Dopamine modulation of Ca(2+) dependent Cl(-) current regulates ciliary beat frequency controlling locomotion in Tritonia diomedea

The physiological mechanisms controlling ciliary beating remain largely unknown. Evidence exists supporting both hormonal control of ciliary beating and control via direct innervation. In the present study we investigated nervous control of cilia based locomotion in the nudibranch mollusc, Tritonia diomedea. Ciliated pedal epithelial (CPE) cells acting as locomotory effectors may be electrically excitable. To explore this possibility we characterized the cells' electrical properties, and found that CPE cells have large voltage dependent whole cell currents with two components. First, there is a fast activating outward Cl(-) current that is both voltage and Ca(2+) influx dependent (I(Cl(Ca))). I(Cl(Ca)) is sensitive to DIDS and 9-AC, and resembles currents of Ca(2+)-activated Cl(-) channels (CaCC). Ca(2+) dependence also suggests the presence of voltage-gated Ca(2+) channels; however, we were unable to detect these currents. The second current, a voltage dependent proton current (I(H)), activates very slowly and is sensitive to both Zn(2+) and changes in pH. In addition we identify a new cilio-excitatory substance in Tritonia, viz., dopamine. Dopamine, in the 10 mumol l(-1)-1 mmol l(-1) range, significantly increases ciliary beat frequency (CBF). We also found dopamine and Tritonia Pedal Peptide (TPep-NLS) selectively suppress I(Cl(Ca)) in CPE cells, demonstrating a link between CBF excitation and I(Cl(Ca)). It appears that dopamine and TPep-NLS inhibit I(Cl(Ca)) not through changing [Ca(2+)](in), but directly by an unknown mechanism. Coupling of I(Cl(Ca)) and CBF is further supported by our finding that DIDS and zero [Cl(-)](out) both increase CBF, mimicking dopamine and TPep-NLS excitation. These results suggest that dopamine and TPep-NLS act to inhibit I(Cl(Ca)), initiating and prolonging Ca(2+) influx, and activating CBF excitation.

Nervous control of ciliary beating by Cl-, Ca2+ and calmodulin inTritonia diomedea

In vertebrates, motile cilia line airways, oviducts and ventricles. Invertebrate cilia often control feeding, swimming and crawling, or gliding. Yet control and coordination of ciliary beating remains poorly understood. Evidence from the nudibranch mollusc, Tritonia diomedea, suggests that locomotory ciliated epithelial cells may be under direct electrical control. Here we report that depolarization of ciliated pedal epithelial (CPE)cells increases ciliary beating frequency (CBF), and elicits CBF increases similar to those caused by dopamine and the neuropeptide, TPep-NLS. Further,four CBF stimulants (zero external Cl-, depolarization, dopamine and TPep-NLS) depend on a common mode of action, viz. Ca2+influx, possibly through voltage-gated Ca2+ channels, and can be blocked by nifedipine. Ca2+ influx alone, however, does not provide all the internal Ca2+ necessary for CBF change. Ryanodine receptor(RyR) channel-gated internal stores are also necessary for CBF excitation. Caffeine can stimulate CBF and is sensitive to the presence of the RyR blocker dantrolene. Dantrolene also reduces CBF excitation induced by dopamine and TPep-NLS. Finally, W-7 and calmidazolium both block CBF excitation by caffeine and dopamine, and W-7 is effective at blocking TPep-NLS excitation. The effects of calmidazolium and W-7 suggest a role for Ca2+-calmodulin in regulating CBF, either directly or via Ca2+-calmodulin dependent kinases or phosphodiesterases. From these results we hypothesize dopamine and TPep-NLS induce depolarization-driven Ca2+ influx and Ca2+ release from internal stores that activates Ca2+-calmodulin, thereby increasing CBF.

Odours detected by rhinophores mediate orientation to flow in the nudibranch mollusc, Tritonia diomedea

Tritonia diomedea is a useful neuroethological model system that can contribute to our understanding of the neural control of navigation. Prior work on both sensory and locomotory systems is complemented by recent field experiments, which concluded that these animals primarily use a combination of odours and water flow as guidance cues. We corroborate these field results by showing similar navigation behaviours in a flow tank. Slugs crawled upstream towards both prey and conspecifics, and turned downstream after crawling into a section of the flow tank downstream of a predator. Controls without upstream odour sources crawled apparently randomly. We then tested whether these behaviours depend on odours detected by the rhinophores. Outflow from a header tank was used to generate prey, predator and unscented control odour plumes in the flow tank. Slugs with rhinophores crawled upstream towards a prey odour plume source, turned downstream in a predator odour plume, and showed no reaction to a control plume. Slugs without rhinophores behaved similarly to controls, regardless of odour plume type. Finally, we used extracellular recordings from the rhinophore nerve to demonstrate that isolated rhinophores are chemosensitive. Afferent activity increased significantly more after application of all three odour types than after unscented control applications. Responses were odour specific. We conclude that rhinophores mediate orientation to flow, and suggest that future work should focus on the integration of mechanosensation and chemosensation during navigation in T. diomedea.

