Mechanisms and Consequences of Locus Coeruleus Activation

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DESCRIPTION (provided by applicant): Neuropathic pain reflects a myriad of changes in the periphery, spinal cord, and supra-spinal structures. Interestingly, despite this baffling array of changes, many approved treatments for neuropathic pain recruit, augment, or mimic bulbospinal inhibition including morphine, noradrenaline (NA) re-uptake inhibitors, and clonidine. Although clinical studies suggest a baseline deficit of descending inhibition in patients with neuropathic pain, we believe that activation of this pathway is a key mechanism engaged by several effective and approved treatments. Gabapentin (GBP) is commonly used for treatment of neuropathic pain but its mechanisms for analgesia are not entirely known. We recently demonstrated GBP activates locus coeruleus (LC) neurons via glutamatergic signaling and that the anti-hypersensitivity effects of GBP in rats with peripheral nerve injury relies on this activation and subsequent spinal NA release. The goals of this proposal are to identify the mechanisms by which GBP regulates glutamate release to activate the LC and also to determine whether the mechanisms of GBP on input to LC neurons apply globally or specifically to subsets involved in analgesia and adverse effects such as somnolence, reduction of attention, and memory retention. Initially, we will examine mechanisms by which GBP modulates GABA regulation of pre-synaptic glutamate release in the LC using synaptosomes and microdialysis. Secondly, we will examine mechanisms by which GBP modulates the unique astroglial-neuronal interaction in the LC to increase extracellular glutamate, using cultured astrocytes and microdialysis. Lastly, we will administrate GBP in the LC and compare NA release in the spinal cord, prefrontal cortex, ventro-lateral preoptic area of the hypothalamus, and the dorsal hippocampus by using microdialysis to test whether GBP-induced activation in the LC is a global phenomenon or selective to projections controlling pain, attention, arousal and memory. The proposed experiments will not only provide important information for understanding analgesic mechanisms of GBP but also lead to novel hypothesis regarding the treatment of neuropathic pain. PUBLIC HEALTH RELEVANCE: Gabapentin is widely used to treat chronic pain, but how it works is not well understood. In this proposal, we will study how gabapentin stimulates a natural pain-relief mechanism to reduce pain but also to cause side effects.