On the motor projection of the stellate ganglion in Octopus vulgaris

A brain atlas for the camouflaging dwarf cuttlefish, Sepia bandensis

The coleoid cephalopods (cuttlefish, octopus, and squid) are a group of soft-bodied marine mollusks that exhibit an array of interesting biological phenomena, including dynamic camouflage, complex social behaviors, prehensile regenerating arms, and large brains capable of learning, memory, and problem-solving.^{1,2,3,4,5,6,7,8,9,10} The dwarf cuttlefish, Sepia bandensis, is a promising model cephalopod species due to its small size, substantial egg production, short generation time, and dynamic social and camouflage behaviors.¹¹ Cuttlefish dynamically camouflage to their surroundings by changing the color, pattern, and texture of their skin. Camouflage is optically driven and is achieved by expanding and contracting hundreds of thousands of pigment-filled saccules (chromatophores) in the skin, which are controlled by motor neurons emanating from the brain. We generated a dwarf cuttlefish brain atlas using magnetic resonance imaging (MRI), deep learning, and histology, and we built an interactive web tool (https://www.cuttlebase.org/) to host the data. Guided by observations in other cephalopods,^{12,13,14,15,16,17,18,19,20} we identified 32 brain lobes, including two large optic lobes (75% the total volume of the brain), chromatophore lobes whose motor neurons directly innervate the chromatophores of the color-changing skin, and a vertical lobe that has been implicated in learning and memory. The brain largely conforms to the anatomy observed in other Sepia species and provides a valuable tool for exploring the neural basis of behavior in the experimentally facile dwarf cuttlefish.

Neural pathways in the pallial nerve and arm nerve cord revealed by neurobiotin backfilling in the cephalopod mollusk Octopus vulgaris

Here, we report the findings after application of neurobiotin tracing to pallial and stellar nerves in the mantle of the cephalopod mollusk Octopus vulgaris and to the axial nerve cord in its arm. Neurobiotin backfilling is a known technique in other molluscs, but it is applied to octopus for the first time to be best of our knowledge. Different neural tracing techniques have been carried out in cephalopods to study the intricate neural connectivity of their nervous system, but mapping the nervous connections in this taxon is still incomplete, mainly due to the absence of a reliable tracing method allowing whole-mount imaging. In our experiments, neurobiotin backfilling allowed: (1) imaging of large/thick samples (larger than 2 mm) through optical clearing; (2) additional application of immunohistochemistry on the backfilled tissues, allowing identification of neural structures by coupling of a specific antibody. This work opens a series of future studies aimed to the identification of the neural diagram and connectome of octopus nervous system.

Peripheral innervation patterns and central distribution of fin chromatophore motoneurons in the cuttlefish Sepia officinalis

Body patterning behavior in unshelled cephalopod molluscs such as squid, octopuses, and cuttlefish is the ability of these animals to create complex patterns on their skin. This behavior is generated primarily by chromatophores, pigment-containing organs that are directly innervated by central motoneurons. The present study focuses on innervation patterns and location of chromatophore motoneurons in the European cuttlefish Sepia officinalis, specifically those motoneurons that control chromatophores of the fin. The fin is known to be innervated by the large, branching fin nerve. This study further characterizes the innervation of fin chromatophores by the fin nerve, generates a reference system for the location of fin nerve branches across individuals, and localizes the neurons whose axons innervate fin chromatophores through the fin nerve. Data from extracellular stimulation of fin nerve branches in intact animals demonstrate topographic innervation of fin chromatophores, while retrograde labeling data reveal the posterior subesophageal mass of the brain as the primary location of fin chromatophore motoneurons.

