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Chuan-Chin Chiao and Roger T. Hanlon
Visual camouflage change is a hallmark of octopus, squid, and cuttlefish and serves as their primary defense against predators. They can change their total body appearance in less than a second due to one principal feature: every aspect of this sensorimotor system is neurally refined for speed. Cephalopods live in visually complex environments such as coral reefs and kelp forests and use their visual perception of backgrounds to rapidly decide which camouflage pattern to deploy. Counterintuitively, cuttlefish have evolved a small number of pattern designs to achieve camouflage: Uniform, Mottle, and Disruptive, each with variation. The expression of these body patterns is based on several fundamental scene features. In cuttlefish, there appear to be several “visual assessment shortcuts” that enable camouflage patterning change in as little as 125 milliseconds. Neural control of the dynamic body patterning of cephalopods appears to be organized hierarchically via a set of lobes within the brain, including the optic lobes, the lateral basal lobes, and the anterior/posterior chromatophore lobes. The motor output of the central nervous system (CNS) in terms of the skin patterns that are produced is under sophisticated neural control of chromatophores, iridophores, and three-dimensional skin papillae. Moreover, arm postures and skin papillae are also regulated visually for additional aspects of concealment. This coloration system, often referred to as rapid neural polyphenism, is unique in the animal kingdom and can be explained and interpreted in the context of sensory and behavioral ecology.
Mindaugas Mitkus, Simon Potier, Graham R. Martin, Olivier Duriez, and Almut Kelber
Diurnal raptors (birds of the orders Accipitriformes and Falconiformes), renowned for their extraordinarily sharp eyesight, have fascinated humans for centuries. The high visual acuity in some raptor species is possible due to their large eyes, both in relative and absolute terms, and a high density of cone photoreceptors. Some large raptors, such as wedge-tailed eagles and the Old World vultures, have visual acuities twice as high as humans and six times as high as ostriches—the animals with the largest terrestrial eyes. The raptor retina has rods, double cones, and four spectral types of single cones. The highest density of single cones occurs in one or two specialized retinal regions: the foveae, where, at least in some species, rods and double cones are absent. The deep central fovea allows for the highest acuity in the lateral visual field that is probably used for detecting prey from a large distance. Pursuit-hunting raptors have a second, shallower, temporal fovea that allows for sharp vision in the frontal field of view. Scavenging carrion eaters do not possess a temporal fovea that may indicate different needs in foraging behavior. Moreover, pursuit-hunting and scavenging raptors also differ in configuration of visual fields, with a more extensive field of view in scavengers.
The eyes of diurnal raptors, unlike those of most other birds, are not very sensitive to ultraviolet light, which is strongly absorbed by their cornea and lens. As a result of the low density of rods, and the narrow and densely packed single cones in the central fovea, the visual performance of diurnal raptors drops dramatically as light levels decrease. These and other visual properties underpin prey detection and pursuit and show how these birds’ vision is adapted to make them successful diurnal predators.
Jimena Perez-Sanchez and Yves De Koninck
One of the most remarkable properties of neural circuits is the ability to restructure their synaptic connections throughout life. This synaptic plasticity allows neurons to structurally reorganize and adapt their function in response to experience. Among the multiple mechanisms that can modulate this property is synaptic inhibition by gamma-Aminobutyric acid (GABA) and/or glycine ionotropic receptors, which allow the flow of chloride and bicarbonate ions through the membrane. Neurons rely upon tight regulation of intracellular chloride for efficient inhibition through these receptors. The maintenance of chloride gradients is important not only to determine the strength of synaptic inhibition but also to determine its nature. Indeed, this inhibition can be hyperpolarizing or depolarizing, or with no outright change in the membrane potential. Despite the fact that membrane depolarization is commonly associated with excitation, depolarizing GABA/glycine can also produce inhibition, thereby highlighting the dual action of these neurotransmitters. Several considerations must be taken into account in order to allow depolarizing GABA/glycine responses to be excitatory. On the other hand, chloride homeostasis is never steady-state and even small changes of chloride across the membrane can impact the strength of inhibition. This dynamic effect has a direct impact on neuronal excitability and makes its regulation by changes in chloride gradients a highly tunable mechanism. Furthermore, increased excitability may also open a window for system refinement changes, such as synaptic plasticity. Indeed, the regulation of chloride homeostasis may underlie periods of enhanced plasticity, such as during early development. Finally, disruption of chloride gradients arises as a hub for pathology, which is evidenced in multiple disorders in the central nervous system.
