Serotonin in the developing stomatogastric system of the lobster, Homarus americanus

Synaptic Dynamics Convey Differential Sensitivity to Input Pattern Changes in Two Muscles Innervated by the Same Motor Neurons

Postsynaptic responses depend on input patterns as well as short-term synaptic plasticity, summation, and postsynaptic membrane properties, but the interactions of those dynamics with realistic input patterns are not well understood. We recorded the responses of the two pyloric dilator (PD) muscles, cpv2a and cpv2b, that are innervated by and receive identical periodic bursting input from the same two motor neurons in the lobster Homarus americanus. Cpv2a and cpv2b showed quantitative differences in membrane nonlinearities and synaptic summation. At a short timescale, responses in both muscles were dominated by facilitation, albeit with different frequency and time dependence. Realistic burst stimulations revealed more substantial differences. Across bursts, cpv2a showed transient depression, whereas cpv2b showed transient facilitation. Steady-state responses to bursting input also differed substantially. Neither muscle had a monotonic dependence on frequency, but cpv2b showed particularly pronounced bandpass filtering. Cpv2a was sensitive to changes in both burst frequency and intra-burst spike frequency, whereas, despite its much slower responses, cpv2b was largely insensitive to changes in burst frequency. Cpv2a was sensitive to both burst duration and number of spikes per burst, whereas cpv2b was sensitive only to the former parameter. Neither muscle showed consistent sensitivity to changes in the overall spike interval structure, but cpv2b was surprisingly sensitive to changes in the first intervals in each burst, a parameter known to be regulated by dopamine (DA) modulation of spike propagation of the presynaptic axon. These findings highlight how seemingly minor circuit output changes mediated by neuromodulation could be read out differentially at the two synapses.

Feeding State-Dependent Modulation of Feeding-Related Motor Patterns

Studies elucidating modulation of microcircuit activity in isolated nervous systems have revealed numerous insights regarding neural circuit flexibility, but this approach limits the link between experimental results and behavioral context. To bridge this gap, we studied feeding behavior-linked modulation of microcircuit activity in the isolated stomatogastric nervous system (STNS) of male Cancer borealis crabs. Specifically, we removed hemolymph from a crab that was unfed for ≥24 h ('unfed' hemolymph) or fed 15 min - 2 h before hemolymph removal ('fed' hemolymph). After feeding, the first significant foregut emptying occurred >1 h later and complete emptying required ≥6 h. We applied the unfed or fed hemolymph to the stomatogastric ganglion (STG) in an isolated STNS preparation from a separate, unfed crab to determine its influence on the VCN (ventral cardiac neuron)-triggered gastric mill (chewing)- and pyloric (filtering of chewed food) rhythms. Unfed hemolymph had little influence on these rhythms, but fed hemolymph from each examined time-point (15 min, 1- or 2 h post-feeding) slowed one or both rhythms without weakening circuit neuron activity. There were also distinct parameter changes associated with each time-point. One change unique to the 1 h time-point (i.e. reduced activity of one circuit neuron during the transition from the gastric mill retraction to protraction phase) suggested the fed hemolymph also enhanced the influence of a projection neuron which innervates the STG from a ganglion isolated from the applied hemolymph. Hemolymph thus provides a feeding state-dependent modulation of the two feeding-related motor patterns in the C. borealis STG.

Development of the Neurochemical Architecture of the Central Complex

The central complex represents one of the most conspicuous neuroarchitectures to be found in the insect brain and regulates a wide repertoire of behaviors including locomotion, stridulation, spatial orientation and spatial memory. In this review article, we show that in the grasshopper, a model insect system, the intricate wiring of the fan-shaped body (FB) begins early in embryogenesis when axons from the first progeny of four protocerebral stem cells (called W, X, Y, Z, respectively) in each brain hemisphere establish a set of tracts to the primary commissural system. Decussation of subsets of commissural neurons at stereotypic locations across the brain midline then establishes a columnar neuroarchitecture in the FB which is completed during embryogenesis. Examination of the expression patterns of various neurochemicals in the central complex including neuropeptides, a neurotransmitter and the gas nitric oxide (NO), show that these appear progressively and in a substance-specific manner during embryogenesis. Each neuroactive substance is expressed by neurons located at stereotypic locations in a given central complex lineage, confirming that the stem cells are biochemically multipotent. The organization of axons expressing the various neurochemicals within the central complex is topologically related to the location, and hence birthdate, of the neurons within the lineages. The neurochemical expression patterns within the FB are layered, and so reflect the temporal topology present in the lineages. This principle relates the neuroanatomical to the neurochemical architecture of the central complex and so may provide insights into the development of adaptive behaviors.

