The purpose of this study is to determine the effect of intrathecal oxytocin on areas and intensity of hyperalgesia and allodynia induced by topical capsaicin.

Purpose: There is a strong experimental basis to support the study of oxytocin by the spinal route for analgesia in humans. Oxytocin containing cells in the dorsal parvocellular division of the paraventricular nucleus (PVN) project to the spinal cord (1). Noxious stimulation activates these cells via the A1 noradrenergic relay in the pons (2) and produces analgesia by spinal release of oxytocin, since intrathecal injection of an oxytocin receptor antagonist worsens pain behaviors from peripheral inflammation (3). Direct electrical stimulation of the PVN reduces dorsal horn neuronal responses to noxious stimulation, and this is blocked by administration of sequestering antibody for oxytocin (4). Similarly, direct electrical stimulation of the PVN reduces behavioral sensitivity in a model of chronic neuropathic pain, and this effect is blocked by an oxytocin receptor antagonist (5). Intrathecal injection of oxytocin in normal rats reduces dorsal horn neuronal responses to noxious stimuli (6) as well as behavioral responses to noxious thermal (3), mechanical (3), and chemical (7) stimuli. Finally, intrathecal injection of oxytocin in rat models of chronic pain also reduces dorsal horn neuronal responses to sensory stimulation (6) as well as behavioral responses to thermal (5) and mechanical (7) stimuli. Rationale: We anticipate that oxytocin will be effective after spinal injection in humans against chemical induced hypersensitivity states. Objectives: Determine the effect of intrathecal oxytocin on areas and intensity of hyperalgesia and allodynia induced by topical capsaicin.