This research will examine the connections, functional properties and chemical content of neurons in the dorsal horn of the spinal cord in decerebrate cats and rats. Further understanding of the role of these cells could lead to better means of treating severe pain in humans. Four related projects are proposed: (1) Superficial dorsal horn neurons of laminae 1-2 and deeper neurons of laminae 3-6 which show responses to stimulation of descending pathways relevant to opiate and 'stimulation-produced' analgesia will be characterized by intracellular dye-labelling and by immunohistochemical staining for peptide or enzyme content. (2) Axon trajectories of neurons in laminae 1-2 will be traced by extracellular and intracellular single unit recording and systematic detailed antidromic mapping using stimulation of varying intensities delivered through low-impedance microelectrodes. Representative units will be marked by intracellular staining and examined immunohistochemically. (3) Activity in pairs of dorsal horn neurons, one in laminae 1-3 and the second deeper in laminae 3-6, will be correlated using simultaneous single unit recordings, spike-triggered averaging, and computation of poststimulus time histograms during repetitive stimulation of descending and afferent systems. If consistent patterns of correlation are found, representative units -- one or both of pair -- will be identified by intracellular staining and by immunohistochemical techniques. (4) Receptive fields and activity of individual neurons in the superficial dorsal horn will be examined before, during and after induction of general anesthesis with pentobarbital or alpha-chloralose. Individual units will be followed by prolonged extracellular single unit recordings with repeated mapping of peripheral receptive fields. Representative units will be penetrated and identified by intracellular dye marking.