Rhythmic and light-induced changes in levels of pineal 5-hydroxyindoles in the pigeon ( Columba livia)

Abstract

Pineal 5-hydroxy- and 5-methoxyindoles of domestic pigeons were measured by differential extractions and spectrofluorometry. The daily rhythms in pineal 5-hydroxytryptamine (serotonin or 5-HT) and related indoles and in pineal 5-hydroxyindole-3-acetic acid (5-HIAA) were described in birds artificially lighted from 04:00 to 18:00. The 5-HT level was lowest at midnight (46.5 ng/pineal) and rose to a maximum of 283.9 ng/pineal at 04:30. This was immediately followed by a 50% decrease; and following the start of darkness an additional drop of 35% occurred. A different rhythm and much lower pineal content of 5-HIAA was found. 5-Hydroxytryptophan and 5-methoxyindole-3-acetic acid were too low in amount for measurement, and a melatonin fraction was too low and variable to provide evidence of a daily rhythm. Experimental changes in the timing of light and darkness showed that: (1) precocious morning rise in 5-HT could be caused by early onset of light, and possibly as well by early start of darkness during the previous evening; (2) delayed start of light did not inhibit the early morning rise; (3) continuation of light through the evening inhibited the usual nocturnal decline in 5-HT. Thus, primary phases in the daily rhythm in pineal 5-HT can be triggered in part by light and darkness. It was shown also that the morning peak in 5-HT could be increased and retained by prior injection of 5-hydroxytryptophan and a monoamine oxidase inhibitor (beta-phenylisopropylhydrazine).