A mosquito neuropeptide in a moth larva (Helicoverpa zea): relation to FMRF-amide immunoreactivity.

The cerebral nervous and midgut endocrine systems of the larval corn earworm, Helicoverpa zea, were examined using light microscopy and immunocytochemistry for RF-amide family peptides. Immunoreactivity for a mosquito neuropeptide, Aedes Head Peptide-I (Aea-HP-I,pERPhPSLKTRFa), is widely distributed in this lepidopteran. Immunostaining for Aea-HP-I is localized (1a) in perikarya and axons of the brain, the subesophageal ganglion, and the first thoracic ganglion, (b) in peripheral axons innervating muscles of the midgut, and (2) in numerous midgut endocrine cells. Aea-HP-I-associated activity generally occurs as a subset of FMRF-amide (FMRFa; a molluscan cardioactive peptide) immunoreactivity. Cross-reactivity studies indicate that Aea-HP-I and FMRFa immunoreactivities are heterogeneous in the cerebral nervous system and in axons innervating the muscles of the midgut, but may be homogeneous in midgut endocrine cells. Radioimmunoassay for Aea-HP-I reveals immunoreactivity in hemolymph, as well as in extracts of midguts and heads.     

Evidence for a FMRFamide-like peptide in the heteronemertine Cerebratulus lacteus Leidy

Circular body wall muscles of Cerebratulus lacteus respond to micromolar concentrations of the neuropeptide Phe-Met-Arg-Pheamide (FMRFa), first isolated from molluscan nervous tissue. Comparison of the relative body wall contractural potencies of various FMRFa analogs indicates that the Arg-Pheamide group is necessary for this activity, but the remaining N-terminal region can be altered considerably without loss of activity. The circular muscle failed to respond to met-enkephalin and many other vertebrate neuropeptides. Acetone-soluble extracts of Cerebratulus contained two FMRFa antibody immunoreactive components separable by reversed phase liquid chromatography. Neither component had the same retention time as FMRFa. Bouin's fixed and paraffin embedded Cerebratulus nervous tissues displayed specific immunofluorescence when incubated with FMRFa polyclonal antibody but not monoclonal antibody specific for the molluscan neuropeptide SCP-B. Some giant neuron somas in the lateral nerve cords and in the ventral cerebral ganglia were immunochemically reactive as were axons in the lateral nerve cord and in the circular and transverse body wall muscles. Pre-exposure of the antibody with FMRFa prevented the reaction. Thus several types of evidence suggest the presence of FMRFa-like neuropeptides in Cerebratulus lacteus.     

Mechanoresponsive neurones in the suboesophageal ganglion of the locust

Abstract. A preparation is described for intracellular recording from the neur-opile of the sub-oesophageal ganglion (SOG) of the locust, while stimulating the labial and maxillary palps with plant material in such a way as to mimic the palpation behaviour which precedes and continues throughout feeding. Twelve neurones responding to simulated palpation were recorded from and stained in the SOG. Axons of three neurones ascended to the brain, six had descending axons and three had all of their processes confined to the suboesophageal ganglion. The major regions of arborization were in the ventrolateral and mediolateral neuropiles of the maxillary and labial neuromeres. All twelve neurones were solely mechanoresponsive. In addition to responding to palpation of one or more of the four palps, five also responded to stimulation of the labrum, one to touching each antenna, and one to mechanical stimulation of each of the six tarsi. In the context of what is known about the role of mechano-stimulation in the control of feeding, and given their particular patterns of input and arborizations, it is suggested that the neurones may be active during food selection and ingestion.     

FMRFamide-like immunocytochemistry in the brain and subesophageal ganglion of <Emphasis Type="Italic">Triatoma infestans</Emphasis> (Insecta: Heteroptera). Coexpression with β-pigment-dispersing hormone and small cardioactive peptide B

