Biphasic effects of ethanol on open-field activity: Sensitivity and tolerance in C57BL/6N and DBA/2N mice.

Male C57BL/6N (C57) and DBA/2N (DBA) inbred mice were found to differ in open-field behavior after an acute ip injection of ethanol and in the development of tolerance to repeated injections. DBA Ss showed only increased activity for 28 min after ethanol doses up to 2.67 g/kg when compared with saline-injected controls; C57 Ss showed dose-related increases in activity during the first 4 min, followed by dose-related decreases in activity. The effects endured for at least 60 min after injection in both strains. In a 3rd experiment, Ss were injected daily with saline or 2 g/kg ethanol and tested on Days 1, 5, 9, and 13 for open-field activity. On the 17th day, all Ss were tested after an ethanol injection; neither strain showed tolerance to the activity-stimulating effect of ethanol. Some evidence for tolerance to the effect of ethanol to reduce activity in C57's was found. In a 4th experiment, twice-daily injections of ethanol for 10 days produced marked tolerance to the depressant effect of an injection on the 11th day in C57 Ss; no tolerance to the stimulant effect of ethanol was found. DBA Ss injected twice daily for 19 days did not display tolerance when tested on Days 10 or 20, instead showing more marked stimulation of activity after ethanol than mice treated chronically with saline. Implications for the genetic control of responses to ethanol are discussed. (21 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Spinal serotonergic receptors mediate facilitation of a nociceptive reflex by subcutaneous formalin injection into the hindpaw in rats

It is documented that spinal nociceptive transmission receives descending facilitatory and inhibitory modulation from supraspinal structures. The rostral ventral medulla (RVM), including the nucleus raphe magnus (NRM), nuclei reticularis gigantocellularis (NGC) and gigantocellularis pars alpha (NGCalpha), is the major bulbar relay of descending modulatory influences. Pharmacological studies show that facilitation of a spinal nociceptive tail-flick (TF) reflex induced by stimulation in the NGC and NGCalpha is mediated by spinal serotonergic receptors. The present series of experiments provide evidence that activation of spinal serotonergic systems are critical for both induction and maintenance of secondary hyperalgesia induced by subcutaneous injection of formalin into one hindpaw. Subcutaneous injection of formalin produced facilitation of tail withdrawal (mechanical) and the TF reflex (thermal). Facilitatory effects persisted for at least 30 min. Peripheral blockade of the activity by local injection of a hydrophilic lidocaine derivative (QX-314, 5%) into the injected hindpaw abolished both mechanical and thermal facilitation, indicating that peripheral input is important to maintain long-lasting facilitation. Intrathecal application of a serotonergic receptor antagonist methysergide at a dose (64 nmol) which completely blocked descending facilitation produced by electrical- or chemical-stimulation in the NGC and NGCalpha also significantly attenuated or completely abolished facilitation of tail withdrawal and the TF reflex induced by formalin. Methysergide was effective whether the injection was performed before or after the formalin injection. These results suggest that activation of descending facilitatory serotonergic influences by a prolonged noxious stimulation could contribute to secondary hyperalgesia observed at the tail.     

Neurochemical and behavioral factors in the development of tolerance to anorectics.

Four tests, with 60 male Sprague-Dawley rats, investigated tolerance and cross-tolerance among several anorectic drugs. In the 1st test, Ss given milk shortly after intraperitoneal injection of 3 mg/kg dextroamphetamine sulfate (controls) developed tolerance to amphetamine anorexia, but Ss given milk when amphetamine's anorectic effects had worn off (experimental Ss) did not develop tolerance in spite of equal drug exposure. In the 2nd test, controls were tolerant to 2 mg/kg apomorphine HCL, a drug with a neurochemical action related to amphetamine. No tolerance to 2 mg/kg apomorphine was shown by experimental Ss. Both groups were tolerant to 1.25 mg/kg apomorphine. The final test replicated part of the 1st test, demonstrating that the control group was tolerant to amphetamine but the experimental group was not. In addition, neither group was tolerant to anorexia produced by 5 mg/kg fenfluramine, a drug whose action is neurochemically different from amphetamine and apomorphine. It appears that both learning and specific neurochemical mechanisms are involved in the development of tolerance to anorectic drugs. (41 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Fixation of spinal reflexes in rats by central and peripheral sensory input.

