Adrenergic effects on the corneal and intraretinal direct-current electroretinogram and on the standing potential of albino rabbit eyes

Isoflurane and ketamine:xylazine differentially affect intraocular pressure-associated scotopic threshold responses in Sprague-Dawley rats

PURPOSE

Amplitudes of electroretinograms (ERG) are enhanced during acute, moderate elevation of intraocular pressure (IOP) in rats anaesthetised with isoflurane. As anaesthetics alone are known to affect ERG amplitudes, the present study compares the effects of inhalant isoflurane and injected ketamine:xylazine on the scotopic threshold response (STR) in rats with moderate IOP elevation.

METHODS

Isoflurane-anaesthetised (n = 9) and ketamine:xylazine-anaesthetised (n = 6) rats underwent acute unilateral IOP elevation using a vascular loop anterior to the equator of the right eye. STRs to a luminance series (subthreshold to -3.04 log scotopic cd s/m²) were recorded from each eye of Sprague-Dawley rats before, during, and after IOP elevation.

RESULTS

Positive STR (pSTR) amplitudes for all conditions were significantly smaller (p = 0.0001) for isoflurane- than for ketamine:xylazine-anaesthetised rats. In addition, ketamine:xylazine was associated with a progressive increase in pSTR amplitudes over time (p = 0.0028). IOP elevation was associated with an increase in pSTR amplitude (both anaesthetics p < 0.0001). The absolute interocular differences in IOP-associated enhancement of pSTR amplitudes for ketamine:xylazine and isoflurane were similar (66.3 ± 35.5 vs. 54.2 ± 24.1 µV, respectively). However, the fold increase in amplitude during IOP elevation was significantly higher in the isoflurane- than in the ketamine:xylazine-anaesthetised rats (16.8 ± 29.7x vs. 2.1 ± 2.7x, respectively, p = 0.0004).

CONCLUSIONS

The anaesthetics differentially affect the STRs in the rat model with markedly reduced amplitudes with isoflurane compared to ketamine:xylazine. However, the IOP-associated enhancement is of similar absolute magnitude for the two anaesthetics, suggesting that IOP stress and anaesthetic effects operate on separate retinal mechanisms.

Serotonin elevates the c-wave of the electroretinogram of the rabbit eye by increasing the transepithelial potential

The influence of serotonin (5-hydroxytryptamine, 5-HT) and serotonin analogues on the direct current electroretinogram (d.c. ERG) and the standing potential of the albino rabbit eye (SP) was studied. After unilateral vitrectomy, corneal recordings were obtained during simultaneous intravitreal perfusion with a control solution alternating with 5-HT at concentrations of 25, 120 and 200 microM. The c-wave increased at 25 and 120 microM when changing from control solution to test solution (P < 0.05) but did not decrease significantly when changing back to control solution (P > 0.05). The c-wave was reversibly elevated at 200 microM (PHS-5-HT, P < 0.01; 5-HT-PHS, P < 0.05). To analyse further the influence on the c-wave, in vivo intraretinal microelectrode recordings were obtained during intravitreal perfusion with 5-HT. The transepithelial potential (TEP) increased (P < 0.01), while the slow PIII was not significantly affected (P > 0.05). The serotonin receptor agonists 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane, 5-methoxytryptamine, alpha-methyl-5-hydroxytryptamine and 2-methyl-5-hydroxytryptamine, caused a significant reversible elevation of the c-wave, whereas 5-carboxyamidotryptamine did not. Tropisetron did not block the serotonin effect and LY53857 had an effect of its own on the c-wave. The results seem to indicate that the influence of serotonin on the c-wave is mainly due to an effect on the retinal pigment epithelium (RPE) and that more than one type of serotonin receptor may be involved.

Effects of quisqualic acid on the corneal and intraretinal direct-current electroretinogram and on the standing potential of the rabbit eye

Quisqualic acid, an excitatory amino acid agonist, has been shown to stimulate inositol phosphate production in the rabbit retina. Inositol trisphosphate serves as a second messenger and increases intracellular calcium. We investigated the influence of quisqualic acid on the direct-current electroretinogram and on the standing potential of the rabbit eye. After unilateral vitrectomy, the corneal direct-current electroretinogram and the standing potential were recorded from both eyes of albino rabbits during simultaneous unilateral intravitreal perfusion with quisqualic acid alternating with control solution. The contralateral eye was used as a control. Intravitreal perfusion with 100-microM and 200-microM quisqualic acid elevated the standing potential significantly. This elevation was accompanied by a significant increase in c-wave amplitude and a significant decrease in b-wave amplitude. Quisqualic acid at 200-microM concentration decreased the a-wave amplitude also. In vivo intraretinal recordings showed that intravitreal perfusion with quisqualic acid at 200-microM concentration significantly increased the retinal pigment epithelial component of the c-wave. We conclude that quisqualic acid influences the direct-current electroretinogram and the standing potential apparently through its action on the retinal pigment epithelium. A possible mode of action is increased production of inositol trisphosphate, followed by an increase in intracellular release of calcium ions and an increase in basal chloride conductance. The decrease in a- and b-wave amplitudes indicates direct effects of quisqualic acid also on the neural retina.

Effects of PhXA41, a prostaglandin analogue, and PGF2 alpha on the corneal and intraretinal d.c. electroretinogram (ERG) of the albino rabbit eye

PhXA41, a prostaglandin F2 alpha analogue, is a new drug alternative for medical treatment of glaucoma. From earlier studies, it is known that certain prostaglandins (pgs) exert an influence on the d.c. electroretinogram (ERG). In the present study, possible effects of intraocular perfusion with PhXA41 (free acid) or PGF2 alpha on the rabbit retina and retinal pigment epithelium (RPE) were evaluated. Unilateral vitrectomy was performed and a continuous intraocular perfusion was established, in which a reference solution was alternated with the test solution. In part one of the experiments, the corneal d.c. ERG and the standing potential of the eye (SP) were recorded from both eyes. In part two, vitreal and intraretinal d.c. ERG were recorded in the vitrectomized eye. PhXA41 and PGF2 alpha produced similar effects, i.e. a significant and reversible c-wave increase at concentrations of 1-10 micrograms/ml. No influence was found in the concentration interval 10-100 ng/ml. There were no changes in a- and b-wave amplitudes at any of the concentrations tested. At a concentration of 10 micrograms/ml, the SP increased significantly. The intraretinal d.c. ERG showed a large reduction of slow PIII accompanied by a small change in transepithelial potential (TEP). Theoretically, a vitreous concentration of less than 1 ng/ml may be reached after topical application. This concentration level does not influence the d.c. ERG in this animal model. At higher concentrations, a significant and reversible c-wave increase seemed to be generated mainly in the inner retina by a reduction of slow PIII generated in the Müller cells. The effect of PhXA41 appears to be mediated by an FP receptor.