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-mou nt preparations combined with histology of serial sections reveals a c onstant number of HA-immunoreactive (HA-ir) neurons in the suboesophag eal (n=8), thoracic (n=4) and abdominal ganglia (females/males n=24/20 ). Except for the suboesophageal and prothoracic ganglion, each thorac ic 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 o f the abdominal ganglia. In the suboesophageal ganglion, processes fro m a bilateral symmetric group of clustered HA-ir cells ascend into the tritocerebrum of the brain and further project into the frontal gangl ion and the recurrent nerve. Ultrastructural analysis reveals dense-co re vesicles, indicative of non-synaptic secretion, in HA-ir elements w ithin the stomatogastric nervous system. Arborisations of HA-ir neuron s 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 g anglionic 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, includ ing neuromodulatory and neurohormonal functions.