Curtis C. Bell - US grants

The long range goal is to understand the processing of electrosensory information in electric fish of the family Mormyridae and to apply the results to general neurobiological problems. This proposal focuses on the electrosensory lobe, a cerebellum-like structure in which electroreceptor afferents of all three types (ampullary, knollenorgan, and mormyromast) terminate, each within a separate region. At the time of an electric organ discharge (EOD), each region is affected not only by the evoked afferent input but also by a corollary discharge of the EOD motor command (EOCD). The form of the EOCD matches the pattern of EOD evoked input. This matching is of two types. An invariant type in which predictable input features are matched by corresponding EOCD features and a modifiable or plastic type. The latter has as yet been seen only in the ampullary region where changes in the EOD evoked input pattern are matched by subsequent changes in the EOCD. This proposal focuses on the mormyromast region because it is the largest region, and because it is the subsystem responsible for active electrolocation. There are two objectives. The first is a physiological and anatomical study of the mormyromast region which seeks to describe the flow of information through the local circuitry. This includes an analysis of EOCD effects and testing for EOCD plasticity. Experiments will be done in curarized fish where the EOCD can be examined in isolation. Single cells and fibers will be recorded both extra- and intracellularly. The effects of electrosensory input, of EOCD's, and the interaction between these two will be examined. Cells will be injected with HRP for anatomical identification and description. Some synaptic structures will be examined with the electron microscope. The second objective of the proposed research is to determine the duration and the site of the plastic change in the EOCD which has been found in the ampullary region and which is expected in the mormyromast region. Methods will include recording, stimulation, and lesions. The general problems to which the proposed work may be relevant include: information processing in local circuits; effects of motor commands on sensory input; storage and retrieval of neural images; and cerebellar function.

The long term goals of this research are to understand synaptic plasticity and sensory-motor processing in the electrosensory lobe of mormyrid electric fish. The electrosensory lobe is a cerebellum-like structure which receives the primary afferent input from electroreceptors. The lobe is also strongly affected by descending input from other central structures. One of the most prominent of these inputs is an electric organ corollary discharge (EOCD) signal associated with the motor command that drives the electric organ to discharge. Many EOCD effects on cells of the electrosensory lobe are plastic and depend upon previous sensory input. Evidence indicates that the plasticity is due to changes in synaptic efficacy within the lobe. The experimental work will be done in the slice preparation and most of the studies will make use of intracellular recording. This proposal has 5 Specific Aims: 1) To determine the synaptic site(s) of EOCD plasticity and to measure its temporal properties in vitro. Certain hypotheses will be tested concerning the site of plastic change. These hypotheses have been derived from in vivo work and include the hypothesis of plastic change at inhibitory synapses. 2) To determine the transmitters and receptor subtypes at the synapses which show plastic change. Inhibitory and excitatory responses will be elicited by artificial electrical stimuli. These responses will be examined for the effects of antagonists to different subtypes of receptors. Agonists will also be tested. 3) To determine the identity and role of the broad dendritic spikes. Broad dendritic spikes which are probably calcium spikes appear to have a role in EOCD plasticity. The role of these spikes in plasticity, the identity of these spikes as calcium spikes, and the role of calcium in EOCD plasticity will be investigated. 4) To complete our knowledge of the morphology of the electrosensory lobe. Morphological work will be done on both cell morphology and connectivity. Studies of the mormyrid electrosensory lobe are expected to contribute to our general knowledge of synaptic plasticity and the role of such plasticity in perceptual and sensory-motor processes.

