Substantia nigra: a site of action of muscle relaxant drugs

NMDA antagonists for treating the non-motor symptoms in Parkinson's disease

Among patients with Parkinson's disease (PD), depression is prevalent and disabling, impacting both health outcomes and quality of life. There is a critical need for alternative pharmacological methods to treat PD depression, as mainstream antidepressant drugs are largely ineffective in this population. Currently, there are no recommendations for the optimal treatment of PD neuropsychiatric symptoms. Given the dual antidepressant and anti-dyskinetic effects of ketamine and other N-methyl-D-aspartate (NMDA) antagonists for PD, this review aims to examine the current evidence of NMDA antagonists for treating neuropsychiatric symptoms, including memantine, amantadine, ketamine, dizoclopine, and d-cycloserine. A comprehensive literature search was conducted using the PubMed database. We also searched the following databases up to March 1, 2018: Ovid MEDLINE, PsycINFO, CINAHL, Google Scholar, Cochrane Central Register of Controlled Trials, and Cochrane Database of Systematic Reviews. The following keywords were used: NMDA antagonist and Parkinson's disease. Two authors independently reviewed the articles identified from the search using specific selection criteria, focusing on studies of mood, psychiatric condition, depression, cognition, and quality of life, and the consensus was reached on the 20 studies included. There is a preliminary evidence that NMDA antagonists may modulate psychiatric symptoms in PD. However, current evidence of psychiatric symptom-modifying effects is inconclusive and requires that further trials be conducted in PD. The repurposing of old NMDA antagonists, such as ketamine for depression and newer therapies, such as rapastinel, suggests that there is an emerging place for modulating the glutamatergic system for treating non-motor symptoms in PD.

[Respiratory failure due to delta-9-tetrahydrocannabinol in a tetraplegic patient]

We report on a patient with an incomplete tetraplegia below C2 who suffered from a post-traumatic abdominal spasticity, spasticity of the legs, and bladder contractions of high intensity. Breathing was possible during the day using accessory respiratory musculature. All standard therapeutic regimes against spasticity failed. Treatment was started with delta-9-tetrahydrocannabinol administered orally in a dosage of 2 x 2.5 mg/day. The spasticity of the legs and the bladder improved with the treatment. After 3 days, the patient complained about dyspnea and shortness of breath. Treatment with delta-9-tetrahydrocannabinol was discontinued after 5 days but the patient needed ventilatory support for 1 week. After 1 week, spontaneous breathing was possible again. The reasons for respiratory failure in endangered patients during treatment with delta-9-tetrahydrocannabinol could be effects such as sedation, combined treatment with baclofen, muscle weakness, or central nervous effects in the medulla oblongata.

Differential sensitivity to the motor and hypothermic effects of the GABA B receptor agonist baclofen in various mouse strains

RATIONALE

Comparison of different mouse strains can provide valuable information about the genetic control of behavioural and molecular phenotypes. Recent evidence has demonstrated the importance of GABA B receptors in anxiety and depression. Investigation of the phamacogenetics of GABA B receptor activation may aid in the understanding of mechanisms underlying the role of GABA B in affect.

OBJECTIVES

The aim of current study was to determine the relative sensitivity of different mouse strains to GABA B receptor agonism in two models of GABA B receptor function, namely hypothermia and motor incoordination.

METHODS

Mice each from 11 strains (BALB/cByJIco, DBA/2JIco, OF1, FVB/NIco, CD1, C3H/HeOuJIco, 129/SvPasIco, NMRI, C57BL/6JIco, A/JOlaHsd and Swiss) were trained to walk on a rotarod for 300 s. On the following day, mice received 0, 3, 6 or 12 mg/kg of L: -baclofen PO. Rectal temperature and rotarod performance were measured at 0, 1, 2 and 4 h after drug application.

RESULTS

L: -Baclofen produced a significant dose-dependent hypothermia and ataxia in most, but not all, mouse strains examined. The magnitude and duration of response was influenced by strain, with mice of the 129/SvPasIco strain showing largest hypothermic response to 12 mg/kg l-baclofen and C3H/HeOuJIco the lowest, whereas the BALB/cByJIco strain demonstrated greatest ataxic response on the rotarod, and NMRI the least. Interestingly, some strains (notably C3H/HeOuJIco) had marked differential hypothermic and ataxic responses, with minimal body temperature responses to L: -baclofen but significant ataxia on the rotarod observed.

