Figure 5.

Recruitment of Orx/Hcrt neurons by a stimulus (S⁺) conditioned to cocaine (COC) but not sweetened condensed milk (SCM). Male Wistar rats were subjected to reinforcement contingencies in which responses at the active lever were differentially reinforced in the presence of a distinct discriminative stimuli S^D associated with reinforcers (cocaine or SCM) availability vs. non-availability. A constant 70 dB white noise served as a discriminative stimulus (S⁺) for availability of the reinforcer (cocaine or SCM), while illumination of a 2.8 W house light located at the top of the chamber’s front panel served as a discriminative stimulus (S⁻) signaling non-availability of the reinforcer (i.e., saline solution instead of cocaine or no consequence instead of SCM). Sessions were initiated by extension of the levers into the chambers and concurrent onset of the respective S^D which remained present until termination of the session by retraction of the levers. In the presence of the S⁺, responses at the right, active lever were reinforced by cocaine or SCM on a fixed-ratio 1 schedule and, similar to training, were followed by a 20 s timeout period signaled by illumination of a cue light above the lever. In the presence of the S⁻, depression of the right active lever was followed by an intermittent tone during which the lever remained inactive for 20 s. Three daily sessions (each lasting 1 h for the cocaine group and 20 min for the SCM group) separated by 30 min intervals were conducted, with two “reward” sessions and one “non-reward” session sequenced in random order. After 8 training days (i.e., a total of 16 “reward” and 8 “non-reward” sessions), both the cocaine and SCM groups were placed on extinction conditions in daily 30 min sessions during which the reinforcers and S^D were withheld until a criterion of ≤ 4 responses/session for 3 consecutive days was reached. Fos-positive (Fos⁺ve) Orx/Hcrt neurons in the lateral hypothalamus (LH) were then counted following COC or SCM S⁺ presentation (reinstatement tests in which both the COC and SCM S⁺ elicited robust recovery of responding; see inset) and compared with Fos-positive counts obtained following the final extinction (EXT) session. Fos-positive orexin neurons: **p < 0.05, compared with EXT; ⁺p < 0.05, COC S⁺ vs. SCM S⁺. Inset: Mean (± SEM) number of responses during reinstatement tests in the presence of the COC S⁺ or SCM S⁺. Both stimuli were equally effective in eliciting responding. **p < 0.01, compared with EXT (n = 8 animals/group).