Ongoing and stimulus-evoked activity of sympathetically correlated neuronsin the intermediate zone and dorsal horn of acutely spinalized rats

We have shown previously that in the acutely spinalized anesthetized rat th e activities of many dorsal horn interneurons (DHN) at the T-10 level are c orrelated positively with both ongoing and stimulus-evoked renal sympatheti c nerve activity (RSNA) and therefore may belong to networks generating RSN A after acute, cervical, spinal transection. In the present study, we recor ded from both DHN and interneurons in the intermediate zone (IZN) of the T- 10 spinal segment in acutely C-1-transected, chloralose-anesthetized, artif icially respired rats. The activities of a similar percentage of IZN and DH N were correlated positively with ongoing RSNA, but the peaks of spike-trig gered averages of RSNA based on the activity of IZN were larger, relative t o dummy averages, than spike-triggered averages of RSNA based on the activi ty of DHN. Sympathetically correlated DHN and IZN differed in their respons es to noxious somatic stimuli. Most correlated DHN had relatively simple so matic fields; they were excited by noxious stimulation of the T-10 and near by dermatomes and inhibited by stimulation of more distal dermatomes. As we have shown previously, the excitatory and inhibitory fields of these neuro ns were very similar to fields that, respectively, excited and inhibited RS NA. On the other hand, the somatic fields of 50% of sympathetically correla ted IZN were significantly more complex, indicating a difference between ei ther the inputs or the processing properties of IZN and DHN. Sympatheticall y correlated IZN and DHN also differed in their responses to colorectal dis tension (CRD), a noxious visceral stimulus. CRD increased RSNA in 11/15 rat s and increased the activity of most sympathetically correlated T-10 IZN. O n the other hand, CRD decreased the activity of a majority of sympathetical ly correlated T-10 DHN. These observations suggest that the same stimulus m ay differentially affect separate, putative, sympathoexcitatory pathways, e xciting one and inhibiting the other. Thus the magnitude and even the polar ity of responses to a given stimulus may be determined by the modality and location of the stimulus, the degree to which multiple pathways are affecte d by the stimulus, and the ongoing activity of presympathetic neurons, at m ultiple rostrocaudal levels, before stimulation. A multipathway system may explain the variability in autonomic responses to visceral and somatic stim uli exhibited in spinally injured patients.