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.

Retrovirus-mediated expression of an artificial beta-endorphin precursor in primary fibroblasts

Peptides are of potential interest in the field of gene therapy but require modification by genetic engineering to facilitate their secretion. Amino terminal addition of a signal peptide is not always sufficient to achieve this goal, as found in this study for beta-endorphin. To overcome this problem, addition of the pre-pro-sequence of mouse nerve growth factor to beta-endorphin was tested. Retrovirus-mediated expression of a hybrid construct of the pre-pro-sequence of nerve growth factor and human beta-endorphin in primary fibroblasts resulted in the secretion of beta-endorphin immunoreactivity at a rate of 620 pg/h/10(6) cells. Analysis of the secreted beta-endorphin immunoreactivity with reverse-phase HPLC, immunoassays using three different antibodies, and an assay for the specific displacement of [3H][D-Ala2,N-MePhe4,Gly-ol5]enkephalin from mu-opioid receptors suggests that the pre-pro-sequence is cleaved off from the pre-pro-sequence/beta-endorphin construct prior to secretion, resulting in bona fide beta-endorphin. Transplantation of beta-endorphin-secreting cells into brain or spinal cord may provide a gene therapy approach for the treatment of chronic, opioid-sensitive pain states.

Studies on the safety of chronically administered intrathecal neostigmine methylsulfate in rats and dogs

BACKGROUND

The spinal delivery of the cholinesterase inhibitor neostigmine yields analgesia in rats and augments the analgesic effects of alpha 2 agonists in sheep. To assess its activity in humans, preclinical toxicology studies to define its safety were required in two species.

METHODS

Rats with chronic intrathecal catheters received daily injections of saline (vehicle) or 5 micrograms/10 microliters or 10 micrograms/10 microliters neostigmine HCl (n = 6/group) for 4 days and were observed for general behavior and nociception (52.5 degrees C hot plate). On day 6, rats were anesthetized and submitted to whole body perfusion/fixation. For dog studies, male beagles were prepared following rigid aseptic precautions with catheters passed from the cisterna magna to the lumbar intrathecal space. Catheters were connected to an external vest-mounted pump. Based on preliminary studies, ten implanted dogs were randomly assigned to receive infusions of neostigmine for 28 days (4 mg/4 ml/day; n = 6) or saline (4 ml/day; n = 4). At 28 days, dogs were anesthetized, cisternal cerebrospinal fluid was obtained, and dogs were submitted to perfusion-fixation. Rat and dog spinal cords were embedded, sectioned, stained, and assessed by the pathologist without knowledge of treatment.

RESULTS

In rats, neostigmine produced a dose-dependent increase in hot plate latency, and no tolerance was observed. Mild tremor was observed but was not debilitating. Histopathology revealed a mild fibrotic reaction to the catheter with mixed signs of moderate, acute, and chronic inflammation with no differences between saline or drug groups. In dogs, neostigmine had no effect on blood pressure or on the skin twitch response but produced bradycardia and an increase in muscle tone. At sacrifice, cerebrospinal fluid protein, specific gravity, and glucose were elevated in both saline and neostigmine groups. Histopathology displayed a local reaction to the spinal catheter and a mixed acute and chronic inflammatory reaction. No group differences were observed. These results suggest that, at the neostigmine concentration of 1 mg/ml in the rat and dog and in doses up to 4 mg/day in the dog, there is no evidence of spinal tissue toxicity that can be attributed to the drug. This result, observed in two species, suggests that intrathecal neostigmine given in this manner is without distinguishable toxicity in these two models.

Reversal of nerve ligation-induced allodynia by spinal alpha-2 adrenoceptor agonists

After nerve injury, a sympathetically dependent allodynia may be observed. Spinal alpha-2 agonists inhibit preganglionic neurons. The nature of the effect of alpha-2 agonists on allodynia induced by L5 and L6 nerve ligation (Chung model) was thus examined. Rats were implanted with spinal intrathecal catheters directed at the upper lumbar (L1-L2) or the lower cervical (C5-C6) spinal levels. After nerve injury, rats displayed a tactile allodynia (mean withdrawal thresholds, < 2.3 g). Lumbar intrathecal injection of alpha-2, but not alpha-1 or an opiate agonist, resulted in a dose-dependent reversal of the allodynia, with the ordering of activity (ED50 in micrograms in parentheses) being dexmedetomidine (0.9) > oxymetazoline (14) = guanfacine (17) = 5-bromo-6-(2-imidazolin-2-ylamino)quinoxaline (19) = 2-(2,6-diethylphenylamino)-2-imidazoline (21) = clonidine (22) > morphine (> 30) = methoxamine (> 30 micrograms). The effects of clonidine and 2-(2,6-diethylphenylamino)-2-imidazoline were reversed by the intrathecal injection of yohimbine. At equivalent doses, clonidine delivered systemically, i.c.v. by chronic ventricular guides, or at the level of the cervical spinal cord, produced substantially less antiallodynic action. These results jointly suggest that the sites of the antiallodynic action of spinal alpha-2 agonists are located at the level of the spinal preganglionic neurons and correspond to their ability to diminish preganglionic sympathetic outflow. The failure of morphine to exert an antiallodynic action reflects the fact that 1) opiates act presynaptically on small primary afferents and the allodynia is mediated by large afferent input and 2) opiates, unlike alpha-2 agonists do not have an effect on autonomic outflow.

