RESUMO
Neurons of the mammalian circadian pacemaker in the hypothalamic suprachiasmatic nuclei exhibit a rhythm in firing rate that can be reset by neuropeptide Y. We recorded the effects of neuropeptide Y on Na+ and K+ conductances of hamster suprachiasmatic nuclei neurons using whole-cell, perforated-patch and cell-attached patch-clamp recordings, both in dissociated and brain slice preparations. While neuropeptide Y had no effect on voltage-gated Na+ currents, neuropeptide Y activated a leak K+ current. Neuropeptide Y phase advances in the suprachiasmatic nuclei brain slice preparation were blocked by a number of K+ channel blockers (tetraethylammonium chloride, dendrotoxin-I, glybenclamide). However, a K+ ionophore, valinomycin, did not shift the rhythm. The inhibition by tetraethylammonium chloride did not persist in the presence of glutamatergic receptor blockers. We have previously shown that glutamate can oppose neuropeptide Y phase-shifting actions, suggesting that K+ channel inhibition acts by inducing glutamate release. Protein synthesis inhibitors had no effect on clock phase when applied during the subjective day, and had no influence on neuropeptide Y-induced phase shifts. On the other hand, glutamate's ability to inhibit neuropeptide Y shifts was abolished by protein synthesis inhibition. Thus, while neuropeptide Y phase shifts do not require protein synthesis, glutamate blocks neuropeptide Y shifts via increased gene expression during the subjective day, at a time when it does not reset the clock. These results indicate that neuropeptide Y phase shifts via a mechanism that does not involve changes in membrane conductance or protein synthesis.
