Plexiform Zone Research Articles

Molecular Characterization, Expression and In Situ Hybridization Analysis of a Pedal Peptide/Orcokinin-type Neuropeptide in Cuttlefish Sepiella japonica.

Neuropeptide pedal peptide (PP) and orcokinin (OK), which are structurally related active peptides, have been widely discovered in invertebrates and constitute the PP/OK neuropeptide family. They have complex structures and play myriad roles in physiological processes. To date, there have been no related reports of PP/OK-type neuropeptide in cephalopods, which possess a highly differentiated multi-lobular brain. Rapid Amplification of cDNA Ends (RACE) was employed to obtain the open reading frame (ORF) of PP/OK-type neuropeptide in Sepiella japonica (termed as Sj-PP/OK). Various software were used for sequence analysis. Semi-quantitative PCR was applied to analyze the tissue distribution profile, quantitative real-time PCR (qRT-PCR) was used to study spatio-temporal expression throughout the entire growth and development period, and in situ hybridization (ISH) was employed to observe the tissue location of Sj-PP/OK. in the present study, we identified the ORF of Sj-PP/OK. The putative precursor of Sj-PP/ OK encodes 22 mature peptides, of which only tridecapeptides could undergo post-translationally amidated at C-terminus. Each of these tridecapeptides possesses the most conserved and frequent N-terminus Asp-Ser-Ile (DSI). Sequence analysis revealed that Sj-PP/OK shared comparatively low identity with other invertebrates PP or OK. The tissue distribution profile showed differences in the expression level of Sj-PP/OK between male and female. qRT-PCR data demonstrated that Sj-PP/OK was widely distributed in various tissues, with its expression level increasing continuously in the brain, optic lobe, liver, and nidamental gland throughout the entire growth and development stages until gonad maturation. ISH detected that Sj-PP/OK positive signals existed in almost all regions of the optic lobe except the plexiform zone, the outer edge of all functional lobes in the brain, epithelial cells and the outer membrane layer of the accessory nidamental gland. These findings suggest that Sj-PP/OK might play a role in the regulation of reproduction, such as vitellogenin synthesis, restoration, and ova encapsulation. The study indicated that Sj-PP/OK may be involved in the neuroendocrine regulation in cephalopods, providing primary theoretical basis for further studies of its regulation role in reproduction.

Morphological alterations induced by the exposure to TiO2 nanoparticles in primary cortical neuron cultures and in the brain of rats.

Nowadays, nanoparticles (NPs) of titanium dioxide (TiO2) are abundantly produced. TiO2 NPs are present in various food products, in paints, cosmetics, sunscreens and toothpastes. However, the toxicity of TiO2 NPs on the central nervous system has been poorly investigated until now. The aim of this study was to evaluate the toxicity of TiO2 NPs on the central nervous system in vitro and in vivo. In cell cultures derived from embryonic cortical brain of rats, a significant decrease in neuroblasts was observed after 24 to 96 h of incubation with TiO2 NPs (5 to 20 μg/ml). This phenomenon resulted from an inhibition of neuroblast proliferation and a concomitant increase in apoptosis. In the same time, a gliosis, characterized by an increase in proliferation of astrocytes and the hypertrophy of microglial cells, occurred. The phagocytosis of TiO2 NPs by microgliocytes was also observed. In vivo, after intraperitoneal injection, the TiO2 NPs reached the brain through the blood brain barrier and the nanoparticles promoted various histological injuries such as cellular lysis, neuronal apoptosis, and inflammation. A reduction of astrocyte population was observed in some brain area such as plexiform zone, cerebellum and subependymal area. An oxidative stress was also detected by immunohistochemistry in neurons of hippocampus, cerebellum and in subependymal area. In conclusion, our study demonstrated clearly the toxic impact of TiO2 NPs on rat brain and neuronal cells and pointed about not yet referenced toxicity impacts of TiO2 such as the reduction of neuroblast proliferation both in vitro and in vivo.

Peculiarities of normal retinal blood supply from the side of retinal pool.

