Abstract
Amines and peptides exert a wide range of physiological actions on both central neurons and peripheral tissues. Among these actions, serotonin and octopamine are known to trigger contrasting postures when injected into freely moving lobsters. Immunocytochemical studies of lobster ganglia have identified presumptive serotonergic neurons, their central and peripheral projections, and their terminal fields of arborization. More than 100 neurons that show serotonin-like immunoreactivity have been found in the lobster nervous system (Beltz and Kravitz, 1983). From immunocytochemical studies it appears that varicosities within peripheral neurosecretory structures and endings in certain central neuropil regions arise from the same 2 pairs of large cells located in the fifth thoracic (T5) and first abdominal (A1) ganglia. Because we believed that such cells could account for the central and peripheral actions of serotonin on the postural system, we chose to study these 2 pairs of neurons in greater detail. In the previous paper, Siwicki et al. (1987) report that these neurons contain the pentapeptide proctolin in addition to serotonin. In this communication, we report that these cells can be identified reliably in living preparations; they have large fields of innervation projecting anteriorly into at least 4 segmental ganglia; these neurons are the origin of the fibers that form the thoracic second root neurosecretory regions; they are generally spontaneously active neurons that have overshooting action potentials in their cell bodies; and the serotonin and proctolin immunoreactivities are first expressed in these cells at widely different times in development.