The goals of this project are to determine the major features of synaptic plasticity and functional circuitry in the electrosensory lobe (ELL) of mon-nyrid electric fish. The ELL is a cerebellum-like structure where the primary afferent fibers from electroreceptors terminate. The ELL and similar cerebellum-like structures in other fish are adaptive sensory processors in {Date Released: 08/08/1997 Date Printed: 01/15/1998} which memory-like predictions about sensory input are generated and subtracted from current sensory input, allowing the neural responses to unexpected or novel input to stand out more clearly. The predictions are based on prior associations between sensory input and various central signals conveyed by parallel fibers such as corollary discharge signals linked to motor commands. Plasticity at the excitatory synapse between parallel fibers and ELL cells could be the cellular mechanism for the generation of these predictions about sensory input, and such plasticity has now been demonstrated in the in vitro slice preparation by the P.I. and his colleagues. The plasticity is observed after a few minutes of pairing of a presynaptic parallel presynaptic parallel fiber input with a postsynaptic spike. The plasticity is observed after a few minutes of pairing of a presynaptic parallel fiber input with a postsynaptic spike. The plasticity is anti-Hebbian in that pairings in which the postsynaptic spike occurs during the parallel fiber-evoked epsp lead to a depression of the epsp, whereas pairings at other delays lead to enhancement. Indications of associative plasticity at inhibitory synapses were also obtained. This project is focused on the mechanisms and features of these different forms of synaptic plasticity and on the functional circuitry of ELL. ELL cells will be recorded intracellularly in the in vitro slice preparation. The roles of NMDA receptors, nitric oxide, postsynaptic spikes and postsynaptic calcium will be examined along with the effects of various pairing protocols. Circuitry will be investigated both morphologically and functionally at the cell to cell level. The results are expected to enhance our understanding of synaptic plasticity at the cellular level and the role of such plasticity in memory-like functions such as the generation of predictions about sensory input.

The goals of this project are to determine the major features of synaptic plasticity and functional circuitry in the electrosensory lobe (ELL) of mon-nyrid electric fish. The ELL is a cerebellum-like structure where the primary afferent fibers from electroreceptors terminate. The ELL and similar cerebellum-like structures in other fish are adaptive sensory processors in {Date Released: 08/08/1997 Date Printed: 01/15/1998} which memory-like predictions about sensory input are generated and subtracted from current sensory input, allowing the neural responses to unexpected or novel input to stand out more clearly. The predictions are based on prior associations between sensory input and various central signals conveyed by parallel fibers such as corollary discharge signals linked to motor commands. Plasticity at the excitatory synapse between parallel fibers and ELL cells could be the cellular mechanism for the generation of these predictions about sensory input, and such plasticity has now been demonstrated in the in vitro slice preparation by the P.I. and his colleagues. The plasticity is observed after a few minutes of pairing of a presynaptic parallel presynaptic parallel fiber input with a postsynaptic spike. The plasticity is observed after a few minutes of pairing of a presynaptic parallel fiber input with a postsynaptic spike. The plasticity is anti-Hebbian in that pairings in which the postsynaptic spike occurs during the parallel fiber-evoked epsp lead to a depression of the epsp, whereas pairings at other delays lead to enhancement. Indications of associative plasticity at inhibitory synapses were also obtained. This project is focused on the mechanisms and features of these different forms of synaptic plasticity and on the functional circuitry of ELL. ELL cells will be recorded intracellularly in the in vitro slice preparation. The roles of NMDA receptors, nitric oxide, postsynaptic spikes and postsynaptic calcium will be examined along with the effects of various pairing protocols. Circuitry will be investigated both morphologically and functionally at the cell to cell level. The results are expected to enhance our understanding of synaptic plasticity at the cellular level and the role of such plasticity in memory-like functions such as the generation of predictions about sensory input.

DESCRIPTION: (Adapted from the Investigator's Abstract) Most sensory regions of the brain receive extensive inputs from other central structures in addition to input from the periphery and most such regions show some form of plasticity. But the functional significance of the central inputs and plasticity are only poorly understood. This study seeks insights into these general issues by examining the electrosensory lobe (ELL) of mormyrid electric fish in which: the sensory inputs can be precisely controlled; the central inputs are relatively well understood; and plasticity has been established at both the systems and synaptic levels. The ELL is a cerebellum-like structure that receives the primary afferent input from electroreceptors as well as afferent input from various central structures. The ELL and similar structures in other fish are adaptive sensory processors in which memory-like predictions about sensory input are generated and subtracted from current sensory input allowing novel inputs to stand out. The predictions being based on prior associations between peripheral and central inputs. The central input in mormyrid fish that has been examined most extensively with regards to its effects and plasticity is an electric organ corollary discharge signal (EOCD) associated with the motor command that drives the EOD. Specific aims of the project are: 1) To determine responses of ELL cells to electrosensory stimuli, to the EOCD and to electrical activation of central afferents. 2) To determine the plasticity of ELL cells' responses to natural activation of other types of descending central afferents besides the EOCD. 3) To determine the circuitry and cellular mechanisms responsible for the plasticity of EOCD responses in ELL cells. 4) To determine the response properties of the central afferents to the ELL; and 5) To determine the anatomical connections of a major source of central input to the ELL. The primary method will be intracellular recording of cells in ELL and in central structures projecting to ELL together with intracellular staining for morphological identification. The results are expected to be a source of hypotheses into the central processing of sensory information in general as well as into the processing of information in other cerebellum-like structures.

