The ultrastructure of a cerebellar analogue in octopus

Cephalopod Brains: An Overview of Current Knowledge to Facilitate Comparison With Vertebrates

Cephalopod and vertebrate neural-systems are often highlighted as a traditional example of convergent evolution. Their large brains, relative to body size, and complexity of sensory-motor systems and behavioral repertoires offer opportunities for comparative analysis. Despite various attempts, questions on how cephalopod 'brains' evolved and to what extent it is possible to identify a vertebrate-equivalence, assuming it exists, remain unanswered. Here, we summarize recent molecular, anatomical and developmental data to explore certain features in the neural organization of cephalopods and vertebrates to investigate to what extent an evolutionary convergence is likely. Furthermore, and based on whole body and brain axes as defined in early-stage embryos using the expression patterns of homeodomain-containing transcription factors and axonal tractography, we describe a critical analysis of cephalopod neural systems showing similarities to the cerebral cortex, thalamus, basal ganglia, midbrain, cerebellum, hypothalamus, brain stem, and spinal cord of vertebrates. Our overall aim is to promote and facilitate further, hypothesis-driven, studies of cephalopod neural systems evolution.

Oxytocin/vasopressin and gonadotropin-releasing hormone from cephalopods to vertebrates

Recent advances in peptide search methods have revealed two peptide systems that have been conserved through metazoan evolution. Members of the oxytocin/vasopressin-superfamily have been identified from protostomian and deuterostomian animals, indicating that the oxytocin/vasopressin hormonal system represents one of the most ancient systems. In most protostomian animals, a single member of the superfamily shares oxytocin-like and vasopressin-like actions. Co-occurrence of two members has been discovered in modern cephalopods, octopus, and cuttlefish. We propose that cephalopods have developed two peptides in the molluscan evolutionary lineage like vertebrates have established two lineages in the oxytocin/vasopressin superfamily. The existence of gonadotropin-releasing hormone (GnRH) in protostomian animals was initially suggested by immunohistochemical analysis using chordate GnRH antibodies. A peptide with structural features similar to those of chordate GnRHs was originally isolated from octopus, and an identical peptide has been characterized from squid and cuttlefish. Novel forms of GnRH-like molecules from other molluscs, an annelid, arthropods, and nematodes demonstrate somewhat conserved structures at the N-terminal regions; but structures of the C-terminal regions critical to gonadotropin-releasing activity are diverse. These findings may be important for the study of the molecular evolution of GnRH in protostomian animals.

The octopus: a model for a comparative analysis of the evolution of learning and memory mechanisms

Comparative analysis of brain function in invertebrates with sophisticated behaviors, such as the octopus, may advance our understanding of the evolution of the neural processes that mediate complex behaviors. Until the last few years, this approach was infeasible due to the lack of neurophysiological tools for testing the neural circuits mediating learning and memory in the brains of octopus and other cephalopods. Now, for the first time, the adaptation of modern neurophysiological methods to the study of the central nervous system of the octopus allows this avenue of research. The emerging results suggest that a convergent evolutionary process has led to the selection of vertebrate-like neural organization and activity-dependent long-term synaptic plasticity. As octopuses and vertebrates are very remote phylogenetically, this convergence suggests the importance of the shared properties for the mediation of learning and memory.

N-methyl-D-aspartate receptor-like immunoreactivity in the brain of Sepia and Octopus

Ionotropic glutamate receptors have been subdivided into N-methyl-D-aspartate (NMDA) and AMPA/kainate classes. NMDA receptor subunit 2A and 2B immunoreactivity is shown to be present in specific regions of the central nervous system (CNS) of the cephalopod molluscs Sepia officinalis and Octopus vulgaris. An antibody that recognizes both mammalian NMDAR2A and NMDAR2B subunits equally was used. SDS-PAGE/Western blot analysis performed on membrane proteins revealed an immunoreactive band at 170 kDa for both species. Immunoreactive bands from both Octopus and Sepia brains disappeared when the antibody was preabsorbed with membrane proteins from rat hippocampus or from their own brains. The same antibody was then used for immunohistochemical staining of serial sections of the CNS to reveal localized specific staining of cell bodies and fibers in several lobes of the brain. Staining was found in lower motor centers, in some higher motor centers, in learning centers, and in the optic lobes. Immunopositivity was also found in the areas of brain that control the activity of the optic gland, a gonadotropic endocrine gland. These findings suggest that glutamate, via NMDA receptors, may be involved as a signaling molecule in motor, learning, visual, and olfactory systems in the cephalopod brain.

