William C. Hall - US grants

Many types of experiments indicate that the superior colliculus plays a key role in initiating orienting movements of the head, eyes and ears toward objects of interest. At least three classes of models of the relationship between its structure and function can be distinguished: those proposing that pathways between the layers of the superior colliculus integrate the sensory and motor systems involved in initiating orienting movements; those arguing that the layers are independent and serve mainly to distribute information to different destinations; and those proposing that connections between compartments within individual layers provide the substrate for interactions between the sensory and motor systems. The specific aims of this application are to test these models. Because of technical limitations, past anatomical studies have provided few clues to intracollicular circuitry. Consequently, current models have been based primarily on physiological properties of collicular neurons, such as the latencies of their responses and properties of their connections, but the postulates are difficult to test with physiological methods. The proposed research will test the current models directly by intracellularly injecting cells of each layer and tracing their intracollicular connections, using living brain slices from the tree shrew, Tupaia belangeri. Relative to other commonly studied species, this primate-like mammal has an especially large and well differentiated superior colliculus, which greatly facilitates the task of analyzing the connections between the layers. In some cases, cells will be prelabeled by retrograde axonal transport so that the injected cells can be identified by the extracollicular destinations of their axons, in addition to their location, morphology and intracollicular connections. In other experiments, sensory or motor pathways to the superior colliculus will be prelabeled to determine whether they converge on collicular neurons of different types. These experiments are designed to provide a framework for new models based on knowledge of intracollicular circuitry and, in this way, will contribute to our understanding of the neural mechanisms underlying sensorimotor integration in the vertebrate brain. receptive and movement fields.

[unreadable] DESCRIPTION (provided by applicant): GABAergic inhibitory circuitry within the intermediate layer of the superior colliculus plays an essential role in generating the premotor signals that initiate shifts in the direction of gaze. However, the spatiotemporal dynamics of this inhibition remain unclear. We will use Clomeleon, a genetically-encoded Cl- indicator, to image in real time the spatial organization of Cl- dependent synaptic inhibition. We have produced transgenic mice that express Clomeleon in premotor cells of the intermediate layer. Clomeleon chronically expressed in these neurons is capable of responding to Cl- fluxes associated with activation of GABAA receptors. We propose to use these Clomeleon mice to determine whether Clomeleon is sufficiently sensitive to detect the changes in [Cl-]i that are generated in the premotor cells by stimulation of GABAergic pathways. To test the hypothesis that activation of premotor cells in a region that commands gaze shifts in one direction inhibits the [unreadable] activity of cells in other regions that would command competing gaze shifts toward stimuli in other directions, we will use Clomeleon to visualize the spatiotemporal distribution of inhibition throughout the intermediate layer that is evoked by activity within one region of this layer's spatial map of gaze vectors. This approach of functional imaging CI-mediated inhibition potentially can be applied to the study of inhibitory circuitry in any brain area. [unreadable] [unreadable] [unreadable]

DESCRIPTION: In this application we have one Specific Aim: To test a new model of saccade generation by the nigro-colliculus pathway by determining the influence of the inhibitory inputs from the substantia nigra pars reticulata (nigra) on identified neuronal cell types within the superior colliculus. To achieve this aim we propose two experiments using the in vitro rodent model. Each of these experiments will be performed on both projection neurons and GAD 67 GFP labeled interneurons within both the intermediate (premotor) layers and the superficial (visual) layers of the colliculus. We shall, 1) introduce hyperpolarizing current into collicular neurons to assess expression of Ih, the hyperpolarization activated cation current and use hyperpolarizing voltage steps under voltage clamp to test for Ih in collicular neurons. 2) Introduce realistic spike trains into the nigra and record from neurons in the colliculus to assess whether nigral activity invokes Ih leading to bursting in superior colliculus neurons. The work proposed here may change the current dogma regarding the role of the nigro-collicular pathway in eye movement initiation by showing that its role is active rather than a passive gating of cortical afferents. The results therefore, may provide a new understanding of how eye movement symptoms occurring in neurological and neuropsychiatric diseases involving the basal ganglia such as Parkinson's disease, Huntington's disease, Tourette syndrome, attention deficit disorder, obsessive compulsive disorder and schizophrenia arise.