Chuanyao Tong

Interaction between intrathecal neostigmine and epidural clonidine in human volunteers.

Background alpha2 ‐Adrenergic agonists are thought to produce analgesia, in part, by activating spinal acetylcholine release. The purpose of the current study was to examine the interaction between intrathecal neostigmine and epidural clonidine for analgesia and side effects in humans. Methods A total of 58 volunteers received an intrathecal injection of 5% dextrose in normal saline (D5NS) or neostigmine (50, 100, or 200 micro gram in D5NS), followed in 1 h by epidural saline or clonidine (computer‐controlled infusion targeted to 50, 100, 200, or 400 ng/ml in cerebrospinal fluid) using an isobolographic design. Visual analog scale pain to a noxious cold stimulus, nausea, weakness, sedation, and other safety variables was measured before and at specified intervals after drug administration. Results The first 21 volunteers randomized to receive intrathecal hyperbaric neostigmine rather than D5NS received the drug while in the sitting position, and had none‐to‐minimal analgesia 1 h later. The remaining volunteers received the drug while in the lateral position, and demonstrated dose‐dependent analgesia in the foot 1 h later. Epidural clonidine also caused dose‐dependent analgesia. The combination of neostigmine and clonidine resulted in an additive enhancement for analgesia, but no enhancement of each drugs side effects, and a reduction in clonidine‐induced hypotension. Neostigmine injected into subjects in the lateral position diminished clonidine‐induced reductions in blood pressure and plasma norepinephrine. Conclusion These results support enhancement of alpha2 ‐adrenergic analgesia by intrathecal neostigmine, but do not demonstrate synergy, as observed in animals. Lack of enhancement of side effects suggests this combination may be clinically useful.

Postoperative analgesia from intrathecal neostigmine in sheep.

Spinal neostigmine produces analgesia in chronically prepared rats, but not in sheep.However, since pain itself activates bulbospinal inhibitory pathways, neostigmine may be more effective in the postoperative period. We examined in sheep the antinociceptive effect of intrathecal neostigmine in the acute postoperative period and determined the muscarinic receptor subtype activated by neostigmine. A cervical intrathecal catheter was inserted via a laminotomy in 14 sheep that then received, in random order 1 mg of spinal neostigmine or saline on postoperative Day 1 and the other injection on postoperative Day 2. Three additional sheep received, on separate days, intrathecal neostigmine alone or with the muscarinic receptor subtype-specific antagonists pirenzepine (M1) 2 mg or AFDX-116 (M2) 2 mg. Antinociception was tested using a mechanical stimulus after each injection. Baseline withdrawal threshold did not change postoperatively. Intrathecal neostigmine, but not saline caused antinociception on both of the first two postoperative days. In contrast, intrathecal neostigmine caused no antinociception in another similar study performed at least 5 days after surgery. Pirenzepine, but not AFDX-116, abolished antinociception from neostigmine, suggesting an action on M1 subtype muscarinic receptors. Intrathecal neostigmine is antinociceptive in sheep during the acute postoperative period, and these data suggest that spinal cholinergic tone, and hence intrathecal neostigmines analgesic effect, may be enhanced during the acute postoperative period. (Anesth Analg 1995;80:1140-4)

Site of Hemodynamic Effects of Intrathecal α2-Adrenergic Agonists

Intrathecally administered alpha 2-adrenergic agonists produce analgesia in humans but may also produce hypotension and bradycardia. To further characterize hemodynamic depression produced by intrathecally administered alpha 2-adrenergic agonists, clonidine (100-1,500 micrograms) was injected into the cervical, thoracic, or lumbar intrathecal space of conscious sheep. Only thoracic intrathecal clonidine injection (100 or 300 micrograms) decreased blood pressure, whereas these doses did not affect blood pressure when injected at other sites. A greater clonidine dose (1,500 micrograms) increased blood pressure to a similar degree at all sites. Hypotension after thoracic intrathecal clonidine injection was inhibited by pretreatment with the alpha 2-adrenergic antagonist idazoxan (1 mg, intrathecally) or the depleter of acetylcholine stores hemicholinium-3 (2 mg, intrathecally), suggesting an action at alpha 2-adrenoceptors on cholinergic preganglionic sympathetic neurons. ST-91, a polar clonidine analog, did not decrease blood pressure after thoracic intrathecal injection. Intrathecal injection of the muscarinic receptor agonist carbamylcholine increased blood pressure. These data describe a complex action of intrathecal alpha 2-adrenergic agonists on hemodynamic parameters that is dependent on site of injection, drug dose, and drug lipophilicity; that can be explained by anatomic factors; and that may possibly be exploited to minimize hemodynamic depression from these agents.