This is a proposal for a collaborative research project between Dr. Angel Caputi of the Instituto de Investigaciones Biologicas Clemente Estable in Montevideo Uruguay and Dr. Curtis Bell of the Neurological Sciences Institute at Oregon Health Sciences University in Portland Oregon. Dr. Bell's grant, MH49792, is the Parent Grant for this project. This project is concerned with the central nervous system of electric fish of the order Gymnotiformes, an order of fish which is native to Uruguay and other parts of South America. More specifically, this project is concerned with gymnotiforrn fish in which the electric organ discharge is a brief pulse of electric current. The goals are to establish the functional circuitry of the electrosensory lobe (ELL) of these fish and to compare it with that of the ELL of pulse electric fish of the order Mormyriformes, an order of fish that is native to Africa and which is being studied in the laboratory of the Principal Investigator, Dr. Bell. The ELLs of both groups of fish are cerebellum-like structures where sensory information from the periphery is first analyzed by the brain. The ELLS of pulse mormyriform fish and the cerebellum-like structures of some other groups of fish have been shown to act as adaptive sensory processors in which predictable features of the sensory inflow are subtracted from current sensory inflow through memory-like processes - allowing novel, information-rich sensory information to stand out more clearly. Plasticity at the synapse between parallel fibers and Purkinje- like cells has been demonstrated in some of these fish, and is the probable mechanism of the memory-like processes. The hypothesis has been advanced that all cerebellum-like sensory structures in vertebrates, including the ELL of pulse gymnotiforms and the dorsal cochlear nucleus of mammals, are functionally similar and act as adaptive sensory processors. Little is known at present concerning the ELL of gymnotiform pulse fish, a major group of electric fish. This project . will determine the circuitry and test for synaptic plasticity in the ELL of these fish using intracellular recording in the in vitro slice preparation. The results will permit a significantly richer comparison of the different cerebellum-like structures and a deeper knowledge of how they analyze and store sensory information.

Sensory information is often acquired through active exploration. Knowledge of the world is gained by exploring a complex surface with hands or a visual scene with eyes. Yet relatively little is known about how neurons encode sensory stimuli in the context of natural patterns of sensing behavior, or about how sensory processing regions in the brain distinguish properties of the external world from the sensory consequences of the animal's own behavior.

A particularly clear example of active sensing is found in mormyrid electric fish. Electric fish use an electrical sense to navigate and find prey in the dark by probing the environment by emitting brief electric organ discharge (EOD) pulses. Nearby objects perturb the electric field around the fish, and these perturbations are detected by electroreceptors in the fish's skin. Each receptor encodes changes in local field strength as small shifts in the precise timing of individual action potentials following the EOD. The fish thus obtains a sequence of "snapshots" of the world, in which information about surrounding objects is encoded in the timing of action potentials.

In nature, the frequency and regularity of this sequence of snapshots varies depending on the behavioral context, whether the fish is probing objects, foraging, or quietly resting. Interestingly, the frequency chosen by the fish has a clear effect on the timing of electroreceptor action potentials within each snapshot: higher rates shift spikes later, and lower rates shift spikes earlier. The size of these effects is comparable to the effects of small invertebrate prey on which these fish feed. How does the fish detect and capture prey when its own sensing behavior has such a strong effect on the sensory input?

This study provides opportunity to explore how sensory processing regions of the fish's brain resolves the ambiguity, and whether a change in the input from electroreceptors is due to an external stimulus or to the animal's own sensing behavior. Neurons at the first stage of electrosensory processing integrate input from electroreceptors with signals from other areas of the fish's brain linked to the motor command that evokes the EOD. Such motor command signals could, in principal, "undo" the effects of EOD rate on electroreceptor input.

The research is expected to lead to a better understanding of how animals use internal knowledge of their actions to distinguish properties of the external world from the sensory consequences of their own behavior. At a more cellular level, the experiements are also expected to lead to a better understanding of how information contained in the precise timing of action potentials is decoded or interpreted by neural circuits.