus) the biogenic amines serotonin and octopamine appear to play important and opposite roles in the regulatio n of aggressive behavior, in the establishment and/or maintenance of domina nt and subordinate behavioral states and in the modulation of the associate d postural stances and escape responses. The octopamine-containing neurosec retory neurons in the thoracic regions of the lobster ventral nerve cord fa ll into two morphological subgroups, the root octopamine cells, a classical neurohemal group with release regions along second thoracic roots, and the claw octopamine cells, a group that selectively innervates the claws. Cell s of both subgroups have additional sets of endings within neuropil regions of ganglia of the ventral nerve cord. Octopamine neurosecretory neurons ge nerally are silent, but when spontaneously active or when activated, they s how large overshooting action potentials with prominent afterhyperpolarizat ions. Autoinhibition after high-frequency firing, which is also seen in oth er crustacean neurosecretory cells, is readily apparent in these cells. The cells show no spontaneous synaptic activity, but appear to be excited by a unitary source. Stimulation of lateral or medial giant axons, which excite serotonergic cells yielded no response in octopaminergic neurosecretory ce lls and no evidence for direct interactions between pairs of octopamine neu rons, or between the octopaminergic and the serotonergic sets of neurosecre tory neurons was found.