Used 2 methods in 5 experiments (55 male hooded Long-Evans rats) to demonstrate retention of postural asymmetries after spinal cord section. In the 1st preparation, postural asymmetries of the hindlimbs were induced by placing electrolytic lesions in the anterior cerebellum. Asymmetry was found to consistently outlast a spinal cord section if 45 min were allowed between brain lesion and cord section. A certain percentage of Ss allowed 35 or 40 min also demonstrated the retention. In the 2nd preparation, postural asymmetries induced by 45 min of direct hindlimb stimulation were also retained after spinal section. Rhizotomy prior to stimulation resulted in a lack of appreciable asymmetry on termination of the stimulation. Retention of a hindlimb-stimulation-induced asymmetry was observed in Ss that underwent a spinal section before stimulation. Results demonstrate that the "spinal fixation" phenomenon can be obtained by induction of postural alterations from central (cerebellar lesion) and peripheral (hindlimb stimulation) sources. Results obtained from spinal Ss indicate that retention of peripherally induced asymmetry is not crucially dependent on higher brain center activity but rather seems to be more dependent on long-term alterations that occur directly in the spinal reflex system. (15 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Influence of stress on morphine-induced hyperthermia: Relevance to drug conditioning and tolerance development.

Three experiments, with 64 Holtzman and 48 Wistar male rats, examined whether Ss would become tolerant, or sensitized, to morphine-induced hyperthermia and the directionality of the conditioned pyretic effects of morphine. Stress produced by temperature-assessment procedures affected Ss' pyretic response to morphine. Under conditions of high stress, Ss first showed diminished, and then enhanced, hyperthermic responding across repeated dosing with morphine sulfate (5 or 35 mg/kg, sc). The diminished hyperthermia can be attributed to habituation to high levels of assessment stress. Repeated morphine doses delivered under conditions of low stress produced only enhanced hyperthermic responding, indicating that Ss became sensitized to morphine's hyperthermic effects. There was little evidence that morphine supported conditioning of pyretic responses. The temperature-assessment stress that produced hyperthermia was mediated by opiate peptides, was blocked by naloxone, and enhanced the agonist effects of morphine. Implications for theories of drug conditioning and tolerance are discussed. (57 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dual ultrastructural immunocytochemical labeling of mu and delta opioid receptors in the superficial layers of the rat cervical spinal cord

The delta opioid receptor (DOR) and mu opioid receptor (MOR) are abundantly distributed in the dorsal horn of the spinal cord. Simultaneous activation of each receptor by selective opiate agonists has been shown to result in synergistic analgesic effects. To determine the cellular basis for these functional associations, we examined the electron microscopic immunocytochemical localization of DOR and MOR in single sections through the superficial layers of the dorsal horn in the adult rat spinal cord (C2-C4). From a total of 270 DOR-labeled profiles, 49% were soma and dendrites, 46% were axon terminals and small unmyelinated axons, and 5% were glial processes. 6% of the DOR-labeled soma and dendrites, and < 1% of the glial processes also showed MOR-like immunoreactivity (MOR-LI). Of 339 MOR-labeled profiles, 87% were axon terminals and small unmyelinated axons, 12% were soma and dendrites, and 2% were glial processes. 21% of the MOR-labeled soma and dendrites, but none of the axon terminals also contain DOR-LI. The subcellular distributions of MOR and DOR were distinct in axon terminals. In axon terminals, both DOR-LI and MOR-LI were detected along the plasmalemma, but only DOR-LI was associated with large dense core vesicles. DOR-labeled terminals formed synapses with dendrites containing MOR and conversely, MOR-labeled terminals formed synapses with DOR-labeled dendrites. These results suggest that the synergistic actions of selective MOR- and DOR-agonists may be attributed to dual modulation of the same or synaptically linked neurons in the superficial layers of the spinal cord.     

