Central nervous mechanisms controlling rhythmic burst generation in the ventilatory motoneurones ofCarcinus maenas

Multibiomarker interactions to diagnose and follow-up chronic exposure of a marine crustacean to Hazardous and Noxious Substances (HNS)

Integrated compensatory responses of physiological systems towards homeostasis are generally overlooked when it comes to analysing alterations in biochemical parameters indicative of such processes. Here an hypothesis-driven multivariate analysis accounting for interactive multibiomarker responses was used to investigate effects of long-term exposure of Carcinus maenas to Hazardous and Noxious Substances (HNS). Adult male crabs were exposed to low and high post-spill levels of acrylonitrile (ACN) or aniline (ANL) for 21d. Bioaccumulation, feeding behaviour, and biomarkers related to mode-of-action (MoA) (detoxification, neurotransmission and energy production) were evaluated over time. Distinct temporal patterns of response to low and high exposure concentrations were depicted, with a main set of interactive multibiomarker predictors identified for each HNS (five for ACN and three for ANL), useful to follow coupled evolvement of biomarker responses. ACN caused peripheral neurotoxic effects coupled with enhanced biotransformation and significant oxidative damage particularly relevant in gills. ANL elicited alterations in central neurotransmission affecting ventilation coupled with very low levels of oxidative damage in gills. Results indicate chronic toxicity data are determinant to improve HNS hazard assessment if the aim is to obtain reliable risk calculations, and develop effective predictive models avoiding overestimation but sufficiently protective. Accounting for multibiomarker interactions brought otherwise overlooked information about C. maenas responses and MoA of ACN and ANL.

Sertraline accumulation and effects in the estuarine decapod Carcinus maenas: importance of the history of exposure to chemical stress

Sertraline is widely prescribed worldwide and frequently detected in aquatic systems. There is, however, a remarkable gap of information on its potential impact on estuarine and coastal invertebrates. This study investigated sertraline accumulation and effects in Carcinus maenas. Crabs from a moderately contaminated (Lima) and a low-impacted (Minho) estuary were exposed to environmental and high levels of sertraline (0.05, 5, 500 μg L(-1)). A battery of biomarkers related to sertraline mode of action was employed to assess neurotransmission, energy metabolism, biotransformation and oxidative stress pathways. After a seven-day exposure, sertraline accumulation in crabs' soft tissues was found in Lima (5 μg L(-1): 15.3 ng L(-1) ww; 500 μg L(-1): 1010 ng L(-1) ww) and Minho (500 μg L(-1): 605 ng L(-1) ww) animals. Lima crabs were also more sensitive to sertraline than those from Minho, exhibiting decreased acetylcholinesterase activity, indicative of ventilatory and locomotory dysfunction, inhibition of anti-oxidant enzymes and increased oxidative damage at ≥ 0.05 μg L(-1). The Integrated Biomarker Response (IBR) index indicated their low health status. In addition, Minho crabs showed non-monotonic responses of acetylcholinesterase suggestive of hormesis. The results pointed an influence of the exposure history on differential sensitivity to sertraline and the need to perform evaluations with site-specific ecological receptors to increase relevance of risk estimations when extrapolating from laboratory to field conditions.

Reciprocal inhibition and postinhibitory rebound produce reverberation in a locomotor pattern generator

The central pattern generator for swimming in the pteropod mollusk Clione limacina consists of at least four pedal interneurons, two each controlling parapodial upstroke and downstroke. The two sets of antagonistic interneurons are linked by reciprocal monosynaptic inhibitory synapses, and all exhibit apparently strong postinhibitory rebound. This simple neuronal network produces reverberating alternate cyclic activity in the absence of tonic drive or apparent feedback modulation.