Daniel J. Bernard, Yining Li, Chirine Toufaily, and Gauthier Schang
The gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are glycoproteins produced by gonadotrope cells of the anterior pituitary gland. The two hormones act on somatic cells of the gonads in both males and females to regulate fundamental aspects of reproductive physiology, including gametogenesis and steroidogenesis. In males, LH stimulates testosterone production and sperm maturation. FSH also regulates spermatogenesis, though the importance of the hormone in this process differs across species. In females, FSH stimulates ovarian follicle maturation. Follicles are structures composed of oocytes surrounded by two types of somatic cells, granulosa and theca cells. FSH stimulates granulosa cells to proliferate and to increase their production of the aromatase enzyme. LH stimulates theca cells to make androgens, which are converted into estrogens by aromatase in granulosa cells. A surge of LH also stimulates ovulation of mature follicles.
Gonadotropin-releasing hormone (GnRH) from the brain is the principal stimulator of gonadotropin synthesis and secretion from the pituitary. The sex steroids (androgens and estrogens) that are produced by the gonads in response to the gonadotropins feedback to the brain and pituitary gland. In the brain, these hormones usually slow the release of GnRH through a process called negative feedback, which in turn leads to decreases in FSH and LH. The steroids also modulate the sensitivity of the pituitary to GnRH in addition to directly regulating expression of the genes that encode the gonadotropin subunits. These effects are gene- and species-specific. In females, estrogens also have positive feedback actions in the brain and pituitary in a reproductive cycle stage-dependent manner. This positive feedback promotes GnRH and LH release, leading to the surge of LH that triggers ovulation.
The gonadotropins are dimeric proteins. FSH and LH share a common α-subunit but have hormone-specific subunits, FSHβ and LHβ. The β subunits provide a means for differential regulation and action of the two hormones. In the case of FSH, there is a second gonadal feedback system that specifically regulates the FSHβ subunit. The gonads produce proteins in the transforming growth factor β (TGFβ) family called inhibins, which come in two forms (inhibin A and inhibin B). The ovary produces both inhibins whereas the testes make inhibin B alone. Inhibins selectively suppress FSH synthesis and secretion, without affecting LH. The pituitary produces additional TGFβ proteins called activins, which are structurally related to inhibins. Activins, however, stimulate FSH synthesis by promoting transcription of the FSHβ subunit gene. Inhibins act as competitive receptor antagonists, binding to activin receptors and blocking activin action, and thereby leading to decreases in FSH.
Together, GnRH, sex steroids, activins, and inhibins modulate and coordinate gonadotropin production and action to promote proper gonadal function and fertility.
Dayna L. Averitt, Rebecca S. Hornung, and Anne Z. Murphy
The modulatory influence of sex hormones on acute pain, chronic pain disorders, and pain management has been reported for over seven decades. The effect of hormones on pain is clearly evidenced by the multitude of chronic pain disorders that are more common in women, such as headache and migraine, temporomandibular joint disorder, irritable bowel syndrome, chronic pelvic pain, fibromyalgia, rheumatoid arthritis, and osteoarthritis. Several of these pain disorders also fluctuate in pain intensity over the menstrual cycle, including headache and migraine and temporomandibular joint disorder. The sex steroid hormones (estrogen, progesterone, and testosterone) as well as some peptide hormones (prolactin, oxytocin, and vasopressin) have been linked to pain by both clinical and preclinical research. Progesterone and testosterone are widely accepted as having protective effects against pain, while the literature on estrogen reports both exacerbation and attenuation of pain. Prolactin is reported to trigger pain, while oxytocin and vasopressin have analgesic properties in both sexes. Only in the last two decades have neuroscientists begun to unravel the complex anatomical and molecular mechanisms underlying the direct effects of sex hormones and mechanisms have been reported in both the central and peripheral nervous systems. Mechanisms include directly or indirectly targeting receptors and ion channels on sensory neurons, activating pain excitatory or pain inhibitory centers in the brain, and reducing inflammatory mediators. Despite recent progress, there remains significant controversy and challenges in the field and the seemingly pleiotropic role estrogen plays on pain remains ambiguous. Current knowledge of the effects of sex hormones on pain has led to the burgeoning of gender-based medicine, and gaining further insight will lead to much needed improvement in pain management in women.
