Identifiable nitrergic neurons in the central nervous system of the nudibranch Melibe leonina localized with NADPH-diaphorase histochemistry and nitric oxide synthase immunoreactivity

Cyclic Guanosine Monophosphate Modulates Locomotor Acceleration Induced by Nitric Oxide but not Serotonin in Clione limacina Central Pattern Generator Swim Interneurons

Both nitric oxide (NO) and serotonin (5HT) mediate swim acceleration in the marine mollusk, Clione  limacina. In this study, we examine the role that the second messenger, cyclic guanosine monophosphate (cGMP), plays in mediating NO and 5HT-induced swim acceleration. We observed that the application of an analog of cGMP or an activator of soluble guanylyl cyclase (sGC) increased fictive locomotor speed recorded from Pd-7 interneurons of the animal's locomotor central pattern generator. Moreover, inhibition of sGC decreased fictive locomotor speed. These results suggest that basal levels of cGMP are important for slow swimming and that increased production of cGMP mediates swim acceleration in Clione. Because NO has its effect through cGMP signaling and because we show herein that cGMP produces cellular changes in Clione swim interneurons that are consistent with cellular changes produced by 5HT application, we hypothesize that both NO and 5HT function via a common signal transduction pathway that involves cGMP. Our results show that cGMP mediates NO-induced but not 5HT-induced swim acceleration in Clione.

The Distribution and Possible Roles of Small Cardioactive Peptide in the Nudibranch Melibe leonina

The neuropeptide small cardioactive peptide (SCP) plays an integrative role in exciting various motor programs involved in feeding and locomotion in a number of gastropod species. In this study, immunohistochemistry, using monoclonal antibodies against SCPB, was used to localize SCPB-like-immunoreactive neurons in the central nervous system, and map their connections to various tissues, in the nudibranch, Melibe leonina. Approximately 28-36 SCPB-like-immunoreactive neurons were identified in the M. leonina brain, as well as one large neuron in each of the buccal ganglia. The neuropil of the pedal ganglia contained the most SCPB-like-immunoreactive varicosities, although only a small portion of these were due to SCPB-like-immunoreactive neurons in the same ganglion. This suggests that much of the SCPB-like immunoreactivity in the neuropil of the pedal ganglia was from neurons in other ganglia that projected through the pedal-pedal connectives or the connectives from the cerebral and pleural ganglia. We also observed extensive SCPB innervation along the length of the esophagus. Therefore, we investigated the impact of SCPB on locomotion in intact animals, as well as peristaltic contractions of the isolated esophagus. Injection of intact animals with SCPB at night led to a significant increase in crawling and swimming, compared to control animals injected with saline. Furthermore, perfusion of isolated brains with SCPB initiated expression of the swim motor program. Application of SCPB to the isolated quiescent esophagus initiated rhythmic peristaltic contractions, and this occurred in preparations both with and without the buccal ganglia being attached. All these data, taken together, suggest that SCPB could be released at night to arouse animals and enhance the expression of both feeding and swimming motor programs in M. leonina.

Cellular, biochemical, and molecular characterization of nitric oxide synthase expressed in the nervous system of the prosobranch Stramonita haemastoma (Gastropoda, Neogastropoda)

Nitric oxide synthase (NOS) has been characterized in several opistobranchs and pulmonates but it was much less investigated in prosobranchs, which include more than 20,000 species and account for most of the gastropod diversity. Therefore, new data from this large group are needed for a better knowledge of the molecular evolution of NOS enzymes in molluscs. This study focused on NOS expressed in the nervous system of the prosobranch neogastropod Stramonita haemastoma. In this study we report compelling evidence on the expression of a constitutive Ca(2+) /CaM-dependent neuronal NOS in the central and peripheral nervous system. The prevailing neuronal localization of NADPHd activity was demonstrated by NADPHd histochemistry in both central and peripheral nervous system structures. L-arginine/citrulline assays suggested that Stramonita NOS is a constitutive enzyme which is both cytosolic and membrane-bound. Molecular cloning of the full-length Stramonita NOS (Sh-NOS) by reverse-transcription polymerase chain reaction (RT-PCR) followed by 5' and 3' RACE showed that Sh-NOS is a protein of 1,517 amino acids, containing a PDZ domain at the N-terminus and sharing similar regulatory domains to the mammalian neuronal NOS (nNOS). Regional expression of the Sh-NOS gene was evaluated by RT-PCR. This analysis showed different expression levels in the nerve ring, the osphradium, the cephalic tentacles, the buccal tissues, and the foot, whereas NOS expression was not found in the salivary glands and the gland of Leiblein. The present data provide a solid background for further studies addressing the specific functions of NO in neogastropods.

