German Barrionuevo - US grants

association learning; neural plasticity; neurophysiology; electrical potential; synapses; epinephrine; electrophysiology; biophysics; online computer;

A well-motivated and common working hypothesis is that learning involves some change in the functional connectivity among nerve cells, probably at their synaptic interconnections. In searching for a likely synaptic candidate for such changes in the mammalian nervous system, the phenomenon of the long-term synaptic potentiation (LTP) is an obvious choice, for three reasons: LTP can be induced by very brief tetanic stimulation, it lasts hours, days, or weeks; and the conditions required for its induction bear a tantalizing similarity to some of the laws of classical conditioning. The present proposal has three interrelated objectives. The first goal is to determine and quantify the spatial and temporal rules governing the induction, magnitude and duration of associative LTP. The second goal is to provide evidence on the basis of the enhanced synaptic efficacy. The third goal is to determine the mechanism responsible for the associativity. The fact that associative LTP occurs in the in vitro hippocampus is extremely important because this is the only known vertebrate preparation in which a promising synaptic model for associative memory has been demonstrated and which is amenable to many of our most powerful analytical neurophysiological techniques.

DESCRIPTION: (Investigator's abstract) The hippocampal formation, consisting of the entorhinal cortex, the dentate gyrus, areas CA3 and CA1, and the subiculum, has long been considered an important component in the circuitry of declarative memory. The entorhinal cortex gives rise to the perforant path fibers, the major excitatory input to the dentate gyrus. Perforant path fibers originating in the superficial layers (II and III) of the entorhinal cortex terminate directly on areas CA3 and CA1 to make monosynaptic contacts onto the distal apical dendrites of pyramidal neurons. Very recently investigations on the direct perforant path project to area CA1 have lead to a new conceptual model of the functional organization of the hippocampal intrinsic circuitry: it has been proposed that the monosynaptic entorhinal inputs to CA3 and CA1 could provide parallel feed forward modulation of activity propagating through the trisynaptic circuitry. One obvious difficulty in evaluating the new model of parallel hippocampal functions is that the biophysical and pharmacological properties of the direct entorhinal projections to CA1 and CA3 have received little attention. The research effort made during the last grant cycle investigating the activity dependent changes in synaptic efficacy in area CA3 particularly focused on the mossy fiber synapse. As a component of the trisynaptic circuitry, the mossy fibers convey disynaptic excitation from layer II entorhinal cortical neurons to area CA3 neurons. The research proposed in this application builds upon these studies and is aimed at investigating the electrophysiological and pharmacological properties of the hitherto neglected direct, monosynaptic perforant path input to CA3 pyramidal neurons as well as its functional interaction with the mossy fiber input. This study will use in vitro hippocampal preparation in conjunction with whole-cell somatic and dendritic recordings to accomplish three major objectives: 1) To identify the electrophysiological effects of perforant path activation of CA3 pyramidal neurons. It is hypothesized that the direct perforant path monosynaptically excites CA3 pyramidal neurons via glutamatergic synapses, and disynaptically inhibits these same cells via GABAergic interneurons located in the s. lacunosum-moleculare. 2) To investigate voltage-gated conductances located in the apical dendrite of the CA3 pyramidal neuron. It is hypothesized that despite its remote dendritic location, the direct, monosynaptic perforant path input to CA3 pyramidal neurons contributes significantly to neuronal excitability via active dendritic conductances. 3) To characterize the interaction between perforant path and mossy fiber synaptic input to CA3 pyramidal cells. It is hypothesized that the reduction in somatic response to input from the direct, monosynaptic perforant path by preceding activation of mossy fibers is due to alterations of voltage-gated dendritic conductances. These pharmacological and biophysical studies will provide new insights into the functional characteristics of the direct, entorhinal input to CA3 pyramidal neurons. It also will provide a foundation for reinterpreting the intrinsic circuitry of the hippocampal formation, and its role in memory and learning function.

