Plasticity of rat central inhibitory synapses through GABA metabolism

1. The production of the central inhibitory transmitter GABA (gamma-aminobutyric acid) varies in response to different patterns of activity. It therefore seems possible that GABA metabolism can determine inhibitory synaptic strength and that presynaptic GABA content is a regulated parameter for synaptic plasticity. 2. We altered presynaptic GABA metabolism in cultured rat hippocampal slices using pharmacological tools. Degradation of GABA by GABA-transaminase (GABA-T) was blocked by gamma-vinyl-GABA (GVG) and synthesis of GABA through glutamate decarboxylase (GAD) was suppressed with 3-mercaptopropionic acid (MPA). We measured miniature GABAergic postsynaptic currents (mIPSCs) in CA3 pyramidal cells using the whole-cell patch clamp technique. 3. Elevated intra-synaptic GABA levels after block of GABA-T resulted in increased mIPSC amplitude and frequency. In addition, tonic GABAergic background noise was enhanced by GVG. Electron micrographs from inhibitory synapses identified by immunogold staining for GABA confirmed the enhanced GABA content but revealed no further morphological alterations. 4. The suppression of GABA synthesis by MPA had opposite functional consequences: mIPSC amplitude and frequency decreased and current noise was reduced compared with control. However, we were unable to demonstrate the decreased GABA content in biochemical analyses of whole slices or in electron micrographs. 5. We conclude that the transmitter content of GABAergic vesicles is variable and that postsynaptic receptors are usually not saturated, leaving room for up-regulation of inhibitory synaptic strength. Our data reveal a new mechanism of plasticity at central inhibitory synapses and provide a rationale for the activity-dependent regulation of GABA synthesis in mammals.

Efficacy of background GABA uptake in rat hippocampal slices

GABA uptake is crucial for the termination of inhibitory synaptic events. In addition, GABA transporters may also control the level of diffusely distributed GABA in the extracellular space. We analysed this function by superfusing rat hippocampal slices with different concentrations of GABA. Whole-cell patch clamp recordings of CA1 pyramidal cells revealed small increases in chloride conductance at 5-10 microM GABA which increased dramatically upon addition of the GABA uptake blocker tiagabine. Tiagabine alone induced a significant chloride conductance indicating that spontaneous release of GABA in hippocampal slices is neutralized by GAT-1, the main hippocampal GABA transporter. Thus, GAT-1 clears the extracellular space in the hippocampus from diffusely distributed GABA with high efficacy.

Presence of gamma-aminobutyric acid transporter mRNA in interneurons and principal cells of rat hippocampus

After release, neurotransmitters are removed from the extracellular space by high-affinity uptake. Specific sodium-dependent transporters serve this function for the inhibitory transmitter gamma-aminobutyric acid (GABA). However, it is largely unknown to which proportion GABA is taken up by GABAergic interneurons, glia cells or principal neurons. We analyzed the distribution of mRNA for the main GABA-transporter subtype in the hippocampus, GAT-1, in adult rats. Most interneurons were strongly stained for GAT-1 mRNA, indicating re-uptake by the GABA-releasing cells. Surprisingly, prominent signals for GAT-1 were also found throughout the principal cell layers (granule and pyramidal cells). These data indicate that GABA transporters may be present in non-GABAergic projection cells of the rat hippocampus which contribute to the clearance of GABA from the extracellular space.

Acute effects of gamma-vinyl-GABA on low-magnesium evoked epileptiform activity in vitro

Vigabatrin (gamma-vinyl-GABA, VGB) is a gamma-aminobutyric acid (GABA) derivative designed to boost synaptic inhibition by inhibiting the degradation of GABA in brain tissue. Indeed, VGB shows potent anti-convulsant activity in animal models of epilepsy and in humans with complex partial seizures. However, details of the mechanism of action of VGB are not well understood and the systemic effects include possible pro-convulsant actions. We therefore analysed the effects of VGB in rat brain slices in the low-Mg(2+) model in vitro. VGB at 100 microM-5 mM showed a concentration- and time-dependent reduction of interictal-like events in the hippocampal CA1 region. Likewise, VGB suppressed epileptiform discharges in the medial entorhinal cortex (mEC), which are known to resist conventional anti-convulsants. In contrast, evoked population spikes in CA1 (which became repetitive after washout Mg(2+)) were not altered by VGB. Our data show that VGB is efficient against epileptiform discharges in temporal structures including pharmacoresistant patterns of activity. The waveform of evoked population spikes in this in vitro model is no indicator for the anti-convulsant properties of drugs.