Neuroplastic and neuropathological changes in the central nervous system of the Gray mussel Crenomytilus grayanus (Dunker) under environmental stress

We studied here neuron ultrastructure, synaptic plasticity and subcellular localization of NADPH-diaphorase (NADPH-d), a cytochemical marker for nitric oxide syntase, in the pedal ganglia of the Gray mussel Crenomytilus grayanus sampled from the polluted and reference sites in Amursky Bay (Sea of Japan) at lower and higher water temperature (in the beginning and the end of August, respectively). At lower temperature, neuroplastic changes in mussel ganglia prevailed: a sharp increase in the number of cytosomes in NADPH-d-positive neurons and a sharp decrease in the number of mitochondria in both NADPH-d-positive and NADPH-d-negative neurons. At higher temperature, neurodegenerative changes prevailed: disruption of a part of NADPH-d-negative axons and interneuronal contacts, formation of concentric lamellar structures in the neuropils, and accumulation of autophagosomes in NADPH-d negative neurons. The results suggest that the stress-induced production of nitric oxide in cytosomes of mussel neurons and plasticity of gap junctions have a neuroprotective effect.

NADPH-diaphorase activity in the central nervous system of the Gray mussel Crenomytilus grayanus (Dunker) under stress conditions: a histochemical study

NADPH-diaphorase (NADPH-d) is a histochemical marker for nitric oxide synthase (NOS) and is widely used to identify nitric oxide (NO) producing cells in the central nervous system (CNS) of both vertebrates and invertebrates. NADPH-d histochemistry was used to quantitatively characterize putative NO-producing neurons in the CNS of the Gray mussel Crenomytilus grayanus subjected to two kinds of stress, environmental pollution and hypoxia, the latter caused by the mollusk transportation in a small volume of water. Mussels were sampled from one relatively clean (reference) and four polluted sites in Amursky and Ussuriysky Bays (Peter the Great Bay, Sea of Japan) in August, 2003. The number of NADPH-d-positive neurons was estimated and enzyme activity was determined from the optical density of the formazan precipitate in the CNS ganglia at 0, 3, and 72 h after sampling. Just after sampling, NADPH-d-positive neurons were found in the cerebropleural, visceral, and pedal ganglia. The number and staining intensity of NADPH-d-positive neurons were significantly higher in the pedal ganglia than the other two ganglia. There were significant differences in the number of NADPH-d-positive neurons and enzyme activity between the mussels from the reference and heavily polluted stations. The proportion and staining intensity of NADPH-d-positive neurons were maximum in the pedal ganglia of the mussels from the heavily polluted station in Amursky Bay. Transportation of mussels in a limited volume of water for 3h resulted in a significant increase in the proportion and staining intensity of NADPH-d-positive neurons in all ganglia. In mollusks from all stations kept in aerated aquaria for 72 h, both the proportion and staining intensity of NADPH-d-positive neurons did not differ significantly from the initial level. However, the differences in the proportion and staining intensity of NADPH-d-positive neurons between the reference and heavily polluted stations were significant. The present results suggest that NO is involved in mollusk nerve cell adaptation to environmental changes.

Nitric oxide inhibits metamorphosis in larvae of Crepidula fornicata, the slippershell snail

This paper concerns the role of nitric oxide (NO) in controlling metamorphosis in the marine gastropod Crepidula fornicata. Metamorphosis was stimulated by the nitric oxide synthase (NOS) inhibitors AGH (aminoguanidine hemisulfate) and SMIS (S-methylisothiourea sulfate) at concentrations of about 100-1000 micromol l(-1) and 50-200 micromol l(-1), respectively. Metamorphosis was not, however, induced by the NOS inhibitor l-NAME (l-N(G)-nitroarginine methyl ester) at even the highest concentration tested, 500 micromol l(-1). Moreover, pre-incubation with l-NAME at 20 and 80 micromol l(-1) did not increase the sensitivity of competent larvae to excess K(+), a potent inducer of metamorphosis in this species; we suggest that either l-NAME is ineffective in suppressing NO production in larvae of C. fornicata, or that it works only on the constitutive isoform of the enzyme. In contrast, metamorphosis was potentiated by the guanylate cyclase inhibitor ODQ (1H-[1,2,4]oxadiazolo[4,3, -a]quinoxalin-1-one) in response to a natural metamorphic inducer derived from conspecific adults. Because NO typically stimulates cGMP production through the activation of soluble guanylate cyclase, this result supports the hypothesis that NO acts as an endogenous inhibitor of metamorphosis in C. fornicata. The expression of NOS, shown by immunohistochemical techniques, was detected in the apical ganglion of young larvae but not in older larvae, further supporting the hypothesis that metamorphosis in C. fornicata is made possible by declines in the endogenous concentration of NO during development.