Background. One of the causes of eye diseases is the damage of the vascular bed components of the eyeball. Objective. To investigate peculiarities of the rat retinal blood supply from retinal pool. Methods. Adult male rats aged from 4,5–7,5 month were used. To investigate the blood supply of their retina, eye vessels were injected with contrast mass. Results. Vascularization of rat retina on micro-level is realized by the branches of central retinal artery that runs deep in the optic nerve and reaching its disk ramifies providing its vascularization. The main branches that ramify from the central retinal artery run from the area of optic disk to the periphery reaching deep into the layer of nerve fibers. These vessels include numerous arterioles and venules of the retina: dorsolateral, ventrolateral, dorsomedial, ventromedial, ventral and dorsal arterioles and venules of the macula. Each branch of the central retinal artery is divided dichotomously forming arterioles (of the first, second and third order). Near the optic nerve the diameter of arterioles of the first order is 80-100 microns. Subsequently these arterioles are dichotomously divided into second order arterioles with the diameter of 40-50 microns. And the second-order arterioles bifurcate into the third-order arterioles with the diameter of 10-20 microns. Arterioles further bifurcate and enter capillaries with the diameter ranging from 4.0 to 9.0 microns. The dense capillary network was traced over the entire area of the retina. In the deep retinal layers two capillary networks (superficial and deep) were visualized. Superficial capillary network was localized in the deep layer of nerve fibers and is clearly seen in all quadrants of the retina. Deep capillary network lies between the outer plexiform zone and the inner nuclear layer. These two capillary networks are interconnected with the help of anastomoses thus forming a morphological basis for compensatory mechanisms. There are non-vascular areas around the retinal artery located symmetrically on both sides of arteriole. The closer arterioles come to the periphery the more clearly we can see gradual disappearance of the deep capillary network and clear visualization of the surface capillary network dominates with capillaries having a wide diameter. The surface layer capillaries of rat’s retina are running radially from the optic nerve toward the edge of the posterior pole of the retina. Peripapillary capillaries are located at a considerable distance, and are rarely anastomosed to each other. Gradually passing into deeper layers of the retina peripapillary capillaries pass into venules. In the area of optic disk the retina contains another capillary network located in the outermost layer of nerve fibers. Capillaries in this region are located in the radial direction. We also found a capillary network in the normal rat’s retina in the area of macula. Loops of capillaries are polymorphous; they are twice longer than wide and run the same direction as branching arterioles. Retinal peripapillary capillary network is characterized by forward direction of capillaries, they have considerable length, with rarely encountered anastomosis and being located deep in the surface layers of nerve fibers of the retina. Conclusion. It was revealed that there are two capillary networks among rat’s retinal layers. Superficial capillary network is localized deep in the layer of nerve fibers and it is clearly seen in all quadrants of the retinal field. Deep capillary network lies between the outer plexiform zone and the inner nuclear layer. Additionaly capillary network was found in the region of optic disk which is located in the outermost layer of nerve fibers.

Cajal, Retzius, and Cajal-Retzius cells.

The marginal zone (MZ) of the prenatal cerebral cortex plays a crucial role in cellular migration and laminar patterning in the developing neocortex and its equivalent in the adult brain – layer I, participates in cortical circuitry integration within the adult neocortex. The MZ/layer I, which has also been called the plexiform layer and cell-poor zone of Meynert, among others, is home to several cell populations including glia, neurons, and Cajal–Retzius (CR) cells. Cajal once said that the MZ is one of the oldest formations in the phylogenetic series, and that the characteristics of layer I in human are similar in all vertebrates except fish (Ramon y Cajal, 1899). Despite the presence of CR cells in the MZ/layer I of all developing and adult vertebrate brains, and more than one hundred years of research, the phenotype and function of layer I cells have still not been clearly defined. Recent technological advances have yielded significant progress in functional and developmental studies, but much remains to be understood about neurons in MZ/layer I. Since the time of Retzius and Cajal, and continuing with modern era research from the likes of Marín-Padilla, the study of CR cells has been based on their morphological characteristics in Golgi staining. However, since Cajal’s initial description, the term “CR cell” has been applied differently and now is often used to indicate reelin (Reln)-positive cells in MZ/layer I. Here we review the history of work by Cajal, Retzius, and others pertaining to CR cells. We will establish a link between original descriptions of CR cell morphology by Cajal, Retzius, and others, and current understandings of the cell populations that reside in MZ/layer I based on the use of cellular markers. We propose to use the term “CR cell” for the class of neurons that express Reln in the MZ/layer I in both prenatal, developing and adult cerebral cortex.

