Chemical activation of cervical cell bodies: effects on responses to colorectal distension in lumbosacral spinal cord of rats.

We have shown that stimulation of cardiopulmonary sympathetic afferent fibers activates relays in upper cervical segments to suppress activity of lumbosacral spinal cells. The purpose of this study was to determine if chemical excitation (glutamate) of upper cervical cell bodies changes the spontaneous activity and evoked responses of lumbosacral spinal cells to colorectal distension (CRD). Extracellular potentials were recorded in pentobarbital-anesthetized male rats. CRD (80 mmHg) was produced by inflating a balloon inserted in the descending colon and rectum. A total of 135 cells in the lumbosacral segments (L(6)-S(2)) were activated by CRD. Seventy-five percent (95/126) of tested cells received convergent somatic input from the scrotum, perianal region, hindlimb, and tail; 99/135 (73%) cells were excited or excited/inhibited by CRD; and 36 (27%) cells were inhibited or inhibited/excited by CRD. A glutamate (1 M) pledget placed on the surface of C(1)-C(2) segments decreased spontaneous activity and excitatory CRD responses of 33/56 cells and increased spontaneous activity of 13/19 cells inhibited by CRD. Glutamate applied to C(6)-C(7) segments decreased activity of 10/18 cells excited by CRD, and 9 of these also were inhibited by glutamate at C(1)-C(2) segments. Glutamate at C(6)-C(7) increased activity of 4/6 cells inhibited by CRD and excited by glutamate at C(1)-C(2) segments. After transection at rostral C(1) segment, glutamate at C(1)-C(2) still reduced excitatory responses of 7/10 cells. Further, inhibitory effects of C(6)-C(7) glutamate on excitatory responses to CRD still occurred after rostral C(1) transection but were abolished after a rostral C(6) transection in 4/4 cells. These data showed that C(1)-C(2) cells activated with glutamate primarily produced inhibition of evoked responses to visceral stimulation of lumbosacral spinal cells. Inhibition resulting from activation of cells in C(6)-C(7) segments required connections in the upper cervical segments. These results provide evidence that upper cervical cells integrate information that modulates activity of distant spinal neurons responding to visceral input.