LARGE DORSAL HORN NEURONS WHICH RECEIVE INPUTS FROM NUMEROUS SUBSTANCE P-LIKE IMMUNOREACTIVE AXON TERMINALS IN THE LAMINAE-I AND LAMINAE-IIOF THE CHICKEN SPINAL-CORD

Large neurons outlined with numerous substance P (SP)-like immunoreact ive (LI) boutons were detected immunocytochemically in the dorsal horn of the chicken spinal cord at the light microscopic level. The cervic al enlargement was mainly used for observations. By electron microscop y, asymmetrical synapses were observed between the SP-LI axon terminal s and the soma and dendrites of the large neurons. Cell bodies of the large neurons were mostly localized in the lamina I and the region lat eral to the lamina I. Some of the cell bodies were also located in the lamina II. Their dendrites extended in the lamina I, in the region la teral to the lamina I, and deeply in the lamina II. In the lamina II, dendrites of these neurons formed synapses with SP-containing central terminals in synaptic glomeruli known to originate from primary affere nts. The findings suggest that these large neurons receive nociceptive information directly from primary afferents. In the light of previous investigations, these neurons are considered to be pain-transmitting long ascending tract neurons. (C) 1997 Elsevier Science Ireland Ltd.