Circadian phase shifts to neuropeptide Y in vitro: Cellular communication and signal transduction

Mammalian circadian rhythms originate in the hypothalamic suprachiasmatic nuclei (SCN), from which rhythmic neural activity can be recorded in vitro. Application of neurochemicals can reset this rhythm. Here we determine cellular correlates of the phase-shifting properties of neuropeptide Y (NPY) on the hamster circadian clock in vitro. Drug or control treatments were applied to hypothalamic slices containing the SCN on the first day in vitro. The firing rates of individual cells were sampled on the second day in vitro. Control slices exhibited a peak in firing rate in the middle of the day. Microdrop application of NPY to the SCN phase advanced the time of peak firing rate. This phase-shifting effect of NPY was not altered by block of sodium channels with tetrodotoxin or block of calcium channels with cadmium and nickel, consistent with a direct postsynaptic site of action. Pretreatment with the glutamate receptor antagonists (DL-2-amino-5- phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione disodium) also did not alter phase shifts to NPY. Blocking GABA(A) receptors with bicuculline (Bic) had effects only at very high (millimolar) doses of Bic, whereas blocking GABA(B) receptors did not alter effects of NPY. Phase shifts to NPY were blocked by pretreatment with inhibitors of protein kinase C (PKC), suggesting that PKC activation may be necessary for these effects. Bathing the slice in low Ca²⁺/high Mg²⁺ can block phase shifts to NPY, possibly via a depolarizing action. A depolarizing high K⁺ bath can also block NPY phase shifts. The results are consistent with direct action of NPY on pacemaker neurons, mediated through a signal transduction pathway that depends on activation of PKC.