Tibor Koos - US grants

[unreadable] DESCRIPTION (provided by applicant): GABAergic inhibition is a primary determinant of the activity of neostriatal projection neurons and recent evidence demonstrates that a significant fraction of this inhibitory control originates from the axon collaterals of the projection neurons themselves. The existence of a functional synaptic circuitry among projection neurons suggests a critical role in the regulation of the spatial and temporal pattern of activity of the neostriatum, and in particular in the control of the balance of activity between the functionally segregated output pathways (the "direct" and "indirect" pathways) of the neostriatum. To extend the understanding of the inhibitory circuitry of neostriatal projection neurons the first objective of the present proposal will be to investigate the connectivity and the biophysical properties of synaptic communication between the striato- nigral (direct) and striato-pallidal (indirect) neostriatal projection neurons with paired whole cell voltage and current clamp recordings in acute brain slices . The second major objective is to test the hypothesis that dopamine regulates synaptic transmission among projection neurons in a circuit dependent manner through two different receptor mechanisms. Our preliminary data demonstrate that synaptic transmission between a subset of pairs of projection neurons is enhanced by dopamine through D1-like receptors while D2-like receptors suppress transmission between a complementary population of pairs. Considering the strict segregation of the two mechanisms demonstrated by our preliminary data, and the known projection pathway selective distribution of dopamine receptors, we hypothesize that the feedback circuitry of projection neurons contributes to the dopaminergic control of the relative activity of the direct and indirect pathways through differential modulation of synaptic connections of projection neurons of the two neostriatal output pathways. These questions will be investigated in electrophysiological and pharmacological experiments by obtaining in vitro paired recordings from projection neurons identified using single-cell RT-PCR profiling of the expression of enkephalin, substance P and the 5 main dopamine receptor subtypes. The basic cellular mechanisms responsible for the differential regulation of activity of the opponent output pathways of the neostriatum has important implications for the understanding of the pathophysiology of several neurological and psychiatric disorders including Parkinson's and Huntington's disease, schizophrenia and drug addiction. In particular, understanding the dopaminergic modulation of the neostriatal circuitry may facilitate the identification of novel, non-dopaminergic primary or adjunctive pharmacotherapies of Parkinson's disease and contribute to the improvement of non-pharmacological treatment modalities of the disorder. [unreadable] [unreadable] [unreadable]

DESCRIPTION (provided by applicant): Neostriatal cholinergic (ChAT) interneurons play an important role in the selection and acquisition of adaptive behavioral actions by encoding the salience and reinforcement value of external events. This information is represented in the precise temporal structure of synchronous multiphasic population responses given to the presentation of behaviorally significant stimuli. An important problem is to understand how these transient signals of ChAT interneurons are detected and decoded in the neostriatal network. Our hypothesis, formulated on the basis of preliminary experiments, is that ChAT interneurons control the activity of the striatal circuitry using nicotinic receptor mediated mechanisms to activate multiple parallel GABAergic mechanisms that elicit kinetically distinct independent inhibitory responses in the spiny projection neurons (SPNs) and in other striatal neurons. These responses originate in part from direct activation of multiple as yet unidentified types of GABAergic interneurons that are distinct from the parvalbumin (PV) containing fast spiking (FS) and NPY expressing interneurons, possibly also involve presynaptic nicotinic facilitation of GABA release from axon terminals and together provide selective, cell type specific inputs to SPNs, ChAT and other interneurons. Using in vitro optogenetic inhibition we also reproduced the population response of ChAT interneurons most commonly observed in behaving animals consisting of a pause of firing followed by weakly synchronous rebound-excitation and demonstrated that this behaviorally directly relevant pattern of activity also engages the inhibitory mechanisms described above. Since these responses are sufficient to inhibit action potential generation in large populations of SPNs they are likely to exert significant effects on the functioning of the basal ganglia and regulate ongoing behavior. The proposed experiments aim at understanding the functional organization of these GABAergic circuits and mechanisms and to quantitatively characterize the effects of reinforcement related population responses of ChAT interneurons on the activity of SPNs in vitro and in vivo. Optogenetic methods will be used to control the activity of ChAT and other interneurons in double transgenic animals allowing the genetic targeting and identification of specific cell types. The disynaptic circuits of ChAT interneurons will be analyzed by identifying the GABAergic interneurons activated by ChAT interneurons and then determining the contribution of individual presynaptic cell types to the various GABAergic response components in SPNs and other neurons by selective optogenetic activation of populations of specific classes of interneurons and by examining the effects of optogenetic inhibition of various interneuron types on IPSCs elicited disynaptically by ChAT interneurons. Finally, we will quantitatively characterize the effects of optogenetically reproduced pause-excitation responses of ChAT interneurons on SPNs in vivo and in vitro. These experiments will describe the functional organization of a powerful novel circuit mechanism of the neostriatum and therefore will significantly advance the understanding of the functioning of the basal ganglia. PUBLIC HEALTH RELEVANCE: Numerous important neurological and psychiatric disorders including Parkinson's and Huntington's diseases, schizophrenia, Tourette's syndrome, obsessive compulsive disorder arise from pathological conditions of the neostriatum. Several of these are directly or indirectly linked to a major neuromodultory system the cholinergic interneurons of this structure, yet critically important issues about the functional role of these interneurons remains unanswered. The proposal will elucidate the organization and functional role of a newly discovered circuit mechanism that is responsible for mediating the reinforcement related population responses of these neurons which under pathological conditions may directly contribute to specific behavioral abnormalities associated with these disorders.

