Benzodiazepines and the mammalian retina. I. Autoradiographic localisation of receptor sites and the lack of effect on the electroretinogram

Retinal and visual function in infants with non-accidental trauma and retinal hemorrhages

PURPOSE

To investigate retinal function and visual outcomes in infants with retinal hemorrhages due to non-accidental trauma (NAT).

METHODS

This is a retrospective review of full-field or multifocal electroretinogram (ERG) recordings, visual acuity in log minimum angle of resolution (logMAR), clinical status, and neuroimaging. Multifocal ERGs from the central 40° were compared to corresponding fundus imaging. Visual acuity was measured by Teller cards at follow-up. ERGs were compared to controls recorded under anesthesia.

RESULTS

Sixteen children met inclusion criteria (14 recorded during the acute phase and 2 during long-term follow-up). During the acute phase, ERGs (n = 4 full field; n = 10 multifocal ERG) showed abnormal amplitude, latency, or both in at least one eye. Ten subjects had significantly reduced responses in both eyes, 3 of which had an ERG dominated by a negative waveform (absent b-wave or P1). The remaining six subjects had responses in one eye that were near normal (≥ 50% of controls). ERGs were sometimes abnormal in local areas without hemorrhage. ERGs could be preserved in local areas adjacent to traumatic retinoschisis. Two subjects with reduced visual acuity had belated ERGs: One had an abnormal macular ERG and the other had a normal macular ERG implying cortical visual impairment. At follow-up, 10 of 14 subjects had significant visual acuity loss (≥ 0.7 age-corrected logMAR); four subjects had mild vision loss (≤ 0.5 age-corrected logMAR). Visual acuity outcome was not reliably associated with the fundus appearance in the acute phase. All subjects with a negative ERG waveform had severe vision loss on follow-up.

CONCLUSIONS

Retinal dysfunction was common during the acute phase of NAT. A near normal appearing fundus did not imply normal retinal function, and ERG abnormality did not always predict a poor visual acuity outcome. However, a negative ERG waveform was associated severe visual acuity loss. Potential artifacts of retinal hemorrhages and anesthesia could not fully account for multifocal ERG abnormalities. Retinal function can be preserved in areas adjacent to traumatic retinoschisis.

Normal clinical electroretinography parameters for poodle, Labrador retriever, Thai ridgeback, and Thai Bangkaew

The purpose of the present study was to establish normal electroretinogram (ERG) parameters using 56 normal eyes of four dog breeds common in Thailand: poodle, Labrador retriever, Thai ridgeback, and Thai Bangkaew. Standard ERG findings were bilaterally recorded using a handheld multi-species ERG unit with an ERG-jet lens electrode for 28 dogs under preanesthesia with diazepam, anesthesia with propofol, and anesthesia maintenance with isoflurane. There were significant differences in the mean values of ERG amplitudes and implicit times among the four dog breeds (p < 0.05) except for the b-wave implicit time of the photopic 30 Hz flicker response with 3 cd.s/m(2) (p = 0.610). Out of the four breeds, Thai Bangkaew had the longest implicit time (p < 0.001) of scotopic low intensity responses, b-wave of scotopic standard intensity responses (3 cd.s/m(2)), a-wave of the higher intensity response (10 cd.s/m(2)), and a-wave of the photopic single flash response (3 cd.s/m(2)). For the b/a ratio, only the ratio of the Cone response was significantly different among the different breeds. In this summary, normal ERG parameters for four dog breeds were reported. Data from the investigation supported the hypothesis that determination of breed-specific limits of normality for ERG responses is necessary for individual clinics and laboratories.