Thomas F. Mathejczyk and Mathias F. Wernet
Evolution has produced vast morphological and behavioral diversity amongst insects, including very successful adaptations to a diverse range of ecological niches spanning the invasion of the sky by flying insects, the crawling lifestyle on (or below) the earth, and the (semi-)aquatic life on (or below) the water surface. Developing the ability to extract a maximal amount of useful information from their environment was crucial for ensuring the survival of many insect species. Navigating insects rely heavily on a combination of different visual and non-visual cues to reliably orient under a wide spectrum of environmental conditions while avoiding predators. The pattern of linearly polarized skylight that results from scattering of sunlight in the atmosphere is one important navigational cue that many insects can detect. Here we summarize progress made toward understanding how different insect species sense polarized light. First, we present behavioral studies with “true” insect navigators (central-place foragers, like honeybees or desert ants), as well as insects that rely on polarized light to improve more “basic” orientation skills (like dung beetles). Second, we provide an overview over the anatomical basis of the polarized light detection system that these insects use, as well as the underlying neural circuitry. Third, we emphasize the importance of physiological studies (electrophysiology, as well as genetically encoded activity indicators, in Drosophila) for understanding both the structure and function of polarized light circuitry in the insect brain. We also discuss the importance of an alternative source of polarized light that can be detected by many insects: linearly polarized light reflected off shiny surfaces like water represents an important environmental factor, yet the anatomy and physiology of underlying circuits remain incompletely understood.
Mathew H. Evans, Michaela S.E. Loft, Dario Campagner, and Rasmus S. Petersen
Whiskers (vibrissae) are prominent on the snout of many mammals, both terrestrial and aquatic. The defining feature of whiskers is that they are rooted in large follicles with dense sensory innervation, surrounded by doughnut-shaped blood sinuses. Some species, including rats and mice, have elaborate muscular control of their whiskers and explore their environment by making rhythmic back-and-forth “whisking” movements. Whisking movements are purposefully modulated according to specific behavioral goals (“active sensing”). The basic whisking rhythm is controlled by a premotor complex in the intermediate reticular formation.
Primary whisker neurons (PWNs), with cell bodies in the trigeminal ganglion, innervate several classes of mechanoreceptive nerve endings in the whisker follicle. Mechanotransduction involving Piezo2 ion channels establishes the fundamental physical signals that the whiskers communicate to the brain. PWN spikes are triggered by mechanical forces associated with both the whisking motion itself and whisker-object contact. Whisking is associated with inertial and muscle contraction forces that drive PWN activity. Whisker-object contact causes whiskers to bend, and PWN activity is driven primarily by the associated rotatory force (“bending moment”).
Sensory signals from the PWNs are routed to many parts of the hindbrain, midbrain, and forebrain. Parallel ascending pathways transmit information about whisker forces to sensorimotor cortex. At each brainstem, thalamic, and cortical level of these pathways, there are one or more maps of the whisker array, consisting of cell clusters (“barrels” in the primary somatosensory cortex) whose spatial arrangement precisely mirrors that of the whiskers on the snout. However, the overall architecture of the whisker-responsive regions of the brain system is best characterized by multilevel sensory-motor feedback loops. Its intriguing biology, in combination with advantageous properties as a model sensory system, has made the whisker system the platform for seminal insights into brain function.
Sabra L. Klein and Jaclyn M. Schwarz
Sex is a biological variable that affects immune responses to both self and foreign antigens (e.g., microbial infections) in the central nervous system (CNS) as well as in peripheral organs. The sex of an individual is defined by the differential determination of the sex chromosomes, the organization of the reproductive organs, and the subsequent sex steroid hormone levels in males and females. Sex is distinct from gender, which includes self-identification as being a male or female as well as behaviors and activities that are determined by society or culture in humans. Male and female differences in immunological responses may be influenced by both sex and gender, with sex contributing to the physiological and anatomical differences that influence exposure, recognition, clearance, and even transmission of microbes in males and females. By contrast, gender may reflect behaviors that influence exposure to microbes, access to health care, or health-seeking behaviors that indirectly affect the course of infection in males and females. Though both sex and gender influence the immune response, the focus of this article is the biological factors that influence immunological differences between the sexes in both the CNS and peripheral tissues to alter the course of diseases across the life span.