Cephalopod chromatophores: neurobiology and natural history

The chromatophores of cephalopods differ fundamentally from those of other animals: they are neuromuscular organs rather than cells and are not controlled hormonally. They constitute a unique motor system that operates upon the environment without applying any force to it. Each chromatophore organ comprises an elastic sacculus containing pigment, to which is attached a set of obliquely striated radial muscles, each with its nerves and glia. When excited the muscles contract, expanding the chromatophore; when they relax, energy stored in the elastic sacculus retracts it. The physiology and pharmacology of the chromatophore nerves and muscles of loliginid squids are discussed in detail. Attention is drawn to the multiple innervation of dorsal mantle chromatophores, of crucial importance in pattern generation. The size and density of the chromatophores varies according to habit and lifestyle. Differently coloured chromatophores are distributed precisely with respect to each other, and to reflecting structures beneath them. Some of the rules for establishing this exact arrangement have been elucidated by ontogenetic studies. The chromatophores are not innervated uniformly: specific nerve fibres innervate groups of chromatophores within the fixed, morphological array, producing 'physiological units' expressed as visible 'chromatomotor fields'. The chromatophores are controlled by a set of lobes in the brain organized hierarchically. At the highest level, the optic lobes, acting largely on visual information, select specific motor programmes (i.e. body patterns); at the lowest level, motoneurons in the chromatophore lobes execute the programmes, their activity or inactivity producing the patterning seen in the skin. In Octopus vulgaris there are over half a million neurons in the chromatophore lobes, and receptors for all the classical neurotransmitters are present, different transmitters being used to activate (or inhibit) the different colour classes of chromatophore motoneurons. A detailed understanding of the way in which the brain controls body patterning still eludes us: the entire system apparently operates without feedback, visual or proprioceptive. The gross appearance of a cephalopod is termed its body pattern. This comprises a number of components, made up of several units, which in turn contains many elements: the chromatophores themselves and also reflecting cells and skin muscles. Neural control of the chromatophores enables a cephalopod to change its appearance almost instantaneously, a key feature in some escape behaviours and during agonistic signalling. Equally important, it also enables them to generate the discrete patterns so essential for camouflage or for signalling. The primary function of the chromatophores is camouflage. They are used to match the brightness of the background and to produce components that help the animal achieve general resemblance to the substrate or break up the body's outline. Because the chromatophores are neurally controlled an individual can, at any moment, select and exhibit one particular body pattern out of many. Such rapid neural polymorphism ('polyphenism') may hinder search-image formation by predators. Another function of the chromatophores is communication. Intraspecific signalling is well documented in several inshore species, and interspecific signalling, using ancient, highly conserved patterns, is also widespread. Neurally controlled chromatophores lend themselves supremely well to communication, allowing rapid, finely graded and bilateral signalling.

Organization of the motor neuron components of the pallial nerve in octopus

Following horseradish peroxidase (HRP) and cobalt (CO2+) application to the pallial nerve of two species of octopus, neurons that control various aspects of mantle behaviors were located in several lobes of the three main regions of the central brain. In the subesophageal region, cells were labeled in the anterior chromatophore, posterior chromatophore, palliovisceral, pedal and vasomotor lobes; in the superior buccal lobe of the supraesophageal region; and in the magnocellular lobe of the periesophageal region. Localization of labeled cells in and around both the anterior and posterior chromatophore lobes suggests a modification of the idea that a single functional area is solely located in an anatomically defined brain lobe of a cephalopod.

Cobalt and horseradish peroxidase tracer studies in the stellate ganglion of octopus

The connectivity of the stellate ganglion of Octopus was investigated using cobalt and horseradish peroxidase (HRP) tracers applied to severed preganglionic and postganglionic nerves. Centripetal cells, defined as cells in the ganglion which send their axons back toward the brain, were discovered by both methods. Application of HRP to the pallial (preganglionic) nerve labeled the preganglionic fibers which enter the ganglion and spread widely within the neuropil. Application of either tracer to a single stellar (postganglionic) nerve labeled motor neuron cell bodies spread over 1/4 to 1/3 of the ganglion. Since there are 25-40 such stellar nerves per ganglion, it may be inferred that there is only a limited topographic relationship between the location of motor neuron cell bodies in the stellate ganglion and the segment of mantle muscle which they innervate. The axon and many side branches of these motor neurons were also labeled. The smooth, diffuse form of HRP labeling produced in pre- and portganglionic axotomized neurons in Oct0pus is similar in appearance to that produced in damaged mammalian neurons.

Retrograde intraaxonal transport of horseradish peroxidase by neurons in octopus

While retrograde axonal transport is the basis of a widely used neuroanatomical method, it has been rigorously demonstrated in vivo only in a few vertebrate species and not yet in an invertebrate. Evidence is presented that motor neurons of the octopus stellate ganglion are capable of retrograde intraaxonal transport of horeseradish peroxidase. This demonstration shows that retrograde transport occurs in widely divergent groups of animals, and may be a general property of neurons.