Ionotropic GABA Receptors and Distal Retinal ON and OFF Responses

In the vertebrate retina, visual signals are segregated into parallel ON and OFF pathways, which provide information for light increments and decrements. The segregation is first evident at the level of the ON and OFF bipolar cells in distal retina. The activity of large populations of ON and OFF bipolar cells is reflected in the b- and d-waves of the diffuse electroretinogram (ERG). The role of gamma-aminobutyric acid (GABA), acting through ionotropic GABA receptors in shaping the ON and OFF responses in distal retina, is a matter of debate. This review summarized current knowledge about the types of the GABAergic neurons and ionotropic GABA receptors in the retina as well as the effects of GABA and specific GABAA and GABAC receptor antagonists on the activity of the ON and OFF bipolar cells in both nonmammalian and mammalian retina. Special emphasis is put on the effects on b- and d-waves of the ERG as a useful tool for assessment of the overall function of distal retinal ON and OFF channels. The role of GABAergic system in establishing the ON-OFF asymmetry concerning the time course and absolute and relative sensitivity of the ERG responses under different conditions of light adaptation in amphibian retina is also discussed.

The effect of antiepileptic drugs on visual performance

Visual disturbances are a common side-effect of many antiepileptic drugs. Non-specific retino- and neurotoxic visual abnormalities, that are often reported with over-dosage and prolonged AED use, include diplopia, blurred vision and nystagmus. Some anticonvulsants are associated with specific visual problems that may be related to the mechanistic properties of the drug, and occur even when the drugs are administered within the recommended daily dose. Vigabatrin, a GABA-transaminase inhibitor, has been associated with bilateral concentric visual field loss, electrophysiological changes, central visual function deficits including reduced contrast sensitivity and abnormal colour perception, and morphological alterations of the fundus and retina. Topiramate, a drug that enhances GABAergic transmission, has been associated with cases of acute closed angle glaucoma, while tiagabine, a GABA uptake inhibitor, has been investigated for a potential GABAergic effect on the visual field. Only mild neurotoxic effects have been identified for patients treated with gabapentin, a drug designed as a cyclic analogue of GABA but exhibiting an unknown mechanism while carbamazepine, an inhibitor of voltage-dependent sodium channels, has been linked with abnormal colour perception and reduced contrast sensitivity. The following review outlines the visual disturbances associated with some of the most commonly prescribed anticonvulsants. For each drug, the ocular site of potential damage and the likely mechanism responsible for the adverse visual effects is described.

Effects of benzodiazepines on frog ERG

Effect of flurazepam (water-soluble benzodiazepine) on the amplitude and time course of ERG waves was investigated in superfused frog eyecups (Rana ridibunda). Flurazepam (50 and 100 microM) had inhibitory effect on the b- and d-wave amplitude, which was not accompanied with significant changes in their implicit time. Flurazepam potentiated the depressant effect of GABA (2.5 and 5 mM) on the b- and d-wave amplitude. The inhibitory effect of flurazepam was not blocked by 50 microM bicuculline (BCC), (GABA(A) antagonist), although the blocker markedly potentiated the b- and d-wave amplitude. The suppressive effect of flurazepam on the b- but not d-wave amplitude was blocked by 100 microM BCC. Our results indicate existence of functional benzodiazepine regulatory sites on GABA(A) receptors in distal frog retina.

Benzodiazepine and kainate receptor binding sites in the RCS rat retina

BACKGROUND

The effect of age and photoreceptor degeneration on the kainate subtype of glutamate receptors and on the benzodiazepine-sensitive gamma-aminobutyric acid-A receptors (GABA(A)) in normal and RCS (Royal College of Surgeons) rats were investigated.

METHODS

[(3)H]Kainate and [(3)H]flunitrazepam were used as radioligands for kainate and GABA(A)/benzodiazepine()receptors, respectively, using the quantitative receptor autoradiography technique.

RESULTS

In both normal and RCS rat retina we observed that [(3)Eta]flunitrazepam and [(3)Eta]kainate binding levels were several times higher in inner plexiform layer (IPL) than in outer plexiform layer (OPL) at all four ages studied (P17, P35, P60 and P180). Age-related changes in receptor binding were observed in normal rat retina: [(3)Eta]flunitrazepam binding showed a significant decrease of 25% between P17 and P60 in IPL,and [(3)Eta]kainate binding showed significant decreases between P17 and P35 in both synaptic layers (71% in IPL and 63% in OPL). Degeneration-related changes in benzodiazepine and kainate receptor binding were observed in RCS rat retina. In IPL, [(3)Eta]flunitrazepam and [(3)Eta]kainate binding levels were higher than in normal retina at P35 (by 24% and 86%, respectively). In OPL, [(3)Eta]flunitrazepam binding was higher in RCS than in normal retina on P35 (74%) and also on P60 (62%).