Effect of continuous intrathecal infusion of omega-conopeptides, N-type calcium-channel blockers, on behavior and antinociception in the formalin and hot-plate tests in rats

The effect of continuous intrathecal infusion of omega-conopeptides in the rat was examined to determine whether antinociception, as measured on the formalin and hot-plate (52.5 degrees C) tests, was altered and whether tolerance developed with chronic infusion of these agents. Infusion of 0.030 and 0.003 nmol/h SNX-111 and 0.290 nmol/h SNX-239 was performed for either 2 days ('acute') or 7 days ('chronic') and was compared to the effect of 20 nmol/h morphine or saline. Both doses of SNX-111 and SNX-239 produced a significant reduction of the response to the hot-plate and formalin tests at both 2 and 7 days of infusion compared to saline infusion. In contrast, morphine only produced a significant effect on day 2, but not on infusion day 7, indicating that tolerance had developed. The effect of SNX-111 was reversible, as shown by a return to nociceptive responses similar to saline-infused rats 2 days after the minipumps had been disconnected after a 7-day infusion period. These data indicate that chronic infusion of omega-conopeptides that block N-type voltage-sensitive calcium channels produce a powerful antinociception, with minimal development of tolerance.

Capsaicin-evoked prostaglandin E2 release in spinal cord slices: relative effect of cyclooxygenase inhibitors

The release of prostaglandin E2 was examined from superfused spinal cord slices. The addition of capsaicin to the perfusate resulted in a dose-dependent increase of prostaglandin E2-like immunoreactivity. Capsaicin (10 microM) evoked prostaglandin E2 release from basal levels of 5.3 +/- 0.8 to 30 +/- 3 fmol/10 min fraction. The capsaicin-evoked release was blocked by the capsaicin receptor antagonist capsazepine (10 microM), but not by removal of extracellular Ca2+ ions. Addition of non-steroidal anti-inflammatory drugs (NSAIDs) to the perfusate had no effect on resting levels of prostaglandin E2, but resulted in a concentration-dependent suppression of capsaicin-evoked release of prostaglandin E2. The IC50 values (in microM) were: indomethacin: 0.7, S(+)-flurbiprofen: 2.0, acetaminophen: 4.4,ketorolac: 5.0, R(-)-flurbiprofen: 8.7, S(+)-ibuprofen: 9.5, and for R(-)-ibuprofen: > 10. The relative potency for the NSAIDs to reduce capsaicin-evoked prostaglandin E2 release, with the exception of acetaminophen, corresponds to their antinociceptive activity after spinal delivery, a finding which further supports the role of prostanoids in spinal nociceptive processing.

Stimulatory pathways and sites of action of intrathecally administered nicotinic agents

Intrathecal (i.t.) administration of nicotinic agonists to rats elicits a receptor-mediated pressor response, a heart rate increase and irritation-nociceptive behavior. We examined the stimulatory pathways and sites of action of spinally administered nicotinic agonists. The thoracic region appears more sensitive to nicotine-elicited pressor actions than the lumbar or cervical regions of the spinal cord. Nicotinic receptors evoking a nociceptive response appear to be located over an area extending from the lumbar rostrally to the thoracic region. Similar to the pressor response, the thoracic spinal cord is the most sensitive region to nicotine in producing the heart rate increase. The cervical region is the least responsive to nicotine, suggesting sites of actions caudal to the brain stem. Intravenous infusion of trimethaphan inhibited the nicotine-elicited pressor response and tachycardia without affecting the irritation response. This suggests that the cardiovascular responses to spinal nicotinic agonists result from enhanced sympathetic outflow. Intrathecal morphine and MK-801 block spinal nicotine-elicited irritation and tachycardia but not the pressor response, indicating that pressor and irritation responses involve independent receptor-mediated pathways. Tachycardia may be associated with the irritation response. In contrast to nicotine, both pressor and irritation responses to cytisine were blocked by morphine with no effect on heart rate. Spinal transection at the T1-2 level eliminated the nociceptive response to nicotine but not the pressor response. Cytisine responses were similar to nicotine; however, cytisine was significantly less potent as a pressor agent in the transected compared with intact animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Augmented responses to intrathecal nicotinic agonists in spontaneous hypertension

