Relationship of putative nicotinic cholinergic receptors in the suprachiasmatic nucleus to levels of pineal serotonin N-acetyltransferase activity in the normally cycling female, the male, and the ovariectomized rat.

We have examined the nocturnal increase in pineal serotonin N-acetyltransferase (SNAT) activity in the rat and the decrease in activity of this enzyme as a result of exposure of the rats to light during the nocturnal period. In females with normal ovulatory cycles and ovariectomized females, levels of the enzyme were similar during the dark (695 +/- 70 pmol/min/gland vs. 590 +/- 68) or after 20 min of lights on at night (107 +/- 5 vs. 105 +/- 8). Carbachol, a cholinergic agonist, was injected at 2400 hr (EST) into the lateral ventricle of orbitally enucleated rats. The agonist simulated the lights on effect in both intact and ovariectomized rats. Highly purified fractions of alpha-bungarotoxin, a nicotinic cholinergic antagonist were injected into either the third ventricle or bilaterally adjacent to the SCN in intact females, intact males, or ovariectomized rats. These injections had no effect on either the nighttime increase or the lights on inhibition of SNAT. This study makes the following observations: 1) As was previously demonstrated [Illnerova, H. Endocrinol. Exp. 9:141-148, 1975], ovariectomy affects neither the daytime (light period) nor nighttime (dark period) activity of pineal SNAT. This study extends previous work by demonstrating that ovariectomy also has no effect on the lights on at night decrease of SNAT activity. 2) This study confirms that intraventricular infusion of carbachol decreases the nocturnal pineal SNAT activity [Zatz, M., and Brownstein, M.J. Brain Res. 213:438-442, 1981]. 3) This study does not confirm the report that the nicotinic cholinergic antagonist alpha-bungarotoxin prevents the loss of nocturnal SNAT activity induced by turning lights on at night [Zatz, M., and Brownstein, M.J. Brain Res. 213:438-442, 1981].