Carole A. Williams

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

Antibody microprobes were used to study release of immunoreactive neurokinins in the dorsal horn of the anaesthetized spinal cat following sustained isometric contraction of ipsilateral hindlimb muscles. Microprobes had immobilized antibodies to neurokinin A (NKA) on their outer surfaces and bound a proportion of released molecules when inserted in the central nervous system. Bound molecules were detected in autoradiographs as zones of reduced binding of 125I-NKA in which microprobes were incubated after withdrawal from the spinal cord. The left hindlimb was immobilized using an epoxy bandage splint and isometric contraction of muscles induced by intermittent tetanic stimulation of a ventral root. A basal presence of immunoreactive neurokinins was detected and this was increased by sustained isometric muscle contraction. It is probable that ergoreceptors contain and release neurokinins.

C2 spinal cord stimulation induces dynorphin release from rat T4 spinal cord: potential modulation of myocardial ischemia-sensitive neurons

During myocardial ischemia, the cranial cervical spinal cord (C1-C2) modulates the central processing of the cardiac nociceptive signal. This study was done to determine 1) whether C2 SCS-induced release of an analgesic neuropeptide in the dorsal horn of the thoracic (T4) spinal cord; 2) if one of the sources of this analgesic peptide was cervical propriospinal neurons, and 3) if chemical inactivation of C2 neurons altered local T4 substance P (SP) release during concurrent C2 SCS and cardiac ischemia. Ischemia was induced by intermittent occlusion of the left anterior descending coronary artery (CoAO) in urethane-anesthetized Sprague-Dawley rats. Release of dynorphin A (1-13), (DYN) and SP was determined using antibody-coated microprobes inserted into T4. SCS alone induced DYN release from laminae I-V in T4, and this release was maintained during CoAO. C2 injection of the excitotoxin, ibotenic acid, prior to SCS, inhibited T4 DYN release during SCS and ischemia; it also reversed the inhibition of SP release from T4 dorsal laminae during C2 SCS and CoAO. Injection of the kappa-opioid antagonist, nor-binaltorphimine, into T4 also allowed an increased SP release during SCS and CoAO. CoAO increased the number of Fos-positive neurons in T4 dorsal horns but not in the intermediolateral columns (IML), while SCS (either alone or during CoAO) minimized this dorsal horn response to CoAO alone, while inducing T4 IML neuronal recruitment. These results suggest that activation of cervical propriospinal pathways induces DYN release in the thoracic spinal cord, thereby modulating nociceptive signals from the ischemic heart.

Activated cranial cervical cord neurons affect left ventricular infarct size and the potential for sudden cardiac death

To evaluate whether cervical spinal neurons can influence cardiac indices and myocyte viability in the acutely ischemic heart, the hearts of anesthetized rabbits subjected to 30 min of LAD coronary arterial occlusion (CAO) were studied 3h after reperfusion. Control animals were compared to those exposed to pre-emptive high cervical cord stimulation (SCS; the dorsal aspect of the C1-C2 spinal cord was stimulated electrically at 50 Hz; 0.2 ms; 90% of motor threshold, starting 15 min prior to and continuing throughout CAO). Four groups of animals were so tested: 1) neuroaxis intact; 2) prior cervical vagotomy; 3) prior transection of the dorsal spinal columns at C6; and 4) following pharmacological treatment [muscarinic (atropine) or adrenergic (atenolol, prazosin or yohimbine) receptor blockade]. Infarct size (IS) was measured by tetrazolium, expressed as percentage of risk zone. C1-C2 SCS reduced acute ischemia induced IS by 43%, without changing the incidence of sudden cardiac death (SCD). While SCS-induced reduction in IS was unaffected by vagotomy, it was no longer evident following transection of C6 dorsal columns or atropinization. Beta-adrenoceptor blockade eliminated ischemia induced SCD, while alpha-receptor blockade doubled its incidence. During SCS, myocardial ischemia induced SCD was eliminated following vagotomy while remaining unaffected by atropinization. These data indicate that, in contrast to thoracic spinal neurons, i) cranial cervical spinal neurons affect both adrenergic and cholinergic motor outflows to the heart such that ii) their activation modifies ventricular infarct size and lethal arrhythmogenesis.

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.

The influence of straining maneuvers on the pressor response during isometric exercise

SummaryExperiments were performed to determine to what extent increments in esophageal and abdominal pressure would have on arterial blood pressure during fatiguing isometric exercise. Arterial blood pressure was measured during handgrip and leg isometric exercise performed with both a free and occluded circulation to active muscles. Handgrip contractions were exerted at 33 and 70% MVC (maximum voluntary contraction) by 4 volunteers in a sitting position and calf muscle contractions at 50 and 70% MVC with the subjects in a kneeling position. Esophageal pressure measured at the peak of inspirations did not change during either handgrip or leg contractions but peak expiratory pressures increased progressively during both handgrip and leg contractions as fatigue occurred. These increments were independent of the tensions of the isometric contractions exerted. Intra-abdominal pressures measured at the peak of either inspiration or expiration did not change during inspiration with handgrip contractions but increased during expiration. During leg exercise, intraabdominal pressures increased during both inspiration and expiration, reaching peak levels at fatigue. The arterial blood pressure also reached peak levels at fatigue, independent of circulatory occlusion and tension exerted, averaging 18.5–20 kPa (140–150 mm Hg) for both handgrip and leg contrations. While blood pressure returned to resting levels following exercise with a free circulation, it declined by only 2.7–3.8 kPa after leg and handgrip exercise, respectively, during circulatory occlusion. These results indicate that straining maneuvers contribute 3.5 to 7.8 kPa to the change in blood pressure depending on body position.

Substance P release in response to cardiac ischemia from rat thoracic spinal dorsal horn is mediated by TRPV1.

Spinal cord stimulation (SCS) inhibits substance P (SP) release and decreases the expression of the transient receptor potential vanilloid 1 (TRPV1) in the spinal cord at thoracic 4 (T4) during cardiac ischemia in rat models (Ding et al., 2007). We hypothesized that activation of TRPV1 in the T4 spinal cord segment by intermittent occlusion of the left anterior descending coronary artery (CoAO) mediates spinal cord SP release. Experiments were conducted in urethane-anesthetized Sprague-Dawley male rats using SP antibody-coated microprobes to measure SP release at the central terminal endings of cardiac ischemic-sensitive afferent neurons (CISAN) in the spinal T4 dorsal horns. Vehicle, capsaicin (CAP; TRPV1 agonist) and capsazepine (CZP; TRPV1 antagonist) were injected into the left T4 prior to stimulation of CISAN by intermittent CoAO (with or without upper cervical SCS). CAP induced endogenous SP release from laminae I and II in the T4 spinal cord above baseline. Conversely, CZP injections significantly inhibited SP release from laminae I-VII in the T4 spinal cord segment below baseline. CZP also attenuated CoAO-induced SP release, while T4 injections of CZP with SCS completely restored SP release to basal levels during CoAO activation. CAP increased the number of c-Fos (a marker for CISAN activation) positive T4 dorsal horn neurons compared to sham-operated animals, while CZP (alone or during CoAO and SCS+CoAO) significantly reduced the number of c-Fos positive neurons. These results suggest that spinal release of the putative nociceptive transmitter SP occurs, at least in part, via a TRPV1 mechanism.

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.

Changes in isometric function following rhythmic exercise

SummarySeven male subjects exercised for 1, 3, 10 and 20 min on a cycle ergometer at 20, 60 and 80%

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

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