Robert P. Vertes - US grants

minority institution research support; secondary schools;

DESCRIPTION (Adapted from applicant's abstract): In previous studies, we demonstrated that the nucleus pontis oralis (RPO) and the median raphe nucleus (MR) are directly involved in the synchronization (theta) and desynchronization of the hippocampal EEG, respectively, and further that the supramammilliary nucleus (SUM) is a critical relay between the brainstem and septum hippocampus in the control of the theta rhythm. The proposed studies are divided into two parts; those done with and without collaborators. Non-collaborative studies and the primary aims of the non-collaborative studies are: (1) to examine whether the SUM exerts an indirect (through the septum) and/or direct influence on the hippocampus in the generation of theta by analyzing the effects on the hippocampal EEG of injections of ibotenic acid or the cholinergic neurotoxin, 192 IgG saporin into the medial septum diagonal band; and (2) to characterize the role of several brainstem/diencephalic nuclei putatively involved in the control of hippocampal synchronization (pedunculopontine tegmental nucleus, nucleus reuniens, mammillary body, posterior nucleus of hypothalamus and SUM) and desynchronization (lateral habenula and MR) by examining the connections of these nuclei, their response to pharmacological agents and their neuronal discharge properties in anesthetized and freely moving rats. Collaborative Studies: These studies will be done in collaboration with Dr. Angel Alonso (electrophysiology in slices), Dr. Csaba Leranth (electron microscopy) and Dr. Matthew L. Shapiro (behavior). The primary aims of the collaborative studies are: 1) to examine the electrophysiological properties and response to various transmitter agents of SUM, MB and MS/DBv neurons in in vitro slices; 2) to examine the synaptic organization of brainstem afferents to the SUM and brainstem diencephalic afferents to the MS/DBv at the ultrastructural level; and 3) to examine the effects of ibotenic acid and 192 IgG saporin induced lesions of the MS/DBv on memory, correlating changes in theta to changes in learning and memory. At present, the candidate is very heavily obligated to non-research related activities (i.e., teaching and service). He is seeking a KO2 award to devote more time to research and to develop several new lines of research. Several reports have shown that long-term potentiation (a candidate mechanism for memory) is optimally induced in the hippocampus by stimulation at the theta frequency, and it has been proposed that theta plays a pivotal role in mnemonic processes in the hippocampus. If theta proves to be critical for memory, it is important to fully understand the neural mechanisms responsible for its generation. This work may have important implications for conditions such as Alzheimer's that involve a loss of cholinergic neurons of the MS/DBv.

neural information processing; hippocampus; central neural pathway /tract; diencephalon; brain stem;

DESCRIPTION (provided by applicant): We have previously demonstrated that the nucleus pontis oralis (RPO) and the median raphe nucleus are directly involved in the synchronization (theta rhythm) and desynchronization of the hippocampal EEG, respectively, and that the suprammillary nucleus serves as a critical relay between RPO and the septum/hippocampus in the control of the theta rhythm. In recent work, we have shown that cells in several structures of "Papez's circuit" fire rhythmically with theta including the mammillary bodies, the ventral tegmental nucleus of Gudden and the anterior ventral nucleus of the thalamus. This suggests that a theta rhythmic signal may resonate throughout Papez's circuit, possibly involved in mnemonic functions of the circuit. The proposed studies will involve three main areas: ascending theta synchronizing systems, ascending hippocampal desynchronizing systems and theta output systems from the hippocampus. Theta synchronizing structures include the supramammillary nucleus and nucleus reuniens of the thalamus; desynchronizing structures include the median raphe nucleus, and theta output structures include the anterior ventral nucleus of thalamus, the presubiculum and the retrosplenial cortex. We will: (1) describe the neurochemical identity of cells together with their physiological profiles using juxtacellular labeling techniques; (2) examine the activity of cells in anesthetized and freely moving rats with respect to the hippocampal EEG; (3) examine the anatomical projections of cells including transmitter specific projections at the light and electron microscopic level; and (4) examine overall patterns of glucose utilization throughout the brain following continuous theta using the 2-DG technique. With this series of studies, we intend to fully characterize systems involved in the generation of theta and those that block its expression in the hippocampus (hippocampal EEG desynchronization) as well as define theta-mediated actions of the hippocampus on extra-hippocampal structures, primarily those of the limbic system and Papez's circuit. An accumulating body of evidence, including several recent reports showing task related theta activity in humans, indicates that the theta rhythm serves a critical role in memory. If, as indicated, theta proves to be critical for memory, we believe it is vitally important to fully understand the neural mechanisms responsible for the generation of theta as well as its actions on other regions of the brain. This work may have important implications for disorders of memory including Korsakoff's syndrome, diencephalic amnesia and Alzheimer's disease.