CONCLUSIONS

The results indicate that postnatal changes occur in kainate and benzodiazepine receptor binding sites in OPL and IPL of the rat retina up to 6 months of age. The data also suggest that the receptor binding changes observed in the RCS retina could be a consequence of the primary photoreceptor degeneration.

The effect of long-term use of benzodiazepines on the eye and retina

Thirty patients on long-term benzodiazepine medication were subjected to full ophthalmoscopic examination. Of these 19 (63.3%) complained of symptoms of irritation, blurred vision or difficulty in reading. None, however, had reduced visual acuities apart from two where the cause was longstanding amblyopia. Thirteen patients had some form of retinal finding, 9 macular and 4 non-macular. Of the 14 who presented for flash and pattern electroretinography (ERG), none showed any abnormality which could be ascribed to the medication, its total dose, or duration. We conclude, on the basis of the evidence from this small cohort, that long-term benzodiazepine medication has little effect upon retinal function as signalled by the ERG.

Insulin resistance syndrome: defective GABA neuromodulation as a possible hereditary pathogenetic factor (the 'GABA hypothesis')

The origin of the insulin resistance syndrome which, mostly through its cardiovascular implications, is characterized by a high incidence of death, is still practically unknown. Energy and glucose homeostasis are under the control of CNS centers and the neurotransmitter GABA modulates the activity of these centers. Alteration of the biochemical structure of GABA receptors is suggested as an interpretation of the origin of the syndrome.

Diazepam and its effects on visual fields

A patient who experienced severe visual field loss whilst taking 100 mg of diazepam that reverted to normal on cessation of the drug is described. Diazepam affects GABA inhibitory neurones and the physiology of this in the retina and visual cortex is reviewed.

Benzodiazepines and the mammalian retina. II. Actions on retinal ganglion cells

The effects of intravenously and iontophoretically applied benzodiazepines, midazolam and flurazepam and their receptor antagonist, flumazepil (RO 15 1788), on visually evoked and spontaneous activities of the mammalian retinal ganglion cells have been studied. Intravenously applied midazolam and flurazepam suppressed both light-evoked and spontaneous firing of rat optic tract fibres. They reduced both the sensitivity to light and the temporal resolution of the fibres. Flumazepil (RO-15-1788), on the other hand, enhanced both light-evoked and spontaneous firing of the optic tract fibres. It increased their light sensitivity but did not affect their temporal resolution. In the cat, iontophoretically applied benzodiazepines suppressed and flumazepil increased the receptive field centre and surround response as well as the spontaneous firing of ON-type retinal ganglion cells which mainly receive GABAergic inputs. However, these drugs did not affect the activities of the OFF-type cells which mainly receive glycinergic inputs. These results suggest not only that the action of benzodiazepines on the retinal ganglion cells is mediated by benzodiazepine receptors that are linked with GABA receptors, but also that the retinal benzodiazepine receptors receive an endogenous benzodiazepine receptor ligand.

Benzodiazepines and the mammalian retina. III. Postnatal development

Iontophoretic effects of the benzodiazepine receptor agonist, flurazepam, and antagonist, flumazepil (RO-15-1788) on the retinal ganglion cells of kittens (7-9 weeks of age) have been compared with those of adult cats (18-22 weeks of age). In the adult retina, flurazepam decreased and flumazepil increased the visually evoked and spontaneous firing of ON-, but produced no effects on the response of OFF-retinal ganglion cells. However, in the kittens retina, in which ON-cells' selectivity to GABA is not fully developed, both the visually evoked and spontaneous activities of ON- and OFF-cells were inhibited by flurazepam and enhanced by flumazepil. This suggests that postnatal development of benzodiazepine action parallels that of GABA action at the retinal ganglion cells in the cat retina.