Pre‐ and postsynaptic excitation and inhibition at octopus optic lobe photoreceptor terminals; implications for the function of the ‘presynaptic bags’

Synaptic transmission was examined in the plexiform zone of Octopus vulgaris optic lobes using field-potential recording from optic lobe slices. Stimulation of the optic nerve produced pre- and postsynaptic field potentials. Transmission was abolished in calcium-free seawater, L- glutamate or the AMPA/Kainate receptor blocker CNQX (EC(50), 40 microm), leaving an intact presynaptic field potential. ACh markedly reduced or blocked and d-tubocurarine augmented both pre- and postsynaptic field potentials, while alpha-bungarotoxin and atropine were without effect. Paired-pulse stimulation showed short-term depression of pre- and postsynaptic components with a half-time of recovery of approximately 500 ms. The depression was partially relieved in the presence of d-tubocurarine (half-time of recovery, 350 ms). No long-term changes in synaptic strength were induced by repetitive stimulation. A polyclonal antibody raised against a squid glutamate receptor produced positive staining in the third radial layer of the plexiform zone. No positive staining was observed in the other layers. Taking into account previous morphological data and our results, we propose that the excitatory terminations of the photoreceptors are in the innermost layer of the plexiform zone where the transmitter is likely to be glutamate and postsynaptic receptors are AMPA/kainate-like. Thus, the function of the terminal bags is to provide a location for a presynaptic cholinergic inhibitory shunt. The results imply that this arrangement provides a temporal filter for visual processing and enhances the perception of moving vs. stationary objects.

Role of FMRFamide in the reproduction of Octopus vulgaris: molecular analysis and effect on visual input

As a part of continuous research on the neurobiology of the cephalopods in general, and the neuroendocrine control of reproduction in Octopus vulgaris in particular, the presence, the molecular analysis and the effect of FMRFamide on the screening-pigment migration in the visual system have been analysed. FMRFamide immunoreactive fibres are present in the outer plexiform layer of the retina as well as in the plexiform zone of the deep retina. These fibres presumably come from optic and olfactory lobes. We isolated an incomplete Octopus FMRFamide cDNA which encodes an amino terminal truncated precursor containing several FMRFamide-related peptides (FaRPs) showing a high degree of identity with the FaRPs encoded in the precursor of Sepia officinalis, except for the presence of an Rpamide related peptide, present only in cnidarians. Finally, stimulation of isolated retina demonstrated that the effect of this tetrapeptide, coupled with dopamine, is the induction of an extreme adaptation of the retina to the light condition. This situation de facto inhibits sexual maturation. Our results on the effect of FMRFamide on the retina confirm the suggested hypothesis that this peptide plays an inhibitory role on the activity of optic gland.

Neurons tend to stop migration and differentiate along the cortical internal plexiform zones in the Reelin signal-deficient mice.

The Reelin molecule plays a fundamental role in corticogenesis. After Reelin binds to its receptors, the Reelin signal is transduced through tyrosine phosphorylation of the intracellular adaptor protein disabled 1 (Dab1). The reelin-gene-deficient mouse, reeler, and Dab1-deficient mouse, yotari, show disrupted positioning of neurons. Several molecules have been identified recently as being involved in Reelin signaling, however, the biological function of Reelin during cortical plate development was still unknown. We observed the migrating behavior of neurons during development in Reelin-signal-deficient mice. To visualize the migrating neurons directly, we introduced green fluorescent protein (GFP)-expression vectors into the ventricular zone with an in utero electroporation system and allowed the embryos to develop in utero until they were analyzed. The result showed that the migrating cells in the mutants were morphologically indistinguishable from those of normal mice. At the stage when the GFP-expressing cells reached the marginal zone near the pial surface and began dendrite formation in normal mice, the GFP-positive cells were found at various deeper positions in the mutant cortex. They had the morphology of migrating cells extending leading processes toward the pial surface. By contrast, in the mutants these cells tended to stop migration along the borders of the internal plexiform zone, the irregular structure consisting mainly of dendrites in the mutant cortex. Postnatally, these neurons began to develop dendrites later than the cells in the normal cortex. During this process, some neurons above the internal plexiform zone extended and developed dendrites in the opposite direction into the internal plexiform zone. These results suggest that the abnormal positioning of neurons in the Reelin-signal-deficient mice is caused, at least in part, by abnormal formation of the internal plexiform zone in the mutant cortex.

