The termination of primary afferents within the rat dorsal horn: Evidence for rearrangement following capsaicin treatment

Abstract The mode of termination of primary afferent fibres within the superficial dorsal horn of the spinal cord was studied with anterograde tracing methods in normal rats and in animals which had been treated with capsaicin (50 mg/kg. s.c.) shortly after birth. In normal animals following injections of horseradish peroxidase (HRP) into lumbar dorsal root ganglia L4, L5, L6, a Golgi‐like filling of primary afferent fibres was seen consistently within laminae I, II, and III of the dorsal horn. Several types of lamina related arborisations were observed, depending upon the survival time after HRP injection. At the earliest survival time (1–2 days) a punctate granular labelling was found in laminae I and II inner (II i ). At 3 days survival a Golgi like labelling of primary afferent axons occurred and three tiers of arborisation were seen within laminae I, II outer (II O ), and II inner (II i ), respectively. By 7 days after injection this pattern was considerably reduced and there was coarser fibre labelling within laminae I, II O , and III. When these tracing experiments were repeated in capsaicin‐treated rats in which up to 90%of unmyelinated fibres had been destroyed, evidence for a considerable loss of some inputs and rearrangement of the remaining fibres was found. At 3–5 days survival the axonal labelling within laminae I and II i was severely depleted yet some input, particularly to lamina II O , was present. At 7 days survival deep fibre labelling within lamina III now extended well into lamina II. Mapping of the primary afferent input following 3 H‐proline injections into the dorsal root ganglion and subsequent autoradiographic processing confirmed that primary afferents terminated throughout the dorsal horn but most heavily in two bands within laminae I and II i . These two dense layers of termination were not present in capsaicin‐treated animals, suggesting that they corresponded to areas of unmyelinated primary afferent fibre input. These data, taken together with histochemical observations on the distribution of substance P and fluoride‐resistant acid phosphatase (FRAP)‐containing primary afferents, suggests that the earliest‐labelled fibre systems (3days) were unmyelinated and replaced at 7 days by predominantly A6 myelinated fibres. It is further suggested that of the three tiers of C fibre terminations seen at 3 days, the most superficial corresponds in part to those primary afferents that can be stained for substance P within lamina I while the most ventral tier within lamina II i are those primary afferents containing FRAP. The presence of the two temporally distinct fibre systems also suggests that laminae I and II o , receive both C and A6 primary afferent input while lamina II i , receives only C fibre input.