CHANGES IN THE RESPONSIVENESS OF PARABRACHIAL NEURONS IN THE ARTHRITIC RAT - AN ELECTROPHYSIOLOGICAL STUDY

1. Rats rendered polyarthritic by injection of Mycobacterium butyricum into the tail were used as a model for the study of ''chronic pain.'' In such rats, anesthetized with halothane in a nitrous oxide-oxygen m ixture, spontaneous activity and responses of parabrachial (PB) neuron s to somatic stimulations were studied in comparison with those in a c ontrol group of healthy animals processed under the same experimental conditions. 2. The size of the somatic receptive held of PB neurons wa s similar in both arthritic and control groups. In the control group 1 3%, 55%, and 32% of the receptive fields were small, medium, and large , respectively. Similarly, in the arthritic group, 10%, 60%, and 30% o f the receptive fields were small, medium, and large, respectively. 3. The spontaneous activity was significantly (P < 0.001) increased in t he arthritic rats (0.1 < 3 < 16 Hz, n = 31; 10th percentile < median < 90th percentile) in comparison with the healthy rats (0.03 < 0.3 < 5 Hz, n = 22). 4. The sensitivity to mechanical stimuli was markedly inc reased in arthritic compared with healthy rats: 1) although PB neurons in normal rats never responded to innocuous stimuli, several PB neuro ns in arthritic rats responded to touch and/or joint movement; 2) the mean mechanical threshold decreased from 15.8 N/cm(2) in normal rats t o 5.9 N/cm(2) in arthritic rats; 3) the mean pressure evoking 50% of t he maximum response decreased from 34 N/cm(2) in normal rats to 21 N/c m(2) in arthritic rats; and 4) the intensity of the maximum response i ncreased from 15.7 Hz in normal rats to 26.3 Hz in arthritic rats. 5. The mechanical encoding properties were clearly modified in arthritic rats compared with healthy rats. In this latter group, the PB neurons exhibited a clear capacity to encode mechanical stimuli in the noxious range: 1) the stimulus-response curves were always positive and monot onic until 48 N/cm(2); and 2) the slope of the mean curve increased pr ogressively from 2 to 8 N/cm(2) before reaching a roughly linear maxim um for a wide range of pressure (8-64 N/cm(2)) and plateauing beyond. In the arthritic rat, the PB neurons also encoded mechanical stimuli, but clearly from a lower pressure range: the slope of the mean curve w as maximum and remained steep from the lowest pressure tested (1 N/cm( 2)) up to 16 N/cm(2); afterward the slope decreased progressively from 16 to 64 N/cm(2) before plateauing. 6. The sensitivity to heat stimul i was only weakly modified. The thermal threshold was weakly, but sign ificantly, increased from 44 degrees C in the normal rat to 45.8 degre es C in the arthritic rat. Other parameters for thermal modality were not changed, with the mean stimulus-response curves being similar in b oth arthritic and normal groups. 7. In conclusion, these experiments d emonstrate that the activity of PB neurons is clearly changed in arthr itic rats. These changes are reminiscent of some behavioral and electr ophysiological modifications observed during arthritis. Considering th e current literature, it is hypothesized that the PB relay could be re sponsible, at least in part, for several affective-emotional, behavior al, autonomic, and energy metabolism changes observed in arthritic rat s.