The ultimate goal of this project is to understand mechanisms responsible for memory processing in the brain. Although several factors contribute to the formation of memories, it is well recognized that the encoding of information in the brain depends on two critical processes: attention and meaning. In effect, events are best remembered when they are attended to and when they are meaningful. In mammals, it is further generally acknowledged that information is initially stored in the hippocampus of the brain. In previous studies using the rat model, the investigator identified a system of connections in the brain that generate the so called theta rhythm of the hippocampus, which appears to serve a direct role in the encoding of significant (or meaningful) information in the hippocampus. Specifically, the hippocampus receives information from various regions of the cortex, and theta may serve to amplify information bearing signals from the cortex, leading to their storage in the hippocampus. In a similar manner, another system of inputs to the hippocampus from the midline thalamus has been identified that appears to code for the dimension of attention to environmental events. This project will, in part, examine the effects of selective activation (or inactivation) of inputs to the hippocampus that are responsible for attention and meaning in memory processing in this structure. More specifically, the manner and degree to which these manipulations will enhance (or impede) learning and memory in rats will be examined. These studies should significantly contribute to an understanding of mechanisms of memory formation in the brain. The researcher engages undergraduate and graduate students in his research projects and continues to disseminate his experimental findings in various professional and lay forums.

DESCRIPTION (provided by applicant): Nuclei lying on the midline of the thalamus (midline nuclei) receive afferent information from regions of the brain that exert activating effects on behavior and on the electrical activity of the cortex. The midline nuclei, in turn, send projection to structures of the forebrain that participate in emotional and cognitive behavior which collectively constitute the limbic system. Two prominent distribution sites of the nucleus reuniens (RE) of the midline thalamus are the hippocampus and medial prefrontal cortex. Both structures serve a prominent role in various aspects of memory. Based on its diverse set of afferents and selective output to structures of the limbic forebrain, the midline nuclei of thalamu appear to serve a prominent role in affective and cognitive functions, perhaps by mediating the effects of arousal/attention on cognitive behavior. In this regard, it is well recognized that destruction of the midline thalamus in humans produces coma; and its partial disruption gives rise to a condition termed the minimally conscious state (MCS) -- or one in which patients are conscious but generally unaware and unresponsive to their surroundings. Schiff and colleagues demonstrated the remarkable findings that deep brain stimulation (DBS) of the midline thalamus in MCS patients restored a range of motor and cognitive functions that had long been dormant (or absent). Surprisingly, despite its apparent direct involvement in a host of limbic-related functions, comparatively little research has been done on the RE/midline thalamus. A few recent studies have shown, however, that lesions of nucleus reuniens in rats severely disrupt learning and memory. We suggest that the RE/midline thalamus represent a critical node in communication between 'lower' and 'higher' regions of the limbic system and among structures of the limbic forebrain. We intend to examine the midline thalamus, and particularly nucleus reuniens (RE), from diverse perspectives with the objective of determining its contribution to affective and cognitive functions. Specifically, with three sets of experiments, we intend to characterize: (1) the effects of selective RE lesions on learning and memory - or on hippocampal- or mPFC-dependent functions; (2) the effects of reversible inactivation of RE on place cell activity in the hippocampus with the aim of determining the contribution of RE to place cell stability and hence to spatial memory; and (3) the potentiating effects of RE stimulation on entorhinal cortical-elicited long term potentiation (LTP) at CA1 of the hippocampus and on hippocampal-elicited LTP at the mPFC. In sum, these studies should provide critical information on the modulatory effects of RE on the limbic forebrain in the control of arousal/attention, affect and cognition with direct implications for disorders of consciousness, schizophrenia and epilepsy.