Neuromodulation in developing motor microcircuits

Neuromodulation confers operational flexibility on motor network output and resulting behaviour. Furthermore, neuromodulators play crucial long-term roles in the assembly and maturational shaping of the same networks as they develop. Although previous studies have identified such modulator-dependent contributions to microcircuit ontogeny, some of the underlying mechanisms are only now being elucidated. Deciphering the role of neuromodulatory systems in motor network development has potentially important implications for post-lesional regenerative strategies in adults.

Identification and developmental expression of the enzymes responsible for dopamine, histamine, octopamine and serotonin biosynthesis in the copepod crustacean Calanus finmarchicus

Neurochemicals are likely to play key roles in physiological/behavioral control in the copepod crustacean Calanus finmarchicus, the biomass dominant zooplankton for much of the North Atlantic Ocean. Previously, a de novo assembled transcriptome consisting of 206,041 unique sequences was used to characterize the peptidergic signaling systems of Calanus. Here, this assembly was mined for transcripts encoding enzymes involved in amine biosynthesis. Using known Drosophila melanogaster proteins as templates, transcripts encoding putative Calanus homologs of tryptophan-phenylalanine hydroxylase (dopamine, octopamine and serotonin biosynthesis), tyrosine hydroxylase (dopamine biosynthesis), DOPA decarboxylase (dopamine and serotonin biosynthesis), histidine decarboxylase (histamine biosynthesis), tyrosine decarboxylase (octopamine biosynthesis), tyramine β-hydroxylase (octopamine biosynthesis) and tryptophan hydroxylase (serotonin biosynthesis) were identified. Reverse BLAST and domain analyses show that the proteins deduced from these transcripts possess sequence homology to and the structural hallmarks of their respective enzyme families. Developmental profiling revealed a remarkably consistent pattern of expression for all transcripts, with the highest levels of expression typically seen in the early nauplius and early copepodite. These expression patterns suggest roles for amines during development, particularly in the metamorphic transitions from embryo to nauplius and from nauplius to copepodite. Taken collectively, the data presented here lay a strong foundation for future gene-based studies of aminergic signaling in this and other copepod species, in particular assessment of the roles they may play in developmental control.

A developmental study of serotonin-immunoreactive neurons in the embryonic brain of the marbled crayfish and the migratory locust: evidence for a homologous protocerebral group of neurons

It is well established that the brains of adult malacostracan crustaceans and winged insects display distinct homologies down to the level of single neuropils such as the central complex and the optic neuropils. We wanted to know if developing insect and crustacean brains also share similarities and therefore have explored how neurotransmitter systems arise during arthropod embryogenesis. Previously, Sintoni et al. (2007) had already reported a homology of an individually identified cluster of neurons in the embryonic crayfish and insect brain, the secondary head spot cells that express the Engrailed protein. In the present study, we have documented the ontogeny of the serotonergic system in embryonic brains of the Marbled Crayfish in comparison to Migratory Locust embryos using immunohistochemical methods combined with confocal laser-scan microscopy. In both species, we found a cluster of early emerging serotonin-immunoreactive neurons in the protocerebrum with neurites that cross to the contralateral brain hemisphere in a characteristic commissure suggesting a homology of this cell cluster. Our study is a first step towards a phylogenetic analysis of neurotransmitter system development and shows that, as for the ventral nerve cord, traits related to neurogenesis in the brain can provide valuable hints for resolving the much debated question of arthropod phylogeny.

Sensory neuron-derived eph regulates glomerular arbors and modulatory function of a central serotonergic neuron

Olfactory sensory neurons connect to the antennal lobe of the fly to create the primary units for processing odor cues, the glomeruli. Unique amongst antennal-lobe neurons is an identified wide-field serotonergic neuron, the contralaterally-projecting, serotonin-immunoreactive deutocerebral neuron (CSDn). The CSDn spreads its termini all over the contralateral antennal lobe, suggesting a diffuse neuromodulatory role. A closer examination, however, reveals a restricted pattern of the CSDn arborization in some glomeruli. We show that sensory neuron-derived Eph interacts with Ephrin in the CSDn, to regulate these arborizations. Behavioural analysis of animals with altered Eph-ephrin signaling and with consequent arborization defects suggests that neuromodulation requires local glomerular-specific patterning of the CSDn termini. Our results show the importance of developmental regulation of terminal arborization of even the diffuse modulatory neurons to allow them to route sensory-inputs according to the behavioural contexts.