Inhibition of morphine-induced tolerance and dependence by a benzodiazepine receptor agonist midazolam in the rat

We investigated whether midazolam administration influenced morphine-induced antinociception and tolerance and dependence in the rat. Antinociception was assessed by the tail-flick (TF) and the hot-plate test (HP 52 degrees C). Morphine tolerance developed after daily single injections of morphine for 11 days. The effect of midazolam on morphine-induced antinociception and tolerance was assessed by giving daily injections of various doses of midazolam for 11 days. The first injection of saline or midazolam was given intraperitoneally and 30 min later morphine (10 mg/kg body weight) was administered subcutaneously. Antinociception was monitored by measuring TF and HP latencies 60 min after the second injection. Midazolam was injected at four different concentrations: 0.03, 0.1, 0.3, and 3 mg/kg body weight. Chronic administration of morphine resulted in the development of tolerance to antinociception in both TF and HP tests, with rats exhibiting baseline antinociception on Day 9. Animals treated with midazolam alone showed little antinociception on Days 3-9. However, midazolam administration in morphine-treated animals attenuated morphine-induced tolerance to antinociception on Days 1-11 as measured by the tail-flick test. Midazolam also decreased the jumping behavior following naloxone injections in morphine-dependent rats. These results suggest that midazolam may prolong the effects of morphine by delaying morphine-induced development of tolerance to antinociception. Midazolam also attenuated a decrease in weight gain induced by chronic injections of morphine.     

Use of the Beck Depression Inventory in veterans with spinal cord injury.

Hypothesized that 7 Beck Depression Inventory items would be poor discriminators between depressed and nondepressed spinal cord injury Ss. To test this hypothesis, 124 veterans (aged 19–79 yrs) with spinal cord injury were administered the Beck Depression Inventory and evaluated for the presence of a major depressive episode. Ss were first administered the Mini-Mental Status Examination, interviewed and then administered the Beck Depression Inventory. Discriminant function analysis revealed that 3 of the 7 items were poor discriminators. Since clinicians are faced with the dilemma of deciding whether endorsement of Beck Depression Inventory items is indicative of depressive symptomatology or the sequelae of spinal cord injury, the authors derived cut scores appropriate for spinal cord injury patients. Data on the sensitivity and specificity of these cut scores are provided. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Effects of local anesthesia on persistence of peripherally induced postural asymmetries in rats.

Hindlimb flexion induced by direct stimulation of the hindlimb has been previously observed subsequent to spinal section if an appropriate time interval was allowed to elapse between onset of flexion and spinal cord section. The present 3 experiments tested the possibility that asymmetry persistence is a product of ongoing cutaneous input that continues after stimulation offset and spinal cord section. A local anesthetic (lidocaine; 25 mg/kg, sc) was injected into the general area of stimulation in 65 hooded rats, and its effects on asymmetry were assessed. Results generally indicate that ongoing cutaneous input is not a sufficient explanation for persistence of flexion in stimulated Ss with intact or severed spinal cords. Data reveal, however, that ongoing cutaneous input may partially explain results of a previous study by the 1st author et al (see record 1982-23053-001) that employed a "stimulation-wait" preparation to obtain peripherally induced spinal fixation. (9 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Conditioning and extinction of tolerance to the hypothermic effect of ethanol in rats.

Examined the contribution of classical conditioning to tolerance to the hypothermic effect of ethanol in 56 male albino rats. During the tolerance acquisition phase, Ss were exposed at 4-day intervals to a distinctive set of environmental cues paired with injections of ethanol (1.4 g/kg, ip). Interspersed between these drug trials were exposures to an alternate set of cues paired with injections of saline. In addition, 3 groups experienced different amounts of stimulation and activity during drug exposure in order to determine whether "behavioral augmentation" of tolerance would occur. In subsequent tests, Ss were tolerant only in the presence of cues previously paired with ethanol. Moreover, this environmentally specific tolerance was associated with a conditioned hyporthermic response to placebo (saline) injections in the drug environment. An extinction procedure designed to weaken tolerance mediated by classical conditioning was also found to be effective. Evidence for conditioned tolerance was weakest in Ss experiencing low levels of activity during the initial drug exposure periods. (19 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Anticipatory hyperexcitability and tolerance to the narcotizing effect of morphine in the rat.

