Rationale: Heroin (diacetylmorphine) administration has been shown to alter the induction of nitric oxide, a molecule known to play a critical role in the regulation of immune responses and resistance to infectious challenges. There is evidence that the physiological and behavioral effects of opioids can be conditioned to environmental stimuli associated with drug administration.
Objectives: The present study tests whether environmental stimuli paired with the administration of heroin alter the expression of inducible nitric oxide synthase (iNOS), one of the isoforms of the enzyme responsible for nitric oxide production.
Methods: The experimental group of rats received three conditioning sessions each involving a subcutaneous injection of heroin (1.0 mg/kg) immediately upon being placed in a chamber that served as the conditioned stimulus. Following the conditioning sessions, the rats remained in their home cages for 6 days to recover from the effects of heroin per se. The rats then were re-exposed to the conditioning chamber in the absence of heroin and immediately following the session were injected subcutaneously with lipopolysaccharide (LPS) to induce iNOS expression. Six hours later, the expression of iNOS messenger ribonucleic acid (mRNA) and protein was determined in spleen, lung, and liver using real-time reverse transcriptase polymerase chain reaction (RT-PCR) and western blotting, respectively. Control procedures were used to determine whether the alterations in iNOS expression were due to conditioning processes.
Results: The results showed that exposure to the conditioned stimulus produced a pronounced reduction in the expression of iNOS mRNA and protein in spleen, lung, and liver tissue. Control procedures showed that this effect was due to conditioning processes.
Conclusions: This study provides the first evidence that heroin-induced alteration of iNOS expression can be conditioned to environmental stimuli, and suggest that environmental stimuli associated with drug use may contribute to the alteration in susceptibility to infection.