Antagonist Reversal Research Articles

Restoration of Glutamatergic Transmission by Dopamine D4 Receptors in Stressed Animals

The prefrontal cortex (PFC), a key brain region for cognitive and emotional processes, is highly regulated by dopaminergic inputs. The dopamine D4 receptor, which is enriched in PFC, has been implicated in mental disorders, such as attention deficit-hyperactivity disorder and schizophrenia. Recently we have found homeostatic regulation of AMPA receptor-mediated synaptic transmission in PFC pyramidal neurons by the D4 receptor, providing a potential mechanism for D4 in stabilizing cortical excitability. Because stress is tightly linked to adaptive and maladaptive changes associated with mental health and disorders, we examined the synaptic actions of D4 in stressed rats. We found that neural excitability was elevated by acute stress and dampened by repeated stress. D4 activation produced a potent reduction of excitatory transmission in acutely stressed animals and a marked increase of excitatory transmission in repeatedly stressed animals. These effects of D4 targeted GluA2-lacking AMPA receptors and relied on the bi-directional regulation of calcium/calmodulin kinase II activity. The restoration of PFC glutamatergic transmission in stress conditions may enable D4 receptors to serve as a synaptic stabilizer in normal and pathological conditions.

Estrogen Receptor α L543A,L544A Mutation Changes Antagonists to Agonists, Correlating with the Ligand Binding Domain Dimerization Associated with DNA Binding Activity

A ligand-dependent nuclear transcription factor, ERα has two transactivating functional domains (AF), AF-1 and AF-2. AF-1 is localized in the N-terminal region, and AF-2 is distributed in the C-terminal ligand-binding domain (LBD) of the ERα protein. Helix 12 (H12) in the LBD is a component of the AF-2, and the configuration of H12 is ligand-inducible to an active or inactive form. We demonstrated previously that the ERα mutant (AF2ER) possessing L543A,L544A mutations in H12 disrupts AF-2 function and reverses antagonists such as fulvestrant/ICI182780 (ICI) or 4-hydoxytamoxifen (OHT) into agonists in the AF2ER knock-in mouse. Our previous in vitro studies suggested that the mode of AF2ER activation is similar to the partial agonist activity of OHT for WT-ERα. However, it is still unclear how antagonists activate ERα. To understand the molecular mechanism of antagonist reversal activity, we analyzed the correlation between the ICI-dependent estrogen-responsive element-mediated transcription activity of AF2ER and AF2ER-LBD dimerization activity. We report here that ICI-dependent AF2ER activation correlated with the activity of AF2ER-LBD homodimerization. Prevention of dimerization impaired the ICI-dependent ERE binding and transcription activity of AF2ER. The dislocation of H12 caused ICI-dependent LBD homodimerization involving the F-domain, the adjoining region of H12. Furthermore, F-domain truncation also strongly depressed the dimerization of WT-ERα-LBD with antagonists but not with E2. AF2ER activation levels with ICI, OHT, and raloxifene were parallel with the degree of AF2ER-LBD homodimerization, supporting a mechanism that antagonist-dependent LBD homodimerization involving the F-domain results in antagonist reversal activity of H12-mutated ERα.

Safety of a 4-factor prothrombin complex concentrate (4F-PCC) versus plasma for vitamin K antagonist reversal: an integrated analysis of phase III clinical trial data

Safety of a 4-factor prothrombin complex concentrate (4F-PCC) versus plasma for vitamin K antagonist reversal: an integrated analysis of phase III clinical trial data

Estrogen receptor α AF-2 mutation results in antagonist reversal and reveals tissue selective function of estrogen receptor modulators

