An identified pleural ganglion interneuron inhibits patterned motor activity in the buccal ganglia of the snail, Helisoma

FMRF-NH2 -related neuropeptides in Biomphalaria spp., intermediate hosts for schistosomiasis: Precursor organization and immunohistochemical localization

Freshwater snails of the genus Biomphalaria serve as intermediate hosts for the digenetic trematode Schistosoma mansoni, the etiological agent for the most widespread form of intestinal schistosomiasis. As neuropeptide signaling in host snails can be altered by trematode infection, a neural transcriptomics approach was undertaken to identify peptide precursors in Biomphalaria glabrata, the major intermediate host for S. mansoni in the Western Hemisphere. Three transcripts that encode peptides belonging to the FMRF-NH2 -related peptide (FaRP) family were identified in B. glabrata. One transcript encoded a precursor polypeptide (Bgl-FaRP1; 292 amino acids) that included eight copies of the tetrapeptide FMRF-NH2 and single copies of FIRF-NH2 , FLRF-NH2 , and pQFYRI-NH2 . The second transcript encoded a precursor (Bgl-FaRP2; 347 amino acids) that comprised 14 copies of the heptapeptide GDPFLRF-NH2 and 1 copy of SKPYMRF-NH2 . The precursor encoded by the third transcript (Bgl-FaRP3; 287 amino acids) recapitulated Bgl-FaRP2 but lacked the full SKPYMRF-NH2 peptide. The three precursors shared a common signal peptide, suggesting a genomic organization described previously in gastropods. Immunohistochemical studies were performed on the nervous systems of B. glabrata and B. alexandrina, a major intermediate host for S. mansoni in Egypt. FMRF-NH2 -like immunoreactive (FMRF-NH2 -li) neurons were located in regions of the central nervous system associated with reproduction, feeding, and cardiorespiration. Antisera raised against non-FMRF-NH2 peptides present in the tetrapeptide and heptapeptide precursors labeled independent subsets of the FMRF-NH2 -li neurons. This study supports the participation of FMRF-NH2 -related neuropeptides in the regulation of vital physiological and behavioral systems that are altered by parasitism in Biomphalaria.

Diversity of the RFamide Peptide Family in Mollusks

Since the initial characterization of the cardioexcitatory peptide FMRFamide in the bivalve mollusk Macrocallista nimbosa, a great number of FMRFamide-like peptides (FLPs) have been identified in mollusks. FLPs were initially isolated and molecularly characterized in model mollusks using biochemical methods. The development of recombinant technologies and, more recently, of genomics has boosted knowledge on their diversity in various mollusk classes. Today, mollusk FLPs represent approximately 75 distinct RFamide peptides that appear to result from the expression of only five genes: the FMRFamide-related peptide gene, the LFRFamide gene, the luqin gene, the neuropeptide F gene, and the cholecystokinin/sulfakinin gene. FLPs display a complex spatiotemporal pattern of expression in the central and peripheral nervous system. Working as neurotransmitters, neuromodulators, or neurohormones, FLPs are involved in the control of a great variety of biological and physiological processes including cardiovascular regulation, osmoregulation, reproduction, digestion, and feeding behavior. From an evolutionary viewpoint, the major challenge will then logically concern the elucidation of the FLP repertoire of orphan mollusk classes and the way they are functionally related. In this respect, deciphering FLP signaling pathways by characterizing the specific receptors these peptides bind remains another exciting objective.

LPLRFamide causes anorexigenic effects in broiler chicks and Bobwhite quail

Although LPLRFamide was the first member of the RFamide family to be isolated from a vertebrate species, its effects on hunger and satiety-related processes are poorly documented. Thus, we intracerebroventricularly administered LPLRFamide (3.0-15.0 nmol) to both Cobb-500 (a broiler type of Gallus gallus) and Bobwhite quail (Colinus virginianus) chicks and measured their food intake. The threshold of anorexigenic response was 7.0 nmol in Cobb-500 chicks and the effect had diminished by 30 min post-injection. In Bobwhite quail all doses of LPLRFamide tested caused anorexia that remained throughout the 60 min observation period. A comprehensive behavior analysis was conducted and Cobb-500 chicks had increased food pecks early in the observation period and spent a greater amount of time in deep rest. Although food pecks were increased pecking efficiency was decreased. In Bobwhite quail, feeding pecks and the number of jumps were reduced after LPLRFamide treatment. We judged that these behaviors in both species were likely not competitive with ingestion and thus did not secondarily contribute to anorexia. These results demonstrate that LPLRFamide is associated with satiety-related processes in Cobb-500 chicks and Bobwhite quail, while threshold of responses are different.

