Hypothalamic regulation of circadian noradrenergic input to the chick pineal gland

Abstract

While the avian pineal gland contains circadian oscillators and photoreceptors capable of producing circadian rhythms of the hormone melatonin, it is extensively innervated by post-ganglionic fibers of the superior cervical ganglia which release norepinephrine (NE) rhythmically. Norepinephrine turnover is higher during subjective day than during subjective night. In mammals, this rhythmic input, which is higher in subjective night than subjective day, derives from the hypothalamic suprachiasmatic nuclei (SCN) and is essential for rhythmic melatonin production. The present study was designed to determine whether one of two candidates for the avian homologue of the mammalian SCN is necessary for rhythmic NE turnover in the chick pineal gland. Either electrolytic lesions or sham lesions were delivered to the periventricular preoptic nuclei (PPN) or to the visual suprachiasmatic nucleus (vSCN). After recovery, the rates of decline in [NE] were determined following pretreatment with alpha-methyl-p-tyrosine, a tyrosine hydroxylase inhibitor, at mid-subjective day or at mid-subjective night. Birds receiving sham surgeries in either PPN or vSCN and birds receiving lesions of the PPN exhibited rhythmicity in NE turnover. No rhythm of NE turnover could be determined in birds with ablated vSCN.