CONCLUSION

There is differential genetic control on specific GABA B receptor populations that mediate hypothermia and ataxia. Further, these studies demonstrate that background strain is an important determinant of GABA B receptor mediated responses, and that hypothermic and ataxic responses may be influenced by independent genetic loci.

Contribution of the serotonin 5-HT1A receptor agonism of 8-OH-DPAT and EMD 128130 to the regulation of haloperidol-induced muscle rigidity in rats

The aim of the present study was to find out whether (+/-)-8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a prototypical 5-HT1A agonist, and (R)-(-)-2-[5-(4-fluorophenyl)-3-pyridylmethylaminomethyl]-chromane HCl (EMD 128130), a compound with serotonin 5-HT1A-agonist and dopamine D2-like antagonist properties, are able to attenuate the haloperidol-induced (1 mg/kg) muscle rigidity in rats. Muscle tone was examined using a combined mechano- and electromyographic (EMG) method that simultaneously measured the mechanical muscle resistance (MMG) of the rat's hind foot to passive movements in the ankle joint, and the EMG activity of two antagonist muscles. Both 8-OH-DPAT (0.125-0.5 mg/kg i.p.) and EMD 128130 (1-10 mg/kg i.p.) dose-dependently decreased the haloperidol-enhanced MMG to passive movements, as well as the tonic and the long-latency reflex EMG activities. Provided these results can be extrapolated to humans, the efficacy of EMD 128130 in relieving the haloperidol-induced muscle rigidity supports the concept that novel antipsychotics with 5-HT1A agonist and dopamine D2 antagonist activities should have a favourable extrapyramidal side-effect profile.

Therapeutic potential of cannabinoids in CNS disease

The major psychoactive constituent of Cannabis sativa, delta(9)-tetrahydrocannabinol (delta(9)-THC), and endogenous cannabinoid ligands, such as anandamide, signal through G-protein-coupled cannabinoid receptors localised to regions of the brain associated with important neurological processes. Signalling is mostly inhibitory and suggests a role for cannabinoids as therapeutic agents in CNS disease where inhibition of neurotransmitter release would be beneficial. Anecdotal evidence suggests that patients with disorders such as multiple sclerosis smoke cannabis to relieve disease-related symptoms. Cannabinoids can alleviate tremor and spasticity in animal models of multiple sclerosis, and clinical trials of the use of these compounds for these symptoms are in progress. The cannabinoid nabilone is currently licensed for use as an antiemetic agent in chemotherapy-induced emesis. Evidence suggests that cannabinoids may prove useful in Parkinson's disease by inhibiting the excitotoxic neurotransmitter glutamate and counteracting oxidative damage to dopaminergic neurons. The inhibitory effect of cannabinoids on reactive oxygen species, glutamate and tumour necrosis factor suggests that they may be potent neuroprotective agents. Dexanabinol (HU-211), a synthetic cannabinoid, is currently being assessed in clinical trials for traumatic brain injury and stroke. Animal models of mechanical, thermal and noxious pain suggest that cannabinoids may be effective analgesics. Indeed, in clinical trials of postoperative and cancer pain and pain associated with spinal cord injury, cannabinoids have proven more effective than placebo but may be less effective than existing therapies. Dronabinol, a commercially available form of delta(9)-THC, has been used successfully for increasing appetite in patients with HIV wasting disease, and cannabinoid receptor antagonists may reduce obesity. Acute adverse effects following cannabis usage include sedation and anxiety. These effects are usually transient and may be less severe than those that occur with existing therapeutic agents. The use of nonpsychoactive cannabinoids such as cannabidiol and dexanabinol may allow the dissociation of unwanted psychoactive effects from potential therapeutic benefits. The existence of other cannabinoid receptors may provide novel therapeutic targets that are independent of CB(1) receptors (at which most currently available cannabinoids act) and the development of compounds that are not associated with CB(1) receptor-mediated adverse effects. Further understanding of the most appropriate route of delivery and the pharmacokinetics of agents that act via the endocannabinoid system may also reduce adverse effects and increase the efficacy of cannabinoid treatment. This review highlights recent advances in understanding of the endocannabinoid system and indicates CNS disorders that may benefit from the therapeutic effects of cannabinoid treatment. Where applicable, reference is made to ongoing clinical trials of cannabinoids to alleviate symptoms of these disorders.

