Immunocytochemical localization of gastrin-releasing peptide/bombesin-like immunoreactive neurons in insects

Neurons in the cockroach nervous system reacting with antisera to the neuropeptide leucokinin I

Antisera were raised against the myotropic neuropeptide leucokinin I, originally isolated from head extracts of the cockroach Leucophaea maderae. Processes of leucokinin I immunoreactive (LKIR) neurons were distributed throughout the nervous system, but immunoreactive cell bodies were not found in all neuromeres. In the brain, about 160 LKIR cell bodies were distributed in the protocerebrum and optic lobes (no LKIR cell bodies were found in the deuto- and tritocerebrum). In the ventral ganglia, LKIR cell bodies were seen distributed as follows: eight (weakly immunoreactive) in the subesophageal ganglion; about six larger and bilateral clusters of 5 smaller in each of the three thoracic ganglia, and in each of the abdominal ganglia, two pairs of strongly immunoreactive cell bodies were resolved. Many of the LKIR neurons could be described in detail. In the optic lobes, immunoreactive neurons innervate the medulla and accessory medulla. In the brain, three pairs of bilateral LKIR neurons supply branches to distinct sets of nonglomerular neuropil, and two pairs of descending neurons connect the posterior protocerebrum to the antennal lobes and all the ventral ganglia. Other brain neurons innervate the central body, tritocerebrum, and nonglomerular neuropil in protocerebrum. LKIR neurons of the median and lateral neurosecretory cell groups send axons to the corpora cardiaca, frontal ganglion, and tritocerebrum. In the muscle layer of the foregut (crop), bi- and multipolar LKIR neurons with axons running to the retrocerebral complex were resolved. The LKIR neurons in the abdominal ganglia form efferent axons supplying the lateral cardiac nerves, spiracles, and the segmental perivisceral organs. The distribution of immunoreactivity indicates roles for leucokinins as neuromodulators or neurotransmitters in central interneurons arborizing in different portions of the brain, visual system, and ventral ganglia. Also, a function in circuits regulating feeding can be presumed. Furthermore, a role in regulation of heart and possibly respiration can be suggested, and probably leucokinins are released from corpora cardiaca as neurohormones. Leucokinins were isolated by their myotropic action on the Leucophaea hindgut, but no innervation of this portion of the gut could be demonstrated. The distribution of leucokinin immunoreactivity was compared to immunolabeling with antisera against vertebrate tachykinins and lysine vasopressin.

Insect tachykinin-like peptide: distribution of leucokinin immunoreactive neurons in the cockroach and blowfly brains

Antisera were raised against leucokinin I, a cockroach myotropic neuropeptide with some resemblance to vertebrate tachykinins. These antisera were used for immunocytochemical mapping of neurons and neurosecretory cells in the brains of a cockroach and a blowfly species. The leucokinin immunoreactive cells are distinct from neurons that can be labeled with antisera against vertebrate type tachykinins. It is suggested that leucokinin-like peptides may have roles as neurohormones and neuromodulators in the insect nervous system.

Substance P-, FMRFamide-, and gastrin/cholecystokinin-like immunoreactive neurons in the thoraco-abdominal ganglia of the flies Drosophila and Calliphora

Immunocytochemical analysis of the thoraco-abdominal ganglia of the flies Drosophila melanogaster and Calliphora vomitoria revealed neurons displaying substance P- (SPLI), FMRFamide-(FLI), and cholecystokinin-like (CCKLI) immunoreactivity. It could be demonstrated that a number of neurons contain peptides reacting with antisera against all the three types of substances, others were either FLI or CCKLI alone. No neurons displayed only SPLI. Instead, the total number (about 30) of SPLI neurons constitute a subpopulation of the FLI/CCKLI neurons. Many of the identifiable immunoreactive neurons seem to be homologous in the two fly species. One set of six large neurons, termed ventral thoracic neurosecretory neurons (VTNCs), are among those that are SPLI, FLI, and CCKLI in both Drosophila and Calliphora. With the present immunocytochemical technique, the detailed morphology of the VTNCs could be resolved. These neurosecretory neurons supply the entire dorsal neural sheath of the thoraco-abdominal ganglia with terminals, thus forming an extensive neurohaemal area. The VTNCs also have processes connecting the thoracic neuromeres to the cephalic suboesophageal ganglion, as well as extensive arborizations in the thoracic ganglia, suggesting an important role in integrating and/or regulating large portions of the central nervous system, in addition to their neurosecretory function. Most of the other SPLI, FLI, and CCKLI neurons in the thoraco-abdominal ganglia seem to be interneurons. However, there are four FLI neurons that appear to be efferents innervating the hindgut and a few abdominal FLI and CCKLI neurons may be additional neurosecretory cells. From the present study it appears as if neuropeptides related to substance P, FMRFamide and CCK have roles as neurotransmitters/neuromodulators and circulating neurohormones in Drosophila and Calliphora.