The distribution of FMRFamide (FMRFa)-like immunoreactivity (LI) was studied in the brain and subesophageal ganglion of Triatoma infestans, the insect vector of Chagas disease. The neuropeptide displayed a widespread distribution with immunostained somata in the optic lobe, in the anterior, lateral, and posterior soma rinds of the protocerebrum, and around the antennal sensory and mechanosensory and motor neuropils of the deutocerebrum. FMRFa-immunoreactive profiles of the subesophageal ganglion were seen in the mandibular, maxillary, and labial neuromeres. Immunostained neurites were detected in the medulla and lobula of the optic lobe, the lateral protocerebral neuropil, the median bundle, the calyces and the stalk of the mushroom bodies, and the central body. In the deutocerebrum, the sensory glomeruli showed a higher density of immunoreactive processes than the mechanosensory and motor neuropil, whereas the neuropils of each neuromere of the subesophageal ganglion displayed a moderate density of immunoreactive neurites. Colocalization of FMRFa-LI and crustacean pigment-dispersing hormone-LI was found in perikarya of the proximal optic lobe, the lobula, the sensory deutocerebrum, and the labial neuromere of the subesophageal ganglion. The distribution pattern of small cardioactive peptide B (SCP_B)-LI was also widespread, with immunolabeled somata surrounding every neuropil region of the brain and subesophageal ganglion, except for the optic lobe. FMRFa- and SCP_B-LIs showed extensive colocalization in the brain of this triatomine species. The presence of immunolabeled perikarya displaying either FMRFa- or SCP_B-LI confirmed that each antisera identified different peptide molecules. The distribution of FMRFa immunostaining in T. infestans raises the possibility that FMRFa plays a role in the regulation of circadian rhythmicity. The finding of immunolabeling in neurosecretory somata of the protocerebrum suggests that this neuropeptide may also act as a neurohormone.This work was sponsored by the Facultad de Ciencias Biomédicas, Universidad Austral. Part of this work was performed at the Division of Neurobiology, Arizona Research Laboratories (Tucson, Arizona) with the support of a Fulbright Research Award to B.P.S.     

Phe-met-arg-phe (FMRF)-amide is a substrate source of NO synthase in the gastropod nervous system

The possible involvement of the L-arginine-containing Phe-met-arg-phe (FMRF)-amide (FMRFa) in neuronal nitric oxide (NO) biosynthesis was studied in a gastropod species. We found NADPH-diaphorase-positive neurons and FMRFa-containing fibers in close proximity in the enteric nervous system. Administration of L-arginine and FMRFa induced quantitatively similar nitrite production in both intact intestinal tissues and tissue homogenates. These changes could be prevented by the presence of NOARG (an NO synthase inhibitor). Neither chemically modified FMRFa (D-arginine instead of L-arginine) nor amino acid constituents of FMRFa (methionine, phenylalanine) affected basal nitrite production. FMRFa-induced alterations were reduced in the presence of Na⁺ channel blockers (tetrodotoxin, amiloride, lidocaine), the Na⁺/K⁺ATPase inhibitor ouabain, or protease inhibitors (leupeptine, pepstatine-a). FMRFa and its amino acid constituents were analyzed by paper chromatography. When FMRFa was added to tissue homogenates, the peptide was eliminated within 1–2 min, whereas methionine, phenylalanine, arginine, and citrulline levels were elevated simultaneously. We tested the effects of FMRFa, L-arginine, and NOARG on intestinal contractile activity. FMRFa relaxed the intestine for 1–2 min and then induced contractions for 20–40 min. In the presence of NOARG, no relaxant effect of FMRFa was recorded. As administration of L-arginine strongly inhibits the mechanical activity of the intestinal muscle, NO production presumably plays a substantial role in the action of FMRFa, at least in the initial phase. Our biochemical data indicate a direct involvement of FMRFa in NO biosynthesis. FMRFa might be hydrolyzed by extracellular peptidases and then the locally released arginine might be transported into the cells and broken-down to produce NO. Depolarization-induced NO production attributable to the activation of amiloride-sensitive Na⁺ channels might also be involved.     

Peripheral nerve connections influence the appearance of neurosecretary material in neural sheath of ventral ganglion of the fly Sarcophaga bullata: an immunocytochemical study

This study examined the role of the brain and peripheral connections with the target organs in the appearance of neurosecretary material within the dorsal neural sheath of the ventral ganglion of the fly S. bullata. Specifically, the accumulation of the neuropeptide FMRFamide (the neurosecretary material) was examined by immunocytochemistry. Immunoreactions were performed on: (1) a normal intact ventral ganglion, (2) an isolated ventral ganglion that was cultured in vivo, and (3) a ventral ganglion that was isolated by transection from the brain, but retained its peripheral nerve connections. The results demonstrate that (a) the neurons of the ganglia survive and exhibit FMRFamide immune reaction independent of their peripheral connections, and (b) the accumulation of neuropeptide in the dorsal neural sheath is controlled by intact peripheral nerve connections with the ganglion. It is suggested that in the absence of their peripheral connections, the axons of FMRFamide immunoreactive neurons fail to invade the neural sheath resulting in the accumulation of neurosecretary material.     

