Sustained isometric contraction of skeletal muscle results in release of immunoreactive neurokinins in the spinal cord of the anaesthetized cat

Effects of neonatal capsaicin deafferentation on neuromuscular adjustments, performance, and afferent activities from adult tibialis anterior muscle during exercise

To investigate the role played by muscle afferents in the sensorimotor loops, we measured the effects of capsaicin injection in newborns on the mechano- and metabosensitive discharges and the running performance at adulthood. Female Sprague Dawley rats received a subcutaneous injection of either 50 mg/kg capsaicin or solvent (10% ethanol, 10% Tween 80 in 0.9% saline) during their second day of life. These two groups were compared with a control, untreated group. Four months later, treadmill running performance and muscle afferent (mechanosensitive and metabosensitive) activities from the tibialis anterior muscle were measured. The capsaicin-treated group demonstrated a reduced maximal exercise capacity (time to exhaustion) and a reduced response of muscle metabosensitive fibers (group III and IV nerve endings) to the examined stimuli (arterial KCl and lactic acid injections, electrically induced fatigue) compared with the sham-injected solvent and control groups. Group IV afferent responses were absent in the compound nerve action potentials evoked by peripheral nerve stimulation. The response to mechanosensitive fibers to tendon vibration was also affected in the capsaicin group compared with the control and sham-injected groups, which presented a bimodal response corresponding to the activation of muscle spindles and Golgi tendon organs. Finally, measurements of the force developed by the tibialis anterior muscle from the beginning to the end of a 3-min muscle stimulation revealed a more significant fall in the capsaicin group compared with the others. The present experiments reveal that the pharmacological alteration of muscular metabosensitive afferent resulted in drastic changes in the neuromuscular sensory encoding and in the central neural network that could accelerate failure of the task during fatigue.

NK(3) receptors in the feline nucleus tractus solitarius are not involved with the muscle pressor response

Isometric muscle contractions cause an increase in mean arterial pressure and heart rate. Previously, we showed that substance P (SP) is released from sites in the feline medial nucleus tractus solitarius (mNTS) in response to isometric muscle contractions, and that it most likely interacted with NK(1) tachykinin receptors at these sites. This study was undertaken to determine whether other tachykinin receptors in this area of the brainstem are involved with the muscle pressor response. Receptor autoradiography, using [(125)I]Bolton-Hunter SP and [(125)I] [MePhe(7)] neurokinin B to label NK(1) and NK(3) receptors, respectively, indicated that NK(3) tachykinin receptors are as abundant as NK(1) and NK(3) receptors, respectively, indicated that NK(3) tachykinin receptors are as abundant as NK(1) receptors in this region of the feline brainstem Injections of the specific NK(3) receptor antagonist, SR 142801 (0.1 to 10 microM) into the mNTS did not modify the pressor response or the heart rate response to isometric muscle contractions. Injection of SR142801 into the NTS prior to the injection of the NK(1) antagonist, GR82334 did not affect the action of GR82334 to attenuate the muscle pressor reflex. We conclude that NK(3) receptors in the NTS are not involved with the regulation of cardiovascular function during activation of the muscle pressor response.

Time-dependent changes in neurokinin(3) receptors and tachykinins during adjuvant-induced peripheral inflammation in the rat

Although considerable evidence exists that spinal neurokinin(1) receptors are involved in central sensitization of nociception, recent evidence from knockout studies indicates that other neurokinin receptors in the spinal cord may mediate a portion of the hyperalgesia caused by substance P and neurokinin A. The present study determined whether the second most abundant class of neurokinin receptors, neurokinin(3) receptors, are regulated during persistent peripheral inflammation. Inflammation in the hind paw of the rat was induced by intraplantar injection of complete Freund's adjuvant. Receptor autoradiography revealed specific binding of [125I]-MePhe(7)-NKB, a selective ligand for neurokinin(3) receptors, in the superficial dorsal horn of the spinal cord. Specific binding of [125I]-MePhe(7)-NKB in the medial dorsal horn was reduced bilaterally two days after unilateral injection of complete Freund's adjuvant. Binding returned to basal levels four days after injection of complete Freund's adjuvant. Neurokinin(3) receptor messenger RNA levels doubled in the dorsal spinal cord at 12h and remained elevated for at least four days. The change in neurokinin(3) receptor binding and messenger RNA during adjuvant-induced inflammation may be a consequence of activation of the receptor. Spinal levels of potential endogenous ligands for spinal neurokinin(3) receptors were measured by radioimmunohistochemistry. Immunoreactive substance P but not neurokinin B peptide 2, a marker for neurokinin B, was reduced bilaterally during adjuvant-induced inflammation.Collectively, these data indicate that spinal neurokinin(3) receptors may play a role in spinal neurotransmission of injured rats and require consideration of other tachykinins as physiologically relevant ligands to spinal neurokinin(3) receptors.