Expression of NMDA receptor subunit mRNAs in neurochemically identified projection and interneurons in the human striatum

N-methyl-D-aspartate (NMDA) receptors are composed of subunits from two families: NR1 and NR2. We used a dual-label in situ hybridization technique to assess the levels of NR1 and NR2A-D messenger ribonucleic acid (mRNA) expressed in projection neurons and interneurons of the human striatum. The neuronal populations were identified with digoxigenin-tagged complementary RNA probes for preproenkephalin (ENK) and substance P (SP) targeted to striatal projection neurons, and somatostatin (SOM), glutamic acid decarboxylase 67 kD (GAD(67)), and choline acetyltransferase (ChAT) targeted to striatal interneurons. Intense NR1 signals were found over all striatal neurons. NR2A signals were high over GAD(67)-positive neurons and intermediate over SP-positive neurons. ENK-positive neurons displayed low NR2A signals, whereas ChAT- and SOM-positive neurons were unlabeled. NR2B signals were intense over all neuronal populations in striatum. Signals for NR2C and NR2D were weak. Only ChAT-positive neurons displayed moderate signals, whereas all other interneurons and projection neurons were unlabeled. Moderate amounts of NR2D signal were detected over SOM- and ChAT-positive neurons; GAD(67)- and SP-positive striatal neurons displayed low and ENK-positive neurons displayed no NR2D hybridization signal. These data suggest that all human striatal neurons have NMDA receptors, but different populations have different subunit compositions that may affect function as well as selective vulnerability.

Brain cannabinoid systems as targets for the therapy of neurological disorders

Unprecedented developments in cannabinoid research within the past decade include discovery of a brain (CB1) and peripheral (CB2) receptor; endogenous ligands, anandamide, and 2-arachidonylglycerol; cannabinoid drug-induced partial and inverse agonism at CB1 receptors, antagonism of NMDA receptors and glutamate, and antioxidant activity; and preferential CB1 receptor localization in areas subserving spasticity, pain, abnormal involuntary movements, seizures, and amnesia. These endogenous structures and chemicals and mechanisms are potentially new pathophysiologic substrates, and targets for novel cannabinoid treatments, of several neurological disorders.

Regional and age specific effects of zolpidem microinfusions in the substantia nigra on seizures

GABAergic (gamma-aminobutyric acid) transmission in the substantia nigra pars reticulata (SNR) is critical for seizure control. The SNR effects on seizures are site-specific within the SNR and developmentally regulated. These age- and site-specific effects may be due to differential regional distribution and functionality of SNR GABA(A) receptor sites. We investigated the role of GABA/benzodiazepine (BZD) receptors in the SNR in the control of seizures as a function of age. In adult rats, we determined the effects of bilateral zolpidem (an agonist of the BZD1 receptor site) microinfusions in the anterior or in the posterior SNR (SNRanterior or SNRposterior, respectively) on flurothyl-induced clonic and tonic-clonic seizures. In SNRanterior, zolpidem microinfusions were anticonvulsant but ineffective in SNRposterior against clonic seizures. Microinfusions of zolpidem in SNRposterior or above SNR, did not alter the threshold to clonic seizures. SNR microinfusions of zolpidem did not alter the threshold to tonic-clonic flurothyl-induced seizures. In 15 day old (PN 15) rats, the SNR microinfusions of zolpidem had anticonvulsant effects on clonic and tonic-clonic seizures. There was no regional specificity. Microinfusions of zolpidem above the SNR, did not alter the threshold to clonic or tonic-clonic seizures. Our data demonstrate that the BZD1 binding sites are involved in the SNR control of flurothyl seizures in adult and PN 15 male rats.