On the neurosecretory system of Dendrobaena atheca Cernosvitov

Four kinds of neurosecretory cells A, B, U and C are distinguished in the central nervous system of Dendrobaena atheca Cernosvitov. A cells, which show different morphological characteristics under different physiological states and during their cyclic changes, are the most active neurosecretory cells. They form the outer layer of the cortical cell zone in the cerebral ganglion. B cells are large and medium sized and are distributed in all parts of the central nervous system. U cells are found only in the sub-pharyngeal ganglion while C cells are distributed in the sub-pharyngeal as well as in the ventral nerve cord ganglion. The number and secretory activity of C cells decrease in caudal direction. Further, Gomori-positive cells are also observed in the ganglia of the vegetative nervous system. A rudimentary neurohaemal organ, the storage zone, has been observed in the cerebral ganglion and there appears to be another neurohaemal area in the ventral nerve cord ganglion. The storage zone is formed by the terminal ends of the axons of A cells. The chrome alum haematoxylin phloxin (CHP) and aldehyde fuchsin (AF) positive substances in the form of granules are found in this area. The cerebral ganglion is richly supplied by blood capillaries. The distal end of the axons of B cells are swollen like a bulb while in some cases the axons are united to form an axonal tract. Extra-cellular material is abundant in different parts of the nervous system. In all cell types, the perinuclear zone is the first to show activity in the secretory cycle. It appears that the nucleus may be involved in the elaboration of the neurosecretory material in the cells.     

Neuroanatomical,immunocytochemical, and physiological studies of the pharyngeal retractor muscle and its putative regulatory neurons playing a role in withdrawal and feeding in the snail, <Emphasis Type="Italic">Helix pomatia</Emphasis>

We describe the neurons regulating two separate functions of the pharyngeal retractor muscle (PRM), namely sustained contraction during body withdrawal and rhythmic phasic contractions during feeding, in the snail, Helix pomatia. The distribution of central neurons innervating the PRM is organized into two main units; one in the buccal-cerebral ganglion complex, the other in the subesophageal ganglion complex. Serotonin- (5-HT-), FMRFamide- (FMRFa-), and tyrosine-hydroxylase-immunostained neurons are present among the PRM neurons that densely innervate the PRM. 5HT both decreases and increases the amplitude of the electrically evoked contraction between concentrations of 0.1 M and 1 M. Dopamine (DA) only decreases the amplitude of contraction at a 1-M threshold concentration. In contrast, FMRFa increases the amplitude of the contraction and slightly elevates the tone of the PRM but requires a higher threshold (10 M). Assay by high-performance liquid chromatography of 5HT and DA in the PRM has shown that the 5HT level decreases during locomotion but increases during feeding, whereas the DA level increases during locomotion but slightly decreases during feeding. Thus, different segments of the PRM are innervated by neurons from different loci within the central nervous system. The segments of the PRM distal to the pharynx are innervated from loci of the subesophageal ganglion complex suggesting that they mediate withdrawal. The proximal segment of the PRM is innervated from cerebral and buccal loci indicating that these neurons mediate the feeding rhythm produced by buccal and cerebral feeding central pattern generators to induce rhythmic phasic contractions in the PRM during feeding.This work was supported by Hungarian Scientific Research Fund (OTKA) grants (T034106, T037389, T037505), the Wellcome Trust CRIG Programme, and the Wellcome Trust Travel Grant.     

Isolation of FMRFa-like peptides from the DB-containing connective tissue of Helix aspersa.

The endocrine dorsal bodies (DB) of Helix aspersa are innervated by axons from the central nervous system, which establish synapse-like structures (SLS) with the DB cells. Previous immunocytochemical studies suggested the presence of FMRFa-like substances in nerves of the DB area and in SLS. This paper reports on biochemical attempts undertaken in order to investigate the nature of these substances: the use of HPLC and RIA confirms the presence of three FMRFa-like peptides in the DB-containing connective tissue among which one is probably the FMRFa itself.     