Expression of c-fos-like immunoreactivity in the feline brainstem in response to isometric muscle contraction and baroreceptor reflex changes in arterial pressure

This study compared whether activation of muscle ergoreceptor afferents caused by isometric muscle contraction, activation of baroreceptor afferents induced by i.v. infusion of phenylephrine, or baroreceptor afferent inactivation, caused by carotid artery occlusion, elicit similar patterns of c-Fos induction in brainstem areas. Adult cats were anesthetized with alpha-chloralose, and in each case, the experimental intervention caused an increase in the arterial blood pressure. There were two sets of control experiments: in both, animals underwent the same surgical procedures but then either remained at rest for the entire study, or the tibial nerve was stimulated, as in the contraction group, following muscle paralysis with tubocurarine. Following the procedures, animals rested for 90 min to allow neuronal expression of c-Fos. Control cats showed very little c-Fos immunoreactivity (c-Fos-ir) in the brainstem. Muscle contraction induced c-Fos-ir expression mainly in the nucleus tractus solitarius, lateral reticular nucleus, lateral tegmental field, vestibular nucleus, subretrofacial nucleus, spinal trigeminal tract and in a lateral region of the periaqueductal grey (P 0.5-1.0). The majority of the c-Fos-ir was found in brainstem areas contralateral to the contracted muscle. In addition, muscle contraction induced c-Fos-ir in the dorsal horns of spinal segments L6-S1 on the ipsilateral side of the spinal cord. Phenylephrine infusion caused c-Fos-ir expression in the nucleus tractus solitarius, spinal trigeminal tract, solitary tract, and dorsal motor nucleus of the vagus. No c-Fos-ir was apparent in the periaqueductal grey. Carotid occlusions induced c-Fos-ir expression in the area postrema, nucleus tractus solitarius, solitary tract, and spinal trigeminal tract. Expression was bilateral. Areas that exhibited c-Fos-ir correspond to sites previously reported to release various neuropeptides in response to muscle contraction or carotid occlusions. These results indicate that the exercise pressor reflex and baroreflex activate similar, but not completely identical, sites in the brainstem.

The role of capsaicin-sensitive muscle afferents in fatigue-induced modulation of the monosynaptic reflex in the rat

1. The role of group III and IV afferent fibres of the lateral gastrocnemious muscle (LG) in modulating the homonymous monosynaptic reflex was investigated during muscle fatigue in spinalized rats. 2. Muscle fatigue was induced by a series of increasing tetanic electrical stimuli (85 Hz, 600 ms) delivered to the LG muscle nerve. Series consisted of increasing train numbers from 1 to 60. 3. Potentials from the spinal cord LG motor pool and from the ventral root were recorded in response to proprioceptive afferent stimulation and analysed before and during tetanic muscle activations. Both the pre- and postsynaptic waves showed an initial enhancement and, after a '12-train' series, an increasing inhibition. 4. The enhancement of the responses to muscle fatiguing stimulation disappeared after L3-L6 dorsal root section, while a partial reflex inhibition was still present. Conversely, after section of the corresponding ventral root, there was only a reduction in the inhibitory effect. 5. The monosynaptic reflex was also studied in animals in which a large number of group III and IV muscle afferents were eliminated by injecting capsaicin (10 mM) into the LG muscle. As a result of capsaicin treatment, the fatigue-induced inhibition of the pre- and postsynaptic waves disappeared, while the response enhancement remained. 6. We concluded that the monosynaptic reflex inhibition, but not the enhancement, was mediated by those group III and IV muscle afferents that are sensitive to the toxic action of capsaicin. The afferents that are responsible for the response enhancement enter the spinal cord through the dorsal root, while those responsible for the inhibition enter the spinal cord through both the ventral and dorsal roots.

