Long-term blockade of brain opioid receptors by the opiate antagonist naltrexone increases methionine-enkephalin content in the striatum and nucleus accumbens (Tempel et al., 1984). To determine whether these changes in peptide levels reflect increased peptide synthesis, we examined preproenkephalin mRNA content in discrete brain regions of control (placebo-treated) and chronic naltrexone-treated animals by Northern analysis. Chronic naltrexone treatment (8 d) led to an approximately 12-fold increase in the striatal content of preproenkephalin mRNA relative to that of control animals. In contrast, no statistically significant change was observed in striatal mRNA for cyclophilin (1B15) or actin. Small increases in preproenkephalin mRNA content occurred in the hippocampus (+40%) and hypothalamus (+19%). No significant changes occurred in the frontal cortex. Increases in levels of the mRNA were seen as early as 24 hr after antagonist treatment. In contrast, changes in opioid receptor density required 3–4 d to reach half-maximal up-regulation after chronic antagonist treatment. Recent evidence has suggested that substance P is regulated by opioid peptides. To determine whether substance P synthesis is altered by chronic antagonist treatment, the mRNA corresponding to the precursor for substance P was examined using a probe for exon-7 of the preprotachykinin gene. Preprotachykinin mRNA content in the striatum was increased 6-fold after chronic antagonist treatment relative to that of control animals. Substance P content was increased 3-fold after chronic antagonist treatment. These data suggest that chronic blockade of brain opioid receptors leads to the increased synthesis of both enkephalin and substance P in the striatum and that these changes are relatively specific.(ABSTRACT TRUNCATED AT 250 WORDS)