Promiscuous G-protein activation by the calcium-sensing receptor

The human calcium-sensing receptor (CaSR) detects fluctuations in the extracellular Ca2+ concentration and maintains Ca2+ homeostasis1,2. It also mediates diverse cellular processes not associated with Ca2+ balance3-5. The functional pleiotropy of CaSR arises in part from its ability to signal through several G-protein subtypes6. We determined structures of CaSR in complex with G proteins from three different subfamilies: Gq, Gi and Gs. We found that the homodimeric CaSR of each complex couples to a single G protein through a common mode. This involves the C-terminal helix of each Gα subunit binding to a shallow pocket that is formed in one CaSR subunit by all three intracellular loops (ICL1-ICL3), an extended transmembrane helix 3 and an ordered C-terminal region. G-protein binding expands the transmembrane dimer interface, which is further stabilized by phospholipid. The restraint imposed by the receptor dimer, in combination with ICL2, enables G-protein activation by facilitating conformational transition of Gα. We identified a single Gα residue that determines Gq and Gs versus Gi selectivity. The length and flexibility of ICL2 allows CaSR to bind all three Gα subtypes, thereby conferring capacity for promiscuous G-protein coupling.

GPCR kinase subtype requirements for arrestin-2 and -3 translocation to the cannabinoid CB1 receptor and the consequences on G protein signalling

Arrestins are key negative regulators of G Protein-Coupled Receptors (GPCRs) through mediation of G protein desensitisation and receptor internalisation. Arrestins can also contribute to signal transduction by scaffolding downstream signalling effectors for activation. GPCR kinase (GRK) enzymes phosphorylate the intracellular C-terminal domain, or intracellular loop regions of GPCRs to promote arrestin interaction. There are seven different GRK subtypes, which may uniquely phosphorylate the C-terminal tail in a type of 'phosphorylation barcode,' potentially differentially contributing to arrestin translocation and arrestin-dependent signalling. Such contributions may be exploited to develop arrestin-biased ligands. Here, we examine the effect of different GRK subtypes on the ability to promote translocation of arrestin-2 and arrestin-3 to the cannabinoid CB1 receptor (CB1) with a range of ligands. We find that most GRK subtypes (including visual GRK1) can enhance arrestin-2 and -3 translocation to CB1, and that GRK-dependent changes in arrestin-2 and arrestin-3 translocation were broadly shared for most agonists tested. GRK2/3 generally enhanced arrestin translocation more than the other GRK subtypes, with some small differences between ligands. We also explore the interplay between G protein activity and GRK2/3-dependent arrestin translocation, highlighting that high-efficacy G protein agonists will cause GRK2/3 dependent arrestin translocation. This study supports the hypothesis that arrestin-biased ligands for CB1 must engage GRK5/6 rather than GRK2/3, and G protein-biased ligands must have inherently low efficacy.

A Novel "Activation Switch" Motif Common to All Aminergic Receptors

Aminergic receptors are G protein-coupled receptors (GPCRs) that transduce signals from small endogenous biogenic amines to regulate intracellular signaling pathways. Agonist binding in the ligand binding pocket on the extracellular side opens and prepares a cavity on the intracellular face of the receptors to interact with and activate G proteins and β-arrestins. Here, by reviewing and analyzing all available aminergic receptor structures, we seek to identify activation-related conformational changes that are independent of the specific scaffold of the bound agonist, which we define as "activation conformational changes" (ACCs). While some common intracellular ACCs have been well-documented, identifying common extracellular ACCs, including those in the ligand binding pocket, is complicated by local adjustments to different ligand scaffolds. Our analysis shows no common ACCs at the extracellular ends of the transmembrane helices. Furthermore, the restricted access to the ligand binding pocket identified previously in some receptors is not universal. Notably, the Trp6.48 toggle switch and the Pro5.50-Ile3.40-Phe6.44 (PIF) motif at the bottom of the ligand binding pocket have previously been proposed to mediate the conformational consequences of ligand binding to the intracellular side of the receptors. Our analysis shows that common ACCs in the ligand binding pocket are associated with the PIF motif and nearby residues, including Trp6.48, but fails to support a shared rotamer toggle associated with activation. However, we identify two common rearrangements between the extracellular and middle subsegments, and propose a novel "activation switch" motif common to all aminergic receptors. This motif includes the middle subsegments of transmembrane helices 3, 5, and 6 and integrates both the PIF motif and Trp6.48.

