Morphological Study Of Substance P Involved In The Central Endogenous Descending Pain Inhibitory System In The Mouse

The rostroventromedial medulla (RVM) is an important component of the endogenous descending pain control system in the central nerve system. Neurons in the RVM are not only modulated by several superior brain regions, but also send axonal projections to the superficial laminae of the spinal dorsal horn, participating in the regulation of the nociceptive transmission.The behavioral experiment indicates that electronical stimulation of the raphe magnus nucleus (RMg) increases the animal's response threshold to the heat and mechanical stimulus, which infers its inhibitory effect on the transmission of nociception. Lesion of the RMg attenuates or eliminates this inhibitory effect. Evidences implicate that the neuronal activity in the RVM is regulated by many neuroactive substances, one of which is opioid. It has been proved that many neural active substances take part in the central endogenous pain control system mediated by the RVM-spinal projecting neurons. To understand the mechanism of the central endogenous pain control system, the studies focusing on these neural active substances and their receptors are very important. Substance P (SP) is an important neuropeptide participating in the modulation of nociceptive information transmission in both central and peripheral nerve system, mediated mainly by neurokinin-1 receptor (NK-1R). Although SP facilitates the afferent nociceptive information peripherally, it mainly plays pain inhibition function in the central nerve system. Immunohistochemical studies show that abundant SP-containing axon fibers and terminals are distributed in the RVM, make complicate synaptic connections with the local neurons. Micro-iontophoretical application of SP increases the frequency of neuronal spikes in the RVM. These results suggest that SP may take part in modulation of neurons in the RVM. However, the origins of SP, the relationship between the SP-ergic terminals and the NK-1R immunoreactive neurons and the projection of the NK-1R expressing neurons in the RVM have not been elaborated yet.By using immunofluorescent histochemical staining, fluorescent tracing and immuno-electron microscopic method, the present study investigated:â‘ the origins of SP-ergic fibers and terminals in the RVM;â‘¡the synaptic connections between SP-ergic fibers and terminals originated from the periaqueductal gray (PAG) and NK-1R immunoreactive neurons;â‘¢the descending projection of NK-1R expressing neurons in the RVM. All the results provide the morphological evidence for that SP participates in the central endogenous descending pain control system.Partâ… The origins of SP immunoreactive fibers and terminals distributed in the rostroventromedial medulla By using the combination of retrograde and anterograde tracing with immunofluorescent histochemical staining, the present study examined the origins of SP-immunoreactive (SP-IR) fibers and terminals distributed in the RVM.After injection of fluoro-gold (FG) into the RMg and its adjacent regions combing with the immunofluorescent histochemical staining for SP, FG labeled SP-immunoreactive neurons were found mainly in the cuneiform nucleus (CnF), dorsal raphe nucleus (DR) and lateral periaqueductal gray subregion (lPAG). By injecting anterograde tracer biotinylated dextran amine (BDA) into the CnF, DR and lPAG, respectively, combined with triple immunofluorescent histochemical staining for SP, NK-1R and BDA, SP and BDA double-labeled fibers and terminals were observed to make synaptic connections with NK-1R immunoreactive neurons in the RMg and the PGi.The results suggest some SP-ergic neurons in the CnF, DR and lPAG project to the RVM and SP may act on the NK-1R expressing neurons. As the RVM is the main component of the central endogenous descending inhibitory system, these results may provide possible morphological evidence for central SP participating in endogenous descending inhibitory system.Partâ…¡Synaptic connections between SP-ergic terminals originated from midbrain periaqueductal gray and neurokinin-1 receptor immunoreactive neurons in the raphe magnus nucleusIn the present study the triple labeling method by employing anterograde tracing combined with double-immunohistochemical staining for both SP and NK-1R was used. The results showed that after injecting BDA into the lateral subregion of the mouse PAG, BDA anterogradely labeled fibers and terminals were found widely distributed in most structures of the brainstem, especially in the RMg. Some of the BDA-labeled fibers and terminals showing SP-immunopositive staining were also observed. A few NK-1R immunoreactive neuronal cell bodies and dendrites were observed. Most of these synapses made by SP/BDA double labeled terminal and NK-1R immunoreactive neurons were asymmetric in form.The present results suggest that SP released from the descending axon terminals originated from the PAG might be one of the most important neural active substances which activate the endogenous pain control system and exert antinociceptive effects in the RMg.Partâ…¢Neurokinin-1 receptor immunoreactive neurons in the rostroventromedial medulla send projections to the superficial laminae of the spinal dorsal hornWe used retrograde tracer FG and NK-1R immunofluorescent histochemichal staining to examine the descending projections of the NK-1R expressing neurons in the RVM to the superficial laminae of the spinal dorsal horn.After the injection of FG into the spinal dorsal horn, FG retrograde labeled neurons were observed mainly in the RVM, Some of which were simultaneously immunoreactive for NK-1R. The above result indicates that some NK-1R expressing neurons in the RVM send axonal projections to the superficial laminae of the spinal dorsal horn, participating in the modulation of neuronal activities in the spinal dorsal horn, may be involved in the modulation of afferent transmission of nociceptive information.