Neurotransmitter-driven activation of transcription factors is important for control of neuronal and neuroendocrine functions. We show with an in vivo approach that the norepinephrine cAMP-dependent rhythmic hormone production in rat pineal gland is accompanied by a temporally regulated switch in the ratio of a transcriptional activator, phosphorylated cAMP-responsive element-binding protein (pCREB), and a transcriptional inhibitor, inducible cAMP early repressor (ICER). pCREB accumulates endogenously at the beginning of the dark period and declines during the second half of the night. Concomitant with this decline, the amount of ICER rises. The changing ratio between pCREB and ICER shapes the in vivo dynamics in mRNA and, thus, protein levels of arylalkylamine-N-acetyltransferase, the rate-limiting enzyme of melatonin synthesis. Consequently, a silenced ICER expression in pinealocytes leads to a disinhibited arylalkylamine-N-acetyltransferase transcription and a primarily enhanced melatonin synthesis.

The CREM (cyclic AMP-responsive element modulator) gene encodes multiple regulators of the cAMP-transcriptional response by alternative splicing. A developmental switch in CREM expression occurs during spermatogenesis, whereby CREM function is converted from an antagonist to an activator (CREM tau; ref. 2) which accumulates to extremely high levels from the premeiotic spermatocyte stage onwards. To define the physiological mechanisms controlling the CREM developmental switch, we have hypophysectomized rats and observed the extinction of CREM tau expression in testis, thereby demonstrating a central role of the pituitary-hypothalamic axis. We then used the seasonal-dependent modulation of spermatogenesis in hamsters to dissect the hormonal programme controlling this developmental process. By this approach, combined with direct administration of pituitary-derived hormones, we have established that follicle-stimulating hormone (FSH) is responsible for the CREM switch. FSH appears to regulate CREM expression by alternative polyadenylation, which results in a dramatic enhancement of transcript stability.

The CREM gene (cAMP‐responsive element modulator) generates both activators and repressors of cAMP‐induced transcription by alternative splicing. We determined the exon structure of the CREM gene and have identified new isoforms. We show that CREM isoforms with different structural characteristics are generated by the shuffling of exons to produce proteins with various combinations of functional domains. CREM proteins bind efficiently to CREs and here we demonstrate that the various isoforms heterodimerize in vivo with each other and with CREB. The two alternative DNA binding domains of CREM, which are differentially spliced in the various isoforms, show distinct binding efficiencies, while CREM alpha/CREB heterodimers exhibit stronger binding than CREM beta/CREB heterodimers to a consensus CRE in vitro. We identify the protein domains involved in activation function and find that the phosphorylation domain and a single glutamine‐rich domain are sufficient for activation. A minimal CREM repressor, containing only the b‐Zip motif, efficiently antagonizes cAMP‐induced transcription. In addition, phosphorylation may reduce repressor function, as a CREM beta mutant carrying a mutation of the serine phosphoacceptor site (CREM beta 68) represses more efficiently than the wild‐type CREM beta.