Serotonin, a major neuromodulatory transmitter, regulates diverse behaviours. Serotonergic dysfunction is implicated in various neuropsychological disorders, such as anxiety and depression, as well as in neurodegenerative disorders. In the central nervous systems, across taxa, serotonergic neurons are often small in number but connect to and act upon multiple brain circuits through their wide-field arborization pattern. We set out to decipher mechanisms by which wide-field serotonergic neurons differentially innervate their target-field to modulate behavior in a context-dependent manner. We took advantage of the sophisticated antennal lobe circuitry, the primary olfactory centre in the adult fruitfly Drosophila melanogaster. Olfactory sensory neurons and projection neurons connect in a partner-specific manner to create glomerular units in the antennal lobe for processing the sense of smell. Our analysis at a single-cell resolution reveals that a wide-field serotonergic neuron connects to all the glomeruli in the antennal lobe but exhibits the glomerular-specific differences in its innervation pattern. Our key finding is that Eph from sensory neurons regulates the glomerular-specific innervation pattern of the central serotonergic neuron, which in turn is essential for modulation of odor-guided behaviours in an odor-specific manner.

Inter-animal variability in the effects of C-type allatostatin on the cardiac neuromuscular system in the lobster Homarus americanus

Although the global effects of many modulators on pattern generators are relatively consistent among preparations, modulators can induce different alterations in different preparations. We examined the mechanisms that underlie such variability in the modulatory effects of the peptide C-type allatostatin (C-AST; pQIRYHQCYFNPISCF) on the cardiac neuromuscular system of the lobster Homarus americanus. Perfusion of C-AST through the semi-intact heart consistently decreased the frequency of ongoing contractions. However, the effect of C-AST on contraction amplitude varied between preparations, decreasing in some preparations and increasing in others. To investigate this variable effect, we examined the effects of C-AST both peripherally and centrally. When contractions of the myocardium were elicited by controlled stimuli, C-AST did not alter heart contraction at the periphery (myocardium or neuromuscular junction) in any hearts. However, when applied either to the semi-intact heart or to the cardiac ganglion (CG) isolated from hearts that responded to C-AST with increased contraction force, C-AST increased both motor neuron burst duration and the number of spikes per burst by about 25%. In contrast, CG output was increased only marginally in hearts that responded to C-AST with a decrease in contraction amplitude, suggesting that the decrease in amplitude in those preparations resulted from decreased peripheral facilitation. Our data suggest that the differential effects of a single peptide on the cardiac neuromuscular system are due solely to differential effects of the peptide on the pattern generator; the extent to which the peptide induces increased burst duration is crucial in determining its overall effect on the system.

Extrasynaptic exocytosis and its mechanisms: a source of molecules mediating volume transmission in the nervous system

We review the evidence of exocytosis from extrasynaptic sites in the soma, dendrites, and axonal varicosities of central and peripheral neurons of vertebrates and invertebrates, with emphasis on somatic exocytosis, and how it contributes to signaling in the nervous system. The finding of secretory vesicles in extrasynaptic sites of neurons, the presence of signaling molecules (namely transmitters or peptides) in the extracellular space outside synaptic clefts, and the mismatch between exocytosis sites and the location of receptors for these molecules in neurons and glial cells, have long suggested that in addition to synaptic communication, transmitters are released, and act extrasynaptically. The catalog of these molecules includes low molecular weight transmitters such as monoamines, acetylcholine, glutamate, gama-aminobutiric acid (GABA), adenosine-5-triphosphate (ATP), and a list of peptides including substance P, brain-derived neurotrophic factor (BDNF), and oxytocin. By comparing the mechanisms of extrasynaptic exocytosis of different signaling molecules by various neuron types we show that it is a widespread mechanism for communication in the nervous system that uses certain common mechanisms, which are different from those of synaptic exocytosis but similar to those of exocytosis from excitable endocrine cells. Somatic exocytosis has been measured directly in different neuron types. It starts after high-frequency electrical activity or long experimental depolarizations and may continue for several minutes after the end of stimulation. Activation of L-type calcium channels, calcium release from intracellular stores and vesicle transport towards the plasma membrane couple excitation and exocytosis from small clear or large dense core vesicles in release sites lacking postsynaptic counterparts. The presence of synaptic and extrasynaptic exocytosis endows individual neurons with a wide variety of time- and space-dependent communication possibilities. Extrasynaptic exocytosis may be the major source of signaling molecules producing volume transmission and by doing so may be part of a long duration signaling mode in the nervous system.

Toward locating the source of serotonergic axons in the tail nerve of Aplysia

Stimulation of the tail nerve (pedal nerve 9, p9) of the mollusk, Aplysia californica, causes release of serotonin (5-HT), which mediates sensitization of withdrawal responses. There are about 35 serotonin-immunoreactive (5-HT-ir) axons in p9, yet the cell bodies of these axons have not been located. Backfills of p9 were combined with 5-HT immunohistochemistry to locate the cell bodies of 5-HT-ir neurons with axons in p9. About 100 neurons had axons in p9. Only about ten neurons, however, were both backfilled and 5-HT-ir. These double-labeled neurons were all located in the pedal ganglion associated with p9, which had a total of approximately 42 5-HT-ir somata. The discrepancy between the number of 5-HT-ir axons and double-labeled cell bodies is not likely due to neurons having multiple axons in the nerve; intracellular fills suggest that these neurons do not branch before entering p9. Additionally, no evidence was found for peripheral 5-HT-ir cell bodies that project axons centrally through p9. Thus, approximately 70% of the neurons that give rise to the 5-HT-ir axons in tail nerve are unaccounted for, but likely to reside in the pedal ganglion.

