Caudate-capsular-induced modulation of single-unit activity in mesencephalic reticular formation

Abstract

Fifty-five sites in and around the caudate nucleus were stimulated at low frequencies to determine their effects on single-unit activity in the mesencephalic reticular formation. Ninety-two units in 15 acutely prepared cats were isolated. Twenty-nine units were inhibited by forebrain stimulation, 15 were excited,and 16 showed both excitation and inhibition. Excitatory influences occurred with a short latency; inhibitory effects had a longer latency. The most effective sites in producing these unit effects were located in the periphery of the caudate nucleus and in the region of the internal capsule and cortex that borders the caudate nucleus by about 1 mm. The effectiveness of a site in modulating unit activity was related to whether that site produced a spindle discharge recorded from the cortex. Ninety-two per cent of the sites that produced a spindle also produced unit effects, whereas only 31 per cent of the nonspindle sites affected units. The type of response produced in a given RF unit appeared to be a property of the unit and not of the site in the forebrain being stimulated. It was concluded that a forebrain excitatory mechanism influences mesencephalic units via a fast-conducting direct pathway; a forebrain inhibitory mechanism influences mesencephalic units via a slow-conducting or multisynaptic pathway (perhaps both); and behavioral effects produced by low-frequency caudate and capsular stimulation may be mediated by an alteration or perhaps disruption of the ongoing neuronal activity in the mesencephalic arousal system.