Distribution of glomeruli with fluoride-resistant acid phosphatase (FRAP)-containing terminals in the substantia gelatinosa of the rat.

In the cervical enlargement of the rat spinal cord, fluoride-resistant acid phosphatase (FRAP) occurs in most of the small dark sinuous primary afferent central terminals (CI-terminals) of type I-synaptic glomeruli of lamina II and is lacking in the large light roundish primary afferent CII-terminals of type II-glomeruli. Reactive CI-terminals are heterogeneously distributed across the dorsoventral thickness of lamina II, with maximal frequency in the zone heavily stained for FRAP in light micrographs, which roughly corresponds to the dorsalmost portion of lamina IIi. In the rest of lamina II many CI-terminals do not contain FRAP. Since all CI-terminals appear to originate from unmyelinated, presumably nociceptive, primary afferents, it is proposed that the FRAP-reactive ones arise from the FRAP-containing subpopulation of DRG small cells, while those lacking FRAP, which have a distinct area of termination in the dorsal horn, belong to the peptide-containing subpopulation.