Age-dependence of the anticonvulsant effects of the GABA uptake inhibitor tiagabine in vitro

Epileptic syndromes frequently start at childhood and therefore it is crucial to test new anticonvulsants at immature stages of the nervous system. We compared the effects of the gamma-aminobutyric acid (GABA) uptake inhibitor tiagabine [(R)-N-(4, 4-bis(3-methyl-2-thienyl)but)3-en-1-yl nipecotic acid] on low-Mg(2+)-induced epileptic discharges in brain slices from rat pups (p 5-8) and juvenile animals (p 15-20). In tissue from rat pups, tiagabine slightly reduced epileptiform activity in hippocampal area CA1 but had no effect in the entorhinal cortex. In juvenile rats, epileptiform discharges were unaffected in CA1 but suppressed by 60% in the entorhinal cortex. While tiagabine increases its efficacy with age, in-situ hybridisation and PCR analysis show that mRNA coding for the neuronal GABA-transporter GAT-1 is already present at p 5. We therefore conclude that the increasing efficacy of tiagabine during ontogenesis is due to functional maturation of GABAergic synapses rather than to up-regulation of GAT-1 expression.

Laminar difference in GABA uptake and GAT-1 expression in rat CA1

1. The axonal plexus of most hippocampal interneurons is restricted to certain strata within the target region. This lamination suggests a possible functional heterogeneity of inhibitory synapses between different interneurons and CA1 pyramidal cells. 2. We therefore compared inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in CA1 pyramidal cells, which were evoked from two stimulation sites (stratum oriens and stratum radiatum). Stimulation in stratum oriens yielded faster decaying IPSPs and IPSCs than stimulation in stratum radiatum. 3. IPSP and IPSC kinetics were regulated by GABA uptake in both layers as indicated by the prolongation of the signals under tiagabine, a GAT-1 (neuronal GABA plasma membrane transporter)-specific GABA-uptake blocker. However, the effect of tiagabine was significantly more pronounced following stimulation in stratum radiatum than in stratum oriens (prolongation of IPSC half-decay time by 167 vs. 115 %, respectively). 4. In situ hybridization with antisense mRNA for the GABA-synthesizing enzyme glutamate decarboxylase (GAD65/67) and the GABA transporter GAT-1 showed that the proportion of interneurons expressing GAT-1 was lower in stratum oriens than in stratum radiatum/lacunosum-moleculare. 5. From these functional and molecular data we conclude that the regulation of IPSP and IPSC kinetics in CA1 pyramidal cells by neuronal GABA uptake differs between layers. Our findings suggest that this laminar difference is caused by a lower expression of GAT-1 in interneurons in stratum oriens than in stratum radiatum/lacunosum-moleculare.

[Esophageal scintigraphy of Zenker's diverticula before and after diverticulotomy]

AIM

The filling and evacuation of Zenker's diverticula were scintigraphically examined before and after operation to quantify their functional relevance. These results were correlated with the symptoms of the patients and the findings of the barium swallow x-ray examination using cineradiography.

METHODS

Sequential and static esophageal scintigraphies were performed in 17 patients with Zenker's diverticulum before and after laser surgical diverticulotomy. We used a gamma camera system in 45 degrees LAO-position after application of 15 ml of tea which was marked with 99mTc-DTPA. Filling and evacuation of the diverticulum were expressed in proportion to the administered activity. Relative volumes of the diverticula were obtained from cineradiography by using the height of the neighbouring cervical vertebra, and the clinical symptoms were divided into 4 groups.

