administration and during opiate withdrawal. It was found that acute opiates 
decrease levels of Fos mRNA and protein in this brain region. This reduced 
level of Fos expression persists with chronic opiate treatment, whereas 
precipitation of opiate withdrawal by an opiate receptor antagonist produces 
a dramatic induction of Fos at the mRNA and protein levels (figure 4). The 
observed changes in Fos levels are consistent with the possibility that 
increased levels of expression of GJG^, cAMP-dependent protein kinase, 
and tyrosine hydroxylase in the LC during chronic opiate treatment could be 
mediated to some extent via decreased levels of Fos and that the return of 
levels of expression of these intracellular messenger proteins toward control 
levels during withdrawal could be achieved as a result of elevated Fos 
expression. Induction of Jun mRNA was also observed during opiate 
withdrawal (Hayward et al. 1990), indicating that several members of the 
Fos/Jun family could be involved in these processes. 
The induction of Fos and Jun during opiate withdrawal was observed in several 
brain regions in addition to the LC, including the NAc, VTA, neostriatum, and 
frontal cortex, but not in other regions, notably hippocampus and whole pons 
(figure 5) (Hayward et al. 1990). Regulation of Fos expression in the NAc and 
VTA indicates that this class of transcription factor could contribute to opiate 
actions on the G proteins and the cAMP system in the mesolimbic dopamine 
system via a mechanism similar to the one suggested above for the LC. 
What does induction of Fos during withdrawal reflect? It has been suggested 
that induction of Fos can be used as a molecular measure of neural activity 
(Morgan and Curran 1991), and indeed, depolarizing stimuli would be 
expected to induce Fos via activation of the calcium pathway. However, 
such a relationship between Fos expression and neural activity is not absolute, 
since Fos can be induced by a cAMP signal (Sheng et al. 1990; Morgan and 
Curran 1991), and cAMP is known to exert negative effects on the firing rate 
of particular neurons (see below for cocaine). Thus, a rigorous interpretation 
of these data is that induction of Fos can be used to map those neurons in the 
brain in which the intracellular cAMP and/or calcium systems become activated 
during opiate withdrawal or other drug treatments. 
The recent development of a back phosphorylation and immunoprecipitation 
procedure has made it possible to study morphine regulation of CREB 
phosphorylation in the rat LC (Guitart et al. 1992). This procedure results in the 
specific immunoprecipitation of a 43-kD phosphoprotein that represents CREB: 
The protein comigrates with purified, phosphorylated CREB on one- and two- 
dimensional gel electrophoresis and yields identical phosphopeptide maps as 
does the purified protein. By use of this procedure, it has been demonstrated 
that acute morphine administration decreases the phosphorylation state of 
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