This project seeks to determine if the central terminals of primary afferent fibers carrying nociceptive information to the CNS are a site of analgesic action of the opiates and opiate peptides. Attention is specifically directed at afferents from the tooth pulp since these fibers are known to conduct impulses primarily, if not exclusively, related to pain. The excitability testing technique will be used to determine the direct action of topically applied opiate on the polarization of central terminals of pulpal afferents. We will also assess the capacity of opiate to modulate the depolarization of these terminals caused by: 1) Topical application of transmitters believed to mediate primary afferent depolarization (PAD) and 2) By electrical stimulation of PAD pathways and of brain stem nuclei known to be involved in opiate analgesia. Induction or re-enhancement of PAD in these nociceptice afferent terminals by opiate would be expected to cause analgesia by decreasing transmitter release from the terminal. Such decrease will be measured directly by assay of putative primary afferent transmitters (glutamate, substance P, somatostatin) released during selective simulation of the dental pulp.