Nathaniel T. McMullen - US grants

Recent computer microscope studies of the rabbit primary auditory cortex have revealed a significant spatial orientation in the dendrite systems of lamina III/IV pyramidal and nonpyramidal neurons. In order to investigate the relationship between the branching characteristics of cortical afferents and these target cells, the terminal zones and spatial arborizations of thalamocortical and callosal afferents to the primary auditory cortex will be examined. The laminar distribution of afferent terminals will be determined using the anterograde transport of HRP-lectin conjugates following an injection into the medial geniculate body or contralateral auditory cortex. The injection site in each case will be determined physiologically using microelectrodes and acoustic stimulation of the contralateral ear. The arborizations of single thalamocortical and callosal afferents will be anterogradely labeled following an iontophoretic injection of HRP-lectin into the physiologically determined acoustic radiations or contralateral auditory field. The arborizations of single afferents will be digitized with an image combining computer microscope and reconstructed in their entirety using computer assisted section alignment and the pursuit of truncated fibers into adjacent sections. Entire arborizations, including branch points and boutons will be examined from different perspectives in order to study their spatial geometry. A variety of spatial analyses will be performed in order to quantitatively characterize and classify auditory cortical afferents. These studies will yield new information on the radial and tangential organization of afferents to the primary auditory cortex and provide a morphological foundation for studying the functional properties of the auditory cortex such as isofrequency lines and binaural interaction columns. In addition, results of the proposed experiments will establish a firm basis for future studies of the role of thalamocortical afferent activity on the development of auditory cortical dendrite systems.

Electrophysiological studies in the visual and somatosensory cortices have shown that activation of inhibitory local circuit neurons (utilizing GABA) generate important, features of cortical sensory maps such as orientations sensitivity and receptive field size. In contrast, nothing is known concerning the role of GABA-ergic interneurons in the generation of auditory cortical maps. Recent pilot data has revealed evidence of a preferential orientation of inhibitory axonal networks which are orthogonal to the isofrequency lines of the tonotopic map (McMullen, 1990). These GABA-ergic cells may participate in the formation of binaural or tonotopic maps through feedforward lateral inhibition. The goal of the present study is to label target cells more reliably and completely using intracellular injection, and to relate their axonal arbors to the patch-like termination of thalamocortical afferents in the same preparation. The intracortical circuits of adjacent pyramidal cells will also be analyzed so that both excitatory and inhibitory cortical pathways may be characterized. Thalamocortical afferents (TC) arising from the medial geniculate body of the thalamus will be anterogradely labeled with the lectin PHA-L or rhodamine. After suitable survival, cortical slices through the auditory cortex will be made with a tissue chopper. Single cells in the living cortical slice will be impaled with a microelectrode and filled iontophoretically with FITC-labeled HRP. Immuno- and histochemical methods will be used to visualize the thalamocortical afferents and the dendritic and axonal arbors of injected cells in the same tissue slice. The axonal and dendritic territories of the filled cells will be reconstructed in 3-dimensions with a computer microscope system. We will also digitize the patchlike terminal territories of TC afferents. The morphological and transmitter-based dichotomy between pyramidal and nonpyramidal cells will enable us to characterize excitatory and inhibitory circuits that form the basis of auditory cortical networks. The proposed studies have the potential for delineating the cellular substrates that generate major functional features of auditory neocortex: isofrequency strips and binaural bands.

This is a Shannon award providing partial support for the research projects that fall short of the assigned Institute's funding range but are in the margin of excellence. The Shannon award is intended to provide support to test the feasibility of the approach; develop further tests and refine research techniques; perform secondary analysis of available data sets; or conduct discrete projects that can demonstrate the PI's research capabilities or lend additional weight to an already meritorious application. The abstract below is taken from the original document submitted by the principal investigator. DESCRIPTION: (Adapted From The Applicant's Abstract.) The anatomical and functional parcelling of sensory neocortex into distinct regions is heavily influenced by afferent axons originating in the thalamus. These findings as well as advances in anterograde and retrograde tracing methods have led to a reexamination of thalamocortical afferent terminations and their relationship to cortical laminar differentiation. In contrast to the visual and somatosensory systems, little is known concerning the morphology and distribution of specific classes of medial geniculate afferents to auditory cortex. Recent findings have revealed a patch-like organization of thalamo-cortical afferents arising from the vMGB. The afferent patches have an intermittent distribution resembling the patchy distribution of binaural interaction classes defined in physiological studies. In the tangential plane, the patches form elongated bands exceeding 2mm in length parallel to isofrequency contours. These results support a model of functionally distinct parallel vMGB projections to cortex. Aim 1 will determine the relationship between the thalamocortical patches and binaural interaction columns in normal rabbits, using physiological mapping in rabbits injected with biocytin in the MGB. Aim 2 will characterize the projections of the separate subdivisions of the MGB on the basis of laminar termination and spatial organization. Aim 3 is a continuation of experiments of Aim 2 with the addition of calcium binding protein immunohistochemistry. In aim 4, the developmental time-course for the arrival, growth and arborization of thalamocortical axons will be compared with the laminar differentiation of auditory cortex. The postnatal remodeling of thalamocortical arbors and their temporal relationship with dendritic resorbtion by target cells within laminae 3 and 4 will be examined. These studies will provide new information on the thalamocortical circuits subserving tonotopic and binaural maps and the morphological substrates for parallel MGB pathways to A1. They will also provide a morphological basis for understanding the trophic role of auditory afferents in cortical development and dendritic growth.