The distribution of histamine-immunoreactive neurons in the ventral nerve cord of the cricket, Gryllus bimaculatus.

The present study demonstrates the immunocytochemical localisation of the biogenic amine, histamine (HA), in interneurons within the ventral nerve cord of the cricket, Gryllus bimaculatus. Analysis of whole-mount preparations combined with histology of serial sections reveals a constant number of HA-immunoreactive (HA-ir) neurons in the suboesophageal (n=8), thoracic (n=4) and abdominal ganglia (females/males n=24/20). Except for the suboesophageal and prothoracic ganglion, each thoracic and abdominal neuromere contains one pair of bilateral-symmetric HA-ir somata in a medio-ventral position. Axons from HA-ir cells in the thorax extend anteriorly and share common projection areas in thoracic associative neuropils; they terminate in the brain. HA-ir cells also display efferent descending axons. Extending posteriorly, these axons give rise to varicose HA-ir fibre plexuses on the surface of nerve 1 of the abdominal ganglia. In the suboesophageal ganglion, processes from a bilateral symmetric group of clustered HA-ir cells ascend into the tritocerebrum of the brain and further project into the frontal ganglion and the recurrent nerve. Ultrastructural analysis reveals dense-core vesicles, indicative of non-synaptic secretion, in HA-ir elements within the stomatogastric nervous system. Arborisations of HA-ir neurons are present in all major neuropil regions of the ventral nerve cord and display characteristic varicose structures also detected in other types of amine-containing cells. Central HA-ir varicose projections in dorsal and ventral neuropils are located in close apposition to the ganglionic surface. The wide-spread innervation of all neuromeres by HA-ir interneurons and the identification of possible neurohemal release sites suggest a general role of HA as a neuroactive substance, including neuromodulatory and neurohormonal functions.