A role for protein kinase intracellular messengers in substance P- and nociceptor afferent-mediated excitation and expression of the transcription factor Fos in rat dorsal horn neurons in vitro

Expression of the inducible transcription factor Fos in the spinal dorsal h orn in vivo is associated with nociceptive afferent activation, but the und erlying stimulation-transcription pathway is less clear. This in vitro spin al cord study concerns the role of protein kinase A and C second messengers in substance P receptor (NK1R)-mediated or nociceptive afferent-evoked neu ronal excitation and Fos expression. Nociceptive afferent (dorsal root) sti mulation of isolated spinal cords (10-14 day old rats) evoked a 'prolonged' excitatory polysynaptic potential (DR-EPSP) that was attenuated (P<0.05) b y: the protein kinase A inhibitor, Rp-cAMP; the protein kinase C inhibitor, bisindolymaleimide 1; and the selective NK1R antagonist, GR82334. Neuronal excitations induced by the NK1R agonist [Sar(9),Met(O-2)(11)]-SP were atte nuated by Rp-cAMP, bisindolymaleimide I and GR82334. Effects of the protein kinase A and C inhibitors on the DR-EPSP or the [Sar(9),Met(O-2)(11)]-SP-i nduced depolarization were nonadditive, suggesting convergence of these int racellular signalling pathways onto a common final target. Nociceptor affer ent-induced Fos, detected by immunohistochemistry in superficial and deep d orsal horn laminae, was attenuated by Rp-cAMP, bisindolymaleimide I and GR8 2334. In spinal cords pretreated with TTX to eliminate indirect neuronal ac tivation, [Sar(9),Met(O-2)(11)]-SP (1-20 <mu>m) elicited a dose-related exp ression of Fos that was reduced by Rp-cAMP, bisindolymaleimide I and GR8233 4. The effects of these inhibitors were most pronounced in the deep laminae . These data support a causal relationship between protein kinase A- or C-d ependent signal transduction, nociceptive afferent- or NK1R-induced neurona l excitation and Fos expression in dorsal horn. Implications for short-vers us long-term modulation of nociceptive circuitry are discussed.