The primary focus of this research is to investigate the synaptic organization of the developing and adult spinal cord in normal and in diseased animals. We have been focusing on the sensory nerves that supply stretch receptors in muscle because they make very simple direct synaptic connections with motoneurons. In collaboration with Dr. Francisco Alvarez (Wright State University, Ohio) we have shown that these sensory nerves also make direct connections onto a class of inhibitory interneurons within the lumbar spinal cord called Renshaw cells. This finding was unexpected because physiological studies in adult cats suggested that such connections did not exist. Using anatomical methods we showed that adult cat Renshaw cells in the adult cat do in fact receive connections from sensory afferents but presumably are functionally inactive. We have also been examining the role of muscle spindle derived factors in determining the strength and specificity of the connections between sensory neurons and motoneurons. We have found that genetically engineered mice in which muscle spindles are greatly reduced in number exhibit normal patterns of connectivity although the strength of the connections is reduced. A similar reduction in the strength of the connections is seen in mice in which muscle spindles no longer express Neurotrophin-3 (NT-3). These finding suggest that NT-3 regulates the functional strength of the connections but not their specificity. ? In collaboration with Dr. Murat Oz (NIDA) we have been investigating the function of neuropeptides in the spinal cord. Following on an earlier study in which the effects of angiotensin II on spinal neurons were examined, we have found that activation Cholecystokinin B type receptors depolarizes spinal neurons through a G-protein coupled mechanism. We are particularly interested in establishing if endogenous neuropeptide release is involved in the activation of locomotor networks by stimulation of the dorsal or ventral roots and are currently examining the role of Calcitonin Gene Related Peptide - known to be present in the terminals of sensory and motor axons -in this process. ? In collaboration with Dr. Rita Balice-Gordons group (University of Pennsylvania) we have been examining the role of gap junction proteins in synchronizing motoneuron activity in the neonatal period. We have found that mice lacking one of the junctional proteins (Connexin-40), exhibit reduced electrical coupling between motoneurons and desynchronized muscle electrical activity. We are currently developing mice that lack two connexins (40 and 36) and investigating whether or not this disrupts the spontaneously occurring high frequency oscillatory activity that is synchronized across several segments of the spinal cord. If so, we will then determine the effects on the development locomotor networks.? We are also investigating whether motoneurons project to spinal interneurons other than Renshaw cells. Such projections may provide an explanation for the observation that stimulation of the ventral roots (containing motoneuron axons) can trigger locomotor activity.? A final project investigates the changes in cellular and synaptic function in a mouse model of spinal muscular atrophy. This project is a collaboration with Dr. K. Fischbeck of NINDS.
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