Circadian variations of neuropeptide Y-like immunoreactivity in the rat pineal gland.

Neuropeptide Y, which is present in the sympathetic nerves innervating pineal glands, has been shown to affect N-acetyltransferase activity and melatonin production. Using enzyme immunoassay, we measured daily patterns of neuropeptide Y (NPY) in rat pineal glands in light-dark cycles (LD) and in constant darkness (DD). NPY within rat pineal glands displayed daily rhythms in LD with a peak at zeitgeber time 16. Similar rhythms in the NPY content were also observed in DD. These rhythms have a similar phase-relationship to that of the rhythm of NAT activity, suggesting that NPY in rat pineal glands plays a role in circadian rhythms of NAT activity.