Neurons without dendrites?--A novel type of neurosecretory cell in locusts

Small-diameter nerves were found that are associated with the lateral peripheral nerves of the unfused abdominal ganglia of locusts. Such small nerves were observed in about 30% of all cases in Locusta migratoria, more than 60% in Schistocerca gregaria. Retrograde staining of these small nerves showed two somata in the posterior, lateral, and ventral region of an abdominal ganglion. These cells give rise to the small nerves that accompany the big lateral nerves and, on their surface, form putative neurohaemal release sites. Astonishingly the cells do not form any dendritic ramifications within the neuropile of the ganglia.

Putative neurohemal areas in the peripheral nervous system of an insect, Gryllus bimaculatus, revealed by immunocytochemistry

The morphology and position of putative neurohemal areas in the peripheral nervous system (ventral nerve cord and retrocerebral complex) of the cricket Gryllus bimaculatus are described. By using antisera to the amines dopamine, histamine, octopamine, and serotonin, and the neuropeptides crustacean cardioactive peptide, FMRFamide, leucokinin 1, and proctolin, an extensive system of varicose fibers has been detected throughout the nerves of all neuromeres, except for nerve 2 of the prothoracic ganglion. Immunoreactive varicose fibers occur mainly in a superficial position at the neurilemma, indicating neurosecretory storage and release of neuroactive compounds. The varicose fibers are projections from central or peripheral neurons that may extend over more than one segment. The peripheral fiber varicosities show segment-specific arrangements for each of the substances investigated. Immunoreactivity to histamine and octopamine is mainly found in the nerves of abdominal segments, whereas serotonin immunoreactivity is concentrated in subesophageal and terminal ganglion nerves. Immunoreactivity to FMRFamide and crustacean cardioactive peptide is widespread throughout all segments. Structures immunoreactive to leucokinin 1 are present in abdominal nerves, and proctolin immunostaining is found in the terminal ganglion and thoracic nerves. Codistribution of peripheral varicose fiber plexuses is regularly seen for amines and peptides, whereas the colocalization of substances in neurons has not been detected for any of the neuroactive compounds investigated. The varicose fiber system is regarded as complementary to the classical neurohemal organs.

The innervation of the closer muscle of the mesothoracic spiracle of the locust

The closer muscle of the mesothoracic spiracle of the locust, Schistocerca gregaria is innervated by two excitatory motoneurones and also by processes of a peripherally located neurosecretory cell. Within the muscle, ultrastructural studies show the presence of two types of excitatory nerve terminal which differ in the content of dense cored vesicles and in their distribution. The ventral segment of the muscle is innervated predominantly by terminals with small clear vesicles and only an occasional dense-cored vesicle. The central part of the muscle is innervated predominantly by terminals with small clear vesicles and larger numbers of dense-cored vesicles. The dorsal segment of the muscle is innervated exclusively by a neurosecretory type innervation. The small neurohaemal organ of the median nerve close to the spiracle muscle is immunoreactive to an antibody raised against bovine pancreatic polypeptide but no immunoreactive processes enter the muscle itself. The muscle possesses specific octopaminergic receptors that increase cyclic AMP levels and the possibility that the neurosecretory input to the muscle is provided by either a central or peripheral octopamine containing neurone is discussed.

Bilaterally projecting neurones in pregenital abdominal ganglia of the locust: anatomy and peripheral targets

The anatomy and physiology of neurones with axons in left and right homologous nerves was studied in abdominal ganglia of the migratory locust, Locusta migratoria, by using a differential cobalt/nickel staining method. These neurones reside within two clusters at the anterior and posterior ends of all unfused abdominal ganglia. Each cluster contains at least seven neurones with bilaterally projecting axons. All neurones of the anterior cluster possess bilaterally projecting axons which leave the ganglion through nerve 2 (= sternal nerve) or nerve 1 (= tergal nerve). Additional axon collaterals, which are present in some of these neurones leave through either nerve 1 or nerve 2 of the right or left side of the ganglion. With few exceptions, the neurones show further asymmetries in their dendritic arborizations. Neurones of the posterior group also have bilaterally projecting axons that leave the ganglion through nerve 1 or nerve 2. Among these neurones are the two large posterior dorsal unpaired median (DUM) neurones with bilaterally symmetrical axons that were described earlier as DUM1 and DUM2. They innervate the skeletal muscles of an abdominal segment. Two other cells of this cluster have anatomical properties similar to the DUM1 neurone and were termed DUMheart1A and DUMheart1B. The rest resemble the bilaterally projecting neurones of the anterior group. With the exception of the two classic DUM neurones (DUM1 and DUM2), all neurones of the anterior and posterior cluster innervate the heart or a neurohaemal area.