Chronic Administration of Lithium or Other Antidepressants Increases Levels of DARPP‐32 in Rat Frontal Cortex

Abstract: We studied the chronic actions of lithium on rat brain by investigating its effects on cyclic AMP‐dependent protein phos‐phorylation by use of a back‐phosphorylation procedure. We identified one heavily regulated phosphoprotein in frontal cortex as the 32‐kDa dopamine‐ and cyclic AMP‐regulated phosphoprotein (DARPP‐32). Immunoblot experiments demonstrated that chronic lithium regulation of DARPP‐32 back‐phosphorylation is associated with equivalent increases in levels of DARPP‐32 immunoreactivity. Lithium regulation of DARPP‐32 immunoreactivity required chronic drug administration and was not observed in several other brain regions examined. Moreover, chronic administration of the antidepressant imipramine or tranylcypromine produced a similar increase in levels of DARPP‐32 in frontal cortex, whereas other types of psychotropic drugs, including haloperidol. morphine, and cocaine, did not influence DARPP‐32 levels. Increased levels of DARPP‐32 could reflect a common functional effect on frontal cortex of long‐term exposure to lithium and some other antidepressant medications, an effect possibly related to the clinical actions of these drugs.