Esther M. Leise

Modular construction of nervous systems: a basic principle of design for invertebrates and vertebrates

The modular construction of brain tissue is not solely a feature of vertebrate nervous tissue, but is characteristic of many invertebrate nervous systems as well. Modern vertebrate and invertebrate modules vary over several orders of magnitude in volume but vary less in diameter. Although the physiological and anatomical differences between the modules discussed herein are overpowering, their importance to nervous system functions are similar. Modules are the serial and parallel processing units that have allowed large-brained animals to evolve. Many invertebrate modules are discrete, hemispherical lobes, visible on the surface of the brain or nerve cord, whereas most mammalian modules are columnar or ellipsoidal tissue compartments that can only be visualized with specific anatomical methods. Lobes from the largest invertebrates can be more voluminous than any neocortical compartments, but these large lobes are usually not single modules. Large invertebrate lobes contain internal compartments that are single modules and of similar size to their vertebrate analogs. However, vertebrate cortical modules or columns, are far more numerous than the compartments in invertebrate brains and in several cases are known to be adjoined laterally into slabs of tissue that extend for several millimeters. Physiological data support the idea that neural modules are not just anatomical entities, but are active local circuits. The specific activities within each type of module will depend upon its neuronal components, both intrinsic and extrinsic, its functional roles and phylogenetic history. Many cellular and intercellular phenomena common to vertebrates and invertebrates underlie the development of modules. Neuronal and glial interactions and their interplay with the extracellular environment depend upon families of molecules with broad phyletic occurrences. The commonalities of growth mechanisms may to a large degree account for the widespread incidence of neuronal processing units. The strategy of enlarging a nervous system through the replication of the basic units is thought to be advantageous for several reasons. This plan allows nervous systems to economize on the branch sizes and lengths needed for interconnections, to ensure that appropriate targets are reached during development and to modulate specific circuits within a larger network.

ON FEEDING MECHANISMS AND CLEARANCE RATES OF MOLLUSCAN VELIGERS

1. Beat of preoral cilia and particle paths were filmed for veligers of Crassostrea gigas, Tritonia diomedea, Nassarius obsoletus and an unidentified prosobranch. Particle captures were filmed for the three identified species. 2. Clearance rates per unit length of velar edge are estimated from the equation (L2-R2)WPF/2C, where L is cilium length, R a correction for recovery stroke, W angular velocity, C/P the ratio of velocities of cilium and particle, and F the fraction of particles captured. The clearance rates are in rough agreement with Baynes values for veligers of Mytilus edulis. 3. In the three identified species, longer preoral cilia clear particles at a higher rate but with less efficiency. Since veligers from larger eggs generally have both longer preoral cilia and a longer velar edge, a larger egg generally produces a veliger with a higher maximum clearance rate when the veliger begins to feed. 4. Angular velocities increase with cilium length in the three identified species of veligers but the larger unidentified species did not continue this trend. 5. Preoral cilia in their effective strokes move 1 to 3 times faster than particles travelling in about the same arc with a mean of about 1.5 times the speed of the particles. In mid effective stroke, the ratio of velocities of cilia and particles is not significantly different for captured and non-captured particles, nor does the ratio vary significantly with angular velocity of cilium. The ratio does vary significantly among species. 6. Particles passing closer to the base of the preoral cilia are more likely to be captured. 7. We hypothesize that suspended particles are concentrated when they are overtaken by preoral cilia in their effective stroke, weakly adhere to the preoral cilia, and are pushed faster than the water. Capture is completed when particles are drawn into the food groove, probably by the action of the recovery stroke of preoral cilia, the current from postoral cilia, or both.