Project Summary The nucleus reuniens (RE) of the thalamus lies ventrally on the midline of the thalamus and is the largest of the midline nuclei of the thalamus. The nucleus reuniens is intimately connected with structures of the limbic system ? or a system of the brain that deals with affect, memory, and cognition. Specifically, RE receives a vast array of input from limbic-related structures, and predominantly directs its output to limbic cortical structures, primarily to the hippocampal formation (HF) and to the prefrontal cortex (PFC). Both structures serve a prominent role in various aspects of memory. With respect to the functional role of RE, it has recently been shown that lesions (or inactivation) RE produce attentional deficits, presumably involving the disruption of RE connections with the PFC, and severe deficits in working memory involving the loss of RE actions on the hippocampus. As RE is strategically positioned to influence executive functions of the prefrontal cortex and mnemonic functions of the hippocampus, it has been directly implicated in several neurological disorders in which these functions are disrupted, including alterations of consciousness, Alzheimer?s disease, epilepsy, and schizophrenia. With the present proposal, we intend to continue our investigation of nucleus reuniens by focusing on its role in mnemonic processing and executive functions, such as attention and behavioral flexibility, which rely on the hippocampus and the orbitomedial prefrontal cortex. This will be done by the following. 1. By examining the activity of single cells of RE during natural sleep- waking states with the expectation that RE cells would show differential changes across stages of sleep and waking that would provide important insight into the state specific effects of RE on the hippocampus and the prefrontal cortex, as revealed by the relationship of RE cellular activity to local field potentials of the HF and prefrontal cortex. 2. By examining the effects of inactivating RE, per se, or RE fibers targeting the HF or the orbitomedial PFC on an odor/texture attentional set shifting task (AST) which measures executive functions such as attention and behavioral flexibility with the expectation that disruption of RE will impair executive functions, particularly so with the suppression of RE fibers projecting to the ventral HF and the orbital cortex. 3. By examining the effects of inactivating nucleus reuniens or RE fibers to targets on a delayed alternation T-maze task that measures spatial working memory (SWM) with the expectation that disruption of RE would significantly alter performance on this task, particularly so with the suppression of RE fibers projecting to the dorsal HF and to the medial PFC. 4. By examining the activity of RE cells in the T-maze task with the expectation that RE neurons would discharge in manner indicative of a transfer of information between the HF and the medial PFC (via RE), leading to the successful performance on this task. We believe that this series of experiments will serve to demonstrate that nucleus reuniens plays a very critical role in working memory and executive functions in conjunction with direct effects on the hippocampus and PFC. In addition, these studies should provide important information on the modulatory role of RE on attentional and cognitive processes which are altered in several neurological disorders including failures of consciousness, Alzheimer?s disease, schizophrenia, and epilepsy.

PROJECT SUMMARY The nucleus reuniens (RE) of the ventral midline thalamus sits as the nonpareil nexus of prefrontal-hippocampal communication. While RE selectively directs its output to cortical limbic targets including the hippocampus (HF) and orbitomedial prefrontal cortex (PFC), RE receives a diverse array of inputs, including a dense supply of serotonergic (5- HT) fibers from the dorsal (DR) and median (MR) raphe nuclei. While DR and MR form two distinct essentially nonoverlapping 5-HT pathways, RE serves as a key site of convergence as MR and DR both distribute heavily to the midline thalamus. While RE's interactions with PFCàHF circuitry have been linked to cognitive behaviors, virtually nothing is known regarding the neuromodulatory role of 5-HT in RE. With the present proposal, we intend to continue our investigation of RE by utilizing pharmacological and chemogenetic approaches in conjunction with behavioral testing to elucidate how 5-HT modulation of RE participates in affective, mnemonic, and executive functioning. We hypothesize that 5-HT influence of RE is essential to flexible behavior and quelling 5-HT input to RE will increase avoidance behavior and impair executive measures, with each of these behaviors recruiting specific raphe pathways. We will utilize a comprehensive battery of validated rat behavioral assays to assess: 1.) anxiety/avoidance behavior using an open field and elevated plus maze paradigm; 2.) spatial working memory and behavioral flexibility using a delayed nonmatch to sample t-maze paradigm; 3.) and attention and behavioral flexibility using an odor tactile attentional set shifting task. First in Aim 1, we will examine how 5-HT availability in RE affects behavior by locally depleting 5-HT using the neurotoxin 5,7- dihydroxytryptamine. In Aim 2a, we will examine the distinct contributions of DR and MR on RE on behavior by using the inhibitory hM4D DREADD receptor to selectively suppress raphe-RE terminals. In Aim 2b, we will use a TPH2-Cre rat model in combination with DREADD technology to selectively inhibit serotoninergic projections from DR or MR to RE. These aims will be the first to outline the specific contributions of 5-HT input to RE on behavior. These results will provide important information on how raphe-thalamic circuitry contributes to emotional and cognitive functioning and importantly how alterations of this system impact this symptomology in depression, anxiety, and other neuropsychiatric disorders.