Activities of citrate synthase, NAD+-linked and NADP+-linked isocitrate dehydrogenases, glutamate dehydrogenase, aspartate aminotransferase and alanine aminotransferase in nervous tissues from vertebrates and invertebrates.
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1. The activities of citrate synthase and NAD+-linked and NADP+-linked isocitrate dehydrogenases were measured in nervous tissue from different animals in an attempt to provide more information about the citric acid cycle in this tissue. In higher animals the activities of citrate synthase are greater than the sum of activities of the isocitrate dehydrogenases, whereas they are similar in nervous tissues from the lower animals. This suggests that in higher animals the isocitrate dehydrogenase reaction is far-removed from equilibrium. If it is assumed that isocitrate dehydrogenase activities provide an indication of the maximum flux through the citric acid cycle, the maximum glycolytic capacity in nervous tissue is considerably greater than that of the cycle. This suggest that glycolysis can provide energy in excess of the aerobic capacity of the tissue. 2. The activities of glutamate dehydrogenase are high in most nervous tissues and the activities of aspartate aminotransferase are high in all nervous tissue investigated. However, the activities of alanine aminotransferase are low in all tissues except the ganglia of the waterbug and cockroach. In these insect tissues, anaerobic glycolysis may result in the formation of alanine rather than lactate. (+info)
Autoinhibition of serotonin cells: an intrinsic regulatory mechanism sensitive to the pattern of usage of the cells.
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After periods of high-frequency firing, the normal rhythmically active serotonin (5HT)-containing neurosecretory neurons of the lobster ventral nerve cord display a period of suppressed spike generation and reduced synaptic input that we refer to as "autoinhibition." The duration of this autoinhibition is directly related to the magnitude and duration of the current injection triggering the high-frequency firing. More interesting, however, is that the autoinhibition is inversely related to the initial firing frequency of these cells within their normal range of firing (0.5-3 Hz). This allows more active 5HT neurons to resume firing after shorter durations of inhibition than cells that initially fired at slower rates. Although superfused 5HT inhibits the spontaneous firing of these cells, the persistence of autoinhibition in saline with no added calcium, in cadmium-containing saline, and in lobsters depleted of serotonin suggests that intrinsic membrane properties account for the autoinhibition. A similar autoinhibition is seen in spontaneously active octopamine neurons but is absent from spontaneously active gamma-aminobutyric acid cells. Thus, this might be a characteristic feature of amine-containing neurosecretory neurons. The 5HT cells of vertebrate brain nuclei share similarities in firing frequencies, spike shapes, and inhibition by 5HT with the lobster cells that were the focus of this study. However, the mechanism suggested to underlie autoinhibition in vertebrate neurons is that 5HT released from activated or neighboring cells acts back on inhibitory autoreceptors that are found on the dendrites and cell bodies of these neurons. (+info)
Urine release in freely moving catheterised lobsters (Homarus americanus) with reference to feeding and social activities.
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Previous studies suggest that urine-borne pheromones play an important role in lobster agonistic and sexual behaviour. This paper investigates the pattern of urine release in catheterised, but otherwise freely moving, adult lobsters with respect to feeding, social and non-social activities. Lobsters on average released 4.1 ml (1 % of body mass) of urine over a 12 h period; this more than doubled to 10.6 ml over the 12 h period after feeding. Hourly monitoring revealed that most urine was released in the first hour after feeding (2.84 ml). With the exception of the first hours after feeding, urine release was intermittent, with pauses lasting up to 17 h. The probability of urine release per hour in unfed lobsters was 0.34 (median); this value increased during agonistic interactions elicited by the introduction of a conspecific (median 0. 63) and during activity initiated by non-social disturbance (median 0.56). Mean urine volume during output hours in unfed lobsters amounted to 1.09 ml h-1. This volume was significantly increased by the presence of a conspecific (1.88 ml h-1) and decreased during activity initiated by non-social disturbances (0.56 ml h-1). No sex-specific differences in urine release were found. The data demonstrate that lobsters control their urine release in a manner dependent on behavioural context. This supports recent findings suggesting the use of urine for chemical signalling in agonistic interactions. (+info)
Cooperative Ca2+ removal from presynaptic terminals of the spiny lobster neuromuscular junction.
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Stimulation-induced changes in presynaptic free calcium concentration ([Ca2+]i) were examined by fluorescent imaging at the spiny lobster excitor motor nerve terminals. The Ca2+ removal process in the terminal was analyzed based on a single compartment model, under the assumption that the Ca2+ removal rate from the terminal cytoplasm is proportional to nth power of [Ca2+]i. During 100 nerve stimuli at 10-100 Hz, [Ca2+]i reached a plateau that increased in a less-than-linear way with stimulation frequency, and the power index, n, was about 2. In the decay time course after stimulation, n changed with the number of stimuli from about 1.4 after 10 stimuli to about 2 after 100 stimuli. With the change of n from 1.4 to 2, the rate became larger at high [Ca2+]i (>1.5 microM), but was smaller at low [Ca2+]i (<1 microM). These results suggest that a cooperative Ca2+ removal mechanism of n = 2, such as mitochondria, may play an important role in the terminal. This view is supported by the gradual increase in the [Ca2+]i plateau during long-term stimulation at 20-50 Hz for 60 s and by the existence of a very slow [Ca2+]i recovery process after this stimulation, both of which may be due to accumulation of Ca2+ in the organelle. (+info)
Thermal compensation in protein and RNA synthesis during the intermolt cycle of the American lobster, Homarus americanus.