Comparative mapping of serotonin-immunoreactive neurons in the central nervous systems of nudibranch molluscs

The serotonergic systems in nudibranch molluscs were compared by mapping the locations of serotonin-immunoreactive (5-HT-ir) neurons in 11 species representing all four suborders of the nudibranch clade: Dendronotoidea (Tritonia diomedea, Tochuina tetraquetra, Dendronotus iris, Dendronotus frondosus, and Melibe leonina), Aeolidoidea (Hermissenda crassicornis and Flabellina trophina), Arminoidea (Dirona albolineata, Janolus fuscus, and Armina californica), and Doridoidea (Triopha catalinae). A nomenclature is proposed to standardize reports of cell location in species with differing brain morphologies. Certain patterns of 5-HT immunoreactivity were found to be consistent for all species, such as the presence of 5-HT-ir neurons in the pedal and cerebral ganglia. Also, particular clusters of 5-HT-ir neurons in the anterior and posterior regions of the dorsal surface of the cerebral ganglion were always present. However, there were interspecies differences in the number of 5-HT-ir neurons in each cluster, and some clusters even exhibited strong intraspecies variability that was only weakly correlated with brain size. Phylogenetic analysis suggests that the presence of particular classes of 5-HT-ir neurons exhibits a great deal of homoplasy. The conserved features of the nudibranch serotonergic system presumably represent the shared ancestral structure, whereas the derived characters suggest substantial independent evolutionary changes in the number and presence of serotonergic neurons. Although a number of studies have demonstrated phylogenetic variability of peptidergic systems, this study suggests that serotonergic systems may also exhibit a high degree of homoplasy in some groups of organisms.

Localization of putative nitrergic neurons in peripheral chemosensory areas and the central nervous system of Aplysia californica

The distribution of putative nitric oxide synthase (NOS)-containing cells in the opisthobranch mollusc Aplysia californica was studied by using NADPH-diaphorase (NADPH-d) histochemistry in the CNS and peripheral organs. Chemosensory areas (the mouth area, rhinophores, and tentacles) express the most intense staining, primarily in the form of peripheral highly packed neuropil regions with a glomerular appearance as well as in epithelial sensory-like cells. These epithelial NADPH-d-reactive cells were small and had multiple apical ciliated processes exposed to the environment. NADPH-d processes were also found in the salivary glands, but there was no or very little staining in the buccal mass and foot musculature. In the CNS, most NADPH-d reactivity was associated with the neuropil of the cerebral ganglia, with the highest density of glomeruli-like NADPH-d-reactive neurites in the areas of the termini and around F and C clusters. A few NADPH-d-reactive neurons were also found in other central ganglia, including paired neurons in the buccal, pedal, and pleural ganglia and a few asymmetrical neurons in the abdominal ganglion. The distribution patterns of NADPH-d-reactive neurons did not overlap with other known neurotransmitter systems. The highly selective NADPH-d labeling revealed here suggests the presence of NOS in sensory areas both in the CNS and the peripheral organs of Aplysia and implies a role for NO as a modulator of chemosensory processing.

Acute and delayed restraint stress-induced changes in nitric oxide producing neurons in limbic regions

RATIONALE

Microinjection into the dentate gyrus of the hippocampus of N(omega)-nitro-l-arginine methyl ester hydrochloride (l-NAME), a nitric oxide synthase (NOS) inhibitor, induces antinociceptive effect 5 days after a single restraint episode. The mechanisms of this stress-antinociceptive modulatory effect have not been investigated but may involve plastic changes in the hippocampal formation (HF).

OBJECTIVE

The objective of the present study was to investigate possible mechanisms of the stress-modulating effect on antinociception induced by NOS inhibition in the hippocampus. We analyzed the effects of restraint stress on neuronal NOS (nNOS) expression and nicotinamide adenine dinucleotide phosphate-diaphorase histochemical activity (NADPH-d) in the HF and related brain regions.

METHODS

Male Wistar rats (n=6-11/group) were submitted to a single (acute stress) or repeated (5 days) episodes of 2-h restraint. Control animals remained in their home cages being all animals daily handled during this period. In the fifth day, animals received unilateral microinjection of l-NAME (150 nmol/0.2 microl) or saline (control) into the dentate gyrus of the dorsal hippocampus (DG). Immediately before and after drug microinjection tail-flick reflex latency or hotplate licking reaction was measured. Animals were killed i. immediately; ii. 5 days after acute stress; or iii. after repeated stress. NADPH-d and nNOS expression were quantified in the HF, caudate-putamen, secondary somatosensorial, entorhinal and piriform cortices and amygdaloid complex.

RESULTS

Five days after one or five restraint episodes l-NAME microinjection into the DG elicited antinociceptive effect (analysis of variance [ANOVA], P<0.05). Acute restraint stress induced a significant increase in the density of neurons expressing NADPH-d and nNOS in the amygdaloid nuclei. nNOS expression increased also in the DG and piriform cortex. Five days after a single or repeated restraint stress there was an additional increase in NADPH-d- and nNOS-positive neurons in CA1, CA3, and entorhinal cortex. No changes were seen in non-limbic regions such as the caudate-putamen and secondary somatosensorial cortex.

