Neuronal mechanisms of learning in teleost fish *

r e s u m e n Se revisaron dos aproximaciones al estudio de la neurobiología del aprendizaje en peces teleósteos: lesiones cerebrales y estimulación química. Respecto al efecto de lesiones cerebrales, la literatura reporta que las ablaciones del telencéfalo producen deficiencias en habituación, mientras que las lesiones en el cerebelo afectan el condicionamiento clásico de retracción ocular y aprendizaje espacial (efectos similares observados en mamíferos sugieren que las funciones del cerebelo pudieron haber evolucionado tempranamente en la historia de los vertebrados). Áreas del Medium Pallium (MP) parecen ser vitales en el aprendizaje emocional de los peces; más aún, las funciones del MP aparentan ser similares a las de la amígdala en mamíferos. Con respecto a procesos neuroquímicos, los antagonistas de los receptores NMDA, mostraron afectar la adquisición de condicionamiento de evitación y miedo. Por último, el óxido nítrico y el guanosín monofosfato cíclico han sido relacionados con los procesos de consolidación del aprendizaje emocional. Palabras clave autor Funcionamiento neuroquímico, peces teleósteos, aprendizaje espacial, procesos de aprendizaje, Medium Pallium Lateral. Palabras clave descriptor Peces teleósteos, neuroquímica, cerebro, lesiones. Para citar este artículo. Hurtado-Parrado, C. (2010). Neuronal mechanisms of learning in teleost fish. Universitas Psychologica, 9 (3), 657-672. * This article was partly supported by a scholarship from the Natural Sciences and Engineering Research Council of Canada (NSERC). The author wishes to thank Kate Dubberley for her help during the preparation of this paper, and to Drs. Randall Jamieson, James Hare, Les Leventhal, and Joseph Pear for their comments on a previous version of this document. ** Correspondence concerning this paper can be sent to 206 Chancellor’s Hall, 177 Dysart Rd. University of Manitoba, Winnipeg, MB R3T 2N2-Canada Email: [email protected] camilo hUrtado-Parrado 664 Un i v e r s i ta s Ps yc h o l o g i ca v. 9 no. 3 s e P t i e m B r e-d i c i e m B r e 2010 This paper reviews the literature specialized in the identification of the neurological mechanisms that underlie different learning processes in teleost fish. Given that one of the main interests of the reviewed research programs has been to demonstrate homologies of brain structures and behavioral functions between mammals and fish, the paper starts describing a) the basic assumptions of the neuroethological approach, followed by b) a brief description of brain anatomy and ontogeny of fish, and c) the evolutionary implications that stem from the analysis of the forebrain development of fish. The main idea of this introductory section is that, contrary to the most accepted assumption held in the comparative field until the late twentieth century, recent evidence supports the existence of homologies in brain and behavioral functions when comparing fish and mammals. After discussing the evolutionary approach and the general hypothesis about possible homologies between fish and mammals, the paper provides details regarding the evidence that supports the relationship between different areas of the fish brain, neurochemical functioning, and several learning phenomena. Habituation, sensitization, Pavlovian conditioning, spatial behavior and emotional learning will be the specific processes reviewed. Furthermore, most of the evidence presented will be based on experiments which tested the effects of complete or partial forebrain ablations, or the effects of chemical stimulation in different parts of the fish brain. In general, strong evidence supports that the pallium areas (medium and lateral), the cerebellum, and the chemical processes involved in Long-Term Potentiation have an important role in the emergence and maintenance of learned behavior. Analysis of brain mechanisms and behavior: The neuroethological approach An area within the neurosciences that is specialized in the comparative study of both the neural anatomy and the functions that underlie animal behavior is termed Comparative Neuroethology (Laming, 1981). As a multidisciplinary area, Comparative Neuroethology integrates knowledge from different disciplines (e.g., evolutionary biology, neuroanatomy, ethology, physiology, and psychology) and has its own methods and sources of evidence. This paper is almost entirely based on neuroethological experimental procedures and related evidence, therefore, some conceptual and methodological assumptions are presented here as guidelines to understand the rationale behind the conclusions and orientations of the research that was reviewed. These assumptions are the following: a) the knowledge regarding the ontogeny of the brain and comparative brain anatomy, physiology and behavior, are two of the most important sources of information to understand the evolution of the vertebrate brain mechanisms; b) the advantages of comparative studies lie in the ability to examine the functions within the brain and the associated behavior in the fully developed animal; c) the constancy of embryological brain development