Mechanisms underlying inhibition of nociceptive jaw-opening reflex produced by stimulating limbic structures

Abstract

The comparative effectiveness of the inhibitory influence of tetanic (100 Hz) stimulation of the hypothalamus, amygdala, and limbic cortex on electromyograph (EMG) response in m. digastricus produced by electrical stimulation of tooth pulp afferents was investigated in cats anesthetized by a mixture of chloralose and Nembutal. It was found that the most pronounced inhibition of nociceptive EMG response was produced by stimulating the medial and lateral structures of the hypothalamus, a less intense response by stimulating central and medial amygdaloid nuclei, and the least reaction by stimulation of different areas of the limbic cortex. It was shown that mechanisms underlying inhibition of the jaw-opening reflex produced by hypothalamic tetanic stimulation is unconnected with the concomitant increase in blood pressure. The inhibitory action of hypothalamic tetanic stimulation following blood pressure stabilization persists; this would suggest a primary but not baroafferent mechanism underlying inhibition of activity in trigeminal nucleus nociceptive neurons. Pronounced and protracted depression of jaw-opening reflex takes place following long-term pressor response produced by injecting noradrenaline intravenously. The same pronounced and protracted pressor response occurs under the effects of angiotensin but without any noticeable change in the amplitude of nociceptive EMG response. Hypothalamic and noradrenergic mechanisms of pain sensitivity are discussed.