Segmental effect of spinal NK-1 receptor blockade on the pressor reflex

The physiological effects of substance P (SP) are mediated via activation of neurokinin-1 (NK-1) receptors. The purpose of this study was to test the hypothesis that blockade of NK-1 receptors in the dorsal horn, both at the site of entry for the primary afferent neurons and adjacent spinal segments, attenuates the pressor reflex evoked by static contraction and stretch of skeletal muscle. Cats were anesthetized with alpha-chloralose and urethan, and a laminectomy was performed. With the exception of the L7 dorsal root, the dorsal and ventral roots from L5 to S2 were sectioned on one side of the spinal cord. Thus the primary afferent fibers mediating the pressor reflex enter the spinal cord via the L7 dorsal root in these experiments. Based on dose-response data, dialysis of the NK-1 receptor antagonist CP-96,345 (5 mM for 2 h) into the L7 dorsal horn ipsilateral to the contracting muscle attenuated the pressor response to static contraction (75 +/- 15 vs. 46 +/- 7 mmHg; n = 5 cats) but not muscle stretch (60 +/- 12 vs. 50 +/- 8 mmHg). Administration of the inactive enantiomer of CP-96,345, CP-96,344 (5 mM for 2 h), into the L7 dorsal horn failed to alter the cardiovascular changes elicited by contraction (45 +/- 7 vs. 43 +/- 6 mmHg) and stretch (31 +/- 8 vs. 32 +/- 11). Dialysis of 5 mM CP-96, 345 into the dorsal horn at the L6 and S1 segments for 2 h decreased the peak pressor response to static contraction (58 +/- 9 vs. 31 +/- 6 mmHg; n = 7) and muscle stretch (61 +/- 6 vs. 44 +/- 8 mmHg). These data suggest that the activation of NK-1 receptors, both at the site of entry and in regions outside of the entry site for afferent neurons, is involved in the spinal processing that produces the pressor reflex evoked by static contraction of skeletal muscle.

Substance P released in the rostral brainstem of cats interacts with NK-1 receptors during muscle pressor response

The release of immunoreactive substance P-like substances (irSP) was measured from sites in the rostral brainstem (at a level 1.3 mm anterior to the obex) of anesthetized cats in response to fatiguing isometric contractions using SP antibody-coated glass microprobes. The contractions caused a pressor and tachycardic response. irSP were released from sites in the medial subnucleus of the nucleus tractus solitarius (mNTS), the solitary tract and lateral tegmental field at this level of the brainstem. Injections of the specific NK-1 receptor antagonist, GR 82334, bilaterally into the mNTS significantly reduced the muscle pressor response, while bilateral injections of the SP NK-1 agonist, GR 73632, into the mNTS significantly increased the pressor and tachycardic responses during the contractions. Neither the antagonist nor the agonist, at the doses tested, affected the resting arterial pressure or heart rate. These data indicate that irSP are released from sites in the mNTS during the reflex pathways activated by isometric contractions and that they interact with NK-1 receptors in the area of the mNTS to affect the cardiovascular responses during the muscle pressor reflex.

Partial transformation from fast to slow muscle fibers induced by deafferentation of capsaicin-sensitive muscle afferents

Mechanical and histochemical characteristics of the lateral gastrocnemius (LG) muscle of the rat were examined 21 days after capsaicin injection into the LG muscle. The capsaicin caused a decrease in generation rate of twitch and tetanic tension and an increase in fatigue resistance of LG muscle. The histochemical muscle fiber profile evaluated by myosin adenosine triphosphatase and reduced nicotinamide adenine dinucleotide tetrazolium reductase methods showed an increase of type I and IIC fibers and a decrease of the type IIB in whole muscle, and a decrease of the IIA, IIX fibers in the red part accompanied by their increase in the white part. Therefore the capsaicin treatment, which selectively eliminated fibers belonging to the III and IV groups of muscle afferents, induced muscle fiber transformation from fast contracting fatiguing fibers to slowly contracting nonfatiguing ones.

