Next, we examined the effects of nicotine on brain-stimulation reward (BSR) thresholds in rats following knockdown of α5* nAChRs in the habenulo-interpeduncular pathway. In the BSR procedure, rats respond vigorously to obtain rewarding electrical self-stimulation via an intracranial stimulating electrode, with the minimal stimulation intensity that maintains self-stimulation behavior termed the reward threshold. Low doses of nicotine (~0.25 mg kg−1) that condition a place preference in rats also lower BSR thresholds36, reflecting drug-induced enhancement of brain reward activity. Conversely, higher doses of nicotine (≥1 mg kg−1) that condition a place aversion can elevate BSR thresholds in rats37. Importantly, rats regulate their pattern of nicotine self-administration behavior at a level that achieves maximal lowering of BSR thresholds36, suggesting that obtaining the stimulatory effects of nicotine on brain reward circuits, whilst avoiding its negative effects, determines the amounts of nicotine consumed by rats. We found that low doses of nicotine (0.125–0.25 mg kg−1; free-base; SC) lowered BSR thresholds by a similar magnitude in the Lenti-Control and Lenti-α5-shRNA rats (Fig. 3b). However, as the dose of nicotine was increased (1–1.5 mg kg−1; free-base;
