Fos expression induced by activation of NMDA and neurokinin-1 receptors in the trigeminal subnucleus caudalis in vitro: Role of protein kinases

Activity-induced neuronal plasticity is partly facilitated by the expression of the immediate-early gene c-fos and the resulting transcription factor Fos. Expression of Fos is associated with nociceptive afferent activation, but a detailed stimulation-transcription pathway for Fos expression has not yet been determined in the trigeminal system. This study utilized a novel in vitro model to determine whether Fos expression can be induced in trigeminal subnucleus caudalis by NMDA or neurokinin-1 receptor activation, and whether inhibition of intracellular kinases has any effect on Fos expression induced by activation of these receptors. Brainstems of male Wistar rats were excised and maintained in artificial cerebrospinal fluid at 37 °C. NMDA or the specific neurokinin-1 receptor agonist [Sar9,Met(O2)11]-SP was applied. These agonists were subsequently tested in the presence of the protein kinase A inhibitor Rp-cAMP or protein kinase C inhibitor chelerythrine chloride. In all experiments the sodium channel blocker tetrodotoxin was used to prevent indirect neuronal activation. Brainstems were processed immunocytochemically for Fos expression, and positive cells were counted in the trigeminal subnucleus caudalis. NMDA and [Sar9,Met(O2)11]-SP significantly increased Fos expression, but these increases could be prevented by chelerythrine chloride. Rp-cAMP had no effect on Fos induced by NMDA but caused a significant reduction in Fos induced by [Sar9,Met(O2)11]-SP. These data demonstrate that in trigeminal subnucleus caudalis activation of either NK1 or NMDA receptors alone induces Fos expression; protein kinases A and C are involved in NK1R-induced Fos while protein kinase A is not required for NMDA receptor-induced Fos.

Research Highlights
► This is the first investigation of Fos expression in the trigeminal system in vitro.
► Activation of NK1-R or NMDA-R induces Fos in trigeminal subnucleus caudalis.
► NK1-R activation induces Fos in caudalis via both PKC- and PKA-dependent pathways.
► NMDA-R activation induces Fos in caudalis via a PKC-dependent pathway.
► Fos induction following NMDA-R activation in caudalis is independent of PKA.