Inhibition of hyperalgesia by ablation of lamina I spinal neurons expressing the substance P receptor

Substance P is released in the spinal cord in response to painful stimuli, but its role in nociceptive signaling remains unclear. When a conjugate of substance P and the ribosome-inactivating protein saporin was infused into the spinal cord, it was internalized and cytotoxic to lamina I spinal cord neurons that express the substance P receptor. This treatment left responses to mild noxious stimuli unchanged, but markedly attenuated responses to highly noxious stimuli and mechanical and thermal hyperalgesia. Thus, lamina I spinal cord neurons that express the substance P receptor play a pivotal role in the transmission of highly noxious stimuli and the maintenance of hyperalgesia.

Noxious cutaneous thermal stimuli induce a graded release of endogenous substance P in the spinal cord: imaging peptide action in vivo

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 degrees, 48 degrees, and 55 degrees C) and cold (10 degrees, 0 degrees, -10 degrees, and -20 degrees C) stimuli, but not innocuous warm (38 degrees C) and cold (20 degrees 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 SPR+ lamina I neurons showing SPR internalization and the number of internalized SPR+ 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.

The pressor reflex evoked by static contraction: neurochemistry at the site of the first synapse

Stimulation of somatic sensory neurons activates the sympathetic nervous system, in turn enhancing cardiovascular function. This has been repeatedly demonstrated when afferent fibers arising from skeletal muscle serve as the sensory neurons. Over the past several years, studies have been performed examining the central nervous system (CNS) mechanisms that cause the reflex increases in arterial blood pressure and heart rate when skeletal muscle contracts. These studies have provided insights into how the CNS alters cardiovascular function, and have helped to enhance our understanding of central sensory transduction processes. Using a variety of techniques, several sites have been identified within the brain and spinal cord that are responsible for producing the reflex pressor response to static contraction. However, the purpose of this manuscript is to review the recent developments concerning only one CNS site: the dorsal horn of the spinal cord. This region serves as the first synapse for afferent fibers from skeletal muscle. The release of neurotransmitters, and possibly neuromodulators, into this region initiates the CNS component of this reflex. In addition, the magnitude of the reflex cardiovascular changes can be modulated at this site. The studies described in this review suggest that the dorsal horn of the spinal cord serves as an important site of integration for sensory signals that influence the cardiovascular system.

Biophysical and functional consequences of receptor-mediated nerve fiber transformation

Stimulation of the nervous system by substance P, a G protein-coupled receptor, and subsequent receptor internalization causes dendrites to change their shape from homogeneous cylinders to a heterogeneous string of swollen varicosities (beads) connected by thin segments. In this paper we have analyzed this phenomenon and propose quantitative mechanisms to explain this type of physical shape transformation. We developed a mathematical solution to describe the relationship between the initial radius of a cylindrical nerve fiber and the average radii of the subsequently created varicosities and connecting segments, as well as the periodicity of the varicosities along the nerve fiber. Theoretical predictions are in good agreement with our own and published experimental data from dorsal root ganglion neurons, spinal cord, and brain. Modeling the electrical properties of these beaded fibers has led to an understanding of the functional biophysical consequences of nerve fiber transformation. Several hypotheses for how this shape transformation can be used to process information within the nervous system have been put forth.

Central projections and entries of capsaicin-sensitive muscle afferents

The entry pathway and central distribution of A delta and C muscle afferents within the central nervous system (CNS) were investigated by combining electron microscopy and electrophysiological analysis after intramuscular injection of capsaicin. The drug was injected into the rat lateral gastrocnemius (LG) and extraocular (EO) muscles. The compound action potentials of LG nerve and the evoked field potentials recorded in semilunar ganglion showed an immediate and permanent reduction in A delta and C components. The morphological data revealed degenerating unmyelinated axons and terminals in the inner sublamina II and in the border of laminae I-II of the dorsal horn at L4-L5 and C1-C2 (subnucleus caudalis trigemini) spinal cord segments. Most degenerating terminals were the central bouton (C) of type I and II synaptic glomeruli. Furthermore, degenerating peripheral axonal endings (V2) presynaptic to normal C were found. Since V2 were previously found degenerated after cutting the oculomotor nerve (ON) or L4 ventral root, we conclude that some A delta and C afferents from LG and EO muscles entering the CNS by ON or ventral roots make axoaxonic synapses on other primary afferents to promote an afferent control of sensory input.

