Effects of stimulating the subcoeruleus-parabrachial region on the non-noxious and noxious responses of T1-T5 spinothalamic tract neurons in the primate.

The effects of electrical stimulation of the subcoeruleus-parabrachial (SC-PB) region on the discharge rate of upper thoracic spinothalamic tract (STT) neurons were investigated in 21 monkeys anesthetized with alpha-chloralose. STT cells were antidromically activated from the medial thalamus (MT) and the ventral posterior lateral nucleus (VPL) and received viscerosomatic convergent input from the cardiopulmonary sympathetic afferents and the left chest-forearm region. Stimulation of the SC-PB region inhibited the activity of all 30 STT neurons studied in the T1-T5 regions of the spinal cord. The minimum average current required to decrease the discharge rate of 22 cells exhibiting spontaneous activity was 89 +/- 10 microA (100 Hz, 100 microseconds duration). Currents as high as 300 microA completely inhibited the activity of most cells. Examination of the importance of frequency of stimulation from the SC-PB area on 8 cells revealed that impulses of at least 40 Hz (208 +/- 37 microA, 100 microseconds duration) were necessary to inhibit the spontaneous activity by 60%. Higher frequencies produced greater degrees of inhibition. Stimulation of the SC-PB region also inhibited the response of 23 of 23 neurons excited by noxious pinch and 11 of 11 wide dynamic range cells stimulated by innocuous input such as blowing or brushing hair. No differences in the inhibition produced by SC-PB stimulation on cells projecting to VPL (n = 20), MT (n = 5), or both VPL and MT (n = 5) were observed. These results demonstrate that the SC-PB region may be an important brainstem site for descending inhibition of both noxious and innocuous somatic input to upper thoracic STT cells in the primate.