Alprazolam dependence prevented by substituting with the beta-carboline abecarnil

Abrupt termination of the treatment of humans with benzodiazepines (BDZs) leads to a rapid onset of discontinuation syndrome characterized by anxiety, muscle spasms, and occasionally convulsions. For this reason, it is recommended in clinical practice to reduce the dose of the BDZs gradually at the end of treatment. Nevertheless, many clinicians report signs of dependence even during gradual reduction of doses (tapering) of the BDZs in a large proportion of patients. Thus, there is considerable interest in discovering means of weaning patients away from BDZs without the risk of discontinuation syndrome. In the present study, mice withdrawn from chronic treatment with alprazolam showed anxiety, muscle rigidity, and seizures between days 1 and 28 after termination of the treatment. Replacement of alprazolam with the beta-carboline abecarnil for 7 days prevented the occurrence of the signs of dependence. In contrast, substitution of the beta-carboline antagonist ethyl-5-isopropoxy-4-methyl-beta-carboline-3-carboxylate (ZK93426) for alprazolam worsened the discontinuation syndrome. Replacement therapy with abecarnil after long-term treatment with the BDZs offers a novel method for rapid tapering.

Tizanidine. A review of its pharmacology, clinical efficacy and tolerability in the management of spasticity associated with cerebral and spinal disorders

The central alpha 2 adrenoceptor agonist tizanidine is a myotonolytic agent used in the treatment of spasticity in patients with cerebral or spinal injury. Wide interpatient variability in the effective plasma concentrations of tizanidine means that the optimal dosage must be titrated over 2 to 4 weeks for each patient (dosages of 2 to 36 mg/day have been used in clinical trials). Maximum effects occur within 2 hours of administration. Antispastic efficacy has been demonstrated for tizanidine in placebo-controlled trials, with reduction in mean muscle tone scores of 21 to 37% versus 4 to 9% for patients receiving placebo. Improvement in muscle tone occurred in 60 to 82% of tizanidine recipients, compared with 60 to 65% of baclofen and 60 to 83% of diazepam recipients. Spasm frequency and clonus are also reduced by tizanidine. The most common adverse effects associated with tizanidine are dry mouth and somnolence/drowsiness. Muscle strength, as assessed by objective means, appears not to be adversely affected by tizanidine and subjective muscle weakness is reported less often by tizanidine recipients than by those receiving baclofen or diazepam. Global tolerability was assessed as good to excellent in 44 to 100% of patients receiving tizanidine, compared with 38 to 90% of baclofen and 20 to 54% of diazepam recipients. In conclusion, tizanidine is an antispastic agent with similar efficacy to that of baclofen and a more favourable tolerability profile. While drowsiness is a frequently reported adverse effect with both agents, subjective muscle weakness appears to be less of a problem with tizanidine than with baclofen. Tizanidine, therefore, appears to be an attractive therapeutic alternative for patients with spasticity associated with cerebral or spinal damage.

Management of chronic pain. Part I

Chronic pain is associated with substantial psychosocial and economic stress, coupled with functional loss and various levels of vocational dysfunction. The role of a pain center is to focus on chronic pain in a multidisciplinary, comprehensive manner, providing the patient with the most effective opportunity to manage his or her chronic disease syndrome. This article focuses on methods to manage many types of chronic pain and describes a broad range of pharmacologic and nonpharmacologic interventions and options available to the patient. Part I of this two-part monograph describes pharmacotherapeutic interventions and regional nerve blocks. Part II focuses on psychologic assessment and treatment and physical therapy. A multimodal management strategy offers patients the greatest improvement potential for specific chronic pain syndromes. Cognitive and behavioral therapies and physical therapies are described. This combination of therapies may provide patients with the skills and knowledge needed to increase their sense of control over pain. The integration of appropriate pharmacotherapeutic regimens, neural blockades, physical therapy, and psychologic techniques maximizes a patient's effectiveness in dealing with chronic pain. Three case studies are presented in Part II.