RESULTS

Zenker's diverticula could be verified visually and quantitatively by scintigraphy. The precise temporal course of the reduction of activity in the diverticulum was exactly determined. The scintigraphic retentions correlated with the x-ray volumes with a coefficient ranging from 0.55 to 0.85. Clinical symptoms also were not very closely related to scintigraphic and x-ray findings, respectively.

CONCLUSION

The esophageal scintigraphy allows quantification of the filling and evacuation of Zenker's diverticula, thus it is suitable for objectivization of the functional relevance of the diverticula. That's why the esophageal scintigraphy should be taken to the diagnosis of diverticula in addition to the clinic and the x-ray examinations. The method is especially useful to evaluate the results after diverticulotomy.

[MRI-controlled biopsies]

Biopsies were the first "intervention" under MR guidance. After initial difficulties concerning ferromagnetic biopsy instruments and the design of MR scanners, the latest technological improvements rendered MR guidance for biopsies more feasible. In this article we illustrate present-day clinical experience in the field of abdominal, breast and bone biopsy. Important aspects regarding the different designs of "interventional" MR scanners and the visualization of instruments for biopsy are discussed.

MR-guided placement of a temporary vena cava filter: technique and feasibility

The purpose of this paper was to investigate the feasibility of MR-guided insertion of a temporary vena cava filter on an open low-field imager. In vivo procedures were performed on four anesthetized pigs using a common nonferromagnetic temporary vena cava filter and a special prototype guidewire developed for vascular interventions guided by low-field MRI. Breath-hold spoiled gradient-echo sequences (fast low-angle shot [FLASH]) with flow compensation were used for position monitoring of the passively visualized intravascular devices. Using the described technique and equipment, all steps of the procedure were feasible in the MR unit. Practicability of the procedure seems to be sufficient for clinical purposes but was inferior compared to the conventional technique of filter placement.

Ösophagusszintigraphie von Zenkerschen Divertikeln vor und nach Divertikulotomie

Ziel: Das Füllungs- und Entleerungsverhalten von Zenkerschen Divertikeln wurde vor und nach Operation szintigraphisch untersucht, um deren funktionelle Relevanz zu quantifizieren. Die Ergebnisse wurden mit den Beschwerden der Patienten und den Befunden der Röntgenkinematographie korreliert. Methoden: Bei 17 Patienten mit Zenkerschem Divertikel wurden vor und nach laserchirurgischer Divertikulotomie Öso-phagussequenzszintigraphien und statische Ösophagusszintigraphien durchgeführt. Die Untersuchungen erfolgten an einer Großfeld-Gamma-kamera in 45° LAO-Position nach Applikation von 15 ml Tee, der mit 99mTc-DTPA markiert war. Divertikelfüllung und -entleerung wurden im Verhältnis zur applizierten Aktivität ausgedrückt. Die relativen Divertikel-volumina wurden aus der Röntgenkinematographie anhand der Höhe des benachbarten Halswirbelkörpers bestimmt, die klinischen Beschwerden nach 4 Schweregraden beurteilt. Ergebnisse: Die Zenkerschen Divertikel ließen sich szintigraphisch visuell und quantitativ nachweisen. Der genaue zeitabhängige Verlauf der Aktivitätsabnahme im Divertikel wurde jeweils exakt bestimmt. Zwischen den szintigraphischen Tracerretentionen und den röntgenologischen Volumina bestand eine geringe bis mäßige Korrelation mit Koeffizienten von 0,55 bis 0,85. Kein sehr enger Zusammenhang bestand weiterhin zwischen der Ausprägung der klinischen Symptome einerseits und den szintigraphischen und röntgenologischen Ergebnissen andererseits. Schlußfolgerung: Die Ösophagusszintigraphie erlaubt die Quantifizierung der Füllung und Entleerung von Zenkerschen Divertikeln und ist somit zur Objektivierung der funktionellen Relevanz der Divertikeln geeignet. Sie sollte deshalb in Ergänzung zur Klinik und zu den röntgenologischen Verfahren in der Divertikeldiagnostik eingesetzt werden. Der besondere Wert der Methode liegt in der postoperativen Kontrolle nach Divertikulotomie.

