cocaine, administered multiple times in a single day, has a dual effect on the 
expression of lEGs: Cocaine administration can either stimulate or suppress 
the expression of c -fos. A single injection stimulates c -fos expression but at 
the same time triggers a process that inhibits c -fos expression. We refer to 
the latter effect as suppression or refractoriness to induction. A single injection 
produces a short-lived suppression of approximately 1 to 2 hours. However, 
multiple injections produce a suppression that can last for days. Thus, the 
duration of suppression is greatly magnified with repeated administration. 
The net effect of the prolonged suppression of IEG expression is to functionally 
uncouple dopaminergic neurotransmission from dopaminergic regulation of 
gene expression. 
MATERIALS AND METHODS 
Treatments 
Male Sprague-Dawley rats (300 g) were housed in groups of three, given 
access to food and water ad libitum, and kept on a 12-hour light/dark 
cycle. Cocaine hydrochloride (HCI) was dissolved in saline and administered 
intraperitoneally (IP) unless specified otherwise, in a volume of 0.2 or 0.3 mL/g 
of body weight. Rats were sacrificed at various times after injection by 
exposure to C0 2 . The brain was rapidly removed, and the frontal cortex, 
caudate-putamen, hippocampus, and cerebellum were dissected out on an 
ice-cold plate. GBR-1 2909 HCI was dissolved as follows: 20 mg was added 
to 40 [iL of glacial acetic acid, followed by 60 of H 2 0, and allowed to 
dissolve completely; next, 1 .7 mL of 200 mM sodium carbonate was added 
and mixed, followed by 200 ^iL of Tween 20. Rats were injected either IP or 
intravenously (0.2 mL/100 g body weight). In some cases animals were 
pretreated IP 30 minutes prior to cocaine with various doses of SCH-23390, 
a dopamine D1 receptor antagonist, or sulpiride, a dopamine D2 receptor 
antagonist (Young et al. 1991). 
Analyses 
RNA was extracted from caudate-putamen, frontal pole or cerebral cortex, 
hippocampus, and cerebellum by the guanidine thiocyanate/cesium chloride 
centrifugation method as described previously (Draisci and ladarola 1989). 
RNA blots were prepared and probed for transcripts with [ 32 P]-labeled, cloned 
rat cDNA c -fos insert (Curran et al. 1987) or oligonucleotide probes to c -fos, 
NGFI-A, prodynorphin, cyclophilin, or p-actin, the latter two being used for 
standardization. 
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