Innervation of the heart and aorta of Manduca sexta.

Innervation of the heart and aorta of Manduca sexta was studied by using anatomic, neuronal tracing and immunocytochemical techniques. The study was undertaken to provide a foundation for investigating the neural mechanisms controlling cardiac reversal in adults. Lateral cardiac nerves were not found in the larval or adult heart. The larval heart and aorta seem to lack innervation, but a neurohemal system for the release of a cardioactive peptide is associated with the larval alary muscles. At adult metamorphosis, this neurohemal system regresses, and, at the same time, processes grow onto the anterior aorta. These processes seem to be neurohemal and originate from two pairs of neurosecretory cells located in the subesophageal ganglion. This system is immunoreactive to cardioactive peptides and may function, therefore, in hormonal modulation of the activity of the adult heart. Also during metamorphosis, synaptic innervation develops on the terminal heart chamber, and this innervation is from axons extending through the seventh and eighth dorsal nerves of the terminal abdominal ganglion. These axons originate from cells that have been identified as serial homologs of motor neuron-1 of other abdominal ganglia. These neurons are immunoreactive to a cardioactive peptide, and this peptide probably modulates the synaptic innervation of the terminal heart chamber. During metamorphosis, the target of the motor neurons-1 of the seventh and eighth segments becomes respecified from larval skeletal muscles to the terminal chamber of the adult heart.

Neuronal control of heart reversal in the hawkmoth Manduca sexta.

Cardiograms demonstrate that heart activity of Manduca sexta changes from larva, to pupa, to adult. The larval heart has only anterograde contractions. During metamorphosis, heart activity becomes a cyclic alternation of anterograde and retrograde contractions. Thus, the adult heart has both an anterograde and a retrograde pacemaker. External stimuli also can initiate cardiac reversal. Cardiac reversal is blocked by tetrodotoxin, indicating that reversal is under neuronal control. A branch of each dorsal nerve 8 innervates the posterior chamber of the heart, the location of the anterograde pacemaker. Only retrograde contractions occur when dorsal nerves 8 are cut. Stimulation of ml(-1) 8 initiates anterograde contractions; when stimulation ceases, the heart reverses to retrograde contractions. These experiments indicate that the anterograde pacemaker receives neural input that makes it the dominant pacemaker. In the absence of neural input this pacemaker is inactive, and the retrograde pacemaker becomes active. Application of crustacean cardioactive peptide accelerates the heart but does not eliminate cardiac reversal. The terminal chamber of the heart is also innervated by a branch of each dorsal nerve 7; stimulation of this nerve increases the strength of contraction of the terminal chamber but has no effect on contractions of the remainder of the heart or on cardiac reversal.

Allatostatin-like-immunoreactive neurons of the tobacco hornworm, Manduca sexta, and isolation and identification of a new neuropeptide related to cockroach allatostatins.

The YXFGLamide C-terminus serves to define most members of a family of structurally related neuropeptides, the YXFGLamides. These peptides have been identified from the nervous system of various insects and include the allatostatins of cockroaches and crickets, the schistostatins of locusts, and the callatostatins of blowflies. The YXFGLamides have been shown to have various functions, including inhibition of juvenile hormone biosynthesis in cockroaches and crickets and inhibition of contraction of certain insect visceral muscles. We wanted to know if these peptides occur in Manduca sexta and what functions they might have. A new peptide, AKSYNFGLamide, was isolated and identified from M. sexta and has been named "lepidostatin-1"; this is the first YXFGLamide to be found in a lepidopteran, and there are indications that additional YXFGLamides occur in M. sexta. An antiserum to cockroach allatostatins (YXFGLamides) was shown to recognize lepidostatin-1 of M. sexta and was used to map YXFGLamide-immunoreactive neurons in larvae. Because immunoreactive interneurons were found to form an extensive neuropil, YXFGLamides probably function as neuromodulators in M. sexta. Neuroendocrine cells in the brain, abdominal ganglia, and their respective neurohemal organs were YXFGLamide immunoreactive and appear to release YXFGLamides as neurohormones. Immunoreactivity to YXFGLamides and M. sexta diuretic hormone were found to be colocalized and appear to be coreleased in these neuroendocrine cells, indicating that YXFGLamides may be involved in regulation of fluid transport. Innervation of the corpora allata by YXFGLamide-immunoreactive processes was very sparse, suggesting that this innervation does not play an important role in allatostasis. Many thoracic motor neurons were YXFGLamide immunoreactive, suggesting that YXFGLamides may have a myomodulatory or myotrophic function in larvae. However, this immunoreactivity disappeared early in metamorphosis and did not reappear in the adult. The YXFGLamide-immunoreactive neurons in the terminal abdominal ganglion were found to innervate the hindgut, indicating that YXFGLamides may be involved in the control of the rate of myogenic contractions of the larval hindgut.