Visualization of magnetic resonance-compatible needles at 1.5 and 0.2 Tesla

PURPOSE

For two types of passively visualizable magnetic resonance (MR)-compatible needles, the size of susceptibility artifacts was investigated at 0.2 and 1.5 Tesla (T) and assessed regarding their suitability for needle visualization.

METHODS

Phantom trials were performed using T1-weighted spin echo (SE), turbospin echo (TSE) and gradient echo (GE) sequences and different angles beta between the needles and the main magnetic field (B0).

RESULTS

Depending on the needle angle beta and the applied pulse sequence, we found artifact diameters of 0-9.7 mm employing SE, of 1.7-9.4 mm employing TSE, and of 1.4-20.6 mm employing GE at 1.5 T. At 0.2 T, we found artifact diameters of 0-5.7 mm employing SE, of 0-6.3 mm employing TSE, and of 0-11.3 mm employing GE.

CONCLUSION

Comparing artifact sizes at 1. 5 T and 0.2 T, low field strength is superior for passive visualization of the needles tested-especially if GE imaging is performed.

[Anti-inflammatory and analgesic actions of of long-chain phenyl substituted alpha-aminoketones]

The investigated phenyl substituted alpha-amino ketones have anti-inflammatory and analgesic activities as could be shown in animal models (rats and mice). The effectiveness depends on the length of the alkyl chain. After i.p. application the most active amino ketones are ten times more effective than acetyl salicylic acid, but the activity did not reach that of indomethacin. The effective analgesic dose is ten times lower than that for anti-inflammatory action. Including the results of the toxicity the analgesic efficiency of the amino ketones could be lead to substances with high analgesic activity. After oral administration only in high doses anti-inflammatory and analgesic effectivities were found. The structural analog keto group instead of the ester or amid group leads not to the expected increase in p.o. effectivity.

[A technic of MRT-guided abdominal drainage with an open low-field magnet. Its feasibility and the initial results]

PURPOSE

To test the practicality of MRT-aided drainage using an open low-field magnet and to report on the early clinical results.

METHODS

So far seven patients have been treated (four subphrenic abscesses, two psoas abscesses and one pancreatic pseudocyst). The planning of the approach and catheter insertion were carried out under MRT control (Magnetom Open, 0.2 T). Subsequent treatment was controlled by CT and fluoroscopy. Initial puncture was carried out with a non-magnetic 18 gauge Chiba needle. The drainage catheter was introduced by Seldinger's technique in six cases and with a trocar in one patient.

RESULTS

In all seven patients drainage could be started successfully. The design of the magnet and coils permitted adequate accessibility of the patient. There were no problems in visualising the puncture needle. Controlling the position of the catheter by MRT was, however, difficult.

CONCLUSION

The first two steps in abscess drainage (planning the approach and inserting the catheter) can be carried out under MRT control. For further catheter control and observing the course of the disease we presently prefer CT or fluoroscopy.

[Technique of MRT-guided core biopsy in the abdomen using an open low-field scanner: feasibility and initial clinical results]

PURPOSE

Due to high soft-tissue contrast and multiplanar imaging capabilities MRI is an interesting modality to perform image-guided biopsies. We checked on the feasibility of MR-guided core biopsies of abdominal masses with an open low-field scanner (0.2 Tesla; vertical field axis).

METHODS

9 patients underwent MR-guided biopsies of abdominal target lesions (6 focal liver lesions, two adrenal masses, one pelvic mass). Different MR-compatible core biopsy instruments were used (needle diameters 14G-18G). MR scans during the procedures were obtained applying T1-weighted gradient echo sequences suitable for breath-hold imaging.

RESULTS

In each case, needle guidance was reliable, so that the biopsy instrument could be positioned correctly. Multiplanar imaging capabilities enabled even angled approaches to upper abdominal masses to be realized safely. The combination of magnet design and table design offered suitable access to the patient.

CONCLUSION

Using an open low-field scanner, MR-guided core biopsies of abdominal masses are practicable.

