Neuropeptide modulation of spinal pain pathways

This thesis examined modulation of pain signaling by spinal neuropeptides. It investigated the action of neuropeptide Y (NPY) and morphine-3β-D-glucuronide (M3G), a morphine metabolite, which may interact with spinal peptidergic mechanisms. The studies herein relate to the development of new therapeutic approaches for the treatment of neuropathic pain.; Experiments were performed using infrared-differential interference contrast (IR-DIC) videomicroscopy and whole-cell patch-clamp recording from neurons in transverse slices of adult rat lumbar spinal cord.; The main findings are: (1) NPY acted at a presynaptic Y2 receptor to attenuate excitatory postsynaptic currents (EPSCs); and (2) NPY acted at a presynaptic Y1 receptor to attenuate inhibitory postsynaptic currents (IPSCs). Postsynaptic Y1 receptors may also be involved in the effect on inhibitory synaptic transmission, but no evidence was found for involvement of a Y2 receptor in attenuation of excitatory transmission. In addition, NPY suppressed GABAergic and glycinergic IPSCs equally. The presynaptic effect of NPY on EPSCs was similar to the μ-opioid agonist, [D-ala2,N-Met-Phe 4,Gly-ol5]-enkephalin (DAMGO), both in terms of efficacy and site of action; NPY and μ-opioid receptors were frequently co-localized on the same presynaptic terminals. NPY also acted at postsynaptic receptors and activated an inwardly-rectifying conductance.; To examine possible interactions of M3G with spinal peptidergic mechanisms its action were compared to DAMGO and the opioid receptor-like1 (ORL1) agonist, nociceptin/orphanin FQ (N/OFQ). DAMGO and N/OFQ suppressed EPSCs and activated an inwardly-rectifying K+ conductance. M3G neither mimicked nor interfered with these peptidergic receptor mechanisms; its only effect was to suppress IPSCs in a naloxone-insensitive manner. IPSCs were also attenuated by DAMGO, but not by N/OFQ. Thus, the effect of M3G on IPSCs may contribute to the allodynia and hyperalgesia observed after intrathecal application of high doses of morphine.; Comparison of the global effects of M3G with the directed effects of NPY and μ-opioids suggests that inhibition of some, but not all, neurons in the inhibitory network of the substantia gelatinosa can lead to analgesia. In addition, because μ-opioid receptors are down-regulated after peripheral nerve injury, Y1 and Y2 receptor agonists may be especially relevant to neuropathic pain management, as their cellular effects resemble those of μ-opioid receptor agonists.