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Submitted on February 18, 2003
Accepted on April 14, 2003
1 Department of Physiology and Department of Medicine, Faculty of Medicine, University of Alberta, 7-26 Medical Sciences Building, Edmonton, Alberta T6G 2H7, CANADA
* To whom correspondence should be addressed. E-mail: anho{at}ualberta.ca.
In this study, we investigated the diurnal changes in the activation state of the 90-kDa ribosomal S6 kinase (p90RSK) in the rat pineal gland. In animals housed under a lighting regimen with 12 h of light, we found an increase in phosphorylated p90RSK during the dark phase and this increase was abolished by treatment with propranolol or continuous exposure to light. To determine the intracellular mechanism involved, rat pinealocytes were treated with norepinephrine. Norepinephrine caused a parallel increase in phosphorylated p42/44 mitogen-activated protein kinase (p42/44MAPK) and p90RSK that was reduced by prazosin or propranolol, indicating involvement of both 
1- and 
-adrenergic receptors. Treatment with dibutyryl cGMP, 4 phorbol 12-myristate 13-acetate or ionomycin mimicked the norepinephrine-stimulated p90RSK phosphorylation whereas dibutyryl cAMP caused a decrease in p90RSK phosphorylation. Inhibition of p42/44MAPK activation by UO126 was effective in reducing the norepinephrine-stimulated p90RSK phosphorylation. Moreover, UO126 had an inhibitory effect on NE-stimulated arylalkyl-N-acetyltransferase activity. These results indicate that the adrenergic-regulated nocturnal increase in p90RSK phosphorylation is mainly mediated through a cGMP 
p42/44MAPK dependent mechanism.
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