Developmental expression of neuromodulators in the central complex of the grasshopper Schistocerca gregaria

The central complex is a major integrative region within the insect brain with demonstrated roles in spatial orientation, the regulation of locomotor behavior, and sound production. In the hemimetabolous grasshopper, the central complex comprises the protocerebral bridge, central body (CB), ellipsoid body, noduli, and accessory lobes, and this modular organization develops entirely during embryogenesis. From a biochemical perspective, a range of neuroactive substances has been demonstrated in these modules of the adult central complex, but little is known about their developmental expression. In this study, we use matrix-assisted laser desorption/ionization-imaging mass spectrometry on single brain slices to confirm the presence of several peptide families (tachykinin, allatostatin, periviscerokinin/pyrokinin, FLRFamide, and neuropeptide F) in the adult central complex and then use immunohistochemistry and histology to examine their developmental expression, together with that of the indolamin serotonin, and the endogenous messenger nitric oxide (NO; via its synthesizing enzyme). We find that each neuromodulator is expressed according to a unique, stereotypic, pattern within the various modules making up the central complex. Neuropeptides such as tachykinin (55%) and allatostatin (65%), and the NO-synthesizing enzyme diaphorase (70%), are expressed earlier during embryonic development than the biogenic amine serotonin (80%), whereas periviscerokinin-like peptides and FLRFamide-like peptides begin to be expressed only postembryonically. Within the CB, these neuroactive substances are present in tangential projection neurons before they appear in columnar neurons. There is also no colocalization of serotonin-positive and peptide-positive projections up to the third larval instar during development, consistent with the clear dorsoventral layering of the neuropil we observe. Our results provide the first neurochemical fingerprint of the developing central complex in an hemimetabolous insect.

5-HT receptors mediate lineage-dependent effects of serotonin on adult neurogenesis in Procambarus clarkii

Background

Serotonin (5-HT) is a potent regulator of adult neurogenesis in the crustacean brain, as in the vertebrate brain. However, there are relatively few data regarding the mechanisms of serotonin's action and which precursor cells are targeted. Therefore, we exploited the spatial separation of the neuronal precursor lineage that generates adult-born neurons in the crayfish (Procambarus clarkii) brain to determine which generation(s) is influenced by serotonin, and to identify and localize serotonin receptor subtypes underlying these effects.

Results

RT-PCR shows that mRNAs of serotonin receptors homologous to mammalian subtypes 1A and 2B are expressed in P. clarkii brain (referred to here as 5-HT_1α and 5-HT_2β). In situ hybridization with antisense riboprobes reveals strong expression of these mRNAs in several brain regions, including cell clusters 9 and 10 where adult-born neurons reside. Antibodies generated against the crustacean forms of these receptors do not bind to the primary neuronal precursors (stem cells) in the neurogenic niche or their daughters as they migrate, but do label these second-generation precursors as they approach the proliferation zones of cell clusters 9 and 10. Like serotonin, administration of the P. clarkii 5-HT_1α-specific agonist quipazine maleate salt (QMS) increases the number of bromodeoxyuridine (BrdU)-labeled cells in cluster 10; the P. clarkii 5-HT_2β-specific antagonist methiothepin mesylate salt (MMS) suppresses neurogenesis in this region. However, serotonin, QMS and MMS do not alter the rate of BrdU incorporation into niche precursors or their migratory daughters.

Conclusion

Our results demonstrate that the influences of serotonin on adult neurogenesis in the crayfish brain are confined to the late second-generation precursors and their descendants. Further, the distribution of 5-HT_1α and 5-HT_2β mRNAs and proteins indicate that these serotonergic effects are exerted directly on specific generations of neuronal precursors. Taken together, these results suggest that the influence of serotonin on adult neurogenesis in the crustacean brain is lineage dependent, and that 5-HT_1α and 5-HT_2β receptors underlie these effects.

Crustacean neuropeptides

Crustaceans have long been used for peptide research. For example, the process of neurosecretion was first formally demonstrated in the crustacean X-organ-sinus gland system, and the first fully characterized invertebrate neuropeptide was from a shrimp. Moreover, the crustacean stomatogastric and cardiac nervous systems have long served as models for understanding the general principles governing neural circuit functioning, including modulation by peptides. Here, we review the basic biology of crustacean neuropeptides, discuss methodologies currently driving their discovery, provide an overview of the known families, and summarize recent data on their control of physiology and behavior.