Intravenous opioids stimulate norepinephrine and acetylcholine release in spinal cord dorsal horn. Systematic studies in sheep and an observation in a human.

Background Opioids produce analgesia by direct effects as well as by activating neural pathways that release nonopioid transmitters. This study tested whether systematically administered opioids activate descending spinal noradrenergic and cholinergic pathways. Methods The effect of intravenous morphine on cerebrospinal fluid and dorsal horn microdialysate concentrations of norepinephrine and acetylcholine was examined in 20 sheep. Animals received either intravenous morphine or fentanyl alone, or morphine plus intravenous naloxone or intrathecal idazoxan. Results Intravenous morphine (0, 0.5, 1 mg/kg, intravenous) produced dose‐dependent increases in cerebrospinal fluid norepinephrine and acetylcholine, but not epinephrine or dopamine. Morphines effect was blocked by intravenous naloxone and by intrathecal idazoxan. In microdialysis experiments, intravenous morphine increased the concentration of norepinephrine and acetylcholine, but not epinephrine or dopamine, in the dorsal horn. In contrast, intravenous morphine exerted no effect on any of these monoamines in the ventral horn. Intravenous naloxone and cervical cord transection each blocked morphines effect on dorsal horn norepinephrine. Conclusions These results support functional studies that indicate that systematically administered opioids cause spinal norepinephrine and acetylcholine release by a naloxone‐sensitive mechanism. Idazoxan blockade of morphines effects on cerebrospinal fluid norepinephrine was unexpected, and suggests that both norepinephrine and acetylcholine release in the spinal cord may be regulated by alpha2 ‐adrenoceptors. Microdialysis experiments suggest increased norepinephrine and acetylcholine levels in cerebrospinal fluid resulted from intravenous morphine‐induced activation of bulbospinal pathways.

Cerebrospinal fluid norepinephrine and acetylcholine concentrations during acute pain.

Painful stimulation increases spinal cord norepinephrine (NE) in animals, and spinally released NE induces acetylcholine (ACh) release to cause analgesia.The purpose of this study was to determine the relationship between NE and ACh in cerebrospinal fluid (CSF) in sheep and humans during painful stimulation. CSF was sampled in anesthetized sheep before and during electrical nerve stimulation at an intensity sufficient to increase mean arterial pressure 15%-20%. To determine whether spinally released NE caused ACh release by stimulation of alpha2-adrenoceptors, seven sheep received intrathecal (IT) idazoxan whereas seven sheep received IT saline before stimulation. To examine the effect of pain on CSF NE and ACh in humans, CSF was sampled in 33 women after at least 4 h of painful labor and in 22 pregnant women without pain. Painful stimulation in sheep increased CSF NE and ACh. IT idazoxan blocked the increase in both NE and ACh. Although mean concentrations of CSF NE and ACh did not differ between parturients with and without pain, there was a significant correlation between NE and ACh concentrations only in those with pain. These data provide evidence in animals for activation of spinal cord noradrenergic-cholinergic systems in response to pain. There is only weak evidence for such activation, however, in women with painful labor. (Anesth Analg 1996;82:621-6)

Alfentanil, but Not Amitriptyline, Reduces Pain, Hyperalgesia, and Allodynia from Intradermal Injection of Capsaicin in Humans