Substance P and neurokinin A metabolism by cultured human skeletal muscle myocytes and fibroblasts

A recent study determined that cultured human skeletal muscle adult myoblasts, myotubes, and fibroblasts degraded angiotensins and kinins via neutral endopeptidase-24.11 (NEP-24.11: EC 3.4.24.11) and aminopeptidase N (APN: EC 3.4.11.2). Due to the possible importance of other peptides to skeletal muscle blood flow and function, the present study looked specifically at the metabolism of the neurokinins substance P (SP) and neurokinin A (NKA) by skeletal muscle peptidases. The results show that SP is degraded not only by NEP-24.11, but also sequentially by dipeptidyl(amino)peptidase IV (DAP IV: EC 3.4.14.5)/APN. NKA is unaffected by DAP IV but is metabolized by NEP-24.11 and APN. NEP-24.11 was inhibited by phosphoramidon (IC50 = 80 nM), thiorphan and ZINCOV, DAP IV by diprotin A (IC50 = 8 microM), and APN by amastatin (IC50 = 50 nM) and bestatin (IC50 = 100 microM). Skeletal muscle myocyte and fibroblast metabolism of SP and NKA may regulate local skeletal muscle vascular and extravascular functions including SP- and NKA-mediated nerve-induced vasodilation. Inhibition of both NEP-24.11 and DAP IV/APN may increase skeletal muscle blood flow and decrease peripheral vascular resistance via potentiation of local neurokinin levels.

Receptor endocytosis and dendrite reshaping in spinal neurons after somatosensory stimulation

In vivo somatosensory stimuli evoked the release of substance P from primary afferent neurons that terminate in the spinal cord and stimulated endocytosis of substance P receptors in rat spinal cord neurons. The distal dendrites that showed substance P receptor internalization underwent morphological reorganization, changing from a tubular structure to one characterized by swollen varicosities connected by thin segments. This internalization and dendritic structural reorganization provided a specific image of neurons activated by substance P. Thus receptor internalization can drive reversible structural changes in central nervous system neurons in vivo. Both of these processes may be involved in neuronal plasticity.

Fatiguing isometric contraction of hind-limb muscles results in the release of immunoreactive neurokinins from sites in the rostral medulla in the anesthetized cat

Antibody-coated microprobes were used to determine whether immunoreactive neurokinins (irNK) were released from sites in the brainstem during fatiguing isometric contractions of the triceps surae muscles in cats anesthetized with alpha-chloralose. Contractions were generated by stimulating the tibial nerve using a microprocessor-controlled stimulator. Microprobes were inserted into the periaqueductal grey (P 0.5-1.0 mm) or the medullary brainstem (either 3.0 or 3.5 mm rostral to the obex) prior to, during and following fatiguing contractions. No release of irNK was detected from the periaqueductal grey as a result of fatiguing isometric contractions. When probes were inserted 3.0 mm rostral to the obex, a basal release of irNK was detected from the medulla but this was inhibited during isometric contractions from sites corresponding to the lateral tegmental field. When probes were inserted into the more rostral site in the medulla (3.5 mm rostral to the obex), irNK were released in response to contractions from sites corresponding to lateral reticular nucleus, ventral regions of the nucleus tractus solitarius and the medial vestibular nucleus. No irNK were released from this site (3.5 mm rostral to obex) either during passive leg flexing, during nerve stimulation following gallamine injection and muscle paralysis or during stimulation of the central end of the cut tibial nerve. These results demonstrate that neurokinins are released from discrete sites in the medulla in response to fatiguing muscle contractions and suggest that tachykinin neurons may be a component of the pathways regulating blood pressure during ergoreceptor activation.

