Abstract
Descending influence from the locus coeruleus/subcoeruleus (LC/SC) on cutaneous and visceral nociceptive transmission in the spinal cord was investigated in the anesthetized rat. Extracellular recordings were made from the L6-S2 segmental level using a carbon filament glass microelectrode (4-6 Mω). Colorectal distention (CRD) was produced by inflating a balloon inside the descending colon and rectum. All neurons tested responded to both CRD and to cutaneous pinch (a force of 613 g/mm2), indicating that nociceptive signals from visceral organs and nociceptive signals from the cutaneous receptive field converge to a neuron. These neurons were divided into two groups based on their response to CRD: short latency-abrupt and short latency-sustained neurons. Electrical stimulation of the LC/SC (30 or 50 μA, 100 Hz, 0.1 ms pulses) failed to inhibit CRD-evoked responses in 10 of 52 short latency-abrupt neurons, whereas cutaneous pinch-evoked responses were inhibited in these convergence neurons. In all short latency-sustained neurons tested (n=15), electrical stimulation of the LC/SC significantly inhibited both cutaneous pinch-evoked responses and CRD-evoked responses. The present result that cutaneous nociceptive transmission was independently inhibited without inhibition of visceral nociceptive transmission may be explained by a synaptic arrangement in which a nerve ending of a descending LC/SC neuron terminates on a cutaneous primary afferent terminal (presynaptic inhibition). [J Physiol Sci. 2008;58 Suppl:S165]
