Figure 5.

Effects of (+)-naltrexone on the discriminative-stimulus effects of cocaine. Abscissae: drug dose in mg/kg (log scale). Ordinates: upper panels, percentage of responses on the cocaine-appropriate lever; lower panels, percentage of response rate after saline administration. Each point represents the mean±SEM (n=5). All drugs were administered i.p. or s.c. at 5 min before sessions. (a, top) Substitution of saline or (+)-naloxone for cocaine. No dose of (+)-naloxone produced cocaine-like discriminative-stimulus effects regardless of the route of administration (i.p. or s.c.). (a, bottom) As already shown for the self-administration results, high doses of (+)-naloxone decreased response rates. (b, c) No dose of (+)-naloxone administered in combination with cocaine blocked the discriminative-stimulus effects of cocaine, regardless of the route of (+)-naloxone (i.p. or s.c.) administration. When combined with cocaine, the decreases in response rates produced by (+)-naloxone were attenuated compared to its effects alone.