Neural basis of a simple behavior: abdominal positioning in crayfish

Crustaceans have been used extensively as models for studying the nervous system. Members of the Order Decapoda, particularly the larger species such as lobsters and crayfish, have large segmented abdomens that are positioned by tonic flexor and extensor muscles. Importantly, the innervation of these tonic muscles is known in some detail. Each abdominal segment in crayfish is innervated bilaterally by three sets of nerves. The anterior pair of nerves in each ganglion controls the swimmeret appendages and sensory supply. The middle pair of nerves innervates the tonic extensor muscles and the regional sensory supply. The superficial branch of the most posterior pair of nerves in each ganglion is exclusively motor and supplies the tonic flexor muscles of that segment. The extension and flexion motor nerves contain six motor neurons, each of which is different in axonal diameter and thus produces impulses of different amplitude. Motor programs controlling each muscle can be characterized by the identifiable motor neurons that are activated. Early work in this field discovered that specific central interneurons control the abdominal positioning motor neurons. These interneurons were first referred to as "command neurons" and later as "command elements." Stimulation of an appropriate command element causes a complex, widespread output involving dozens of motor neurons. The output can be patterned even though the stimulus to the command element is of constant interval. The command elements are identifiable cells. When a stimulus is repeated in a command element, from either the same individual or from different individuals, the output is substantially the same. This outcome depends upon several factors. First, the command elements are not only identifiable, but they make many synapses with other neurons, and the synapses are substantially invariant. There are separate flexion-producing and extension-producing command elements. Abdominal flexion-producing command elements excite other flexion elements and inhibit extensor command elements. The extension producing elements do the opposite. These interactions insure that interneurons of a particular class (flexion- or extension-producing) synaptically recruit perhaps twenty others of similar output, and that command elements promoting the opposing movements are inhibited. This strong reciprocity and the recruitment of similar command elements give a powerful motor program that appears to mimic behavior.

Gating of afferent input by a central pattern generator

Intracellular recordings from the sole proprioceptor (the oval organ) in the crab ventilatory system show that the nonspiking afferent fibers from this organ receive a cyclic hyperpolarizing inhibition in phase with the ventilatory motor pattern. Although depolarizing and hyperpolarizing current pulses injected into a single afferent will reset the ventilatory motor pattern, the inhibitory input is of sufficient magnitude to block afferent input to the ventilatory central pattern generator (CPG) for approximately 50% of the cycle period. It is proposed that this inhibitory input serves to gate sensory input to the ventilatory CPG to provide an unambiguous input to the ventilatory CPG.

Dopamine and nicotine, but not serotonin, modulate the crustacean ventilatory pattern generator

Dopamine (DA) causes a dose-dependent increase in the frequency of motor neuron bursts [virtual ventilation (fR)] produced by deafferented crab ventilatory pattern generators (CPGv). Domperidone, a D2-specific DA antagonist, by itself reversibly depresses fR and also blocks the stimulatory effects of DA. Serotonin (5HT) has no direct effects on this CPGv. Nicotine also causes dramatic dose-dependent increases in the frequency of motor bursts from the CPGv. The action is triphasic, beginning with an initial reversal of burst pattern typical of reversed-mode ventilation, followed by a 2- to 3-min period of depression and then a long period of elevated burst rate. Acetylcholine chloride (ACh) alone is ineffective, but in the presence of eserine is moderately stimulatory. The inhibitory effects of nicotine are only partially blocked by curare. The excitatory action of nicotine is blocked by prior perfusion of domperidone, but not by SKF-83566.HCl, a D1-specific DA antagonist. SKF-83566 had no effects on the ongoing pattern of firing. These observations support the hypothesis that dopaminergic pathways are involved in the maintenance of the CPGv rhythm and that the acceleratory effects of nicotine may involve release of DA either directly or via stimulation of atypical ACh receptors at intraganglionic sites.