Neuroanatomy and immunocytochemistry of the median neuroendocrine cells of the subesophageal ganglion of the tobacco hawkmoth, Manduca sexta: immunoreactivities to PBAN and other neuropeptides.

The median neuroendocrine cells of the subesophageal ganglion, important components of the neuroendocrine system of the tobacco hawkmoth, Manduca sexta, have not been well investigated. Therefore, we studied the anatomy of these cells by axonal backfills and characterized their peptide immunoreactivities. Both larvae and adults were examined, and developmental changes in these neuroendocrine cells were followed. Processes of the median neuroendocrine cells project to terminations in the corpora cardiaca via the third and the ventral nerves of this neurohemal organ, but the ventral nerve of the corpus cardiacum is the principal neurohemal surface for this system. Cobalt backfills of the third cardiacal nerves revealed lateral cells in the maxillary neuromere and a ventro-median pair in the labial neuromere. Backfills of the ventral cardiacal nerves revealed two ventro-median pairs of cells in the mandibular neuromere and two ventro-median triplets in the maxillary neuromere. The efferent projections of these cells are contralateral. The anatomy of the system is basically the same in larvae and adults. The three sets of median neuroendocrine cells are PBAN- and FMRFamide-immunoreactive, but only the mandibular and maxillary cells are proctolin-immunoreactive. During metamorphosis, the mandibular and maxillary cells also acquire CCK-like immunoreactivity and the labial cells become SCP- and sulfakinin-immunoreactive. Characteristics of FMRFamide-like immunostaining suggest that the median neuroendocrine cells may contain one or more of the FLRFamides that have been identified in M. sexta. The mandibular and maxillary neuroendocrine cells appear to produce the same set of hormones, and a somewhat different set of hormones is produced by the labial neuroendocrine cells. Two pairs of interneurons immunologically related to the neurosecretory cells are associated with the median maxillary neuroendocrine cells. These cells are PBAN-, FMRFamide-, SCP-, and sulfakinin-immunoreactive and project to arborizations in the brain and all ventral ganglia. These interneurons appear to have extensive modulatory functions in the CNS.

Biotin-containing proteins of the insect nervous system, a potential source of interference with immunocytochemical localization procedures.

When the biotinylated Manduca sexta adipokinetic hormone gene was used as a probe for in situ hybridization, the intrinsic neurosecretory cells were stained with a biotin detection system that contained streptavidin or avidin. Further experiments showed that the DNA probe was not necessary for staining these cells by streptavidin-alkaline phosphatase, and that they were not stained by alkaline phosphatase alone. Similarly, the intrinsic neurosecretory cells were stained directly by streptavidin conjugated to a fluorescent dye. Other parts of the central nervous system could also be stained with streptavidin-alkaline phosphatase but not as readily as the intrinsic neurosecretory cells of the corpora cardiaca. Further analysis demonstrated three biotin-containing proteins in the intrinsic neurosecretory cells of the corpora cardiaca and in the brain. The most abundant of these proteins, when analyzed by sodium dodecyl sulfate polyacrylamide gel electrophoresis, was found to have a molecular weight of 130,000, which is the size of the subunits of pyruvate carboxylase, a biotin-containing enzyme. The same protein was recognized by an antiserum against an insect pyruvate carboxylase, indicating that this protein is probably pyruvate carboxylase. The results reported here indicate that the intrinsic neurosecretory cells of the corpora cardiaca may contain pyruvate carboxylase in a concentration higher that other cells of the central nervous system. We also note that caution is necessary to avoid false positive results if an avidin containing detection system is used for in situ hybridization or immunocytochemistry.