Spectral analyses reveal the presence of adult-like activity in the embryonic stomatogastric motor patterns of the lobster, Homarus americanus

The stomatogastric nervous system (STNS) of the embryonic lobster is rhythmically active prior to hatching, before the network is needed for feeding. In the adult lobster, two rhythms are typically observed: the slow gastric mill rhythm and the more rapid pyloric rhythm. In the embryo, rhythmic activity in both embryonic gastric mill and pyloric neurons occurs at a similar frequency, which is slightly slower than the adult pyloric frequency. However, embryonic motor patterns are highly irregular, making traditional burst quantification difficult. Consequently, we used spectral analysis to analyze long stretches of simultaneous recordings from muscles innervated by gastric and pyloric neurons in the embryo. This analysis revealed that embryonic gastric mill neurons intermittently produced pauses and periods of slower activity not seen in the recordings of the output from embryonic pyloric neurons. The slow activity in the embryonic gastric mill neurons increased in response to the exogenous application of Cancer borealis tachykinin-related peptide 1a (CabTRP), a modulatory peptide that appears in the inputs to the stomatogastric ganglion (STG) late in larval development. These results suggest that the STG network can express adult-like rhythmic behavior before fully differentiated adult motor patterns are observed, and that the maturation of the neuromodulatory inputs is likely to play a role in the eventual establishment of the adult motor patterns.

Mass spectral comparison of the neuropeptide complement of the stomatogastric ganglion and brain in the adult and embryonic lobster, Homarus americanus

Neuropeptides in the stomatogastric ganglion (STG) and the brain of adult and late embryonic Homarus americanus were compared using a multi-faceted mass spectral strategy. Overall, 29 neuropeptides from 10 families were identified in the brain and/or the STG of the lobster. Many of these neuropeptides are reported for the first time in the embryonic lobster. Neuropeptide extraction followed by liquid chromatography coupled to quadrupole-time-of-flight mass spectrometry enabled confident identification of 24 previously characterized peptides in the adult brain and 13 peptides in the embryonic brain. Two novel peptides (QDLDHVFLRFa and GPPSLRLRFa) were de novo sequenced. In addition, a comparison of adult to embryonic brains revealed the presence of an incompletely processed form of Cancer borealis tachykinin-related peptide 1a (CabTRP 1a, APSGFLGMRG) only in the embryonic brain. A comparison of adult to embryonic STGs revealed that QDLDHVFLRFa was present in the embryonic STG but absent in the adult STG, and CabTRP 1a exhibited the opposite trend. Relative quantification of neuropeptides in the STG revealed that three orcokinin family peptides (NFDEIDRSGFGF, NFDEIDRSGFGFV, and NFDEIDRSGFGFN), a B-type allatostatin (STNWSSLRSAWa), and an orcomyotropin-related peptide (FDAFTTGFGHS) exhibited higher signal intensities in the adult relative to the embryonic STG. RFamide (Arg-Phe-amide) family peptide (DTSTPALRLRFa), [Val(1)]SIFamide (VYRKPPFNGSIFa), and orcokinin-related peptide (VYGPRDIANLY) were more intense in the embryonic STG spectra than in the adult STG spectra. Collectively, this study expands our current knowledge of the H. americanus neuropeptidome and highlights some intriguing expression differences that occur during development.

Understanding Circuit Dynamics Using the Stomatogastric Nervous System of Lobsters and Crabs

Studies of the stomatogastric nervous systems of lobsters and crabs have led to numerous insights into the cellular and circuit mechanisms that generate rhythmic motor patterns. The small number of easily identifiable neurons allowed the establishment of connectivity diagrams among the neurons of the stomatogastric ganglion. We now know that (a) neuromodulatory substances reconfigure circuit dynamics by altering synaptic strength and voltage-dependent conductances and (b) individual neurons can switch among different functional circuits. Computational and experimental studies of single-neuron and network homeostatic regulation have provided insight into compensatory mechanisms that can underlie stable network performance. Many of the observations first made using the stomatogastric nervous system can be generalized to other invertebrate and vertebrate circuits.