Substance P-like immunoreactive neurons in the nervous system of Drosophila

With an antiserum against substance P a small number of neurons could be resolved in great detail in the nervous system of the fruitfly Drosophila melanogaster. In the brain, 10 substance P-like immunoreactive (SPLI) neurons were individually identified. Two of these form extensive bilateral connections with dorsal and ventral protocerebral neuropil. Another two neurons have cell bodies located ventrally in the subesophageal ganglion and processes throughout subesophageal neuropil. In the thoracico-abdominal ganglia 10 SPLI neurons could be identified. Eight of these have large cell bodies located ventrally in thoracic ganglia and two have small cell bodies located posteriorly in the abdominal ganglia. Six of the 8 thoracic SPLI neurons could be resolved in detail and were found to form: (1) processes in dorsal thoracic and abdominal neuropil as well as processes running through the cervical connective into the subesophageal ganglia; and (2) processes running into the dorsal neural sheath of the thoracic ganglia. The latter processes form an extensive network of varicose terminals over the thoracic ganglia. Our results indicate that a substance P-like neuropeptide can act as a neurohormone released into the circulation from terminals in the neural sheath as well as a neurotransmitter/neuromodulator released by interneurons in the brain.

Immunocytochemical demonstration of vertebrate peptides in invertebrates: the homology concept

During the last years many positive immunocytochemical reactions have been described in invertebrates using antisera to vertebrate regulatory peptides. However, due to the specificity problems associated with immunocytochemistry, the significance of the majority of these findings remains unclear, as so far only a few of the substances causing the immunoreactions in invertebrates have been isolated. It is proposed that comparing the localizations of "vertebrate" peptides in different and not closely related species of an invertebrate group may give a clue to the physiological relevance of the immunoreactive substances.

Diversity in neurohaemal organs for homologous neurosecretory cells in different insect species as demonstrated by immunocytochemistry with an antiserum to molluscan cardioexcitatory peptide

Neurosecretory cells were histochemically identified in the thoracic ganglia of the Colorado potato beetle and Drosophila melanogaster with an antiserum to the molluscan cardioexcitatory neuropeptide FMRFamide (Phe-Met-Arg-Phe-NH2). These cells and the FMRFamide-immunoreactive neurosecretory cells identified in a locust are most likely homologous. The neurosecretory axon terminals in the 3 species use different neurohaemal organs. Such a structural diversity in release sites of apparently homologous neurosecretory cells in different species may account for the large structural variability in neurohaemal organs in insects in general.

Immunocytochemical localization of peptidergic neurons and neurosecretory cells in the neuro-endocrine system of the Colorado potato beetle with antisera to vertebrate regulatory peptides

A large number of antisera to regulatory vertebrate peptides was tested immunocytochemically on the nervous system of the Colorado potato beetle to further characterize the peptidergic cells of the neuro-endocrine system and to reveal cells participating in endocrine control mechanisms. Neurons, neurosecretory cells, axons and axon terminals were revealed by antisera to ACTH, gastrin, CCK, alpha-endorphin, beta-endorphin, gamma 1-MSH, insulin, motilin, human calcitonin, growth hormone, somatostatin, CRF, ovine prolactin and rat prolactin. Together with previously described results these findings demonstrate that at least 19 different peptidergic cell types are present in the Colorado potato beetle. Several of these cell types are identical with the known neurosecretory cells, while others have not been identified before. The functions of the immunoreactive neurons are as yet unclear, although in two cases the localization of these cells gives some clues. Thus the lateral neurosecretory cells, which are immunoreactive with antisera to beta-endorphin and ovine prolactin, may regulate corpus allatum activity, whereas a CRF immunoreactive substance seems to be used as neurotransmitter by antennal receptors. These immunocytochemical findings do not imply that the immunoreactive substances are evolutionarily related to the vertebrate peptides to which the antisera were raised. It is postulated that if the part of the substance recognized by a certain antiserum is functionally important for the insect, which should be so if the insect peptide is evolutionarily related to its vertebrate homologue, the antiserum should reveal homologous cells in different insect species. The consequence of this hypothesis is, that if an antiserum does not reveal homologous neurons in different insect species, the immunologically demonstrated substance is probably of little physiological importance, and will not be related evolutionarily to the vertebrate analogue. The positive immunocytochemical results in the Colorado potato beetle are discussed in relation to these considerations.