THE JOURNAL OF COMPARATIVE NEUROLOGY 307:647-657 (1991) Histamine-Immunoreactive Neurons in the Midbrain and Suboesophageal Ganglion of the Sphinx Moth Manduca sexta U. HOMBERG AND J.G. HILDEBRAND Fakultat fur Biologie, Universitat Konstanz, 7750 Konstanz, Federal Republic of Germany (U.H.) and Arizona Research Laboratories, Division of Neurobiology, University of Arizona, Tucson, Arizona 85721 (J.G.H.) ABSTRACT This paper describes the distribution of histamine-like immunoreactivity in the midbrain and suboesophageal ganglion of the sphinx moth Manduca sextu. Intense immunocytochemical staining was detected in ten bilateral pairs of neurons in the median protocerebrum and in one pair of neurons in the suboesophageal ganglion. Whereas most areas of the brain and suboesophageal ganglion are innervated by one or more of these neurons, typically no immunoreactive fibers were found in the mushroom bodies, the protocerebral bridge, and the lateral horn of the protocerebrum. The 11 histamine-immunoreactive neurons were recon- structed from serial sections. Ten neurons have bilateral arborizations, often with axonal projections in symmetric areas of both hemispheres. One neuron, whose soma resides in the lateral protocerebrum, has only unilateral projections. Of the 11 neurons, 6 occur in pairs with similar morphological features. In addition to these neurons, weak histamine-like immunoreac- tivity was detected in 7-13 interneurons that were not reconstructed individually. The central projections of the ocellar nerves from the intracranial ocelli also exhibit histamine-like immunoreactivity. The single-cell reconstructions reveal similarities between the organization of histamine- and serotonin-immunoreactive neurons in the brain and suboesophageal ganglion of this insect. Key words: immunocytochemistry, biogenic amines, neurotransmitters, insect brain The presence and neuroactive role of histamine have been demonstrated in the vertebrate nervous system (see reviews by Prell and Green, '86; Schwartz et al., '86; Hough, '88) and in several species of invertebrates (Elias and Evans, '83; Claiborne and Selverston, '84; Weinreich, '85; Hardie, '87, '88, '89; Simmonds and Hardie, '88; Callaway and Stuart, '89; McClintock and Ache, '89; Wik- gren et al., '90). In the vertebrate brain, histamine- containing neurons, as revealed by immunocytochemistry, largely or exclusively reside in the posterior hypothalamus (Hough, '88; Inagaki et al., '88; Airaksinen and Panula, '88, '90; Airaksinen et al., '89). These neurons have divergent projections to many areas in the brain, resembling in part the organization of other aminergic pathways (Airaksinen and Panula, '90; Brodin et al., '90). In the marine snail Aplysiu cuZifornicu, an identified histaminergic afferent neuron, the C2 neuron, is involved in various aspects of feeding behavior (Weinreich, '85; McCaman and Weinreich, '85; Chiel et al., '90). A neuro- transmitterimodulator role for histamine has also been proposed in several species of arthropods (Maxwell et al., '78; Elias and Evans, '83; Claiborne and Selverston, '84; Hardie, '87, '88, '89; Pirvola et al., '88; McClintock and Ache, '89; Orona et al., '90). Recent physiological (Hardie, '87, '88, '89; Simmonds and Hardie, '88; Callaway and Stuart, '89) and immunocytochemical (Pirvola et al., '88; Nassel et al., '88; Callaway et al., '89) evidence suggests that histamine is the transmitter of arthropod photorecep- tors. In addition, biochemical (Maxwell et al., '78; Elias and Evans, '83) and immunocytochemical (Nassel et al., '88, '90; Pirvola et al., '88) studies have also demonstrated the presence of histamine in the insect central nervous system. In the sphinx moth Munducu sextu, histamine synthesis and the presence of small amounts of histamine have been demonstrated in all parts of the central nervous system (Maxwell et al., '78; Elias and Evans, '83). Immunocy- tochemistry revealed a unique pair of ascending histamine- immunoreactive neurons with somata in the mesothoracic ganglion and putatively axonal projections in the suboesoph- aged ganglion (SOG) and antennal lobes (Homberg, '90). Here we report on the anatomy of histamine-immunoreac- tive neurons in the midbrain and suboesophageal ganglion Accepted January 31,1991. o 1991 WILEY-LISS. INC.