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finding that the before group was oore tolerant than the after group when 

 comparisons were made after 30 days of dosing, and by the greater shift to the 

 right in the nicotine dose-effect functions of the before group than the after' 

 group when these functions were redetermined during chronic dosing. 



The tolerance reported here was not only dependent on behavioral factors 

 stemming from nicotine-induced disruption of FR responding, however, since tht> 

 after group also showed some tolerance. This finding suggests that the 

 development of tolerance to nicotine under the conditions used here' involves 

 two components. One component evident" only in the before group involves a 

 behavioral adaptation of the organism to the disruptive effects of nicotine on 

 schedule-controlled responding. This is consistent with the reinforcement 

 density hypothesis which states that tolerance to the behavioral effecs of a - 

 compound is more likely to occur when the compound interferes with an 

 organism's ability to satisfy the requirements for reinforcement; that is, when 

 exposure to the compound results in a loss of reinforcement (Schuster et al ., 

 1966; see Corfield-Sumner and Stolerman, 1978 for a review). The other compon- 

 ent of tolerance more directly implicates certain adaptation mechanisms result- 

 ing from the repeated exposure to nicotine; these mechanisms would be expected 

 to operate in both the before and after groups. These mechanisms are typically 

 thought of as alterations in distribution and/or receptor sensitivi^j[Levine, 

 1978). It is presently unclear how behavioral, dispos itional and rraptor 

 sensitivity factors interact in situations where behavioral tolerance occurs. 



In contrast to the present results, Hendry and Rosecrans (1982) concluded 

 that behavioral variables do not influence the development of tolerance to 

 nicotine. In their study, male mice responded under a FR 25 schedule of 

 sweetened milk reinforcement. Tolerance to the rate-decreasing effect of 1.2 

 mg/kg of nicotine (free base) developed in the before group after 30 days of 



