DNA sequence organization in the mollusc Aplysia californica.

The sequence organization of the DNA of the mollusc Aplysia californica has been examined by a combination of techniques. Close-spaced interspersion of repetitive and single copy sequences occurs throughout the majority of the genome. Detailed examination of the DNA of this protostome reveals great similarities to the pattern observed in the two deuterostome organisms previously examined in detail in this laboratory, Xenopus laevis and Strongylocentrotus purpuratus. Labeled and unlabeled Aplysia DNA were prepared from developing embryos and sheared to a fragment length of 400 nucleotides. The kinetics of reassociation were studied by means of hydroxyapatite chromatography, single-strand-specific S1 nuclease, and optical methods of assay. Aplysia DNA of this fragment length contains at least five resolvable kinetic fractions. One classification of these fractions, listed with their reassociation rate constants (l M-1 sec-1) is: single copy (0.00057), slow (0.047), fast (2.58), very fast (4000), and foldback (greater than 10(5)). Sequence arrangement was deduced from: the kinetics of reassociation of DNA fragments of length 400 or 2000 nucleotides; the hyperchromicity of reassociated fragments containing duplex regions; the size of duplex regions resistant to S1 nuclease; and the reassociation of labeled fragments of various lengths with short driver fragments. More than 80% of the single copy DNA sequences are interspersed with repetitive sequences. The maximum spacing of the repeats is about 2000 nucleotides, and the average less than 1000. The very fast fraction does not show interspersion with single copy sequences or with other kinetic fractions. The foldback fraction sequences are fairly widely interspersed. The slow fraction sequences are interspersed with the fast fraction, and possibly also with the single copy DNA. The fast fraction is the dominant interspersed repetitive fraction. Its sequences are adjacent to the great majority of the single copy sequences and have an average length of about 300 nucleotides.     

Pharmacological dissociation of different forms of synaptic plasticity in the marine mollusc Aplysia

The actions of the neuromodulator serotonin (5HT) in Aplysia sensory neurons can be dissociated on the basis of their sensitivity to the 5HT receptor antagonist cyproheptadine, and their concentration requirement to 5HT. Here we summarise a series of experiments that suggest that mechanistically distinct processes contribute to the different physiological components of short- and long-term synaptic plasticity.     

The effects of delta9-tetrahydrocannabinol on action potentials in the mollusc Aplysia.

The effects of delta9-tetrahydrocannabinol (delta9-THC) on action potentials were examined during intrasomatic recordings from the isolated buccal and parieto-visceral ganglia of Aplysia californica. When added to the saline solution bathing the preparation, the compound (in concentrations 10(-4) - 10(-5) M) caused a reduction in spike overshoot (15-20% of total amplitude) and increased the lability of responses to electrical stimulation. The somatic membrane appeared to be more affected than the axonal membrane. Diffusion barriers in the ganglion probably account for the high degree of variability in drug response, such that both of the characteristic changes were observed in only about 30% of the tests. This is the first report to describe effects of delta9-THC on invertebrate neurones. The results indicate that delta9-THC causes a depression in nerve cell excitability, and these data are consistent with reported effects of THC compounds in mammals.     

Hydrolyses and transglycosylations performed by purified alpha-D-glucosidase of the marine mollusc Aplysia fasciata

The purification and characterisation of the alpha-glucosidase from the marine mollusc Aplysia fasciata are reported. Overall substrate specificity of the pure enzyme for both hydrolytic and transglycosylation reactions was studied. Remarkable characteristics of this enzyme are indicated by the results of the interesting survey of transglycosylation reactions reported: pyridoxine glucosylation, synthesis of chromophoric (pNP) di- and trisaccharides, glucosylation of cellobiose and sucrose. For these last two acceptors both the yields of reactions and the concentrations of products are comparable to those obtained using glycosyl transferases; in addition, synthesis of pyridoxine and chromophoric glycosides were still possible using a 1:1 ratio maltose:acceptor which is a very interesting characteristic from a synthetic point of view (effortless purification, productivity of each reaction batch, etc.).     

Rhombic particle arrays in gill epithelium of a mollusc, Aplysia californica.

Gill epithelia from adult and juvenile Aplysia were examined by conventional thin section and freeze-fracture methods. Freeze-fracture replicas of adult gill epithelium revealed septate and gap junctions, which served as membrane markers for the epithelial cells. In these same cell membranes, non-junctional rhombic arrays of intramembranous particles were observed on prominent ridges on the membrane P fracture face of some epithelial cells. In thin sections of adult epithelium, nerve terminals were observed abutting the lateral plasma membranes near the basal lamina of some epithelial cells. Correlative areas of plasma membrane in freeze-fracture replicas showed a close association between rhombic particle arrays and abutting nerve terminals. In thin sections of juvenile Aplysia, nerve terminals abutting the epithelial cells were not recognizable, and rhombic arrays were not observed in freeze-fracture replicas. This suggested that a developmental association existed between the appearance of rhombic arrays in adult epithelia and their innervation. It is not known with certainty if, in invertebrates, rhombic arrays are an essential structural entity of all innervated cell membranes; however, in the cells thus far studied, there appears to be an associative condition. In the case of the gill epithelium of Aplysia, rhombic arrays are located in the same vicinity as the abutting nerve terminals. Similar arrays of intramembranous particles have been observed in myoneural postjunctional complexes of other invertebrates and have been interpreted to be the morphological expression of neurotransmitter receptors. An analogous explanation is put forth, namely that rhombic arrays may represent the structural correlates of neurotransmitter receptors and/or ionic channels in innervated membranes of invertebrates.     