Understanding of the brain mechanisms regulating reproductive behaviors in female laboratory animals has been aided greatly by our knowledge of estrogen receptors in the brain. Hypothalamic neurons that express the gene for estrogen receptor-alpha regulate activity in the neural circuit for the simplest female reproductive response, lordosis behavior. In turn, many of the neurotransmitter inputs to the critical hypothalamic neurons have been studied using electrophysiological and neurochemical techniques. The upshot of all of these studies is that lordosis behavior presents the best understood set of mechanisms for any mammalian behavior.
Jacques Balthazart and Gregory F. Ball
It is well established that testosterone from testicular origin plays a critical role in the activation of male sexual behavior in most, if not all, vertebrate species. These effects take place to a large extent in the preoptic area although other brain sites are obviously also implicated. In its target areas, testosterone is actively metabolized either into estrogenic and androgenic steroids that have specific behavioral effects or into inactive metabolites. These transformations either amplify the behavioral activity of testosterone or, alternatively, metabolism to an inactive compound dissipates any biological effect. Androgens and estrogens then bind to nuclear receptors that modulate the transcription of specific genes. This process is controlled by a variety of co-activators and co-repressors that, respectively, enhance or inhibit these transcriptional processes. In addition, recent work has shown that the production of estrogens by brain aromatase can be modulated within minutes by changes in neural activity and that these rapid changes in neuroestrogen production impact sexual behavior, in particular sexual motivation within the same time frame. Estrogens thus affect specific aspects of male sexual behavior in two different time frames via two types of mechanisms that are completely different. Multiple questions remain open concerning the cellular brain mechanisms that mediate testosterone action on male sexual behavior.
Yeonjoo Yoo and Fabrizio Gabbiani
Computational modeling is essential to understand how the complex dendritic structure and membrane properties of a neuron process input signals to generate output signals. Compartmental models describe how inputs, such as synaptic currents, affect a neuron’s membrane potential and produce outputs, such as action potentials, by converting membrane properties into the components of an electrical circuit. The simplest such model consists of a single compartment with a leakage conductance which represents a neuron having spatially uniform membrane potential and a constant conductance summarizing the combined effect of every ion flowing across the neuron’s membrane. The Hodgkin-Huxley model introduces two additional active channels; the sodium channel and the delayed rectifier potassium channel whose associated conductances change depending on the membrane potential and that are described by an additional set of three nonlinear differential equations. Since its conception in 1952, many kinds of active channels have been discovered with a variety of characteristics that can successfully be modeled within the same framework. As the membrane potential varies spatially in a neuron, the next refinement consists in describing a neuron as an electric cable to account for membrane potential attenuation and signal propagation along dendritic or axonal processes. A discrete version of the cable equation results in compartments with possibly different properties, such as different types of ion channels or spatially varying maximum conductances to model changes in channel densities. Branching neural processes such as dendrites can be modeled with the cable equation by considering the junctions of cables with different radii and electrical properties. Single neuron computational models are used to investigate a variety of topics and reveal insights that cannot be evidenced directly by experimental observation. Studies on action potential initiation and on synaptic integration provide prototypical examples illustrating why computational models are essential. Modeling action potential initiation constrains the localization and density of channels required to reproduce experimental observations, while modeling synaptic integration sheds light on the interaction between the morphological and physiological characteristics of dendrites. Finally, reduced compartmental models demonstrate how a simplified morphological structure supplemented by a small number of ion channel-related variables can provide clear explanations about complex intracellular membrane potential dynamics.
