Figure 8.

Calculated brain cocaine concentrations in two representative rats of the High and Low frequency groups. Whole-brain levels of cocaine were mathematically evaluated every 10 s and are represented over session 7 (a–d), session 23 (b–e), and over the last session (c–f) for one High frequency rat (rat 1, upper line) and one Low frequency rat (rat 2, lower line). Session 7 was the penultimate session of acquisition with the 40-s time-out (TO) condition for all rats. Cocaine infusions were limited to 30. Session 23 was the 15th session of the TO-specific procedure promoting High and Low frequency self-administration; cocaine infusions were limited to 25. In both cases, the session ended after 180 min or when the maximal number of infusions allowed was reached. The last session (session 29) was the one immediately preceding sacrifice for brain sampling. Cocaine infusions were limited to eight, and session length was limited to 90 min. For each rat, sacrifice was performed 80 min after the end of its session. Rats 1 and 2 showed a comparable time course of brain cocaine concentrations in session 7. In session 23 and in the last session, they showed very distinct time courses of cocaine brain concentrations.