Field behavior of the nudibranch mollusc Tritonia diomedea

The nudibranch mollusc Tritonia diomedea has been a useful model system for studies of how the brain controls behavior. However, no broad study of T. diomedea field behavior exists--an important deficit since laboratory behaviors may differ from what occurs in nature. Here we report analysis of time-lapse video of the slugs in their natural habitat to describe behaviors and their relationships to sensory cues. We found that movements relative to conspecifics, prey, and predators correlated with direction of water flow. These observations lead to three new navigational hypotheses: regardless of the actual heading to the target, T. diomedea crawls (1) upstream toward potential mates, (2) upstream toward food, and (3) downstream away from predators. We also describe both the behavior and its sensory context for feeding, escape swims, mating, and egg-laying, among other behaviors. Field behaviors were similar to published descriptions of laboratory behavior. However, the field observations add contextual detail, including preceding and subsequent behaviors and interactions with suites of habitat features not present in the laboratory. For example, the escape swim, previously studied as an isolated behavior in response to a single stimulus, appears to be affected by multiple sensory modalities and coordinated with several other behaviors. Our work will provide a basis for future neuroethological experimentation and also is the first step in the study of navigation in T. diomedea.

Orientation and navigation relative to water flow, prey, conspecifics, and predators by the nudibranch mollusc Tritonia diomedea

Progress in understanding sensory and locomotory systems in Tritonia diomedea has created the potential for the neuroethological study of animal navigation in this species. Our goal is to describe the navigational behaviors to guide further work on how the nervous system integrates information from multiple senses to produce oriented locomotion. Observation of T. diomedea in its habitat has suggested that it uses water flow to navigate relative to prey, predators, and conspecifics. We test these hypotheses in the field by comparing slug orientation in time-lapse videos to flow direction in circumstances with and without prey, predators, or conspecifics upstream. T. diomedea oriented upstream both while crawling and after turning. This trend was strongest before feeding or mating; after feeding or mating, the slugs did not orient significantly to flow. Slugs turned downstream away from an upstream predator but did not react in control situations without an upstream predator. These data support the hypothesis that T. diomedea uses a combination of odors (or some other cue transported downstream) and water flow to navigate relative to prey, predators, and conspecifics. Understanding the context-dependent choice between upstream and downstream crawling in T. diomedea provides an opportunity for further work on the sensory integration underlying navigation behavior.

Immunological localization of Tritonia peptide in the central and peripheral nervous system of the terrestrial snail Helix aspersa

We report here evidence that the pedal peptides (Peps) first discovered in mollusks may be neurotransmitters with a general role in control of molluscan somatic and visceral muscles. Using Tritonia peptide (TPep) antiserum we obtained morphological evidence for such a role in Helix aspersa. We localized 1,200-1,400 small and medium-sized (5-40 microm) TPep-IR neurons in the central nervous system of Helix and demonstrated the presence of these neurons in each ganglion. Many TPep-immunoreactive (IR) neurons were motoneurons that sent axons to almost all peripheral nerves. TPep-IR fibers innervated the foot, esophagus, hermaphroditic duct, optic tentacles, salivary gland, heart, and proximal and distal aorta. In peripheral tissues TPep-IR fiber ramifications were mostly associated with muscles and with ciliated epithelia. In addition, TPep-IR fibers were in the neuropil of the ganglia, the commissures, and the connectives, and they formed axosomatic terminals in the central nervous system. TPep-IR neurons were found in the esophagus and hermaphroditic duct and as sensory receptors in the bulb of the optic tentacles. These results from Helix, and those reported elsewhere from other mollusks, suggest a general involvement of TPep-like substances in control of muscle- and ciliary-driven motor activities, including perhaps their antecedent sensory and central axosomatic integrative activity.

Localization of serotonin and its possible role in early embryos of Tritonia diomedea(Mollusca: Nudibranchia)

A classical neurotransmitter serotonin (5-HT) was detected immunochemically using laser scanning microscopy at the early stages of Tritonia diomedea development. At the one- to eight-cell stages, immunolabeling suggested the presence of 5-HT in the cytoplasm close to the animal pole. At the morula and blastula stages, a group of micromeres at the animal pole showed immunoreactivity. At the gastrula stage no immunoreactive cells were detected, but they arose again at the early veliger stage. Antagonists of 5-HT(2) receptors, ritanserin and cyproheptadine, as well as lipophilic derivatives of dopamine blocked cleavage divisions or distorted their normal pattern. These effects were prevented by 5-HT and its highly lipophilic derivates, serotoninamides of polyenoic fatty acids, but not by the hydrophilic (quaternary) analog of 5-HT, 5-HTQ. The results confirm our earlier suggestion that endogenous 5-HT in pre-nervous embryos acts as a regulator of cleavage divisions in nudibranch molluscs.