Inhibitory retinal efferents from dopaminergic cells in the optic lobe of the octopus

The neural basis of visual processing and behavior in cephalopods

Coleoid cephalopods (octopuses, squids and cuttlefishes) are the only branch of the animal kingdom outside of vertebrates to have evolved both a large brain and camera-type eyes. They are highly dependent on vision, with the majority of their brain devoted to visual processing. Their excellent vision supports a range of advanced visually guided behaviors, from navigation and prey capture, to the ability to camouflage based on their surroundings. However, their brain organization is radically different from that of vertebrates, as well as other invertebrates, providing a unique opportunity to explore how a novel neural architecture for vision is organized and functions. Relatively few studies have examined the cephalopod visual system using current neuroscience approaches, to the extent that there has not even been a measurement of single-cell receptive fields in their central visual system. Therefore, there remains a tremendous amount that is unknown about the neural basis of vision in these extraordinary animals. Here, we review the existing knowledge of the organization and function of the cephalopod visual system to provide a framework for examining the neural circuits and computational mechanisms mediating their remarkable visual capabilities.

Invaginating Presynaptic Terminals in Neuromuscular Junctions, Photoreceptor Terminals, and Other Synapses of Animals

Typically, presynaptic terminals form a synapse directly on the surface of postsynaptic processes such as dendrite shafts and spines. However, some presynaptic terminals invaginate-entirely or partially-into postsynaptic processes. We survey these invaginating presynaptic terminals in all animals and describe several examples from the central nervous system, including giant fiber systems in invertebrates, and cup-shaped spines, electroreceptor synapses, and some specialized auditory and vestibular nerve terminals in vertebrates. We then examine mechanoreceptors and photoreceptors, concentrating on the complex of pre- and postsynaptic processes found in basal invaginations of the cell. We discuss in detail the role of vertebrate invaginating horizontal cell processes in both chemical and electrical feedback mechanisms. We also discuss the common presence of indenting or invaginating terminals in neuromuscular junctions on muscles of most kinds of animals, and especially discuss those of Drosophila and vertebrates. Finally, we consider broad questions about the advantages of possessing invaginating presynaptic terminals and describe some effects of aging and disease, especially on neuromuscular junctions. We suggest that the invagination is a mechanism that can enhance both chemical and electrical interactions at the synapse.

Dopamine modulates synaptic activity in the optic lobes of cuttlefish, Sepia officinalis

The effects of dopamine on spontaneous excitatory postsynaptic currents (sEPSCs) and inhibitory postsynaptic currents (sIPSCs) in three different classes of neurones within the optic lobe of cuttlefish were investigated using whole-cell voltage clamp techniques in a slice preparation. The neuronal types were centrifugal and amacrine neurones, located in the inner granular cell layer, and medullar interneurones, located within the central medulla of the optic lobes. The results demonstrate that bath application of dopamine (50 microM) reversibly reduced both the frequency and amplitude of sEPSCs and of sIPSCs in these optic lobe neurones. The inhibitory effects of DA were dose-dependent and neither D1- nor D2-like receptors appear to be implicated, but probably D4-like receptors are involved in these actions. By pre-applying tetrodotoxin (TTX, 0.5 microM), to block action potential-dependent EPSCs and IPSCs, it is shown that dopamine has no effect on the amplitude, frequency or decay time constant of the mEPSCs or mIPSCs. The results are the first to identify a specific physiological action of dopamine on cephalopod brain activity, they indicate that this effect is probably presynaptic to the specific classes of cells recorded from, and they provide information on the pharmacological profile of the receptors involved. The widespread inhibitory effect of dopamine on the activity of cuttlefish optic lobe neurones is discussed in the context of comparable data from vertebrate preparations and the actions of other neuromodulators in the cuttlefish brain.

Cholinergic and glutamatergic spontaneous and evoked excitatory postsynaptic currents in optic lobe neurons of cuttlefish, Sepia officinalis

Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded from two different classes of neurons in the optic lobes of the cuttlefish brain and their synaptic activities analyzed and compared. The cell types were as follows: efferent centrifugal neurons, with cell bodies in the inner granule layer and axons projecting to the retina, and interneurons local to the medulla. For both neuronal groups, the sEPSCs reversal potentials were around 0 mV and there were no significant differences in their mean amplitude and rise times. However, the sEPSCs from the centrifugal neurons had a significantly higher frequency and faster decay time constant than those recorded from the medulla. Tetrodotoxin (TTX) reduced the mean frequency of the sEPSCs from both the medulla and centrifugal neurons by 69.66 +/- 4.05% and 57.80 +/- 3.87%, respectively, implying that more than half of these excitatory synaptic inputs were due to action potential-mediated release of neurotransmitter. Pharmacological examination revealed that the centrifugal neurons were driven by spontaneous synaptic inputs mediated by glutamatergic and cholinergic receptors, because co-application of the glutamate antagonist kynurenic acid (KYNA) and the nicotinic antagonist mecamylamine hydrochloride (MCM) resulted in complete blockade of these excitatory inputs. For the medulla neurons, the synaptic inputs were driven by glutamate and other transmitters yet to be identified. Evoked EPSCs (eEPSCs) were recorded from both types of neurons by stimulating the appropriate optic nerve bundles; in centrifugal neurons, the eEPSCs were blocked by co-application of KYNA and MCM, whereas in the medulla neurons, KYNA alone either totally or partially blocked the eEPSCs.

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.

Membrane particles and gap junctions in the retinas of two species of cephalopods, Octopus ocellatus and Sepiella japonica

To study correlation between membrane structure and photoreceptor function, we compared the size and density of intramembrane particles (IMPs) in various membrane compartments of freeze-fractured retinas in a cuttle-fish, Sepiella japonica, and an octopus, Octopus ocellatus. Distribution of gap junctions in the retinas was also examined. Similar results were obtained in the two species. P-faces of both rhabdomeric microvillar membrane and non-rhabdomeric plasma membrane of the apical process were characterized by a random distribution of dense IMPs (ca. 5500-6500/microns2), which showed a unimodal size distribution with a mean diameter of ca. 10 nm. Unlike other invertebrate ocelli, the plasma membrane of the cell body in both the outer and inner segments had significantly denser P-face particles (ca. 7500-8000/microns2) than the rhabdomeric microvillar membrane. The size distribution of IMPs in each part of the membrane was also unimodal, but with a mean diameter of ca. 8 nm. In tangential fractures, each lamella of the myeloid body showed a patchwork of P-faces with irregularly arranged, dense particles and E-faces with orderly patterened granulation. Density and size distribution of the P-face particles in the myeloid membrane resembled those in the rhabdomeric microvillar membrane. The plasma membranes of the supporting cell and the gial cell had relatively sparse P-face particles (ca. 1500-3000/microns2). In addition to the previously reported gap junctions, which connected visual cell inner segments with each other, directly or via collaterals, small gap junctions were found between the visual cell axons and presumed efferent nerve fibres in the plexiform layer. Large-sized gap junctions provided mutual connections for both supporting cells and glial cells. In conclusion, IMPs of 10 nm in mean diameter in the microvillar and non-microvillar parts of the apical process plasma membrane and in the myeloid membrane represent the molecules or their clusters of two photopigments in the cephalopod visual cell, rhodopsin and retinochrome, respectively, and electrical transmission plays a role in visual cell-efferent nerve interactions.