Abnormal central cholinergic activity has been reported to be responsible in part for the pathogenesis of high blood pressure in spontaneously hypertensive rats (SHR). Administration of cholinergic agonists in brain and spinal cord results in exaggerated pressor responses in SHR. Studies to date have focused largely on the muscarinic cholinergic system. Recently, we demonstrated that intrathecal administration of nicotinic agonists results in pressor, tachycardic, and irritation responses. In the present study we examine the cardiovascular and behavioral responses to nicotine and cytisine administered intrathecally in La Jolla strain (LJ) SHRLJ and age-matched Wistar-Kyoto (WKYLJ) rats. Nicotinic agonists produced augmented pressor, heart rate, and irritation responses in SHRLJ compared with normotensive rats. In both SHRLJ and WKYLJ rats, cytisine elicited a greater nociceptive response and greater spinobulbar component to the pressor response than nicotine. SHRLJ and WKYLJ rats also differ in that the SHRLJ strain shows a diminished tendency for desensitization to cytisine. As in Sprague-Dawley rats, in SHRLJ and WKYLJ rats the cardiovascular and behavioral responses to intrathecal nicotine were significantly inhibited by mecamylamine, dihydro-beta-erthyroidine, and methyllycaconitine. However, methyllycaconitine, which effectively blocked cytisine-elicited cardiovascular and behavioral responses in Sprague-Dawley and WKYLJ rats, was unable to inhibit the maximal rise in cystine-elicited blood pressure, heart rate, and irritation responses in SHRLJ. In contrast to the heightened cardiovascular and behavioral responses, the number of nicotinic binding sites in spinal cord membranes was significantly decreased in the hypertensive rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term perfusion of the cerebroventricular system of dogs without leakage to the peripheral circulation

Methods developed previously for studying the effect of cerebroventricular injection or ventriculocisternal perfusion of test substances are unsatisfactory because the test substance is not confined to the central compartment. Most likely the test substance enters the peripheral circulation via the arachnoid villi. The purpose of this paper is to describe a method for perfusing the cerebroventricular system of conscious dogs without passage of test substances to the peripheral circulation. With the method described, the mean (+/- SE) cerebroventricular pressure in conscious dogs was 7.4 +/- 0.8 cmH2O (n = 16), and the mean (+/- SE) production of cerebrospinal fluid (CSF) was 25 +/- 0.3 microliter/min (n = 16). Endogenously occurring migrating myoelectric complexes (MMCs) of the small intestine were recorded in dogs before catheters were implanted in the left and right lateral ventricles and the fourth ventricle and after catheter implantation during cerebroventricular perfusion with artificial CSF alone or with CSF containing sulfated (S-CCK-OP) or nonsulfated cholecystokinin octapeptide (NS-CCK-OP). Only cerebroventricular perfusion with S-CCK-OP (1.2 pmol.kg-1.min-1; n = 20) replaced spontaneously occurring MMCs with a fed-like pattern of myoelectric activity. The results suggest that replacement of the fasting pattern of myoelectric activity with a fed-like pattern in the fasted dog was mediated by CCK-A receptors located in one or more brain nuclei surrounding the third ventricle.

Analgesic and neurotoxic effects of intrathecal corticosteroids in rats

BACKGROUND

Despite the widespread use of epidurally administered corticosteroids in the treatment of sciatica and the failure of animal studies to demonstrate neurotoxicity from epidermally administered corticosteroids, controversy remains regarding the mechanism of action as well as the safety of this treatment. The goal of this study was to determine whether spinally administered corticosteroids have any analgesic effects, and whether repeated intrathecal administration causes any neuronal damage to the spinal cord.

METHODS

Chronic lumbar intrathecal catheters were implanted in rats. Formalin testing was carried out 1 h after the intrathecal administration of 400 micrograms methylprednisolone sodium succinate, 48 h after intrathecal administration of triamcinolone diacetate 250 micrograms, or 24 h after the last of a series of four injections of triamcinolone diacetate 250 micrograms given at 5-day intervals. Histologic sections of multiple levels of spinal cord from the animals receiving repeat intrathecal steroid injections were compared to those from animals that received intrathecal saline at the same intervals.