Receptor inactivation by dye-neuropeptide conjugates. 3. Comparative binding of dye-neuropeptide conjugates to FMRFamide receptors of Helix aspersa and Loligo pealei

Three neuropeptide analogues of FMRFamide (FMRFa) were covalently attached to a tethered derivative of methylene blue to form dye-neuropeptide conjugates. The comparative binding of the latter to FMRFa receptors was subsequently examined in both Helix aspersa (circumesophageal ganglia) and squid (optic lobe membrane). In Helix, the FMRFa analogue CFMRFamide (CFMRFa) inhibited the specific binding of the FMRFa ligand [¹²⁵I]daYFnLRFa in a dose-dependent manner. Az-CFMRFa, one of the dye-neuropeptide conjugates, also dose-dependently inhibited the specific binding of [¹²⁵I]daYFnLRFa. Moreover, their potencies equaled or exceeded that of FMRFamide. In squid, the binding of CFMRFa and FMRFa was similar. However, the dye-neuropeptide conjugate (IC₅₀ of 14 nM) was about 44-fold less potent than FMRFa. The conjugates were synthesized as part of a study seeking to target and inactivate preselected receptors with heretofore unattainable selectivity and permanence.     

One receptor type mediates two independent effects of FMRFa on neurosecretory cells of Lymnaea

Structure activity relations (SAR) of FMRFa on the transient hyperpolarizing response and long lasting depression of excitability of neurosecretory caudo dorsal cells (CDCs) of the pond snail Lymnaea stagnalis were examined. Although these effects to FMRFa occur independently, the SARs for the induction of both responses were identical suggesting that CDCs possess a single type of FMRFa receptors. Native GDPFLRFa and SDPFLRFa were equipotent to FMRFa receptors. It is concluded that activation of the receptor requires [Arg3-Phe4]-NH2, whereas N-terminal amino acids are involved in binding.     

The survival of chick retinal ganglion cells in response to brain-derived neurotrophic factor depends on their embryonic age

The survival effects of brain-derived neurotrophic factor (BDNF) on the ganglion cells of the chick retina were studied in vitro at different embryonic ages. We found these effects to be strongly age-dependent: at E5, when the first ganglion cell axons have crossed the optic chiasm, but not yet reached the tectum, ganglion cells survived on a laminin substrate irrespective of the presence or absence of BDNF. At E6, when the axons of the first-generated ganglion cells reached the rostral pole of the tectum, the ganglion cells began to show a dependency on BDNF for survival, but the majority of them were alive after 2 days in vitro in the absence of BDNF. With increasing age, the BDNF dependency for survival increased, and at E11, the majority of the ganglion cells plated were dependent on BDNF for survival. It is at this age that the maximal number of axons can be found in vivo in the optic nerve, the subsequent elimination of ganglion cells and their axons resulting in the loss of hundreds of thousands of them over the next few days. Taken together, these data indicate that retinal ganglion cells depend on BDNF for survival only when their axons have reached their target in vivo. This situation is reminiscent of that described in the peripheral nervous system for the nerve growth factor responsiveness of mouse trigeminal sensory neurons during the period of innervation of their target.     

The Sense Organs and Ventral Ganglion of Sagitta (Chaetognatha)

This paper describes some features of the chaetognath nervous system from ultrastructural observations and observations on material stained with specific techniques for nervous tissue, and from records of the activity of the locomotor muscles and ventral ganglion. Sensory cells grouped on the ventral surface of the head bear ciliary processes (some with multiple tubules), and are probably in connexion with the central nervous system by their own axons, unlike the sensory cells of the hair fan vibration receptors of head and body. The ventral ganglion is motor to the locomotor muscles of the body, and controls the rhythmic locomotor activity of the animal. Electrical events associated with contraction of these muscles are compound non-overshooting spike-like potentials. The ventral ganglion contains several large nerve fibres constant in position and connexions in different individuals. Some of these arise from cells in the ganglia of the head, and pass to the ventral ganglion, others from cells within the ventral ganglion, and probably supply the ciliary hair fan receptors of the body, whilst the motor axons issuing from the ventral ganglion are smaller in diameter. The ganglion is arranged on a ladder-like plan, and axons of the lateral cell bodies cross the central neuropil transversely before they contribute to the longitudinal tracts or pass out in the radial nerves. Synapses in the neuropil contain 30–40 nm electron lucent vesicles; the transmitter is unknown, but is unlikely to be either acetylcholine or l -glutamate. Occasional larger electron dense vesicles up to 70 nm in diameter are also found within nerve fibres of the neuropil. It is concluded that the arrangement of the peripheral nervous system is unlike that of several groups which have been suggested as related to chaetognaths.     