[A new laser puncture guidance device for CT- and MRT-guided punctures of the trunk]

A stereotactic device was developed to facilitate CT- or MR-guided punctures of the body. The correct needle angulation is checked by the reflection of fan-shaped red and green laser beams upon the needle. Due to its principle of function and design the device has considerable advantages: The puncture angle can be controlled in three dimensions. The freedom of movement within the operation area will not be affected. The device does not have to be permanently installed and can be employed in MRI at any field strength. Furthermore, all kinds of puncture instruments can be used.

[The von Hippel-Lindau syndrome. Its differential diagnosis from cystic kidneys in adulthood]

A 51-year-old patient with severe back pain had undergone resection of a benign cerebellar tumour when aged 15 years. In addition, polycystic kidney disease was diagnosed 24 years ago, bilateral phaeochromocytoma 2 years ago, and for 4 months before the present admission he had been on haemodialysis. The family history indicated autosomal dominant inheritance of the polycystic renal disease. His general condition was found to have deteriorated, he had pain on pressure over the upper thoracic and lower lumbar vertebrae, and the kidneys were enlarged on palpation. There were increased concentrations of calcium (3.01 mmol/l), parathormone (2.0 ng/l), carcinoembryonic antigen (13.5 micrograms/l) and TPA (69 U/l). Computed tomography demonstrated cystic and solid parts of much enlarged kidneys. Biopsy revealed a poorly differentiated clear-cell renal carcinoma. Further information concerning the previously removed brain tumour showed this to have been an haemangioblastoma of the cerebellar tonsils indicating the diagnosis of v. Hippel-Lindau disease. Nine other family members had been affected, but none had the full-blown picture of the disease. The patient died 3 weeks later from the rapidly advancing tumour. Autopsy showed the bilateral renal carcinoma, bilateral phaeochromocytoma and metastases to the sternum, femurs, vertebrae and liver.

[Digital picture archiving, processing and communication in diagnostic imaging with a Siemens Network (SIENET): initial results]

The concept of Picture Archiving and Communication System (PACS) in digital imaging offers substantial advantages: better data security, faster access to archived images, simultaneous availability of images at different sites, minimal space demand for archive medium, retrospective image post-processing opportunities and lower costs resulting from reduced film consumption and length of stay. However, practical experiences concerning reliability and efficiency of real PAC networks in radiological departments have to be made yet in order to improve network structure, software design and hardware performance. The ongoing stepwise installation and test of a Siemens PACS (SIENET) at the Department of Radiology at the Medical University of Lübeck will support the introduction of PACS concept into radiological practice.

[Densitometry in suspected preclinical osteoporosis: quantitative computerized tomography versus dual energy roentgen absorptiometry]

The bone mineral density of 85 patients with suspicion of a preclinical osteoporosis was measured at the lumbar spine by using quantitative computed tomography (QCT) in single-energy technique (SEQCT) and dual-energy technique (DEQCT) as well as by using dual-energy X-ray absorptiometry (DEXA). Additionally, the bone density of 63 of these patients was measured at the left femoral neck using DEXA. DEQCT und DEXA of the lumbar spine showed only a moderate correlation (R = 0.75) and differed considerably concerning the classification of the patients as normals or individuals with a mineral deficit when compared with age-matched normals (relative mineral deficit). The DEXA proved to be susceptible to degenerative alterations of the lumbar spine. Because of the extremely low dose of radiation and the good reproducibility the DEXA could nevertheless be recommended as a method for the long-time progress control especially for younger patients. SEQCT and DEQCT showed a very strong correlation (R = 0.98). The SEQCT with its lower dose of radiation should be sufficient for a long-time progress control and in many cases also for the initial diagnostics. Significant but only moderate correlations were found between the bone density at the femoral neck and the DEXA or DEQCT results (R = 0.68 respectively R = 0.63) for the lumbar spine, so that the linear regression did not render any useable approximations. Sufficiently exact information about the mineralization status of a certain skeletal site can only be obtained by direct measurement.