Embryonic differentiation of serotonin-containing neurons in the enteric nervous system of the locust (Locusta migratoria)

The enteric nervous system (ENS) of the locust consists of four ganglia (frontal and hypocerebral ganglion, and the paired ingluvial ganglia) located on the foregut, and nerve plexus innervating fore- and midgut. One of the major neurotransmitters of the ENS, serotonin, is known to play a vital role in gut motility and feeding. We followed the anatomy of the serotonergic system throughout embryonic development. Serotonergic neurons are generated in the anterior neurogenic zones of the foregut and migrate rostrally along the developing recurrent nerve to contribute to the frontal ganglion. They grow descending neurites, which arborize in all enteric ganglia and both nerve plexus. On the midgut, the neurites closely follow the leading migrating midgut neurons. The onset of serotonin synthesis occurs around halfway through development-the time of the beginning of midgut closure. Cells developing to serotonergic phenotype express the serotonin uptake transporter (SERT) significantly earlier, beginning at 40% of development. The neurons begin SERT expression during migration along the recurrent nerve, indicating that they are committed to a serotonergic phenotype before reaching their final destination. After completion of the layout of the enteric ganglia (at 60%) a maturational phase follows, during which serotonin-immunoreactive cell bodies increase in size and the fine arborizations in the nerve plexus develop varicosities, putative sites of serotonin release (at 80%). This study provides the initial step for future investigation of potential morphoregulatory functions of serotonin during ENS development.

Evolution and development of neural circuits in invertebrates

Developmental mechanisms can shed light on how evolutionary diversity has arisen. Invertebrate nervous systems offer a wealth of diverse structures and functions from which to relate development to evolution. Individual homologous neurons have been shown to have distinct roles in species with different behaviors. In addition, specific neurons have been lost or gained in some phylogenetic lineages. The ability to address the neural basis of behavior at the cellular level in invertebrates has facilitated discoveries showing that species-specific behavior can arise from differences in synaptic strength, in neuronal structure and in neuromodulation. The mechanisms involved in the development of neural circuits lead to these differences across species.

Central pattern generation and the motor infrastructure for suck, respiration, and speech

The objective of the current report is to review experimental findings on centrally patterned movements and sensory and descending modulation of central pattern generators (CPGs) in a variety of animal and human models. Special emphasis is directed toward speech production muscle systems, including the chest wall and orofacial complex during patterned motor output. Experimental results indicate that CPGs subserving orofacial motor behavior can be modulated via descending and sensory inputs. This feature of control may also operate in the control of other centrally patterned motor behaviors including speech breathing, suck, mastication, and the recombination of CPG processes for the development and production of speech.

LEARNING OUTCOMES

Readers will be able to: (1) define the salient characteristics of CPGs, (2) list five factors which influence the development and operation of a CPG over the lifespan, (3) define sensorimotor entrainment of CPGs, and (4) describe one new application for therapeutic training of the non-nutritive suck in premature infants.

Invertebrate central pattern generation moves along

Central pattern generators (CPGs) are circuits that generate organized and repetitive motor patterns, such as those underlying feeding, locomotion and respiration. We summarize recent work on invertebrate CPGs which has provided new insights into how rhythmic motor patterns are produced and how they are controlled by higher-order command and modulatory interneurons.

Embryogenesis of the serotonergic system in the earthwormEisenia fetida (Annelida, Oligochaeta): Immunohistochemical and biochemical studies

Organization of the serotonergic system and changes of the serotonin (5-HT) content were studied during the embryogenesis of the earthworm Eisenia fetida, using immunocytochemistry and HPLC. A gradual emergence of 5-HT immunoreactive (IR) cells and their axon projections in the several ganglia of the central (CNS) and peripheral nervous system are described in the context of a staged time-scale of development. The first 5-HT-IR neurons appear in the subesophageal ganglion at an early embryonic stage (E2), followed by neurons in some rostrally located ventral ganglia. In the cerebral ganglion, 5-HT-IR cells can be detected only from stage E5. The number of labeled cells in each ganglion of the embryo increases until hatching, when it is still considerably lower than that observed in adults. This shows that the development of the 5-HTergic system is far from complete by the end of embryogenesis. Organization of 5-HT-IR innervation of the body wall starts by stages E3 to E4. In the stomatogastric nervous system the first 5-HT-IR fibers can be detected by stage E5. By stage E9 5-HT immunopositive neurons can be observed in both the stomatogastric ganglia and the enteric plexus. Both 5-HT levels and the numbers of the labeled cells show a significant increase before hatching, which indicate a functional maturation of the 5-HTergic system. Based on the early appearance of 5-HT, we suppose that it may play a regulatory role in both the gangliogenesis and the maturation of peripheral functions necessary during postembryonic life.