Methods for automating the analysis of live-cell single-molecule FRET data

Single-molecule FRET (smFRET) is a powerful imaging platform capable of revealing dynamic changes in the conformation and proximity of biological molecules. The expansion of smFRET imaging into living cells creates both numerous new research opportunities and new challenges. Automating dataset curation processes is critical to providing consistent, repeatable analysis in an efficient manner, freeing experimentalists to advance the technical boundaries and throughput of what is possible in imaging living cells. Here, we devise an automated solution to the problem of multiple particles entering a region of interest, an otherwise labor-intensive and subjective process that had been performed manually in our previous work. The resolution of these two issues increases the quantity of FRET data and improves the accuracy with which FRET distributions are generated, increasing knowledge about the biological functions of the molecules under study. Our automated approach is straightforward, interpretable, and requires only localization and intensity values for donor and acceptor channel signals, which we compute through our previously published smCellFRET pipeline. The development of our automated approach is informed by the insights of expert experimentalists with extensive experience inspecting smFRET trajectories (displacement and intensity traces) from live cells. We test our automated approach against our recently published research on the metabotropic glutamate receptor 2 (mGluR2) and reveal substantial similarities, as well as potential shortcomings in the manual curation process that are addressable using the algorithms we developed here.

Cannabinoid 1 (CB1 ) receptor arrestin subtype-selectivity and phosphorylation dependence

BACKGROUND AND PURPOSE

Arrestin or G protein bias may be desirable for novel cannabinoid therapeutics. Arrestin-2 and arrestin-3 translocation to CB₁ receptor have been suggested to mediate different functions that may be exploited with biased ligands. Here, the requirement of a recently described phosphorylation motif 'pxxp' (where 'p' denotes phosphorylatable serine or threonine and 'x' denotes any other amino acid) within the CB₁ receptor C-terminus for interaction with different arrestin subtypes was examined.

EXPERIMENTAL APPROACH

Site-directed mutagenesis was conducted to generate nine different phosphorylation-impaired CB₁ receptor C-terminal mutants. Bioluminescence resonance energy transfer (BRET) was employed to measure arrestin-2/3 translocation and G protein dissociation of a high efficacy agonist for each mutant. Immunocytochemistry was used to quantify receptor expression.

KEY RESULTS

The effects of each mutation were shared for arrestin-2 and arrestin-3 translocation to CB₁ receptor pxxp motifs are partially required for arrestin-2/3 translocation, but translocation was not completely inhibited until all phosphorylation sites were mutated. The rate of arrestin translocation was reduced with simultaneous mutation of S425 and S429. Desensitisation of G protein dissociation was inhibited in different mutants proportional to the extent of their respective loss of arrestin translocation.

CONCLUSIONS AND IMPLICATIONS

These data do not support the existence of an 'essential' pxxp motif for arrestin translocation to CB₁ receptor. These data also identify that arrestin-2 and arrestin-3 have equivalent phosphorylation requirements within the CB₁ receptor C-terminus, suggesting arrestin subtype-selective biased ligands may not be viable and that different regions of the C-terminus contribute differently to arrestin translocation.

Putative Synthetic Cannabinoids MEPIRAPIM, 5F-BEPIRAPIM (NNL-2), and Their Analogues Are T-Type Calcium Channel (CaV3) Inhibitors

