Despite the great importance of glycine receptors in key areas of the nervous system with relevance for human health, remarkably little is known about the regulation of glycinergic synapse strength, and even less about glycinergic synapses in functional circuits. Glycinergic synapses comprise much of the inhibitory drive controlling networks in the spinal cord, brainstem and midbrain, regulating motor behavior, rhythm generation, somatosensory, auditory, and retinal signaling, and coordination of reflex responses. Our long-term goal is to understand how to control the strength of glycinergic synapses in the central nervous system to provide novel drug targets for disorders of spinal cord and brainstem circuits. The objective of this research proposal is to define how potentiation of glycinergic synapses is triggered and maintained, using functional studies in intact spinal cord slices from adolescent mice. Using electrophysiological recordings in spinal cord slices, we find that the inflammatory cytokine, IL-1beta, rapidly upregulates inhibitory glycine receptors on inhibitory neurons in the dorsal horn. To our knowledge, this is the first example of long-term potentiation (LTP) of glycine receptors anywhere in the CNS. The rapid inhibition of inhibitory dorsal horn neurons is expected to promote the transmission of pain signals to the brain, likely contributing to the known nociceptive effects of intrathecal IL-1beta. Our preliminary data support the hypothesis to be tested in this application: that IL-1beta released in the dorsal horn by peripheral injury activates cell adhesion molecules and intracellular protein kinase cascades, rapidly increasing synaptic glycinergic receptor numbers. The rationale for the proposed research is that by identifying the signaling cascades that normally control glycinergic synapse strength, we will provide novel therapeutic targets to treat pain and other glycine receptor-dependent disorders. Proposed experiments will elucidate the signaling pathways and receptor subtypes involved in glycinergic LTP (Aims 1 and 2), primarily relying on sensitive electrophysiological recordings in spinal cord slices. Our preliminary results also indicate that inflammation in vivo potentiates glycinergic synapses, similarly to IL-1beta potentiation observed in vitro. We will therefore identify the role of glycine receptor LTP after peripheral inflammation (Aim 3), using electrophysiological and behavioral assays. The proposed work is innovative, in our opinion, because 1) we have identified the first example of LTP at glycinergic synapses in the mammalian CNS, and 2) as synaptic plasticity can underlie pathology, delineating the underlying mechanisms offers a new way to control glycinergic synapses in disease. The contributions of this research will be the elucidation of as yet entirely unknown mechanisms underlying glycine receptor signaling and synaptic potentiation in a developed tissue setting. These contributions are significant because they constitute critical first steps towards the development of new treatments for pain, respiratory and motor disorders, and auditory disorders.
Our work provides a novel approach to modifying synapses in the first processing station for pain signals, and is relevant to public health because it could provide treatment strategies for chronic pain and other glycine receptor-dependent neuropathies. Our work is also within the mission of the NIH as it represents a fundamental creative discovery that will further our understanding of a poorly understood neurotransmitter system.