RESULTS

The animals receiving repeated intrathecal triamcinolone diacetate demonstrated mild, statistically significant reduction of pain behavior (hindlimb flinching) during the second but not the first phase of the formalin test when compared to controls. No analgesic effects were demonstrated after methylprednisolone sodium succinate or a single injection of triamicinolone diacetate. Animals that received methylprednisolone sodium succinate demonstrated transient segmental allodynia. No behavioral or neurologic abnormalities were seen in any other group. Some histologic evidence neuronal damage (the presence of argyrophilic neurons was seen in the chronically implanted animals in areas of the cord adjacent to the spinal catheters, but there was no difference in incidence of these changes between the steroid and control groups.

CONCLUSIONS

Intrathecal steroid injections have no analgesic effect and do not suppress spinal sensitization when administered acutely. After chronic administration, there is a mild effect on nociceptor-driven spinal sensitization (phase 2 of the formalin test), but no analgesic effect on an acute noxious stimulus (phase 1 of the formalin test). Repeated intrathecal administration of triamcinolone diacetate (0.8 mg/kg) is not associated with spinal neurotoxic effects during the time period studied.

Characterization of muscarinic receptors involved in tracheal CGRP release

Application of acetylcholine (ACh) to isolated rat trachea induces an increase in calcitonin gene-related peptide (CGRP) outflow in the perfusates. The elevation of CGRP release by ACh was absent in capsaicin-desensitized preparations, suggesting that the release of peptide is derived from capsaicin-sensitive afferent nerves. ACh-induced release was not altered by hexamethonium, but was significantly attenuated by atropine, indicating involvement of the muscarinic receptor. Effects of three selective muscarinic subtype antagonists, pirenzepine (M1), methoctramine (M2) and 4-DAMP (M3) on ACh-evoked release were examined. The ordering of antagonist potency was: 4-DAMP (ED50 = 14 nM) > pirenzepine (3.8 microM) > methoctramine (> 10 microM). The results suggest that the muscarinic receptor mediating tracheal CGRP release resembles the M3 receptor subtype.

Effect of graded hypothermia (27 degrees to 34 degrees C) on behavioral function, histopathology, and spinal blood flow after spinal ischemia in rat

BACKGROUND AND PURPOSE

We used a rat model of reversible spinal ischemia to assess the effect of spinal cord temperature on the development of neurological and histopathologic changes after 20 minutes of reversible aortic occlusion. Spinal cord blood flow and CO2 reactivity was tested by using laser Doppler before and 60 minutes after ischemia.

METHODS

In halothane (1%)-anesthetized rats, the spinal cord temperature as assessed by using thermocouple in the paraspinal muscles was lowered to 34 degrees, 31 degrees, or 27 degrees C. After ischemia, spinal cord temperature was raised to 37 degrees C for the next 30 minutes. Animals were maintained in this normothermic condition for 8 hours, after which motor and sensory function were assessed. All animals were then anesthetized and perfused with 10% formalin for light microscopic analysis of spinal cords.

RESULTS

In normothermic animals, 20 minutes of ischemia resulted in a loss of CO2 reactivity and hind limb paraplegia with an attendant allodynia that persisted for the 8 hours of reperfusion. Even mild (34 degrees C) hypothermia resulted in significant improvement of neurological function compared with the normothermic group. In paraplegic animals, lumbosacral interneuronal pools localized primarily in laminae III through VII displayed heavy argyrophilic neurons and areas of localized necrosis. In moderate and deep hypothermic animals preservation of CO2 responsivity and complete recovery of neurological function were seen with no detectable histopathologic changes.

CONCLUSIONS

These results show that a slight decrease in spinal cord temperature in the peri-ischemic period provides significant protection as measured by histopathology and neurological function.

Spinal delivery of sufentanil, alfentanil, and morphine in dogs. Physiologic and toxicologic investigations

BACKGROUND

This study examines the behavioral effects and potential neurotoxicity of sufentanil, alfentanil, and morphine after chronic daily epidural (15-day) and intrathecal (28-day) administration in dogs.

METHODS

Dogs were chronically implanted with a lumbar intrathecal or epidural catheter and received daily injections for 28 or 15 days, respectively, of saline or one of three mu agonists: sufentanil (intrathecal 5, 25, or 50 micrograms/0.5 ml; epidural 10, 50, or 100 micrograms/2.0 ml), alfentanil (intrathecal 40 or 400 micrograms/0.5 ml; epidural 80 or 800 micrograms/2.0 ml), or morphine (intrathecal 0.5 or 5 mg/0.5 ml; epidural 1 or 10 mg/2.0 ml). Dogs were examined for antinociception (skin twitch) and neurobehavioral changes. When the animals were killed, cisternal cerebrospinal fluid was taken for clinical chemistry, and after perfusion fixation, spinal cord tissue was taken for histologic analysis.