Synthetic cannabinoid receptor agonists (SCRAs) are a large and growing class of new psychoactive substances (NPSs). Two recently identified compounds, MEPIRAPIM and 5F-BEPIRAPIM (NNL-2), have not been confirmed as agonists of either cannabinoid receptor subtype but share structural similarities with both SCRAs and a class of T-type calcium channel (Ca_V3) inhibitors under development as new treatments for epilepsy and pain. In this study, MEPIRAPIM and 5F-BEPIRAPIM and 10 systematic analogues were synthesized, analytically characterized, and pharmacologically evaluated using in vitro cannabinoid receptor and Ca_V3 assays. Several compounds showed micromolar affinities for CB₁ and/or CB₂, with several functioning as low potency agonists of CB₁ and CB₂ in a membrane potential assay. 5F-BEPIRAPIM and four other derivatives were identified as potential Ca_V3 inhibitors through a functional calcium flux assay (>70% inhibition), which was further confirmed using whole-cell patch-clamp electrophysiology. Additionally, MEPIRAPIM and 5F-BEPIRAPIM were evaluated in vivo using a cannabimimetic mouse model. Despite detections of MEPIRAPIM and 5F-BEPIRAPIM in the NPS market, only the highest MEPIRAPIM dose (30 mg/kg) elicited a mild hypothermic response in mice, with no hypothermia observed for 5F-BEPIRAPIM, suggesting minimal central CB₁ receptor activity.

Defining Steric Requirements at CB1 and CB2 Cannabinoid Receptors Using Synthetic Cannabinoid Receptor Agonists 5F-AB-PINACA, 5F-ADB-PINACA, PX-1, PX-2, NNL-1, and Their Analogues

Synthetic cannabinoid receptor agonists (SCRAs) are a diverse class of new psychoactive substances (NPS). They commonly comprise N-alkylated indole, indazole, or 7-azaindole scaffolds with amide-linked pendant amino acid groups. To explore the contribution of the amino acid side chain to the cannabinoid pharmacology of SCRA NPS, a systematic library of side chain-modified SCRAs was prepared based on the recent detections of amino acid derivatives 17 (5F-AB-PINACA), 18 (5F-ADB-PINACA), 15 (PX-1), 19 (PX-2), and 20 (NNL-1). In vitro binding affinities and functional activities at cannabinoid type 1 and 2 receptors (CB₁ and CB₂, respectively) were determined for all the library members using radioligand competition experiments and a fluorescence-based membrane potential assay. Binding affinities and functional activities varied widely across compounds (K_i = 0.32 to >10 000 nM, EC₅₀ = 0.24-1259 nM), with several clear structure-activity relationships (SARs) emerging. Affinity and potency at CB₁ changed as a function of the heterocyclic core (indazole > indole > 7-azaindole) and the pendant amino acid side chain (tert-butyl > iso-propyl > iso-butyl > benzyl > ethyl > methyl > hydrogen). Ensemble docking at CB₁ revealed a clear steric basis for observed SAR trends. Interestingly, although 15 (PX-1) and 19 (PX-2) have been detected in recreational drug markets, they failed to induce centrally CB₁-mediated effects (e.g., hypothermia) in mice using radiobiotelemetry. Together, these data provide insights regarding structural contributions to the cannabimimetic profiles of 17 (5F-AB-PINACA), 18 (5F-ADB-PINACA), 15 (PX-1), 19 (PX-2), 20 (NNL-1), and other SCRA NPS.

Pharmacological selection of cannabinoid receptor effectors: Signalling, allosteric modulation and bias

The type-1 cannabinoid receptor (CB₁) is a promising drug target for a wide range of diseases. However, many existing and novel candidate ligands for CB₁ have shown only limited therapeutic potential. Indeed, no ligands are currently approved for the clinic except formulations of the phytocannabinoids Δ⁹-THC and CBD and a small number of analogues. A key limitation of many promising CB₁ ligands are their on-target adverse effects, notably including psychoactivity (agonists) and depression/suicidal ideation (inverse agonists). Recent drug development attempts have therefore focussed on altering CB₁ signalling profiles in two ways. Firstly, with compounds that enhance or reduce the signalling of endogenous (endo-) cannabinoids, namely allosteric modulators. Secondly, with compounds that probe the capability of selectively targeting specific cellular signalling pathways that may mediate therapeutic effects using biased ligands. This review will summarise the current paradigm of CB₁ signalling in terms of the intracellular transduction pathways acted on by the receptor. The development of compounds that selectively activate CB₁ signalling pathways, whether allosterically or via orthosteric agonist bias, will also be addressed.