Examined the role of Pavlovian conditioning in tolerance to the narcotizing effect of a high dose of morphine in 32 male albino Sprague-Dawley rats. Initially, 2 groups received 9 injections of morphine (40 mg/kg), and 2 groups received 9 injections of saline. One group administered each substance was injected in 1 of 2 distinctive environments: the animal colony or a distinctive room. Subsequently, Ss in all groups received 5 morphine injections in the distinctive room. Analyses of videotape records of postinjection behavior indicated that Ss tested in the presence of the usual predrug cues were more tolerant to the narcotizing effect of morphine than Ss tested with cues different from those previously associated with morphine. In addition, Ss tested with the usual predrug cues exhibited more anticipatory "hyperexcitable" behavior than Ss tested in the absence of the usual predrug cues. Results provide further evidence that compensatory pharmacological conditional responses partially mediate tolerance and suggest that these drug-anticipatory responses contribute to so-called "withdrawal symptoms." (18 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Comparison of lidocaine and saline for epidural top-up during combined spinal-epidural anesthesia in volunteers

This study was designed to determine the efficacy of saline as an epidural top-up to prolong spinal anesthesia during combined spinal-epidural anesthesia (CSEA). Eight volunteers received three separate CSEAs with intrathecal lidocaine (50 mg). After two-segment regression, each subject received either a saline (10 mL), lidocaine 1.5% (10 mL), or control sham (0.5 mL saline) epidural injection in a randomized, double-blind, triple cross-over fashion. Sensory block was assessed by pinprick and tolerance to transcutaneous electrical stimulation (TES) equivalent to surgical stimulation at the knee and ankle. Motor strength was assessed with iso-metric force dynamometry. Data were analyzed with a repeated measures analysis of variance and a paired t-test. Sensory block to pinprick was prolonged in the thoracolumbar dermatomes only by lidocaine (P < 0.05). Neither lidocaine nor saline prolonged the duration of tolerance to TES at the tested sites. Instead, saline decreased the duration of tolerance to TES by 20 and 24 min at the knee and ankle (P < 0.05). Recovery from motor block at the quadriceps was prolonged by an epidural injection of lidocaine (P < 0.05). We conclude that when 10 mL of epidural saline is administered after two-segment regression, it is an ineffective top-up and may decrease the duration of spinal anesthesia during CSEA.     

Tolerance to the hyperthermic effect of morphine in the rat is a learned response.

Results of 3 experiments with 84 male Wistar rats indicate that the hyperthermic effect of morphine in rats becomes attenuated over the course of successive administrations by a conditional, compensatory, hypothermic response elicited by cues present at the time of morphine administration, thus accounting for hyperthermic tolerance. Ss with a history of morphine administration displayed a tolerant response to the hyperthermic effect of the drug and a compensatory hypothermia following a placebo if these substances were administered following cues that previously signaled morphine—neither the tolerant reaction to morphine nor the hypothermic response to the placebo resulted when Ss were injected following cues that previously signaled injection of physiological saline (Exp I). Presenting environmental cues previously associated with morphine, but without the drug, abolished established tolerance, that is, pyretic tolerance could be extinguished (Exp II). Placebo sessions interspersed between morphine sessions impeded the acquisition of tolerance, that is, pyretic tolerance was retarded by partial reinforcement (Exp III). (52 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Inescapable shock-induced potentiation of morphine analgesia in rats: Sites of action.

Inescapable shock (IS) enhances analgesia to systemic morphine (MOR) 24 hr later. IS activates serotonin neurons in the dorsal raphe nucleus (DRN), rendering them hyperexcitable. These studies tested whether IS potentiates the analgesic effect of MOR microinjected in the DRN, as predicted by this hypothesis. To test site specificity, the effect of previous IS was examined on MOR microinjected lateral to the DRN and into 2 other sites that support MOR analgesia, the nucleus raphe magnus (NRM) and spinal cord. Twenty-four hours after IS, potentiated analgesia was observed after 0.5 μg MOR microinjected into, but not lateral to, the DRN. Potentiated analgesia was also observed after NRM (1.0 μg) and spinal cord (3.0 μg) MOR microinjections. These data suggest that IS-induced excitability changes within the DRN synergize with opiates microinjected in other analgesia areas and that this potentiates the responses to opiates 24 hr after IS. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Chronic morphine increases biosynthesis of nitric oxide synthase in the rat spinal cord

The present study was undertaken to determine the influence of chronic morphine treatment on the biosynthesis of nitric oxide synthase (NOS) in the rat spinal cord using in situ hybridization and immunohistochemical methods. Repeated administration of morphine (20-100 mg/kg/day; 10 days) increased the NOS mRNA level in laminae I-IV and X 3 h after the last injection. That effect was accompanied by an increase in both the number of NOS-positive cells (24 h) and the optical density of NOS-immunoreactivity (3 and 24 h). The results indicate that repeated morphine administration increases NOS biosynthesis in the rat spinal cord, which may reflect adaptive changes accounting for development of opiate tolerance and dependence.     