NMDA receptor stimulation induces temporary α-tubulin degradation signaled by nitric oxide-mediated tyrosine nitration in the nervous system of Sepia officinalis

Biochemical and immunohistochemical evidence is reported, showing basal protein nitration in specific regions of the optic lobes of Sepia officinalis, mainly in the fiber layers of the plexiform zone. SDS–PAGE analysis of optic lobe extracts revealed an intense 3-nitrotyrosine immunoreactive band identified as α-tubulin by immunoprecipitation and partial purification. Stimulation of NMDA receptors resulted in a selective decrease in α-tubulin levels within 30 min with partial recovery after 4 h. The effect was suppressed by the NO synthase (NOS) inhibitor l-nitroarginine. Incubation of optic lobes with free 3-nitrotyrosine resulted likewise in a selective loss of α-tubulin, due apparently to incorporation of the amino acid into the C-terminus of detyrosinated α-tubulin to give the nitrated protein purportedly more susceptible to degradation. Overall, these results point to a novel potential physiologic role of NO and free 3-nitrotyrosine in the control of the α-tubulin tyrosination/detyrosination cycle and turnover in Sepia nervous tissue.

Neuronal Circuits in Cephalopod Vision

Cephalopods have a sophisticated visual system that is comparable in overall performance to many vertebrate visual systems. Although the eyes of cephalopods and vertebrates have strong similarities, their central processing systems seem anatomically very different. In this paper we investigate the mechanism used by cephalopods for analysing visual signals, with a view to comparing this with vertebrate systems. The cephalopod retina is comparatively simple containing only photoreceptors, supporting cells, and an efferent innervation. The retina is topographically mapped onto the optic lobe via a chiasma that inverts the visual map. Within the optic lobe most photoreceptor axons terminate in a complex ncuropil area, the outer plexiform zone. This zone receives connections from many different cell types and stains positively for most of the major neurotransmitters; this is undoubtedly an important processing area. Using 400 μm slices from the optic lobe, field potentials were recorded from this area in response to stimulation of the optic nerves; these showed a complex potential in the plexiform layers, indicating synaptic activity in this zone. This activity could be abolished by synaptic blocking agents. The spatial extent of this activity was revealed by an optical recording method, whereby the slice was stained with a voltage-sensitive dye and a fast video system was used to image the responses to electrical stimulation. This confirmed that the activity from the retinal photoreceptors excites follower cells in the plexiform zone, probably the small amacrine cells, and that this spreads along the margin of the zone to cover a distance of about 400 μm. The signal could not be followed further in transverse slices. These experiments provide evidence for the first steps in our understanding of visual processing in cephalopods and demonstrate the advantages of combining electrical and optical recordings in the analysis of complex neural processing.

Monoaminergic afferents to the neocortex: A developmental histofluorescence study in normal and reeler mouse embryos

The patterns of distribution of monoaminergic (MA) afferents during early histogenesis of the neocortex of normal and Reeler mice are studied by histofluorescence microscopy. Fluorescing fibers appear rostrally in the cortex of both genotypes on the 14th embryonic day (E14), which is within 24 h of the development of the cortical plate. They are distributed to all regions of the cortex by the time of birth. Although the patterns of intracortical deployment differ in the two genotypes, the fibers appear to have homologous target structures. These are: (1) the polymorphic cells of the subplate in the depths of the normal and in the superplate near the surface of the mutant cortex; and (2) the zones of consolidation of apical dendrites of pyramidal cells: the external plexiform zone of normal and a series of intracortical plexiform planes in the mutant cortex. By contrast, the axons of this system do not branch significantly among the compactly ordered somata of pyramidal cells within the cortical plate of either genotype.

The central nervous system of Loligo. I. The optic lobe.

