Cholinergic Basal Forebrain Structures Are Involved in the Mediation of the Arousal Effect of Noradrenaline

Cholinergic basal forebrain structures are implicated in cortical arousal and regulation of the sleep-wake cycle. Cholinergic neurones are innervated by noradrenergic terminals, noradrenaline excites them via alpha-1 receptors and microinjection of noradrenaline into the basal forebrain enhances wakefulness. However, it is not known to what extent the cholinergic versus non-cholinergic basal forebrain projection neurones contribute to the arousing effects of noradrenaline. To elucidate the roles of cholinergic basal forebrain structures we administered methoxamine, an alpha-1-adrenergic agonist into the basal forebrain, in intact animals and again after selective destruction of the basal forebrain cholinergic cells by 192 IgG-saporin. In eight male Han-Wistar rats implanted with electroencephalogram/electromyogram electrodes, a microdialysis probe targeted into the basal forebrain was perfused with artificial cerebrospinal fluid for 6 h on a baseline day, and with cerebrospinal fluid in the first and with methoxamine in the second 3-h period of the subsequent day. The sleep-wake activity was recorded for 24 h on both days. Saporin was then injected into the basal forebrain and 2 weeks later the same experimental schedule (with cerebrospinal fluid and methoxamine) was repeated. In the intact animals, methoxamine exhibited a robust arousing effect and non-rapid eye movement (NREM) and REM sleep was suppressed. Lesioning of the basal forebrain cholinergic neurones abolished almost completely the NREM sleep-suppressing effect of methoxamine, whereas the REM sleep-suppressing effect remained intact. Thus, the basal forebrain cholinergic neurones mediate, at least in part, cortical arousal and non-REM sleep-suppression, but they are not involved in the REM sleep-suppressing effects of noradrenaline.