Serotonin Injections Induce Metamorphosis in Larvae of the Gastropod Mollusc Ilyanassa obsoleta

Bath-applied serotonin (5-HT) induces competent larvae of the marine snail Ilyanassa obsoleta to metamorphose. Previously, the mode of action of 5-HT, whether as an external ligand or as an internal neurotransmitter, was unknown. Larvae were injected with 10-4 M 5-HT and other pharmacological agents to provide evidence that serotonergic neurons are necessary for metamorphosis in Ilyanassa larvae and that serotonin functions as a neurotransmitter or neuromodulator during this process. About 50% of 5-HT-injected animals metamorphose within 48 hours. Fluoxetine, a 5-HT reuptake inhibitor, and alpha-methyl-5-hydroxytryptamine (αm5HT), a 5-HT agonist, were also effective inducers of metamorphosis. Gramine (3-[dimethylaminomethyl]indole), a 5-HT antagonist, inhibited the inductive activity of 5-HT, while the amino acid gammaaminobutyric acid (GABA) resulted in rates of morphological restructuring similar to those of controls. Collectively, the results of our experiments support the idea that serotonergic neurons are active during larval metamorphosis of Ilyanassa and that 5-HT does not induce metamorphosis by binding to epidermal chemoreceptors.

Metamorphosis in the marine snail Ilyanassa obsoleta, yes or NO?

Metamorphosis is a crucial life-history event that can change an organisms form, function, behavior, and ecological interactions. In the Mollusca, several neurotransmitters and neuromodulators play inductive or inhibitory roles in the pathways that govern larval metamorphosis. Nitric oxide (NO) has been implicated in developmental processes in vertebrates and arthropods, but not previously in molluscs. We determined that NO donors block pharmacologically induced metamorphosis in the mud snail Ilyanassa obsoleta, whereas injections of inhibitors of nitric oxide synthase (NOS) allow competent larvae to become juveniles. We describe a new developmental role for NO, as an endogenous inhibitor of molluscan metamorphosis.

NADPH-Diaphorase Activity Changes During Gangliogenesis and Metamorphosis in the Gastropod Mollusc Ilyanassa obsoleta

Gaseous nitric oxide (NO) is produced through the action of the enzyme nitric oxide synthase (NOS) and acts as a neurotransmitter (Jacklet and Gruhn, 1994b; Elphick et al., 1995a; Jacklet, 1995) in the nervous systems of adult gastropod molluscs. By comparison, little or no information appears to exist about the ontogeny of molluscan NOS‐containing neurons. NADPH‐diaphorase (NADPHd) has been determined biochemically and histochemically to colocalize with NOS immunoreactivity in neurons; NOS is an isoform of NADPHd (Dawson et al., 1991; Hope et al., 1991). We used NADPHd histochemistry to map the distribution of NOS activity in the nervous systems of larvae, including metamorphosing individuals, and juveniles of the marine snail Ilyanassa obsoleta. Several ganglionic neuropils displayed reaction product throughout development. The most intense NADPHd staining occurred in the neuropil of the apical ganglion, a specialized larval structure. Intermediate staining levels occurred in neuropils of the cerebral, pedal, and pleural ganglia. Larval buccal and intestinal ganglia showed little reaction product, with slight increases arising in metamorphically competent larvae. NADPHd activity conspicuously decreased in the central nervous systems of metamorphosing larvae. The osphradial ganglion, which was present in young larvae, showed only weak NADPHd activity. Our results provide evidence for the existence of a nitrergic signalling system in molluscan larvae and juveniles.

Serotonin and Nitric Oxide Regulate Metamorphosis in the Marine Snail Ilyanassa obsoleta

SYNOPSIS. Several neuroactive compounds have been implicated as playing roles in the circuitry that controls larval metamorphosis in marine molluscs. For the caenogastropod Ilyanassa obsoleta, results of neuroanatomical studies suggest that the production of nitric oxide (NO) increases throughout the planktonic stage and that NO production is necessary for the maintenance of the larval state, especially as it becomes metamorphically competent. Bath application or injection of exogenous serotonin (5HT) can initiate metamorphosis in competent larvae, and exogenous NO can inhibit such serotonergically-induced metamorphosis. Inhibition of endogenous nitric oxide synthase (NOS) can also trigger larval metamorphosis. The production of endogenous NO appears to decrease concurrently with the initiation of metamorphosis, but the specific interactions between serotonergic and nitrergic neurons are unknown. Evidence in support of NO acting to up-regulate the enzyme guanylyl cyclase (GC) is still equivocal. Thus, we do not yet know if NO exerts its effects through the actions of cyclic 39,59-guanosine monophosphate (cGMP) or by a cGMP-independent mechanism. The ubiquity of nitrergic signalling and its significance for developing molluscan embryos and larvae are still the subject of speculation and require further investigation.

