Circadian Phase Shifts to Neuropeptide YIn Vitro: Cellular Communication and Signal Transduction

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Abstract
Mammalian circadian rhythms originate in the hypothalamic suprachiasmatic nuclei (SCN), from which rhythmic neural activity can be recordedin 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 clockin vitro. Drug or control treatments were applied to hypothalamic slices containing the SCN on the first dayin vitro. The firing rates of individual cells were sampled on the second dayin 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 GABAAreceptors with bicuculline (Bic) had effects only at very high (millimolar) doses of Bic, whereas blocking GABABreceptors 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.