INTERACTION BETWEEN SUBSTANCE P-IMMUNOREACTIVE CENTRAL TERMINALS AND GAMMA-AMINOBUTYRIC ACID-IMMUNOREACTIVE ELEMENTS IN SYNAPTIC GLOMERULI IN THE LAMINA-II OF THE CHICKEN SPINAL-CORD

We investigated the interaction between gamma-aminobutyric acid (GABA) -immunoreactive (IR) elements and substance P (SP)-IR central terminal s in synaptic glomeruli in lamina II of the chicken spinal cord in ord er to ascertain how pain information is modulated in the spinal dorsal horn. We combined the peroxidase-antiperoxidase (PAP) technique and t he protein A-gold (PAG) technique to observe the synaptic relationship between these two components. At the light microscopic level, we obse rved both GABA-IR and SP-IR elements in the lamina II. GABA-IR element s were also observed in the lamina III. At the electron microscopic le vel, the following three GABA-IR elements formed synapses with the SP- IR central terminals in synaptic glomeruli: (1) elements which appeare d to be axon terminals containing tightly-packed pleomorphic clear ves icles; (2) elements which appeared to be vesicle-containing dendrites with loosely-packed clear and dense-cored vesicles (DCVs); and (3) den drites without synaptic vesicles. The first type of element was always presynaptic to the SP-IR central terminal. The second type was postsy naptic, presynaptic or in some cases reciprocal to the SP-IR central t erminals. The third type was postsynaptic to the SP-IR central termina l. These results suggest that the SP-containing primary afferents acti vate GABA-containing dendrites and that the SP-containing primary affe rents are inhibited presynaptically by GABA-containing neurons through axo-axonic and dendro-axonic synapses.