Corinna Darian-Smith and Karen Fisher
Spinal cord injury (SCI) affects well over a million people in the United States alone, and its personal and societal costs are huge. This article provides a current overview of the organization of somatosensory and motor pathways, in the context of hand/paw function in nonhuman primate and rodent models of SCI. Despite decades of basic research and clinical trials, therapeutic options remain limited. This is largely due to the fact that (i) spinal cord structure and function is very complex and still poorly understood, (ii) there are many species differences which can make translation from the rodent to primate difficult, and (iii) we are still some way from determining the detailed multilevel pathway responses affecting recovery. There has also been little focus, until recently, on the sensory pathways involved in SCI and recovery, which are so critical to hand function and the recovery process. The potential for recovery in any individual depends on many factors, including the location and size of the injury, the extent of sparing of fiber tracts, and the post-injury inflammatory response. There is also a progression of change over the first weeks and months that must be taken into account when assessing recovery. There are currently no good biomarkers of recovery, and while axon terminal sprouting is frequently used in the experimental setting as an indicator of circuit remodeling and “recovery,” the correlation between sprouting and functional recovery deserves scrutiny.
Andrew J. Parker
Humans and some animals can use their two eyes in cooperation to detect and discriminate parts of the visual scene based on depth. Owing to the horizontal separation of the eyes, each eye obtains a slightly different view of the scene in front of the head. These small differences are processed by the nervous system to generate a sense of binocular depth. As humans, we experience an impression of solidity that is fully three-dimensional; this impression is called stereopsis and is what we appreciate when we watch a 3D movie or look into a stereoscopic viewer. While the basic perceptual phenomena of stereoscopic vision have been known for some time, it is mainly within the last 50 years that we have gained an understanding of how the nervous system delivers this sense of depth. This period of research began with the identification of neuronal signals for binocular depth in the primary visual cortex. Building on that finding, subsequent work has traced the signaling pathways for binocular stereoscopic depth forward into extrastriate cortex and further on into cortical areas concerning with sensorimotor integration. Within these pathways, neurons acquire sensitivity to more complex, higher order aspects of stereoscopic depth. Signals relating to the relative depth of visual features can be identified in the extrastriate cortex, which is a form of selectivity not found in the primary visual cortex. Over the same time period, knowledge of the organization of binocular vision in animals that inhabit a wide diversity of ecological niches has substantially increased. The implications of these findings for developmental and adult plasticity of the visual nervous system and onset of the clinical condition of amblyopia are explored in this article. Amblyopic vision is associated with a cluster of different visual and oculomotor symptoms, but the loss of high-quality stereoscopic depth performance is one of the consistent clinical features. Understanding where and how those losses occur in the visual brain is an important goal of current research, for both scientific and clinical reasons.
Alyssa L. Pedersen and Colin J. Saldanha
Given the profound influence of steroids on the organization and activation of the vertebrate central nervous system (CNS), it is perhaps not surprising that these molecules are involved in processes that restructure the cytoarchitecture of the brain. This includes processes such as neurogenesis and the connectivity of neural circuits. In the last 30 years or so, we have learned that the adult vertebrate brain is far from static; it responds to changes in androgens and estrogens, with dramatic alterations in structure and function. Some of these changes have been directly linked to behavior, including sex, social dominance, communication, and memory. Perhaps the most dramatic levels of neuroplasticity are observed in teleosts, where circulating and centrally derived steroids can affect several end points, including cell proliferation, migration, and behavior. Similarly, in passerine songbirds and mammals, testosterone and estradiol are important modulators of adult neuroplasticity, with documented effects on areas of the brain necessary for complex behaviors, including social communication, reproduction, and learning. Given that many of the cellular processes that underlie neuroplasticity are often energetically demanding and temporally protracted, it is somewhat surprising that steroids can affect physiological and behavioral end points quite rapidly. This includes recent demonstrations of extremely rapid effects of estradiol on synaptic neurotransmission and behavior in songbirds and mammals. Indeed, we are only beginning to appreciate the role of temporally and spatially constrained neurosteroidogenesis, like estradiol and testosterone being made in the brain, on the rapid regulation of complex behaviors.
