GABAergic neurons in the mouse superficial dorsal horn with special emphasis on their relation to primary afferent central terminals.

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Immunocytochemical studies were carried out on the morphological relation between primary afferent central terminals (C-terminals) and GABAergic neurons in the mouse superficial dorsal horn. The superficial dorsal horn is composed of many synaptic glomeruli comprising two types: Type I with centrally located CI-terminals surrounded by several dendrites and few axonal endings, and Type II with centrally located CII-terminals surrounded by several dendrites and a few axonal endings. The CI-terminals are sinuous or scalloped with densely packed agranular synaptic vesicles, a few granular synaptic vesicles and mitochondria, and show an electron dense axoplasm, whereas the CII-terminals are large and round or rectangular with evenly distributed agranular synaptic vesicles, a number of granular synaptic vesicles and mitochondria, and show an electron opaque axoplasm. The immunoreaction of GABA was remarkable in the superficial laminae of the dorsal horn. Many interneuronal somata in the substantia gelatinosa showed GABAergic immunoreactivity. The immunoreaction was seen in the entire GABAergic neuroplasm, but not in the nucleus and its envelope. Most GABAergic features appeared as dendrites making postsynaptic contact with CI- or CII-terminals; i.e., numerous C-terminals made presynaptic contact with GABAergic dendrites. GABA immunoreactivity was seen over round synaptic vesicles and mitochondrial membranes. A few CII-terminals made presynaptic contact with GABAergic interneuronal somata. Previous physiological and anatomical studies have suggested that not only the cutaneous nociceptive primary afferent C-terminals but also mechanoreceptive primary afferent C-terminals make presynaptic contact with the GABAergic dendrites, boutons and soma. The presynaptic relation of these primary afferents with GABAergic neurons seems to provide morphological support for the essential feature of the gate control theory: primary afferent fibers may play a part in the modulation of nociceptive information via GABAergic neurons in the superficial dorsal horn. Small GABAergic terminals were found to make contact with blood capillaries suggesting the release of GABA into circulation.