CIRCADIAN PHASE-SHIFTS TO NEUROPEPTIDE-Y IN-VITRO - CELLULAR COMMUNICATION AND SIGNAL-TRANSDUCTION

Mammalian circadian rhythms originate in the hypothalamic suprachiasma tic 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 neu ropeptide Y (NPY) on the hamster circadian clock in vitro. Drug or con trol 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-shift ing effect of NPY was not altered by block of sodium channels with tet rodotoxin or block of calcium channels with cadmium and nickel, consis tent 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 pha se shifts to NPY. Blocking GABA(A) receptors with bicuculline (Bic) ha d effects only at very high (millimolar) doses of Bic, whereas blockin g 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. B athing the slice in low Ca2+/high Mg2+ can block phase shifts to NPY, possibly via a depolarizing action. A depolarizing high K+ bath can al so block NPY phase shifts. The results are consistent with direct acti on of NPY on pacemaker neurons, mediated through a signal transduction pathway that depends on activation of PKC.