The crustacean stomatogastric nervous system contains a set of distinct but interacting rhythmic motor circuits that control movements of the foregut. When isolated, these circuits produce activity patterns that are almost perfect replicas of their behavior in vivo. The ease with which distinct circuit neurons are identified, recorded, and manipulated has provided considerable insight into the general principles of how motor circuits operate and are controlled at the cellular level. The small number of relatively large neurons has facilitated several technical advances in neuroscience research and allowed the identification of one of the earliest circuit connectomes. This enabled, for the first time, studies of circuit dynamics using the relationships between all component neurons of a nervous center. A major discovery was that circuits are not dedicated to producing a single neuronal activity pattern, and that the involved neurons are not committed to particular circuits. This flexibility results predominantly from the ability of neuromodulators to change the cellular and synaptic properties of circuit neurons. The relatively unique access to, and detailed documentation of, identified circuit, sensory, and descending pathways has also started new avenues into examining how individual modulatory neurons and transmitters affect their target cells. Groundbreaking experimental and modeling work has further demonstrated that the intrinsic properties of neurons depend on their recent history of activation and that neurons and circuits counterbalance destabilizing influences by compensatory homeostatic regulation of ionic conductances. The stomatogastric microcircuits continue to provide key insight into neural circuit operation in numerically larger and less accessible systems.
Thomas W. Cronin, N. Justin Marshall, and Roy L. Caldwell
The predatory stomatopod crustaceans, or mantis shrimp, are among the most attractive and dynamic creatures living in the sea. Their special features include their powerful raptorial appendages, used to kill, stun, or disable other animals (whether predators, prey, or competitors), and their highly specialized compound eyes. Mantis shrimp vision is unlike that of any other animal and has several unique features. Their compound eyes are optically triple, each having three separate regions that produce overlapping visual fields viewing certain regions of space. They have the most diverse set of spectral classes of receptors ever described in animals, with as many as 16 types in a single compound eye. These receptors are based on a highly duplicated set of opsin molecules paired with strongly absorbing photostable filters in some photoreceptor types. The receptor set includes six ultraviolet types, all spectrally distinct, many themselves tuned by photostable filters. There are as many as eight types of polarization receptors of up to three spectral classes (including an ultraviolet class). In some species, two sets of these receptors analyze circularly polarized light, another unique capability. Stomatopod eyes move independently, each capable of visual field stabilization, image foveation and tracking, or scanning of image features. Stomatopods are known to recognize colors and polarization features and evidently use these in predation and communication. Altogether, mantis shrimps have perhaps the most unusual vision of any animal.
We live in an approximately 24-hour world and circadian rhythms have evolved to adapt organisms to the opportunities presented by Earth’s 24-hour cycle of light and dark. A “master clock” located in the suprachiasmatic nucleus (SCN) of the brain orchestrates daily rhythms in all manner of behavioral, endocrine, metabolic, autonomic, and homeostatic systems in our bodies. The SCN is comprised of about 20,000 neurons and about one third as many astroglia. How can so few neurons and astroglia guide so many rhythms? How do neurons time out an interval as long as a day? The answers are a case study in understanding how genes within cells, and cells within circuits, function together to perform complex activities and optimize bodily functions. While individual clock cells are found in virtually all bodily tissues, the unique connectome of the SCN, its specialized afferent inputs from the retinohypothalamic tract, and its neural and humoral outputs enable its “babel” of neuronal types to synchronize their activity and signal time to the rest of the body.
At the molecular-cellular level, circadian rhythms are regulated by a 24-hour transcriptional–translational feedback loop. At the SCN tissue level, individual SCN neurons coordinate their gene expression and electrical activity, working together in circuits that sustain coherent rhythms. The SCN has many distinct cell types based on their neurotransmitters, neuropeptides, and afferent and efferent connections. There has been much progress in unraveling the dynamic network organization that underlies the SCN network’s communications. Though the precise anatomical connections underlying interneuronal communication in the SCN are not completely understood, key signaling mechanisms that sustain the SCN’s intrinsic rhythmicity have been tackled using intersectional genomic tools. Transgenic animals that permit the visualization of clock gene–protein expression have enabled analysis of SCN network activity over time. Availability of animals bearing mutations in clock genes or proteins enable the determination of changes within neurons, among neurons in networks, and their impact on behavior. The use of continuous readouts of circadian activity that track behavior, or clock gene expression, or electrical activity changes over time, within an SCN or a single neuron, leads the way to unraveling mechanisms sustaining the circadian timing system. Because the results of circadian studies generate huge amounts of data, the entry of mathematical modelers and statisticians into the field has begun to yield useful and testable predictions on how these multiplexed systems work to adapt to our 24-hour world.
