A daily rhythm of activator protein-1 activity in the rat pineal is dependent upon trans-synaptic induction of JunB.
The daily cycle of phenotypic variation in the mammalian pineal provides a unique model for the investigation of the molecular mechanisms that regulate neurotransmitter synthesis. In the rat, a circadian adrenergic mechanism directs a change in serotonin metabolism that results in the nocturnal production of melatonin. Activity of the activator protein-1 transcriptional regulatory complex, as demonstrated by band-shift assays of rat pineal gland extracts, has now been shown to exhibit a rhythm, in vivo, which is temporally correlated with the rhythm of melatonin synthesis. Thus, nocturnal activator protein-1 activity (23.00 h) is markedly elevated, being 8-fold higher than the level of light-phase activity (P < 0.005). The nocturnal activator protein-1 protein complex is induced through a trans-synaptic, beta-adrenoceptor-linked mechanism and is characterized by the prolonged participation of JunB as demonstrated using antibodies for specific activator protein-1 proteins. Indeed, JunB appears to be a major component of nocturnal changes in activator protein-1 activity, JunD forming an additional, constitutive component which is not affected by the nocturnal adrenergic signal. The alpha 1-adrenoreceptor-linked c-Fos protein, which is coordinately induced with JunB, does not form a stable component of nocturnal activator protein-1 activity. In contrast, parallel experiments showed that c-Fos does form a major component of the hippocampal activator protein-1 complex that is induced in rats following kainic acid treatment. In the pineal, a similar, although not identical, pattern of activator protein-1 activation has also been demonstrated in cultured glands following treatment with norepinephrine. Immunoblotting has demonstrated parallel accumulation of JunB and c-Fos protein in pineal nuclear fractions following stimulation both in vivo and in vitro. The results provide evidence of posttranscriptional selection of neurotransmitter-stimulated activator protein-1 protein complexes, a mechanism which complements the differential induction of fos and jun genes in the pineal, and serves to generate a specific activator protein-1 transcription factor complex. This finding has general implications for the functional interpretation of fos and jun gene induction in neuronal systems. The stable JunB complex demonstrated here may be considered as one component of a timing mechanism which acts to perpetuate synaptic signals and thereby maintain an appropriate period of nocturnal pineal function.