in vertebrates and the recapitulation of evolution that occurs during ontogeny have provided important information for understanding brain morphology; and d) the information about brain morphology is, nevertheless, less helpful when further understanding of physiology and behavior are attempted, especially due to technical and interpretative limitations. Lastly, one of the major difficulties for the neuroethological studies is the extrapolation of results from a particular vertebrate species to others. This limitation applies especially in those cases where common ancestors ceased to exist a long time ago and today’s species are not members of a linear phylogenetic scale (instead they tend to represent variations along a “tree” whose trunk does not exist anymore, Laming, 1981). Although extrapolations in general are carefully reviewed under the neuroethological approach, they still seem to be possible alternatives, especially when they relate to basic brain functions such as learning (Laming, 1981). neUronal mechanisms of learning in teleost fish Un i v e r s i ta s Ps yc h o l o g i ca v. 9 no. 3 s e P t i e m B r e-d i c i e m B r e 2010 665 General characteristics of the fish brain and its ontogeny A common finding among vertebratesis that the nervous system is one of the earliest groups of tissues to develop embryologically. As Laming (1981) points out, the neural tube is fully developed by the time that 10-15% of embryonic life has passed. Moreover, from an evolutionary perspective, the thickening of the neural tube is evidence of the cephalization of both the sense organs and integrative centers that has occurred in vertebrates. According to Laming (1981): (...) Outgrows of this early developed nervous system and connections with nervous tissue outside the neural tube form the spinal and cranial nerves. These are the routes by which the animal receives information from its own tissues and the environment [; also, those structures] relay commands to muscles, glands and sense organs through which a response is mediated. (p. 9) The neural tube develops into three components: the prosencephalon (forebrain), mesencephalon (mid-brain), and rhombencephalon (hindbrain). These three vesicles are traditionally associated with the three primary senses-olfaction, vision, and audition respectively which together, constitute the brainsteam in adult vertebrates. Further in development, each one of these vesicles develops a secondary outgrowth: telencephalon (cerebrum), optic tectum, and cerebellum respectively (Davis & Northcutt, 1983; Laming, 1981). A general overview of the major brain regions, divisions, locations and their respective abbreviations is displayed in Figure 1. Evolution and development of the forebrain in fish This paper is mainly dedicated to the description of the neuroethological research that supports the idea that certain structures of the forebrain are critical for the learning processes of one of the oldest vertebrate family. Therefore, it is necessary Figure 1 Overview of a generalized lower vertebrate brain Lateral (a) and dorsal (b) views of a generalized lower vertebrate brain (e.g., teleost fish) to show the main superficial features. Cer, cerebellum; Di, diencephalon; Ep, epiphysis (pineal); Inf, inferior lobe of hypothalamus; Med, medulla oblongata; OB, olfactory bulb; OT, olfactory tract; Sp, spinal cord; Tec, optic tectum; Vag, vagal lobes; Tel, telencephalon; 1-11, cranial nerves (Adapted from Laming, 1981, p. 8). camilo hUrtado-Parrado 666 Un i v e r s i ta s Ps yc h o l o g i ca v. 9 no. 3 s e P t i e m B r e-d i c i e m B r e 2010 to present the general findings and theories regarding the development of the vertebrate forebrain and its evolutionary implications. In early stages of development, the vertebrate forebrain has the shape of a vesicle, with dorsal and ventral walls that are thin and membranous. Laming (1981), by means of both ontogenetic and comparative anatomical evidence, presents an tentative overview of the telencephalon’s evolution. Laming’s proposed line of evolution is constituted in part by existing and extinct species; it begins with a hypothetical species that has a primitive non-differentiated telencephalon and evolves into three different “branches” ending in Teleosts, Amphibians, and Elasmobranchs. In Amphibians and Elasmobranchs there is a pattern of evagination of the thick sidewalls of the telencephalic vesicle which causes both walls to meet in the median plane (forming two lateral ventricules) (Braford, 1995; Laming, 1981). Conversely, in holosteans and teleosts the same lateral walls of the telencephlalon evert. Laming (1981) found that the differences in forebrain development and evolution suggested that the comparisons between forebrain functions in vertebrates rarely can be made on the basis of homology, and instead, analogy seemed to be a better approach. Nevertheless, more recent evidence that will be introduced here contradicts Laming’s suggestion of using an “analogy approach” (e.g., Salas et al., 2006). Specifically the following sections will describe a research program which follows a more “conservative” approach