Cholinergic modulation of covert attention in the rat.

RATIONALE: Nicotine is known to facilitate attentional processing, but its role in processing spatial and non-spatial cues is not well established in rodents.

OBJECTIVE: These experiments tested the hypothesis that nicotine facilitates the orienting of attention in space but has no effect on non-spatial cues and that the benefits are blocked by the nicotinic antagonist mecamylamine.

METHODS: Eight male rats were trained to insert their noses in an opening, which triggered the presentation of cue and target lights in a modified covert orienting task. Four types of trials were presented: valid cues (cue and target lights on the same side of the nose hole), invalid cues (cue and targets on opposite sides), double cues (both cue lights illuminated, target on either side), and no cue (cue lights omitted, targets on either side). The reaction time required to withdraw the nose from the fixation hole (RT) and the time for the rat to move to the feeder (MT) were measured.

RESULTS: Nicotine decreased all RTs in a dose-dependent manner but significantly lowered the invalid cue RTs and the validity effect (invalid-valid cue RT). Mecamylamine slowed RTs in a dose-dependent fashion and reduced the validity effect by significantly slowing the valid cue RTs. With mixtures of a fixed strength of nicotine and an increasing dose of mecamylamine, RTs showed nicotine-like effects at low doses and mecamylamine-like effects at high doses. Neither of these drugs had a major effect on non-orienting trials (double and no cue RTs), and the alerting effect (no cue RTs-double cue RTs).

CONCLUSIONS: Taken together with recent work in humans and non-human primates, these results suggest that the nicotinic cholinergic modulation of visual covert orienting is conserved across species despite different ecological niches.