N-type calcium channels regulate release of glutamate and substance P from primary sensory afferents in the superficial dorsal horn of the spinal cord. Presynaptic N-type calcium channels in the spinal dorsal horn are major targets of drugs to treat neuropathic and chronic pain syndromes. However, we don't yet understand why N-type calcium channel blockade is so effective against neuropathic pain and we don't understand how important they are in mediating the spinal analgesic actions of opiates. In this second phase of our project we will test our hypothesis that a distinct isoform of N-type calcium channels is essential for spinal level analgesic actions of a sub-set of drugs including opiates in vivo. During the 1st funding period of this grant we discovered that this novel splice isoform the N-type calcium channel, CaV2.2-e37a, is significantly more sensitive to drugs that act through G protein coupled receptors, including morphine. This is important because i) we previously showed that CaV2.2-e37a channels are enriched in nociceptors whereas CaV2.2-e37b channels are found throughout the nervous system, and ii) spinal level analgesic actions of morphine and other drugs are, at least in part, mediated by inhibition of presynaptic N-type channels in the dorsal horn. We are now ready to test the functional significance of CaV2.2-e37a in vivo in normal and disease states. We have developed four novel mouse lines using single exon gene targeting. These mice are genetically modified to express exclusively N-type channels containing either CaV2.2-e37a or CaV2.2-e37b but not both. We have developed all the necessary technical expertise to assess these mice at electrophysiology, biochemical, immunohistochemical, and behavioral levels. We will ask if CaV2.2-e37a channels mediate basal nociception and basal analgesic effects of morphine. We will also ask if CaV2.2-e37a channels are required for hyperalgesia to develop with neuropathic pain, and if they are necessary to mediate the analgesic effects of morphine during neuropathic pain. Our studies are highly relevant to the challenge of finding effective treatment for neuropathic and more generally chronic pain. Peripheral neuropathies can develop following acute nerve injury, and are strongly associated with diabetes, autoimmune disorders, malnutrition, and infections. There are no effective treatments for the millions of neuropathic and chronic pain sufferers.
Our studies are highly relevant to finding effective treatment for neuropathic and more generally chronic pain. By studying a novel form of the N-type calcium channel in the pain pathway, our studies of novel knock-in mice should lead to new strategies for inhibiting the activity of this N-type calcium channel and alleviating pain sufferers. )
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