EXPERIMENTAL STUDIES IN SENSORY PATHWAYS

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Normal basal ganglia function is highly dependent on the activities of dopaminergic (DA) neurons in the SNc and GABAergic neurons in SNr/EP. However, many aspects of the mechanisms controlling SNc and SNr neural activity which influence dopamine and GABA release at their projection targets are still not well understood. Recently there has been a growing interest in converging inputs from the tegmental pedunculopontine (PPN) and subthalamic nucleus (STN) to the SNc DA and SNr GABA neurons. Although the functional role of these inputs is poorly understood, our previous studies suggested that acetylcholine (ACh) inputs from PPN depolarize the resting membrane potential of DA and GABA neurons by a combination of increased cationic influx and decreased K+ efflux, and reduces Ca2+ entry during the pacemaker-like slow depolarization (PLSD), which decreases the amplitude and duration of the subsequent spike after hyperpolarization. We, therefore, hypothesize that (1) the action of ACh increases the tendency for SNc DA and SNr GABAergic neurons to exhibit burst firing in response to glutamatergic (glu) input from the STN and PPN and (2) in addition to glu and ACh excitatory projections from the PPN and STN, but also GABAergic inhibitory projections to SN (causing disinhibition of GABA system by phasic inhibition of the SNr neurons) are modulating SNc DA and SNr GABAergic neuronal firing behavior. To test these hypothesis, we will utilize the newly developed in vitro organotypic culture preparation consisting of PPN, SN, STH and STR to conduct morphological and electrophysiological studies. Morphological studies will involve light and electron microscopy to (1) identify ACh, glu and GABAergic projection patterns and their terminal sites on the SNc DA and SNr GABA neurons, (2a) identify the nuclear origin of ACh and glu projections to SNc DA and SNr GABAergic neurons with a combined biocytin intracellular labeling, and (2b) identify the locations of the intracellularly labeled biocytin terminals making synapses with SNc DA and SNr GABA neurons and (3) identify the location of ACh, glu and GABA receptors on the SNc DA and SNr GABA neurons. Electrophysiological studies will involve use of whole-cell (current) clamp or intracellular sharp electrode recording to delineate the effect of (1) cholinergic receptor activation on Ca2+ entry during the PLSD and AHP,(2) the effect of excitatory synaptic inputs from PPN and STN and GABAergic inhibitory inputs from the STR on firing properties and bursting of SNc DA and SNr GABAergic neurons. Understanding the role of multiple inputs in the control and/or modulation of SNc and SNr cellular activity which affect their target structures (i.e. striatum and thalamus) is of fundamental importance relative to motor and behavioral function as well as clinical entities such as Parkinson's disease.