Exploring Stereochemical and Conformational Requirements at Cannabinoid Receptors for Synthetic Cannabinoids Related to SDB-006, 5F-SDB-006, CUMYL-PICA, and 5F-CUMYL-PICA

Synthetic cannabinoid receptor agonists (SCRAs) represent the most rapidly expanding class of new psychoactive substances (NPSs). Despite the prevalence and potency of recent chiral indole-3-carboxamide SCRAs, few pharmacological data are available regarding the enantiomeric bias of these NPSs toward human CB1 and CB2 receptors. A series of homochiral indole-3-carboxamides derived from (S)- and (R)-α-methylbenzylamine and featuring variation of the 1-alkyl substituent were prepared, pharmacologically evaluated, and compared to related achiral congeners derived from cumyl- and benzylamine. Competitive binding assays demonstrated that all analogues derived from either enantiomer of α-methylbenzylamine (14-17) showed affinities for CB1 (K_i = 47.9-813 nM) and CB2 (K_i = 47.9-347 nM) that were intermediate to that of the corresponding benzylic (10-13, CB1 K_i = 550 nM to >10 μM; CB2 K_i = 61.7 nM to >10 μM) and cumyl derivatives (6-9, CB1 K_i = 12.6-21.4 nM; CB2 K_i = 2.95-24.5 nM). In a fluorometric membrane potential assay, all α-methylbenzyl analogues (excluding 17) were potent, efficacious agonists of CB1 (EC₅₀ = 32-464 nM; E_max = 89-104%) and low efficacy agonists of CB2 (EC₅₀ = 54-500 nM; E_max = 52-77%), with comparable or greater potency than the benzyl analogues and much lower potency than the cumyl derivatives, consistent with binding trends. The relatively greater affinity and potency of (S)-14-17 compared to (R)-14-17 analogues at CB1 highlighted an enantiomeric bias for this series of SCRAs. Molecular dynamics simulations provided a conformational basis for the observed differences in agonist potency at CB1 pending benzylic substitution.

Signalling profiles of a structurally diverse panel of synthetic cannabinoid receptor agonists

Synthetic cannabinoid receptor agonists (SCRAs) represent the most rapidly proliferating class of "designer drugs" or "new psychoactive substances". SCRAs offer unregulated alternatives to cannabis that evade routine drug tests, but their use is increasingly associated with severe toxicity and death worldwide. Little is currently known about SCRA molecular pharmacology, or the mechanisms underpinning their toxicity, although the effects are believed to be primarily mediated by the type 1 cannabinoid receptor (CB₁). In this study, we aimed to characterise the signalling profiles of a structurally diverse panel of novel SCRAs at CB₁. We compare SCRAs to traditional reference cannabinoids CP55,940, WIN55,212-2, and THC. The activity of the SCRAs was assessed in key receptor signalling and regulatory pathways, including cAMP production, translocation of β-arrestin 1 and 2, and receptor internalisation. The activity profiles of the ligands were also evaluated using operational analysis to identify ligand bias. Results revealed that SCRAs activities were relatively balanced in the pathways evaluated (compared to WIN55,212-2), although 5F-CUMYL-P7AICA and XLR-11 possessed partial efficacy in cAMP stimulation and β-arrestin translocation. Notably, the SCRAs showed distinct potency and efficacy profiles compared to THC. In particular, while the majority of SCRAs demonstrated robust β-arrestin translocation, cAMP stimulation, and internalisation, THC failed to elicit high efficacy responses in any of these assays. Further study is required to delineate if these pathways could contribute to SCRA toxicity in humans.

Do Toxic Synthetic Cannabinoid Receptor Agonists Have Signature in Vitro Activity Profiles? A Case Study of AMB-FUBINACA

