Responses of the leptomeningeal microcirculation to hypothalmic stimulation

Abstract

Studies were made on the effects of hypothalamic stimulation on the cerebral blood supply system at the parietal and parieto-occipital regions. Magnification angiography with and without fluorescein, radioactive krypton, and heat clearance techniques were used. Stimuli were delivered through bipolar stainless-steel needle electrodes using 15-sec train stimuli of 0.5-msec-wide square waves at a 100/sec pulse repetition rate and close to threshold intensities. It was found that stimulation of specific hypothalamic sites causes an immediate transient opening or somesimes closing of microvessel beds. This effect appears to be mediated by a vasoactive neurogenic mechanism presumably exerted on precapillary arterioles or possibly precapillary sphincters. This assumption was made on the grounds that in many cases the microvessel volume changes took place even without significant alterations in systemic blood pressure. Adaptation phenomena were observed in each case: Just as the effect on the microvessel beds reached its maximum and even while the stimulation continued, there was observed a sudden reversal of the effect towards the prestimulus state. A recruitment phenomenon was also observed, i.e., a late additional poststimulus increase in the effect prior to return to prestimulus state. A long-term adaptation was also detected when the duration of the phenomena was examined. It is suggested that the observed phenomena reflect the capacity of the cerebral circulatory system to undergo changes in the blood supply, i.e., in rate of blood flow and/or blood volume, independently, through the agency of neurogenic mechanisms. The localized changes described here were observed at different discrete brain sites as relatively fast, joint or even independent events, and this is highly suggestive of neurally mediated effects.