RESULTS

Bolus intrathecal and epidural injections of sufentanil, alfentanil, and morphine produced dose dependent antinociception, bradycardia, an initial tachypnea followed by a decrease in respiratory rate, hypothermia and somnolence. The order of potency was sufentanil > alfentanil > morphine on all measures. Over the extended period of drug delivery, a loss of response (tolerance) was observed on all measures. No abnormal morphologic or histologic effects were found when comparing the drug and dose groups. An inflammatory reaction secondary to the catheter was found in all animals. Intrathecal, but not epidural, catheters resulted in significant increases in cerebrospinal fluid protein and cell counts in vehicle animals. Values in drug treated animals did not differ significantly from the respective vehicle controls. A rapid systemic redistribution of all three drugs was observed. No differences were found in the pharmacokinetic parameters measured at day 1 and at the day of killing for any route.

CONCLUSIONS

This large-animal model demonstrates the expected pharmacologic potency of these three agents and tolerance development. Based on cerebrospinal fluid and systematic histopathologic analyses, these three spinally administered agents showed no evidence of neurotoxicity over the range of doses/concentrations employed when given by the intrathecal or epidural route as compared to vehicle controls. Consideration of the toxicokinetics in this canine model suggests that it provides an appropriate test of the safety of these agents in concentrations which exceed those employed for daily intermittent epidural and intrathecal drug delivery in humans.

Antinociception and side effects of liposome-encapsulated alfentanil after spinal delivery in rats

We investigated the spinal antinociceptive and supraspinally mediated side effects of intrathecal (IT) alfentanil after delivery in saline or when encapsulated in liposomes of different lipid constituencies in rats. Rats prepared with chronic IT catheters received IT injections of alfentanil (1, 3, 10, 30, or 50 micrograms) prepared in either saline or in one of three liposome formulations (dipalmitoyl phosphatidyl choline [DPPC], DPPC containing 20% by weight of dipalmitoyl phosphatidyl glycerol [DPPC-DPPG], or DPPC containing 20 weight percent of cholesterol [DPPC-CHOL]). Antinociception was measured by hot-plate (HP) test (52.5 degrees C). In separate groups of halothane-anesthetized rats, plasma alfentanil concentrations were measured (2-120 min) after 50 micrograms IT alfentanil given in either saline or liposomes. Antinociception was measured by tail withdrawal upon its immersion in water 52.5 degrees C. Supraspinal side effects of the drug were tested by measuring catalepsy and the eye blink evoked by touching the cornea. IT alfentanil in saline produced a dose-dependent increase in the HP response latency and this effect was accompanied by a similar dose-dependent increase in the incidence of catalepsy and blockade of corneal responses, indicating a rapid supraspinal redistribution. The HP dose-response curve for IT alfentanil delivered in liposomes was shifted slightly to the right, as compared to saline vehicle, but liposome encapsulation totally abolished the side effects that were otherwise observed at the highest IT alfentanil dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacology and toxicology of chronically infused epidural clonidine.HCl in dogs

To evaluate the physiological effects and toxicity of epidural clonidine.HCl, male Beagle dogs were prepared with chronic lumbar epidural catheters and administered constant infusions of either saline (N = 10), or 80 micrograms/hr (N = 6), 200 micrograms/hr (N = 6), or 320 micrograms/hr (N = 12) clonidine.HCl at a rate of 4 ml/24 hr for 28 days. Saline infusion had no effect upon any behavioral measure. Epidural clonidine produced a dose-dependent increase in thermal skin-twitch response latency (antinociception), lowering of respiration rate, heart rate, and blood pressure, and increased sedation. The effects were maximum from approximately Day 1 to Day 3 when, with the exception of respiration which remained depressed, a progressive adaptation was observed over the course of the study. There were no negative effects on body weight, body temperature, motor function, bowel or bladder function, or clinical pathology values. After 28 days of continuous infusion, the dogs were deeply anesthetized and terminated. Cisternal cerebrospinal fluid taken at termination displayed no clinically significant differences in protein or glucose concentration. All groups, including control, had dogs which had a chronic inflammatory response in the epidural space, as represented by fibrosis, foreign body giant cells, and lymphocytes, but no spinal cord pathology. Both the steady-state plasma and CSF concentrations of clonidine were proportional to the dose; the ratio of CSF to plasma concentration was approximately 0.5. The failure to see any change in CSF composition, significant spinal cord pathology, or signs of tissue or organ toxicity emphasizes the safety of epidurally administered clonidine at infusion rates up to 320 micrograms/hr and at infusate concentrations up to 2 mg/ml.