Effect of nitric oxide synthase inhibition on the manipulative behaviour of Sepia officinalis

Nitric oxide (NO), produced by nitric oxide synthase (NOS) in brain tissue, is essential for a variety of kinds of learning in vertebrates. In invertebrates, there are clear examples of an association between NO signalling and olfaction, feeding behaviour and learning. The role of NO as a neurotransmitter in the manipulative behaviour of Sepia officinalis was tested. Manipulative behaviour requires extensive chemotactile sensory processing, fine motor control and probably motor learning processes. NADPH-diaphorase activity (a reliable histochemical marker for nitric oxide synthase) was found in sensory epithelia and in the axial nerve cord of the arms. NOS inhibitor injections (L-NAME) produced an increase in the latency of prey paralysis. By placing mechanical constraints on the base of the fifth periopods of the crab, we prevented the cuttlefish from injecting cephalotoxin and, thus, forced it to change injection sites. We showed that L-NAME pretreatment did not affect the flexibility of the manipulative behaviour. The implications of the involvement of NO in the acquisition of chemo-tactile information and in the programming of the motor skills of the manipulative behaviour is discussed.

Nitric oxide synthase (NOS) in the brain of the cephalopod Sepia officinalis

Nitric oxide synthase-like protein (NOS) is shown to be present in specific regions of the central nervous system (CNS) of the cephalopod mollusc Sepia officinalis (cuttlefish). NOS activity, which is Ca(2+)/calmodulin-dependent, was determined by measuring the conversion of L-[(14)C]arginine in L-[(14)C]citrulline. The partially purified NOS from brain and optic lobes exhibited on SDS-PAGE a band at 150 kDa that was immunolabelled by antibodies raised against the synthetic peptide corresponding to the amino acids 1,414-1,429 of the C-terminus of rat nNOS. This same antibody was then used for immunohistochemical staining of serial sections of the cuttlefish CNS to reveal localized specific staining of cell bodies and fibers in several lobes of the brain. Staining was found in many lower motor centers, including cells and fibers of the inferior and superior buccal lobes (feeding centers); in some higher motor centers (anterior basal and peduncle lobes); in learning centers (vertical, subvertical, and superior frontal lobes); and in the visual system [retina and deep retina (optic lobe)]. Immunopositivity was also found in the olfactory lobe and organ and in the sucker epithelium. These findings suggest that nitric oxide (NO) may be involved as a signaling molecule in feeding, motor, learning, visual, and olfactory systems in the cuttlefish brain. The presence of NOS in the cephalopod "cerebellum" and learning centers is discussed in the context of the vertebrate CNS.

Galanin-immunoreactive neuronal system and colocalization with serotonin in the optic lobe and peduncle complex of the octopus (Octopus vulgaris)

Immunohistochemical techniques were used to investigate the distribution of galanin-like immunoreactivity and colocalization with serotonin (5-HT) in the optic lobe and peduncle complex of the octopus, Octopus vulgaris. Galanin immunoreactive (Gal-IR) fibers, but not cells, were seen in the plexiform layer of the optic lobe cortex. Gal-IR cells were scattered in the cell-islands of the optic lobe medulla and Gal-IR varicose fibers were observed to be abundant in the neuropil surrounding the islands. All Gal-IR cells were immunoreactive for 5-HT, and a few cells showed only 5-HT-like immunoreactivity. In the peduncle lobe, no Gal-IR cells were seen in the basal zone or spine, but in the basal zone, many Gal-IR fibers were seen. In the anterior olfactory lobule, only a few pyramidal Gal-IR cells were observed in the cell layer, and their apical processes were traced to the central neuropil. In the median olfactory lobule, ovoid Gal-IR cells were scattered in the peripheral cell layer. All Gal-IR cells in the anterior and median olfactory lobules showed 5-HT-like immunoreactivity. In the posterior olfactory lobule, ovoid and triangular Gal-IR cells were scattered in the cell layer. Some of them showed 5-HT-like immunoreactivity. Western blot analysis indicated an Gal-IR band at approximately 15.4 kDa. These results suggest the association of galanin-like substance and 5-HT with the visual system of octopus and that the main form of the octopus galanin might have a different molecular weight from vertebrate galanins.

NADPH-diaphorase containing cells and fibers in the central nervous system of squid, Loligo bleekeri keferstein

Distribution of NADPH-diaphorase in the central nervous system of squid was determined using histochemical technique. We found NADPH-diaphorase positive cell bodies and fibers both in the optic and the posterior anterior lobe and fibers in the peduncle lobe. These results clarify the biochemical similarity between two structurally similar organs of invertebrate and vertebrate, the peduncle lobe and the anterior basal lobe, and the cerebellum. NADPH-diaphorase positive fibers innervated the inner granule layer and the outer plexiform layer of the outer cortex of the optic lobe. This is in good agreement with avian centrifugal projection from isthmo-optic nucleus to retina where nitric oxide synthase is known to be contained. There may be at least two distinct neural systems, the motor control system and the visual information processing system, which use nitric oxide as a transmitter or modulator in the squid central nervous system.

