The encoding of photoperiodic information ensues in terms of the daily profile in the expression of cyclic AMP (cAMP)-inducible genes such as the arylalkylamine N-acetyltransferase (AA-NAT) gene that encodes the rate-limiting enzyme in melatonin formation. In the present study, we compared the influence of the photoperiodic history on the cAMP-inducible genes AA-NAT, inducible cyclic AMP early repressor (ICER), fos-related antigen-2 (FRA-2), mitogen-activated protein kinase phosphatase-1 (MKP-1), nerve growth factor inducible gene-A (NGFI-A) and nerve growth factor inducible gene-B (NGFI-B) in the pineal gland of rats. For this purpose, we monitored the daily profiles of each gene in the same pineal gland under a long (light/dark 16:8) and a short (light/dark 8:16) photoperiod by measuring the respective mRNA amounts by real-time polymerase chain reaction analysis. We found that, for all genes under investigation, the duration of increased nocturnal expression is lengthened and, in relation to light onset, the nocturnal rise is earlier under the long photoperiod (light/dark 16:8). Furthermore, with the exception of ICER, all other cAMP-inducible genes tend to display higher maximum expression under light/dark 8:16 than under light/dark 16:8. Photoperiod-dependent changes persist for all of the cAMP-inducible genes when the rats are kept for two cycles under constant darkness. Therefore, all cAMP-inducible genes are also influenced by the photoperiod of prior entrained cycles. Our study indicates that, despite differences regarding the expressional control and the temporal phasing of the daily profile, cAMP-inducible genes are uniformly influenced by photoperiodic history in the rat pineal gland.