The optic nerve fibres project on to the optic lobe in a regular manner, being precisely re-assorted after passing through a chiasma. In the outer plexiform zone the optic nerve fibres end in contact with the dendrites of second-order visual cells. These presumably serve to classify the visual input and four types can be recognized anatomically: (1) The smallest have minute circular dendritic fields, in contact with one or few optic nerve fibres. (2) There are also larger circular fields. (3) Many cells have very elongated narrow dendritic fields each running straight in one direction and thus perhaps sensitive to edges. (4) The largest second-order visual cells have enormous oval dendritic fields, several millimetres long, orientated in the long axis of the lobe. Each type of field occupies a different level, producing the characteristic layering of the outer plexiform zone. Numerous amacrine cell processes end in the outer plexiform layer, some are very small with restricted branches, others have wide trees with fibres passing first inwards then outwards several times. There are thus possibilities of establishing uniform conditions of excitation or inhibition over small or large areas of the visual field. The dendrites of the centrifugal cells with axons passing to the retina spread in the various layers of the plexiform zone. They could serve to project information of the areas excited, or inhibited, out to the retina. The axons of the second-order visual cells form radial columns in the outer part of the medulla of the optic lobe. Those with the smaller dendritic fields end more superficially, the largest ones about half-way through the lobe. Each column contains fibres and neuropil at its centre, surrounded by multipolar and bipolar amacrine cells, whose branches enter the neuropil among the endings of the second-order visual cells. Horizontal multipolar cells of various sizes link the columns. Third-order visual cells send dendrites into these columns and axons deeper into the lobe, some directly to the optic tract. The giant cells of the magnocellular lobe can thus be activated by a visual pathway involving only two previous synapses (as well as by a direct static pathway involving none). Central to the zone of radial columns is a zone where many of the connexions are tangential. There is an increasing number of large cells passing centrally, many being presumably fourth-order visual neurons. They send axons either elsewhere within the lobe or to the optic tract. Fibres reaching the lobe from the central brain or opposite lobe are distributed in this region and also reach out into the radial columns. In many of the tracts leaving the optic lobes for other centres the fibres maintain precise topographical relations, as also do those of the optic commissure. This regularity is especially clear in the bundles that pass to the motor centres (peduncle lobes and anterior basal lobes) but may be present in others. There is thus a regular mapping of the visual field throughout much of the system. Other pathways show complex interweaving, for instance those for colour control, where the response pattern is not topographically related to the visual input.

Time of neuron origin in the hippocampus and dentate gyrus of normal and reeler mutant mice: an autoradiographic analysis.

AbstractThe hippocampus and dentate gyrus of reeler mutant mice contain the normal neuronal classes. The majority of cells are included in laminar aggregates reminiscent of normal structure though these are irregular in outline, interrupted at points, and less densely packed. Many hippocampal neurons and granule cells of the dentate gyrus are scattered in the external plexiform zone (strata radiatum and lacunosum‐moleculare) of the hippocampus rather than being included in the laminar aggregates.Neurons in the reeler are generated at the same time as their normal homologs. However, the relationship of adult cell position to sequence of cell generation is abnormal in two general ways in the mutant. First, although in both the mutant and normal, cells in the external plexiform zone of the hippocampus are among the earliest formed, a relatively greater proportion of cells generated through a relatively longer embryonic time are destined to be scattered in this distal location in the mutant. Secondly, within the laminar aggregates of the hippocampus in reeler, the more superficial cells tend to be formed earlier, and the more deeply located ones are the latest formed. A reversed relationship exists in the normal in which the deepest cells are formed earliest, the more superficial ones last. The observations are consistent with the view that a mechanism which provides for orderly lamination of cells in the normal is defective in reeler.

Receptive fields of the visual system of the squid.

IN an octopus some second order visual cells that receive synapses in the plexiform layer from the optic nerve fibres have dendritic fields with an oval orientation lying in the horizontal plane1. It has now been shown that in a squid there are cells with several distinct types of receptive field, which are suitable to act as detectors of visual contours of differing size, shape and orientation. The largest have oval dendritic fields, several millimetres long, predominantly orientated along the vertical (dorso-ventral) axis of the optic lobe (Figs. 1 and 2). Each makes contact with the endings of many thousands of optic nerve fibres and therefore they are probably activated only by very large visual contours. Their large axons reach deep into the optic lobe, from which the axons of third order cells reach direct to the first order giant cells. This may therefore be the pathway for escape reactions from large objects, perhaps also for schooling responses to other squids. The cell bodies of these second order visual neurones lie at the base of the inner granule cell layer and their trunks bend over in the second tangential layer of the plexiform zone to make recurrent axons (Figs. 1 and 3).

Cholinesterase of squid optical ganglia.