Multiple mechanisms for the effects of capsaicin, bradykinin and nicotine on CGRP release from tracheal afferent nerves: role of prostaglandins, sympathetic nerves and mast cells

Application of capsaicin (CAP), bradykinin (BK) or nicotine (NIC) to intraluminally perfused rat tracheas induced an increase in calcitonin gene-related peptide (CGRP) levels in the perfusates. Depletion of sensory afferent CGRP with systemic CAP pretreatment resulted in a significant reduction of CGRP release evoked by CAP, BK or NIC. Chemical destruction of sympathetic nerve fibres by systemic pretreatment with 6-hydroxydopamine reduced CGRP release evoked by NIC, but did not alter the release produced by CAP or BK. Elimination of the tracheal mast cell population by pretreatment with compound 48/80 did not alter the effects of CAP, BK or NIC. CGRP release evoked by BK and NIC, but not CAP, was diminished by indomethacin, suggesting that cyclooxygenase products mediate the actions of BK and NIC. Prostaglandins, PGE1, PGE2, PGF2 alpha and PGI2, displayed stimulatory effects on CGRP release in the trachea. There are evidently multiple mechanisms mediating CGRP release from sensory terminals in rat trachea. It appears that CAP exerts a direct action on sensory nerves, while the effects of BK and NIC are mediated by PG synthesis. Sympathetic activation may be involved in NIC, but not BK, induced PG-mediated CGRP release.

Tolrestat treatment prevents modification of the formalin test model of prolonged pain in hyperglycemic rats

This study examined the effects of hyperglycemia and treatment with the aldose reductase inhibitor, Tolrestat, on the pain behavior evoked by injection of formalin into the dorsum of a single hind paw. In control rats, injection of formalin (50 microliters of a 5% solution) evoked two phases of flinching of the injected paw (phases 1 and 2), separated by a quiescent period. Four weeks of streptozotocin-induced diabetes or galactose intoxication did not alter the frequency of flinching during either of the active phases but significantly (P < 0.001 and P < 0.05, respectively) enhanced flinch frequency during the quiescent period. Concurrent treatment with Tolrestat (50 mg/kg/day by gavage) during hyperglycemia prevented the accumulation of the polyol pathway metabolites sorbitol and fructose in the nerve and spinal cord of streptozotocin-diabetic rats and also significantly (P < 0.05) reduced the enhanced flinching of diabetic rats during the quiescent period. These data demonstrate that hyperglycemia induces a period of Tolrestate-preventable hyperalgesia in a paradigm that is used to model persistent pain and suggest that exaggerated flux through aldose reductase may initiate changes in nociceptive pathways that could contribute to some of the pain states experienced by patients with diabetic neuropathy.

Voltage-sensitive calcium channels in spinal nociceptive processing: blockade of N- and P-type channels inhibits formalin-induced nociception

The role of spinal voltage-sensitive calcium channels (VSCC) in a behavioral model of prolonged nociception was examined in rats. Blockade of VSCC by the trivalent cations neodymium (NdCl3) and lanthanum (LaCl3) resulted in a dose-dependent suppression of both phases of the response to formalin. omega-Conopeptides, which selectively block N-type VSCC, also produced a dose-dependent inhibition of both the initial behavior [phase 1; ED50 (nmol): SNX-111 (0.003) > SNX-185 (0.010) > SNX-239 (0.16) >> SNX-159 (> 0.26); SNX-199 (> 0.30)] and the facilitated response [phase 2; ED50 (nmol): SNX-111 (0.003) > SNX-185 (0.009) > SNX-239 (0.020) > SNX-159 (0.120) = SNX-199 (0.230)]. In contrast, SNX-231 (0.24 nmol), which is selective for a non-L/non-N site and also the L-type VSCC blockers nifedipine (24 nmol), nimodipine (29 nmol), verapamil (200 nmol), and diltiazem (220 nmol), had minimal effects on either phase of the formalin test at the highest dose examined. The P-type channel blocker omega-agatoxin IVA produced a 40% inhibition of phase 1 at the highest dose and phase 2 was suppressed in a dose-dependent fashion (ED50, 0.001 nmol). The response latency to a high-threshold thermal stimulus (the 52.5 degrees C hot plate) was moderately (20%) increased by NdCl3 (0.30 nmol) and SNX-111 (0.008 nmol), but not verapamil (200 nmol) and omega-agatoxin IVA (0.006 nmol). High doses of the N-type VSCC produced characteristic shaking behavior, serpentine-like tail movements, and impaired coordination. However, at antinociceptive doses there was no significant motor effect, though three of the N-type antagonists produced some tail movements. These studies demonstrate that VSCC of the N- and P-type, but not L-type, are involved in facilitated nociceptive processing at the spinal level.

