Modulatory effect of locus coeruleus stimulation on visually evoked potentials in freely moving rats

Interactions between light-flash stimulation and electrical single impulse stimulation of locus coeruleus (LC) were investigated in 9 freely moving rats with chronically implanted electrodes during different behavioral states. The interaction of both showed different immediate and long-lasting effects. Simultaneous visual and electrical stimulation caused a clear decrease of trigger rate of photically evoked after-discharges from 60% to 15% and a reduction of the early primary component of visually evoked potentials (VEP) in area 17 compared with the values without LC stimulation during relaxed wakefulness (RW). On the contrary, the simultaneous LC stimulation was ineffective during exploratory behavioral states (E). At 100 ms stimulus interval and more there was no detectable LC stimulus effect on VEP components during RW, whereas the LC stimulation under beta-blockade (2.5 mg/kg propranolol i.p.) during RW-like behavior clearly reduced the early VEP component to 75% compared with values without LC stimulation. On the other hand, LC stimulation facilitated mainly the primary VEP component to about 160% during E compared with the flash response alone. This effect was observed from 100 to 300 ms after LC stimulus and was unaffected by muscarinic blockade (0.5 mg/kg scopolamine i.p.). We conclude from these data that the short-time effects are comparable with the influence of spontaneous behavioral activation and probably result from a costimulation of cholinergic neurons around LC, whereas the long-lasting effects may be attributed to the influence of noradrenergic LC cells. The noradrenergic-dependent facilitation of early VEP component counteracts the cholinergically-induced reduction.