Individual self-administration of nicotine by rats

Self-administration (SA) of nicotine (N) was studied in 20 male and 19 female N:NIH rats using the two-bottle method. The experimental protocol consisted of seven consecutive periods each lasting 6 days: Period (P)1, choice of water (W) and 0.003% N; P2, choice of W and 0.006% N; P3, choice of W and 0.012% N; P4, W only; P5, choice of W and 0.006% N; P6, 0.006% N only; and P7, choice of W and 0.006% N. Group means showed that males and female rats consumed similar amounts of N during Ps 1-3. After an N-free period (P4), a small decline was observed in the subsequent voluntary intake of N (P5). Forced N (P6) exposure did not affect a subsequent N intake (P7) in males but increased it slightly in females. A survey of individual animals, however, showed that the voluntary N consumption varied greatly among animals, but was quite consistent for a particular rat. Values ranged from 0.43 to 7.59 for males and from 0.35 to 4.69 mg/kg/day for females for Ps 1-3. The N-free (P4) and the forced-N (P6) periods each affected a subsequent voluntary N intake (P5, P7) of the rats very differently, but again consistently, in that some rats decreased, some increased and some did not change their N choice. The results indicate that group means can be misleading in their conclusions and strongly support the assumption that the response of an individual animal to N, and not N per se, is the determining force of its SA.