Background: Intradermal injection of capsaicin produces brief pain followed by hyperalgesia and allodynia in humans, and the latter effects are mediated by spinal N‐methyl‐D‐aspartate mechanisms. Amitriptyline recently was shown to antagonize N‐methyl‐D‐aspartate receptors, and in this study, the authors sought to determine the effect of amitriptyline alone and with the opioid alfentanil on hyperalgesia and allodynia produced by intradermal injection of capsaicin. Methods: Forty‐six healthy volunteers in the general clinical research center received repeated intradermal injections of capsaicin (100 micro gram) alone or before and after systemic injection of 4 mg midazolam, 25 mg amitriptyline, alfentanil by computer‐controlled infusion, or amitriptyline plus alfentanil. Acute pain and areas of mechanical hyperalgesia and allodynia were determined at specified intervals. Blood was obtained for alfentanil and amitriptyline assay. Results: Capsaicin injection produced acute pain followed by hyperalgesia and allodynia. Alfentanil reduced these pain responses in a plasma‐concentration‐dependent manner, and reduction in hyperalgesia and allodynia correlated with reduction in acute pain. Amitriptyline alone had no effect and did not potentiate alfentanil. Alfentanil produced concentration‐dependent nausea, an effect diminished by amitriptyline. Discussion: These data correspond with previous studies in volunteers demonstrating reduction in hyperalgesia and allodynia after intradermal injection of capsaicin by systemically administered opioids, and they suggest that this reduction may be secondary to reduced nociceptive input by acute analgesia. These data do not support the use of acute systemic administration of amitriptyline for acute pain, hyperalgesia, and allodynia, although the roles of chronic treatment and spinal administration are being investigated.

The vascular mechanism of ephedrine's beneficial effect on uterine perfusion during pregnancy.

Sensitivity to the vasoconstricting actions of adrenergic agents is altered during pregnancy and is drug-, regional vascular bed-, and endothelium-dependent. To examine whether the uterine perfusion-sparing property of ephedrine is due to local actions, we examined the effects in vitro of ephedrine and the alpha-adrenergic agonist metaraminol (10(-10)-10(-3) M) in uterine and femoral vessels, with and without functional endothelium, from nonpregnant and pregnant ewes. Both agents produced dose-dependent contractions in all vascular rings. In all cases metaraminol was more potent (by analysis of the concentration producing a 50% maximal response [EC50]) and efficacious (by maximal effect). Pregnancy increased constriction from both agents in femoral arterial rings, whereas pregnancy decreased constriction from both agents in uterine arterial rings. However, the ratio of maximal effect at femoral versus uterine rings during pregnancy was greater for ephedrine (5.2 +/- 0.6) than metaraminol (1.9 +/- 0.3). This difference was further accentuated by endothelium removal. Constriction to both agents was abolished by phentolamine (10(-5) M). These data suggest that both ephedrine and metaraminol constrict uterine and systemic vessels by actions on alpha adrenoceptors, and that ephedrine may spare uterine perfusion during pregnancy due to more selective constriction of systemic vessels than that caused by metaraminol.

Neostigmine Counteracts Spinal Clonidine-induced Hypotension in Sheep

Background:Intraspinal clonidine Injection produces analgesia free of respiratory depression, but also decreases blood pressure and causes sedation. Spinal neostigmine injection alone Increases blood pressure in animals and enhances clonidine-induced analgesia. Methods:To test whether neostigmine would alter clonidine-induced hypotension, nine chronically prepared sheep received intrathecal injections of saline or neostigmine (150, 300, 1,000 μg) followed in 15 min by 200 μg clonidine. Results:Clonidine plus saline decreased mean arterial pressure by 12 ± 3% associated with small, statistically nonsignificant decreases in heart rate, cardiac output, and systemic vascular resistance. Prior injection of neostigmine diminished hypotension 60 min after clonidine injection in a dose-dependent manner. To further define the time course and pharmacology of this interaction, seven other sheep received intrathecal saline, neostigmine (1,000 μg), or neostigmine plus methylatropine (1,000 μg) 75 min prior to 200 μg clonidine. With this longer Interval between injections, neostigmine abolished clonidine-induced hypotension, and this protective effect was inhibited by methylatropine. To test whether rostral spread of neostigmine in cerebrospinal fluid would alter its hemodynamic effects, we injected intrathecal neostigmine into the upper cervical site. intrathecal neostigmine increased mean arterial pressure and heart rate at this site to a degree similar to that in the thoracic area, with no effect on behavioral or arterial blood gas tensions. Conclusions:These data are consistent with neostigmines counteraction of clonidine-induced hypotension by a spinal muscarinic mechanism and support investigation of spinal α2-adrenergic-cholinergic combinations for pain therapy.