Direct enzymatic glucosylation of naringin in grapefruit juice by alpha-D-glucosidase from the marine mollusc Aplysia fasciata

The enzymatic glucosylations of naringin, performed using alpha-D-glucosidase, identified in the Mediterranean mollusc Aplysia fasciata is reported. The enzyme actively operates on maltose and has an interesting transglycosylation potential using this donor. We also investigated the use of this marine alpha-glucosidase for a food-compatible glucosylation of naringin to produce new enzymatically modified carbohydrate possessing naringin derivatives. The regioselective formations of the beta-gluco-C6 alpha-glucosyl derivative and of the corresponding isomaltosyl diglucoside of naringin were obtained in high yield and efficiency of reaction. Suspensions of naringin can be used up to approximately 90 mg/mL initial acceptor concentration. In different experiments it was demonstrated that the enzyme was still active after 48 h in presence of this high amount of acceptor and that one of the diasteromers of the naringin is preferred by the enzyme from A. fasciata during glucosylation/deglucosylation enzymatic steps. Finally, the feasibility of efficient naringin glucosylation in grapefruit juice was also demonstrated at optimal pH of the enzyme and low maltose concentrations.     

Local hormonal modulation of neural activity in Aplysia

The abdominal ganglion of Aplysia provides a convenient experimental system for cellular studies on the roles of peptides as chemical messengers in the nervous system. There are indications that the bag cells, a group of neuroendocrine cells, synthesize and release egg laying hormone (ELH), a peptide with an apparent molecular weight of 6000. Our recent investigations indicate that a burst of impulse activity in the bag cells produces five types of long-lasting responses, some excitatory, others inhibitory, in 26 identified neurons and 2 identified cell clusters located near the bag cells in the abdominal ganglion. The responses have slow, smoothly graded onsets, and many of them result in modulation of neuronal activity for 3 hours or more. Physiological and ultrastructural data support the hypothesis that they are induced by a bag cell hormone (or hormones) that is released into vascular and interstitial spaces of the ganglion to act on the target neurons. Local application of purified ELH to one of the target neurons provides evidence that the bag cell effect is mediated by ELH. Many of the target neurons are known to be parts of neuronal circuits that control specific behavioral and homeostatic processes. Since egg laying is initiated by the bag cell discharge and is associated with a stereotyped behavior pattern lasting several hours, the actions of these peptide-secreting neurons on the central nervous system may serve to regulate certain elements of behavior and homeostasis during egg laying.     

Markovian neural networks

The neural network that efficiently and nearly optimally solves difficult optimization problems is defined. The convergence proof for the Markovian neural network that asynchronously updates its neurons' states is also presented. The comparison of the performance of the Markovian neural network with various combinatorial optimization methods in two domains is described. The Markovian neural network is shown to be an efficient tool for solving optimization problems.     

Cephalopod neural networks

Cephalopods have arguably the largest and most complex nervous systems amongst the invertebrates; but despite the squid giant axon being one of the best studied nerve cells in neuroscience, and the availability of superb information on the morphology of some cephalopod brains, there is surprisingly little known about the operation of the neural networks that underlie the sophisticated range of behaviour these animals display. This review focuses on a few of the best studied neural networks: the giant fiber system, the chromatophore system, the statocyst system, the visual system and the learning and memory system, with a view to summarizing our current knowledge and stimulating new studies, particularly on the activities of identified central neurons, to provide a more complete understanding of networks within the cephalopod nervous system.     

Bayesian neural networks

A neural network that uses the basic Hebbian learning rule and the Bayesian combination function is defined. Analogously to Hopfield's neural network, the convergence for the Bayesian neural network that asynchronously updates its neurons' states is proved. The performance of the Bayesian neural network in four medical domains is compared with various classification methods. The Bayesian neural network uses more sophisticated combination function than Hopfield's neural network and uses more economically the available information. The naive Bayesian classifier typically outperforms the basic Bayesian neural network since iterations in network make too many mistakes. By restricting the number of iterations and increasing the number of fixed points the network performs better than the naive Bayesian classifier. The Bayesian neural network is designed to learn very quickly and incrementally.     

Oscillatory neural networks in the rabbit hippocampus

A model is described to account for damped oscillatory activity of two interacting neural populations, pyramidal cells and interneurons. This network in the hippocampus is treated as a lumped system with tine delays between elements. The physiological mechanism underlying the oscillatory activity appears to involve neural population interaction and cannot be described in terms of a network composed of but two neurons, a single pyramidal cell and a single interneuron. An unusual aspect of the model is the explicit incorporation of an ongoing background input to raise the mean level of activity of the pyramidal cell population. This model has evolved from a series of studies previously performed on cats. To test the model experiments were performed on rabbits. The data showing oscillatory activity following fornix stimulation in the rabbit indicate that the model can be applied not only to the cat but also to the rabbit. In additions, for commissural stimulation oscillatory potentials of neural populations and individual pyramidal cells were evoked as predicted by the model.     

Hypnosis modelling in neural networks

In the framework of the neural network theory effects similar to hypnotic displays are constructed. They are based on the associative paradigm involving non-linear interaction of excitatory and inhibitory channels with synaptic memory. The non-linearity of long-term memorizing processes may cause effects exhibited by blind spots, which are interpreted as the first stage of hypnosis. More complicated phenomena are discussed in terms of a two-layer network.