Project Summary/Abstract The neostriatum is directly involved in the pathophysiology of several clinically significant neurological and neuropsychiatric conditions. A crucially important question is to understand how dopamine regulates the function of this brain area. Recently, we conducted preliminary experiments to examine the possible roles of newly discovered GABAergic interneurons in the dopaminergic control of the neostriatum. Surprisingly, we found that cell-type specific neurotoxic lesion of tyrosine hydroxylase expressing GABAergic interneurons (THINs) dramatically alters the behavioral response of mice to the administration of the psychostimulant amphetamine. These results suggest that despite their very small population size, THINs have a crucial role in regulating the dopaminergic control of the neostriatum and that of striatal-dependent behavior. The proposal evaluates the feasibility of a new avenue of investigation into the functioning of THINs and their role in the dopaminergic control of behavior. The proposal addresses the following aims. First, we will develop and optimize techniques that will enable the reversible and effective inactivation of THINs in transgenic mice. Second, we will test if transient inactivation of THINs using these tools recapitulates the results of the neurotoxic lesion experiments. Preliminary results indicate that partial, transient pharmacogenetic inhibition can qualitatively reproduce the phenomena observed with irreversible methods. Next, we will test the hypothesis that reversible inactivation of THINs enhances the effect of amphetamine resulting in long- lasting changes in the striatal circuitry, in particular the excitatory cortical inputs to projection neurons. Investigation of the function of THINs may lead to significant breakthroughs in elucidating the mechanism of action of psychostimulants and more generally numerous problems of dopaminergic signaling including the control of movement, learning and reinforcement as well as the processes that contribute to the development of pathological conditions ranging from Parkinson's disease to drug abuse.

Project Summary/Abstract The striatum is the main input structure of the basal ganglia, a system that is crucial not only for voluntary motor control, but also for reinforcement-mediated learning and higher cognitive functions. The importance of the striatum is illustrated by the severe disabilities associated with numerous neurological and neuropsy- chiatric conditions that affect this brain structure. The recent introduction of methods for targeting and manipulating genetically and physiologically defined cell types is currently revolutionizing our understanding of the neostriatum. Our preliminary studies, together with recent research demonstrate that the classically known cell types represent less than a third of the interneuron classes in the striatum, and reveal an intricate and precisely organized circuitry of these neurons. The prosed study will test 4 novel hypotheses which were formulated on the basis of preliminary data and capture functionally important principles of organization of the network of striatal interneurons. First, we will investigate the connectivity of genetically identified interneurons and test the hypotheses that their connectivity is highly cell type specific and structured to form at least 2 separate interneuron sub- networks within the neostriatum. Second, building on our previous research and novel preliminary data we hypothesize that cholinergic interneurons (CINs), which traditionally have been thought of as neuromodulatory elements, participate in a fast bi-directional synaptic network with multiple GABAergic interneurons utilizing nicotinic excitatory connections. Importantly, our preliminary data also suggest the existence of a novel source of nicotinic excitation from the brainstem that appears to selectively target a distinct subset of GABAergic interneurons. Third, we will test the hypothesis that a subset of interneurons are hierarchically organized in the sense that one or more classes of interneurons exist which are specialized to control the activity of other interneurons. These interneuron-specific interneurons are of great interest because their impact may be widely distributed and disproportionally amplified via hierarchical control of other interneurons. Finally, based on preliminary data we hypothesize that a class of novel GABAergic interneurons are essential for normal goal-directed behavior, and we will explore the mechanism of action of these neurons using in vivo electrophysiological and Ca2+-imaging methods. These experiments will yield important new insights into the functioning of the striatum and may help to identify new cellular substrates for therapeutic interventions in a variety of neurological and neuropsychiatric disorders.