Differential temperature sensitivity of ischemia-induced glutamate release and eicosanoid production in rats

The effect of mild and moderate hypothermia on ischemia-induced glutamate release and eicosanoid production was evaluated in WKY rats subjected to incomplete forebrain ischemia. Under isoflurane anesthesia, microdialysis probes were inserted into the hippocampus and caudate nucleus. In four groups of rats, the intraischemic temperature was maintained at either 38 degrees C (normothermia), 36 degrees C, 34 degrees C (mild hypothermia) and 30 degrees C (moderate hypothermia). In these groups, normothermia was restored immediately upon reperfusion. In two additional groups, both intra- and post-ischemic temperatures were maintained at either 34 degrees C or 30 degrees C. The levels of glutamate were measured in the dialysate collected during ischemia and the levels of TxB2, 6-keto-PGF1 alpha and PGF2 alpha were measured in dialysate collected prior to and after ischemia. As expected, hypothermia reduced ischemia-induced glutamate release in both structures. However, the application of mild hypothermia did not attenuate post-ischemic levels of all eicosanoids measured. Moderate hypothermia (30 degrees C) attenuated the post-ischemic increase in the levels of PGF2 alpha. The data suggest that the processes that lead to eicosanoid formation are less sensitive to temperature reduction than those that lead to glutamate release.

Cardiovascular and behavioral responses to nicotinic agents administered intrathecally

We have examined the role of the spinal nicotinic receptors in mediating cardiovascular and behavioral responses in conscious rats. Intrathecal administration of nicotinic agonists to the lumbosacral region of the spinal cord caused a dose-dependent increase in systolic blood pressure, heart rate and a nociceptive (behavioral) response. The order of potencies for the pressor response was l-nicotine > or = cytisine > N-methylcarbamylcholine > or = dimethylphenylpiperazinium > d-nicotine. However, cytisine was the most potent in producing the heart rate increase and nociceptive response. Unlike the other agonists, cytisine also exhibited marked desensitization of the three responses upon repeated administration. The effects of nicotine were antagonized in a dose-dependent fashion by mecamylamine, hexamethonium, alpha-lobeline, dihydro-beta-erythroidine and methyllycaconitine. By contrast, cytisine-induced responses were blocked effectively by mecamylamine and methyllycaconitine, but not by alpha-lobeline or dihydro-beta-erythroidine. However, when alpha-lobeline or dihydro-beta-erythroidine antagonism of the pressor response to cytisine was monitored during the initial minute following intrathecal administration, both antagonists significantly inhibited the response. The competitive ganglionic blocker, trimethaphan, or the elapid alpha-toxin, alpha-bungarotoxin, when administered intrathecally, had no effect on nicotine- or cytisine-elicited responses. The cardiovascular responses to intrathecal nicotine and cytisine have two components. The first is likely mediated through direct sympathetic output and desensitizes rapidly to cytisine, the second is coupled indirectly to the nociceptive response and shows a diminished capacity for rapid desensitization. Agonist and antagonist specificities indicate that the spinal nicotinic receptor differs from those in ganglia and those characterized in brain to date. Although antagonist specificity of the blockade of nicotine and cytisine elicited responses differ, this may be due to the unique desensitization capacity of cytisine rather than an action mediated by distinct receptor subtypes.

Ligand specificity of nicotinic acetylcholine receptors in rat spinal cord: studies with nicotine and cytisine

Administration of nicotinic agonists to the spinal cord leads to a variety of cardiovascular and behavioral responses. The discrete localization of afferent and efferent fibers presents a system in which occupation of nicotinic receptor subtypes might be correlated with specific pharmacologic responses. To this end, we examined [3H]cytisine and [3H]nicotine binding to membranes isolated from regions of the rat spinal cord. [3H]Cytisine showed saturable, noncooperative (nH congruent to 1) binding to a single-class of sites with a Kd of 0.44 +/- 0.01 nM and total saturable sites of 19.9 +/- 0.9 fmol/mg of protein. [3H]Cytisine binding to membranes from intermediolateral cell column and dorsal and ventral sections of the lumbosacral regions each revealed a single class of binding sites with virtually identical Kd values. However, the dorsal sections of the lumbar spinal cord contained a higher number of total binding sites than ventral lumbar or intermediolateral sections. The rank order potencies of the nicotinic agonists competing for [3H]cytisine binding was cytisine > I-nicotine > N-methylcarbamylcholine > dimethylphenylpiperazinium > acetylcholine > d-nicotine > carbamylcholine. Competitive antagonists also competed with high affinities (Ki as low as nanomolar) with the order of potencies being alpha-lobeline > or = dihydro-beta-erythroidine >> methyllycaconitine, whereas the channel blockers, mecamylamine and hexamethonium, only competed at concentrations > or = 100 microM. Competitive ganglionic blockers such as d-tubocurarine or trimethaphan and neurotoxins such as alpha-bungarotoxin, alpha-cobratoxin or neuronal bungarotoxin had weak affinities for cytisine sites. Similar to [3H]cytisine, [3H]nicotine also revealed a saturable single class of binding sites, but of lower affinity. The rank order of Ki values of the agonists, antagonists and neurotoxins for competing with [3H]nicotine binding was similar to the order for [3H]cytisine. Nicotinic receptors in the spinal cord membrane show a specificity for both agonists and antagonists that differ from neuronal receptors in the ganglia or the regions of the brain characterized to date. These findings, when correlated with the pharmacological responses documented in the comparison article suggests that the spinal nicotinic receptors may define a new member of the neuronal nicotinic receptor family.