Programmed Cell Death in the Apical Ganglion During Larval Metamorphosis of the Marine Mollusc Ilyanassa obsoleta

The apical ganglion (AG) of larval caenogastropods, such as Ilyanassa obsoleta, houses a sensory organ, contains five serotonergic neurons, innervates the muscular and ciliary components of the velum, and sends neurites into a neuropil that lies atop the cerebral commissure. During metamorphosis, the AG is lost. This loss had been postulated to occur through some form of programmed cell death (PCD), but it is possible for cells within the AG to be respecified or to migrate into adjacent ganglia. Evidence from histological sections is supported by results from a terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay, which indicate that cells of the AG degenerate by PCD. PCD occurs after metamorphic induction by serotonin or by inhibition of nitric oxide synthase (NOS) activity. Cellular degeneration and nuclear condensation and loss were observed within 12 h of metamorphic induction by NOS inhibition and occur before loss of the velar lobes, the ciliated tissue used for larval swimming and feeding. Velar disintegration happens more rapidly after metamorphic induction by serotonin than by 7-nitroindazole, a NOS inhibitor. Loss of the AG was complete by 72 h after induction. Spontaneous loss of the AG in older competent larvae may arise from a natural decrease in endogenous NOS activity, giving rise to the tendency of aging larvae to display spontaneous metamorphosis in culture.

Gangliogenesis in the Prosobranch Gastropod Ilyanassa obsoleta

We determined that the larval nervous system of Ilyanassa obsoleta contains paired cerebral, pleural, pedal, buccal, and intestinal ganglia and unpaired apical, osphradial, and visceral ganglia. We used a modified form of NADPH diaphorase histochemistry to compare the neuroanatomy of precompetent (including specimens 6, 8, and 12 days after hatching), competent, and metamorphosing larvae with postmetamorphic juveniles. This method highlighted ganglionic neuropils and allowed us to identify individual ganglia at various stages of development, thereby laying a foundation for concurrent histochemical studies.

An inducer of molluscan metamorphosis transforms activity patterns in a larval nervous system

Larvae of the nudibranch mollusc Phestilla sibogae metamorphose in response to a small organic compound released into seawater by their adult prey, the scleractinian coral Porites compressa. The transformations that occur during metamorphosis, including loss of the ciliated velum (swimming organ), evacuation of the shell, and bodily elongation, are thought to be controlled by a combination of neuronal and neuroendocrine activities. Activation of peripheral chemosensory neurons by the metamorphosis-inducing compound should therefore elicit changes within the central nervous system. We used extracellular recording techniques in an attempt to detect responses of neurons within the larval central ganglia to seawater conditioned by P. compressa, to seawater conditioned by the weakly inductive coral Pocillopora damicornis, and to non-inductive seawater controls. The activity patterns within the nervous systems of semi-intact larvae changed in response to both types of coral exudates. Changes took place in two size classes of action potentials, one of which is known to be associated with velar ciliary arrests.

The osmium-ethyl gallate procedure is superior to silver impregnations for mapping neuronal pathways

Ganglia processed through the osmium-ethyl gallate procedure (OEG)19 retain more structural integrity than those processed through various silver impregnation methods. However, the OEG method continues to be neglected by most neuroanatomists. Both types of procedures have been used to trace large neuronal tracts, but during silver impregnation the neuropils lose many of their identifying characteristics. We demonstrate here the advantages of the OEG procedure by comparing it with two silver techniques, Rowells and Holmess. The OEG method yields consistent and reliable results and is easier to carry out than silver protocols. Most importantly, the better preservation of the neuropils has led to the discovery and study of regional specializations that were previously undetected from silver preparations.