DAILY CHANGES IN PRESYNAPTIC CHOLINERGIC ACTIVITY OF RAT SYMPATHETIC SUPERIOR CERVICAL-GANGLION

The in vitro capacity of sympathetic superior cervical ganglia (SCG) t o take up [H-3]choline from the extracellular medium, to synthesize ac etylcholine from [H-3]choline, and to release [3H]acetylcholine in res ponse to a high K+ concentration, were examined in rats throughout a 2 4-h cycle. Both the release of [H-3]acetylcholine and the synthesis of [H-3]acetylcholine from [H-3]choline exhibited significant diurnal va riations, showing maxima during the first half of the night. After the se maxima, nocturnal acetylcholine release and synthesis decayed to da ytime levels and remained low until the end of the night. [H-3]Choline uptake by rat SCG did not vary significantly throughout a 24-h period . A 1.5-h exposure of rats to darkness at the 5th hour of light phase of the daily photoperiod did not change significantly any parameter st udied. A 20-min, 5-Hz, electrical stimulation of the preganglionic tru nk of SCG excised from rats at noon increased significantly subsequent K+-induced [H-3]acetylcholine release but did not change [H-3]acetylc holine synthesis. In decentralized SCG of rats subjected to a unilater al SCG decentralization and a contralateral sham-operation 7 days earl ier, [H-3]acetylcholine release and synthesis were highly reduced or a bolished at the decentralized side, while [H-3]choline uptake remained unaltered. The present results suggest that an activation of pregangl ionic rat SCG neurons takes place during the first half of the scotoph ase.