A seasonal pineal peptide rhythm persists in superior cervical ganglionectomized rats

Neurohypophyseal peptide hormone activity is present in the pineal gland of mammals, and varies over a seasonal cycle. Pineal peptide levels, measured by arginine vasotocin (AVT) radioimmunoassay, increase dramatically for a brief time during August each year. The manner in which this cycle is regulated is as yet unknown. Input to the pineal from sympathetic axons arising in the superior cervical ganglia (SCG) is essential for the generation and regulation of the circadian rhythm in melatonin synthesis, and is the only pathway known to regulate pineal biochemical processes. It was of interest then to determine the impact of the SCG on the seasonal peptide cycle. Levels of pineal arginine vasotocin immunoactivity (iAVT) were monitored during August, 1984, in rats which had been superior cervical ganglionectomized (SCGX), in sham-operated and intact controls (L:D 12:12), and in rats subjected to L:D 22:2. The results indicate that SCGX does not abolish the seasonal cycle, but may influence the timing of the iAVT peak. Inhibition of pineal melatonin synthesis by exposure of rats to L:D 22:2 did not mimic the phase delay seen with SCGX, but did cause a significant increase in the amplitude of the August iAVT activity peak.