Optical ganglia of squid (Ommatostrephes sloanei‐pacificus) possess very high cholinesterase activity. Squid cholinesterase is located on the level of layers r2–r3 of plexiform zone in ganglion cortex. The activity of catalytic site of squid cholinesterase was equal to 1.2 × 105 min−1. The hydrolysis rates of acetylcholine, butyrylcholine and acetyl‐β‐methylcholine in the presence of squid cholinestarse were the same (in optimal substrate concentrations). But their Km values were different (the lowest ones for acetylcholine); squid cholinesterase did not hydrolyze benzoylcholine. All organophosphorus inhibitors studied possessed a high activity with regard to squid cholinesterase. It was shown that O‐ethyl‐S(3,3‐dimethyl‐butyl)‐methyl‐thiophosphonate (LG‐56) is a selective inhibitor of squid cholinesterase. Substrate and inhibitor specificity and catalytic activity of squid cholinesterase differed essentially from those of acetylcholinesterase and serum cholinesterase.

The central nervous system ofNautilus

The central nervous system of Nautilus shows greater similarity to that of coleoid cephalopods than appears at first sight. In the area where the three main cords of the nervous system meet there is a region comparable in position to the magnocellular lobe of coleoids, and it contains large cells. It receives some static nerve fibres and is the origin of the nerves of the ocular tentacles. The anterior suboesophageal cord is not a single entity. The brachial nerves and nerves of the hood arise from its anterior part, which is directly continuous with the cerebral cord. The funnel nerves arise from a distinct part, continuous with the magnocellular and palliovisceral regions. If the tentacles are innervated from a region derived from the cerebral cord then they cannot be closely compared with the foot of other molluscs. The cerebral cord shows no clear internal division into lobes, but it is nevertheless organized on a plan recognizably like that of coleoids. Its anterior portion contains large cells and gives rise to the connectives that control the buccal mass. It receives the labial nerves and probably gustatory fibres. In the hinder part of the cerebral cord four regions are recognized. An outer dorsal plexiform zone receives afferents from many sources and perhaps serves to allow responses to combinations of inputs. It is especially developed as lateral cerebral lobes at the entry of the brachial nerve fibres. This zone may be compared with the inferior and superior frontal lobes of Octopus. Fibres pass from the plexiform zone through a layer of small cells to a laminated zone of specialized neuropil. This region corresponds approximately to the vertical lobe of coleoids, but the similarity is not very great. The centre of the cerebral cord contains larger cells, probably providing the output channels to other centres. The ventral portion contains commissural bundles. The olfactory lobes are relatively larger and the optic lobes smaller than in coleoids. Both are lateral continuations of the cerebral cord and have the same basic structure as the latter. The optic nerve fibres do not form a chiasma between the retina and the optic lobe. The optic lobe shows a general similarity to that of coleoids but there is no external granular layer and no peduncle lobe. There is no distinct optic gland but cells that perhaps represent optic gland tissue occur between the optic and cerebral lobes. The statocyst is a simple sack with no signs of macula or crista. Its duct remains open in the adult. The static nerve fibres run partly to the magnocellular lobe, partly to the cerebral cord. The plan of the cerebral cord of Nautilus thus appears as a general sketch of the system that exists in coleoids. The ‘higher’ centres for producing responses from combinations of inputs and perhaps for memory storage are only beginning to emerge from an undivided centre for the reflex control of the operations of feeding. The fact that Nautilus has remained macrosmatic and has poor vision may be connected with the relative simplicity of its higher centres. Nevertheless, its nervous system contains vastly more channels and complex parts than are found in any non-cephalopod mollusc.

Neurobiological studies on cephalopods III. Histochemistry of 24 enzymes in the optic system