The estrogen receptor (ER) is a ligand-dependent transcription factor containing two transcriptional activation domains. AF-1 is in the N terminus of the receptor protein and AF-2 activity is dependent on helix 12 of the C-terminal ligand-binding domain. Two point mutations of leucines 543 and 544 to alanines (L543A, L544A) in helix 12 minimized estrogen-dependent transcriptional activation and reversed the activity of the estrogen antagonists ICI182780 (ICI) and tamoxifen (TAM) into agonists in a similar manner that TAM activated WT ERα through AF-1 activation. To evaluate the physiological role of AF-1 and AF-2 for the tissue-selective function of TAM, we generated an AF-2-mutated ERα knock-in (AF2ERKI) mouse model. AF2ERKI homozygote female mice have hypoplastic uterine tissue and rudimentary mammary glands similar to ERα-KO mice. Female mice were infertile as a result of anovulation from hemorrhagic cystic ovaries and elevated serum LH and E2 levels, although the mutant ERα protein is expressed in the AF2ERKI model. The AF2ERKI phenotype suggests that AF-1 is not activated independently, even with high serum E2 levels. ICI and TAM induced uterotropic and ER-mediated gene responses in ovariectomized AF2ERKI female mice in the same manner as in TAM- and E2-treated WT mice. In contrast, ICI and TAM did not act as agonists to regulate negative feedback of serum LH or stimulate pituitary prolactin gene expression in a different manner than TAM- or E2-treated WT mice. The functionality of the mutant ERα in the pituitary appears to be different from that in the uterus, indicating that ERα uses AF-1 differently in the uterus and the pituitary for TAM action.

Use of Prothrombin Complex Concentrate for Vitamin K Antagonist Reversal before Surgical Treatment of Intracranial Hemorrhage

Background:Oral anticoagulant therapy (OAT) is used to prevent/treat thromboembolism. Major bleeding is common in patients on OAT; eg, warfarin increases intracranial hemorrhage (ICH) risk.Case:A 71-year-old male on warfarin (to reduce stroke risk) presented at Accident and Emergency Minor Injuries Unit with headache after reportedly sounding ‘drunk’. On triage, the patient appeared lucid and well. However, International Normalized Ratio (INR) was 4.1. Head computed tomography (CT) indicated a large right-sided subdural hematoma. Prothrombin complex concentrate (PCC; Beriplex® P/N, CSL Behring) with vitamin K normalized the INR within minutes of administration. The patient underwent neurosurgery without complications, and was discharged after 5 days, with no residual neurological symptoms.Conclusions:ICH patients can present with no neurological signs. In OAT patients with headache, INR must be established; if ≥3.0, normalization of INR and head CT are essential. PCC is the best option to rapidly reverse anticoagulation and correct INR pre-surgery.

Initial conditions influence the characteristics of ballistic contractions in the ankle dorsiflexors

This study investigated the influence of different initial conditions on a subsequent fast (ballistic) isometric contraction of the ankle dorsiflexor muscles. Surface electromyograms (EMGs) of dorsiflexor and plantarflexor muscles were recorded during ballistic contractions performed without any pre-activation (BAL) and in ballistic contractions preceded by a sustained submaximal contraction (20% MVC) that was followed either by a rapid voluntary relaxation of the agonist muscle (VRBAL) or by a rapid antagonist (reversal) contraction (ARBAL). In the latter condition, three different antagonist torque levels were compared (25, 50 and 75% MVC). The results showed that the mean average rate of torque development was significantly (P < 0.001) greater for the ARBAL condition (968.5 ± 183.9% MVC/s) compared with the VRBAL (509.3 ± 78.7% MVC/s) and BAL (461.8 ± 79.9% MVC/s) conditions. Furthermore, the mean value recorded for VRBAL was significantly (P < 0.05) greater than for BAL condition. The faster increases in torque during the VRBAL and ARBAL conditions were associated with a greater agonist EMG activity. Compared with VRBAL, performance during the ARBAL condition was improved by a greater level of antagonist coactivation and, in some trials, by the presence of a silent EMG period between the end of the antagonist activation and the onset of the agonist ballistic contraction. Together, these results indicate that the initial conditions can have a substantial influence on the rate of torque development during ballistic contractions performed in isometric conditions.

Prothrombin complex concentrate in surgical patients: retrospective evaluation of vitamin K antagonist reversal and treatment of severe bleeding

