An immunohistochemical study of neuronal populations containing neuropeptides or γ-aminobutyrate within the superficial layers of the rat dorsal horn

Abstract Immunohistochemical techniques have been used to characterise a number of interneuronal types found within the superficial layers of the rat dorsal horn. Substance P, methionine-enkephalin, neurotensin, avian pancreatic polypeptide, somatostatin and glutamic acid decarboxylase-like immunoreactivities were found within discrete populations of cell bodies within layers I to III of the spinal cord after colchicine pretreatment. Each immunoreactive cell type was distinct in terms of the distribution of terminals, perikaryal size and position within the dorsal horn. Cell bodies containing substance P were predominantly within layers I, II and III, while surgical isolation of the dorsal horn to remove terminals of extrinsic origin indicated that the terminals of the local substance P-containing interneurones were present throughout layers I and II 0 . Enkephalin-immunoreactive neurones were most numerous within layer II, with less frequently observed larger enkephalin immunoreactive neurones in layer I. Neurotensin-containing neurones were found in a fairly narrow layer along the II/III border, with dendritic arborisations extending into both layers II and III. Neurotensin-immunoreactive terminals occurred in two bands, a dense band within layer II, and a less dense one within layers I and II 0 . Somatostatin-like immunoreactivity was found in occasional large neurones within layer II i . Avian pancreatic polypeptide immunoreactivity was localized to neurones within layers I, II and III, as was terminal immunoreactivity. Glutamic acid decarboxylase-containing neurones were large and were found predominantly within layers I, II 0 and on the II/III border, where terminal immunoreactivity was also concentrated. Hemisection combined with dorsal root section indicated that the majority of these immunohistochemically identifiable neurones were interneurones. The interconnections and relations of these neurones to dorsal horn projection neurones are considered in the light of the reported analgesic properties of a number of these substances when applied directly to the spinal cord.