Cholinergic basal forebrain structures are involved in the mediation of the arousal effect of noradrenaline

Abstract

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 6h 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 24h on both days. Saporin was then injected into the basal forebrain and 2weeks 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.