IntroductionProthrombin complex concentrates are recommended for rapid reversal of vitamin K anticoagulants. As they normalize levels of vitamin K dependent clotting factors and re-establish hemostasis, they may also be used as adjunctive therapy in patients with major bleeding. The aim of this study was to retrospectively evaluate the efficacy of prothrombin complex concentrates in the surgical setting.MethodsThe case notes of 50 patients requiring urgent oral anticoagulation reversal (n = 12) or with severe perioperative coagulopathic bleeding (n = 38) who received an infusion of prothrombin complex concentrate (Beriplex P/N(R) 500) at the surgical department of the University of Munich Hospital, Germany were retrospectively reviewed. Efficacy of prothrombin complex concentrate application was evaluated using the Quick test, reported as an international normalized ratio, hemodynamic measurements and requirement for blood products. Safety assessments included whole blood hemoglobin levels and specific parameters of organ dysfunction.ResultsBaseline characteristics were comparable, except that mean baseline international normalized ratio and hemoglobin levels were significantly higher (P < 0.01) in anticoagulation reversal than in bleeding patients. In anticoagulation reversal, the international normalized ratio was significantly reduced (from 2.8 +/- 0.2 at baseline to 1.5 +/- 0.1, P < 0.001) after one prothrombin complex concentrate infusion (median dose 1500 IU; lower quartile 1,000, upper quartile 2,000). No major bleeding was observed during surgery after prothrombin complex concentrate administration. Only one patient received platelets and red blood cell transfusion after prothrombin complex concentrate administration. In bleeding patients, infusion of prothrombin complex concentrate (median dose 2,000 IU; lower quartile 2,000, upper quartile 3,000) significantly reduced the INR from 1.7 +/- 0.1 at baseline to 1.4 +/- 0.1 (P < 0.001). This decrease was unrelated to fresh frozen plasma or vitamin K administration. Bleeding stopped after prothrombin complex concentrate administration in 4/11 (36%) patients with surgical bleeding and 26/27 (96%) patients with diffuse bleeding. Hemoglobin levels increased significantly from baseline in bleeding patients (P < 0.05) and mean arterial pressure stabilized (P < 0.05). No thrombotic events or changes in organ function were reported in any patient.ConclusionsProthrombin complex concentrate application effectively reduced international normalized ratios in anticoagulation reversal, allowing surgical procedures and interventions without major bleeding. In bleeding patients, the improvement in coagulation after prothrombin complex concentrate administration was judged to be clinically significant.

5-HT 6 receptor antagonist reversal of emotional learning and prepulse inhibition deficits induced by apomorphine or scopolamine

5-HT 6 receptors have been implicated in consolidation of visuospatial and reward-based learning tasks. Since 5-HT 6 receptors may be important in modulation of sensory gating which is often affected in schizophrenic patients, we tested whether Ro 4368554, a 5-HT 6 selective antagonist at a dose of 10 mg/kg, could reverse the loss of prepulse inhibition from apomorphine or scopolamine. In addition, we also tested whether Ro 4368554 altered fear conditioning using fear potentiated startle, a model for emotional learning. Prepulse inhibition of startle was disrupted by apomorphine (0.5 mg/kg) when prepulse emissions were 5 dB above background but not above 15 dB, while scopolamine (0.5 mg/kg) caused disruption at both prepulse levels. Scopolamine-mediated disruption was not reversed by Ro 4368854 but apomorphine-mediated disruption was significantly ameliorated by 5-HT 6 inhibition. For fear potentiated startle, scopolamine and/or Ro 4368554 were administered before two daily fear conditioning sessions; rats were tested on the following day. Rats that received scopolamine displayed no fear potentiated startle but Ro 4368554 reversed this scopolamine deficit. Additionally, we mapped Fos induction in rats treated with scopolamine and/or Ro 4368554; scopolamine increased Fos expression in the central nucleus of the amygdala and this was attenuated by Ro 4368554. In summary, we have demonstrated the efficacy of 5-HT 6 antagonists in modulating sensory gating and fear conditioning, and thus may be of therapeutic use for schizophrenia-related disorders.

