Chick pineal melatonin synthesis: Light and cyclic AMP control abundance of serotonin N-acetyltransferase protein

Melatonin production in the pineal gland is high at night and low during th e day. This rhythm reflects circadian changes in the activity of serotonin N-acetyltransferase [arylalkylamine N-acetyltransferase (AA-NAT); EC 2.3.1. 87], the penultimate enzyme in melatonin synthesis. The rhythm is generated by an endogenous circadian clock. In the chick, a clock is located in the pinealocyte, which also contains two phototransduction systems. One control s melatonin production by adjusting the clock and the other acts distal to the clock, via cyclic AMP mechanisms, to switch melatonin synthesis on and off. Unlike the clock in these cells, cyclic AMP does not appear to regulat e activity by altering AA-NAT mRNA levels. The major changes in AA-NAT mRNA levels induced by the clock seemed likely (but not certain) to generate co mparable changes in AA-NAT protein levels and AA-NAT activity. Cyclic AMP m ight also regulate AA-NAT activity via changes in protein levels, or it mig ht act via other mechanisms, including posttranslational changes affecting activity. We measured AA-NAT protein levels and enzyme activity in cultured chick pineal cells and found that they correlated well under all condition s, They rose and fell spontaneously with a circadian rhythm. They also rose in response to agents that increase cyclic AMP. They were raised by agents that increase cyclic AMP, such as forskolin, and lowered by agents that de crease cyclic AMP, such as light and norepinephrine. Thus, both the clock a nd cyclic AMP can control AA-NAT activity by altering the total amount of A A-NAT protein. Effects of proteosomal proteolysis inhibitors suggest that c hanges in AA-NAT protein levels, in turn, reflect changes in the rate at wh ich the protein is destroyed by proteosomal proteolysis. It is likely that cyclic AMP-induced changes in AA-NAT protein levels mediate rapid changes i n chick pineal AA-NAT activity. Our results indicate that light can rapidly regulate the abundance of a specific protein (AA-NAT) within a photorecept ive cell.