prefrontal lobe /cortex; dopamine receptor; schizophrenia; neural conduction; synapses; dopamine; voltage gated channel; disease /disorder model; electrostimulus; pyramidal cells; sodium channel; neural transmission; calcium channel; video microscopy; laboratory rat; voltage /patch clamp; Macaca fascicularis; electrophysiology; tissue /cell culture;

DESCRIPTION: (Adapted from the Investigator's Abstract) Pyramidal cell dendrites combine excitatory postsynaptic potential from tens of thousands of synapses spread across hundreds of microns of membrane. These voltage transients are then propagated to the soma. Until recently, it has been hypothesized that the integration and transfer of synaptic inputs involved only passive properties of dendrites. However, recent work on the properties of pyramidal cell dendrites has shown that the somatic response to input to the apical dendrites is modulated by voltage-dependent conductances in the soma and dendrite and thus is not passive. This observation raises the possibility that summation of synaptic responses is influenced by dendritic non-linearities, and that the summation will be modified by changes in the properties of postsynaptic voltage dependent channels. Modeling studies have suggested that dendritic non-linearities introduced by the addition of voltage dependent sodium and calcium channels may provide a mechanism whereby pyramidal cell dendrites sum synaptic inputs super-linearly, allowing dendritic branches to perform complex computions or to detect coincident inputs. The central hypothesis of the proposal is that conductances underlying the active propagation of perforant path (PP) synaptic input are modulated by: i) changes in membrane voltage, ii) modulatory inputs. PP is the output of the entorhinal cortex and a major source of ipsilateral extrinsic afferents to the dentate gyrus granule cells, and to hippocampal pyramidal cells in areas CA1 and CA3. Thus, the PP input plays a central role in the neural coding and integration in cognitive functions subserved by the hippocampal formation, i.e., memory and learning. The central hypothesis will be tested with whole-cell recordings, anatomical labeling, electrical stimulation and flash photolysis of caged compounds. These investigations will address the following Specific Aims: 1) To determine the properties and mechanisms of active summation of PP EPSPs with other voltage changes in CA3 pyramidal cells. We will investigate how PP EPSPs are affected by changes in membrane potential. 2) To determine the mechanisms of the actions of neuromodulators on the boosting of PP EPSP. The postsynaptic actions of two neuromodulators known to play important roles in hippocampal function, acetylcholine (Ach) and norepinephrine (NE), will be investigated. 3) To determine the properties and mechanisms of active summation of PP EPSPs in CA3 interneurons. We will investigate the characteristics of PP input to these interneurons

DESCRIPTION (provided by applicant): The central hypothesis of the proposal is that interneurons localized in the stratum lacunosum moleculare (SLM Is) in hippocampal area CAS are capable of expressing mossy fiber long-term potentiation (MF LTP). The central hypothesis will be tested with whole-cell recordings from SLM Is, anatomical labeling, and electrical stimulation of MF and PP pathways. These investigations will address the following two Specific Aims. Aim 1: To identify the requirements for the induction and maintenance of LTP at MF to SLM I synapses in area CAS of the hippocampus. More specifically, we will test the following hypotheses: a) LTP at MF to SLM I synapses is mediated by postsynaptic Ca2+ via activation of group I mGluR receptors; b) LTP at MF to SLM I synapses is mediated by PKA and PKC activation; and c) The early maintenance phase of LTP at MF to SLM I synapses depends on protein synthesis. Aim 2: To examine the changes in efficacy of synaptic integration in SLM I produced by LTP induction at MF to SLM I synapses in area CAS of the hippocampus. More specifically, we will test the following hypotheses: a) MF to SLM I synapses is associated with an increase in supralinear summation between MF and PP synaptic inputs to SLM I; b) The increase in supralinear summation between MF and PP synaptic inputs to SLM I after LTP at MF to SLM I synapses is due to an increase in Ca 2+ currents mediated by the T/R channel subtype; and c) The increase in supralinear summation between MF and PP synaptic inputs to SLM I after LTP at MF to SLM I synapses is blocked by inhibition of protein kinases in the postsynaptic cell.