http://www.back-in-business-physiotherapy.com/images/files/DescendingControl.pdf

Descending control of pain.

Cholecystokinin/opioid interactions

The distribution of substance P receptor (SPR) protein in the rat periaqueductal gray matter (PAG) was investigated with a polyclonal antibody in the four subdivisions obtained by cytochrome-oxidase histochemistry (Co-hi). At light microscopic analysis, immunoreactivity appeared particularly dense in the dorsal subdivision of the PAG, was less intense in the other subdivisions, and formed several longitudinally organized columns. SPR-like immunoreactivity (SPR-i) was localized mostly to cell bodies and dendrites of small and medium-sized neurons, which constituted about 6% of the total neuronal population of the PAG. At the electron microscopic level, SPR-i could be observed on postsynaptic as well as on nonsynaptic regions of both cell bodies and dendrites. A small proportion of axons (4.2%) and axon terminals (5.3%) showed SPR-i, the majority of labeled axon terminals, amounting to about 70% of synapsing elements, formed asymmetric synapses with dendrites. Rare astroglial processes displaying SPR-i were also observed scattered throughout the neuropil of all PAG subdivisions. Our observations suggest that 1) also in the PAG, SP may act in a diffuse, nonsynaptic manner, probably on targets that are distant from its sites of release; and 2) SP may modulate excitatory neurotransmission acting presynaptically on those labeled axons that form asymmetric synapses. J. Comp. Neurol. 398:473–490, 1998. © 1998 Wiley-Liss, Inc.

Immunocytochemical localization of substance P receptor in rat periaqueductal gray matter: a light and electron microscopic study.

The analgesic effect of activation of 5-HT3 receptors at the spinal cord is attenuated by the opioid antagonist naloxone. Enkephalin-immunoreactive neurons in the spinal cord superficial dorsal horn are innervated by 5-HT-immunoreactive fibers. This prompted us to examine whether enkephalinergic dorsal horn neurons express 5-HT3 receptors. Using the technique of single-cell RT-PCR adapted for small neurons in the superficial dorsal horn, methionine-enkephalin sequence-specific PCR products were observed in about half of the neurons studied. Furthermore, 5-HT3 receptor mRNA was detected in approximately 25% of enkephalinergic neurons. These observations suggest that at least part of the antinociception elicited by activation of 5-HT3 receptors at the spinal cord may involve enkephalinergic dorsal horn neurons.

Enkephalinergic neurons express 5-HT3 receptors in the spinal cord dorsal horn: single cell RT-PCR analysis

Peripheral axotomy influences the in vivo release of cholecystokinin in the spinal cord dorsal horn—possible involvement of cholecystokinin-B receptors

Substance P (SP) nerve terminals innervate the intermediolateral cell column (IML) of the thoracic spinal cord, where SP coexists with serotonin (5-HT), neurokinin A (NKA) and thyrotropin-releasing hormone (TRH). Neither the depolarization-induced release of SP nor the presence of other neurochemicals in the regulation of SP release has been directly studied in this system. In the present study, basal and K+-stimulated release of SP from the microdissected intermediate area (including the IML, intercalated nucleus and central autonomic nucleus) of the rat thoracic spinal cord, and the regulation of SP release by presynaptic autoreceptors and by coexisting neurochemicals (5-HT, NKA and TRH) were studied using an in vitro superfusion system. Potassium evoked a concentration- and extracellular Ca2+-dependent release of SP. In rats pretreated with the serotoninergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT), both SP content and the absolute amount of SP released were decreased. However, the fraction of the remaining tissue content of SP released by K+ depolarization was not changed subsequent to 5,7-DHT treatment. Moreover, 5-HT, 5-HT1B agonists (CGS-12066B and RU 24969) and a 5-HT3 agonist (2-methyl-5-HT) did not alter the K+-evoked release of SP. These data demonstrate that SP is released from the intermediate area of the rat thoracic spinal cord and some of the SP released comes from serotoninergic nerve terminals. Although 5-HT coexists with SP in the IML, neither endogenous 5-HT nor 5-HT receptor ligands appear to regulate the release of SP. Other colocalized neuropeptides (NKA and TRH) are not involved in the regulation of SP release because neither NKA, a NK2 agonist (GR 64349) nor a TRH analog (MK-771) changed the K+-evoked release of SP.



A neurokinin-1 (NK1) antagonist (GR 82334) dose-dependently (10-9-10-7 M) increased the K+ stimulated release of SP. These data suggest the presence of presynaptic inhibitory NK1 autoreceptors. Whereas, NK1 agonists, [GR 73632 (10-9-10-6 M) and [Sar9, Met (O2)1]] SP (10-8-10-6 M)], increased the basal and K+-stimulated release of SP, the excitatory effects of GR 73632 were not blocked by the NK1 antagonist. Moreover, GR 73632 increased the efflus of SP to a greater extent in the absence of peptidase inhibitors. Thus, the effect of NK1 agonists on the release of SP may be related to an inhibition of peptide degradation rather than activation of NK1 autoreceptors. © 1996 Wiley-Liss, Inc.1

Characterization of substance P release from the intermediate area of rat thoracic spinal cord.

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Characterization of substance P release from the intermediate area of rat thoracic spinal cord.