Anatomy and fine structure of nerves associated with the corpus allatum and foregut musculature of Periplaneta americana (L.) (Dictyoptera : Blattidae)

Identification of the cellular target for eclosion hormone in the abdominal transverse nerves of the tobacco hornworm, Manduca sexta

The isolated abdominal central nervous system of Manduca sexta undergoes an increase in cyclic GMP (cGMP) when exposed to the insect peptide eclosion hormone (EH) before pupal ecdysis. Previously, cGMP immunocytochemistry revealed that the EH-stimulated increase in cGMP was contained in numerous filamentous processes within the transverse nerve associated with each abdominal ganglion. These processes seemed to be the axons of neurosecretory cells projecting to this neurohemal organ. In the present paper, we now show that the EH-stimulated cGMP is not present in neurosecretory terminals. There is no colocalization of the EH-stimulated cGMP with immunoreactivity of two peptides, known to be present in axons in the transverse nerves. Furthermore, there is no colocalization of EH-stimulated cGMP with the synaptic vesicle protein, synaptotagmin. The neurosecretory axons are localized to a narrow band at the anterior margin of the transverse nerve, whereas the cellular elements showing an EH-stimulated cGMP increase are primarily present in the posterior region. There are two cell types in this region: a granular and a nongranular type. The cGMP immunoreactivity seems to be contained within the nongranular type. During adult development, the cells of the posterior compartment spread in a thin layer between the transverse and dorsal nerves, become positive for myosin immunoreactivity between pupal stages 5 and 8, and seem to form the adult ventral diaphragm muscles. We conclude that the EH-sensitive filaments in the transverse nerves of Manduca are most likely to be intrinsic cells that subsequently develop into the ventral diaphragm muscles of the adult.

Distribution of sulfakinin-like peptides in the central and sympathetic nervous system of the American cockroach, Periplaneta americana (L.) and the field cricket, Teleogryllus commodus (Walker)

We describe the distribution of sulfakinin-like neuropeptides in the central and sympathetic nervous system of the American cockroach Periplaneta americana (L.) (Blattodea) and the field cricket Teleogryllus commodus (Walker) (Othoptera), using an antisulfakinin primary antibody and confocal laser scanning microscopy. We conclude that, in the cockroach, sulfakinin-like material is produced in ten pairs of anterior cells in the pars intercerebralis, as well as two pairs of medial and one major pair of lateral posterior brain cells. This contrasts with findings in other insects, including the cricket, where only the posterior cell groups express sulfakinin-immunoreactive material. Extensive arborization of dendrites containing sulfakinin-like peptides occurs within the neuropile of both species, suggesting a neurotransmitter/neuromodulator function. In the cockroach, there is clear evidence of direct distribution of sulfakinin-like peptides along axons to the foregut tissue, and a plexus of retrocerebral nerves is likely to serve as a neurohaemal release site. Neurohaemal release into the dorsal aorta is also postulated. Sulfakinin-immunoreactive axons do not innervate the hindgut in either cockroaches or crickets. Sulfakinin may function as a gut myotropin in the Blattodea, in addition to functioning as a neurotransmitter within the central nervous system. This latter function appears to be general across insect orders, while the neurohaemal distribution and myotropic activity are restricted to the Blattodea.

Neurons projecting from the brain to the corpora allata in orthopteroid insects: anatomy and physiology

Retrograde and orthograde labeling of neurons projecting to the corpus allatum was performed in locust, grasshopper, cricket, and cockroach species in order to identify brain neurons that may be involved in the regulation of juvenile hormone production. In the acridid grasshopper Gomphocerus rufus L., and the locusts Locusta migratoria (R.&F.) and Schistocerca gregaria Forskal, the corpora allata are innervated by two morphologically distinguishable types of brain neurons. One group of 9-13 neurons (depending on species) with somata in the pars lateralis extend axons via the nervus corporis cardiaci 2 and nervus corporis allati 1 to the ipsilateral corpus allatum, whereas two cells in each pars lateralis have bilateral projections and innervate both glands. No direct connection between the pars intercerebralis and corpus allatum has been found. In contrast, neurons with paired axons innervating both glands are not present in Periplaneta americana (L.) and Gryllus bimaculatus de Geer. Instead, two cells in each pars lateralis project only to the gland contralateral to their somata. Electrophysiological experiments on acridid grasshoppers have confirmed the existence of a direct conduction pathway between the two glands via the paired axons of four cells that have been identified by neuroanatomy. These cells are not spontaneously active under experimental conditions. Ongoing discharges in the left and right nerves are unrelated, suggesting that the corpora allata receive independent neuronal inputs from the brain.

Axonal projections within the brain-retrocerebral complex of the cricket, Teleogryllus commodus

The cerebral origins and axonal trajectories of neurons projecting to the retrocerebral complex of the cricket, Teleogryllus commodus, were examined in silver-intensified nickel preparations. Spatially separate groups of somata in the pars intercerebralis (PI) and in the pars lateralis (PL), commonly accepted as neurosecretory loci, were found to give rise to axons which terminate in the nervus corporis allati 2, the corpus allatum, or the corpus cardiacum. Additional findings demonstrated a distinct group of somata from the PI whose axons run in the esophageal nerve (stomatogastric nervous system), nine somata in the subesophageal ganglion with axons projecting into the nervus corporis allati 2, and also a small cluster of tritocerebral perikarya with axons terminating in the corpus cardiacum. Somata residing in the PI and PL were found to be compartmentally organized based upon the retrocerebral destinations of their axons. Possible functional consequences of these results with respect to the insect neurosecretory system are discussed.