Jose M. Alonso and Harvey A. Swadlow
The thalamocortical pathway is the main route of sensory information to the cerebral cortex. Vision, touch, hearing, taste, and balance all depend on the integrity of this pathway that connects the thalamic structures receiving sensory input with the cortical areas specialized in each sensory modality. Only the ancient sense of smell is independent of the thalamus, gaining access to cortex through more anterior routes. While the thalamocortical pathway targets different layers of the cerebral cortex, its main stream projects to the middle layers and has axon terminals that are dense, spatially restricted, and highly specific in their connections. The remarkable specificity of these thalamocortical connections allows for a precise reconstruction of the sensory dimensions that need to be most finely sampled, such as spatial acuity in vision and sound frequency in hearing. The thalamic axon terminals also segregate topographically according to their stimulus preferences, providing a simple principle to build cortical sensory maps: neighboring values in sensory space are represented by neighboring points within the cortex.
Thalamocortical processing is not static. It is continuously modulated by the brain stem and corticothalamic feedback based on the level of attention and alertness, and during sleep or general anesthesia. When alert, visual thalamic responses become stronger, more reliable, more sustained, more effective at sampling fast changes in the scene, and more linearly related to the stimulus. The high firing rates of the alert state make thalamocortical synapses chronically depressed and excitatory synaptic potentials less dependent on temporal history, improving even further the linear relation between stimulus and response. In turn, when alertness wanes, the thalamus reduces its firing rate, and starts generating spike bursts that drive large postsynaptic responses and keep the cortex responsive to sudden stimulus changes.
Maintaining water balance is critical for survival, but our bodies are constantly losing more water than we produce. Consuming water, therefore, is needed to restore what is lost by sweating, bleeding, vomiting, urinating, even breathing. Because the fluid in the body is divided into intracellular and extracellular compartments, and because depletion can happen in one compartment without affecting the other, separate detection mechanisms for losses in each are required. Moreover, the relatively high concentration of sodium in the extracellular space means that sodium loss accompanies extracellular dehydration. Accordingly, the behavioral response to loss of fluid from the extracellular space needs to include sodium intake. Activity of osmoreceptors (in the case of intracellular loss), or baroreceptors and the renin-angiotensin system (in the case of extracellular loss), underlies the responses to perturbations of fluid balance, and promotes the appropriate behaviors needed to restore balance to the system. The peptide angiotensin II (AngII) is a key component of these responses. Studies of AngII in drinking have been critical in our understanding of how a peripherally derived peptide can act in the brain without transport across the blood–brain barrier, and AngII-induced drinking has served as an important model for the study of intracellular signaling pathways that affect behavior. Although much has been discovered about these systems and how they respond to fluid deficits, the precise means by which the systems generate a behavioral response and the mechanism that mediates satiety remains poorly understood. Nevertheless, ongoing experiments on these open questions have already started to provide a new perspective on the negative reinforcement that is provided by drinking under conditions of thirst.
Susan C. P. Renn and Nadia Aubin-Horth
Several species show diversity in reproductive patterns that result from phenotypic plasticity. This reproductive plasticity is found for example in mate choice, parental care, reproduction suppression, reproductive tactics, sex role, and sex reversal. Studying the genome-wide changes in transcription that are associated with these plastic phenotypes will help answer several questions, including those regarding which genes are expressed and where they are expressed when an individual is faced with a reproductive choice, as well as those regarding whether males and females have the same brain genomic signature when they express the same behaviors, or if they activate sex-specific molecular pathways to output similar behavioral responses. The comparative approach of studying transcription in a wide array of species allows us to uncover genes, pathways, and biological functions that are repeatedly co-opted (“genetic toolkit”) as well as those that are unique to a particular system (“genomic signature”). Additionally, by quantifying the transcriptome, a labile trait, using time series has the potential to uncover the causes and consequences of expressing one plastic phenotype or another. There are of course gaps in our knowledge of reproductive plasticity, but no shortage of possibilities for future directions.