CONCLUSION

The results confirm that the dorsal hippocampus participates in the modulation of stress consequences. They also show that a single stress episode causes acute changes in nitric oxide system in the amygdala complex and delayed modifications in the HF. The delayed (5 days) antinociceptive effect of NOS inhibition in the HF after a single restraint episode suggests that those latter modifications may have functional consequences. It remains to be tested if the acute amygdala and delayed hippocampal changes are causally related.

New techniques for whole-mount NADPH-diaphorase histochemistry demonstrated in insect ganglia

Fixation-resistant NADPH-diaphorase (NADPHd) activity is used widely as a marker for nitric oxide synthase (NOS). In frozen sections, NADPHd histochemistry yields high anatomic definition. In whole-mounts, however, poor penetration of the reagents, background staining, and tissue opacity severely limit its application. Here we report a combination of new methods that significantly improves whole-mount NADPHd staining. We demonstrate these methods in the thoracic ganglia of a large insect, the locust Schistocerca gregaria, in which NADPHd has been analyzed previously using both whole-mounts and serial section reconstructions. The penetration of the staining reagents was markedly improved after fixation in methanol/formalin compared to phosphate-buffered formaldehyde. Methanol/formalin also reduced nonspecific NADPHd and enhanced the selective staining. Penetration was further enhanced by incubation regimens that exploit the temperature- or pH-dependence of NADPHd. In combination with methanol/formalin fixation, this permitted staining to develop evenly throughout these comparatively large invertebrate ganglia. These improvements were complemented by a new clearing technique that preserves the NADPHd staining, gives excellent transparency, and avoids distortion of specimen morphology. The new methods revealed the three-dimensional architecture of NADPHd expression in locust ganglia in unprecedented detail and may similarly improve whole-mount detection of NADPHd in other invertebrate and vertebrate preparations.

Neural correlates of swimming behavior in Melibe leonina

The nudibranch Melibe leonina swims by rhythmically bending from side to side at a frequency of 1 cycle every 2-4 s. The objective of this study was to locate putative swim motoneurons (pSMNs) that drive these lateral flexions and determine if swimming in this species is produced by a swim central pattern generator (sCPG). In the first set of experiments, intracellular recordings were obtained from pSMNs in semi-intact, swimming animals. About 10-14 pSMNs were identified on the dorsal surface of each pedal ganglion and 4-7 on the ventral side. In general, the pSMNs in a given pedal ganglion fired synchronously and caused the animal to flex in that direction, whereas the pSMNs in the opposite pedal ganglion fired in anti-phase. When swimming stopped, so did rhythmic pSMN bursting; when swimming commenced, pSMNs resumed bursting. In the second series of experiments, intracellular recordings were obtained from pSMNs in isolated brains that spontaneously expressed the swim motor program. The pattern of activity recorded from pSMNs in isolated brains was very similar to the bursting pattern obtained from the same pSMNs in semi-intact animals, indicating that the sCPG can produce the swim rhythm in the absence of sensory feedback. Exposing the brain to light or cutting the pedal-pedal connectives inhibited fictive swimming in the isolated brain. The pSMNs do not appear to participate in the sCPG. Rather, they received rhythmic excitatory and inhibitory synaptic input from interneurons that probably comprise the sCPG circuit.

Modulation of swimming in the gastropod Melibe leonina by nitric oxide

Nitric oxide (NO) is a gaseous intercellular messenger produced by the enzyme nitric oxide synthase. It has been implicated as a neuromodulator in several groups of animals, including gastropods, crustaceans and mammals. In this study, we investigated the effects of NO on the swim motor program produced by isolated brains and by semi-intact preparations of the nudibranch Melibe leonina. The NO donors sodium nitroprusside (SNP, 1 mmol l–1) and S-nitroso-N-acetylpenicillamine (SNAP, 1 mmol l–1) both had a marked effect on the swim motor program expressed in isolated brains, causing an increase in the period of the swim cycle and a more erratic swim rhythm. In semi-intact preparations, the effect of NO donors was manifested as a significant decrease in the rate of actual swimming. An NO scavenger, reduced oxyhemoglobin, eliminated the effects of NO donors on isolated brains, supporting the assumption that the changes in swimming induced by donors were actually due to NO. The cGMP analogue 8-bromoguanosine 3′,5′-cyclic monophosphate (1 mmol l–1) produced effects that mimicked those of NO donors, suggesting that NO is working via a cGMP-dependent mechanism. These results, in combination with previous histological studies indicating the endogenous presence of nitric oxide synthase, suggest that NO is used in the central nervous system of Melibe leonina to modulate swimming.