Nonspiking interneurons in the ventilatory central pattern generator of the shore crab, Carcinus maenas

Eight nonspiking interneurons were identified that are elements of the central pattern generator controlling ventilation in the shore crab, Carcinus maenas. Intracellular recordings from these neurons in an isolated ganglion preparation revealed that these cells exhibit large amplitude oscillations in their membrane potentials, which are in-phase with the ventilatory motor pattern. These oscillations are present during the expression of the two distinct ventilatory motor output patterns corresponding to forward and reversed ventilation, and the oscillations stopped during pauses in the ventilatory rhythm. Injection of intracellular current pulses into these interneurons caused a resetting of the ongoing ventilatory rhythm, indicating that these cells are part of the ventilatory central pattern generator. The structure of each interneuron was determined by the intracellular injection of Lucifer Yellow dye. These neurons have a large diameter main neurite ranging from 10 to 20 microns in diameter with very restricted primary and secondary branching from the main neurite. All of the interneurons are restricted to a single hemiganglion and perturbation of these cells with intracellular current pulses only affect the motor output of the hemiganglion containing the interneuron. These eight nonspiking interneurons appear to be the primary components of the central pattern generator underlying ventilation in the crab.

Neural control of ventilation in the shore crab, Carcinus maenas. II. Frequency-modulating interneurons

1. We have identified a class of nonspiking interneurons which can control the frequency of ventilation in a graded manner. These frequency modulating interneurons (FMis) also receive synaptic inputs in-phase with the ventilatory motor output providing a functional positive feedback loop in the ventilatory system. The class of FMis is composed of three morphologically and physiologically distinct interneurons, FMi1, FMi2 and FMi3. 2. Depolarization of FMi1 increases the rate of ventilation, while hyperpolarization decreases the rate (Fig. 1). This control is restricted to a single ventilatory central pattern generator (CPG), (Fig. 2), although FMi1 sends processes into the neuropils of both hemiganglionic CPGs (Fig. 3). 3. Hyperpolarization of FMi2 increases the rate of both ventilatory CPGs while depolarization of this cell slows and eventually arrests the rhythm (Figs. 5 and 6). FMi2 receives a synaptic input correlated with the motor output of each of the ventilatory CPGs (Fig. 4). During periods of reversed ventilation, this cell is abruptly hyperpolarized and continues to be driven in-phase with the ventilatory motor output (Fig. 7). 4. Hyperpolarization of FMi3 increases the rate of ventilation and depolarization decreases the rate of ventilation produced by both CPGs (Fig. 10). This control of the ventilatory rate by FMi3 is graded (Fig. 11). There is no apparent change in the membrane potential of FMi3 during reversed ventilation and it is morphologically distinct from FMi2. 5. FMi2 and FMi3 may be involved in the switch in ventilatory motor pattern from forward to reversed ventilation. Hyperpolarization of FMi2 and depolarization of FMi3 can elicit bouts of reversed ventilation from both CPGs (Fig. 13). 6. These results suggest that the FM interneurons act in parallel to control the frequency of ventilation and may act as integrating elements between spiking 'command' fibers in the circumesophageal connectives and the nonspiking interneurons of the ventilatory CPG.

Neuronal structure and synaptic distribution of a uropod closer motor neuron in the crayfish terminal ganglion

One of the uropod closer muscles in the crayfish, the adductor exopodite, is innervated by two large identified motor neurons. They were injected intracellularly with horseradish peroxidase or nickel chloride to reveal the structure and distribution of the input and output synapses using electron microscopy. The development of nickel with rubeanic acid greatly improved the tissue preservation at the ultrastructural level compared with ammonium sulphide. Cell bodies of the motor neurons lying in the ventro-lateral cortex of the ganglion are extensively invaginated by glial cells. Input synapses occur directly upon the primary neurite within the neuropil or upon the major anterior neurite. They are most abundant, however, upon the numerous smaller neurites of the motor neuron. The primary neurite in the dorsal region of the neuropil, upon which no synapses were made, is wrapped with glial cells. Occasionally, these two adductor exopodite motor neurons were found as adjacent postsynaptic profiles at the same synapse when both cells were stained simultaneously in the same preparation. In the present study we could not locate any sites of synaptic output which strictly fulfil the structural criteria of a synapse on the processes of the motor neuron. This result is inconsistent with physiological evidence which suggests that spikeless interactions occur between the two adductor exopodite motor neurons and their synergists. This might be the result of two possible features of the interaction: the sites of synaptic output may be limited to a few restricted branches, and the interaction between these motor neurons may depend largely upon electrical synapses.