Dorsal root ganglia (DRG) neurons synthesize and transport substance P (SP) to the spinal cord where it is released in response to intense noxious somatosensory stimuli. We have shown previously that SP release in vivo causes a rapid and reversible internalization of SP receptors (SPRs) in dorsal horn neurons, which may provide a pharmacologically specific image of neurons activated by SP. Here, we report that noxious heat (43°, 48°, and 55°C) and cold (10°, 0°, Ϫ10°, and Ϫ20°C) stimuli, but not innocuous warm (38°C) and cold (20°C) stimuli, applied to the hindpaw of anesthetized rats induce SPR internalization in spinal cord neurons that is graded with respect to the intensity of the thermal stimulus. Thus, with increasing stimulus intensities, both the total number of SPRϩ lamina I neurons showing SPR internalization and the number of internalized SPRϩ endosomes within each SPR immunoreactive neuron showed a significant increase. These data suggest that thermal stimuli induce a graded release of SP from primary afferent terminals and that agonist dependent receptor endocytosis provides evidence of a spatially and pharmacologically unique "neurochemical signature" after specific somatosensory stimuli.
Key words: substance P receptor; tachykinin; neurokinin-1; nociception; pain; sensory neuronNeurons with cell bodies located in dorsal root ganglia (DRG) convey somatosensory information from peripheral tissues to the CNS. These DRG neurons synthesize several neurotransmitters that are released into the spinal cord to signal somatosensory stimulation. To understand f urther how DRG neurons convey somatosensory information and how somatosensory information is processed by spinal cord neurons, several issues need to be clarified. First, it remains to be determined whether release of a particular sensory neurotransmitter(s) in the spinal cord is correlated with stimulation of a particular sensory modality and with excitation of a specific group of primary afferent fibers. Second, it is important to determine whether transmitter release is graded with stimulus intensity. Last, correlation of the intensity of sensory stimulation with the neurochemical changes that take place in the spinal cord will begin to provide data on how somatosensory stimuli are encoded within the spinal cord and whether different intensities and modalities of sensory stimulation generate a unique "neurochemical signature" within the spinal cord.Substance P (SP), an undecapeptide that is present in 20 -30% of DRG neurons, is one of the most extensively studied primary afferent neurotransmitters. Several lines of evidence suggest that SP is involved in signaling nociceptive information in the spinal cord. First, SP is contained primarily in small-diameter afferent fibers and is released in the spinal cord after a noxious stimulus (Hokfelt et al., 1975;Dalsgaard et al., 1984;Boehmer et al., 1989;Levine et al., 1993). Second, iontophoresis of SP onto the spinal cord has been reported to excite selectively the spinal nociceptive neurons with minimal...