The effects of capsaicin denervation on leucocyte and complement components of the inflammatory response

Primary afferent unmyelinated nerves containing substance P modulate the inflammatory response to injury through a process known as neurogenic inflammation. Rats treated as neonates with capsaicin, to destroy these nerves permanently, have a reduced capacity to mount an inflammatory response as measured by swelling or plasma extravasation. In this study we examined some aspects of the immunological response to injury in capsaicin-treated rats. One question addressed was whether changes in the inflammatory response are due to abnormal innervation or to a direct toxic effect of capsaicin. It was observed that the histological appearance of thymus, splenic white pulp and popliteal lymph nodes was unaltered in capsaicin-treated rats. However, there was an increase in leucocyte number in splenic red pulp. There was also an increase in the white cell numbers in the peripheral circulation (7 +/- 1 X 10(6)/mL for normal rats and 14 +/- 2 X 10(6)/mL in capsaicin-treated rats, p less than 0.001, Student's t-test). This affected all white cell subgroups. Complement levels in capsaicin-treated rats were not different from normals and both groups showed a similar depletion 6 days following antigenic stimulation with sheep red cells. These data support the view that the reduced cellular response to injury in the capsaicin-treated rat is secondary to the effect of the chemical on afferent nerves and not to a primary toxic effect on cells of the inflammatory response.