3. Provide direct evidence for a causal role of a specific transcription factor by 
transfection or viral infection studies. 
Together, these studies establish the precise molecular steps through which 
a drug of abuse alters the expression of some target protein and thereby 
produces addictive changes in the CNS. 
IDENTIFICATION OF BIOCHEMICAL ADAPTATIONS INVOLVED IN DRUG 
ADDICTION 
Studies In the Locus Coeruleus 
The locus coeruleus (LC) is the major noradrenergic nucleus in brain, located 
on the floor of the fourth ventricle in the rostral pons. The electrophysiological 
effects of opiates in the LC are well established. Acutely, opiates inhibit LC 
neurons via the activation of a K + channel and the inhibition of a slowly 
depolarizing Na + channel (Aghajanian and Wang 1987; North et al. 1987). 
Both actions are mediated via pertussis toxin-sensitive G proteins (Gj and/or 
GJ (Aghajanian and Wang 1986; North et al. 1987), and regulation of the Na + 
channel is mediated by reduced levels of neuronal cAMP and of activated 
cAMP-dependent protein kinase (Wang and Aghajanian 1987, 1990; Alreja 
and Aghajanian 1991). Chronically, LC neurons develop tolerance to these 
acute inhibitory actions as neuronal firing rates recover toward control levels 
(Aghajanian 1978; Christie et al. 1987). The neurons also become dependent 
on opiates after chronic exposure in that abrupt cessation of opiate treatment, 
such as through administration of an opiate receptor antagonist, leads to a 
severalfold elevation in LC firing rates above control levels in vivo (Aghajanian 
1978; Rasmussen et al. 1990). A variety of pharmacological and behavioral 
studies have indicated that such regulation of LC neuronal excitability 
contributes to physical aspects of opiate addiction, namely, physical 
dependence and withdrawal (Rasmussen et al. 1990). 
The tolerance and dependence that occur in LC neurons during chronic opiate 
exposure cannot be explained by changes in opiate receptors (Christie et al. 
1987), and this led our laboratory several years ago to consider the possibility 
that intracellular messenger pathways may be involved. Since that time, it has 
been demonstrated that chronic administration of opiates leads to a dramatic 
up-regulation of the cAMP system at every major step between receptor and 
physiological response. Chronic opiates increase levels of G ia and G^ (Nestler 
et al. 1989), adenylate cyclase (Duman et al. 1988), cAMP-dependent protein 
kinase (Nestler and Tallman 1988), and several phosphoprotein substrates 
for the protein kinase (Guitart and Nestler 1989, 1990), including tyrosine 
hydroxylase (Guitart et al. 1990), the rate-limiting enzyme in the biosynthesis 
96 