Pharmacological manipulation of serotonin levels in the nervous system of the opisthobranch mollusc Tritonia diomedea

Serotonin-related disorders can be treated by manipulating serotonin synthesis with the serotonin precursor 5-hydroxytryptophan (5-HTP) or other pharmacological agents. The mollusc Tritonia diomedea is a model for investigating the effects of altering serotonin content on the functions of identified neurons. We used high-performance liquid chromatography and immunohistochemistry to examine the amount and localization of 5-HTP, serotonin, and the serotonin breakdown product 5-hydroxyindolacetic acid (5-HIAA) in the Tritonia brain after various pharmacological treatments. Exposure to 5-HTP (2 mM for 30 min-1 h) caused an immediate and massive increase in total 5-HTP content, which lasted more than 20 h, and the widespread appearance of 5-HTP immunoreactivity in neurons. Serotonin levels rose gradually, but only a restricted number of additional neurons displayed serotonin immunoreactivity. 5-HTP treatment also caused an increase in the total amount of 5-HIAA and the appearance of 5-HIAA immunoreactivity throughout the brain. Treatment with the synthesis cofactor tetrahydrobiopterin, the initial precursor tryptophan, or serotonin itself had no persistent effect on total serotonin content. The amino acid decarboxylase inhibitor hydroxybenzylhydrazine (NSD-1015) also had no effect on the total serotonin content, although it caused an accumulation of 5-HTP. Thus, serotonin levels in the brain of T. diomedea appear to be maintained by a homeostatic mechanism that can be disrupted by 5-HTP.

Development of central pattern generating circuits

The networks that generate rhythmic motor patterns in invertebrates and vertebrates are ideal for studying the mechanisms by which functional circuits are formed during development. Rhythmic motor patterns and movements are seen embryonically, before they are needed for behavior; recent work suggests that activity in immature spinal cord networks is important for circuit formation and transmitter specification. Despite significant advances in describing the patterns of transcription factor expression in both invertebrate nervous systems and vertebrate spinal cord, a real understanding of how central pattern generators develop is hindered by our lack of knowledge of the organization of these circuits in adults.

Synaptic and Extrasynaptic Secretion of Serotonin

Serotonin is a major modulator of behavior in vertebrates and invertebrates and deficiencies in the serotonergic system account for several behavioral disorders in humans. The small numbers of serotonergic central neurons of vertebrates and invertebrates produce their effects by use of two modes of secretion: from synaptic terminals, acting locally in "hard wired" circuits, and from extrasynaptic axonal and somatodendritic release sites in the absence of postsynaptic targets, producing paracrine effects. In this paper, we review the evidence of synaptic and extrasynaptic release of serotonin and the mechanisms underlying each secretion mode by combining evidence from vertebrates and invertebrates. Particular emphasis is given to somatic secretion of serotonin by central neurons. Most of the mechanisms of serotonin release have been elucidated in cultured synapses made by Retzius neurons from the central nervous system of the leech. Serotonin release from synaptic terminals occurs from clear and dense core vesicles at active zones upon depolarization. In general, synaptic serotonin release is similar to release of acetylcholine in the neuromuscular junction. The soma of Retzius neurons releases serotonin from clusters of dense core vesicles in the absence of active zones. This type of secretion is dependent of the stimulation frequency, on L-type calcium channel activation and on calcium-induced calcium release. The characteristics of somatic secretion of serotonin in Retzius neurons are similar to those of somatic secretion of dopamine and peptides by other neuron types. In general, somatic secretion by neurons is different from transmitter release from clear vesicles at synapses and similar to secretion by excitable endocrine cells.

Arthropod 5-HT2 receptors: a neurohormonal receptor in decapod crustaceans that displays agonist independent activity resulting from an evolutionary alteration to the DRY motif

The stomatogastric nervous system (STNS) is a premiere model for studying modulation of motor pattern generation. Whereas the cellular and network responses to monoamines have been particularly well characterized electrophysiologically, the transduction mechanisms that link the different monoaminergic signals to specific intracellular responses are presently unknown in this system. To begin to elucidate monoaminergic signal transduction in pyloric neurons, we used a bioinformatics approach to predict the existence of 18 monoamine receptors in arthropods, 9 of which have been previously cloned in Drosophila and other insects. We then went on to use the two existing insect databases to clone and characterize the 10th putative arthropod receptor from the spiny lobster, Panulirus interruptus. This receptor is most homologous to the 5-HT2 subtype and shows a dose-dependent response to 5-HT but not to any of the other monoamines present in the STNS. Through a series of pharmacological experiments, we demonstrate that this newly described receptor, 5-HT2betaPan, couples with the traditional G(q) pathway when expressed in HEK293 cells, but not to G(s) or G(i/o). Moreover, it is constitutively active, because the highly conserved DRY motif in transmembrane region 3 has evolved into DRF. Site-directed mutagenesis that reverts the motif back to DRY abolishes this agonist-independent activity. We further demonstrate that this receptor most likely participates in the modulation of stomatogastric motor output, because it is found in neurites in the synaptic neuropil of the stomatogastric ganglion as well as in the axon terminals at identified pyloric neuromuscular junctions.

