Basal forebrain and hypothalamic influences upon brain stem neurons.

Intracellular recordings were performed from bulbar and caudopontine (BcP), rostropontine (rP) and mesencephalic (Mes) neurons in acute encéphale isolé preparations during low- and high-frequency basal forebrain (BF) and hypothalamic (Hyp) stimulation. Low-frequency BF stimulation induced long-latency (7-8 msec) EPSPs on 31% of BcP neurons. High-frequency stimulation of the same regions induced excitation in 58% of them. No inhibition was observed. The descending influences on BcP neurons disappeared after bilateral acute sensorimotor decortication. Low- and high-frequency BF stimulation influenced a very low percentage of rP and Mes neurons. The only effect was a long-latency slow rising EPSP with or without spikes. No inhibition was found in these neurons. Low- and high-frequency Hyp stimulation produced short-latency EPSPs with or without spikes on a large percentage of BcP, rP and Mes neurons. The effect was more constant on rostral (rP) than on caudal (BcP) neurons and more evident when high-frequency stimulation was applied. At high frequency the activation of brain stem neurons paralleled an EEG desynchronization. IPSPs were seen in a low percentage of Mes and rP neurons. Mes neurons could be also activated antidromically. The results suggest that BF regions do not antagonize directly the mesencephalic activating system and may induce electrocortical synchronization through the activation of caudal brain stem neurons. This circuit involves sensorimotor cortical neurons. The facilitatory effect of Hyp stimulation on rostral brain stem neurons indicates that the EEG desynchronization which follows high-frequency Hyp activation may be mediated by the ascending reticular system. Other mechanisms mainly at the diencephalo-telecephalic level may however be involved in this effect.