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1. The in vitro rates of incorporation of precursors into protein and RNA and the concentration of RNA were measured in tissues of intermolt and premolt lobsters acclimated to 5 degrees C and 20 degrees C. Midgut gland, abdominal muscle and gill of intermolt lobsters respond to temperature acclimation by a compensatory translation of the rate-temperature (R-T) curves with respect to the rates of incorporation of 3H-leucine and 3H-uridine into the acid-insoluble fraction. Midgut gland and muscle of premolt animals exhibit either no compensation or inverse compensation; gill tissue exhibits a rotation of the R-T curve. 2. The existence of the complete de novo pathway of pyrimidine biosynthesis is demonstrated in the class Crustacea. NaH14 CO2 is incorporated into orotic acid and orotic-14 C-acid is incorporated into the acid-insoluble fraction. 3. Both the concentration of RNA and the rates of incorporation of precursors of both the salvage and de novo pyrimidine pathways are enhanced in the midgut gland of premolt lobsters, relative to intermolt tissue, under conditions of warm-acclimation. (+info)
Effects of phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate on a Na+-gated nonselective cation channel.
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Olfactory receptor neurons in the lobster express a nonselective cation channel that is activated by intracellular Na+ and carries a substantial part of the depolarizing receptor current. Here, we show that phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and phosphatidylinositol 4-phosphate [PI(4)P] applied to the intracellular face of cell-free patches activate the channel in the absence of Na+ and that antibodies against the respective phospholipids irreversibly inhibit the evoked activity. Further, we show that applying PI(4,5)P2 or PI(4)P in the presence of Na+ decreases the concentration of Na+ required to activate the channel from an EC50 of 74 to 22 mM for PI(4,5)P2 and to 29 mM for PI(4)P, respectively. Na+-gated channel activity was irreversibly inhibited by monoclonal antibodies against PI(4,5)P2 and PI(4)P in patches never exposed to exogenous phosphatidylinositols, suggesting that endogenous inositol phospholipids are required for the activation of the channel by intracellular Na+. Our findings suggest that PI(4,5)P2 and/or PI(4)P may serve as intracellular signaling molecules in these primary sensory neurons and provide a general mechanism to explain how the sensitivity of Na+-gated channels to Na+ could be much greater in intact cells than in excised membrane patches. (+info)
Evidence for novel caffeine and Ca2+ binding sites on the lobster skeletal ryanodine receptor.
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1. The effects of Ca2+, ATP and caffeine on the gating of lobster skeletal muscle ryanodine receptors (RyR) was investigated after reconstitution of the channels into planar phospholipid bilayers and by using [3H]-ryanodine binding studies. 2. The single channel studies reveal that the EC50 (60 microM) for activation of the lobster skeletal RyR by Ca2+ as the sole ligand is higher than for any other isoform of RyR studied. 3. Inactivation of the channel by Ca2+ (EC50 = 1 mM) occurs at concentrations slightly higher than those required to inactivate mammalian skeletal RyR (RyR1) but lower than those required to inactivate mammalian cardiac RyR (RyR2). 4. Lifetime analysis demonstrates that cytosolic Ca2+, as the sole activating ligand, cannot fully open the lobster skeletal RyR (maximum Po approximately 0.2). The mechanism for the increase in open probability (Po) is an increase in both the frequency and the duration of the open events. 5. ATP is a very effective activator of the lobster RyR and can almost fully open the channel in the presence of activating cytosolic [Ca2+]. In the presence of 700 microM Ca2+, 1 mM ATP increased Po to approximately 0.8. 6. Caffeine, often used as a tool to identify the presence of RyR channels, is relatively ineffective and cannot increase Po above the level that can be attained with Ca2+ alone. 7. The results reveal that caffeine increases Po by a different mechanism to that of cytosolic Ca2+ demonstrating that the mechanism for channel activation by caffeine is not 'sensitization' to cytosolic Ca2+. 8. By studying the mechanisms involved in the activation of the lobster RyR we have demonstrated that the channel responds in a unique manner to Ca2+ and to caffeine. The results strongly indicate that these ligand binding sites on the channel are different to those on mammalian isoforms of RyR. (+info)
From embryo to adult: persistent neurogenesis and apoptotic cell death shape the lobster deutocerebrum.
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Neuronal plasticity and synaptic remodeling play important roles during the development of the invertebrate nervous system. In addition, structural neuroplasticity as a result of long-term environmental changes, behavioral modifications, age, and experience have been demonstrated in the brains of sexually mature insects. In adult vertebrates, persistent neurogenesis is found in the granule cell layer of the mammalian hippocampus and the subventricular zone, as well as in the telencephalon of songbirds, indicating that persistent neurogenesis, which is presumably related to plasticity and learning, may be an integral part of the normal biology of the mature brain. In decapod crustaceans, persistent neurogenesis among olfactory projection neurons is a common principle that shapes the adult brain, indicating a remarkable degree of life-long structural plasticity. The present study closes a gap in our knowledge of this phenomenon by describing the continuous cell proliferation and gradual displacement of proliferation domains in the central olfactory pathway of the American lobster Homarus americanus from early embryonic through larval and juvenile stages into adult life. Neurogenesis in the deutocerebrum was examined by the in vivo incorporation of bromodeoxyuridine, and development and structural maturation of the deutocerebral neuropils were studied using immunohistochemistry against Drosophila synapsin. The role of apoptotic cell death in shaping the developing deutocerebrum was studied using the terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling method, combined with immunolabeling using an antiphospho histone H3 mitosis marker. Our results indicate that, in juvenile and adult lobsters, birth and death of olfactory interneurons occur in parallel, suggesting a turnover of these cells. When the persistent neurogenesis and concurrent death of interneurons in the central olfactory pathway of the crustacean brain are taken into account with the life-long turnover of olfactory receptor cells in crustacean antennules, a new, highly dynamic picture of olfaction in crustaceans emerges. (+info)