Dopaminergic signalling in the rodent neonatal suprachiasmatic nucleus identifies a role for protein kinase A and mitogen-activated protein kinase in circadian entrainment.

The circadian clock of the suprachiasmatic nuclei (SCN) of perinatal rodents is entrained by maternally derived cues. The SCN of neonatal Syrian hamsters express high-affinity D1 dopamine receptors, and the circadian activity-rest cycle of pups can be entrained by maternal injection of dopaminergic agonists. The present study sought to characterize the intracellular pathways mediating dopaminergic signalling in neonatal rodent SCN. Both dopamine and the D1 agonist SKF81297 caused a dose-dependent increase in phosphorylation of the transcriptional regulator Ca2+/cyclic AMP response element (CRE) binding protein (CREB) in suprachiasmatic GABA-immunoreactive (-IR) neurons held in primary culture. The D1 antagonist SCH23390 blocked this effect. Dopaminergic induction of pCREB-IR in GABA-IR neurons was also blocked by a protein kinase A (PKA) inhibitor, 5-24, and by the MAPK inhibitor, PD98059, whereas KN-62, an inhibitor of Ca2+/calmodulin-dependent (CAM) kinase II/IV was ineffective. Treatment with NMDA increased the level of intracellular Ca2+ in the cultured primary SCN neurons in Mg2+-free medium, but SKF81297 did not. Blockade of CaM kinase II/IV with KN-62 inhibited glutamatergic induction of pCREB-IR in GABA-IR neurons, whereas 5-24 was ineffective, confirming the independent action of Ca2+- and cAMP-mediated inputs on pCREB. SKF81297 caused an increase in pERK-IR in SCN cells, and this was blocked by 5-24, indicative of activation of MAPK via D1/cAMP. These results demonstrate that dopaminergic signalling in the neonatal SCN is mediated via the D1-dependent activation of PKA and MAPK, and that this is independent of the glutamatergic regulation via Ca2+ and CaM kinase II/IV responsible for entrainment to the light/dark cycle.