Recreational consumption of synthetic cannabinoid receptor agonists (SCRAs) is a growing crisis in public health in many parts of the world. AMB-FUBINACA is a member of this class of drugs and is responsible for a large proportion of SCRA-related toxicity both in New Zealand and internationally. Strikingly, little is currently known about the mechanisms by which SCRAs exert toxic effects or whether their activity through the CB₁ cannabinoid receptor (the mediator of cannabinoid-related psychoactivity) is sufficient to explain clinical observations. The current study therefore set out to perform a basic molecular pharmacology characterization of AMB-FUBINACA (in comparison to traditional research cannabinoids CP55,940, WIN55,212-2, and Δ⁹-THC) in fundamental pathways of receptor activity, including cAMP inhibition, pERK activation, ability to drive CB₁ internalization, and ability to induce translocation of β-arrestins-1 and -2. Activity pathways were then compared by operational analysis to indicate whether AMB-FUBINACA may be a biased ligand. Results revealed that AMB-FUBINACA is highly efficacious and potent in all pathways assayed. However, surprisingly, bias analysis suggested that Δ⁹-THC, not AMB-FUBINACA, may be a biased ligand, with it being less active in both arrestin pathways than predicted by the activity of the other ligands tested. These data may help predict molecular characteristics of SCRAs. However, more research is required to determine whether these molecular effects manifest in toxicity at tissue/system level.

CUMYL-4CN-BINACA Is an Efficacious and Potent Pro-Convulsant Synthetic Cannabinoid Receptor Agonist

Synthetic cannabinoid receptor agonists (SCRAs) are the largest class of new psychoactive substances (NPS). New examples are detected constantly, and some are associated with a series of adverse effects, including seizures. CUMYL-4CN-BINACA (1-(4-cyanobutyl)-N-(2-phenylpropan-2-yl)indazole-3-carboxamide) is structurally related to potent, cumylamine-derived SCRAs such as 5F-CUMYL-PINACA, but is unusual due to a terminal aliphatic nitrile group not frequently encountered in SCRAs or pharmaceuticals. We report here that CUMYL-4CN-BINACA is a potent CB1 receptor agonist (K i = 2.6 nM; EC50 = 0.58 nM) that produces pro-convulsant effects in mice at a lower dose than reported for any SCRA to date (0.3 mg/kg, i.p). Hypothermic and pro-convulsant effects in mice could be reduced or blocked, respectively, by pretreatment with CB1 receptor antagonist SR141716, pointing to at least partial involvement of CB1 receptors in vivo. Pretreatment with CB2 receptor antagonist AM-630 had no effect on pro-convulsant activity. The pro-convulsant properties and potency of CUMYL-4CN-BINACA may underpin the toxicity associated with this compound in humans.

Development of selective, fluorescent cannabinoid type 2 receptor ligands based on a 1,8-naphthyridin-2-(1H)-one-3-carboxamide scaffold

Cannabinoid type 2 (CB2) receptor has been implicated in several diseases and conditions, however no CB2 receptor selective drugs have made it to market. The aim of this study was to develop fluorescent ligands as CB2 receptor tools, to enable an increased understanding of CB2 receptor expression and signalling and thereby accelerate drug discovery. Fluorescent ligands have been successfully developed for other receptors, however none with adequate subtype selectivity or imaging properties have been reported for CB2 receptor. A series of 1,8-naphthyridin-2-(1H)-one-3-carboxamides with linkers and fluorophores appended in the N1 and C3-positions were developed. Molecular modelling indicated the C3 cis-cyclohexanol-linked compounds directed the linker out of the CB2 receptor between transmembrane helices 1 and 7. Herein we report fluorescent ligand 32 (hCB2 pK i = 6.33 ± 0.02) as one of the highest affinity, selective CB2 receptor fluorescent ligands reported. Despite 32 displaying poor specific labelling of CB2 receptor, the naphthyridine scaffold with this linker remains highly promising for future development of CB2 receptor tools.

Paradox of gastric cardia: it becomes more acidic following meals while the rest of stomach becomes less acidic

INTRODUCTION

The proximal cardia region of the stomach has a high incidence of inflammation, metaplasia and neoplasia. It demonstrates less acid buffering following meals than the more distal stomach. Novel high definition pHmetry was employed to investigate acidity at the cardia under fasting conditions and in response to a meal.

METHODS

15 healthy subjects were studied. A custom-made 12-electrode pH catheter was clipped at the squamocolumnar junction with four electrodes recording proximal to and eight distal to the squamocolumnar junction. The most distal pH electrode was located at the catheter tip, and nine electrodes in the region of the squamocolumnar junction were 11 mm apart.

