Baclofen's suppression of epileptiform-like activity: a retinal model

The influence of established and new antiepileptic drugs on visual perception. 1. A placebo-controlled, double-blind, single-dose study in healthy volunteers

The influence of single oral dosages of carbamazepine (CBZ), valproic acid, vigabatrin (VGB), lamotrigine (LTG), gabapentin (GBP), and losigamone (LSG) on visual perception was investigated in ten healthy volunteers according to a double-blind, placebo-controlled, cross-over study design. The test battery comprised visual acuity, the Lanthony-D-15-désaturé colour perception test, increment, postadaptation and transient tritanopia threshold measurements, perception threshold assessment for monochromatic and chromatic gaussian dots, monochromatic gratings and gratings of differing spatial frequency, and critical flicker fusion tests with various stimuli. The only consistent and partly significant effects were seen after VGB and GBP. After VGB, increment, postadaptation and transient tritanopia thresholds and the critical flicker fusion increased, whereas GBP led to a somewhat converse profile. The other tests were not influenced consistently by any antiepileptic drug (AED). We conclude that: (i) gamma-amino-butyric acid-(GABA)-related properties as under the prototype drug VGB result in specific alterations of the transient tritanopia phenomenon which is consistent with the physiological hypothesis for this retinal paradigm based on extracellular recordings in primates. The possible mechanisms why VGB improved critical flicker fusion as the only AED in this trial are discussed. The profile of GBP indicates a unique mechanism of action. We have not observed specific influences on visual perception under AEDs which act mainly via alterations of ion membrane conductance. The transient tritanopia and flicker fusion paradigms we used appear to be promising to investigate antiepileptic drugs with hitherto unknown modes of actions in human noninvasively.

The influence of established and new antiepileptic drugs on visual perception. II. A controlled study in patients with epilepsy under long-term antiepileptic medication

In this study, we investigated visual performance under chronic antiepileptic drug treatment. Patients were under carbamazepine (CBZ) (n = 18), valproic acid (VPA) (n = 9), CBZ and vigabatrin (VGB) (n = 4), CBZ and gabapentin (GBP) (n = 8), and under CBZ and topiramate (TPR) (n = 6), respectively. Seven untreated patients with epilepsy and 42 healthy volunteers served as controls. The test battery comprised the Lanthony-D15-désaturé colour perception test, increment, postadaptation and transient tritanopia (TT) threshold measurements, visual perception threshold assessments for monochromatic and chromatic gratings and gaussian dots, and critical flicker fusion (CFF) tests. No differences were seen between naive patients and healthy controls. Patients under drug treatment always showed alterations of visual perception. Postadaptation and TT thresholds were altered under each drug regimen after short delays between switching off the adaptation light and switching on the blue test light. Threshold elevations were maximum under the combination of CBZ and TPR and lowest under CBZ and GBP. Consistent impairment of the CFF was seen under combined CBZ and TPR whereas VPA as well as combined CBZ and VGB led to ameliorations the mechanisms of which are discussed. The other tests were less sensitive. In conclusion, alterations of visual function were apparent under chronic antiepileptic drug treatment both with established and new agents. However, it may be difficult to distinguish between effects based on specific modes of action and nonspecific retino- and neurotoxicity.

The physiology of GABAB receptors in the vertebrate retina

Writing a chapter on retinal GABAB receptors is premature, as evidenced by the paucity of citations more than two years old. Despite that, this area of retinal pharmacology has made significant strides and, although it is a story without an ending, it has had an exciting beginning. To date, the experiments indicate that horizontal cell feedback to cones is mediated, at least in part, by the GABAB receptor system which probably regulates a potassium conductance. In the inner retina, GABAB receptors are found on bipolar cells, amacrines, and ganglion cells. Here, the actions are a subtle regulation of channel conductance, but the effects are a dramatic reorganization of a fundamental coding property of the retina, namely the distinction between tonic and phasic responses to light. In both the distal and proximal retina, the GABAB receptor does not appear to work alone, but instead works in concert with the GABAA receptor. The full significance of these interactions has yet to be determined. Although the discovery of the GABAB receptor has led to the resolution of several retinal mysteries, the case is far from closed. At this juncture, what can be said is that the GABAB receptor represents a unique and ubiquitous system that reveals the power of regulating calcium and potassium conductances, as opposed to the more familiar properties of the glutamate/acetylcholine regulated cationic conductances or the GABAA/glycine controlled chloride channels.