in terms of forebrain structures and the associated and learning phenomena. Evolution of the brain and behavior in vertebrates: From fish to mammals Salas et al. (2006) contrast two important theories about the evolution of the brain and its functions from ancient vertebrates to mammals. The dominant “classical theories” of the early twentieth century proposed that brain evolution occurred over several successive stages, and consequently, the complexity of structures and functions have increased to what we can now see in the advanced cognitive capabilities of mammals. In terms of these theories, the fish telencephalon would consist mainly of a subpallium and a very small and primitive paleocortex, both entirely dedicated to sensory functions (particularly olfaction) and with relatively simple neural circuits. Moreover, given that more complex structures (e.g., caudate, putamen, hippocampus, and neocortex) were considered to appear later in more recent species, fish behavior was, therefore, assumed to be mainly reflexive or instinctive (Salas et al., 2006). Savage’s (1969) experiments constitute examples of the evidence that supported the aforementioned understanding of fish neurobehavioral functioning held during the early and mid-twentieth century. Savage reported that the failure in acquisition and retrieval of shock avoidance tasks in forebrainless fish was neither due to a reduced sensitivity to aversive stimulus (shock), nor a failure in a manifestation of signs of fear. Savage also reported that a) normal and forebrainless fish showed similar speeds in feeding, even when the levels of food deprivation and appetitive behavior were comparable; b) the removal of the forebrain did not interfere with the ability to learn simple simultaneous spatial discrimination; and c) the introduction of a five second delay between response and reward caused the extinction of discrimination in the telencephalon ablated fish, but not, however, in the non operated fish. Finally, Savage reported that the removal of the forebrain did not have an important effect on feeding rate. As Salas et al. (2006) pointed out, because across different research programs the systematic removal of the telencephalon of fish did not show any impairment in sensory, motor, or motivational processes (as Savage’s experiments clearly showed), the idea about the progressive evolution of the brain from fish to other more evolved vertebrates was maintained. Moreover, Laming (1981) defended a similar position when he stated that any functional or anatomic similarity between early vertebrates (e.g., fish) and more recent species would be better explained as analogies. Nevertheless, at the end of the twentieth century researchers began to find unique ways to study neUronal mechanisms of learning in teleost fish Un i v e r s i ta s Ps yc h o l o g i ca v. 9 no. 3 s e P t i e m B r e-d i c i e m B r e 2010 667 the relationship between fish behavior and certain areas of the brain. The use of more precise techniques and experimental procedures revealed evidence that the forebrain in teleost fish is involved in emotional, social and, reproductive behavior, as well as in learning and memory (e.g., Flood & Overmier, 1981; Overmier & Hollis, 1983, 1990; Savage, 1980). In conjunction with this evidence, other neuroethological, comparative, developmental and neuroanatomical evidence lead to a different understanding of vertebrate brain evolution. Instead of assuming a continuous and progressively linear complex evolution of the vertebrate brain, it seemed more plausible to assume that parallel radiations evolved independently from a remote common ancestor, from which vertebrates inherited some basic features of brain and behavior organization. Consequently, the increases in brain size and complexity occurred in different periods and in many members of the vertebrate family, including fish (Laming, 1980; Salas et al., 2003). As shown in Figure 2, the brain of extant and hypothetical vertebrates, although displaying noticeable morphological differences, can be understood as a combination of both primitive and derived characteristics. Following Salas et al. (2006), theFigure 2 Three hypothetical lines of telencephalon evolution The graphic shows transversal brain sections of present, extant and hypothetical vertebrates which, although showing noticeable morphological differences, can be understood as a mixture of both primitive and derived characteristics. E = Eversion; Evag. = Evagination; I = Inversion; CS = corpus striatum; N = Neocortex; P = Pyriform; S = Septum; H = Hippocampus; V = lateral ventricule. Adapted from Laming (1981, p. 12). camilo hUrtado-Parrado 668 Un i v e r s i ta s Ps yc h o l o g i ca v. 9 no. 3 s e P t i e m B r e-d i c i e m B r e 2010 se morphological variations “can be conceived as variations of a common vertebrate plan” (p. 158), and it seems that the evidence today supports a more conservative understanding. Whereas Laming (1981), Flood and Overmier (1981), Davis and Northcutt (1983), and Overmier and Hollis (1983, 1990) constitute the most complete reviews about the brain structures associated