Descending inhibition from medial and lateral midbrain of spinal dorsal horn neuronal responses to noxious and nonnoxious cutaneous stimuli in the cat.

Abstract

Lumbar dorsal horn neurons responding to electrical hindlimb nerve stimulation at A-fiber strength were recorded in cats anesthetized with sodium pentobarbital and N2O. Of 52 U gave additional responses to nerve stimulation at C-fiber strength and responded to a range of tactile and noxious heat stimuli (class 2), while 22 U receiving only A-fiber input responded only to weak mechanical stimuli (class 1). All units selected responded to brushing of hairs in the receptive field. To investigate descending effects on dorsal horn neurons produced by midbrain stimulation, velocity-controlled brushing of hairs and temperature-controlled noxious heating of glabrous skin of the hindlimb were used. The brush-evoked responses of most class 1 and 2 U were inhibited during stimulation (mean, 30 Hz; 450 or 600 .mu.A) in midbrain periaqueductal gray (PAG) and lateral reticular formation (LRF). Inhibition during stimulation in LRF (to 72% of control) was stronger than that during stimulation in PAG (to 81%) in the mean of all class 1 and 2 U responses to brush. Stimulation in both PAG and LRF inhibited noxious heat-evoked (50.degree. C, 10 s) responses of class 2 U to a mean of about 55%, whereas the brush-evoked responses of the same units were inhibited to 81 and 72% from PAG and LRF, respectively. Both class 1 and 2 U gave graded responses to brush stimuli applied at different velocities, such that the velocity response relationship was monotonically increasing over most of the velocity range. Discharge rates in class 2 U were linearly related to the temperature of noxious skin heating. PAG and LRF stimulation resulted in a variety of alterations in the brush velocity- and noxious heat-response curves, including reductions in slope of the curves and/or increases in unit response thresholds. There is apparently relative, but not absolute, differential midbrain control of nociceptive vs. nonnociceptive spinal transmission. The possibility of differential controls from PAG and LRF, synaptic mechanisms and functional implications are discussed.