Actions of glycine on non-dopaminergic neurons of the rat substantia nigra

The effects of glycine on non-dopaminergic cells in rat substantia nigra pars compacta and pars reticulata maintained in vitro were investigated using intracellular recording techniques. Glycine, superfused at a concentration between 30 microM and 1 mM, reversibly blocked the spontaneous firing of these neurons. The inhibition of firing discharge was associated with a hyperpolarization of the membrane (potassium acetate-filled electrodes) and an increase in conductance. Under voltage-clamp experiments (holding potential between -57 and -65 mV), glycine produced an outward response which reversed polarity at about -74 mV. However, when the recording electrodes were filled with KCl, the glycinergic response was mainly depolarizing/inward and reversed at about -43 mV. Thus, it appeared to be due to an increase in chloride permeability. Furthermore, the effects of glycine were reversibly antagonized by strychnine (between 300 nM and 1 microM). Our findings demonstrate that glycine is a potent inhibitory agent on non-dopaminergic cells of the substantia pars compacta and par reticulata that acts by activating strychnine-sensitive receptors.

Kindling increases the sensitivity of rats to adverse effects of certain antiepileptic drugs

Development of novel antiepileptic drugs (AEDs) requires determining the margin between the desired anticonvulsant effect and undesired adverse effects (AE) (therapeutic index). For this purpose, drug-induced "minimal neurological deficits" (e.g., motor dysfunctions) are commonly quantified by simple tests, such as the rotarod test, in normal, i.e., nonepileptic animals. However, increasing evidence shows that chronic brain dysfunction associated with epilepsy may increase susceptibility to the AE of certain AEDs, e.g., N-methyl-D-aspartate (NMDA) receptor antagonists. The increased AE potential of such investigational drugs can be predicted by using kindled rats instead of normal rodents in preclinical drug evaluation studies. In the present experiments, we wished to determine whether kindled rats also exhibit an altered susceptibility to neurological adverse effects of standard AEDs, i.e., carbamazepine (CBZ), phenobarbital (PB), valproate (VPA), and diazepam (DZP). Abecarnil, a novel benzodiazepine (BZD) receptor agonist, was included in the study for comparison. All drugs were administered in diverse doses in kindled and nonkindled rats, and all behavioral alterations were scored in the cage and open field. Furthermore, the rotarod test was used to detect and quantify motor impairment induced by drug treatments. Kindled rats were more susceptible than nonkindled rats to motor impairment (ataxia and/or rotarod failures) induced by high doses of AEDs, although differences were noted between the drugs tested. VPA was the only drug that induced stereotyped behavior; it was much more potent in this respect in kindled than nonkindled rats. Abecarnil did not differ substantially in its AE in either subgroup of animals. Our data indicate that epileptogenesis induced by kindling renders the brain more susceptible to certain AE of AEDs.(ABSTRACT TRUNCATED AT 250 WORDS)

Gonadectomy and sex modulate spontaneous activity of substantia nigra pars reticulata neurons without modifying GABA/benzodiazepine responsiveness

Gonadal steroid hormones or their derivatives can alter the GABA receptor complex and GABA-mediated responses. This study examines the influences of the in vivo gonadal steroid milieu on neuronal responses to GABA and benzodiazepine agonists in the substantia nigra pars reticulata (SNr) of rats. Spontaneous activity and microiontophoretic sensitivity to GABA of single SNr neurons were analyzed in chloral hydrate anesthetized intact male, intact female, orchidectomized male, and ovariectomized female rats using extracellular electrophysiological techniques. Benzodiazepine responses in each hormone group were assessed as 1) the ability of the iontophoretically applied midazolam to enhance GABA sensitivity and 2) the ability of systemically administered diazepam to decrease SNr firing rate. The results indicate that neither sex nor castration modified GABA sensitivity or benzodiazepine responsiveness in the SNr. However, a heightened level of basal SNr activity was observed in males compared to orchidectomized, intact female, or ovariectomized rats. Elevated SNr activity was also observed in males compared with other hormone groups following iontophoretic application of the GABA antagonist SR95531, suggesting that this augmentation in firing rate may be independent of nigral GABAergic control. Regulation of in vivo spontaneous SNr activity may be associated with gonad-related influences on the nigrostriatal system, but appears unrelated to intrinsic alterations in GABA/benzodiazepine responses in this area.