Expression of the allatostatin gene in endocrine cells of the cockroach midgut.

Cockroach allatostatins are neuropeptides that have been isolated from the brain of Diploptera punctata and shown to inhibit juvenile hormone production by the corpora allata. Enzyme-linked immunoassay and immunocytochemistry with antisera to two allatostatins, ASB2 (AYSYVSEYKRLPVYNFGL-NH2) and ASAL (APSGAQRLYGFGL-NH2), revealed that allatostatins were located not only in the insect brain but also in several peripheral tissues including the cockroach midgut and hindgut. Allatostatin-like immunoreactivity was found in nerve fibers of the stomatogastric nervous system as well as in intrinsic endocrine cells of the midgut. Midgut extracts were shown to be biologically active in an allatostatin bioassay and to contain several allatostatin-like peptides, including the octadecapeptide ASB2, which was identified by mass spectrometry following HPLC purification. Reverse transcription of brain mRNA followed by PCR with degenerate oligonucleotides for ASB2 and ASAL yielded a 338-bp fragment of the allatostatin gene that encoded six allatostatins. In situ hybridization with this probe confirmed that an allatostatin gene is expressed in intrinsic endocrine cells of the midgut. Reverse transcription of midgut mRNA followed by PCR and sequencing of the product revealed that the same gene is expressed in the midgut and in the brain. Allatostatins are thus an example of insect "brain-gut peptides" and we suggest that their function may not be restricted to the regulation of juvenile hormone production.

Leucokinin and diuretic hormone immunoreactivity of neurons in the tobacco hornworm, Manduca sexta, and co-localization of this immunoreactivity in lateral neurosecretory cells of abdominal ganglia.

Because leucokinins stimulate diuresis in some insects, we wished to identify the neurosecretory cells in Manduca sexta that might be a source of leucokinin-like neurohormones. Immunostaining was done at various stages of development, using an antiserum to leucokinin IV. Bilateral pairs of neurosecretory cells in abdominal ganglia 3-7 of larvae and adults are immunoreactive; these cells project via the ipsilateral ventral nerves to the neurohemal transverse nerves. The immunoreactivity and size of these lateral cells greatly increases in the pharate adult, and this change appears to be related to a period of intensive diuresis occurring a few days before adult eclosion. Relationships of these neurons to cells that are immunoreactive to a M. sexta diuretic hormone were also investigated. Diuretic hormone and leucokinin immunoreactivity are co-localized in the lateral neurosecretory cells and their neurohemal projections. A median pair of leucokinin-immunoreactive, and a lateral pair of diuretic hormone-immunoreactive neurons in the larval terminal abdominal ganglion project to neurohemal release sites within the cryptonephridium. The immunoreactivity of these cells is lost as the cryptonephridium is eliminated during metamorphosis. This loss appears to be related to the change from the larval to adult pattern of diuresis.

A comparative study of leucokinin-immunoreactive neurons in insects.

Antisera were raised against leucokinin IV, a member of the leucokinin peptide family. Immunohistochemical localization of leucokinin immunoreactivity in the brain of the cockroach Nauphoeta cinerea revealed neurosecretory cells in the pars intercerebralis and pars lateralis, several bilateral pairs of interneurons in the protocerebrum, and a group of interneurons in the optic lobe. Several immunoreactive interneurons were found in the thoracic ganglia, while the abdominal ganglia contained prominent immunoreactive neurosecretory cells, which projected to the lateral cardiac nerve. The presence of leucokinins in the abdominal nerve cord was confirmed by HPLC combined with ELISA. Leucokinin-immunoreactive neurosecretory cells were also found in the pars intercerebralis of the cricket Acheta domesticus and the mosquito Aedes aegypti, but not in the locust Schistocerca americana or the honey bee Apis mellifera. However, all these species have leucokinin-immunoreactive neurosecretory cells in the abdominal ganglia. The neurohemal organs innervated by abdominal leucokinin-immunoreactive cells were different in each species.