Detection of immunoreactive substance P-like substances from cat brainstem sites during fatiguing isometric contractions

Isometric contractions were generated on the left hindlimb muscles from adult cats (n = 11) anesthetized with alpha-chloralose (75 mg/kg) to determine whether immunoreactive substance P (irSP) was released from either the right periaqueductal grey (PAG) or ventrolateral medulla (VLM), sites shown to be involved with the integration of the muscle pressor response. The release of immunoreactive SP was measured using SP antibody-coated microelectrodes that were inserted into the PAG or the VLM during periods of rest, fatiguing isometric contractions and post-contraction. Mean arterial pressure increased by 78 +/- 11 mmHg during the contractions. There was a release of irSP from sites in the medulla during the contractions compared to the non-contraction periods but none was detected from the PAG in response to muscle stimulation. These results provide further evidence that SP-like substances may be involved with the central integration of the muscle pressor response.

Inhibition in the release of immunoreactive beta-endorphin from the periaqueductal grey during isometric contractions of cat hind-limb muscles: the effects of clonidine

Glass microelectrodes, coated with antibodies specific for beta-endorphin, were inserted into the right periaqueductal grey (PAG) (at PO.5-1.0 mm, LR 2.0 mm and 6.0 mm below the dorsal surface of the colliculi) of cats anesthetized with alpha-chloralose to determine whether immunoreactive beta-endorphins (ir-END) were released in response to fatiguing isometric contractions of the hind-limb muscles. Probes were inserted into the PAG prior to, during and following muscle contraction in the absence or presence of clonidine. ir-END was released from the PAG up to 3 h after surgery was completed while cats remained at rest. In the absence of clonidine, mean arterial pressure increased by 65 +/- 12 mmHg during contractions and the release of ir-END was inhibited during the contraction periods compared to resting periods. Clonidine abolished the pressor response to muscular contraction when injected into the PAG, but did not cause the release of ir-END during fatiguing isometric contractions. These data suggest that isometric contractions of skeletal muscle do not induce the release of ir-END-like substances from the PAG and clonidine does not attenuate the muscle pressor response by causing the release of ir-END from this level in the PAG.

Effects of graded muscle contractions on spinal cord substance P release, arterial blood pressure, and heart rate

The release of substance P (SP)-like immunoreactivity (SP-LI) in the dorsal horn of the spinal cord and the cardiovascular changes to both high-tension (HT) and low-tension (LT) contractions were determined using alpha-chloralose-anesthetized cats. Over a 10-minute period, seven contractions (HT or LT) were induced. Each contraction was 20 seconds in duration and was followed by an 80-second quiescent period. The tension-time index (TTI) for the HT contractions was 2751 +/- 348 kg.s (mean +/- SD), which was greater than the TTI of 813 +/- 167 kg.s for the LT contractions. The HT contractions caused a greater release of SP-LI than the LT contractions: SP-LI increased from 0.18 +/- 0.02 to 0.32 +/- 0.03 fmol/100 microL and from 0.18 +/- 0.02 to 0.25 +/- 0.04 fmol/100 microL for the two types of contractions, respectively. Concomitant with this greater SP-LI release, HT contractions caused larger increases in mean arterial pressure (34 +/- 16 versus 11 +/- 4 mm Hg) and heart rate (18 +/- 7 versus 8 +/- 4 beats per minute) than did the LT contractions. These changes in SP-LI, mean arterial pressure, and heart rate were virtually abolished when the contractions were repeated after sectioning the L-5-S-2 dorsal and ventral roots or when the electrical stimulation of the ventral roots was repeated after muscle paralysis with gallamine triethiodide. These results demonstrate that contraction-evoked SP-LI release in the dorsal horn is related to the developed tension. Furthermore, these data provide additional support for the hypothesis that the release of SP from the central terminations of muscle afferents plays a role in mediating the cardiovascular responses to static contraction of skeletal muscle.