with different behavioral phenomena in teleost fish until the end of the twentieth century, more recent reviews are found in Portavella, Vargas, Torres, and Salas (2002) and Salas et al. (2006). The following section of the paper presents a general review, and if available, an update of the findings regarding the brain structures associated with memory, learning, and emotional behavior in fish. A special emphasis on emotional phenomena is made primarily because there is more available evidence coming from related research programs, and because some methodological alternatives derived from Hineline’s (1977) parametric analysis of negative reinforcement and avoidance in species other than fish may lead to future comparative analysis and systematic replications. Non-associative learning Habituation is the reduction of responding as a consequence of repeated presentations or a prolonged exposure to a stimulus. Peeke, Peeke, and Willinston (1972) reported that after complete ablation of goldfish telencephalon, long-term habituation impairment was observed. Moreover, similar effects are reported by Laming and Ennis (1982), who explored the habituation of fright and arousal responses of goldfish and roaches. In Laming and Ennis’ report, fright response appeared first and showed habituation in few trials, arousal response, however, appeared later but also habituated. The authors concluded that telencephalon ablation severely impaired habituation of arousal, though not fright responses. Furthermore that similar results (i.e. long-term impairment) have been found with telencephalon ablated Bettasplendens (Marino-Neto & Sabatino, 1983). In the case of sensitization, which refers to the increase in responding that results from the repeated presentations of a stimulus, and which is not attributable to peripheral processes, there was no report of learning impairments in fishwith telencephalic ablation (Overmier & Curnow, 1969; Overmier & Hollis, 1990). Associative learning Pavlovian conditioning is generally understood as the repeated pairing of a “neutral” stimulus and a hedonically powerful stimulus in such an arrangement that the neutral stimulus (CS) predicts the hedonic one (US). As a consequence of these pairings, the CS acquires behavior-controlling properties or Conditioned Response (CR) (Overmier & Hollis, 1983, 1990). Following the reviews of Overmier and Hollis (1983, 1990) and Salas et al. (2006), every experiment that tested the effects of telencephalic ablation on Pavlovian conditioning did not find any impairment of the learning process. This is a very general finding across a wide variety of procedures, associative indices, and range of delays (Overmier & Hollis, 1983, 1990; Salas et al., 2006). In fact, Overmier and Hollis (1990) reported that telencephalon-ablated fish, exposed to high complexity classical conditioning processes, did not display impairment in their performance. Nonetheless, it is of special relevance that practically all the experiments on classical conditioning and its associated brain functions in fish have utilized electric shock or the presentation of a bright light as aversive stimulus(US). The relevance of this variable (i.e. type of aversive stimulation) may be the starting point for a research program that has not yet been developed. Contrasting the lack of effects of telencephalic ablations, research conducted by Gómez, Durán, Salas, and Rodríguez (2010), Yoshida and Hirano (2010), and Rodríguez et al. (2005), exemplifies cumulated evidence supporting the notion that teleost’s cerebellum is essential for the classical conditioning of several types of behavior (in the same way that it is for mammals). Rodriguez et neUronal mechanisms of learning in teleost fish Un i v e r s i ta s Ps yc h o l o g i ca v. 9 no. 3 s e P t i e m B r e-d i c i e m B r e 2010 669 al. (2005), and Gomez et al. (2010) analyzed the involvement of the cerebellum in the classical conditioning of motor and emotional responses and on spatial cognition. The authors reported that a) cerebellum lesions in goldfish impair the classical conditioning of a simple eye-retraction response, a phenomenon analogous to the eye blink conditioning described in mammals; b) autonomic emotional responses (e.g., heart rate classical conditioning) were also impaired by cerebellum lesions and; c) goldfish with cerebellum lesions presented a severe impairment in spatial cognition. Lastly, the authors reported that the observation of normal swimming activity or obstacle avoidance indicated that cerebellum lesions did not produce any observable motor deficit, and the lesions did not interfere with the occurrence of unconditioned motor or emotional responses. Rodriguez et al. (2005), Yoshida and Hirano (2010), and Gómez et al. (2010) reached the same conclusion; the functional involvement of the teleost’s cerebellum in learning and memory is very similar to that of mammals. Subsequently, the authors suggest that the cognitive and emotional functions of the cerebellum may have evolved early in vertebrate evolution, having been conserved along the phylogenetic history of the extant vertebrate groups.