Neural Adaptations to Resistive Exercise

It is generally accepted that neural factors play an important role in muscle strength gains. This article reviews the neural adaptations in strength, with the goal of laying the foundations for practical applications in sports medicine and rehabilitation. An increase in muscular strength without noticeable hypertrophy is the first line of evidence for neural involvement in acquisition of muscular strength. The use of surface electromyographic (SEMG) techniques reveal that strength gains in the early phase of a training regimen are associated with an increase in the amplitude of SEMG activity. This has been interpreted as an increase in neural drive, which denotes the magnitude of efferent neural output from the CNS to active muscle fibres. However, SEMG activity is a global measure of muscle activity. Underlying alterations in SEMG activity are changes in motor unit firing patterns as measured by indwelling (wire or needle) electrodes. Some studies have reported a transient increase in motor unit firing rate. Training-related increases in the rate of tension development have also been linked with an increased probability of doublet firing in individual motor units. A doublet is a very short interspike interval in a motor unit train, and usually occurs at the onset of a muscular contraction. Motor unit synchronisation is another possible mechanism for increases in muscle strength, but has yet to be definitely demonstrated. There are several lines of evidence for central control of training-related adaptation to resistive exercise. Mental practice using imagined contractions has been shown to increase the excitability of the cortical areas involved in movement and motion planning. However, training using imagined contractions is unlikely to be as effective as physical training, and it may be more applicable to rehabilitation. Retention of strength gains after dissipation of physiological effects demonstrates a strong practice effect. Bilateral contractions are associated with lower SEMG and strength compared with unilateral contractions of the same muscle group. SEMG magnitude is lower for eccentric contractions than for concentric contractions. However, resistive training can reverse these trends. The last line of evidence presented involves the notion that unilateral resistive exercise of a specific limb will also result in training effects in the unexercised contralateral limb (cross-transfer or cross-education). Peripheral involvement in training-related strength increases is much more uncertain. Changes in the sensory receptors (i.e. Golgi tendon organs) may lead to disinhibition and an increased expression of muscular force. Agonist muscle activity results in limb movement in the desired direction, while antagonist activity opposes that motion. Both decreases and increases in co-activation of the antagonist have been demonstrated. A reduction in antagonist co-activation would allow increased expression of agonist muscle force, while an increase in antagonist co-activation is important for maintaining the integrity of the joint. Thus far, it is not clear what the CNS will optimise: force production or joint integrity. The following recommendations are made by the authors based on the existing literature. Motor learning theory and imagined contractions should be incorporated into strength-training practice. Static contractions at greater muscle lengths will transfer across more joint angles. Submaximal eccentric contractions should be used when there are issues of muscle pain, detraining or limb immobilisation. The reversal of antagonists (antagonist-to-agonist) proprioceptive neuromuscular facilitation contraction pattern would be useful to increase the rate of tension development in older adults, thus serving as an important prophylactic in preventing falls. When evaluating the neural changes induced by strength training using EMG recording, antagonist EMG activity should always be measured and evaluated.

The reversal of antagonists facilitates the peak rate of tension development

Gabriel DA, Basford JR, An K-N. The reversal of antagonists facilitates the peak rate of tension development. Arch Phys Med Rehabil 2001;82:342-6. Objective: The present study was designed to test the effects of the reversal of antagonists on the peak rate of tension development (dF/dtmax) of the elbow extensors. Design: Experimental, with matched controls. Setting: A biomechanics research laboratory. Participants: Twenty-six healthy women without a history of upper extremity injury or neurologic disorder, randomly assigned to experimental (n = 13) or control (n = 13) groups. Interventions: Two groups of healthy subjects followed identical exercise protocols, except that the control group performed maximal isometric contractions of the elbow extensors and the experimental group executed a maximal isometric elbow flexion contraction immediately before a maximal elbow extension contraction. Both groups performed 5 cycles of a 2-second contraction with 22-second rest periods between agonist muscle contractions and were evaluated at 4 test sessions spaced 2 weeks apart. Main outcome measures: All measurements were done with the shoulder and elbow at 90° of flexion in the sagittal plane to ensure reproducibility. A load cell was used to measure elbow extension moment and to calculate the peak rate of tension development (dF/dtmax). Biceps and triceps brachii surface electromyographic activity was monitored concurrently. The electromyographic measures were mean spike (peak-to-peak) amplitude and mean spike frequency of the biceps and triceps brachii activity. Results: Intraclass dF/dtmax and electromyographic reliability was good (r ≥.72) in both groups. Because biceps electromyographic measures were considerably less reliable (r ≤.53), they were not included in our analysis. While dF/dtmax increased quadratically in both groups (p <.05), the experimental group was on average 36.1Nm · s−1 (63%) greater across sessions 2 to 4 (p <.05). In contrast, triceps electromyographic activity did not differ significantly between groups (p >.05). The means averaged across groups exhibited a quadratic increase from session 1 to session 4: 91μV or 48% for mean spike amplitude (p <.05) and 7Hz or 16% for mean spike frequency (p <.05). Conclusions: The greater dF/dtmax for the experimental group was not associated with increased electromyographic activity. The experimental group appeared to use the biomechanic properties of the pretensioned extensor muscle-tendon complex, rather than neurologic biasing, to accomplish its power gains.

