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 thresho lds to ureter pressure increases and encode stimuli within the noxious range. However, little is known about how these properties are reflec ted 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. Sin gle-unit recordings were made from 76 neurons with somatic receptive f ields on the left flank, of which 57 were fully characterized and test ed by raising the ureter pressure to 80 mmHg for 30 s. Of these 57 neu rons, 24 (42%) were influenced by the ureter stimulus, as follows: 18 were excited, 2 were inhibited, and 4 showed changes in background act ivity and/or in somatic receptive field area, without a time-locked ch ange in firing rate. The remaining 33 cells (58%) showed no changes in firing rate, background activity, somatic receptive field area, or in put properties as a result of ureter stimulation. 3. Neurons respondin g to the 80-mmHg stimulus were further tested with a range of ureter p ressures (5-100 mmHg). No responses were evoked by stimuli of <20 mmHg , and responses observed were proportional to stimulus intensity. Exci tatory 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 ac tivity (80 vs. 27%) and more likely to have bilateral somatic receptiv e fields (30 vs. 6%). Neurons with ureter input had higher rates of ba ckground activity and larger somatic receptive fields. Ureter stimulat ion also produced changes in the somatic receptive field area of neuro ns excited or inhibited by the stimulus, indicating a high degree of p lasticity in the ureteric nociceptive pathway. 5. We conclude that the characteristics of the responses of dorsal horn neurons with ureter i nput to noxious and innocuous ureter stimulation indicate that they re ceive ureteric input mainly from high-threshold afferents, and that th eir response properties correlate well with ureteric pain sensation in humans.