The supramedullary neurons of fish: present status and goals for the future

Cell Cycle Re-entry in the Nervous System: From Polyploidy to Neurodegeneration

Terminally differentiated cells of the nervous system have long been considered to be in a stable non-cycling state and are often considered to be permanently in G0. Exit from the cell cycle during development is often coincident with the differentiation of neurons, and is critical for neuronal function. But what happens in long lived postmitotic tissues that accumulate cell damage or suffer cell loss during aging? In other contexts, cells that are normally non-dividing or postmitotic can or re-enter the cell cycle and begin replicating their DNA to facilitate cellular growth in response to cell loss. This leads to a state called polyploidy, where cells contain multiple copies of the genome. A growing body of literature from several vertebrate and invertebrate model organisms has shown that polyploidy in the nervous system may be more common than previously appreciated and occurs under normal physiological conditions. Moreover, it has been found that neuronal polyploidization can play a protective role when cells are challenged with DNA damage or oxidative stress. By contrast, work over the last two and a half decades has discovered a link between cell-cycle reentry in neurons and several neurodegenerative conditions. In this context, neuronal cell cycle re-entry is widely considered to be aberrant and deleterious to neuronal health. In this review, we highlight historical and emerging reports of polyploidy in the nervous systems of various vertebrate and invertebrate organisms. We discuss the potential functions of polyploidization in the nervous system, particularly in the context of long-lived cells and age-associated polyploidization. Finally, we attempt to reconcile the seemingly disparate associations of neuronal polyploidy with both neurodegeneration and neuroprotection.

Electrical coupling between hippocampal neurons: contrasting roles of principal cell gap junctions and interneuron gap junctions

There is considerable experimental evidence, anatomical and physiological, that gap junctions exist in the hippocampus. Electrical coupling through these gap junctions may be divided into three types: between principal neurons, between interneurons and at mixed chemical (glutamatergic)/electrical synapses. An approach, combining in vitro experimental with modeling techniques, sheds some light on the functional consequences of electrical coupling, for network oscillations and for seizures. Additionally, in vivo experiments, using mouse connexin knockouts, suggest that the presence of electrical coupling is important for optimal performance on selected behavioral tasks; however, the interpretation of such data, in cellular terms, has so far proven difficult. Given that invertebrate central pattern generators so often depend on both chemical and electrical synapses, our hypothesis is that hippocampus-mediated and -influenced behaviors will act likewise. Experiments, likely hard ones, will be required to test this intuition.

Autonomic control of glands and secretion: a comparative view

The autonomic nervous system together with circulating and local hormones control secretion from glands. This article summarizes histochemical and functional studies on the autonomic innervation and control of secretory glands in non-mammalian vertebrates, including secretion of saliva in the mouth and gastric acid in the stomach, secretion of enzymes and bicarbonate from the pancreas and gut wall, secretion of mucus in the gut epithelium and onto the skin, and salt secretion from salt glands and rectal glands. Cholinergic and adrenergic nerves, directly or indirectly, in combination with different types of peptidergic and other nerves appear to innervate gland tissues and affect secretion in all investigated species.

Endoreplication: a molecular trick during animal neuron evolution

The occurrence of endoreplication has been repeatedly reported in many organisms, including protists, plants, worms, arthropods, molluscs, fishes, and mammals. As a general rule, cells possessing endoreplicated genomes are large-sized and highly metabolically active. Endoreplication has not been frequently reported in neuronal cells that are typically considered to be fully differentiated and non-dividing, and which normally contain a diploid genome. Despite this general statement, various papers indicate that giant neurons in molluscs, as well as supramedullary and hypothalamic magnocellular neurons in fishes, contain DNA amounts larger than 2C. In order to study this issue in greater detail here, we review the available data about endoreplication in invertebrate and vertebrate neurons, and discuss its possible functional significance. As a whole, endoreplication seems to be a sort of molecular trick used by neurons in response to the high functional demands that they experience during evolution.

Macroglial cells of the teleost central nervous system: a survey of the main types

Following our previous review of teleost microglia, we focus here on the morphological and histochemical features of the three principal macroglia types in the teleost central nervous system (ependymal cells, astrocyte-like cells/radial glia and oligodendrocytes). This review is concerned with recent literature and not only provides insights into the various individual aspects of the different types of macroglial cells plus a comparison with mammalian glia, but also indicates the several potentials that the neural tissue of teleosts exhibits in neurobiological research. Indeed, some areas of the teleost brain are particularly suitable in terms of the establishment of a "simple" but complete research model (i.e. the visual pathway complex and the supramedullary neuron cluster in puffer fish). The relationships between neurons and glial cells are considered in fish, with the aim of providing an integrated picture of the complex ways in which neurons and glia communicate and collaborate in normal and injured neural tissues. The recent setting up of successful protocols for fish glia and mixed neuron-glia cultures, together with the molecular facilities offered by the knowledge of some teleost genomes, should allow consistent input towards the achievement of this aim.

Morphological, cytochemical, and cytofluorimetric features of supramedullary neurons of the fish Solea ocellata

Various teleost species belonging to different orders possess a particular neuronal system formed by giant supramedullary neurons (SNs). In some species, SNs are scattered along the spinal cord; in others they are organized in a compacted and well-defined cluster located at the boundary between the medulla oblongata and spinal cord. In addition to the many morphological, physiological, and histochemical studies performed both in vivo and in vitro by several authors since the end of the 19th century, quantitative microfluorometric evaluation of the DNA content of SNs has showed that clustered SNs but not aligned SNs have a DNA content much greater than the normal value of 2C. Such a high DNA content is exceptional for vertebrate neurons. In the present study, we extend this analysis of SNs to the fish Solea ocellata. Our results show that the organization of the SNs of S. ocellata is neither strictly aligned nor clustered, but somewhere in between, and that this is also true of both their morphological characteristics and DNA content values. Interspecific differences in the distribution and morphology of SNs may reflect functional differences, possibly related to environmental or behavioral differences among species. In addition, the possible functional significance of endoreplication in SNs is discussed.