Effects of the Reversal of Antagonists on the Peak Rate of Tension Development.

Effects of the Reversal of Antagonists on the Peak Rate of Tension Development.

The Influence of Varied Resistance and Speed of Concentric Antagonistic Contractions on Subsequent Concentric Agonistic Efforts

Traditionally, near maximal isometric efforts have been used to elicit the beneficial facilitations and inhibitions associated with strength and flexibility improvements using proprio-ceptive neuromuscular facilitation (PNF) and modified PNF techniques. Although concentric reversal of antagonists has been used clinically, there is little evidence regarding the concentric contraction speed and resistance levels necessary to elicit an augmented neuromuscular response. This study addresses the question of speed and resistance specificity and their subsequent PNF effects on strength and power outputs. This was accomplished through the use of a double-acting concentric dynamometer (DACD). Twelve males, aged 18–25 years, participated in this study. Each subject performed a reversal of an antagonist maneuver simulating a horizontal bench pull and bench press, all using combinations of slow speed of contraction, fast speed of contraction, and isometric contraction. Within the limitations of this study, it appears that a low-resistance, fast speed of antagonistic contraction significantly increases both peak and average force during a fast agonistic effort (p < 0.05). Neither maximal isometric nor slow concentric contraction of the antagonist (against heavy resistance) seemed to augment the succeeding concentric effort of the agonist under either slow or fast conditions.

Reversal of antagonists: Effect on elbow extension strength and endurance

Objective: To evaluate the reversal of elbow antagonists designed to facilitate the extensors relative to agonist-only contractions of the extensors, while using a protocol previously demonstrated to result in an increase in strength through motor learning. Design:Two-group convenience sample composed of healthy subjects. Setting: Clinic research laboratory. Subjects: Twenty-eight healthy women without a history of upper extremity injury or neurologic disorder. Measurements: Elbow extension strength and endurance were measured during maximal effort isometric reversal of the elbow antagonists (experimental group) and elbow extension only (control group) resistance exercise. Biceps brachii short head (BBS), triceps brachii lateral head (TLAT), and triceps brachii long head (TLNG) electromyographic (EMG) activity was measured concurrently. Results: There were no significant differences between groups for either elbow extension strength, BBS, TLAT, or TLNG EMG activity. The experimental and control groups exhibited a significant linear ( p < .01) and quadratic ( p < .01) increase in baseline strength of 5.2N · m (30.5%). EMG activity from each muscle group increased linearly ( p < .01) across all test sessions. Similar results were observed for elbow extension strength and biceps and triceps EMG activity during fatigue testing. Conclusions: The reversal of antagonists technique was not superior to agonist-only resistance exercise, nor did it interfere with the acquisition of muscular strength or endurance through motor learning.

Knee extension torque during repeated knee extension-flexion reversals and separated knee extension-flexion dyads.

I measured the knee extension torque, produced by eight hemiparetic patients during repeated knee extension-flexion reversals and during separated knee extension-flexion dyads, to determine the effect of "reversal of antagonists" on knee extension torque. The hemiparesis in these eight patients was secondary to intracranial lesions. I tested their involved lower extremity on an isokinetic dynamometer at 60 degrees/sec. Using a sequential medical-trials design, I found knee extension torque to be significantly greater during repeated knee extension-flexion reversals than during separated knee extension-flexion dyads (p less than .05). I, therefore, concluded that reversal of antagonists may facilitate knee extension torque production in the type of patients tested, under the specific conditions of their testing.

Characterization of an adenylate cyclase-linked serotonin (5-HT1) receptor in a neuroblastoma X brain explant hybrid cell line (NCB-20).