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, Helix pomatia

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 microM and 1 microM. Dopamine (DA) only decreases the amplitude of contraction at a 1-microM 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 microM). 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.

Multilevel inhibition of feeding by a peptidergic pleural interneuron in the mollusc Lymnaea stagnalis

The pleural interneuron PlB is a white neuron in the pleural ganglion of the snail Lymnaea. We test the hypothesis that it inhibits neurons at all levels of the feeding system, using a combination of anatomy, physiology and pharmacology. There is just one PlB in each pleural ganglion. Its axon traverses the pedal and cerebral ganglia, running into the buccal ganglia. It has neuropilar branches in the regions of the cerebral and buccal ganglia where neurons that are active during feeding also branch. Activation of the PlB blocks fictive feeding, whether the feeding rhythm occurs spontaneously or is driven by a modulatory interneuron. The PlB inhibits all the neurons in the feeding network, including protraction and retraction motoneurons, central pattern generator interneurons, buccal modulatory interneurons (SO, OC), and cerebral modulatory interneurons (CV1, CGC). Only the CV1 interneuron shows discrete 1:1 IPSPs; all other effects are slow, smooth hyperpolarizations. All connections persist in Ca(2+)/Mg(2+)-rich saline, which reduces polysynaptic effects. The inhibitory effects are mimicked by 0.5 to 100 micromol l(-1) FMRFamide, which the PlB soma contains. We conclude that the PlB inhibits neurons in the feeding system at all levels, probably acting though the peptide transmitter FMRFamide.

A population of pedal-buccal projection neurons associated with appetitive components of Aplysia feeding behavior

Backfills of the cerebral-buccal connective (CBC) of Aplysia californica revealed a cluster of five to seven pedal-buccal projection neurons in the anterolateral quadrant of the ventral surface of each pedal ganglion. Intra- and extracellular recordings showed that the pedal-buccal projection neurons shared common electrophysiological properties and synaptic inputs. However, they exhibited considerable heterogeneity with respect to their projection patterns. All pedal-buccal projection neurons that were tested received a slow excitatory postsynaptic potential from the ipsi- and contralateral cerebral-pedal regulator (C-PR) neuron, a cell that is thought to play a key role in the generation of a food-induced arousal state. Tests were conducted to identify potential synaptic follower neurons of the pedal-buccal projection neurons in the cerebral and buccal ganglia, but none were detected. Finally, nerve recordings revealed projections from the pedal-buccal projection neurons in the nerves associated with the buccal ganglion. In tests designed to determine the functional properties of these peripheral projections, no evidence was obtained supporting a mechanosensory or proprioceptive role and no movements were observed when they were fired. It is proposed that peripheral elements utilized in consummatory phases of Aplysia feeding may be directly influenced by a neuronal pathway that is activated during the food-induced arousal state.

The neuronal basis of feeding in the snail, Helisoma, with comparisons to selected gastropods

Research on identified neurons during the last quarter century was forecast at a conference in 1973 that discussed "neuronal mechanisms of coordination in simple systems." The focus of the conference was on the neuronal control of simple stereotyped behavioral acts. Participants discussing the future of such research called for a comparative approach; emphasis on structure-function interactions; attention to environmental and behavioral context; and the development of new techniques. Significantly, in some cases amazing progress has been made in these areas. Major conclusions of the last quarter century are that so-called simple behaviors and the neural circuitry underlying them tend to be less simple, more flexible, and more highly modulated than originally imagined. However, the comparative approach has, as yet, failed to reach its potential. Molluscan preparations, along with arthropods and annelids, have always been at the forefront of neuroethological studies. Circuitry underlying feeding has been studied in a handful of species of gastropod molluscs. These studies have contributed substantially to our understanding of sensorimotor organization, the hierarchical control of behavior and coordination of multiple behaviors, and the organization and modulation of central pattern generators. However, direct interspecific comparisons of feeding circuitry and potentially homologous neurons have been lacking. This is unfortunate because much of the vast radiation of the class Gastropoda is associated with variations in feeding behaviors and feeding apparatuses, providing ample substrates for comparative studies including the evolution of defined circuitry. Here, the neural organization of feeding in the snail, Helisoma, is examined critically. Possible direct interspecific comparisons of neural circuitry and potentially homologous neurons are made. A universal model for central pattern generators underlying rasping feeding is proposed. Future comparative studies can be expected to combine behavioral, morphological, electrophysiological, molecular and genetic techniques to identify neurons and define neural circuitry. Digital resources will undoubtedly be exploited to organize and interface databases allowing illumination of the evolution of homologous identified neurons and defined neural circuitry in the context of behavioral change.