Morphology of the vasopressin-like immunoreactive (VPLI) neurons in many species of grasshopper.

It has previously been shown that the pair of vasopressin-like immunoreactive (VPLI) neurons of the locust, Locusta migratoria, have cell bodies on the ventral midline of the suboesophageal ganglion and extensive arborisations in all ganglia of the central nervous system. In the present study, we have stained vasopressin-like immunoreactive neurons in 16 additional species of grasshopper, and consistently find this pair of extensive neurons: we assume these to be interspecies homologues. However, the anatomy of these neurons falls into two morphological types: the first, typified by Schistocerca gregaria, has most of its processes distributed in dorsal and lateral neuropil of all ganglia; the second, typified by Locusta migratoria, is equally extensive in its arborisation, but the distribution of branches is shifted peripherally into the optic lobes and the proximal portions of peripheral nerves. It has been suggested that the peripheral fibres in Locusta migratoria are neurohaemal organs for the release of a vasopressin-like diuretic peptide. Our sample of 17 Acridoid species has deliberately selected animals from very different habitats, but our extensive survey of VPLI anatomy shows that peripheral fibres are only present in species from the subfamily Oedipodinae (of which Locusta migratoria is a member) and that no peripheral fibres are present in any of the species from the 4 other subfamilies of the Acridoidea that we have examined. The presence of peripheral fibres is therefore determined by phylogeny and not by habitat. The absence of peripheral VPLI fibres in most grasshopper species examined in this study probably means that the release of putative diuretic hormone from VPLI to control water homeostasis cannot be a conserved function of this ubiquitous neuron. In contrast, the extensive central arborisations and rare antigenicity, which are highly conserved features of the VPLI neuron in all those grasshoppers we have examined, suggests that any conserved role is more likely to be central. A central role for the VPLI neuron has yet to be determined.

Vasopressin-immunoreactive neurons and neurohemal systems in cockroaches and mantids.

Vasopressin-like neuropeptides of insects are of special interest because of their possible function as hormones and neuromodulators. Therefore, this study was undertaken by using whole-mount immunofluorescent staining by two antisera that recognize different types of vasopressin-like immunoreactive groups of neurons in the cockroaches Periplaneta americana, Leucophaea maderae, Nauphoeta cinerea, Diploptera punctata, and Blaberus discoidalis and in the mantids Litaneuria minor and Tenodera aridifolia sinensis. Using an antiserum to Arg/vasopressin, only two cells, the paired ventral paramedian (PVP) neurons, were immunostained in the central nervous system (CNS) of the cockroaches. These cells are located in the subesophageal ganglion, project throughout the CNS, and appear to be neurosecretory. Their varicose collaterals extend into the dorsal (motor) neuropil of the segmental ganglia, and this neuropil may be the principal site of the release of their neurosecretion. The PVP neurons were also stained by an antiserum to Lys/vasopressin; in addition, this antiserum stained several other groups of neurons, most of which appeared to be neurosecretory. Two pairs of Lys/vasopressin-immunoreactive cells are similar to the PVP neurons in that they are located in the subesophageal ganglion, extend through the ventral nerve cord, have collaterals in the dorsal neuropil of the segmental ganglia, and appear to be neurosecretory within the CNS. In addition, midventral and anteroventral clusters of Lys/vasopressin-immunoreactive neurosecretory neurons in the subesophageal ganglion project neurohemal release sites on the corpora allata. Other types of Lys/vasopressin-immunoreactive neurons include median and lateral neurosecretory cells of the protocerebrum and neurosecretory cells in the tritocerebrum, all of which project to the corpora cardiaca. In the abdominal ganglia there are posterolateral clusters of Lys/vasopressin neurosecretory neurons, and these cells extend to neurohemal release sites on the transverse and lateral cardiac nerves. In mantids the anti-Arg/vasopressin and anti-Lys/vasopressin antisera stained most of the same groups of neurons that these antisera recognized in cockroaches. The results of this study suggest that there are two or more vasopressin-like peptides in cockroaches and mantids and that these peptides may be released either as hormones in the blood or as neurosecretions within the CNS. Their function(s) in these insects remains to be determined.

