SYMPATHETICALLY CORRELATED ACTIVITY OF DORSAL HORN NEURONS IN SPINALLY TRANSECTED RATS

In mammals with an intact neuraxis, most sympathetic nerve activity is generated by brain stem systems. Therefore these systems have attract ed much more attention than spinal systems that generate excitatory in puts to sympathetic preganglionic neurons. The purpose of this study w as to determine whether, within hours of C-1 spinal cord transection, spinal dorsal horn neurons (DHNs) play a role in generating sympatheti c nerve activity. Experiments were conducted in chloralose-aneschetize d rats. We recorded renal sympathetic nerve activity (RSNA) in the lef t renal nerve, and we recorded the activity of neurons located in the left dorsal horn at T-2, T-8, T-10, T-13, and L-2. We also recorded th e activity of neurons in the right dorsal horn at T-10. The somatic fi elds and cutaneous modalities of most neurons were determined. Spike-t riggered averaging was used to determine relationships between the ong oing activity of DHNs and ongoing RSNA. In the left dorsal horn, burst s of ongoing activity of 16% of DHNs at T-8 and 43% of DHNs at T-10 we re positively correlated with bursts of ongoing RSNA at latencies of 5 9 +/- 8 (SE) ms. At no other level on tile left side, nor in the T-10 segment on the right side, was the activity of DHNs correlated with RS NA. DHNs with activity correlated with RSNA were located only in dorsa l horn laminae III-V. Deeper laminae were not investigated in these ex periments. The activity of all sympathetically correlated DHNs exhibit ed bursts of action potentials with interspike intervals of < 10 ms. A ll but one of the sympathetically correlated DHNs exhibited wide-dynam ic-range modalities. The modalities of sympathetically uncorrelated ne urons were more heterogeneous. Brief (5 - 10 s) noxious cutaneous stim ulation of mid- and lower thoracic dermatomes on the left side excited all sympathetically correlated DHNs and simultaneously increased RSNA . The excitatory cutaneous fields of sympathetically correlated neuron s were circumscribed by the excitatory fields for RSNA. The excitatory cutaneous fields of some sympathetically uncorrelated DHNs extended b eyond the excitatory fields for RSNA. Noxious cutaneous stimulation of the extremities on the left side that decreased RSNA simultaneously d ecreased the activity of all sympathetically correlated DHNs. These da ta provide electrophysiological evidence that, in spinally transected rats, a population of DHNs map generate or convey excitatory input to renal sympathetic preganglionic neurons.