RESULTS

The electrode situated in the cardia 5.5 mm distal to the squamocolumnar junction differed from all other intragastric electrodes during fasting in recording minimal acidity (pH <4 = 2.2%) while all other intragastric electrodes recorded high intragastric acidity (pH <4 =or>39%) (p<0.05). The cardia also differed from the rest of the stomach, showing a marked increase in acidity in response to the meal (from 2.2% fasting to 58.4% at 60-70 min after the meal; p<0.05) while the electrodes distal to the cardia all showed a marked decrease in acidity (p<0.05). These changes in acidity at the cardia following the meal caused the gastric acidity to extend 10 mm closer to the squamocolumnar junction.

CONCLUSION

Whereas the rest of the stomach shows a marked fall in acidity on ingesting a meal, the cardia paradoxically increases in acidity to become the most acidic region throughout the postprandial period.

The management of renal failure in patients with liver disease. Experiences from a district general hospital

AIM

To compare the pre-existing management of patients with Hepatorenal Syndrome (HRS) in the gastroenterology unit of the Royal Alexandra Hospital, Renfrewshire, with the published evidence based studies.

METHOD

A retrospective, 6-month, case record review of patients diagnosed with HRS was performed. An evidence-based protocol for the diagnosis and management of HRS was introduced into the unit, to aid patient treatment prospectively. After 6 months, both compliance with the protocol, and patient outcomes were analysed.

RESULTS

Eleven patients were identified in the first part of the audit cycle, all of whom died. Seven were identified in the second cycle. Two had their renal function successfully corrected and one was discharged from hospital. Renal impairment and staging of liver disease was equivalent in both groups. The second group received more appropriate and aggressive therapy. Alcohol was the causative aetiology of liver disease in all patients.

CONCLUSIONS

Targetted therapy in patients with severe liver disease and HRS can improve renal parameters. Previous studies have shown this to be linked with improved patient outcomes.

Severe reflux disease is associated with an enlarged unbuffered proximal gastric acid pocket

BACKGROUND

An unbuffered pocket of highly acidic juice is observed at the gastric cardia after a meal in healthy subjects.

AIMS

To compare the postprandial acid pocket in healthy subjects and patients with severe reflux disease and define its position relative to anatomical and manometric landmarks.

METHODS

12 healthy subjects and 16 patients with severe reflux disease were studied. While fasted, a station pull-through was performed using a combined dual pH and manometry catheter. Position was confirmed by radiological visualisation of endoscopically placed radio-opaque clips. The pull-through study was repeated 15 min after a standardised fatty meal. Barium meal examination was performed before and following the meal.

RESULTS

A region of unbuffered acid (pH <or=2) immediately distal to the proximal gastric folds was more frequent in reflux patients (23/32 studies) than in healthy subjects (11/24) (p<0.05). This unbuffered acid pocket was longer in the reflux patients than in the healthy subjects (median length 3 cm (range 1-15) vs 2 cm (range 1-5); p<0.05). The acid pocket extended proximally as far as the proximal gastric folds in the patients but stopped a median of 1.1 cm distal in healthy subjects (p = 0.005). In healthy subjects the acid pocket occupied the distal portion of the sphincter which opened postprandially, whereas in reflux patients it corresponded to the proximal displacement of the gastric folds--that is, hiatus hernia.

CONCLUSION

This enlarged region of unbuffered postprandial acidic juice observed in the patients just below the gastro-oesophageal junction may contribute to the aetiology of severe reflux disease.

Intensive cognitive-behaviour group therapy for diagnostically heterogeneous groups of patients with psychiatric disorder

This uncontrolled study evaluates the efficacy of a combined treatment of medications (for the majority of patients) and a closed group, intensive (two-week) cognitive-behaviour therapy programme for heterogeneous groups of psychiatric patients. Five hundred and thirty-one patients at a private psychiatric clinic were included in the study. Self-report measures of depression, anxiety, self-esteem and "locus of control" were administered before and after treatment and at intervals up to one year later. Statistically and clinically significant improvements were found in all measures and these improvements were maintained up to one year. The results provide support for the efficacy of the treatment.

Massive ingestion of sustained-release verapamil with a concretion and bowel infarction

Gastric concretions secondary to a drug overdose are uncommon but potentially fatal if not recognized and treated. They may continue to release drug into the stomach for hours or days after the ingestion, complicating diagnosis and treatment. We describe the case of a man with the previously unreported association of bowel infarction with a verapamil ingestion and concretion. This case illustrates the need for a heightened awareness of this potential complication.