Peptide-immunocytochemistry of neurosecretory cells in the brain and retrocerebral complex of the sphinx moth Manduca sexta.

Antisera against a variety of vertebrate and invertebrate neuropeptides were used to map cerebral neurosecretory cells in the sphinx moth Manduca sexta. Intense immunoreactive staining of distinct populations of neurosecretory cells was obtained with antisera against locust adipokinetic hormone, bovine pancreatic polypeptide, FMRFamide, molluscan small cardioactive peptide (SCPB), leucine-enkephalin, gastrin/cholecystokinin, and crustacean beta-pigment dispersing hormone (beta PDH). Other antisera revealed moderate to weak staining. Each type of neurosecretory cell is immunoreactive with at least one of the antisera tested, and most of these neurons can be identified anatomically. The staining patterns provide additional information on the organization of cerebral neurosecretory cells in M. sexta. Based upon anatomical and immunocytochemical characteristics, 11 types of neurosecretory cells have been recognized in the brain, one type in the suboesophageal ganglion, and one in the corpus cardiacum. Extensive colocalization experiments show that many neurosecretory cells are immunoreactive with several different antisera. This raises the possibility that these cells may release mixtures of neuropeptides into the hemolymph, as has been demonstrated in certain other systems. The immunocytochemical data should be helpful in efforts to identify additional peptide neurohormones released from the brain of this and other insects.

Neurosecretory neurons and their projections to the serotonin neurohemal system of the cockroach Periplaneta americana (L.), and identification of mandibular and maxillary motor neurons associated with this system.

The neuroanatomy of a serotonin neurohemal system in the head of Periplaneta americana was studied by means of immunohistochemistry, cobalt backfilling, transmission electron microscopy, and nerve transection. This neurohemal system is supplied by bilateral groups of two or three neurons whose somata are located ventrally in the subesophageal ganglion, near the root of each mandibular nerve. Axons of these serotoninergic neurons extend into all of the nerves of the mouth parts but reach most of these nerves by a very circuitous route. Initially the axons extend from the subesophageal ganglion, through the ipsilateral mandibular nerve trunk, and into the third branch of the mandibular nerve. From here the axons extend into the second branch of the maxillary nerve by way of a link nerve, and then they project retrogradely to reenter the subesophageal ganglion. In the ganglion, branches of these axons extend into the labial nerves, and the axons run dorsally through the subesophageal ganglion, circumesophgeal connectives, and tritocerebrum to reach the labral nerves. In the nerves of the mouth parts the serotoninergic axons give rise to numerous secondary branches that form an extensive neurohemal system at the surface of these nerves. The relatively large surface and cephalic location of this system probably indicate that the timely release of relatively large amounts of serotonin plays an important role in the physiology of feeding in this insect. The somata, neurites, and dendritic fields of the serotonin neurohemal neurons and those of the motor neurons of the mandibular abductor muscle occur together, and some of the mandibular abductor motor neurons also stain for serotonin. In order to distinguish clearly between these neurohemal and motor neurons, the anatomy of the mandibular abductor motor neurons has also been determined. Similarly, in the course of this study it has been necessary to work out the anatomy of the motor neurons of the maxillary retractor and cardo rotator muscles in order to distinguish them from the serotoninergic neurons. A nonserotoninergic peripheral neuron is associated with the serotonin neurohemal system, and its soma is located on the mandibular-maxillary link nerve. This link nerve neuron appears to be neurosecretory.

Improved methods for cobalt filling and silver intensification of insect motor neurons.

A Parafilm disk floating on saline and bearing a drop of cobalt solution in which the nerve stump is bathed provides a convenient and versatile method for in vitro or in vivo filling of neurons for cobalt sulfide staining. Silicone grease around the edge of the disk provides an effective seal around the nerve as it passes between the two solutions. Using a modified developer, silver intensification of cobalt sulfide stained neurons may be done in the light at room temperature, and the time of optimum intensification may be observed under a dissecting microscope.