Proctolin in the lobster nervous system.

The pentapeptide proctolin (Arg-Tyr-Leu-Pro-Thr) is present in high concentrations in neurosecretory organs of the lobster, Homarus americanus. The central nervous system contains ca. 1400 proctolin-immunoreactive neurons, which appear to serve a variety of different functions. Some of these neurons have been specifically identified and analyzed biochemically to determine which classical neurotransmitters coexist with the peptide. These include: serotonin-proctolin cell pairs in the fifth thoracic and first abdominal ganglia; a large dopamine-proctolin neuron in the circumesophageal ganglion; and cholinergic-proctolin sensory neurons which innervate a mechanoreceptor in the scaphognathite. With these identified neurons we have begun to investigate the physiological actions of proctolin, the interactions between cotransmitters, and the development of multiple transmitter phenotypes in individual neurons.