Axonal regeneration in the central nervous system of aplysia californica determined by anterograde transport of biocytin

Rhythmic biting, a component of consummatory feeding behavior in the sea hare Aplysia californica, is eliminated following bilateral cerebral-buccal connective (CBC) crushes and recovers within 14 days postlesion. To assess axonal regeneration after CBC lesions, we used biocytin backfills of CBCs followed by fluorescence labeling with streptavidin-lissamine rhodamine. Anterograde transport of biocytin showed up to 1 mm of outgrowth by regenerating axons at 3 days postlesion. At 7 days postlesion, the regenerated axons approached or had entered the ipsilateral buccal neuropil and exhibited numerous varicosities; the average rate of axonal growth was 326 microm/day for the longest, most rapidly growing axons labeled in the CBC. The number of varicosities on labeled axons, suggestive of intercellular interactions, was increased dramatically at all times postlesion. At 14 and 20 days postlesion, regenerated axons branched extensively in the ipsilateral buccal neuropil, entered the contralateral buccal neuropil, and entered peripheral nerves on both sides of the midline. At these later times postlesion, some labeled axons encircled unlabeled buccal cell bodies and exhibited branches containing numerous varicosities, indicative of axosomatic contacts. Some regenerating axons were observed in the sheath of the CBC, but the vast majority of labeled axons remained confined to the connective core, as in control preparations. The bilateral projections within the buccal ganglia of labeled cerebral-to-buccal axons and the large number of varicosities present on these processes are indicative of regenerating axons and synapses that likely contribute to the functional recovery of rhythmic biting.

Characterization of an identified cerebrobuccal neuron containing the neuropeptide APGWamide (Ala-Pro-Gly-Trp-NH2) in the snail Lymnaea stagnalis

A bilaterally symmetrical pair of cerebrobuccal neurons in Lymnaea stagnalis shows immunoreactivity for the molluscan neuropeptide APGWamide. The neuron somata are whitish in colour and located on the ventral surface of each cerebral ganglion between the roots of the labial nerves. A single axon travels via the ipsilateral cerebrobuccal connective into the buccal ganglia, where it gives rise to fine neuritic branching. Based upon these characteristics, the neuron has been named the cerebrobuccal white cell (CBWC). In isolated CNS preparations, in the absence of feeding motor output, the CBWC is silent and receives few, low amplitude, synaptic inputs. During generation of fictive feeding, the CBWC bursts in phase with cycles of feeding motor output. Tonic or phasic stimulation of CBWC leads to initiation of rhythmic feeding motor output. However, evoked bursts of activity in CBWC, which mimic its normal burst pattern, cannot entrain the buccal rhythm, suggesting that CBWC is not itself a major component of the feeding central pattern generator (CPG). Strong stimulation of CBWC during ongoing feeding motor output leads to a reduction in frequency and/or intensity of the buccal rhythm. Bath application of synthetic APGWamide (10(-7)M-10(-4)M) to the isolated CNS can activate feeding motor output in quiescent preparations after a delay, but disrupts ongoing buccal rhythms. This study represents the first description of a peptidergic cerebrobuccal neuron in the well described gastropod feeding system and also provides new information about the role of a novel molluscan neuropeptide.

Photoinactivation of neurones axonally filled with the fluorescent dye 5(6)-carboxyfluorescein in the pond snail, Lymnaea stagnalis

We describe a new, simple and reliable technique to fill molluscan neurones from their cut axons with sufficient fluorescent dye for photoinactivation experiments. The fluorescent dye 5(6)-carboxyfluorescein (5-CF) travels quickly up the nerves of the gastropod mollusc, Lymnaea stagnalis into the buccal ganglia and fills the cell bodies in 1-3 h. 5-CF filled neurones can be located in the intact ganglia with low intensity blue light. Impalement shows that they are alive and show normal resting, action and synaptic potentials. Intense laser light (wavelength 442 nm, intensity 0.5 MW.m-2) kills all the 5-CF filled cells in less than 5 min in laboratory reared snails. Unstained neurones are not killed. 5-CF fills neurones quicker than Lucifer yellow (LY) when the dye is applied axonally. Neurones stained with Lucifer yellow do not contain sufficient dye to be killed with 5 min laser illumination, but this irradiation reduces the membrane resistance to less than 25%.