Neuropeptide Y receptor-expressing dorsal horn neurons: Role in nocifensive reflex responses to heat and formalin

Lumbar intrathecal injection of neuropeptide Y (NPY) is antinociceptive, particularly in models of nerve injury and inflammation. Intrathecal NPY does not alter nociception in mice null for the Y1 neuropeptide Y receptor (Y1R) and these mice show enhanced nocifensive reflex responses to aversive thermal, mechanical, visceral and chemical stimuli. Y1R and NPY receptor type 2 (Y2R) are present in the spinal dorsal horn presynaptically on primary afferent, and possibly interneuron terminals, but only Y1R is found postsynaptically on dorsal horn neurons. In the present study, we sought to assess the anatomic effects of lumbar intrathecal disulfide conjugate of neuropeptide Y and saporin (NPY-sap) and to determine the role of Y1R-expressing dorsal horn neurons in nocifensive responses to aversive thermal and chemical stimulation. Lumbar intrathecal injection of NPY-sap was used to selectively destroy Y1R-expressing lumbar dorsal horn neurons followed by testing nocifensive reflex responses on the hotplate and after hind-paw formalin injection. NPY-saporin decreased superficial dorsal horn staining for Y1R, but not neurokinin-1 receptor, mu opiate receptor or NPY peptide, and had no effect on Y1R cell counts in fourth lumbar spinal segment dorsal root ganglia. Loss of Y1R-expressing dorsal horn neurons was associated with increased first response latencies on the 44 °C hotplate and reduced total time rats spent licking and guarding hind paws during 600 s trials at 44 °C or 200 s trials at 47 °C. First hind-paw response latencies to high intensity phasic stimulation at 52 °C were unaffected. NPY-sap also reduced formalin-induced nocifensive behaviors during both interphase and phase II. These data demonstrate that selective destruction of Y1R-expressing superficial dorsal horn neurons, probably excitatory interneurons and/or projection neurons, reduces nocifensive reflex responses, particularly to activation of C nociceptors, and suggest a possible role for Y1R-expressing dorsal horn neurons in pain.