The Effect of Motor Preparation on Changes in H Reflex Amplitude During the Response Latency of a Warned Reaction Time Task

Monosynaptic Hoffman reflexes (H reflexes) were recorded from the soleus muscle during the response latency of a warned reaction time (RT) task that required plantarflexion of the foot. The task was done under four conditions of predictability of the response signal (RS), created by the factorial combination of foreperiod duration (1 and 4 s) and variability (fixed and variable). RT varied systematically with RS predictability and was facilitated in conditions that favored prediction of the RS. The response latency was divided into two successive phases by the onset of reflex augmentation: a premotor phase of constant reflex amplitude and a succeeding motor phase marked by progressively increasing reflex amplitude. Reflex augmentation during the motor phase was coupled more closely to the imminent movement than to the preceding signal to respond. The duration of the premotor phase was unaffected by RS predictability, but the duration of the motor phase (like RT) was shorter when the RS was more predictable. The maximum H reflex amplitude reached during the motor phase was greater when the RS was more predictable. The tonic level of H reflex amplitude during the premotor phase was greater in conditions that made prediction of the RS difficult. A second experiment showed that this difference was present throughout the foreperiod. These results suggest that conditions that favor prediction of the RS enhance motor preparation. changes in motor preparation (which affect RT) affect the processes underlying reflex amplitudes in the premotor phase and throughout the preceding foreperiod, in conditions that make prediction of the RS difficult, appear to reflect heightened general arousal.

MR imaging of syringomyelia and hydromyelia

The relative effectiveness of plain computed tomography (CT), metrizamide CT, conventional myelography, and magnetic resonance (MR) imaging was compared for the examination of cystic spinal cord lesions. Intramedullary cavities in 18 patients were demonstrated by MR imaging: cavities were uncomplicated in 13 patients, associated with spinal tumors in two, and studied after occipital craniectomy for treatment of Chiari malformation and syringomyelia in two. Cavities were shown by MR imaging in all enlarged spinal cords, but a cavity was shown in only one of four small cords. The rostral limits of the cavities were demonstrated better than were the caudal extensions. Ventricular communication was not demonstrated. Chiari malformation was shown only in cavities that did not involve the medulla. Syringes associated with tumor were indistinguishable from uncomplicated cavities, but the tumor had abnormal signal on long spin-echo sequences in two cases. Cystic cord tumor (one case) had an inhomogeneous appearance. Caudal displacement of the cerebellar hemisphere through the surgical defect associated with compression of the fourth ventricle was shown in two cases after posterior fossa craniectomy. Thirteen patients were studied with metrizamide CT also. MR imaging proved to be as accurate as metrizamide CT in the diagnosis of intramedullary cavities that result in spinal cord enlargement, but it was less sensitive in detecting cavities within normal-sized or diminished spinal cords. It had the advantage that tumor tissue could be distinguished from associated syrinx cavities by differences in signal characteristics; and cerebellar ectopia was evaluated easily on sagittal MR views.

Magnetic resonance imaging of meningiomas

Twenty-eight patients with 32 meningiomas were studied on a 0.5-T superconductive magnetic resonance (MR) imager. This common, benign treatable tumor was more clearly seen on computed tomography (CT) than MRI in 53% of cases. This is a result of poor contrast between the tumor and the adjacent brain on all spin-echo and inversion-recovery pulse sequences. Those sequences that provide the greatest anatomic detail were best for identifying this low-contrast lesion. Inversion-recovery scans in particular demonstrated the tumor as a discrete hypointense mass (relative to nearby white matter) with excellent visualization of the dural base and white matter buckling indicative of extracerebral mass effect. Other characteristic features include: a hypointense rim because of the venous capsule (66%); mottling due to hypervascularity; a well defined edema collar that demarcates the tumor from adjacent brain; and hyperostosis with thickening of the calvaria and obliteration of its normal landmarks. MRI did not demonstrate tumor calcification but did demonstrate vascular encasement, displacement, and occlusion better than CT and as well as digital venous angiography.