Many studies have shown that brain stem regions such as the nucleus raphe magnus and periaqueductal gray can control the inflow of sensory information to the spinal cord. It has recently been found that selective analgesia may result from stimulating these brain centers and that morphine analgesia may be mediated by the same mechanisms. Pathways implicated in this "descending control" extend from enkephalin-containing neurons in the mesencephalic central gray which project to serotonergic neurons in the nucleus raphe magnus, which in turn project to the spinal cord marginal zone (MZ) (lamina I), substantia gelatinosa (SG) (lamina II), and nucleus proprius (lamina V). However, many questions still remain unanswered, including: 1) the types of neurotransmitters involved at the central nervous system (CNS) level and 2) the morphological and physiological substrates by which these descending pathways produce their effects. The aim of this research proposal is to explore these issues by studying: 1) the morphology of MZ and SG neurons and identified projection neurons that are modulated by focal brainstem stimulation; 2) the synaptic conductance changes evoked by focal brainstem stimulation; 3) the types of neurotransmitters released by brainstem neurons. Conventional single unit recordings, both extra and intracellular, will be used to physiologically identify interneurons and projection neurons. Iontophoresis of intracellular labels will be used to identify the recorded neurons for examination at light and EM levels. Microiontophoresis and pressure injection will be used to locally apply specific antagonists to putative neurotransmitters, and finally, immunocytochemistry combined with anatomical tracers will be used to establish the relationship between descending connections and the various putative neurotransmitters found in the spinal SG. The results of these studies should lead to a basic understanding of the mechanisms whereby the CNS can modulate somatosensory information at the spinal level.