The developmental role of serotonin: news from mouse molecular genetics

New genetic models that target the serotonin system show that transient alterations in serotonin homeostasis cause permanent changes to adult behaviour and modify the fine wiring of brain connections. These findings have revived a long-standing interest in the developmental role of serotonin. Molecular genetic approaches are now showing us that different serotonin receptors, acting at different developmental stages, modulate different developmental processes such as neurogenesis, apoptosis, axon branching and dendritogenesis. Our understanding of the specification of the serotonergic phenotype is improving. In addition, studies have revealed that serotonergic traits are dissociable, as there are populations of neurons that contain serotonin but do not synthesize it.

Constant amplitude of postsynaptic responses for single presynaptic action potentials but not bursting input during growth of an identified neuromuscular junction in the lobster,Homarus americanus

As lobsters grow from early juveniles to adults their body size increases more than 20-fold, raising the question of how function is maintained during these ongoing changes in size. To address this question we studied the pyloric 1 (p1) muscle of the stomach of the lobster, Homarus americanus. The p1 muscle receives multiterminal innervation from one motor neuron, the lateral pyloric neuron of the stomatogastric ganglion. Staining with antibodies raised against synaptotagmin showed that as the muscle fibers increased in length, the spacing between the terminal innervation increased proportionally, so the number of synaptic contact regions/muscle fiber did not change. Muscle fibers were electrically coupled in both juveniles and adults. The amplitude of single intracellularly recorded excitatory junctional potentials evoked by motor nerve stimulation was the same in both juveniles and adults. Nonetheless, the peak depolarizations reached in response to ongoing pyloric rhythm activity or in response to high-frequency trains of stimuli similar to those produced during the pyloric rhythm were approximately twofold larger in juveniles than in adults. This suggests that homeostatic regulation of synaptic connections may operate at the level of the amplitude of the single synaptic potential rather than on the summed depolarization evoked during strong rhythmic activity.

A crustacean serotonin receptor: Cloning and distribution in the thoracic ganglia of crayfish and freshwater prawn

Serotonin (5-HT) is involved in regulating important aspects of behavior and a variety of systemic physiological functions in both vertebrates and invertebrates. These functions are mediated through binding to 5-HT receptors, of which approximately 13 have been characterized in mammals. In crustaceans, important model systems for the study of the neural basis of behaviors, 5-HT is also linked with higher-order behaviors, associated with different 5-HT receptors that have been identified at the physiological and pharmacological levels. However, no crustacean 5-HT receptors have been identified at the molecular level. We have cloned a putative 5-HT(1) receptor (5-HT(1crust)) from crayfish, prawn, and spiny lobster and have raised antibodies that recognize this protein in all three organisms. 5-HT(1crust) immunoreactivity (5-HT(1crust)ir) was observed surrounding the somata of specific groups of neurons and as punctate staining within the neuropil in all thoracic ganglia of crayfish and prawn. In the crayfish, 5-HT(1crust)ir was also found in boutons surrounding the first and second nerves of each ganglion and on the 5-HT cells of T1-4. In the prawn, 5-HT(1crust)ir was also found in axons that project across the ganglia and along the connectives. We found examples of colocalization of 5-HT(1crust) with 5-HT, consistent with the short-term modulatory role of 5-HT, as well as cases of serotonergic staining in the absence of a 5-HT(1crust) signal, which might imply that other 5-HT receptors are found at these locations. We also observed receptors that did not possess counterpart 5-HT staining, suggesting that these may also mediate long-term neurohormonal functions of serotonin.

Dopamine and histamine in the developing stomatogastric system of the lobster Homarus americanus

Dopamine and histamine are neuromodulators found in the adult stomatogastric nervous system (STNS) of several crustacean species. We used antibodies against tyrosine hydroxylase (TH) and histamine to map the distribution and developmental acquisition of the dopamine and histamine neurons in the STNS of the lobster, Homarus americanus. Embryos, larvae, juvenile and adult animals were studied. TH labeling was present in the STNS as early as E80-85 (80-85% of embryonic development). A subset of preparations in embryos, larvae, juveniles, and adults contained 1-5 labeled somata in the stomatogastric ganglion. Histamine staining appeared in the STNS as early as E50. The distribution of both TH and histamine staining remained relatively constant through development. Electrophysiological recordings demonstrated that receptors for both amines are present in the embryo. Bath application of dopamine increased the frequency of the pyloric rhythm in embryos, and evidence for dopaminergic activation of peripherally initiated spiking in motor axons was seen. In embryos and adults, histamine inhibited the motor patterns produced by the stomatogastric ganglion (STG). These data suggest that the dopaminergic and histaminergic systems in H. americanus appear relatively early in development and that the effects of each are largely maintained through development.