1. 1. In four species of cephalopods an investigation was made of the presence of several histochemically localizable enzymes. 2. 2. Among the enzymes studied were the cholinesterases, β-glucuronidase, various dehydrogenases, NADH 2- and NADPH 2-tetrazolium reductases, cytochrome c-oxidase, and several phosphatases. 3. 3. The main sites of hydrolysis of acetylcholine were the retinal plexus, some of the inner layers of the plexiform zone of the deep retina and the fibers of the optic ganglion. In decapods butyrylcholine was split at the same places, but in octopods the outer granular layer of the deep retina and the outer layers of the plexiform zone were the sole sites of hydrolysis in the optic lobe. 4. 4. β-glucoronidase was found in the distal segment of the retina and in the proximal segment in a zone just distal to the nuclei of the recepter cells. No other sites of activity could be demonstrated. 5. 5. NADH 2-terazolium reductase occurred in all parts of the retina, in the optic nerves and in the optic lobe, the highest concentration being found in the plexiform zone. The same is true for the NAD-linked dehydrogenases, of which isocitrate dehydrogenase and α-cerophosphate dehydrogenase are the only enzymes to be found in high amounts. NADPH 2-tetrazolium reductase occurred in the proximal segment of the retina, in the optic nerves and in the optic lobe, again in highest concentration in the plexiform zone. Succinate dehydrogenase shows the same localization as NADH 2-tetrazolium reductase but occurs in essentially smaller amounts. Cytochrome c-oxidase is present in the same localization as NADH 2-tetrazolium reductase. 6. 6. Alkaline and acid phosphatases were found in the retina and in all cells of the optic lobe. No activity could be detected in the nerve fibers or in the capillaries. A phosphatase splitting creatine phosphate at low pH, essentially different from acid phosphatase, has been demonstrated. Adenosine-monophosphatase and adenosinetriphosphatase occur in cells as well as axons of the optic system. The former enzyme is found in the highest concentration in the second radial layer of the plexiform zone, the latter is the only specific phosphatase found in the capillary walls. 7. 7. The main difference between the mammalian nervous system and the cephalopod optic ganglion were: the divergent behavior of true and aspecific cholinesterase towards specific inhibitors, the uniform moede of distribution of the reductases, dehydrogenases and cytochrome oxidase and the occurrence of alkaline phosphatase in the perikarya.

ELECTRON MICROSCOPY OF OPTIC NERVES AND OPTIC LOBES OF OCTOPUS AND ELEDONE.

Each optic nerve contains several bundles of axons. The axons have their surface membranes directly apposed and the bundles lie in troughs of the elongated Schwann cells. The axons have pronounced varicosities along their length. The axons enter the optic lobe and run between the granule cells to synapse in the plexiform zone. The granule cells are small neurons. Their cytoplasmic organelles include endoplasmic reticulum, ribosomes, agranular reticulum and of special interest, oval or spherical bodies with a lamellated cortex and granular medulla. The elongated varicose presynaptic bags of the optic axons contain mitochondria in the proximal region, numerous synaptic vesicles and, sometimes, neurofilaments. Below the mitochondrial zone, synaptic contacts are made with small spines invaginated into the bags. The spines probably originate from the trunks of the granule cells. Tunnel fibres that are probably trunks of the outer granule cells, run through channels in the synaptic bags.

The optic lobes ofOctopus vulgaris

The optic lobes provide a system for coding the visual input, for storing a record of it and for decoding to produce particular motor responses. There are at least three types of optic nerve fibre, ending at different depths in the layered dendritic systems of the plexiform zone. Here the optic nerve fibres meet the branches of at least four types of cell. (1) Centripetal cells passing excitation inwards. The dendrites of these are very long, with fields orientated more often in horizontal and vertical than in other directions. (2) Numerous amacrine cells, with cone-shaped dendritic fields but no determinable axon. (3) Centrifugal cells conducting back to the retina. (4) Commissural fibres from the opposite optic lobe, and other afferents. After section of the optic nerves the plexiform layer of the corresponding part of the optic lobe becomes reduced, but the tangential layers of dendrites remain. There is a reduction in the thickness of the layers of amacrine and other cells and a shrinkage of the whole lobe. Conversely the tangential layers can be degenerated, leaving the optic nerve fibres, by severing the arteries to the optic lobe. The centre of the optic lobe contains cells with spreading dendritic trees of many forms. Some run mainly tangentially, others are radial cones. Those towards the centre send axons to the optic tract. Small multipolar cells accompany the large neurons of the cell islands. About 2 x 107optic nerve fibres visible with the light microscope enter the lobes but only 0-5 x 106, or less, leave in the optic tract, these being distributed to some ten centres in the supraoesophageal lobes. It is suggested that the variety of shapes of the dendritic trees within the optic lobes provides the elements of the coding system by which visual input is classified.

The Plexiform Zone and Shape Recognition in the Octopus

The Plexiform Zone and Shape Recognition in the Octopus