Nociceptive Role of SST and NT receptor-expressing dorsal horn neurons

Veterans Health Administration, Decatur PA

Investigators

Abstract

DESCRIPTION (provided by applicant): Successful management of chronic pain is a major ongoing and unmet clinical challenge. The superficial dorsal horn (SDH) of the spinal cord has long been identified as the first key gateway for nociceptive information entering the CNS. Interventions applied at the level of the dorsal horn have the potential to selectively interdict the flow of specific types of nociceptive information to the brain with greater efficacy and fewer side effects than the currently available alternatives, medications or surgery. The focus of the present proposal are two types of inhibitory nocifensive neurons in the SDH and the therapeutic potential of two neuropeptides, which given intrathecally activate receptors on inhibitory SDH interneurons and produce analgesia in humans, somatostatin (SST), or anti-nociception in animals, neurotensin (NTS). Both of these neuropeptides are abundantly present in the SDH, and neurons with the cognate receptors are located in lamina II (substantia gelatinosa). SST receptors (sst2a) are expressed entirely on GABAergic inhibitory interneurons suggesting that SST produces naloxone-insensitive analgesia by exciting inhibitory interneurons that normally inhibit nociceptive projection neurons. Neurotensin is primarily excitatory, is produced/released by glutamatergic (excitatory) interneurons and also produces naloxone-insensitive anti-nociception by presumably exciting inhibitory interneurons which, in turn, inhibit nociceptive projection neurons. The aim of the present proposal is to evaluate SDH inhibitory interneurons as targets for achieving sustained, but ultimately reversible, non-opioid analgesia by enhancing the activity of these inhibitory nociceptive interneurons, i.e. reversing the inhibitory deficit thought important in the pathogenesis of chronic neuropathic pain. The specific study objectives for SST are: 1 - determine the precise anatomic effects (target cell loss, neuropathology) and nocifensive behavioral effects produced by lumbar intrathecal injection of the targeted cytotoxin, SST-saporin, in normal rats and rat models of chronic inflammatory and neuropathic pain; 2 - determine effects of tonically activating sst2a receptor-expressing SDH interneurons with lumbar intrathecal injection of SST-cholera toxin A subunit conjugate (SST-CTA) in normal rats and rat models of chronic inflammatory and neuropathic pain; and 3 - then determine the effects of lumbar intrathecal injection of SST conjugated to the gene for CTA (SST-polyplex-CTA DNA) in normal rats and rat models of inflammatory and neuropathic pain. The specific objectives for NTS are: 1 - determine the precise anatomic effects (target cell loss, neuropathology) and nocifensive behavioral effects produced by lumbar intrathecal the targeted cytotoxin, NTS-saporin, in normal rats and rat models of chronic inflammatory and neuropathic pain; 2 - determine effects of tonically activating NTS1 receptor-expressing SDH interneurons with lumbar intrathecal injection of NTS-cholera toxin A subunit conjugate (NTS-CTA) in normal rats and rat models of chronic inflammatory and neuropathic pain; and 3 - then determine the effects of lumbar intrathecal injection of NTS conjugated to the gene for CTA (NTS-polyplex-CTA DNA) in normal rats and rat models of inflammatory and neuropathic pain. Anatomic studies will determine efficacy and selectivity of the delivery of CTA or the CTA gene using immunohistochemical markers for excitatory and inhibitory neurotransmitters and receptors and for phosphoCREB (a marker for activation of adenylyl cyclase by CTA). Behavioral analyses will include a standard reflex nociceptive test (thermal plate lick/guard) and operant tests (thermal escape and thermal preference) that require cerebral participation in making choices based on the degree of discomfort. Preliminary results with NTS-saporin produces robust spontaneous behaviors (scratching) highly suggestive of ongoing discomfort and increased hotplate responses while lumbar intrathecal SST-saporin also showed spontaneous scratching. These novel strategies for tonically exciting inhibitory interneurons with CTA,may open a new chapter in the search for safer, more effective ways to manage chronic, intractable pain.