Synaptic architecture of glomeruli in lamina II of the chicken spinal cord, as revealed using ultrathin section and freeze fracture techniques.

Abstract

Synaptic glomeruli in lamina II of the chicken dorsal horn were studied using the freeze fracture technique, and the results were compared with those obtained using the ultrathin section technique. Our findings using the freeze fracture technique were as follows. (1) On the presynaptic P-face of the central terminal, intramembrane particles (IMPs) were arranged circularly around a small dimple which was reported to be a synaptic vesicle attachment site. A distinct area with aggregated large IMPs was found on the postsynaptic E-face of some peripheral neuronal elements. (2) The area with small IMPs intermingled with several dimples and the area with aggregated large IMPs were present juxtaposed on the same central terminal P-face. The area with aggregated large IMPs indicates that the central terminal functions as a postsynaptic element; accordingly, the two areas represent a reciprocal synapse. (3) Distinct IMP aggregates were observed on the P-face of vesicle-containing dendrites which did not face the central terminal. (4) A fractured septate junction was revealed as numerous parallel-lined furrows on the E-face of the central terminal. The distribution of IMPs in the synaptic glomerulus supports the hypothesis that the synaptic glomerulus is the site of the local inhibitory feedback circuit for pain transmission.