Distribution and functional significance of Met-enkephalin-Arg6-Phe7-and Met-enkephalin-Arg6-Gly7-Leu8-like peptides in the blowflyCalliphora vomitoria

Summary Neuronal pathways in the retrocerebral complex and thoracico-abdominal ganglionic mass of the blowflyCalliphora vomitoria have been identified immunocytochemically with antisera against the extended-enkephalins, Met-enkephalin-Arg⁶-Phe⁷ (Met-7) and Met-enkephalin-Arg⁶-Gly⁷-Leu⁸ (Met-8). Neurons of the hypocerebral ganglion, immunoreactive to Met-8, have axons in the crop duct nerve and terminals in muscles of the crop and its duct. Certain neurons of the hypocerebral ganglion are also immunoreactive to Met-7, and axons from these cells innervate the heart. Met-8 immunoreactive nerve terminals invest the cells of the corpus allatum. The source of this material is believed to ve a single pair of lateral neurosecretory cells in the brain. There is no Met-7 immunoreactive material in the corpus allatum. In the corpus cardiacum neither Met-7 nor Met-8 immunoreactivity is present in the cells. However, in the neuropil of the gland certain fibres, with their origins elsewhere, do contain Met-8 immunoreactivity. The most prominent neurons in the thoracic ganglion are the Met-7 immunoreactive ventral thoracic neurosecretory cells, axons from which project to neurohaemal areas in the dorsal neural sheath and also, via the ventral connective, to the brain. Co-localisation studies show that the perikarya of these cells are immunoreactive to antisera raised against several vertebrate-type peptides, such as Met-7, gastrin/cholecystokinin and pancreatic polypeptide. However, their axons and terminals show varying amounts of the peptides, suggesting differential transport and utilisation. Only a few cells in the thoracic ganglion are immunoreactive to Met-8 antisera. These lie close to the nerve bundles suppling the legs. In the abdominal ganglion, Met-8 immunoreactive neurons project to the muscles of the hindgut. This study suggests that the extended enkephalin-like peptides ofCalliphora may have a variety of different roles: as neurotransmitter or neuromodulator substances; in the direct innervation of effector organs; and as neurohormones.     

Comparative analysis defines a broader FMRFamide-gated sodium channel family and determinants of neuropeptide sensitivity

FMRFamide (Phe-Met-Arg-Phe-amide, FMRFa) and similar neuropeptides are important physiological modulators in most invertebrates, but the molecular basis of FMRFa activity at its receptors is unknown. We therefore sought to identify the molecular determinants of FMRFa potency against one of its native targets, the excitatory FMRFa-gated sodium channel (FaNaC) from gastropod mollusks. Using molecular phylogenetics and electrophysiological measurement of neuropeptide activity, we identified a broad FaNaC family that includes mollusk and annelid channels gated by FMRFa, FVRIamides, and/or Wamides (or myoinhibitory peptides). A comparative analysis of this broader FaNaC family and other channels from the overarching degenerin (DEG)/epithelial sodium channel (ENaC) superfamily, incorporating mutagenesis and experimental dissection of channel function, identified a pocket of amino acid residues that determines activation of FaNaCs by neuropeptides. Although this pocket has diverged in distantly related DEG/ENaC channels that are activated by other ligands but enhanced by FMRFa, such as mammalian acid-sensing ion channels, we show that it nonetheless contains residues that determine enhancement of those channels by similar peptides. This study thus identifies amino acid residues that determine FMRFa neuropeptide activity at FaNaC receptor channels and illuminates the evolution of ligand recognition in one branch of the DEG/ENaC superfamily of ion channels.     