Transient spinal ischemia in rat: characterization of spinal cord blood flow, extracellular amino acid release, and concurrent histopathological damage

Extracellular concentrations of amino acids in halothane-anesthetized rats were measured using a microdialysis fiber inserted transversely through the dorsal spinal cord at the level of the lumbar enlargement in conjunction with HPLC and ultraviolet detection. After a 2-h washout and a 1-h control period, 20 min of reversible spinal cord ischemia was achieved by the inflation of a Fogarty F2 catheter passed through the femoral artery to the descending thoracic aorta. After 2 h of postischemic reperfusion, animals were transcardially perfused with saline followed by 10% formalin or 4% paraformaldehyde. The glutamate concentration in the dialysate was significantly elevated after 10 min of occlusion and returned to near-baseline during the first 30 min of reperfusion. Taurine was elevated significantly 0.5 h postocclusion and continued to increase throughout the 2 h of reperfusion. Glycine concentrations showed a tendency to be slightly above baseline during the reperfusion period. Glutamine concentrations modestly increased following 2 h of reperfusion. No significant changes in aspartate, asparagine, and serine were detected. In control animals no significant changes in any amino acids were detected. To assess the role of complete spinal ischemia on spinal glutamate release, studies were carried out using cardiac arrest. Twenty minutes after induction of cardiac arrest, the glutamate concentration was increased about 350-400%. In a separate group of animals, spinal cord blood flow (SCBF) and its response to decreased CO2 were measured using a laser probe implanted into the epidural space at the level of the L2 vertebral segment. SCBF decreased to 5-6% of the control during aortic occlusion. After reversible ischemia, marked hyperemia was seen for the first 15 min, followed by hypoperfusion at 60 min. Under control-preischemic conditions a decrease in arterial CO2 content caused a decrease in SCBF of about 25%. This autoregulatory response was almost completely absent when assessed 60 min after a 20-min interval of aortic occlusion. Histopathological analysis of spinal cord tissue from these animals demonstrated heavy neuronal argyrophilia affecting small and medium-sized neurons located predominantly in laminae III-V. These changes corresponded to signs of irreversible damage at the ultrastructural level. Occasionally, small areas of focal necrosis, located in the dorsolateral part of the dorsal horn and anterolateral part of the ventral horn, were found. The results are consistent with a role for glutamate in ischemically induced spinal cord damage and suggest that taurine elevation detected during the early reperfusion period may serve as an important indicator of irreversible spinal cord neuronal damage.

Quantitative assessment of tactile allodynia in the rat paw

We applied and validated a quantitative allodynia assessment technique, using a recently developed rat surgical neuropathy model wherein nocifensive behaviors are evoked by light touch to the paw. Employing von Frey hairs from 0.41 to 15.1 g, we first characterized the percent response at each stimulus intensity. A smooth log-linear relationship was observed, with a median 50% threshold at 1.97 g (95% confidence limits, 1.12-3.57 g). Subsequently, we applied a paradigm using stimulus oscillation around the response threshold, which allowed more rapid, efficient measurements. Median 50% threshold by this up-down method was 2.4 g (1.81-2.76). Correlation coefficient between the two methods was 0.91. In neuropathic rats, good intra- and inter-observer reproducibility was found for the up-down paradigm; some variability was seen in normal rats, attributable to extensive testing. Thresholds in a sizable group of neuropathic rats showed insignificant variability over 20 days. After 50 days, 61% still met strict neuropathy criteria, using survival analysis. Threshold measurement using the up-down paradigm, in combination with the neuropathic pain model, represents a powerful tool for analyzing the effects of manipulations of the neuropathic pain state.