indicating that subpopulations of neurons respond differentially to chronic 
cocaine. 
Future Studies 
The types of studies described above have been and will continue to be 
useful in identifying specific types of transcription factors that could be involved 
in opiate and cocaine regulation of gene expression in specific target brain 
regions. However, at best, these studies can offer only circumstantial evidence 
for the possible role of a given transcription factor in mediating drug action on 
the expression of specific target proteins. Ultimately, more direct evidence is 
required to establish a causal relationship between a given transcription factor 
and target gene. Such direct evidence could theoretically be obtained by 
monitoring the transcriptional activity of a target gene under conditions where 
the activity of a specific transcription factor (or of a second messenger/protein 
phosphorylation pathway involved in the functioning of that factor) is specifically 
altered. 
Our laboratory is testing the use of a nonpathogenic strain of herpes simplex 
virus-1 (HSV-1) (Dobson et al. 1990) as a vehicle for introducing DNA into 
specific neuronal cell types in vivo. In this procedure, the recombinant virus is 
injected in the vicinity of a brain region of interest, where it appears to be able 
to infect a certain percentage of the nearby neurons (C.M. Bergson and E.J. 
Nestler, unpublished observations, June 1991). An example of how this 
procedure could be used to more directly study possible transcriptional 
mechanisms involved in drug addiction is described below for the case of 
opiate regulation of tyrosine hydroxylase gene expression in the LC. A fusion 
gene subcloned into the recombinant HSV-1 , consisting of part of the regulatory 
region of the tyrosine hydroxylase gene coupled to a reporter gene— any gene 
whose expression can be easily monitored (e.g., genes for (3-galactosidase, 
chloramphenicol acetyltransferase, or luciferase)— can be introduced into LC 
neurons via HSV-1 infection. If chronic morphine regulation of tyrosine 
hydroxylase in the LC occurs at the level of gene expression as inferred from 
the mRNA data (see above), treatment of rats with chronic morphine would 
also be expected to increase expression of the reporter gene. Next, 
introduction of fusion genes with mutated response elements in the tyrosine 
hydroxylase promoter could then be used to determine which response 
element (or combination of response elements) is required for such 
transcriptional regulation. The direct involvement of a specific transcription 
factor in this process could also be tested by introducing DNA that, when 
transcribed, encodes an RNA strand that is antisense for a given transcription 
factor mRNA. The antisense RNA would hybridize with the sense mRNA and 
thereby prevent it from being translated into the transcription factor under 
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