Anatomy and in vivo activity of neurons connecting the crustacean stomatogastric nervous system to the brain

In decapod crustaceans, the inferior ventricular nerve connects the cerebral ganglia (brain) with the stomatogastric nervous system (STNS). In the ivn of the crayfish, eight axons with diameters between 3.5 microm and 10 microm were found in close proximity to the oesophageal ganglion. Two of these axons terminate with their cell body within the ivn. The projections of the other six axons spread inside many neuropiles of the brain, mainly within the protocerebrum and the neuropils of the first and second antennae. Several fibers also send neurites via the circumoesophageal connectives toward the paired commissural ganglia and further down to the ventral nerve cord. The activity of motoneurons within the STNS and of axons in the ivn was recorded with implanted electrodes before, during and after times of feeding. At the beginning of feeding all tonically active ivn neurons accelerated their discharge rate and initially silent neurons also started to fire. Spike frequency was correlated with the quantity of food consumed. The ivn response was accompanied by a corresponding increase in pyloric frequency and an initiation of a gastric rhythm. The two motor rhythms showed a strong phasic interaction, but there was no phase coupling to the ivn activity.     

Structures with GABA-like and GAD-like immunoreactivity in the cervical sympathetic ganglion complex of adult rats

Summary The distribution of gamma-aminobutyric acid (GABA)-like and glutamate decarboxylase (GAD)-like immunoreactivity was studied in the cervical sympathetic ganglion complex of rats, including the intermediate and inferior cervical ganglia and the uppermost thoracic ganglion. GABA-positive axons may enter the ganglion complex via its caudal end. Others apparently arise from small GABA-positive cell bodies which are scattered among principal neurons, within clusters of SIF cells and in bundles of GABA-negative axons. The majority of these cells is located in the lower half of the ganglion complex. Principal neurons did not react with antibodies against GABA or GAD. An unevenly distributed meshwork of GABA-immunoreactive axons was seen in each of the ganglia. Immunoreactive axons formed numerous varicosities. Some of them were aggregated in a basket-like form around a subpopulation of GABA-negative principal ganglion cell bodies. Electron-microscopic immunocytochemistry revealed that GABA-positive nerve fibers establish asymmetric synaptic junctions with dendritic and somatic spines of principal neurons, whereas postsynaptic densities are inconspicous or absent on dendritic shafts and somata. The results suggest that in the cervical sympathetic ganglion complex principal neurons are not GABAergic, but are innervated by axons which react with both antibodies against GAD and/ or GABA antibodies and originate from a subpopulation of small neurons.     

Sensory Projections from Ectopic Appendages in an insect: Inherent Specificity and Influence of Location

Peripheral and central pathfinding by sensory axons from appendages was investigated in the fly Sarcophaga bullata . (a) Supernumerary appendages (haltere, wing, antenna and leg) were produced by imaginal disc transplantation at various ectopic sites, (b) Leg neuropil was deafferented by leg disc extirpation and in its place another leg disc was implanted. (c) The basal stalk of a leg disc connecting it with the thoracic ganglion was transected. Using cobalt chloride and HRP backfilling methods the pathways taken by the afferents from these experimentally altered appendages was examined. The results indicate that the larval nerves and the imaginal disc stalks act as guides for growing axons to locate their correct entry sites within the ventral ganglion. In the absence of these guides the axons follow any peripheral nerve, such as abdominal nerve, and enter the ganglion at inappropriate sites. However, within the ganglion they take particular routes, almost identical to those taken by axons from in situ appendages suggesting the existance of some kind of a labelled pathway. Deafferentation does not make the leg neuropil more attractive to ingrowing ectopic sensory axons.     

Serotonin immunoreactivity of neurons in the gastropod Aplysia californica

Serotonergic neurons and axons were mapped in the central ganglia of Aplysia californica using antiserotonin antibody on intact ganglia and on serial sections. Immunoreactive axons and processes were present in all ganglia and nerves, and distinct somata were detected in all ganglia except the buccal and pleural ganglia. The cells stained included known serotonergic neurons: the giant cerebral neurons and the RB cells of the abdominal ganglion. The area of the abdominal ganglion where interneurons are located which produce facilitation during the gill withdrawal reflex was carefully examined for antiserotonin immunoreactive neurons. None were found, but two bilaterally symmetric pairs of immunoreactive axons were identified which descend from the contralateral cerebral or pedal ganglion to abdominal ganglion. Because of the continuous proximity of this pair of axons, they could be recognized and traced into the abdominal ganglion neuropil in each preparation. If serotonin is a facilitating transmitter in the abdominal ganglion, these and other antiserotonin immunoreactive axons in the pleuroabdominal connectives may be implicated in this facilitation.