Behavioural and neurohistological changes in aging Sepia

The life cycle of the cuttlefish in the English Channel is characterized by a succession of homogeneous population cohorts. These conditions provide an excellent opportunity for the study of aging in this cephalopod. In a first longitudinal study, we considered the oldest animals and compared their success rates at the first capture attempt. During the first weeks of the study, the results remained constant and then, during the weeks immediately preceding the natural death, a dramatic drop was observed. This deterioration may be due to defects of visuomotor coordination. In a second study, we used an associative learning protocol with negative reinforcement and the performances of young and old animals were compared. The most striking results showed that the performances of the oldest animals during the retention test were very mediocre. Such results suggest that the long-term memory process is affected. Finally, a modification of the Fink-Heimer silver stain enabled us to draw a map of spontaneous terminal degeneration in the central nervous system of the oldest animals. The structures which are characterized by the presence of multimodal inputs (the spine of the peduncle lobe and the basal lobes) present the most obvious signs of degeneration.

The anterior basal lobe and control of prey-capture in the cuttlefish (Sepia officinalis)

The predatory behaviour of the cuttlefish comprises several stages: prey-detection, orientation, translation and prey-seizing. In this neuroethological study, lesions to the anterior basal lobe were made by an electrolytic method and the animals were allowed to attack their prey in an unrestricted way so that motor defects, functional recovery and the emergence of new adaptative behavioural strategies could be studied. Lesions to the central region of the anterior basal lobe suppress the orientating behaviour, thus only prey situated in the frontal visual field can be seized. Less extensive lesions in this region are associated with similar defects. Without head orientation, the cuttlefish still rotates with its fins. This rotation, however, is usually underestimated, tentacular ejection thus missing the prey. Dorsal lesions cause an underestimation of tentacular strike often associated with defects in maintaining ocular convergence. These results demonstrate the heterogenous function of the anterior basal lobe and its complex role in the control of predatory behaviour.

Distribution of immunoreactive somatostatin (ISRIF) in the nervous system of the squid, Loligo pealei

Somatostatin (SRIF) is a neuropeptide with a widespread distribution in the mammalian CNS. In the present study we have examined the distribution of immunoreactive-like SRIF (ISRIF)-containing elements in the nervous system of the cephalopod mollusk Loligo pealei, or the Woods Hole squid. ISRIF was localized by light immunocytochemistry in sections of the squid-optic lobe, circumesophageal ganglia-and in stellate ganglion. In the optic lobe, ISRIF neurons were found in the internal granule cell layer and medulla and immunoreactive fibers were seen throughout the lobe and in the optic tract but were absent from the optic nerve, i.e., the projection between the retina and optic lobe. In the supraesophageal complex, ISRIF neurons were found in all lobes, but primarily in the vertical, subvertical, and frontal. In the subesophageal ganglion, ISRIF neurons were seen mainly following unilateral pallial nerve lesions; these neurons were primarily small-to-medium sized. ISRIF fibers were seen in many of the nerves exiting from the brain and in nerves extending between the sub- and supra-esophageal ganglia. In the stellate ganglion, ISRIF was present in many neurons as well as in a plexus of fibers within the ganglion; the peptide was absent from the second-order fibers and the giant axon. The data suggest that a molecule immunologically similar to vertebrate SRIF may be a major transmitter/modulator in this invertebrate. These results provide a foundation for further studies to evaluate the role of this molecule.

The central afferent and efferent organization of the gravity receptor system of the statocyst of Octopus vulgaris

In Octopus vulgaris, the projections of the afferent fibers from, and the locations of cell bodies of the efferent fibers of, the nerve innervating and macula (statocyst gravity receptor epithelium) were studied within the CNS using iontophoretic whole nerve injection of either cobalt chloride or Lucifer Yellow CH.13 Afferent fibers from the macula nerve project to subesophageal, periesophageal and supraesophageal areas of the brain. Large numbers of such fibers were found in the subeosophageal lateral pedal and posterior lateral pedal lobes, palliovisceral lobe, and the magnocellular commissure. Afferent fibers were also found in the periesophageal ventral and dorsal magnocellular lobes. Supraesophageal macular nerve afferent projections were seen to the peduncle lobe and the ipsilateral median basal lobe. There is evidence for two different macular nerve afferent projections to the contralateral median basal lobe, via and suprapedal commissure and the macula-to-contralateral-median-basal-lobe tract. Evidence is presented for the location of at least some of the macula's efferent fiber's cell bodies in the lateral and posterior lateral pedal lobes, and magnocellular lobes. The differences between the results obtained here with the two different staining methods are discussed. The results imply that processing of static information occurs in many areas of the Octopus CNS, and is much more complex than previously thought. Some of the possible physiological consequences are considered.

Evidence for visual mapping in the peduncle lobe of octopus

The first use of horseradish peroxidase in the central nervous system of a cephalopod revealed apparent topographic connections between optic and peduncle lobes in octopus. Optic lobe efferent fibers within the peduncle lobe neuropil and labeled cells in the peduncle lobe exhibited alignment along the anteroposterior and dorsoventral axes of the peduncle lobe. The location and number of labeled peduncle lobe cells depended on the extent of the injection site in the optic lobe.