Spinal dorsal horn neurons responding to noxious distension of the ureter in anesthetized rats

1. Stimulation of the ureter in humans evokes only painful sensations. A large proportion of ureteric afferents show high activation thresholds to ureter pressure increases and encode stimuli within the noxious range. However, little is known about how these properties are reflected in the central processing of ureteric information. In this study, dorsal horn neurons recorded in the left side of the T-12-L(1) spinal cord of anesthetized rats have been tested for responses to innocuous and noxious pressure stimuli applied to the ipsilateral ureter. 2. Single-unit recordings were made from 76 neurons with somatic receptive fields on the left flank, of which 57 were fully characterized and tested by raising the ureter pressure to 80 mmHg for 30 s. Of these 57 neurons, 24 (42%) were influenced by the ureter stimulus, as follows: 18 were excited, 2 were inhibited, and 4 showed changes in background activity and/or in somatic receptive field area, without a time-locked change in firing rate. The remaining 33 cells (58%) showed no changes in firing rate, background activity, somatic receptive field area, or input properties as a result of ureter stimulation. 3. Neurons responding to the 80-mmHg stimulus were further tested with a range of ureter pressures (5-100 mmHg). No responses were evoked by stimuli of <20 mmHg, and responses observed were proportional to stimulus intensity. Excitatory responses showed a long onset latency (median = 23 s) and long afterdischarges (median = 145 s). 4. All neurons with ureter input had nociceptive somatic inputs. When compared with neurons without ureter input, cells with ureter input were more likely to show background activity (80 vs. 27%) and more likely to have bilateral somatic receptive fields (30 vs. 6%). Neurons with ureter input had higher rates of background activity and larger somatic receptive fields. Ureter stimulation also produced changes in the somatic receptive field area of neurons excited or inhibited by the stimulus, indicating a high degree of plasticity in the ureteric nociceptive pathway. 5. We conclude that the characteristics of the responses of dorsal horn neurons with ureter input to noxious and innocuous ureter stimulation indicate that they receive ureteric input mainly from high-threshold afferents, and that their response properties correlate well with ureteric pain sensation in humans.