The role of neurokinin and N-methyl-D-aspartate receptors in synaptic transmission from capsaicin-sensitive primary afferents in the rat spinal cord in vitro

The rat spinal cord with connected dorsal root ganglia was used to study neurokinin and N-methyl-D-aspartate receptors involved in the sensory synaptic transmission of dorsal horn cells. Selective C-fibre excitation was produced by capsaicin (200-500 nM) administered to the dorsal root ganglions. Sixty-nine per cent of dorsal horn cells responded with a postsynaptic depolarization and enhanced synaptic activity, recorded via intracellular electrodes, to capsaicin-activated primary afferent input. Dorsal horn neurons activated by the capsaicin-evoked input were also excited by a 1-min perfusion of the neurokinin-1 receptor agonists substance P methyl ester or GR73 632 and by the neurokinin-2 agonist neurokinin-A. These cells were also depolarized by N-methyl-D-aspartate. Responses to substance P methyl ester and GR73 632 were selectively reduced by the neurokinin-1 receptor antagonist CP96,345, and responses to neurokinin-A were completely blocked by the neurokinin-2 receptor antagonist MEN10 376. The depolarization evoked by N-methyl-D-aspartate was not altered by either of the antagonists, but was completely blocked by the selective N-methyl-D-aspartate receptor antagonist (-)-2-amino-5-phosphonovaleric acid. Capsaicin-evoked responses in the dorsal horn were inhibited by MEN10,376 (63 +/- 13% inhibition) but no significant change was observed with CP96,345. The N-methyl-D-aspartate receptor antagonist (-)-2-amino-5-phosphonovaleric acid consistently inhibited the capsaicin-induced response by 76 +/- 14%. Combination of (-)-2-amino-5-phosphonovaleric acid and MEN10,376 produced an almost complete abolition of the capsaicin-evoked depolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

Substance P release in the spinal cord during the exercise pressor reflex in anaesthetized cats

1. The purpose of this study was to determine if static skeletal muscle contraction causes the release of substance P(SP) in the L7-dorsal horn region of the spinal cord. A laminectomy was performed to expose the spinal cord of alpha-chloralose anaesthetized cats. The L6 spinal root was cut. A microdialysis probe was inserted into the L7 dorsal horn region ipsilateral to the contracting triceps surae muscle. The probe was perfused with a buffer solution at 3 microliters/min. Substance P-like immunoreactivity (SP-LI) was measured, from the microdialysis samples, by radioimmunoassay. 2. A 5-9 min contraction of the triceps surae muscle was evoked by alternate electrical stimulation of the peripheral ends of the cut L7 and S1 ventral roots. Basal SP-LI release was 0.20 +/- 0.03 fmol/100 microliters and was increased to 0.54 +/- 0.05 fmol/100 microliters (mean +/- S.D.) by static muscle contraction. This increase was greatly attenuated after cutting the L7 and S1 dorsal roots (0.23 +/- 0.03 to 0.39 +/- 0.08 fmol/100 microliters) or completely abolished by muscle paralysis (0.27 +/- 0.03 to 0.31 +/- 0.01 fmol/100 microliters). Muscle contraction also increased mean arterial blood pressure (MAP) 29 +/- 20 mmHg and heart rate (HR) 11 +/- 5 beats/min (mean +/- S.D.). These cardiovascular changes to muscle contraction were abolished by sectioning the dorsal roots or when the ventral roots were electrically stimulated after the cats were paralysed. 3. These results demonstrate that static contraction of skeletal muscle increases the release of SP-LI in the dorsal horn of the spinal cord. Furthermore, these data support the hypothesis that SP plays a role in mediating the cardiovascular responses evoked during static exercise.

Release of immunoreactive enkephalinergic substances in the periaqueductal grey of the cat during fatiguing isometric contractions

Antibody-coated microprobes were used to determine whether immunoreactive enkephalins were released in response to fatiguing isometric contractions of the hind-limb muscles in cats anesthetized with alpha-chloralose. Contractions were performed by stimulating the tibial nerve via a microprocessor-controlled stimulator. Microprobes were inserted into the periaqueductal grey (P 0.5-1.0 mm) prior to, during and following fatiguing contractions. During fatiguing contractions, mean arterial blood pressure increased by 76 +/- 9 mmHg above resting and recovery levels. Levels of immunoreactive enkephalins were elevated in the dorsolateral periaqueductal grey during the isometric contraction when compared to resting levels. It is possible that isometric muscle contraction causes the release of Met-enkephalin-like substances in the periaqueductal grey.