Pregnancy and ephedrine increase the release of nitric oxide in ovine uterine arteries

Ephedrine is the preferred vasoconstrictor for the treatment of hypotension after epidural and spinal anesthesia in obstetrics because it preserves uterine perfusion better than pure alpha-adrenergic agonists. Previous studies of uterine vascular rings in vitro suggested that direct uterine vasoconstriction from ephedrine is reduced during pregnancy. This study examined the hypothesis that nitric oxide synthase (NOS) is up-regulated in uterine arteries during pregnancy, and that ephedrine stimulates NOS to release nitric oxide (NO) and diminish direct vasoconstriction. Uterine arterial vessels were obtained from 12 pregnant and 9 nonpregnant ewes, and vessel tension was monitored in vitro in response to escalating concentrations of ephedrine or metaraminol. In some experiments, vascular endothelium was mechanically removed, while in others antagonists of NO synthesis (Nomega-nitro-L-arginine methyl ester [L-NAME]), NO diffusion (hemoglobin [Hgb]), or guanylate cyclase (methylene blue [MB]) were included. In other experiments, solutions containing ephedrine were superfused over uterine arteries from pregnant ewes onto uterine arteries from nonpregnant ewes. Finally, NOS activity, determined by14 C-citrulline generation, was determined in uterine arteries from pregnant and nonpregnant ewes. Both ephedrine and metaraminol caused concentration-dependent constriction of uterine arterial rings from pregnant and nonpregnant animals. Pregnancy reduced maximum constriction from ephedrine more than metaraminol. Similarly, ephedrine-induced constriction was increased more than that of metaraminol in uterine arteries from pregnant animals treated to diminish the effects of nitric oxide (L-NAME, Hgb, MB, endothelium removal). Ephedrines constriction of uterine arteries from nonpregnant animals was reduced when it was superfused over uterine arteries from pregnant animals. NOS activity was increased in uterine arteries from pregnant compared to nonpregnant animals. These studies confirm decreased direct uterine arterial vasoconstriction during pregnancy from ephedrine and support the hypothesis of increased release of an endogenous vasodilator (NO), either from the vascular endothelium or the vessel wall, as the cause for this decreased vasoconstriction. (Anesth Analg 1996;82:288-93)

Cardiorespiratory and Spinal Cord Blood Flow Effects of Intrathecal Neostigmine Methylsulfate, Clonidine, and Their Combination in Sheep

Background Intrathecal neostigmine may produce analgesia by itself and may enhance analgesia from spinal clonidine. Before clinical trials, the spinal cord blood flow effects of these drugs alone and in combination should be examined in animals. Methods Conscious, nonpregnant ewes with indwelling vascular and thoracic spinal catheters received intrathecal injection of 0.2 or 2 mg neostigmine, 0.2 mg clonidine, or 2 mg neostigmine plus 0.2 mg clonidine. Mean systemic and pulmonary arterial and central venous pressures, heart rate, and cardiac output were monitored, arterial blood was sampled for blood gas tensions and pH, and spinal cord blood flow was determined by colored microsphere injection before and at 15, 60, and 240 min after spinal study drug injection. Results Neostigmine alone did not affect cardiorespiratory variables or spinal cord blood flow. Intrathecal clonidine alone decreased systemic arterial and central venous pressures, whereas these effects were not observed with addition of neostigmine. Clonidine or neostigmine alone or the combination of clonidine and neostigmine did not affect spinal cord blood flow. Conclusions Intrathecal neostigmine alone or in combination with clonidine does not reduce spinal cord blood flow, an important preclinical toxicity issue. These results provide additional support for initial clinical trials of intrathecal neostigmine for analgesia.