Clonal cell line NCB-20 (a hybrid of mouse neuroblastoma N18TG2 and Chinese hamster 18-day embryonic brain explant) expressed both high- (KD 180 nM) and low-affinity (greater than 3000 nM) binding sites for [3H]serotonin (5-HT) which were absent from the parent neuroblastoma. The low-affinity binding site was eliminated by 1 microM spiperone. The order of drug potency for inhibition of high-affinity [3H]5-HT binding was consistent with a 5-HT1 receptor (5,6 - dihydroxytryptamine = 5-HT = methysergide = 5-methoxytryptamine greater than cyproheptadine = clozapine = mianserin greater than spiperone greater than dopamine = morphine = ketanserin = norepinephrine). [3H]5-HT binding was inhibited by guanine nucleotides (e.g., GTP and Gpp(NH)p), whereas antagonist binding was not; ascorbate was also inhibitory. A 30-min exposure of cells to 1-2 microM 5-HT or other agonists produced a three- to fivefold stimulation of cyclic AMP levels. The order of potency for 5-HT agonist stimulation of basal cyclic AMP levels and 5-HT antagonist reversal of agonist-stimulated levels was the same as the order of drug potency for inhibition of high-affinity [3H]5-HT binding, suggesting linkage of the 5-HT1 receptor to adenylate cyclase in NCB-20 cells.

Effects of antagonistic contraction on elbow-flexion movement time in man.

The use of neuromuscular facilitation has long been advocated by physical therapists as a technique for inducing the greatest possible activity in affected muscles. Increased central excitation enhances voluntary activity of paralyzed or paretic muscles and speeds recovery considerably. To a lesser extent, coaches and physical educators have employed similar techniques to improve athletic performance. One facilitation technique which has recently been shown to increase isokinetic knee extensor strength (1) is the reversal of antagonists. Based upon Sherrington's law of successive induction (3), it involves the isometric contraction of antagonists followed by the immediate contraction of agonists. Since muscular strength and another important characteristic of performance, speed, are usually thought to be unrelated (2), it was unclear how this technique might effect movement time. A horizontal elbow-flexion arm movement, executed at maximal speed and timed from 130 to 100 of flexion was studied in nine male subjects to determine the effects of a pre-stimulus antagonistic isometric contraction on movement time. Subjects reacted to an auditory stiaulus, immediately preceded by one of three conditions: complete muscular relaxation of both agonist and antagonist arm muscles prior to the response cue, or pre-stimulus isometric antagonist contractions at two different intensities. All subjects performed 20 trials at each pre-stimulus tension level (0, 5, 10 kg), with tension treatments presented in random order across subjects. Movement time (30 of displacement) and average angular velocity were determined. All test-retest reliability coefficients were high, i.e., r > 0.87. A variance analysis, corrected for repeated measures, showed that movement times and average angular velocities were significantly improved after pre-stimulus antagonistic contractions were performed. Movement times decreased 11.59% and 16.60% and average angular velocities increased 23.60% and 31.0% on trials where pre-stimulus antagonist contractions were performed at intensities of 5 and 10 kg, respectively. Present results taken together with previous findings (1) suggest chat the principle of successive induction is applicable to voluntary movement as well as reflex activity. The efficacy of applying this technique to specific athletic activity remains to be investigated.

Differential reversal of various dopamine antagonists by anticholinergics in Sidman avoidance: possible relationship to adrenergic blockade.

Various dopamine receptor antagonists have divergent clinical and neurochemical properties. The relative ability of anticholinergics (benztropine and scopolamine) to reverse these drugs was assessed in squirrel monkeys and rats performing a Sidman avoidance task. In monkeys, benztropine markedly attenuated the effects of oxiperomide, metoclopramide, halopemide, tiapride and mezilamine as well as haloperidol. Chlorpromazine and fluphenazine were antagonized to a moderate extent; thioridazine and perlapine were not antagonized; and clozapine was actually potentiated by benztropine. In the rat, benztropine antagonized haloperidol strongly but reversed fluphenazine, thioridazine or clozapine only weakly or not at all. The overall effects of scopolamine in both species were similar to those of benztropine. The dopamine receptor antagonists that were most completely reversed by benztropine were found to inhibit 3H-spiroperidol more strongly than 3H-WB-4101 binding in calf caudate, while the reverse was true for drugs that were antagonized only moderately or not at all by benztropine. These results support a previous suggestion that anticholinergic reversal is less marked against dopamine antagonists with alpha-adrenergic blocking properties. Benztropine reversal of experimental dopamine receptor antagonists in the squirrel monkey Sidman avoidance test may contribute to their preclinical characterization.

Muscular Power with successive induction and reversal of antagonists

Muscular Power with successive induction and reversal of antagonists