BACKGROUND To evaluate directly the possibility that the potent exogenous opioid analgesic morphine may alter neuronal expression of opioid peptide genes, we assessed the effect of subarachnoid morphine on basal and noxious stimulation-induced expression of preproenkephalin in spinal cord neurons. METHODS Twenty male Sprague-Dawley rats were prepared 48 h in advance with lumbar subarachnoid catheters. In the first phase, basal expression was evaluated in rats that received morphine 10 micrograms or saline intrathecally (n = 5 per group). Subsequently, the experiment was repeated (n = 5 per group), except that 10 min after morphine or saline administration rats received a hindpaw footpad injection of 50 microliters 5% formalin. Rats were killed during pentobarbital anesthesia 2 h later, and messenger RNA transcribed from preproenkephalin was measured in lumbar spinal cord with quantitative in situ hybridization with a complementary sulfur 35-labeled oligonucleotide probe and emulsion autoradiography. RESULTS In control (nonstimulated) rats, 20% of the neurons in laminae I-II and 10% of those in laminae III-IV expressed preproenkephalin. Injection of formalin increased the fraction of positive neurons by 34% (P < 0.05) and 20% (P < 0.05) in laminae I-II and V-VI, respectively, but had no effect on expression in laminae III-IV. Subarachnoid morphine did not alter basal expression of preproenkephalin but markedly attenuated the noxious stimulation-induced increase in laminae I-II (P < 0.01) and V-VI (P < 0.05) by preventing the stimulation-evoked recruitment of preproenkephalin-expressing neurons that otherwise would have occurred. CONCLUSIONS Subarachnoid morphine does not acutely alter basal expression of preproenkephalin in spinal cord neurons but inhibits the increase in preproenkephalin expression that would otherwise occur after noxious stimulation.