Effects of environmental conditioning on the development of nicotine sensitization: behavioral and neurochemical analysis.

We have investigated the effects of environmental conditioning on the induction of nicotine sensitization of locomotion, stereotypy and nucleus accumbens dopamine release. Sprague-Dawley rats, some of which had been previously implanted with a microdialysis guide cannula over the nucleus accumbens, were sensitized with 5 days of repeated nicotine (0.6 mg/kg per day, SC) or saline injections (1 ml/kg per day). During nicotine treatment the drug administration was either paired with the microdialysis/activity monitor testing chamber (conditioned) (n = 6) or with the animal's home cage (unconditioned) (n = 6) and after 60 min the animal was returned to home cage and received a second injection of saline 15 min later. A third group received saline in the testing apparatus followed by nicotine in the home cage (pseudo-conditioned) (n = 6). In the guide cannulated animals, 2 mm microdialysis probes were inserted after completing day 5 of treatment and all animals were tested for their response to nicotine (0.6 mg/kg, SC) on day 6. Both locomotor activity and nucleus accumbens dopamine release showed a larger response subsequent to nicotine challenge in the nicotine versus saline pretreated animals in the conditioned group, but not in the unconditioned group. In the pseudo-conditioned group there was an increase in the stereotypy responses to nicotine, however the locomotor and dopamine release responses were not significantly enhanced. The results from the conditioned group were confirmed in animals which were tested for behavioral activation and dopamine release simultaneously (n = 5). These findings indicate that nicotine sensitization of locomotor activity and nucleus accumbens dopamine release (using a 5-day pretreatment protocol) is dependent on conditioning the animal to the testing environment during nicotine pretreatment.