Spasticity in rats with sacral spinal cord injury

We have investigated sacral spinal cord lesions in rats with the goal of developing a rat model of muscular spasticity that is minimally disruptive, not interfering with bladder, bowel, or hindlimb locomotor function. Spinal transections were made at the S2 sacral level and, thus, only affected the tail musculature. After spinal transection, the muscles of the tail were inactive for 2 weeks. Following this initial period, hypertonia, hyperreflexia, and clonus developed in the tail, and grew more pronounced with time. These changes were assessed in the awake rat, since the tail is readily accessible and easy to manipulate. Muscle stretch or cutaneous stimulation of the tail produced muscle spasms and marked increases in muscle tone, as measured with force and electromyographic recordings. When the tail was unconstrained, spontaneous or reflex induced flexor and extensor spasms coiled the tail. Movement during the spasms often triggered clonus in the end of the tail. The tail hair and skin were extremely hyperreflexive to light touch, withdrawing quickly at contact, and at times clonus could be entrained by repeated contact of the tail on a surface. Segmental tail muscle reflexes, e.g., Hoffman reflexes (H-reflexes), were measured before and after spinalization, and increased significantly 2 weeks after transection. These results suggest that sacral spinal rats develop symptoms of spasticity in tail muscles with similar characteristics to those seen in limb muscles of humans with spinal cord injury, and thus provide a convenient preparation for studying this condition.     

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.     

Associative factors in tolerance to analgesia produced by electrical stimulation in the brainstem.

Tolerance to the analgesic effects of electrical stimulation of the periaqueductal gray was confirmed, and it was demonstrated that this effect was mediated by environmental conditional stimuli (CSs). First, tolerance was extinguished after repeated presentation of CSs in the absence of brain stimulation. Second, environment-specific tolerance was demonstrated. Analgesia was reinstated after brain stimulation in a different test environment. Omission of brain stimulation in the stimulation environment failed to reveal a hyperalgesic response in tolerant Ss. Brainstimulation analgesia should be beneficial for analyzing neural mechanisms underlying conditioned tolerance to analgesia. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Overshadowing effects in the stimulus control of morphine analgesic tolerance.

Attempted to replicate previous demonstrations of classical conditioning of morphine analgesic tolerance and to determine whether stimulus overshadowing effects might explain previous conflicting findings. In Exp I, 8 groups of male Sprague-Dawley rats received a series of 10 morphine (5 mg/kg) and/or saline injections, differing only with respect to the contingency between a compound visual-auditory CS and the substance injected. When tested for analgesic responding to morphine in the presence of the compound CS, only those groups for which the CS and morphine injections were paired during the acquisition sequence evidenced tolerance. In Exp II, tolerant Ss were tested in the presence of 1 component of the compound CS. When a loud tone (85 db) was used in the compound, less analgesic tolerance was elicited later by the weaker visual stimulus alone. This differential stimulus control of the analgesic response suggests that overshadowing may contribute to failures to replicate conditioned morphine tolerance. It is possible that internal morphine-produced stimuli may overshadow external cues. (18 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Tolerance to amphetamine anorexia: Role of learning versus body weight settling point.

Conducted 3 experiments with male Sprague-Dawley rats to study the development of tolerance to amphetamine (AM) anorexia. In Exp I, 19 Ss that had become tolerant to the suppressant effect of AM on milk intake were anorexic when offered other foods or water. These results appear to support a conditioning interpretation. In Exps II (38 Ss) and III (24 Ss), Ss made tolerant with milk as the diet showed prolonged anorexia when switched to Purina pellets or slightly bitter milk; but when switched from pellets or adulterated milk to milk, tolerant Ss were anorexic only 1 day and then ingested significantly more of the new diet. Results are inconsistent with a conditioning interpretation. Tolerant Ss maintained their weight below the level of saline controls despite the recovery of food intake, and the level at which they maintained their weight varied with the palatability of the diet. These results suggest that AM lowers the settling point for body weight and that tolerance to this effect does not develop. Thus, the reinstatement of prolonged anorexia when apparently tolerant Ss were switched to a less palatable diet can be understood as an attempt to attain a lower weight level. (32 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)