et al. 1988; Hyman et al. 1989). The middle element, ENKCRE-2, contains 
the sequence TGCGTCA. Multimers of this sequence can confer cAMP and 
Ca ++ induction on a minimal proenkephalin promoter in a fashion similar to the 
observed regulation of the endogenous proenkephalin gene in many cell types 
(Nguyen et al. 1990; Kobierski et al. 1991). The ENKCRE-2 sequence is similar 
to the consensus binding site for CREB (TGACGTCA) and for AP-1 proteins 
(TGAC/GTCA). Although ENKCRE-2 contains the CGTCA motif that is required 
for cAMP regulation in many genes (Hyman et al. 1988), it binds AP-1 proteins 
with very high affinity (Hyman et al. 1988; Comb et al. 1988; Kobierski et al. 
1991). 
Stimulus paradigms in brain, in which both lEGs and the proenkephalin gene 
are known to be regulated, include seizures (White and Gall 1987; White et 
al. 1987; Gall 1988), antipsychotic drug administration (Sabol et al. 1983; 
Tang et al. 1983; Sivam and Hong 1986; Romano et al. 1987), and precipitated 
opiate withdrawal (Lightman and Young 1987). Seizures induced by electrical 
or chemical means (e.g., pentylenetetrazol) produce dramatic effects on 
expression of proenkephalin in the dentate gyrus of the hippocampus and in 
entorhinal cortex. Seizures increase levels of enkephalin immunoreactivity 
and decrease levels of dynorphin immunoreactivity in mouse hippocampus (Gall 
1988). Electrical stimulation has also been shown to increase proenkephalin 
mRNA levels and to decrease prodynorphin mRNA levels in rat hippocampus 
(Morris et al. 1988). Because electrical stimulation and seizures also induce 
c -fos and other lEGs (Morgan et al. 1987; White and Gall 1987; Dragunow 
and Robertson 1987) with a time course that precedes proenkephalin mRNA 
accumulation, the proenkephalin gene has been investigated as a model of 
lEG-target gene interaction in seizure paradigms. Based on the temporal 
relationship of induction, on the observation that AP-1 binding activity is induced 
by seizures, and on activation of the proenkephalin promoter in undifferentiated 
F9 cells by cotransfected c- fos and c -jun, Sonnenberg and colleagues (1989a) 
concluded that proenkephalin is likely to be a target gene for c-Fos and c-Jun 
induced in hippocampal neurons by seizures. 
Although heterodimers of c-Fos and c-Jun may regulate proenkephalin in 
F9 cells, the many proteins that can regulate gene expression via binding to 
AP-1 sites (including Fos, Jun, and ATF family members) suggests that for 
AP-1 -regulated genes such as proenkephalin, the relevant transcriptional 
regulators may differ from cell type to cell type and, even within a given cell 
type, may vary under different circumstances. It is also important to recall 
that not all proteins that bind to AP-1 sites are lEGs. Some, such as jun- D, 
are constitutively expressed, with little if any change in level of expression 
following stimulation. This is relevant in the present context because it has 
recently been shown that jun - D can positively regulate proenkephalin gene 
29 
